Lattice distortion effects on the magneto-structural phase transition of Mn As

MnAs is a magnetic-metal with magneto-elastic and magneto-caloric effects that makes it very attractive for technological applications such as transducers and refrigeration devices. In recent years, the interest on MnAs has increased due to the possibility of epitaxial growth on III-V semiconductors. The integration of ferromagnetic materials with semiconductors is a main topic for spintronics and MnAs grown on GaAs is a strong candidate for spin injection devices. MnAs presents a first-order phase transition at ∼40 deg C, changing from ferromagnetic/ hexagonal (α-phase) to paramagnetic/orthorhombic(β-phase). In the case of MnAs films grown on GaAs the first-order phase transition is non-abrupt with a coexistence of the two phases in the form of periodical/y alternated stripes for a large temperature range (∼20 deg C). As a result of this phase coexistence, a considerable fraction of the volume of the MnAs epitaxial films is usual/y in the paramagnetic phase at ∼30 deg C, which is a strong limitation for room temperature applications. The growth of Mn As films on different crystal orientations has been suggested as an alternative that can provide higher phase-transition temperatures. The detailed mechanism that associates the crystal distortion (lattice parameter variation) with the phase transition temperature is, however, still unclear. In order to get more insight into the Mn As phase transition, we have designed an X-ray diffraction experiment and performed theoretical calculations to investigate in detail the dependence of the phase transition temperature on crystal distortions. (author)