First-order phase transition at the curie temperature in MnAs and MnAs _0.9Sb_0.1

Structural transformations of MnAs and MnAs_0.9Sb_0.1 were investigated by X-ray diffraction in high magnetic fields up to 5 T. The temperature dependence of the magnetization was measured in a magnetic field of 0.01 T and the Curie temperature T_C was determined to be 315 K for MnAs and 290 K for MnAs_0.9Sb_0.1 during heating process. For both compounds, a metamagnetic transition from a paramagnetic to a ferromagnetic state was observed above T_C. The X-ray diffraction profile at 319 K for MnAs showed a single phase of an orthorhombic MnP-type structure in zero field. An applied magnetic field of 3 T induced the appearance of a hexagonal NiAs-type structure. On further increase of the magnetic field, a single phase with a hexagonal structure was realized above 3.5 T in a forced-ferromagnetic state. The X-ray diffraction profile at 295 K for MnAs _0.9Sb_0.1 showed a hexagonal NiAs-type structure. The coexistence of ferromagnetic and paramagnetic states with different lattice parameters was confirmed in a magnetic field of 2.5 T. The volume expansion induced by a magnetic field was found to be 2.1% for MnAs and 1.1% for MnAs _0.9Sb_0.1.