Synthesis, Structure, and Magnetic Properties of Co, Ni, and Co−Ni Alloy Nanocluster-Doped SiO2 Films by Sol−Gel Processing
Identifieur interne : 000619 ( Istex/Corpus ); précédent : 000618; suivant : 000620Synthesis, Structure, and Magnetic Properties of Co, Ni, and Co−Ni Alloy Nanocluster-Doped SiO2 Films by Sol−Gel Processing
Auteurs : G. Mattei ; C. De Julián Fernández ; P. Mazzoldi ; C. Sada ; G. De ; G. Battaglin ; C. Sangregorio ; D. GatteschiSource :
- Chemistry of Materials [ 0897-4756 ] ; 2002.
Abstract
The optical, structural, and magnetic properties of Co, Ni, and CoxNi100-x (0 < x < 100) alloy nanoclusters in silica host obtained by the sol−gel route are presented. Throughout the entire range of composition investigated, from pure Ni to pure Co, the nanoclusters exhibit an fcc structure, with lattice parameters increasing with the Co fraction in the system. This indicates Co−Ni alloy formation. It has also been observed that the average cluster diameter increases with increasing Co fraction in the system. An enhancement of the magnetic moment was exhibited in the pure Co and Ni samples, and good agreement between the measured values and those of the corresponding bulk alloys was found for the other samples. All composites are superparamagnetic at room temperature.
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DOI: 10.1021/cm021106r
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Marta 2137,I-30123 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</mods:affiliation></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">ISTEX</idno><idno type="RBID">ISTEX:D5F926784691B48FE5A9B062D997E614373EEE5E</idno><date when="2002" year="2002">2002</date><idno type="doi">10.1021/cm021106r</idno><idno type="url">https://api.istex.fr/ark:/67375/TPS-641Q40Q9-V/fulltext.pdf</idno><idno type="wicri:Area/Istex/Corpus">000619</idno><idno type="wicri:explorRef" wicri:stream="Istex" wicri:step="Corpus" wicri:corpus="ISTEX">000619</idno></publicationStmt><sourceDesc><biblStruct><analytic><title level="a" type="main" xml:lang="en">Synthesis, Structure, and Magnetic Properties of Co, Ni,
and Co−Ni Alloy Nanocluster-Doped SiO<hi rend="subscript">2</hi> Films by
Sol−Gel Processing</title><author><name sortKey="Mattei, G" sort="Mattei, G" uniqKey="Mattei G" first="G." last="Mattei">G. Mattei</name><affiliation><mods:affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</mods:affiliation></affiliation><affiliation><mods:affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</mods:affiliation></affiliation><affiliation><mods:affiliation> Corresponding author. Fax: +39.049.8277003. Phone: +39.049.8277045. E-mail: mattei@padova.infm.it.</mods:affiliation></affiliation></author><author><name sortKey="De Julian Fernandez, C" sort="De Julian Fernandez, C" uniqKey="De Julian Fernandez C" first="C." last="De Julián Fernández">C. De Julián Fernández</name><affiliation><mods:affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</mods:affiliation></affiliation><affiliation><mods:affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</mods:affiliation></affiliation></author><author><name sortKey="Mazzoldi, P" sort="Mazzoldi, P" uniqKey="Mazzoldi P" first="P." last="Mazzoldi">P. Mazzoldi</name><affiliation><mods:affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</mods:affiliation></affiliation><affiliation><mods:affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</mods:affiliation></affiliation></author><author><name sortKey="Sada, C" sort="Sada, C" uniqKey="Sada C" first="C." last="Sada">C. Sada</name><affiliation><mods:affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</mods:affiliation></affiliation><affiliation><mods:affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</mods:affiliation></affiliation></author><author><name sortKey="De, G" sort="De, G" uniqKey="De G" first="G." last="De">G. De</name><affiliation><mods:affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</mods:affiliation></affiliation><affiliation><mods:affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</mods:affiliation></affiliation></author><author><name sortKey="Battaglin, G" sort="Battaglin, G" uniqKey="Battaglin G" first="G." last="Battaglin">G. Battaglin</name><affiliation><mods:affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</mods:affiliation></affiliation><affiliation><mods:affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</mods:affiliation></affiliation></author><author><name sortKey="Sangregorio, C" sort="Sangregorio, C" uniqKey="Sangregorio C" first="C." last="Sangregorio">C. Sangregorio</name><affiliation><mods:affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</mods:affiliation></affiliation><affiliation><mods:affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</mods:affiliation></affiliation></author><author><name sortKey="Gatteschi, D" sort="Gatteschi, D" uniqKey="Gatteschi D" first="D." last="Gatteschi">D. Gatteschi</name><affiliation><mods:affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</mods:affiliation></affiliation><affiliation><mods:affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</mods:affiliation></affiliation><affiliation><mods:affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</mods:affiliation></affiliation></author></analytic><monogr></monogr><series><title level="j" type="main">Chemistry of Materials</title><title level="j" type="abbrev">Chem. Mater.</title><idno type="ISSN">0897-4756</idno><idno type="eISSN">1520-5002</idno><imprint><publisher>American Chemical Society</publisher><date type="e-published" when="2002-07-12">2002</date><date when="2002-08-19">2002</date><biblScope unit="vol">14</biblScope><biblScope unit="issue">8</biblScope><biblScope unit="page" from="3440">3440</biblScope><biblScope unit="page" to="3447">3447</biblScope></imprint><idno type="ISSN">0897-4756</idno></series></biblStruct></sourceDesc><seriesStmt><idno type="ISSN">0897-4756</idno></seriesStmt></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract">The optical, structural, and magnetic properties of Co, Ni, and CoxNi100-x (0 < x < 100) alloy nanoclusters in silica host obtained by the sol−gel route are presented. Throughout the entire range of composition investigated, from pure Ni to pure Co, the nanoclusters exhibit an fcc structure, with lattice parameters increasing with the Co fraction in the system. This indicates Co−Ni alloy formation. It has also been observed that the average cluster diameter increases with increasing Co fraction in the system. An enhancement of the magnetic moment was exhibited in the pure Co and Ni samples, and good agreement between the measured values and those of the corresponding bulk alloys was found for the other samples. All composites are superparamagnetic at room temperature.</div></front></TEI><istex><corpusName>acs</corpusName><keywords><teeft><json:string>phys</json:string><json:string>saed</json:string><json:string>magnetic moment</json:string><json:string>annealing</json:string><json:string>sio2</json:string><json:string>nanoparticles</json:string><json:string>magn</json:string><json:string>chem</json:string><json:string>lattice parameter</json:string><json:string>appl</json:string><json:string>nanoclusters</json:string><json:string>molar ratio</json:string><json:string>room temperature</json:string><json:string>magnetic property</json:string><json:string>saed analysis</json:string><json:string>molar</json:string><json:string>matrix</json:string><json:string>optical absorption spectrum</json:string><json:string>good agreement</json:string><json:string>optical spectrum</json:string><json:string>nanostructured material</json:string><json:string>alloy</json:string><json:string>mater</json:string><json:string>silica</json:string><json:string>silica host</json:string><json:string>bulk alloy</json:string><json:string>saed pattern</json:string><json:string>superparamagnetic behavior</json:string><json:string>lattice</json:string><json:string>compositional analysis</json:string><json:string>intermediate alloy</json:string><json:string>corresponding bulk alloy</json:string><json:string>heating rate</json:string><json:string>singh nalwa</json:string><json:string>physical property</json:string><json:string>magnetic field</json:string><json:string>silica slide</json:string><json:string>academic press</json:string><json:string>sio2 film chem</json:string><json:string>planar view</json:string><json:string>average value standard deviation</json:string><json:string>alloy nanoclusters</json:string><json:string>alloy formation</json:string><json:string>film thickness</json:string><json:string>other sample</json:string><json:string>demagnetization curve</json:string><json:string>small particle size</json:string><json:string>systematic structural</json:string><json:string>structural feature</json:string></teeft></keywords><author><json:item><name>MATTEI G.</name><affiliations><json:string>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</json:string><json:string>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</json:string><json:string>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</json:string><json:string>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</json:string><json:string>Corresponding author. Fax: +39.049.8277003. Phone: +39.049.8277045. E-mail: mattei@padova.infm.it.</json:string></affiliations></json:item><json:item><name>DE JULIáN FERNáNDEZ C.</name><affiliations><json:string>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</json:string><json:string>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</json:string><json:string>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</json:string><json:string>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</json:string></affiliations></json:item><json:item><name>MAZZOLDI P.</name><affiliations><json:string>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</json:string><json:string>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</json:string><json:string>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</json:string><json:string>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</json:string></affiliations></json:item><json:item><name>SADA C.</name><affiliations><json:string>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</json:string><json:string>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</json:string><json:string>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</json:string><json:string>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</json:string></affiliations></json:item><json:item><name>DE G.