Torque Differential Magnetometry Using the qPlus Mode of a Quartz Tuning Fork

A quartz tuning fork is the key component of high-resolution atomic force microscope. Because of its high quality factor, a quartz tuning fork can also be used for high-sensitivity magnetometry. We develop a highly sensitive torque differential magnetometry using the qPlus mode of a quartz tuning fork. The tuning fork is driven by an ac voltage, and its deflection is measured by the resultant ac current. We observe a sharp resonance of the quartz tuning fork at low temperatures down to 1.6 K. We calibrate our torque differential magnetometry by measuring the angular dependence of the hysteresis loop in single-crystal Fe0.25TaS2. Furthermore, we demonstrate the high sensitivity of the torque differential magnetometry by measuring the quantum oscillations of a bismuth single crystal. The extracted Fermi-surface cross sections are consistent with those of bismuth crystals.