Ultrasonic Surface Acoustic Wave Investigation of Thin Ferromagnetic Films.

An experimental work consisting of the preparation of thin ferromagnetic films and the study of the magnetoelastic interaction between these films and ultrasonic Rayleigh -mode surface acoustic waves (SAW) has been accomplished with the principal goal of clarifying the dependence of surface wave attenuation response on sample magnetic structure. The magnetic-field dependence of attenuation rm A(vec H) was mostly measured at 295 K, 600 MHz, for sample thicknesses from 50-500 A, and sample compositions Ni_{rm x}Fe _{rm 1-x}, (0 <=q x <=q 1). For selected Ni films, either the temperature dependence of rm A(vec H) was studied or relative velocity Deltarm V(vec H) measurements were performed. As an adjunct to this study it was found that the quasi-static response of sample magnetization to dc magnetic bias fields H is very conveniently represented by the ferromagnetic magnetoresistive effect, Deltarm R(vec H). Barkhausen-type discontinuities in Deltarm R(vec H) indicated multidomain film structure and magnetization reversal through domain wall motion. The rm A(vec H) data were analyzed in terms of a phenomenological model wherein the SAW interact, via inverse magnetostrictive coupling, with the magnetization of Stoner -Wohlfarth-type uniaxially anisotropic films. Although the model was originally developed for idealized single -domain samples with magnetization reversal occurring only through coherent spin rotation, here it was found that even in films with multidomain magnetic structure and non -coherent domain wall motion good qualitative agreement between theory and experiment can be achieved. The Deltarm R(vec H) effect was developed as an experimental probe for determining the direction and angular dispersion of a sample's internal "uniaxial anisotropy field" vec H_{A } and the degree of sample magnetic saturation. For a given film composition and thickness, these characteristics were found to critically affect SAW propagation, with low anisotropy dispersion and high saturability being characteristic of samples with large rm A(vec H), Deltarm V(vec H) and Deltarm R(vec H) responses. Effective sample characterization has made feasible the determination and control of essential preparation parameters and techniques involved in obtaining "good" (large response) consistently reproducible results from the film fabrication process of vacuum evaporation. It was found that inverse magnetostriction, combined with differential thermal expansion between film and substrate, also largely determines vec H_{A} and thus determines the magnitude of sample rm A(vec H) , Deltarm V(vec H) and Deltarm R(vec H) response.