</name><affiliations><json:string>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</json:string><json:string>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</json:string><json:string>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</json:string><json:string>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</json:string></affiliations></json:item><json:item><name>BATTAGLIN G.</name><affiliations><json:string>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</json:string><json:string>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</json:string><json:string>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</json:string><json:string>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</json:string></affiliations></json:item><json:item><name>SANGREGORIO C.</name><affiliations><json:string>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</json:string><json:string>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</json:string><json:string>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</json:string><json:string>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</json:string></affiliations></json:item><json:item><name>GATTESCHI D.</name><affiliations><json:string>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</json:string><json:string>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</json:string><json:string>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</json:string><json:string>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</json:string></affiliations></json:item></author><arkIstex>ark:/67375/TPS-641Q40Q9-V</arkIstex><language><json:string>unknown</json:string></language><originalGenre><json:string>research-article</json:string></originalGenre><abstract>The optical, structural, and magnetic properties of Co, Ni, and CoxNi100-x (0 > x > 100) alloy nanoclusters in silica host obtained by the sol−gel route are presented. Throughout the entire range of composition investigated, from pure Ni to pure Co, the nanoclusters exhibit an fcc structure, with lattice parameters increasing with the Co fraction in the system. This indicates Co−Ni alloy formation. It has also been observed that the average cluster diameter increases with increasing Co fraction in the system. An enhancement of the magnetic moment was exhibited in the pure Co and Ni samples, and good agreement between the measured values and those of the corresponding bulk alloys was found for the other samples. All composites are superparamagnetic at room temperature.</abstract><qualityIndicators><score>7.827</score><pdfWordCount>4351</pdfWordCount><pdfCharCount>25229</pdfCharCount><pdfVersion>1.2</pdfVersion><pdfPageCount>8</pdfPageCount><pdfPageSize>612 x 792 pts (letter)</pdfPageSize><pdfWordsPerPage>544</pdfWordsPerPage><pdfText>true</pdfText><refBibsNative>true</refBibsNative><abstractWordCount>123</abstractWordCount><abstractCharCount>778</abstractCharCount><keywordCount>0</keywordCount></qualityIndicators><title>Synthesis, Structure, and Magnetic Properties of Co, Ni, and Co−Ni Alloy Nanocluster-Doped SiO2 Films by Sol−Gel Processing</title><genre><json:string>research-article</json:string></genre><host><title>Chemistry of Materials</title><language><json:string>unknown</json:string></language><issn><json:string>0897-4756</json:string></issn><eissn><json:string>1520-5002</json:string></eissn><volume>14</volume><issue>8</issue><pages><first>3440</first><last>3447</last></pages><genre><json:string>journal</json:string></genre></host><ark><json:string>ark:/67375/TPS-641Q40Q9-V</json:string></ark><categories><wos><json:string>1 - science</json:string><json:string>2 - materials science, multidisciplinary</json:string><json:string>2 - chemistry, physical</json:string></wos><scienceMetrix><json:string>1 - applied sciences</json:string><json:string>2 - enabling & strategic technologies</json:string><json:string>3 - materials</json:string></scienceMetrix><scopus><json:string>1 - Physical Sciences</json:string><json:string>2 - Materials Science</json:string><json:string>3 - Materials Chemistry</json:string><json:string>1 - Physical Sciences</json:string><json:string>2 - Chemical Engineering</json:string><json:string>3 - General Chemical Engineering</json:string><json:string>1 - Physical Sciences</json:string><json:string>2 - Chemistry</json:string><json:string>3 - General Chemistry</json:string></scopus><inist><json:string>1 - sciences appliquees, technologies et medecines</json:string><json:string>2 - sciences exactes et technologie</json:string></inist></categories><publicationDate>2002</publicationDate><copyrightDate>2002</copyrightDate><doi><json:string>10.1021/cm021106r</json:string></doi><id>D5F926784691B48FE5A9B062D997E614373EEE5E</id><score>1</score><fulltext><json:item><extension>pdf</extension><original>true</original><mimetype>application/pdf</mimetype><uri>https://api.istex.fr/ark:/67375/TPS-641Q40Q9-V/fulltext.pdf</uri></json:item><json:item><extension>zip</extension><original>false</original><mimetype>application/zip</mimetype><uri>https://api.istex.fr/ark:/67375/TPS-641Q40Q9-V/bundle.zip</uri></json:item><json:item><extension>txt</extension><original>false</original><mimetype>text/plain</mimetype><uri>https://api.istex.fr/ark:/67375/TPS-641Q40Q9-V/fulltext.txt</uri></json:item><istex:fulltextTEI uri="https://api.istex.fr/ark:/67375/TPS-641Q40Q9-V/fulltext.tei"><teiHeader><fileDesc><titleStmt><title level="a" type="main">Synthesis, Structure, and Magnetic Properties of Co, Ni,
and Co−Ni Alloy Nanocluster-Doped SiO2 Films by
Sol−Gel Processing</title></titleStmt><publicationStmt><authority>ISTEX</authority><publisher>American Chemical Society</publisher><availability><licence>Copyright © 2002 American Chemical Society</licence><p>American Chemical Society</p></availability><date type="e-published" when="2002-07-12">2002</date><date when="2002-08-19">2002</date><date type="Copyright" when="2002">2002</date></publicationStmt><notesStmt><note type="content-type" source="research-article" scheme="https://content-type.data.istex.fr/ark:/67375/XTP-1JC4F85T-7">research-article</note><note type="publication-type" scheme="https://publication-type.data.istex.fr/ark:/67375/JMC-0GLKJH51-B">journal</note></notesStmt><sourceDesc><biblStruct type="article"><analytic><title level="a" type="main" xml:lang="en">Synthesis, Structure, and Magnetic Properties of Co, Ni,
and Co−Ni Alloy Nanocluster-Doped SiO<hi rend="subscript">2</hi> Films by
Sol−Gel Processing</title><author xml:id="author-0000" role="corresp"><persName><surname>Mattei</surname><forename type="first">G.</forename></persName><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy
</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation><affiliation role="corresp"> Corresponding author. Fax: +39.049.8277003. Phone: +39.049. 8277045. E-mail: mattei@padova.infm.it.</affiliation></author><author xml:id="author-0001"><persName><surname>de Julián Fernández</surname><forename type="first">C.</forename></persName><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy
</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation></author><author xml:id="author-0002"><persName><surname>Mazzoldi</surname><forename type="first">P.</forename></persName><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy
</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation></author><author xml:id="author-0003"><persName><surname>Sada</surname><forename type="first">C.</forename></persName><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy
</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation></author><author xml:id="author-0004"><persName><surname>De</surname><forename type="first">G.</forename></persName><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</affiliation><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy
</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation></author><author xml:id="author-0005"><persName><surname>Battaglin</surname><forename type="first">G.</forename></persName><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</affiliation><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy
</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation></author><author xml:id="author-0006"><persName><surname>Sangregorio</surname><forename type="first">C.</forename></persName><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy
</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation></author><author xml:id="author-0007"><persName><surname>Gatteschi</surname><forename type="first">D.</forename></persName><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy
</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</affiliation></author><idno type="istex">D5F926784691B48FE5A9B062D997E614373EEE5E</idno><idno type="ark">ark:/67375/TPS-641Q40Q9-V</idno><idno type="DOI">10.1021/cm021106r</idno></analytic><monogr><title level="j" type="main">Chemistry of Materials</title><title level="j" type="abbrev">Chem. Mater.</title><idno type="acspubs">cm</idno><idno type="coden">cmatex</idno><idno type="pISSN">0897-4756</idno><idno type="eISSN">1520-5002</idno><imprint><publisher>American Chemical Society</publisher><date type="e-published" when="2002-07-12">2002</date><date when="2002-08-19">2002</date><biblScope unit="vol">14</biblScope><biblScope unit="issue">8</biblScope><biblScope unit="page" from="3440">3440</biblScope><biblScope unit="page" to="3447">3447</biblScope></imprint></monogr></biblStruct></sourceDesc></fileDesc><profileDesc><abstract><p>The optical, structural, and magnetic properties of Co, Ni, and Co<hi rend="italic"><hi rend="subscript">x</hi></hi>Ni<hi rend="subscript">100</hi><hi rend="subscript">-</hi><hi rend="italic"><hi rend="subscript">x</hi></hi> (0 < <hi rend="italic">x</hi> < 100)
alloy nanoclusters in silica host obtained by the sol−gel route are presented. Throughout
the entire range of composition investigated, from pure Ni to pure Co, the nanoclusters exhibit
an fcc structure, with lattice parameters increasing with the Co fraction in the system. This
indicates Co−Ni alloy formation. It has also been observed that the average cluster diameter
increases with increasing Co fraction in the system. An enhancement of the magnetic moment
was exhibited in the pure Co and Ni samples, and good agreement between the measured
values and those of the corresponding bulk alloys was found for the other samples. All
composites are superparamagnetic at room temperature.
