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 Pr0.1La0.9Be13 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.
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The local field (Hloc) for Pr nuclei depends on the Van-Vleck susceptibility (χVV): Hloc = Hext(1 + hfχVV) ≡ Hext(1 + K), where hf = 187.7 mol emu−1 is the hyperfine coupling constant for Pr nuclei.
