Development of high-resolution capacitive Faraday magnetometers for sub-Kelvin region

High-sensitivity capacitive Faraday magnetometers were developed for static DC magnetization measurements in a sub-Kelvin region that can be used with ³He-⁴He dilution refrigerators (∼50 mK) and ³He refrigerators (∼0.28 K). For high-resolution magnetization measurements, the background magnetization of the force-sensing capacitor should be as small as possible, compared with the magnetization value of a measured specimen. In this study, we succeeded in reducing the background of the capacitor in both low- and high-field regions by compensating for the diamagnetic response of a thin quartz plate, making use of Pauli-paramagnetic alloys and Van Vleck paramagnets as a counter magnetization for a diamagnetic signal. Having established an ultra-high-sensitivity capacitor, we achieved a resolution of 10^-5 (∼10^-5-10^-6) emu in the low- (high-) field region below (above) 1 T. In particular, the newly developed capacitors with a Van Vleck paramagnet Pr_0.1La_0.9Be₁₃ and paramagnetic MgAl alloys are demonstrated to be very useful for high-resolution magnetization measurements at milli-Kelvin temperatures in low and high magnetic fields, respectively.