</p></abstract><textClass ana="subject"><keywords scheme="document-type-name"><term>Article</term></keywords></textClass><langUsage><language ident="zxx"></language></langUsage></profileDesc></teiHeader></istex:fulltextTEI></fulltext><metadata><istex:metadataXml wicri:clean="corpus acs not found" wicri:toSee="no header"><istex:xmlDeclaration>version="1.0" encoding="UTF-8"</istex:xmlDeclaration><istex:document><article article-type="research-article" specific-use="acs2jats-1.1.23" dtd-version="1.1d1"><front><journal-meta><journal-id journal-id-type="acspubs">cm</journal-id><journal-id journal-id-type="coden">cmatex</journal-id><journal-title-group><journal-title>Chemistry of Materials</journal-title><abbrev-journal-title>Chem. Mater.</abbrev-journal-title></journal-title-group><issn pub-type="ppub">0897-4756</issn><issn pub-type="epub">1520-5002</issn><publisher><publisher-name>American Chemical Society</publisher-name></publisher><self-uri>pubs.acs.org/cm</self-uri></journal-meta><article-meta><article-id pub-id-type="doi">10.1021/cm021106r</article-id><article-categories><subj-group subj-group-type="document-type-name"><subject>Article</subject></subj-group></article-categories><title-group><article-title>Synthesis, Structure, and Magnetic Properties of Co, Ni,
and Co−Ni Alloy Nanocluster-Doped SiO<sub>2</sub> Films by
Sol−Gel Processing</article-title></title-group><contrib-group><contrib contrib-type="author" corresp="yes"><name name-style="western"><surname>Mattei</surname><given-names>G.</given-names></name><xref rid="cm021106rAF1">*</xref></contrib><contrib contrib-type="author"><name name-style="western"><surname>de Julián Fernández</surname><given-names>C.</given-names></name></contrib><contrib contrib-type="author"><name name-style="western"><surname>Mazzoldi</surname><given-names>P.</given-names></name></contrib><contrib contrib-type="author"><name name-style="western"><surname>Sada</surname><given-names>C.</given-names></name></contrib><aff>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy
</aff></contrib-group><contrib-group><contrib contrib-type="author"><name name-style="western"><surname>De</surname><given-names>G.</given-names></name></contrib><aff>Sol−Gel Division, Central Glass and Ceramic Research Institute,
196 Raja S.C. Mullick Road, Kolkata 700032, India
</aff></contrib-group><contrib-group><contrib contrib-type="author"><name name-style="western"><surname>Battaglin</surname><given-names>G.</given-names></name></contrib><aff>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,
I-30123 Padova, Italy
</aff></contrib-group><contrib-group><contrib contrib-type="author"><name name-style="western"><surname>Sangregorio</surname><given-names>C.</given-names></name></contrib><contrib contrib-type="author"><name name-style="western"><surname>Gatteschi</surname><given-names>D.</given-names></name></contrib><aff>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,
I-50019 Sesto Fiorentino (Firenze), Italy
</aff></contrib-group><author-notes><corresp id="cm021106rAF1">
Corresponding author. Fax: +39.049.8277003. Phone: +39.049.
8277045. E-mail: mattei@padova.infm.it.</corresp></author-notes><pub-date pub-type="epub"><day>12</day><month>07</month><year>2002</year></pub-date><pub-date pub-type="ppub"><day>19</day><month>08</month><year>2002</year></pub-date><volume>14</volume><issue>8</issue><fpage>3440</fpage><lpage>3447</lpage><history><date date-type="received"><day>11</day><month>01</month><year>2002</year></date><date date-type="rev-recd"><day>22</day><month>05</month><year>2002</year></date><date date-type="asap"><day>12</day><month>07</month><year>2002</year></date><date date-type="issue-pub"><day>19</day><month>08</month><year>2002</year></date></history><permissions><copyright-statement>Copyright © 2002 American Chemical Society</copyright-statement><copyright-year>2002</copyright-year><copyright-holder>American Chemical Society</copyright-holder></permissions><abstract><p>The optical, structural, and magnetic properties of Co, Ni, and Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic> (0 < <italic toggle="yes">x</italic> < 100)
alloy nanoclusters in silica host obtained by the sol−gel route are presented. Throughout
the entire range of composition investigated, from pure Ni to pure Co, the nanoclusters exhibit
an fcc structure, with lattice parameters increasing with the Co fraction in the system. This
indicates Co−Ni alloy formation. It has also been observed that the average cluster diameter
increases with increasing Co fraction in the system. An enhancement of the magnetic moment
was exhibited in the pure Co and Ni samples, and good agreement between the measured
values and those of the corresponding bulk alloys was found for the other samples. All
composites are superparamagnetic at room temperature.
</p></abstract><custom-meta-group><custom-meta><meta-name>document-id-old-9</meta-name><meta-value>cm021106r</meta-value></custom-meta></custom-meta-group></article-meta></front><body><sec id="d7e193"><title>1. Introduction</title><p>The study of the physical and chemical properties of
nanoparticulated composites has received much interest
because of their new physical properties and their
potential applications.<xref rid="cm021106rb00001" ref-type="bibr"></xref> In particular, it has been observed that magnetic particles with sizes in the nanometer range exhibit features that are quite different
from those of the corresponding bulk magnets, including
modifications of the intrinsic properties,<named-content content-type="bibref-group"><xref rid="cm021106rb00002" ref-type="bibr"></xref>,<xref rid="cm021106rb00003" ref-type="bibr"></xref></named-content> superparamagnetism,<named-content content-type="bibref-group"><xref rid="cm021106rb00003" ref-type="bibr"></xref>−<xref rid="cm021106rb00004" specific-use="suppress-in-print" ref-type="bibr"></xref><xref rid="cm021106rb00005" ref-type="bibr"></xref></named-content> enhanced coercitivity,<xref rid="cm021106rb00003" ref-type="bibr"></xref> shifts of the
hysteresis loops,<named-content content-type="bibref-group"><xref rid="cm021106rb00006" ref-type="bibr"></xref>,<xref rid="cm021106rb00007" ref-type="bibr"></xref></named-content> or the absence of magnetic saturation
at high fields.<xref rid="cm021106rb00007" ref-type="bibr"></xref> Also, nanoparticles embedded in dielectric matrixes present enhanced magnetoresistance<named-content content-type="bibref-group"><xref rid="cm021106rb00008" ref-type="bibr"></xref>,<xref rid="cm021106rb00009" ref-type="bibr"></xref></named-content>
and good magnetooptical features.<xref rid="cm021106rb00010" ref-type="bibr"></xref> This departure from
bulklike behavior might be due to several reasons:
modifications of the electronic structure,<xref rid="cm021106rb00011" ref-type="bibr"></xref> the interplay
between surface and volume properties,<xref rid="cm021106rb00003" ref-type="bibr"></xref> chemical interactions between the nanocluster and the embedding
matrix,<named-content content-type="bibref-group"><xref rid="cm021106rb00012" ref-type="bibr"></xref>,<xref rid="cm021106rb00013" ref-type="bibr"></xref></named-content> or interparticle interactions.<xref rid="cm021106rb00005" ref-type="bibr"></xref> As a consequence, magnetic nanoclusters have been gaining importance in recent days because of their possible applications in magnetic recording media<sup>3</sup> and catalysis.<xref rid="cm021106rb00014" ref-type="bibr"></xref>
Moreover, alloy-based nanocomposites can add a further
degree of freedom, i.e., the composition, which is expected to play a crucial role in tailoring new magnetic
properties in such systems.<xref rid="cm021106rb00015" ref-type="bibr"></xref> Therefore, careful control
of these parameters is of paramount importance in the
synthesis of new alloy-based materials. This control can
be made difficult by the different chemical reactivities
of two metals with the matrix, which can promote
separation (via oxidation, for instance) instead of alloying of the species.
