Exploring the impact of Cr-doping on the crystallographic and magnetic structure of Mn5Si3 antiferromagnetic alloy

The role of Cr-doping on the structural and magnetic ground states of Mn5Si3 alloy has been investigated through temperature-dependent neutron powder diffraction (NPD) and x-ray absorption fine structure (XAFS) techniques. All the Cr-doped alloys of nominal composition Mn5-xCrxSi3 (for x = 0.05, 0.1, and 0.2) undergo two first-order magnetostructural phase transitions, i.e., hexagonal (space group P63/mcm) paramagnetic → orthorhombic (space group Ccmm) collinear antiferromagnetic → orthorhombic (space group Cc2m) noncollinear antiferromagnetic phase, on cooling from room temperature. NPD studies at different constant temperatures indicate that both antiferromagnetic phases are commensurate in nature and can be represented by the q=(0,1,0) magnetic propagation vector for all the Cr-doped alloys. Such doping at the Mn site results in a significant modification of the noncollinear antiferromagnetic structure (both moment size and orientation) and hence affects the unusual magnetic properties, such as inverted hysteresis loop, thermomagnetic irreversibility, etc. The XAFS measurements were performed to interpret the local environment of doped Cr atoms in detail, which is critical for a microscopic understanding of the unusual properties of this class of Cr-doped Mn5Si3 alloys. The analysis confirms the elemental state of Cr in the doped alloys and indicates a high degree of preservation of local crystallographic structure with varying Cr concentration and sample temperature. Doping induces intriguing changes in XAFS patterns, elucidated through different types of scattering mechanisms associated with the central absorbing Cr atom.