On the Ferromagnetic Resonance in Nickel and Supermalloy

Both the real and imaginary part of the high frequency permeability of flat specimens of nickel and supermalloy have been investigated at 9030 mc/sec. and 24400 mc/sec., with a constant field $H_0$ parallel to the plane of the specimen. For the higher frequency, experiments were extended beyond the Curie point (358°C for nickel and about 400°C for supermalloy). The decrease in saturation magnetization causes the maximum and the minimum of ferromagnetic absorption to shift toward higher field strengths. The $g$ factor is independent of temperature within the limit of error (2 percent); $g=2.12\mathrm{}$ for supermalloy and $g=2.20\mathrm{}$ for nickel.

The line width in nickel increases first slowly, but above 200°C more rapidly from 250 gauss at room temperature to 700 gauss at the Curie point, the width being defined as half the width between the points where the imaginary part of the susceptibility has dropped to half its maximum value. In supermalloy the width is 110 gauss between 20°C and 300°C and then rises sharply to 350 gauss at 420°C. There is no discontinuity at the Curie point, but the intensity of the absorption drops very rapidly beyond that temperature. At the lower frequency the width is reduced by approximately a factor 1.6.

The observed data are compared with existing theories. Only interactions of the ferromagnetic spins with the lattice vibrations or the conduction electrons could probably account for the observed order of magnitude of the width. It is not clear, however, how the width could be independent over a large temperature interval, if these interactions were the only broadening agent. A pseudo-dipolar interaction, introduced by Van Vleck, might give an explanation for the line width at lower temperatures.