Magnetic properties and anisotropy of (Fe1−xCox)7Se8

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Abstract
(Fe1−xCox)7Se8 samples have NiAs‐type structures with or without ordered arrangements (3c or 4c) of cation vacancies. With increasing x, the lattice parameter a remains unchanged but c exhibits a fairly large reduction (∼10% by variation of x from 0 to 1). The Curie temperature decreases with increasing x and the ferrimagnetism disappears at x≊0.6. The effective magnetic moments (∼5.5 μB for x≤0.3) evaluated from the high‐temperature portion (range of vacancy disorder) of the 1/χ versus temperature curves are close to the value expected on the ionic model. Results show that the magnetic states up to at least x=0.3 are those attained by a simple replacement of magnetic moments of Fe ions by those of Co ions. Samples with x>0.6 are weakly magnetic. Nearly temperature‐independent ac susceptibility at x≊0.7 indicates the absence of localized moments. In this connection, a correlation between lattice parameters and magnetic character is pointed out for 7 : 8 isomorphs of transition element selenides. The easy‐axis rotation from in the c plane towards the c axis with lowering temperature occurs in a different way in two superstructures: The easy axis in 4c structures rotates gradually according to temperature change. In 3c structures with x≤0.1, the rotation takes place at some definite temperature (e.g., 165 K for x=0.1). Corresponding torque curves clearly indicate a first order transition. Details of this anisotropy transition are interpreted as a combined effect of the spin‐orbit interaction and other additional terms.

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