We describe a new type of directional magnetometer based on a novel process utilizing the hysteretic properties of a soft cylindrical ferromagnetic thin film having circumferential anisotropy. The physics on which the operation of our sensor is based deals with the rotation of the magnetization vector of a soft magnetic material under a selected working geometry of applied fields and provides the computation of the resulting "field-voltage" transfer function. The principal difficulties encountered during development work are discussed. The trickiest part was the preparation of the thin film itself. The essential steps of our best production process includes multiple vacuum evaporations to give a composite of several non-magnetostrictive Fe-Ni films separated by SiO films. Data representing the performance of our magnetometer are presented.

Description of a thin film directional magnetometer

This work is concerned with improving the properties of single-domain uniaxially anisotropic thin ferromagnetic films for use as sensor elements in an inductance-variation magnetometer. Spurious signals originating from small reverse closure domains at the easy direction ends of the film contribute noise to the output signal and limit the sensitivity of the magnetometer. Their number and size can be controlled by tapering the film edge to form a region where the locally high coercivity discourages their formation. Examination by several techniques of a range of films with differing taper angles has established an optimum angle of taper which correlates closely with theoretical considerations of the magnetisation curling distance at the film edge. Films incorporating this feature have a greatly improved noise performance.

Minimising the noise in a thin film inductance variation magnetometer

Caracterisation du bruit d'un capteur magnetometrique a couche mince

The magnetic properties and low-frequency noise measurements of thin films of Permalloy are described. The films are obtained by electrodeposition. They are polycrystalline and single domain. The noise due to the film is shown to be an increasing function of the dispersion angle α 90 . A model for the noise mechanism taking into account the magnetostatic stray field arising in the grain boundaries and the thermal agitation energy is given. It is in qualitative agreement with the experimental results.

J. Dumas

Papers

Magnetic properties and noise in cylindrical Permalloy thin-film magnetometers