Constrained phase coexistence in thin MBE-grown MnAs films on GaAs

We present experimental evidence for the equilibrium coexistence between crystalline phases in thin MnAs layers grown epitaxially on GaAs. The phases, which can coexist in the bulk system only at one temperature point, are simultaneously present in the heterostructures over a temperature interval of more than 20 °C, which varies with the thickness of the MnAs layer. This phase coexistence is explained by the constraint on the lateral expansion of epitaxial layers which gives rise to strain in the layer. For the thicker MnAs layer a hysteresis of the temperature dependence of the phase composition is observed. The curvature of the samples is strongly anisotropic, with the larger curvature along the c-axis of the MnAs layer, and originates from thermal contraction of the film on cooling from growth temperature.