Effect of Stress-induced Magnetic Anisotropy on the Properties of Giant Magnetostrictive Single Layer and Multilayer Thi
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Effect of Stress-induced Magnetic Anisotropy on the Properties of Giant Magnetostrictive Single Layer and Multilayer Thin Films J. H. Tan1, V. H. Guerrero2, R. C. Wetherhold2 and W. A. Anderson1 University at Buffalo, SUNY [1] Electrical Engineering Department, 208 Bonner Hall [2] Mechanical and Aerospace Engineering Department, 318 Jarvis Hall Buffalo, New York 14260
ABSTRACT Giant magnetostrictive thin films deposited on nonmagnetic substrates can constitute effective sensors and actuators for microdevices. In this work, we investigated the effects of a stress-induced anisotropy on the magnetic properties of Tb0.4Fe0.6, Fe0.5Co0.5 single layer films and [Tb0.4Fe0.6/Fe0.5Co0.5]n multilayers deposited on Si substrates. The magnetostrictive thin films were fabricated by means of RF sputtering and were subjected to a post-deposition annealing treatment. The uniaxial magnetic anisotropy was induced by bending the substrate before deposition and then allowing it to resume its original flat shape after depositing the film. The heat treatment was performed in a vacuum system with a vacuum of 10-6 Torr. The magnetic properties of the fabricated specimens were measured using a SQUID. SEM and XRD analyses were performed to ensure that the thermal treatment would relax the internal stresses induced during the deposition process without crystallizing the film. The thickness of the single layer thin films studied was between 300 and 800 nm while multilayer samples consisted of 6 layers with each layer thickness ranged from about 20 to 40 nm. Compared to single layer samples, multilayer samples with stress anneal growth exhibited an improvement in magnetic saturation by a factor of two while maintaining a low coercive field. Manipulations of the magnitude and direction of magnetic anisotropy was observed by introducing various values of tensile and compressive stress into the film. INTRODUCTION For the development of micro-actuators and sensors in MEMS, giant magnetostrictive materials have become increasingly important. By combining exchange-couple giant magnetostrictive materials (amorphous Tb0.4Fe0.6) and materials with large magnetic polarizations (FeCo), multilayer magnetic thin films exhibit significant improvement in terms of high magnetostriction at low field and high magnetostrictive susceptibility compared to single layer thin films [1]. The amorphous rare earth transition metal TbFe has rather low magnetization due to its rare earth nature. FeCo is magnetically soft, has a very high magnetization and a sizable magnetostriction. In order to achieve the polarization enhancement and the anisotropy reduction, careful study of the magnetic, magnetostrictive and mechanical characteristic of these multilayers must be done [1]. To further improve the magnetostrictive properties by controlling the uniaxial anisotropy, stress annealing growth was introduced to these multilayer magnetostrictive thin films structured as [TbFe/FeCo]n. These could lead to the potential commercial applications in magnetostrictive devices.
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