Layer-preferential substitutions and magnetic properties of pyrrhotite-type Fe_7−yM_yX₈ chalcogenides (X = S, Se; M = Ti, Co)

A comparative study of four series of pyrrhotite-type chalcogenide compounds Fe_7−yM_yX₈ (X = S, Se) with substitution of Ti or Co for iron has been performed by means of x-ray and neutron powder diffraction, and by magnetization measurements. In Fe_7−yM_yX₈ compounds having a ferrimagnetic order at $y=0$, the substitution of either Ti or Co for iron is observed to result in a monotonous decrease of the magnetic ordering temperature, while the resultant magnetization shows a non-monotonous behavior with a minimum around $y=1.0$–1.5 in all the Fe_7−yM_yX₈ families except Fe_7−yCo_ySe₈. Suppression of a magnetically ordered state with substitutions in Fe_7−yM_yX₈ is ascribed to nearly zero values of Ti and Co magnetic moments, while the non-monotonous changes of the resultant magnetization are explained by the compensation of the sublattice magnetizations due to the non-random substitutions in alternating metallic layers. The difference in the cation partitioning observed in Fe_7−yTi_yX₈ and Fe_7−yCo_yX₈ is attributed to the difference in the spatial extension of Ti and Co 3d orbitals. High coercive field values (20–24 kOe) observed at low temperatures in the Ti-containing compounds Fe_7−yTi_yX₈ with y ⩾ 3 are suggested to result from the enhancement of Fe orbital moment due to the Ti for Fe substitution. (paper)