Validity of the Néel-Arrhenius model for highly anisotropic Co_xFe_{3-x}O_4 nanoparticles

Abstract

We report a systematic study on the structural and magnetic properties of Co_{x}Fe_{3-x}O_{4} magnetic nanoparticles with sizes between $5$ to $25$ nm, prepared by thermal decomposition of Fe(acac)_{3} and Co(acac)_{2}. The large magneto-crystalline anisotropy of the synthesized particles resulted in high blocking temperatures ($42$ K \leqq $T_B$ $\leqq 345$ K for $5\leqq$ d $\leqq 13$ nm ) and large coercive fields ($H_C\approxeq 1600$ kA/m for $T=5$ K). The smallest particles ($<d>=5$ nm) revealed the existence of a magnetically hard, spin-disordered surface. The thermal dependence of static and dynamic magnetic properties of the whole series of samples could be explained within the Néel-Arrhenius relaxation framework without the need of ad-hoc corrections, by including the thermal dependence of the magnetocrystalline anisotropy constant $K_1(T)$ through the empirical Brükhatov-Kirensky relation. This approach provided $K_1(0)$ values very similar to the bulk material from either static or dynamic magnetic measurements, as well as realistic values for the response times (${\tau }_0\simeq 10^{-10}$ s). Deviations from the bulk anisotropy values found for the smallest particles could be qualitatively explained based on Zener\'{}s relation between $K_1(T)$ and M(T).

References in corpus (5)

• Spin Disorder and Magnetic Anisotropy in Fe3O4 Nanoparticles
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• Validity of the Néel-Arrhenius model for highly anisotropic Co_xFe_{3-x}O_4 nanoparticles
• Ferrimagnetism and spin canting of ZnFe2O4 nanoparticles embedded in ZnO matrix
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• A Detailed Investigation of the Onion Structure of Exchanged Coupled Magnetic Fe(3-delta)O4@CoFe2O4@Fe(3-delta)O4 Nanoparticles
• Superparamagnetic dynamics and blocking transition in Fe${}_3$O${}_4$ nanoparticles probed by vibrating sample magnetometry and muon spin relaxation