With increasing interest in micro machine and micro mechatronics, high interest is given to micro devices using magnetostrictive alloys with small sizes. The accurate measurement of magnetostritions ranging from several tens ppm to 1000 ppm of small alloy samples is not easy, and the establishment of measurement methods of magnetostriction of small samples is needed. In this study, four different methods were examined to measure magnetostrictions of small metal samples (cubic samples with sizes smaller than 5 mm x 5 mm x 5 mm ) : Optical interference (OI) method, cantilever (CL) method, strain gauge (SG) method and capacitance (CP) method. The OI method was found to have a high sensitivity, however, too sensitive to measure small deformations of a small sample because of mechanical disturbance induced by surrounding mechanical noises. The CL, SG, and CP methods exhibited similar values of magnetostriction with high stability and reproducibility. This paper reports and discusses results of these measurement methods.
The crystal growth of Tb-Fe giant magnetostrictive materials under microgravity (μG) and terrestrial gravity (1G) was investigated. The microgravity conditions were obtained by free fall in drop tower facility at the Japan Microgravity Center (JAMIC). TbFe1.83 alloy with 1 gram and cubic form was prepared for unidirectional solidification under microgravity environment. The samples were melted just before drop and solidified by contact chill against a sample at the period of microgravity after dropping. The microstructure of μG sample was columnar structure and growth direction was aligned in thermal gradient. In 1G sample, the microstructure was weak aligned in thermal gradient. The composition was measured by EDX. The TbFe3 phase was observed in 1G sample, and no TbFe3 phase was observed in μG sample, caused by reduction of thermal convection in microgravity environment. In μG sample, the columnar structure that aligned thermal gradient was oriented orientation. The magnetostriction of parallel direction to the thermal gradient was larger than perpendicular direction in μG and 1G. The magnetostriction of μG sample, the measurement direction was parallel to the thermal gradient, was larger than 1G sample caused by microstructure.