Ni-doping assisted modification of the non-collinear antiferromagnetic ordering in Mn₅Si₃ alloy

Ni-doped Mn₅Si₃ alloys of nominal compositions Mn_5−xNi_xSi₃ (for x = 0.05, 0.1, and 0.2) have been investigated through detailed neutron powder diffraction (NPD) studies in zero magnetic field and ambient pressure. At room temperature, all three Ni-doped alloys crystallize with D8₈ type hexagonal structure having P6₃∕mcm space group. These alloys undergo paramagnetic → collinear antiferromagnetic → non-collinear antiferromagnetic transitions on cooling from room temperature. A significant decrease in collinear to non-collinear antiferromagnetic transition temperature has been observed with increasing Ni concentration. The magnetic structure of both antiferromagnetic phases can be described by the magnetic propagation vector k = (0,1,0). However, the moment size and the orientation in the non-collinear antiferromagnetic phase are found to be notably affected by the Ni-doping. Approaching near-parallel arrangement of Mn-moments with increasing Ni-doping is found to be responsible for the gradual disappearance of unusual magnetic properties (inverted hysteresis loop, thermomagnetic irreversibility, etc.) observed in Mn₅Si₃ alloy.