Magnetic First-Order Phase Transition in Single-Crystal MnAs

General Electric Research Laboratory, Schenectady, New York

 (Received 8 January 1963) 

Abstract : This paper describes an experimental and theoretical examination of a 10-/zg 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 4.55X10-12 dyn-1 cm2 , a lattice thermal expansion coefficient a. of 5.71X10~5 /°C, an apparent paramagnetic Curie temperature To* of 277.1°K, and a Curie-temperature dependence on volume strain, (3 [ = (dTc/To){dV/VQ)~X ~\, of 18.9, where T0 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, Ki-\-2K2+3K2, to be about — 7.6 and — 12.0X 106 ergs/cm3 at 299 and 77°K, respectively.

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