Magnetization Process Associated with Rearrangement of Martensite Variants in Iron-Based Ferromagnetic Shape Memory Allo
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Magnetization Process Associated with Rearrangement of Martensite Variants in Iron-Based Ferromagnetic Shape Memory Alloys Takashi Fukuda1,Tatsuaki Sakamoto1 Tomoyuki Terai1 Tomoyuki Kakeshita1and Kohji Kishio2 1 Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan 2 Department of Superconductivity, Graduate School of Engineering, University of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ABSTRACT Magnetization processes of Fe-31.2Pd(at.%) and Fe3Pt (S ≈ 0.8) single crystals in martensite state have been examined in order to confirm the propriety of the condition for the rearrangement of variants under magnetic field: τmag>τreq, where τmag is the magnetic shear stress and τreq is the shear stress required for the rearrangement. When the magnetic field is applied along the [001] direction of each specimen, the magnetization curve shows a large hysteresis due to the rearrangement of variants. Its area, i.e., energy dissipation, is nearly the same as that obtained by stress-strain curves, suggesting the path of the rearrangement of variants by magnetic field is essentially the same as that by external stress. From the magnetization curve, the uniaxial magnetocrystalline anisotropy constant Ku is estimated: it is about 350 kJ/m3 for Fe-31.2Pd at 77 K, and is about 500 kJ/m3 for Fe3Pt at 4.2 K. The maximum of τmag, being evaluated from Ku and twinning shear, is about 2.8 MPa for Fe-31.2Pd at 77K and is about 4.3 MPa for Fe3Pt at 4.2K. For Fe-31.2Pd, the value of τreq is obtained by tensile tests at 80 K to be 0.5-2.5MPa, and the above condition is satisfied. The above condition is also confirmed to be adequate by examining the influence of field direction on the magnetic filed-induced strain. INTRODUCTION Some ferromagnetic shape memory alloys exhibit a magnetic field-induced strain (MFIS) due to rearrangement of martensite variants. The first report of this phenomenon was made by Ullakko et al. in a Ni-Mn-Ga alloy[1,2]. Subsequently, the giant MFIS was reported in ironpalladium alloys containing about 30at.% of Pd [3-5] and in an ordered Fe3Pt with degree of order of about 0.8 [5,6]. In addition to these alloys, Co-Ni-Al, Ni-Fe-Al etc. [7,8] are also considered to be candidates exhibiting a giant MFIS due to rearrangement of martensite variants. The condition for the rearrangement of variants by magnetic field has been considered as the following: the magnetic stress due to magnetocrystalline anisotropy energy should be larger than the stress required for the rearrangement. This condition has been confirmed to be adequate for some Ni-Mn-Ga alloys by evaluating magnetocrystalline anisotropy constant and the stress for the rearrangement of variants[9-12]. However, the propriety of this condition is not confirmed for Fe-Pd and Fe3Pt, although its confirmation is of importance for understanding the mechanism of the rearrangement of variants by magnetic field. According to our experiments[4-6], the characteristics of
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