Detection of Low-Intensity Magnetic Fields by Means of Ferromagnetic Films

Abstract

This paper describes a sensitive field magnetometer which utilizes a ferromagnetic film as the sensitive element. Typical vacuum‐evaporated films of Ni–Fe of approximately 80% Ni composition are used. These films are chosen to have small directional dispersion in the uniaxial anisotropy. It is well known that in the absence of other magnetic fields the application of a cyclically varying field (of amplitude greater than the anisotropy field) perpendicular to the average anisotropy axis causes the film to split into domains parallel to that axis. The net flux which cuts a sensing coil, the axis of which is parallel to the domains, remains zero during such cyclic magnetization. If there is a small component of field along the anisotropy axis, however, the net flux is proportional to that field. The induced voltage in the sensing coil indicates the direction and magnitude of the axial component of an ambient field. This principle forms the basis of the present device. In practice, a sinusoidal rf (4 Mc) field drives the film along the perpendicular direction. A modulation field is applied along the average anisotropy axis. The output of the sensing coil is a modulated rf signal which is diode‐rectified and phase‐detected. In the absence of an ambient field along the anisotropy axis, the detected signal is of a frequency twice that of the modulation field. An ambient field along the axis causes the fundamental frequency of the modulation field to appear. The amplitude of the fundamental is proportional to the ambient field for small fields, while the phase is determined by the direction of the field. Fields having frequencies from dc to roughly 0.1 the modulation frequency can be detected in this manner. The smallest field so far detected by this device is of the order of 10⁻⁶ Oe in a dc to 1 cps bandpass.