Microwave Transmission Through Permalloy Films

A new interference effect has been observed in the transmission of microwave signals through Permalloy films. This effect, which involves interference between the eddy-current microwave transmission and the magnetic field associated with the excited spin waves, permits identification of the symmetry and surface pinning behavior of the standing spin waves. Previous studies of spin waves in Permalloy films have involved a determination of the surface impedance in the vicinity of spin-wave excitation. The present measurements show a decrease in transmitted microwave power for the lowest-order spin-wave mode and for those higher-order modes whose magnetic excitation has even symmetry with respect to the plane of the film. However, corresponding to intermediate modes, which have odd symmetry, we observe a transmission maximum. Thus we observe alternate minima and maxima on transmission. The measured fields correspond exactly with the field positions of the alternate strong and weak maxima in surface impedance, as observed on reflection. This correspondence between transmission and reflection supports the generally accepted interpretation of spin-wave mode symmetry. In addition, the direction and amplitude of this interference as a function of the angle between the magnetic-field direction and the plane of the film agree with the partial-pinning model of Nisenoff and Terhune.