Development of helium-free torque magnetometer

点击翻译按钮获取中文摘要

We have developed a new torque magnetometer on the basis of a 4-K refrigerator. The system temperature can be lowered down to 1.5 K by pumping liquefied helium from a top loading sample space. A piezoresisitor bridge on a cantilever is used to detect torque acting on a sample. A transverse magnetic field is supplied by a variable field permanent magnet up to 10 KG. We find that a sensitivity of our torque magnetometer is 10^<-10> Nm. Our cryostat consists of a 4-K refrigerator (Sumitomo Heavy Industries, Ltd.) and an exchange gas cooling system (Janis). The top-loading sample space is inserted into a bore of a permanent magnet (magnetic Solutions MM-1000-48), which produces a transverse magnetic field. The magnet consists of the two concentric dipole-ring magnets. The inner magnet as well as the outer magnet can be rotated independently by stepper motors and positioning systems. Thus, the magnetic field as a vector sum can be altered from 0 to 10 kG at any orientation angle. A torque sensor attached to a copper sample holder is a microfabricated Si substrate cantilever based on a bridge circuit. This consists of the four-constantan piezoresistors (Quantum Design). The off balance signal of the bridge is quickly measured by a direct trigger link between an anovoltmeter (Keithley 2182) and a current source (Keithley 2400). A torque chip is 25 μm in thickness, a sample stage is 2 X 2mm, and an arm is A mm in length. Torsion of the cantilever yields a change in piezoresistors. The torque sensor can be rotated by using a stepper motor (Oriental Motor) equipped at the top of the insert cryost at. Angular resolution of the motor is 0.0036 degrees. A sample temperature can be controlled by a temperature controller (Lakeshore 340). The system is totally controlled by a LabVIEW program.