Temperature dependence and annealing effects on magnetoelastic SAW attenuation and magnetoresistance in Ni films

The magnetic field dependencies of 600 MHz magnetoelastic Rayleigh-mode surface acoustic wave (SAW) attenuation and dc magnetoresistance of Ni thin films were measured as a function of temperature and correlated with variations of the samples´ anisotropy field. Thin film samples (<20 nm) were prepared by thermal evaporation in a vacuum of 10^-7 Torr onto ST-cut, X-propagating single-crystal quartz substrates. The piezoelectric substrates were equipped with 1.5 μm interdigital transducers and contacts for measuring the samples´ microwave SAW attenuation and dc magnetoresistance in situ. The temperature dependence of as-deposited samples was measured between 237-400 K and for in-film-plane applied fields between ±90 Oe. Heating the samples to about 380 K produced substantial permanent improvements (>10 dB/cm) in SAW attenuation response. This effect may be due to irreversible magnetoelastic annealing effects between the thin film and substrate. After annealing, the subsequent temperature dependence of sample response repeatably correlated with a linear (-0.6 Oe/K) temperature dependence of the samples´ uniaxial anisotropy field. The temperature dependent anisotropy may be caused by the combined effects of the magnetostriction and differential thermal expansion between the polycrystalline Ni films and the anisotropic quartz substrates. These effects can be used to provide a method for enhancement of the sample magnetoelastic SAW attenuation and magnetoresistive response by controlling the magnitude and direction of the sample´s anisotropy field