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Giant

Magnetostriction in Ferromagnetic Shape-Memory Alloys

Tomoyuki Kakeshita and Kari Ullakko Abstract Shape-memory alloys are now widely used because they exhibit a large recoverable strain, which is caused by the conversion of variants in the martensite phase. The conversion of variants is usually promoted by the application of external stress. Recently, however, it was found that the conversion of variants can also be promoted by the application of a magnetic field to induce the martensitic state in ferromagnetic Ni2MnGa shape-memory alloys. Since then, the research in this field has focused considerable attention on applications for using the materials as actuators and sensors because their response to a magnetic field is much faster than their response to heating or cooling. Furthermore, the mechanism of the conversion of variants by the magnetic field has attracted academic interest from many researchers. In this article, we show giant magnetostrictive behavior in three ferromagnetic shape-memory alloys—Ni2MnGa, Fe-Pd, and Fe3Pt—and review the investigations performed so far by many researchers, including the present authors. Keywords: single crystal, magnetic materials, phase transformation, shape memory.

Introduction The martensitic transformation, which occurs in many Fe-, Cu-, and Ti-based alloys and in ceramics, is a typical first-order structural phase transformation without atom diffusion. It has been widely studied in order to know its characteristics from the physical, metallographic, crystallographic, and technological points of view.1 This martensitic transformation is known to be extensively influenced by external fields2 such as temperature, uniaxial stress, and hydrostatic pressure. Therefore, to clarify the effect of external fields on martensitic transformations, it is very important to understand the essential problems of the transformation, such as its thermodynamics, kinetics, and the origin of the transformation, and it is also important to obtain technological information related to developing structural and smart mate-

MRS BULLETIN/FEBRUARY 2002

rials. A magnetic field is another external field that influences martensitic transformations because of the difference in magnetic moment between the parent and martensitic states. The effect of magnetic fields on martensitic transformations has been studied by many researchers, especially in Sadovsky’s group in Russia3 and recently by Kakeshita et al.4 As a result, many interesting phenomena have been found associated with magnetic-fieldinduced martensitic transformations, and these have been quantitatively analyzed.3,4 Recently, Ullakko et al.5,6 found by studying off-stoichiometric Ni2MnGa ferromagnetic shape-memory alloys that a magnetic field can control the crystallographic domains, or variants, and its related strain is quite large (103 –102). Since then, the research in this field has focused considera-

ble attention on applications for using the materials as actuators and sensors, partly because their response to a magnetic fi