Development of Novel Magnetostrictive Fe-Co-B Thin Films as a High Frequency Sensor Platform
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Development of Novel Magnetostrictive Fe-Co-B Thin Films as a High Frequency Sensor Platform Zhizhi Sheng and Z.-Y. Cheng Materials Research and Education Center, Auburn University, Auburn, AL 36849, U.S.A. ABSTRACT Fe-Co-B was identified as a potential candidate for the development of high-frequency sensors and high-frequency actuators. To fabricate high-frequency magnetostrictive resonators, Fe-Co-B magetostrictive thin films were prepared by combining electrochemical deposition and microfabrication processes. It is crucial to obtain thin films with proper microstructure and composition. Results showed that Fe-rich Fe-Co-B thin films exhibited better resonance behavior than those with Co-rich and equiatomic Fe and Co compositions. It is also found that the deposition condition plays an important role on the performance of the films. Fe55Co28B17 thin films were fabricated under the same current density for different times. The films exhibited nanocrystalline structure, circular nodules as surface morphology and good resonance behavior. Fe/Co ratio on surface and cross section slightly decreased with increasing the deposition time. The resonance frequency slightly increased and the Q value was found to decrease with increasing deposition time. INTRODUCTION Fe-Co-B thin films have been considered as potential candidates for high-frequency sensors and high-frequency actuators. Compared with crystalline magnetostrictive materials such as Ni80Fe20, TbFe2, SmFe2, Terfenol-D (Tb0.3Dy0.7Fe1.9) and Gafenol (Fe-Ga), Fe-Co-B amorphous or nanocrystalline alloys have low anisotropy so that they can be used for miniaturized devices, and they are potentially used for high-frequency application. Compared with commercial amorphous materials such as Metglas 2826 MB (Fe40Ni38Mo4B18) [1, 2] and Metglas 2605SC (Fe81B13.5Si3.5C2)[3, 4], the easy fabrication of Fe-Co-B thin film sensors in micro or nano scale could be achieved and the homogeneity could be guaranteed. Sensors with both higher sensitivity and higher Q value (quality merit factor) are desirable. It is generally accepted that the smaller the size is, the higher the sensitivity. With smaller size, a higher resonance frequency can also be achieved. Fe-rich Fe-Co-B alloys were reported as superior to Co-rich Fe-Co-B alloys due to larger magnetostriction. For metal-metalloid amorphous alloys, Fe-rich alloys showed positive magnetostriction on the order of 10-5, while Co-rich alloys exhibited negative magnetostriction on the order of 10-6, and zero magnetostriction composition was found to be (Co0.95Fe0.05)100xMx[5]. Fe-rich amorphous alloys with large magnetostriction can provide a strong resonance signal. It is yet unknown if Fe-rich Fe-Co-B alloys have good sensor performance. The highest magnetic moment was among 10 at.% to 20 at.% for both Fe-B and Co-B alloys[6]. For Fe-Co-B ternary alloy, the atomic ratio of Fe/Co and Boron content for better magnetostriction is under investigation. It is also necessary to know the suitable composition for higher sensitivity. In my recent
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