</p><p>The sol−gel technique is able to control both cluster<xref rid="cm021106rb00016" ref-type="bibr"></xref>
and matrix<xref rid="cm021106rb00017" ref-type="bibr"></xref> composition. From the point of view of
magnetic applications, this technique has been used to
synthesize alloy nanoclusters involving mainly transition metals, e.g., Nd−Fe,<xref rid="cm021106rb00018" ref-type="bibr"></xref> Fe−Cr,<xref rid="cm021106rb00019" ref-type="bibr"></xref> Fe−Ni,<xref rid="cm021106rb00020" ref-type="bibr"></xref> Cu−Ni,<xref rid="cm021106rb00021" ref-type="bibr"></xref>
and Fe−Co.<xref rid="cm021106rb00022" ref-type="bibr"></xref> In this paper, we report the sol−gel
preparation and systematic structural and compositional analysis of a series of nanoclusters consisting of
the ferromagnetic 3d elements Co and Ni and their
intermediate alloys embedded in a silica host. The
general molar composition of the nanoclusters is
Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic> with <italic toggle="yes">x</italic> = 100, 80, 75, 70, 66, 50, 33, and 0 to
cover the entire phase diagram of the Co−Ni system
from pure Co (<italic toggle="yes">x</italic> = 100) to pure Ni (<italic toggle="yes">x</italic> = 0). The Co−Ni
bulk alloy phase diagram presents, for Co compositions
larger than 70%, a structural transition from fcc to hcp
that modifies all of the physical properties of the
system.<xref rid="cm021106rb00023" ref-type="bibr"></xref> Indeed, Co−Ni alloys within this range of
composition present higher values of coercivity and
remanence and are used for magnetic recording media.<xref rid="cm021106rb00024" ref-type="bibr"></xref></p></sec><sec id="d7e317"><title>2. Experimental Section</title><p>Film compositions (in moles) are given in Table <xref rid="cm021106rt00001"></xref>. The pure
Co, pure Ni, and Co−Ni codoped silica films were prepared
by the sol−gel dip-coating technique. The sols were prepared
starting from Si(OC<sub>2</sub>H<sub>5</sub>)<sub>4</sub> (TEOS), Co(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O, Ni(NO<sub>3</sub>)<sub>3</sub>·6H<sub>2</sub>O, catalytic amount of HNO<sub>3</sub>, H<sub>2</sub>O, <italic toggle="yes">n</italic>-propanol, and isobutanol. The film composition [i.e., the molar ratio of total
metal (pure or mixed Co + Ni) to SiO<sub>2</sub>] was kept constant in
all of the films at 10 equiv mol % metal/90% SiO<sub>2</sub> (see Table
<xref rid="cm021106rt00001"></xref>). The general preparation method of these films is as
follows: The required amount of TEOS was first dissolved in
<italic toggle="yes">n</italic>-propanol (50% of the total amount). To this mixture, the
required amount of metal salt solution (dissolved in water and
then acidified with HNO<sub>3</sub>) was added with stirring. A residual
amount of <italic toggle="yes">n</italic>-propanol (50%) was then added, and the solution
was stirred for 30 min. After this period, isobutanol was added,
and the sol was stirred for another hour. The total H<sub>2</sub>O/TEOS
and HNO<sub>3</sub>/TEOS molar ratios were 6 and 0.01, respectively.
The water of crystallization of the metal salts was also taken
into account. The total equivalent oxide (including dopants)
content of the sols was about 5.5 wt % in all cases. Thoroughly
cleaned silica glasses (type II, Heraeus) were used as substrates. For the dip-coating preparation, a withdrawal velocity
of 10−12 cm/min was maintained. The resulting films were
dried at 60 °C. Then, the samples were first heated to 500 °C
in air (the heating rate was about 200 °C/h) to remove the
organic matter and to obtain pure and/or mixed cobalt−nickel
oxide-doped silica films. The temperature was then raised from
500 to 800 °C (the heating rate was 150 °C/h) in an 8% H<sub>2</sub>−92% N<sub>2</sub> gas atmosphere (in the following, indicated simply as
H<sub>2</sub>−N<sub>2</sub>) and held for 1 h. After the holding time, the furnace
was allowed to cool naturally to 450 °C (about 70 min) while
the H<sub>2</sub>−N<sub>2</sub> atmosphere was maintained. Then, the samples
were pushed to a cooler part of the furnace (about 300 °C) and
kept there for 15 min, after which they were further pushed
to a 40 °C region and kept there for 15 min. All operations
were done in H<sub>2</sub>−N<sub>2</sub> atmosphere. These final (i.e., after H<sub>2</sub>−N<sub>2</sub> annealing) samples are labeled Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic>, where <italic toggle="yes">x</italic> = 100,
80, 75, 70, 66, 50, 33, and 0, and have a general film
composition of 10 mol % metal−90% silica.
<table-wrap id="cm021106rt00001" position="float" orientation="portrait"><label>1</label><caption><p>Sol−Gel-Synthesized Co−Ni Nanocluster-Doped SiO<sub>2</sub> Films: Measured Film Thickness <italic toggle="yes">t</italic></p></caption><oasis:table colsep="2" rowsep="2"><oasis:tgroup cols="4"><oasis:colspec colnum="1" colname="1"></oasis:colspec><oasis:colspec colnum="2" colname="2"></oasis:colspec><oasis:colspec colnum="3" colname="3"></oasis:colspec><oasis:colspec colnum="4" colname="4"></oasis:colspec><oasis:tbody><oasis:row><oasis:entry namest="1" nameend="1">sample</oasis:entry><oasis:entry namest="2" nameend="2"><italic toggle="yes">t</italic> (nm)</oasis:entry><oasis:entry namest="3" nameend="3"><italic toggle="yes">C</italic><sub>Co</sub>/<italic toggle="yes">C</italic><sub>Ni</sub>/<italic toggle="yes">C</italic><sub>Si</sub><italic toggle="yes"><sup>a</sup></italic><sup></sup>
(nominal)</oasis:entry><oasis:entry namest="4" nameend="4">(<italic toggle="yes">C</italic><sub>Co</sub><sub> </sub>+ <italic toggle="yes">C</italic><sub>Ni</sub>)<italic toggle="yes"><sup>b</sup></italic><sup></sup>/<italic toggle="yes">C</italic><sub>Si</sub>
(RBS ± 5%)
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>100</sub></oasis:entry><oasis:entry colname="2">148 ± 3<italic toggle="yes"><sup>c</sup></italic><sup></sup></oasis:entry><oasis:entry colname="3">10:0:90
</oasis:entry><oasis:entry colname="4">9.5:90.5
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>80</sub>Ni<sub>20</sub></oasis:entry><oasis:entry colname="2">134 ± 3<italic toggle="yes"><sup>c</sup></italic><sup></sup></oasis:entry><oasis:entry colname="3">8:2:90
</oasis:entry><oasis:entry colname="4">10.5:89.5
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>75</sub>Ni<sub>25</sub></oasis:entry><oasis:entry colname="2">135 ± 3<italic toggle="yes"><sup>c</sup></italic><sup></sup></oasis:entry><oasis:entry colname="3">7.5:2.5:90
</oasis:entry><oasis:entry colname="4">10:90
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>70</sub>Ni<sub>30</sub></oasis:entry><oasis:entry colname="2">148 ± 3<italic toggle="yes"><sup>c</sup></italic><sup></sup></oasis:entry><oasis:entry colname="3">7:3:90
</oasis:entry><oasis:entry colname="4">10:90
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>66</sub>Ni<sub>33</sub></oasis:entry><oasis:entry colname="2">160 ± 3<italic toggle="yes"><sup>c</sup></italic><sup></sup></oasis:entry><oasis:entry colname="3">6.6:3.3:90
</oasis:entry><oasis:entry colname="4">9.9:90.1
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>50</sub>Ni<sub>50</sub></oasis:entry><oasis:entry colname="2">150 ± 15<italic toggle="yes"><sup>d</sup></italic><sup></sup></oasis:entry><oasis:entry colname="3">5:5:90
</oasis:entry><oasis:entry colname="4">10.5:89.5
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>33</sub>Ni<sub>66</sub></oasis:entry><oasis:entry colname="2">140 ± 14<italic toggle="yes"><sup>d</sup></italic><sup></sup></oasis:entry><oasis:entry colname="3">3.3:6.6:90
</oasis:entry><oasis:entry colname="4">9.6:90.4
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Ni<sub>100</sub></oasis:entry><oasis:entry colname="2">148 ± 3<italic toggle="yes"><sup>c</sup></italic><sup></sup></oasis:entry><oasis:entry colname="3">0:10:90
</oasis:entry><oasis:entry colname="4">10:90</oasis:entry></oasis:row></oasis:tbody></oasis:tgroup></oasis:table><table-wrap-foot><p><italic toggle="yes"><sup>a</sup></italic><sup></sup> Nominal molar concentrations <italic toggle="yes">C</italic> of Co, Ni, and Si.<italic toggle="yes"><sup>b</sup></italic><sup></sup> RBS
measured Co plus Ni molar concentration (the associated error is
about 5%).<italic toggle="yes"><sup>c</sup></italic><sup></sup> Measured by TEM.<italic toggle="yes"><sup>d</sup></italic><sup></sup> Measured by RBS.</p></table-wrap-foot></table-wrap></p><p>The cobalt−nickel, silicon, and oxygen contents in the films
were determined by Rutherford backscattering spectrometry
(RBS). A <sup>4</sup>He<sup>+</sup> beam at an energy of 2.2 MeV was used. Optical
absorption spectra were measured in the wavelength region
from 200 to 800 nm by using a Cary UV−vis−NIR dual-beam
spectrophotometer. Samples for transmission electron microscopy (TEM) were prepared in planar and cross-sectional views
as reported in ref <xref rid="cm021106rb00016" specific-use="ref-style=base-text" ref-type="bibr"></xref> and examined at the CNR-LAMEL
Institute in Bologna, Italy, with a Philips CM30-T TEM
instrument operating at 300 kV equipped with an EDAX
energy-dispersive X-ray spectrometer (EDS) for compositional
analysis. The magnetic characterization was carried out using
a Cryogenic S600 SQUID magnetometer with the magnetic
field applied in the plane of the silica slides and the magnetic
moment measured in that direction.
