Site Occupancies and Their Effects on the Physical Properties of Spinel : An Ab Initio Study

Abstract

Recent experiments on Fe substituted spinel CoCr 2 O 4 have discovered multiple functional properties in the system such as temperature and composition dependent magnetic compensation, and tunable exchange bias. These properties are attributed to the renormalization of the inter-atomic magnetic exchange interactions arising due to the non-regular site occupancies of the magnetic cations. We perform ab initio electronic structure calculations by DFT þ U method and combine with a generalized thermodynamic model to compute the site occupancy patterns of magnetic cations, structural properties and magnetic exchange interactions of Co Cr 1Àx Fe x ð Þ 2 O 4 for the entire composition range 0 < x < 1. We find that the substituting Fe atoms prefer to occupy the tetrahedral sites of the spinel structure for the entire range of x, in agreement with the experimental inferences. By computing the variations of the various inter-atomic magnetic exchange interactions as a function of site occupancy, we provide a microscopic picture of the evolution of a collinear structure from a non-collinear one due to substitution of Fe in CoCr 2 O 4 . The computed results are analyzed using the electronic structures. This work is a systematic study exploring the interrelations of site occupancy, composition, electronic structure and magnetic interactions in Fe-substituted CoCr 2 O 4 which helps gain fundamental knowledge of this material.