EFFECTS OF PRESSURE ON MAGNETIC PROPERTIES OF MNAS

Using a vibrating-coil magnetometer, we have studied the effect of hydrostatic pressures of up to 11 kbar on the magnetic properties of MnAs. These experiments were carried out between 60 and 500A°K. The boundary between the ferromagnetic hexagonal (B81) phase and the orthorhombic (B31) phase has been determined. In addition, the existence of ordered metamagnetic and ferromagnetic structures within the high-pressure B31 phase is established and their boundaries delineated in the pressure-temperature phase space. The existence of regions within this space in which the paramagnetic susceptibility of the B81 and B31 phases follow a Curie-Weiss law has enabled us to confirm directly the hypothesis of Goodenough, Ridgley, and Newman that the manganese moment goes from a low-spin to a high-spin configuration on going from the B31 to the B81 phase. The pressure dependence of the paramagnetic Curie point (dθdp) was found to be negative in the B81 phase and positive in the B31 phase. This is shown to be in contradiction with the Bean-Rodbell model as it has been applied to MnAs. We show that the minimum generalization of their model to permit consistency with our results requires consideration of multiple exchange interactions, anisotropic strain effects, and higher-order contributions to the elastic energy. Finally, we give an interpretation of our results within the framework of the band spectrum of MnAs. © 1969 The American Physical Society.