Validity of the Néel-Arrhenius model for highly anisotropic CoₓFe₃₋ₓO₄ nanoparticles

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We report a systematic study on the structural and magnetic properties of CoₓFe₃₋ₓO₄ 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 TB 345 K for 5 d 13 nm) and large coercive fields (HC 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 K₁(T) through the empirical Brükhatov-Kirensky relation. This approach provided K₁(0) values very similar to the bulk material from either static or dynamic magnetic measurements, as well as realistic values for the response times (τ₀ ≃ 10⁻¹⁰ s). Deviations from the bulk anisotropy values found for the smallest particles could be qualitatively explained based on Zeners relation between K₁(T) and M(T).