Modelling of magneto‐mechanical hysteresis loops in Ni‐Mn‐Ga shape memory alloys

Abstract

A predictive model of field-induced strain in Ni-Mn-Ga ferromagnetic shape memory alloys is proposed. In this study, magnetocrystalline anisotropy K1 is introduced to permit magnetisation rotation in martensite platelets. The demagnetisation field induced by the shape of platelets is investigated. The proposed model is identified on experiments performed by Straka et al. This identification shows that observed macroscopic hysteresis loops, i.e. magnetisation and detwinning strain versus applied magnetic field, correspond exactly to a successive activation of three mechanisms: Movement of 180° domain walls, rotation of magnetisation and martensite detwinning. As expected, magnetization and strain -induced by magnetic field- in a single crystal are mainly given by the mobility of twin boundaries and magnetic anisotropies (due to the martensite crystallographic structure and to the shape of platelets). (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)