Quantitative Model of Large Magnetostrain Effect in Ferromagnetic Shape Memory Alloys

Abstract

A quantitative model describing large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni2MnGa is briefly reported.The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties of a similar type materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Poisson's rule. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetisation. A simple model of magnetisation and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni2MnGa.

The model of magnetic-field-controlled shape memory effect in NiMnGa

“…The last model has been also successfully applied to explain quantitatively the results of experimental study of large magneto-strain effects in IS^MnGa earlier obtained in [2]. In particular, according to our model estimation made in [3] a giant magneto-strain value can be achieved only in materials with very low (< 2 MPa) twinning stress values. Mechanical testing results obtained for giant magneto-strain Ni-Mn-Ga samples have completely confirmed these estimations.…”

“…In this case, the main thermodynamic driving forces should have a magnetic nature and be connected with high magnetization anisotropy and significant differences in magnetization free energies for different twin variants of martensite [1][2][3][4][5][6][7]. For instance, Fig.l shows an example of giant 5% magneto-strain effect associated with the direct and reverse magnetic-field-induced transformation between two single twin variants in Ni 4 8Mn 3 oGa22 sample.…”

“…Large magnetically driven strain effects are expected have to occur in these systems. Up to date, the largest magneto-strain effects were achieved in Ni-Mn-Ga ferromagnetic shape memory alloys [1][2][3][4][5][6][7]. The best results on the field-induced strain reported short time ago [2] never exceeded 0.2-0.3% strain value in a magnetic field of order 8 kOe in martensitic state for nearly stoichiometric single-crystal samples of Ni2MnGa alloy.…”

“…O'Handley [5] first included the rotation magnetization effects together with usual Zeeman energy term JOURNAL DE PHYSIQUE IV into consideration. Likhachev and Ullakko [3] have developed a general thermodynamic approach based on the Maxwell's relationships and derived the main magneto-mechanical equation representing both the magnetic and the mechanical driving force balance in ferromagnetic shape memory materials. The last model has been also successfully applied to explain quantitatively the results of experimental study of large magneto-strain effects in IS^MnGa earlier obtained in [2].…”

Magnetic phase separation in ordered alloys

“…It has been suggested that the control of shape-memory properties by application of an external magnetic field is a principle for operation of the recently developed actuator materials. 20,21 In Cu-Al-Mn the martensitic transition exists 14 only for compositions that are very far from the stoichiometry (Cu 2 AlMn) where the L2 1 ordered phase is paramagnetic. 22 Nevertheless, the influence of magnetism coming from Mn has been revealed in several experiments.…”