Spin reorientation in premartensite and austenite Ni2MnGa

Abstract

Abstract Symmetry breaking in the pre-martensite state, sometimes called the martensite precursor state, has been hotly debated for decades. Incompletely softened phonon modes and the diffuse scattering suggest dynamical effects without a static symmetry breaking. Here we use careful measurement of magnetization curves of magnetically closed samples to evidence local symmetry breaking, at and above the pre-martensite transition in off-stoichiometry Ni-Mn-Ga Heusler alloy. During the heating cycle after the martensite transformation the differential susceptibility curve slowly transforms from a typical one-peak corresponding to the magnetization along the easy axis to two-peaks typical for magnetization along the harder magnetic axis. It is explained by change of the macroscopic magnetic easy axis from [100] to [110]. Above the premartensite transformation temperature the magnetic easy axis slowly changes back to [100]. Reorientation of the easy axis with temperature can be explained by the phonon domains model.