</p></sec><sec id="d7e688"><title>3. Results and Discussion</title><p>Silica films containing pure Co and Ni and six
intermediate Co−Ni alloys were prepared and their
compositions were checked by Rutherford backscattering (RBS) technique. As the atomic weights of Co and
Ni are similar, these metals cannot be differentiated by
RBS; therefore, only the total concentration of Co and
Ni was obtained by this technique. The results, reported
in Table <xref rid="cm021106rt00001"></xref>, were in good agreement with the nominal
compositions of the film samples.
</p><p><bold>3.1. Optical Absorption.</bold> After the air annealing at
500 °C for 30 min to remove the organic material, the
sol−gel films were faintly bluish in color and showed
characteristic absorption bands of Ni<sup>2+</sup> and Co<sup>2+</sup> ions
having <italic toggle="yes">T</italic><italic toggle="yes"><sub>d</sub></italic> symmetry in an amorphous silica host as has
also been observed by other workers.<named-content content-type="bibref-group"><xref rid="cm021106rb00025" ref-type="bibr"></xref>−<xref rid="cm021106rb00026" specific-use="suppress-in-print" ref-type="bibr"></xref><xref rid="cm021106rb00027" ref-type="bibr"></xref></named-content> These 500 °C
air-baked films were further heated in H<sub>2</sub>−N<sub>2</sub> atmosphere to 800 °C to reduce the metal ions. The evolution
of the optical spectrum of the sample Co<sub>50</sub>Ni<sub>50</sub> (after H<sub>2</sub>−N<sub>2</sub> annealing for 1 h) is presented in Figure <xref rid="cm021106rf00001"></xref>.
<fig id="cm021106rf00001" position="float" orientation="portrait"><label>1</label><caption><p>Evolution of the optical absorption spectrum of the sample Co<sub>50</sub>Ni<sub>50</sub> after air annealing for 30 min at 500 °C and
after a subsequent annealing in H<sub>2</sub>−N<sub>2</sub> for 1 h at 800 °C.</p></caption><graphic xlink:href="cm021106rf00001.tif" position="float" orientation="portrait"></graphic></fig></p><p>The optical spectra of pure Ni and Co nanocluster-doped films obtained after annealing at 800 °C in H<sub>2</sub>−N<sub>2</sub> atmosphere are displayed in Figure <xref rid="cm021106rf00002"></xref>a and are
similar to the theoretical spectra simulated considering
5-nm-diameter clusters embedded in SiO<sub>2</sub> (<italic toggle="yes">n</italic> = 1.46)
using Mie theory<xref rid="cm021106rb00028" ref-type="bibr"></xref> (see Figure <xref rid="cm021106rf00002"></xref>b). As can be seen, the
surface plasmon resonance (which is very sharp in noble
metals<sup>28</sup>) is almost completely damped by the interband
transitions involving partially filled d bands. Therefore,
the optical spectra exhibit only a very broad band
centered near 340 nm and a shoulder near 300 nm for
Ni- and Co-cluster doped SiO<sub>2</sub>, respectively. This is a
qualitative indication that metallic reduction took place
in both cases. The optical spectra of the samples
containing either Co or Ni are presented in Figure <xref rid="cm021106rf00002"></xref>c
and exhibit absorption bands that are intermediate
between those of pure Ni and Co systems. However,
because of the small differences in the optical spectra
as a function of composition, it is not possible to make
quantitative conclusions on the actual cluster composition.
<fig id="cm021106rf00002" position="float" orientation="portrait"><label>2</label><caption><p>(a) Optical absorption spectra of the samples Co<sub>100</sub>
and Ni<sub>100</sub> after annealing in H<sub>2</sub>−N<sub>2</sub> for 1 h at 800 °C. (b) Mie
simulation of the extinction cross section of 5-nm-diameter Co
or Ni clusters in SiO<sub>2</sub> (refractive index <italic toggle="yes">n</italic> = 1.46). (c) Optical
absorption spectra of the samples Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic> with <italic toggle="yes">x</italic> = 100, 75,
66, 50, 33, and 0. The spectra are vertically shifted for better
clarity.</p></caption><graphic xlink:href="cm021106rf00002.tif" position="float" orientation="portrait"></graphic></fig></p><p><bold>3.2. TEM. </bold>A systematic structural and compositional
investigation was carried out on the samples by TEM.
The bright-field cross-sectional micrographs of Co−Ni
samples are shown in Figure <xref rid="cm021106rf00003"></xref>. In all of the cases
investigated, well-defined spherical particles with dimensions in the nanometer range can be observed on
the amorphous background of the matrix in a layer of
about 120−150 nm thickness from the surface. The
sample Ni<sub>100</sub> exhibits some small voids inside the film
(Figure <xref rid="cm021106rf00003"></xref>a), suggesting that incomplete densification of
the matrix occurred during annealing. This phenomenon
is absent in all of the other samples. The film thickness
measured by TEM is reported in Table <xref rid="cm021106rt00001"></xref>, except for the
samples Co<sub>33</sub>Ni<sub>66</sub> and Co<sub>50</sub>Ni<sub>50</sub>, examined as planar
sections, for which the values obtained by RBS are
reported. Figure <xref rid="cm021106rf00004"></xref> shows the histograms of the diameter
distribution for all of the samples as obtained by TEM
analysis: nonlinear fits to this distribution with log−normal and Gaussian functions results in slightly lower
χ<sup>2</sup> values for the Gaussian distribution. A monotonic
evolution of the average particle size from 〈<italic toggle="yes">D</italic>〉 = 3.1 ±
1.1 nm (average value ± standard deviation of the
experimental distribution) to 〈<italic toggle="yes">D</italic>〉 = 6.5 ± 1.1 nm was
observed with increasing Co fraction in the system, as
reported in Figure <xref rid="cm021106rf00005"></xref> (the error bar at each experimental
point is the standard deviation).
<fig id="cm021106rf00003" position="float" orientation="portrait"><label>3</label><caption><p>Cross-sectional TEM bright-field images of the samples Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic>: (a) <italic toggle="yes">x</italic> = 0 (pure Ni), (b) <italic toggle="yes">x</italic> = 33 (planar view), (c) <italic toggle="yes">x</italic>
= 50 (planar view), (d) <italic toggle="yes">x</italic> = 66, (e) <italic toggle="yes">x</italic> = 70, (f) <italic toggle="yes">x</italic> = 75, (g) <italic toggle="yes">x</italic> = 80, (h) <italic toggle="yes">x</italic> = 100 (pure Co).</p></caption><graphic xlink:href="cm021106rf00003.tif" position="float" orientation="portrait"></graphic></fig><fig id="cm021106rf00004" position="float" orientation="portrait"><label>4</label><caption><p>Size distribution histograms from a TEM analysis of the samples Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic>: (a) <italic toggle="yes">x</italic> = 0 (pure Ni), (b) <italic toggle="yes">x</italic> = 33, (c) <italic toggle="yes">x</italic> = 50,
(d) <italic toggle="yes">x</italic> = 66, (e) <italic toggle="yes">x</italic> = 70, (f) <italic toggle="yes">x</italic> = 75, (g) <italic toggle="yes">x</italic> = 80, (h) <italic toggle="yes">x</italic> = 100 (pure Co).</p></caption><graphic xlink:href="cm021106rf00004.tif" position="float" orientation="portrait"></graphic></fig><fig id="cm021106rf00005" position="float" orientation="portrait"><label>5</label><caption><p>Evolution of the average diameter as a function of the Co fraction in the system deduced from the TEM size distribution in the samples Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic>.
