Non-equilibrium cation influence on the Néel temperature in ZnFe₂O₄

We present results on the thermal dependence of the magnetization above ambient conditions of ZnFe₂O₄ nanoferrites having different degrees of inversion (c ∼ 0.05-0.40) and grain sizes (D ∼ 6-50 nm). The disordered ferrites present a net magnetization in the 400-510 K range that mainly depends on the degree of inversion. The reciprocal magnetization deviate from different temperature values of the linear behavior showed at high temperatures. Our results show that, contrary to similar nanoferrite systems, a simple scaling law cannot be applied to explain the Néel temperature T_N dependency with the grain size. These results can be interpreted in terms of the lack of correlation between size and inversion that occurs in ZnFe₂O₄ and the strong T_N dependence on the superexchange J_AB interaction strength. A model assuming a random distribution of superexchange interactions can reproduce the Curie-Weiss temperature and the saturation magnetization values of the disordered ferrites.