Sensitive Automatic Torque Balance for Thin Magnetic Films

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Abstract
An automatic torque balance has been constructed for use in studying the magnetic properties of thin ferro‐magnetic films. The automatic servo force balancing technique using a photoelectric pickoff has been applied to a fused‐silica torsion fiber suspension. A sample up to 600 mg in weight can be handled. The data are automatically recorded by an X‐Y plotter. Sensitivity of 10−6 dyn‐cm and a response time of a fraction of a second have been obtained, providing a significant improvement over similar instruments previously used. An appropriate set of coils provides the magnetic field H. Since the torque on a sample is given by M×H=L, measurement of torque affords a means of studying the behavior of the magnetization M. Provision is made for observing torque either parallel or perpendicular to the film surface. The sensitivity available will permit the study of 1‐cm‐diam films as thin as one‐half an atomic layer under ideal conditions. Examples are given of the measurement of the saturation magnetization, remanent magnetization, anisotropy constant, and coercive field on a 35‐Å film, showing the quality of the data. The anisotropy field and the squareness are calculated from the above quantities. Observations of rotational hysteresis loss are also discussed.

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