Origin of Magnetic Anisotropy Energy of Fe7S8 and Fe7Se8

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Abstract
Based on the electronic energy state of Fe2+ ion in the crystalline fields in Fe7S8 and Fe7Se8 (4c) with a pseudo NiAs structure, the expression of magnetic anisotropy energy were derived by perturbation method under an appropriate approximation. Then the experimental results of magnetization curves were analyzed from this theory and the physical constants for the anisotropy energy, such as orbital angular momentum in the lowest state and the anisotropy energies of the second‐ and the fourth‐order, were estimated. Using these constants, the experimental results of the temperature dependence on the spin angle (the second‐order transformation) without magnetic field were well explained. On the other hand, a possibility of the occurrence of the first‐order phase transformation for spin angle appearing in Fe7Se8 (3c) and FeS (α‐point) was discussed from the same basis of the theory, and was given an explanation for the nuclear quadrupole shift at this transformation point in the Mössbauer experiment.

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