Sensitive Flux Measurement of Thin Magnetic Films

Abstract

The two main difficulties encountered in the design of a sensitive hysteresis loop tracer for thin magnetic films are the flux calibration and the reduction of noise. The aim of this paper is to examine their nature and to show the possible solution for the design of very sensitive hysteresis loop tracers. The study of the flux distribution around a thin magnetic film specimen permits determination of the merits of various pickup coil configurations, as well as the form which optimizes the signal-to-noise ratio. The various disturbing voltages and the ways to eliminate them are examined. Optimization of the amplifier noise figure, proper choice of the integration network, dc restoration, and hum synchronization are described for the reduction of the output noise after integration and amplification. Two instruments built according to these principles are outlined. The first has a single wire pickup and is well suited for measurement of the flux distribution around a thin magnetic film and for experiments in vacuum at elevated temperatures; the second is more flexible and sensitive. By using different pickup coils covering a frequency range from 50 cps to 10 kc, its sensitivity is sufficient to measure flux values of 2×10−12 v-sec at a frequency of 500 cps.