A revised magnetic phase diagram for FexMn5-xSi3 intermetallics

The magnetic and crystal structures of the series FexMn5-xSi3 (x=4,3.5,3,2,1) have been reinvestigated by neutron powder diffraction between 315 and 8 K. Fe atoms are found to preferentially occupy the 4d site of the hexagonal D8(8) structure (P6(3)/mcm). The compound Fe4MnSi3 orders ferromagnetically below 300 K. Refinement of the neutron data for this compound indicates that the moments are aligned at similar to40degrees off the c axis. A high moment at the 6g site is due to a large Mn moment which is forced into a ferromagnetic alignment with the Fe moments. Associated with this moment is a moderate magnetic entropy change, with a maximum value of 4 J kg-1 K-1 for a magnetic field change of 5 T. The magnetic structure of Fe3.5Mn1.5Si3 is similar to that of Fe4MnSi3, but the Curie point is lower, below 240 K. A feature for this composition is the appearance of a spiral structure below 60 K. The compounds with x=1, 2, and 3 are all antiferromagnets with a spiral structure observed for Fe3Mn2Si3 at all temperatures up to the Neel point of 150 K. Two separate antiferromagnetic structures exist for Fe2Mn3Si3, one between 120 and 45 K and another structure for temperatures below 45 K. Similar features in the magnetic ordering are exhibited by FeMn4Si3. At temperatures between 70 and 95 K, the magnetic structure is similar to that of the low temperature phase of Fe2Mn3Si3. Accompanying the magnetic transition at 70 K is a structural distortion to space group Cmcm, a feature also observed for Mn5Si3.