An alternating-gradient magnetometer (invited)

Abstract

This paper describes a type of vibrating-sample magnetometer capable of sensitivity exceeding 10−8 emu. The instrument is 1000 times more sensitive than a conventional VSM with comparable working space, and is much quicker to use than a SQUID magnetometer, which generates point-by-point data. The magnetic sample is mounted on the end of a cantilevered rod that incorporates a piezoelectric element. The sample is magnetized by a dc field (variable in magnitude), and is simultaneously subjected to a small alternating field gradient. The alternating field gradient exerts an alternating force on the sample, proportional to the magnitude of the field gradient and to the magnetic moment of the sample. The resulting deflection of the cantilever rod is measured by the voltage output of the piezoelectric element. By operating at or near a mechanical resonance frequency of the cantilever, the output signal is greatly amplified. In practice, the operating frequency is 100–1000 Hz, with mechanical Q values of 25–250. Mechanical and acoustic noise in the environment limits the sensitivity. Measurements have been made with a signal-to-noise ratio of about 500 on a 25-μm sphere with a moment of 3.7×10−6 emu; this corresponds to a sensitivity of at least 10−8 emu. A complete hysteresis loop over ±10 kOe can be made in about 100 s. Measurements have been made over a temperature range from 77 to 400 K.