</p></caption><graphic xlink:href="cm021106rf00005.tif" position="float" orientation="portrait"></graphic></fig></p><p>We also analyzed the samples with selected area
electron diffraction (SAED) to investigate the crystalline
structure of the clusters. For all of the analyzed compositions, the SAED pattern exhibits Debye−Scherrer
rings of a single fcc phase, as shown in Figure <xref rid="cm021106rf00006"></xref>a for
the Co<sub>70</sub>Ni<sub>30</sub> sample. Considering that the bulk Co−Ni
alloy presents (according to the bulk phase diagram) a
single hcp alloy phase when the Co fraction is greater
than 75%,<xref rid="cm021106rb00029" ref-type="bibr"></xref> this result is clear evidence of the stabilization of an otherwise metastable phase occurring either
as a size effect or as a result of an increased interaction
between the particle surface and the matrix. The fact
that this stabilization of the fcc phase is not simply
triggered by the Ni content can be further understood
by considering that the Co<sub>100</sub> sample also exhibits a
single fcc phase. Indeed, the bulk stable Co structure
at room temperature is hcp (<italic toggle="yes">P</italic>6<sub>3</sub>/<italic toggle="yes">mmc</italic>, with <italic toggle="yes">a</italic> = 0.2505
nm and <italic toggle="yes">c</italic> = 0.4060 nm), which transforms via a
martensitic transition into a Co fcc phase (<italic toggle="yes">Fm</italic>3<italic toggle="yes">m</italic>, <italic toggle="yes">a</italic> =
0.354 47 nm) that is stable above 450 °C.
<fig id="cm021106rf00006" position="float" orientation="portrait"><label>6</label><caption><p>(a) SAED pattern of Co<sub>70</sub>Ni<sub>30</sub> after annealing in
H<sub>2</sub>−N<sub>2</sub> for 1 h at 800 °C. (b) Radially averaged SAED profiles
of some Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic> samples after annealing in H<sub>2</sub>−N<sub>2</sub> for 1 h
at 800 °C: (upper panel) <italic toggle="yes">x</italic> = 50, (middle panel) <italic toggle="yes">x</italic> = 75, (lower
panel) <italic toggle="yes">x</italic> = 100 (pure Co). Vertical lines indicate the reflections
for bulk fcc Co.</p></caption><graphic xlink:href="cm021106rf00006.tif" position="float" orientation="portrait"></graphic></fig></p><p>Bulk nickel has an fcc structure (<italic toggle="yes">Fm</italic>3<italic toggle="yes">m</italic>, <italic toggle="yes">a</italic> = 0.352 38
nm), and its lattice parameter differs from that of the
bulk Co fcc phase by 0.0021 nm, which is at the limit of
SAED quantification technique. Therefore, to better
calibrate the SAED pattern, an internal Si standard was
used (a piece of monocrystalline Si was glued in front
of the sample during TEM sample preparation). The
SAED spectra were radially averaged,<xref rid="cm021106rb00030" ref-type="bibr"></xref> and from the
resulting intensity profiles (presented in Figure <xref rid="cm021106rf00006"></xref>b), the
lattice parameter was extracted. The results of the
SAED analysis are summarized in Table <xref rid="cm021106rt00002"></xref> for all of the
compositions investigated. Just for comparison, Table
<xref rid="cm021106rt00002"></xref> also reports the fcc lattice parameters of a theoretical
Vegard Co−Ni alloy calculated at the Co fractions
corresponding to the Co/Ni molar ratios measured by
EDS at the Co K and Ni K shells (last column of Table
<xref rid="cm021106rt00002"></xref>). Even if the error bars in the SAED quantification
are not much smaller than the difference between lattice
parameters of bulk fcc Co and fcc Ni structures, Figure
<xref rid="cm021106rf00007"></xref> shows an increase in the SAED measured lattice
constant as a function of Co fraction in the system,
indicating Co−Ni fcc alloy formation. No evidence of
crystalline cobalt−nickel oxides or silicates (which
would give reflections at 1/<italic toggle="yes">d</italic> values lower than those
actually observed) can be extracted from the SAED
analysis (see, for instance, the SAED pattern in Figure
<xref rid="cm021106rf00006"></xref>a). Up to now, it has not been clear why the Co−Ni
system also stabilizes in our samples in the fcc phase
when the Co molar fraction in the system is near 1. It
is interesting to mention that Co−Ni alloy clusters with
similar or smaller sizes, embedded in silica, have been
synthesized by our group<xref rid="cm021106rb00031" ref-type="bibr"></xref> by sequential ion implantation of Co and Ni at different doses without postimplantation annealing. In that case, above Co fractions
of about 0.7, the coexistence of fcc and hcp phases was
evidenced by SAED analysis. This is an indication that
size is not the main factor triggering the final structure
of the clusters. We stress the fact that, during sol−gel
synthesis, both the matrix and the Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic> clusters
are formed at high temperature (800 °C), where the fcc
phase is the stable phase for the bulk. It is well-known
that the martensitic transformation from fcc to hcp bulk
Co generally ends with coexistence of the two phases
and that it is not suppressed by fast cooling rates.<xref rid="cm021106rb00032" ref-type="bibr"></xref>
Therefore, a possible explanation of our results could
be the fact that, during the cooling process (which lasts
for about 2 h and, therefore does not quench the system),
there is no strong driving force that constrains the
clusters to change their phase, allowing the system to
relax into the fcc state, whose stabilization at room
temperature can be further assisted by the nanometric
dimensions of the particles.<xref rid="cm021106rb00033" ref-type="bibr"></xref><fig id="cm021106rf00007" position="float" orientation="portrait"><label>7</label><caption><p>Evolution of the fcc lattice parameter (filled circles) as a function of the Co fraction in the system obtained from SAED analysis of the samples Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic>. For comparison, the
solid line represents the calculated Vegard lattice parameters
of a Co−Ni fcc alloy.</p></caption><graphic xlink:href="cm021106rf00007.tif" position="float" orientation="portrait"></graphic></fig><table-wrap id="cm021106rt00002" position="float" orientation="portrait"><label>2</label><caption><p>TEM−SAED Results on Co−Ni Nanocluster-Doped SiO<sub>2</sub> Films</p></caption><oasis:table colsep="2" rowsep="2"><oasis:tgroup cols="6"><oasis:colspec colnum="1" colname="1"></oasis:colspec><oasis:colspec colnum="2" colname="2"></oasis:colspec><oasis:colspec colnum="3" colname="3"></oasis:colspec><oasis:colspec colnum="4" colname="4"></oasis:colspec><oasis:colspec colnum="5" colname="5"></oasis:colspec><oasis:colspec colnum="6" colname="6"></oasis:colspec><oasis:tbody><oasis:row><oasis:entry namest="1" nameend="1">sample</oasis:entry><oasis:entry namest="2" nameend="2">cluster
diameter<italic toggle="yes"><sup>a</sup></italic><sup></sup>
(nm)</oasis:entry><oasis:entry namest="3" nameend="3"><italic toggle="yes">a</italic><sub>fcc</sub><italic toggle="yes"><sup>b</sup></italic><sup></sup> (nm)
(SAED)</oasis:entry><oasis:entry namest="4" nameend="4"><italic toggle="yes">a</italic><sub>fcc</sub><italic toggle="yes"><sup>c</sup></italic><sup></sup><sub> </sub>(nm)
(Vegard's
law)</oasis:entry><oasis:entry namest="5" nameend="5"><italic toggle="yes">C</italic><sub>Co</sub>/<italic toggle="yes">C</italic><sub>Ni</sub><italic toggle="yes"><sup>d</sup></italic><sup></sup>
(nominal)</oasis:entry><oasis:entry namest="6" nameend="6"><italic toggle="yes">C</italic><sub>Co</sub>/<italic toggle="yes">C</italic><sub>Ni</sub><italic toggle="yes"><sup>e</sup></italic><sup></sup>
(EDS)
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>100</sub></oasis:entry><oasis:entry colname="2">6.5 ± 1.7
</oasis:entry><oasis:entry colname="3">0.3545(12)
</oasis:entry><oasis:entry colname="4">0.354 47
</oasis:entry><oasis:entry colname="5">−
</oasis:entry><oasis:entry colname="6">−
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>80</sub>Ni<sub>20</sub></oasis:entry><oasis:entry colname="2">6.1 ± 1.7
</oasis:entry><oasis:entry colname="3">0.3539(12)
