Validity of the Néel-Arrhenius model for highly anisotropic Co_xFe_3−xO₄ nanoparticles

We report a systematic study on the structural and magnetic properties of Co_xFe_3−xO₄ magnetic nanoparticles with sizes between 5 and 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 < T_B < 345 K for 5 < d < 13 nm) and large coercive fields (H_C ≈ 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 Neel–Arrhenius relaxation framework by including the thermal dependence of the magnetocrystalline anisotropy constant K₁(T), without the need of ad-hoc corrections. This approach, using the empirical Brükhatov-Kirensky relation, 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 ≈ 10⁻¹⁰s). Deviations from the bulk anisotropy values found for the smallest particles could be qualitatively explained based on Zener's relation between K₁(T) and M(T)