Magnetic Structures and Exchange Interactions in the Mn-Pt System

The magnetic structures and transformations in the ordered phases of the Mn-Pt system have been investigated in a wide concentration range by magnetic, x-ray, and neutron diffraction methods. The properties of the ${\mathrm{Mn}}_3{\mathrm{Pt}}_{1-y}{\mathrm{Rh}}_y$ and ${\mathrm{Mn}}_{3-z}{\mathrm{Fe}}_z\mathrm{Pt}$ systems have also been studied. The triangular and the collinear antiferromagnetic structures, both found in the ${\mathrm{Mn}}_3$Pt phase, undergo a first-order transformation into each other at a critical value of the lattice parameter where the next-nearest-neighbor interaction changes sign. In the MnPt phase a simple antiferromagnetic structure occurs with the directions of the magnetic moments dependent on concentration and temperature. There is no direct connection between the anisotropy energy and the lattice dimensions. The Mn${\mathrm{Pt}}_3$ phase has simple ferromagnetic structure. The measured transition temperatures are summarized in magnetic phase diagrams. The magnetic structures and transformations of the Mn-Pt system are explained by assuming nearest- and next-nearest-neighbor interactions dependent on the interatomic distances. The magnetic phase diagram of the ${\mathrm{Mn}}_3$Pt phase calculated in the molecular-field approximation is in agreement with the experimental observations.