A new system for measuring alternating current magnetic susceptibility of natural materials over a wide range of frequencies

Abstract

A new measurement system has been developed for detecting frequency dependence of low-field alternating current (AC) magnetic susceptibility in natural samples. Instead of employing intricate AC bridge circuits, this system configuration is simple and consists of a set of primary, secondary (pickup), and compensation coils; a function generator; and a two-phase lock-in amplifier. This system can measure both in-phase and out-of-phase components of AC susceptibility. The operating frequency can be chosen at any value between 1 Hz and 2.5 kHz, and the AC magnetic field intensity is changeable up to 1.0 × 10 3 A/m. The resolution is 1 × 10 -8 m 3 /kg (mass-specific SI) for the 10-10 2 Hz range and 2-5 × 10 -8 m 3 /kg for >10 2 Hz. Reconnaissance measurements made on Chinese loess and paleosol samples revealed a stronger frequency dependence for the paleosol than for the loess over the entire frequency range. This result suggests that the frequency dependence spectrum over wide band of frequencies can be useful, especially in environmental magnetism, as a new rock magnetic property to estimate the grain size distribution of superparamagnetic particles. © 2010 by the American Geophysical Union.