Development and validation of a 10 kHz–1 MHz magnetic susceptometer with constant excitation field

Abstract

The design and validation of a mutual inductance AC susceptometer with constant excitation field of up to 4.25 Oe, operating at frequencies from 10 kHz to 1 MHz, is presented. Considerations such as parasitic capacitances between wire turns and sensing bridge electronics were taken into account in order to extend the operating frequency range. An 18AWG wire with considerable insulator thickness was used for coil construction to keep parasitic capacitive reactance negligible relative to coil inductive reactance, and to obtain controlled field operation. A high speed instrumentation amplifier (slew rate over 33 V/μs) was designed and constructed using voltage feedback LM7171 operational amplifiers. The system was calibrated with Dy2O3 to account for mismatches in signal amplitude and phase shifts due to the electronics, coil coupling and imperfections, and external disturbances. AC susceptometer operation in the 10 kHz–1 MHz frequency range was validated by measuring the complex susceptibility of cobalt ferrite nanoparticles suspended in solvents of different viscosities. Good agreement was found between the experimental Brownian relaxation times and those predicted theoretically from the viscosity of the suspending media and the hydrodynamic diameter of the nanoparticles.