Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature

Abstract

The α − β magneto-structural phase transition in MnAs/GaAs(111) epilayers is investigated by elastic neutron scattering. The in-plane parameter of MnAs remains almost constant with temperature from 100 K to 420 K, following the thermal evolution of the GaAs substrate. This induces a temperature dependent biaxial strain that is responsible for an α − β phase coexistence and, more important, for the stabilization of the ferromagnetic α-phase at higher temperature than in bulk. We explain the premature appearance of the β-phase at 275 K and the persistence of the ferromagnetic α-phase up to 350 K with thermodynamical arguments based on the MnAs phase diagram. It results that the biaxial strain in the hexagonal plane is the key parameter to extend the ferromagnetic phase well over room temperature.

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