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.