Unveiling the Hidden Entropy in ZnFe2O4

Ferrimagnetic Clusters as the Origin of Anomalous Curie-Weiss Behavior in ZnFe2O4 Antiferromagnetic Susceptibility

Different studies carried out in the last three decades on the magnetic susceptibility of the spinel ZnFe₂O₄ ferrite have revealed the positive character of its Curie–Weiss temperature, contradicting its observed antiferromagnetic behavior which is characterized by a well-defined susceptibility peak centered around the Neel temperature (10 K). Some approaches based on ab initio calculations and mixture of interactions have been attempted to explain this anomaly. This work shows how for very low values of the inversion parameter, the small percentage of Fe atoms located in tetrahedral sites gives rise to the appearance of ferrimagnetic clusters around them. Superparamagnetism of these clusters is the main cause of the anomalous Curie–Weiss behavior. This finding is supported experimentally from the thermal dependence of the inverse susceptibility and its evolution with the degree of inversion.

New Insights on the Spin Glass Behavior in Ferrites Nanoparticles

The magnetic properties of nanocrystalline M_xFe_3-xO₄ ferrites with M=Fe, Co, and Zn were investigated. The data support a core-shell model, where the core is ferrimagnetically ordered, and the shell shows a spin glass type behavior. The reduced magnetizations of spin glass components follow an m_g = (1 - b/H^-1/2) field dependence. The b values are strongly correlated with the intensities of exchange interactions. The field dependences of the magnetoresistances of Fe₃O₄ and Zn_xFe_3-xO₄ nanoparticles pellets, experimentally determined, are well described if instead of the core reduced magnetization, commonly used, that of the shell is taken into account. For similar compositions of the nanoparticles, identical b values are obtained both from magnetization isotherms and magnetoresistances studies. The half-metallic behavior of spinel Fe₃O₄ based nanoparticles is discussed comparatively with those of double perovskites.