High-Pressure Study of the First-Order Phase Transition in MnAs

The first-order magnetic-transition temperature in MnAs has been measured as a function of both increasing and decreasing pressure. A critical pressure $P_c\sim 4.6$ kbar has been obtained for the range of stability of the hexagonal phase. The pressure hysteresis increases with decreasing temperatures. Cooling to 77°K under 5 kbar and then releasing pressure gives the $B31\mathrm{}$ phase at atmospheric pressure, and it remains stable on heating to 138°K, where there is an increase of magnetization of over a factor of 50. These data, together with earlier magnetic measurements on the system $\mathrm{Mn}{\mathrm{As}}_{1-x}{\mathrm{P}}_x$, demonstrate that the origin of the first-order phase transition is a large exchange striction in the basal planes plus a volume-dependent Weiss molecular field and manganese moment. This volume dependence is due to electron rearrangements associated with a high-spin-to-low-spin transition. The sharpness of the high-spin-to-low-spin transition cannot be accounted for by variations in crystal-field splitting with volume. It is suggested that a bandwidth is increasing with decreasing volume through the maximum bandwidth for spontaneous band ferromagnetism. The critical band appears to consist of $t_{\pm }$ orbitals, which are primarily influenced by the Mn-Mn interactions within basal planes.