Effects of Pressure on the 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 500°K. The boundary between the ferromagnetic hexagonal ($B{8}_1$) phase and the orthorhombic ($B31\mathrm{}$) phase has been determined. In addition, the existence of ordered metamagnetic and ferromagnetic structures within the high-pressure $B31\mathrm{}$ 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 $B{8}_1$ and $B31\mathrm{}$ 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 $B31\mathrm{}$ to the $B{8}_1$ phase. The pressure dependence of the paramagnetic Curie point ($\frac{d\theta }{\mathrm{dp}}$) was found to be negative in the $B{8}_1$ phase and positive in the $B31\mathrm{}$ 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.