</oasis:entry><oasis:entry colname="4">0.354 05
</oasis:entry><oasis:entry colname="5">4.0
</oasis:entry><oasis:entry colname="6">3.6 ± 0.2
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>75</sub>Ni<sub>25</sub></oasis:entry><oasis:entry colname="2">4.7 ± 1.3
</oasis:entry><oasis:entry colname="3">0.3540(12)
</oasis:entry><oasis:entry colname="4">0.353 95
</oasis:entry><oasis:entry colname="5">3.0
</oasis:entry><oasis:entry colname="6">2.9 ± 0.2
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>70</sub>Ni<sub>30</sub></oasis:entry><oasis:entry colname="2">4.7 ± 1.2
</oasis:entry><oasis:entry colname="3">0.3536(12)
</oasis:entry><oasis:entry colname="4">0.353 84
</oasis:entry><oasis:entry colname="5">2.33
</oasis:entry><oasis:entry colname="6">2.2 ± 0.2
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>66</sub>Ni<sub>33</sub></oasis:entry><oasis:entry colname="2">4.1 ± 1.3
</oasis:entry><oasis:entry colname="3">0.3538(15)
</oasis:entry><oasis:entry colname="4">0.353 77
</oasis:entry><oasis:entry colname="5">2.0
</oasis:entry><oasis:entry colname="6">1.9 ± 0.1
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>50</sub>Ni<sub>50</sub></oasis:entry><oasis:entry colname="2">3.5 ± 1.0
</oasis:entry><oasis:entry colname="3">0.3532(15)
</oasis:entry><oasis:entry colname="4">0.353 42
</oasis:entry><oasis:entry colname="5">1.0
</oasis:entry><oasis:entry colname="6">1.0 ± 0.1
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Co<sub>33</sub>Ni<sub>66</sub></oasis:entry><oasis:entry colname="2">3.3 ± 1.2
</oasis:entry><oasis:entry colname="3">0.3528(18)
</oasis:entry><oasis:entry colname="4">0.353 08
</oasis:entry><oasis:entry colname="5">0.5
</oasis:entry><oasis:entry colname="6">0.51 ± 0.04
</oasis:entry></oasis:row><oasis:row><oasis:entry colname="1">Ni<sub>100</sub></oasis:entry><oasis:entry colname="2">3.1 ± 1.0
</oasis:entry><oasis:entry colname="3">0.3519(12)
</oasis:entry><oasis:entry colname="4">0.352 38
</oasis:entry><oasis:entry colname="5">0
</oasis:entry><oasis:entry colname="6">0</oasis:entry></oasis:row></oasis:tbody></oasis:tgroup></oasis:table><table-wrap-foot><p><italic toggle="yes"><sup>a</sup></italic><sup></sup> Average value ± standard deviation.<italic toggle="yes"><sup>b</sup></italic><sup></sup> fcc lattice parameter
from SAED analysis.<italic toggle="yes"><sup>c</sup></italic><sup></sup> fcc lattice parameter from Vegard's law
assuming the Co/Ni molar ratio measured by EDS.<italic toggle="yes"><sup>d</sup></italic><sup></sup> Nominal Co/Ni molar ratio.<italic toggle="yes"><sup>e</sup></italic><sup></sup> Co/Ni molar ratio measured by EDS.</p></table-wrap-foot></table-wrap></p><p><bold>3.3. Magnetic Properties. </bold>The formation of the alloy
in these materials has been further studied through an
analysis of the magnetic moment per atom as a function
of the Co/Ni ratio. In bulk Co−Ni alloys, the magnetic
moment per atom varies almost linearly between the
Ni and the fcc Co values, which are 0.6 μ<sub>B</sub> and 1.80 μ<sub>B</sub>,<xref rid="cm021106rb00023" ref-type="bibr"></xref>
respectively. For each sample, the magnetic field dependence of the magnetization was measured at 3 K up
to a maximum applied field of 6 T. The magnetic
contributions of the sample holder and of the silica slide
were separately measured and subtracted from the
measured data. The magnetic moment per atom was
calculated (with an accuracy of about 10%) considering
all Co and Ni atoms, whose concentration was obtained
from RBS measurements. The corrected magnetization
curves are shown in Figure <xref rid="cm021106rf00008"></xref>. For all samples, a
technical saturation was reached above about 2 T. In
Figure <xref rid="cm021106rf00009"></xref>, the magnetic moment per atom at 6 T is
reported for all of the samples (circles), together with
the corresponding values for the bulk alloys (dashed
line). A good agreement between the magnetic moment
values of the present Co−Ni samples and the corresponding bulk samples is observed, except for the values
of the pure Ni and pure Co samples, which are larger.
A reason that might explain the observed enhancement
is that the annealing (and reduction) at 800 °C is not
able to fully eliminate free ions trapped in the matrix.
In fact, free Co<sup>2+</sup> and Ni<sup>2+</sup> ions have large magnetic
moments (the spin-only magnetic moments are 3.87 μ<sub>B</sub>
and 2.82 μ<sub>B</sub>, respectively). However the occurrence of a
fraction of ions large enough to cause the observed
magnetic moment increase would make the magnetization not saturate even at the highest applied field,
whereas we observe saturation at field of ca. 2 T. Even
the presence of cobalt or nickel oxides and silicates can
be ruled out, in agreement with the TEM results. In
fact, these compounds are antiferromagnetic at 4 K,
which would make the total magnetic moment smaller
than that expected if all of the nanoparticles were
metallic.
<fig id="cm021106rf00008" position="float" orientation="portrait"><label>8</label><caption><p>Variation of the magnetic moment per atom as a function of the applied field at 3 K for the Co<sub>100</sub> (down
triangles), Co<sub>80</sub>Ni<sub>20</sub> (circles), Co<sub>33</sub>Ni<sub>66</sub> (squares), and Ni<sub>100</sub> (up
triangles) samples.</p></caption><graphic xlink:href="cm021106rf00008.tif" position="float" orientation="portrait"></graphic></fig><fig id="cm021106rf00009" position="float" orientation="portrait"><label>9</label><caption><p>Magnetic moment per atom at 3 K for a 6-T applied magnetic field (filled circles) as a function of the Co fraction in the system obtained for the samples Co<italic toggle="yes"><sub>x</sub></italic>Ni<sub>100</sub><sub>-</sub><italic toggle="yes"><sub>x</sub></italic>. For comparison, the dashed line represents the values for bulk Co−Ni alloys.</p></caption><graphic xlink:href="cm021106rf00009.tif" position="float" orientation="portrait"></graphic></fig></p><p>Theoretical and experimental results indicate that 3d-
and 4d-metal nanoclusters can present enhanced magnetic moments as a result of a reduction in the atomic
coordination at their surfaces.<named-content content-type="bibref-group"><xref rid="cm021106rb00011" ref-type="bibr"></xref>,<xref rid="cm021106rb00034" ref-type="bibr"></xref>−<xref rid="cm021106rb00035" specific-use="suppress-in-print" ref-type="bibr"></xref><xref rid="cm021106rb00036" ref-type="bibr"></xref></named-content> This effect has
been observed both in Ni nanoparticles<sup>11,34</sup> smaller than
(about 2 nm) or comparable to ours and in Co nanoparticles larger or much smaller<named-content content-type="bibref-group"><xref rid="cm021106rb00007" ref-type="bibr"></xref>,<xref rid="cm021106rb00037" ref-type="bibr"></xref>,<xref rid="cm021106rb00038" ref-type="bibr"></xref></named-content> than ours. We
observed an enhancement with respect to the bulk value
around 30% for pure Co nanoparticles, whereas values
reported in the literature so far are at most 20% even
for particles much smaller than those synthesized in the
present work. Because the magnetic moment generally
increases with decreasing particle size,<xref rid="cm021106rb00034" ref-type="bibr"></xref> we can conclude that the size is not the only parameter to be
considered. On the other hand, in nanostructured
materials (granular, epitaxial, and multilayered films),
the chemical bonding between the surface of the nanostructure and the surrounding matrix plays a major
role.<named-content content-type="bibref-group"><xref rid="cm021106rb00002" ref-type="bibr"></xref>,<xref rid="cm021106rb00003" ref-type="bibr"></xref></named-content> Even though the mechanism underlying the
increase of the magnetic moment is still not clear, it is
reasonable to assume that the modification at the
surface caused by the interaction with the silica matrix
is responsible for the observed behavior. The fact that
we observed an increase that, within the experimental
error, is among the highest ever reported can be
ascribed to the particular sol−gel technique used.
