X-Ray and Susceptibility Study of the First-Order Magnetic Transformation in Mn3Pt

The first‐order transformation from triangular to collinear antiferromagnetic structure (AF‐AF transformation) has been investigated in the ${\mathrm{Mn}}_3{\mathrm{Pt}}_{\text{1−x}}{\mathrm{Rh}}_x$ and ${\mathrm{Mn}}_{\text{3+y}}{\mathrm{Pt}}_{\text{1−y}}$ systems by x‐ray diffraction and magnetic methods. At the AF‐AF transformation temperature $T_t$ a discontinuous change in the lattice parameter is observed with a magnitude depending on concentration but with always the same lattice parameter value of the collinear phase. This implies the existence of a critical lattice parameter value for the AF‐AF transformation. Below the Néel temperature $T_N$ an anomaly in the thermal expansion occurs indicating exchange interactions dependent on the interatomic separation. This anomaly differs in sign for the collinear‐paramagnetic and triangular‐paramagnetic transitions. The susceptibility shows an abrupt change at $T_t$ and a maximum at $T_{\mathrm{N}}$ in the triangular phase. No maximum occurs for the collinear phase. The observed transition temperatures are in agreement with the earlier neutron diffraction data.