Influence of Superexchange Interaction on the Ferromagnetic Properties of Manganites and Cobaltites
Tóm tắt
The role of superexchange interaction in the formation of the ferromagnetic state of cobaltites in the La0.5Sr0.5Co1–xMexO3 (Me = Cr, Ga, Fe) systems and manganites La0.7Sr0.3Mn0.85M0.15O3 (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 (ТС = 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 La0.7Sr0.3Mn0.85Nb0.15O3 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.
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