Electronic structure and phase stability of the magnetocaloric compound (B31) β-MnAs"

K. Sachtleben, C. B. Dias de Góes, and C. Paduani

Dept. of Physics, UFSC, SC, Florianópolis, Brazil.

E-mail: kewinsachtleben@gmail.com

The MnAs compound has the hexagonal NiAs-type B81 structure (α-phase, s.g. P63/mmc), ferromagnetic at low temperatures, and which, at the Curie temperature of 318 K, exhibits a first-order phase transition to an orthorhombic MnP-type (Pnma) B31 structure (β-phase), one of the most common derivatives of the NiAs-type structure among the pnictides. The MnP-type structure can be viewed as a distorted NiAs structure, where the unit cell contains four (instead of two) formula units. The atomic distance between Mn and As atoms changes from 2.479 Å, in the α-phase, to 2.581 Å in the β-phase. MnAs undergoes also a structural-phase transformation from NiAs-type to a zinc-blende structure, under large volume expansion. These characteristics put MnAs into the special class of magnetocaloric compounds. β-MnAs has already been reported in the literature as non ferromagnetic, as well as unlikely to be paramagnetic. However, it has been also pointed out that the α-β structural transition is magnetically driven, from the high symmetry ferromagnetic state to a lower symmetry antiferromagnetic state. In this contribution is investigated the band structure of MnAs in both hexagonal (B81) and orthorhombic (B31) phases. The dependence of the Mn moment on the unit cell volume is investigated. The characteristics of the calculated Fermi surfaces indicate that the α-β structural transition is driven by nesting of the Fermi surfaces, and provide explanation for the experimentally observed larger resistivity in β-MnAs.

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