Measuring the remanent magnetic moment of a kilogram-level test mass for TianQin

In the TianQin space-based gravitational wave (GW) detection mission, the measurement of remanent magnetic moment ($\stackrel{\ensuremath{\rightarrow}}{m}$) of the kilogram-level test mass is a prerequisite before the launch. Current measurement methods have shown a measured deviation since the assumption of a uniform distribution of $\stackrel{\ensuremath{\rightarrow}}{m}$ in the test mass volume, and the unconsidered interference from ambient magnetic fields. We develop a new torsion pendulum with a modulated 2D Helmholtz coils to measure the $\stackrel{\ensuremath{\rightarrow}}{m}$. The 2D Helmholtz coils are set to get rid of the assumption and avoid the undesired systematic interference. The measurement precision of $\stackrel{\ensuremath{\rightarrow}}{m}$ by our instrument is better than 1 nA ${\mathrm{m}}^{2}$. The instrument is particularly useful for measuring the extremely low magnetic properties of the materials for use in the construction of TianQin space-based GW detection and other precision scientific apparatus.