Structural phase transitions in Ni₂MnGa induced by low-temperature uniaxial stress

An experimental study is presented of the multiple phase transitions occurring in the single crystals of Ni₂MnGa, which had first been stressed and unloaded at liquid nitrogen temperature. The measurements of the specific heat, of the low-field magnetic susceptibility and of the ultrasonic velocity reveal that the material experiences the first-order structural phase transitions at T₃ = 190 K, at T₂ = 230 K, and at T₁ = 290 K. The relative deformation not resulting in the plastic flow of the crystal was ΔL/L = 0.05. After unloading the dimensions and the shape of the samples were restored upon heating. The new phases appear in Ni₂MnGa because of an anomalous instability of the lattice of this compound with respect to atomic displacements along a 〈110〉 direction in the (110) plane.