</p><p>What we found more surprising is that the same
increase was not observed for the alloy-based nanocomposites. Electronic structures of alloys are different
from those of metals because of the different crystal
structures and, mainly, the chemical order. Co−Ni
alloys are chemically very stable: this is one of the
reasons these materials are largely used for magnetic
recording media.<xref rid="cm021106rb00039" ref-type="bibr"></xref> Probably, the chemical stability is
conserved when these materials are reduced to nanosize, thus reducing the chemical interaction effects that
lead to the magnetic moment enhancement.
</p><p>The magnetic behavior at room temperature was
studied by hysteresis measurements. Figure <xref rid="cm021106rf00010"></xref> shows
the first quadrant of the demagnetization curves of the
Co<sub>80</sub>Ni<sub>20</sub>, Co<sub>70</sub>Ni<sub>30</sub>, and Co<sub>50</sub>Ni<sub>50</sub> samples. For all of the
samples, neither hysteretic behavior nor remanent
magnetization are observed. The absence of irreversible
processes is characteristic of magnetic nanoparticles
with superparamagnetic behavior.<named-content content-type="bibref-group"><xref rid="cm021106rb00003" ref-type="bibr"></xref>,<xref rid="cm021106rb00004" ref-type="bibr"></xref>,<xref rid="cm021106rb00040" ref-type="bibr"></xref></named-content> This behavior
appears when thermal demagnetization effects are so
large that the magnetic moment can overcome the
magnetic barriers that hinder the free inversion of the
magnetization. These barriers depend on the particle
volume and the magnetic anisotropy of the material. In
our case, either the small particle size or the reduced
anisotropy barrier associated with the fcc structure of
the Co−Ni nanoparticles<sup>5</sup> determines the occurrence of
superparamagnetic behavior at room temperature. Studies are in progress to analyze the relationship between
the superparamagnetic behavior and the morphological
or structural features of these alloy-based nanoparticles.
<fig id="cm021106rf00010" position="float" orientation="portrait"><label>10</label><caption><p>Demagnetization curve (represented in terms of the magnetic moment per film volume) at 300 K for the Co<sub>50</sub>Ni<sub>50</sub>, Co<sub>70</sub>Ni<sub>30</sub>, and Co<sub>80</sub>Ni<sub>20</sub> samples.</p></caption><graphic xlink:href="cm021106rf00010.tif" position="float" orientation="portrait"></graphic></fig></p></sec><sec id="d7e1535"><title>4. Conclusion</title><p>In this work, the formation of Co−Ni alloy nanoparticles embedded in a silica host and synthesized with
the sol−gel technique has been reported. The clusters
exhibit an fcc structure for all of the compositions
investigated, with a lattice parameter that is a growing
function of the Co fraction in the samples. The excellent
morphological and structural features (small particle
size, narrow particle distribution, and small volumetric
particle concentration) of the composites prepared by
this sol−gel procedure are ideal for studying the influence of size in the alloy-based nanoparticles, and in the
Co−Ni case, for analyzing the effects on the magnetic
properties of stabilization of the fcc structural phase in
Co-rich alloys. An enhancement of the magnetic moment
was detected in the pure Co and Ni samples, and good
agreement between the measured values and the values
for the corresponding bulk alloys was found in the other
cases. The different behaviors of the alloy-based nanoparticles and the metal-based nanoparticles can be
related to the greater chemical stability of the alloy-based ones. At room temperature, an absence of remanence and coercivity is observed in the hysteresis loops,
which is related to the superparamagnetic behavior of
the fcc nanoparticles.
</p></sec></body><back><ack><title>Acknowledgments</title><p>This work was partially supported by the MURST National University Research
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Marta 2137,I-30123 Padova, Italy</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</affiliation><affiliation> Corresponding author. Fax: +39.049.8277003. Phone: +39.049.8277045. E-mail: mattei@padova.infm.it.</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="family">DE JULIáN FERNáNDEZ</namePart><namePart type="given">C.</namePart><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="family">MAZZOLDI</namePart><namePart type="given">P.</namePart><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="family">SADA</namePart><namePart type="given">C.</namePart><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="family">DE</namePart><namePart type="given">G.</namePart><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="family">BATTAGLIN</namePart><namePart type="given">G.</namePart><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="family">SANGREGORIO</namePart><namePart type="given">C.</namePart><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</affiliation><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="family">GATTESCHI</namePart><namePart type="given">D.</namePart><affiliation>INFM, Dipartimento di Fisica, Università di Padova, via Marzolo 8, I-35131 Padova, Italy</affiliation><affiliation>Sol−Gel Division, Central Glass and Ceramic Research Institute,196 Raja S.C. Mullick Road, Kolkata 700032, India</affiliation><affiliation>INFM, Dipartimento di Chimica Fisica, Università di Venezia, Calle Larga S. Marta 2137,I-30123 Padova, Italy</affiliation><affiliation>Dipartimento di Chimica, Università di Firenze, via della Lastruccia 5,I-50019 Sesto Fiorentino (Firenze), Italy</affiliation><role><roleTerm type="text">author</roleTerm></role></name><typeOfResource>text</typeOfResource><genre type="research-article" displayLabel="research-article" authority="ISTEX" authorityURI="https://content-type.data.istex.fr" valueURI="https://content-type.data.istex.fr/ark:/67375/XTP-1JC4F85T-7">research-article</genre><originInfo><publisher>American Chemical Society</publisher><dateCreated encoding="w3cdtf">2002-07-12</dateCreated><dateIssued encoding="w3cdtf">2002-08-19</dateIssued><copyrightDate encoding="w3cdtf">2002</copyrightDate></originInfo><abstract>The optical, structural, and magnetic properties of Co, Ni, and CoxNi100-x (0 < x < 100) alloy nanoclusters in silica host obtained by the sol−gel route are presented. Throughout the entire range of composition investigated, from pure Ni to pure Co, the nanoclusters exhibit an fcc structure, with lattice parameters increasing with the Co fraction in the system. This indicates Co−Ni alloy formation. It has also been observed that the average cluster diameter increases with increasing Co fraction in the system. An enhancement of the magnetic moment was exhibited in the pure Co and Ni samples, and good agreement between the measured values and those of the corresponding bulk alloys was found for the other samples. All composites are superparamagnetic at room temperature.</abstract><relatedItem type="host"><titleInfo><title>Chemistry of Materials</title></titleInfo><titleInfo type="abbreviated"><title>Chem. Mater.</title></titleInfo><genre type="journal" authority="ISTEX" authorityURI="https://publication-type.data.istex.fr" valueURI="https://publication-type.data.istex.fr/ark:/67375/JMC-0GLKJH51-B">journal</genre><identifier type="ISSN">0897-4756</identifier><identifier type="eISSN">1520-5002</identifier><identifier type="acspubs">cm</identifier><identifier type="coden">CMATEX</identifier><identifier type="uri">pubs.acs.org/cm</identifier><part><date>2002</date><detail type="volume"><caption>vol.</caption><number>14</number></detail><detail type="issue"><caption>no.</caption><number>8</number></detail><extent unit="pages"><start>3440</start><end>3447</end></extent></part></relatedItem><identifier type="istex">D5F926784691B48FE5A9B062D997E614373EEE5E</identifier><identifier type="ark">ark:/67375/TPS-641Q40Q9-V</identifier><identifier type="DOI">10.1021/cm021106r</identifier><accessCondition type="use and reproduction" contentType="restricted">Copyright © 2002 American Chemical Society</accessCondition><recordInfo><recordContentSource authority="ISTEX" authorityURI="https://loaded-corpus.data.istex.fr" valueURI="https://loaded-corpus.data.istex.fr/ark:/67375/XBH-X5HBJWF8-J">ACS</recordContentSource><recordOrigin>Copyright © 2002 American Chemical Society</recordOrigin></recordInfo></mods><json:item><extension>json</extension><original>false</original><mimetype>application/json</mimetype><uri>https://api.istex.fr/ark:/67375/TPS-641Q40Q9-V/record.json</uri></json:item></metadata><annexes><json:item><extension>tiff</extension><original>true</original><mimetype>image/tiff</mimetype><uri>https://api.istex.fr/document/D5F926784691B48FE5A9B062D997E614373EEE5E/annexes/tiff</uri></json:item></annexes><serie></serie></istex></record>Pour manipuler ce document sous Unix (Dilib)
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