Abstract
Cobalt ferrite, CoFe2O4, nanoparticles in the size range 2–15 nm have been prepared using a non-aqueous solvothermal method. The magnetic studies indicate a superparamagnetic behavior, showing an increase in the blocking temperatures (ranging from 215 to more than 340 K) with the particle size, D TEM. Fitting M versus H isotherms to the saturation approach law, the anisotropy constant, K, and the saturation magnetization, M S, are obtained. For all the samples, it is observed that decreasing the temperature gives rise to an increase in both magnetic properties. These increases are enhanced at low temperatures (below ~160 K) and they are related to surface effects (disordered magnetic moments at the surface). The fit of the saturation magnetization to the T 2 law gives larger values of the Bloch constant than expected for the bulk, increasing with decreasing the particle size (larger specific surface area). The saturation magnetization shows a linear dependence with the reciprocal particle size, 1/D TEM, and a thickness of 3.7 to 5.1 Å was obtained for the non-magnetic or disordered layer at the surface using the dead layer theory. The hysteresis loops show a complex behavior at low temperatures (T ≤ 160 K), observing a large hysteresis at magnetic fields H > ~1000 Oe compared to smaller ones (H ≤ ~1000 Oe). From the temperature dependence of the ac magnetic susceptibility, it can be concluded that the nanoparticles are in magnetic interaction with large values of the interaction parameter T 0, as deduced by assuming a Vogel–Fulcher dependence of the superparamagnetic relaxation time. Another evidence of the presence of magnetic interactions is the almost nearly constant value below certain temperatures, lower than the blocking temperature T b, observed in the FC magnetization curves.
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Acknowledgments
This study was partly funded by the Spanish Science and Technology Ministry, Spain (Project Nos. MAT2008-06503/NAN and NanoBioMed CONSOLIDER-INGENIO 2010), Xunta de Galicia, Spain (Project INCITE08PXIB209049PR), and the European Union (Contract Nos. 037465-FLUOROMAG, EU-FP6 Framework Programme LIFESCI-HEALTH-6, and No. NMP3-LA-2008-214685-MAGISTER, EU-FP7 Large Collaborative Project).
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Vázquez-Vázquez, C., López-Quintela, M.A., Buján-Núñez, M.C. et al. Finite size and surface effects on the magnetic properties of cobalt ferrite nanoparticles. J Nanopart Res 13, 1663–1676 (2011). https://doi.org/10.1007/s11051-010-9920-7
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DOI: https://doi.org/10.1007/s11051-010-9920-7