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Open AccessArticle

Magnetocrystalline and Surface Anisotropy in CoFe2O4 Nanoparticles

1
Institute of Structure of Matter–CNR, Monterotondo Stazione, 00016 Rome, Italy
2
Institute of Physics, Mathematics and Information Technology, Immanuel Kant Baltic Federal University, 236041 Kaliningrad, Russia
3
Department of Physics, University of Oviedo, 33204 Gijón, Spain
4
The Department of Physical and Chemical Science, University of L’Aquila, Via Vetoio, Coppito, 67100 L’Aquila, Italy
5
Department of Geological and Chemical Sciences, University of Cagliari, Cittadella Universitaria, 09042 Monserrato, Italy
6
National Interuniversity Consortium of Materials Science and Technology (INSTM), Via Giuseppe Giusti 9, 50121 Firenze, Italy
7
Department of Chemistry and Industrial Chemistry (DCIC), University of Genova, 16146 Genova, Italy
*
Author to whom correspondence should be addressed.
Nanomaterials 2020, 10(7), 1288; https://doi.org/10.3390/nano10071288
Received: 19 May 2020 / Revised: 13 June 2020 / Accepted: 26 June 2020 / Published: 30 June 2020
(This article belongs to the Special Issue Applications and Properties of Magnetic Nanoparticles)
The effect of the annealing temperature Tann on the magnetic properties of cobalt ferrite nanoparticles embedded in an amorphous silica matrix (CoFe2O4/SiO2), synthesized by a sol-gel auto-combustion method, was investigated by magnetization and AC susceptibility measurements. For samples with 15% w/w nanoparticle concentration, the particle size increases from ~2.5 to ~7 nm, increasing Tann from 700 to 900 °C. The effective magnetic anisotropy constant (Keff) increases with decreasing Tann, due to the increase in the surface contribution. For a 5% w/w sample annealed at 900 °C, Keff is much larger (1.7 × 106 J/m3) than that of the 15% w/w sample (7.5 × 105 J/m3) annealed at 700 °C and showing comparable particle size. This indicates that the effect of the annealing temperature on the anisotropy is not only the control of the particle size but also on the core structure (i.e., cation distribution between the two spinel sublattices and degree of spin canting), strongly affecting the magnetocrystalline anisotropy. The results provide evidence that the magnetic anisotropy comes from a complex balance between core and surface contributions that can be controlled by thermal treatments. View Full-Text
Keywords: magnetic nanoparticles; cobalt ferrite; magnetic anisotropy magnetic nanoparticles; cobalt ferrite; magnetic anisotropy
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MDPI and ACS Style

Omelyanchik, A.; Salvador, M.; D’Orazio, F.; Mameli, V.; Cannas, C.; Fiorani, D.; Musinu, A.; Rivas, M.; Rodionova, V.; Varvaro, G.; Peddis, D. Magnetocrystalline and Surface Anisotropy in CoFe2O4 Nanoparticles. Nanomaterials 2020, 10, 1288.

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