Magnetic anisotropy, sress, and martensitic transformation in Ni-Mn-Ga thin films on Si(100) wafer
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0998-J06-09
Magnetic anisotropy, sress, and martensitic transformation in Ni-Mn-Ga thin films on Si(100) wafer Michael Hagler1, Volodymyr A. Chernenko2,3, Makoto Ohtsuka4, Stefano Besseghini3, and Peter M¸llner1 1 Materials Science and Engineering, Boise State University, 1910 University Dr., Boise, ID, 83725 2 Institute of Magnetism, Kyiv, 03142, Ukraine 3 CNR-IENI, Lecco, 23900, Italy 4 IMRAM, Tohoku University, Sendai, 980-8577, Japan Abstract Ni-Mn-Ga magnetic shape memory alloys (MSMAs) tend to undergo a large deformation upon the application of a magnetic field. This deformation is attributed to twin boundary motion in the martensitic phase. In an effort to harness the shape memory effect for use in sensors, actuators, and micro-devices, the behavior of Ni-Mn-Ga thin films is attracting attention. Substrate curvature measurements were done with Ni-Mn-Ga films with a thickness of 2.0 µm sputter-deposited on Si(100) wafer having amorphous 500 nm thick SiNx buffer layer. During the wafer bow curvature measurements, stress levels of 0.65 GPa were attained. The martensitic transformation is manifested by a stress-temperature hysteretic loop. Measurements of magnetization curves were carried out on Ni-Mn-Ga films with thickness between 0.5 and 3.0 µm. A change of the magnetization behavior from the easy-plane type for thin films to the out-of-plane easy-axis type for thick films is observed. This effect is caused by the interplay between different contributions to the overall anisotropy of film. INTRODUCTION Magnetic Shape Memory Alloys (MSMAs) tend to deform up to 10% upon the application of a magnetic field [1-3]. Magnetic-field-induced deformation (magnetoplasticity [46], and magnetoelasticity [7,8]) is of interest for actuator applications [9-11]. The inverse effect, deformation-induced change of magnetization [12-14] bears a potential for sensor applications. Magnetic-field-induced deformation is carried by the motion of twin boundaries [5,9] requiring their high mobility and large magnetic anisotropy resulting in a magnetostress larger than the twinning stress (see e.g. [15]). In MSMA, highly mobile twin boundaries are formed due to a martensitic transformation (MT). Ni-Mn-Ga is the most studied MSMA (see e.g. [11]). The interest in the miniaturization of sensors, actuators, and other devices has led to the study of MSMA thin films. Recently, results on structure [16], texture [17], transformation behavior [16-18], and magnetic anisotropy [16,19] have been published. In the present work, the MT of a Ni-Mn-Ga thin film on Si(100) wafer is studied using the substrate curvature measurement method [20]. The magnetic anisotropy was studied with a vibrating sample magnetometer (VSM).
EXPERIMENTAL Ni-Mn-Ga thin films with thicknesses between 0.1 and 3.0 µm were deposited by r.f. magnetron sputtering, with a target of nominal composition Ni49.5Mn28.0Ga22.5 (numbers indicate atomic percent). A 500 nm amorphous SiNx layer was used as diffusion barrier between the 0.5 mm thick Si(100) substrate and the Ni-Mn-Ga film.
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