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

J. B. GOODENOUGH AND J. A. KAFALAS 

Lincoln Laboratory* Massachusetts Institute of Technology', Lexington, Massachusetts

 (Received 21 November 1966)  

Abstract:

The first-order magnetic-transition temperature in MnAs has been measured as a function of both increasing and decreasing pressure. A critical pressure Pc'"^'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 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 MnAsi-sP*, 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 JOY 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± orbitals, which are primarily influenced by the Mn-Mn interactions within basal planes.

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http://journals.aps.org.sci-hub.tw/pr/abstract/10.1103/PhysRev.157.389