Magnetocrystalline anisotropy and twinning stress in Ni-Mn-Ga ferromagnetic shape memory alloys

Abstract

The condition for the rearrangement of martensite variant (RMV) by magnetic field has been studied quantitatively in a wide temperature range for two types of alloys: a Ni2MnGa with 10M martensite and a Ni 2.02Mn1.09Ga0.89 with 14M martensite. The RMV by magnetic field occurs at any temperature for the 10M martenste but only in a limited temperature range for the 14M martensite. The magnetic field at which the RMV starts to occur, Hs, increases with decreasing temperature for both alloys. The condition for the RMV by magnetic field is explained by two shear stresses: the magnetic shear stress acting across twinning plane t mag (which is evaluated as the difference in magnetic energy between variants divided by twinning shear) and the shear stress required for twinning plane movement treq. That is, the RMV by magnetic field starts to occur when tmag exceeds treq regardless of temperature and structure of martensite. © 2006 IOP Publishing Ltd.