Influence of Superexchange Interaction on the Ferromagnetic Properties of Manganites and Cobaltites

Abstract

The role of superexchange interaction in the formation of the ferromagnetic state of cobaltites in the La_0.5Sr_0.5Co_1–xMexO₃ (Me = Cr, Ga, Fe) systems and manganites La_0.7Sr_0.3Mn_0.85M_0.15O₃ (M–Nb, Mg) with a perovskite structure has been studied. It was found that the ferromagnetic state in cobaltites can be implemented in some compositions without the mixed-valence effect of cobalt ions. The initial compound (x = 0) is ferromagnetic (Т_С = 247 K) with the saturation magnetization close to 2μB (at T = 30 K) per formular unit. It has been shown that the chemical substitution of cobalt by chromium reduces the spontaneous magnetization to 0.3μB (at х = 0.2), while the substitution of cobalt by iron ions (х = 0.2) does not alter the magnetization. The obtained data are interpreted in the model of positive superexchange interactions between cobalt and iron ions and negative ones between cobalt and chrome. It has been shown that the La_0.7Sr_0.3Mn_0.85Nb_0.15O₃ composition is ferromagnetic with ТС=145K, with a magnetic moment of 3.1 μB/Mn at 10 K, and no evidence of a cooperative orbital ordering in the manganite compounds has been revealed. Partial chemical substitution of Nb⁵⁺ ions by Mg²⁺ ones leads to the formation of Mn⁴⁺ ions, while it does not strengthen the ferromagnetic state. The strengthening of the structural distortions reduces the ferromagnetic component. It is assumed that the ferromagnetic state is caused by a significant hybridization of eg-orbitals of the manganese and oxygen ions, which strengthens the positive component of the superexchange interactions.