FABRICATION AND CHARACTERIZATION OF MAGNETOSTRICTIVE THIN FILMS USING THE COMBINATORIAL METHOD
Hunter, Dwight Denroy
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Magnetostrictive materials are smart materials which show dimensional and magnetization changes in response to magnetic fields. Presently, there is growing interest to find magnetostrictive thin films for many microsystem applications, especially in microelectromechanical systems (MEMS) as powerful transducers for microactuators. But to exploit their capabilities and meet the stringent needs of microactuator and sensor applications, small driving magnetic fields on the order of mT are desirable. Current magnetostrictive materials such as rare-earth containing Terfenol, despite exhibiting giant magnetostriction, is at an extreme disadvantage due to the high saturation field (H > 0.1 T) imposed by its large magnetocrystalline anisotropy. Recently, large magnetostriction was observed in Fe-Ga alloys, and has sparked widespread research into other Fe-based alloys for possible replacements of Terfenol. Furthermore, it is becoming increasingly important to find rare-earth free compounds from a cost and availability point of view. In this thesis, we investigated the composition dependent magnetostrictive and micro-structural properties of several binary (Fe-Ga, Co-Fe, Fe-Zn, Fe-W, Fe-Mo) and ternary (Fe-Ga-Zn, Fe-Co-Al) Fe-based thin film alloys prepared using a co-sputtering based composition spread approach. This technique facilitates synthesis and screening of large compositional landscapes in individual studies and allows rapid identification of compositions with enhanced physical properties. Magnetostriction measurements were performed on as-deposited and on some annealed composition spread films, which were fabricated on arrays of micro-machined cantilevers substrates. From this study, binary Co-Fe thin film alloys emerged as a large magnetostrictive material with effective magnetostrictive values in excess of 260 ppm at a low saturation field ¡Ö 10 mT, which were quenched following a vacuum anneal at 800 <super>o</super>C for 1 hour. This substantial increase in magnetostriction was observed for compositions near the (fcc+bcc)/bcc phase boundary (Co<sub>0.65</sub>Fe<sub>0.5</sub>), and was found to depend on the cooling rate from the annealing temperature. Structural characterization by synchrotron micro-diffraction and transmission electron microscopy (TEM) reveals that this large increase in magnetostriction is associated with the presence of an equilibrium Co-rich fcc phase that precipitates into a Fe-rich bcc host phase upon annealing. The Co-Fe system is compared with Fe-Ga alloys, in which DO<sub>3</sub> nanoprecipitates dispersed in the host A2 matrix were observed at compositions (Fe<sub>0.8</sub>Ga<sub>0.2</sub>), which displays enhanced magnetostriction. The DO<sub>3</sub> nanoprecipitates in the Fe-Ga alloys are believed to behave as tetragonal defects in the matrix and their orientations can be changed by the application of a magnetic field, leading to magnetostriction. It is speculated that the Co-rich precipitates in our Co-Fe films function in much the same way as the DO<sub>3</sub> precipitates in the Fe-Ga alloys, implying that the mechanisms which give rise to magnetostriction in both systems are similar. The results on the as-deposited Fe-Ga-Zn and Fe-Co-Al ternary thin film spreads are somewhat encouraging from the point of view of finding new magnetostrictive materials. In the Fe-Ga-Zn alloys, we found that the magnetostriction value around the Fe<sub>0.6</sub>Ga<sub>0.1</sub>Zn<sub>0.3</sub> compositions was reasonably high, &lambda<sub>eff</sub> ~ 80 ppm. This could be interesting from an application and cost points of view: this means that a less expensive metal such as Zn could be substituted for Gab, while still preserving the magnetostriction. For the Fe-Co-Al ternary, the highest effective magnetostriction, &lambda<sub>eff</sub> ~ 80 ppm, was observed near the Fe<sub>0.5</sub>Co<sub>0.25</sub>Al<sub>0.25</sub> composition.