The low frequency magnetic properties of nickel-iron films and their investigation by BH curve plotter techniques.

Abstract

The present thesis deals with the low frequency or quasistatic properties of thin vacuum evaporated nickel—iron films, in particular with extending the range and accuracy with which these properties can be measured by audio-frequency BH plotter techniques. An integral part of this work has consequently been the design and construction of a suitable BH plotter, together with a critical examination of the theoretical and practical factors limiting the performance of an instrument of this type. Considered in detail are the optimum integration transfer functions for low noise, and the best geometrical shape of the sense coil for the maximum coupling with the film sample. Circuits are given for the transistorized, high accuracy, phase compensated preamplifier and feedback integrator used in the BH plotter. Theoretical and experimental results are given in the use of the BH plotter for the measurement of such quantities as anisotropy torque curves to 0.1 per cent accuracy, which have previously required more complex and delicate instruments. Easy axis hysteresis loops are examined and the switching processes, and consequently the coercive force, shown to be dependent on the time rate of change of the measuring field. The effects of anisotropy dispersion is considered throughout, and a rigorous analysis is given of the wavelength and amplitude of the resulting magnetizational ripple. The properties of thin films are critically dependent on the conditions during deposition. This thesis also describes a self—supporting electron bombardment source with instrumentation for the control of film thickness and the stabilization of the deposition rate.