This paper describes an experimental and theoretical examination of a 10-μg $c$-axis-oriented single crystal of MnAs. The critical magnetic field for the first-order transition to the ferromagnetic phase has been measured as a function of temperature (15 to 65°C) and pressure (0 to 1000 bars gauge), using miniature-coil pulsed fields to 110 kOe. Data analysis substantiates the thesis of the recent Bean-Rodbell theory on magnetic first-order phase transitions that the transition in MnAs near 45°C is between ferromagnetic and paramagnetic phases and arises from a sufficiently sensitive dependence of exchange energy on lattice strain and a sufficiently high compressibility. The match between theory and experiment yields a compressibility $K$ of 4.55×${10}^{-12\mathrm{}}$ ${\mathrm{dyn}}^{-1\mathrm{}}$ ${\mathrm{cm}}^2$, a lattice thermal expansion coefficient $\alpha$ of 5.71×${10}^{-5\mathrm{}}$/°C, an apparent paramagnetic Curie temperature ${T_0}^{*}$ of 277.1°K, and a Curie-temperature dependence on volume strain, $\beta \left[\equiv \right(\frac{d{T}_c}{T_0}\left){\left(\frac{\mathrm{dV}}{V_0}\right)}^{-1\mathrm{}}\right]$, of 18.9, where $T_0$ is the Curie temperature of the unstrained specimen at 0°K. Discrepancies in the match suggest a primary need to include short-range order in the theory. Auxiliary experiments show the magnetocrystalline anisotropy sum, $K_1+2\mathrm{}{K}_2+3\mathrm{}{K}_3$, to be about -7.6 and -12.0×${10}^6$ ergs/${\mathrm{cm}}^3$ at 299 and 77°K, respectively.

• Received 8 January 1963

DOI:https://doi.org/10.1103/PhysRev.130.1347