Thermally Induced Magnetic Anisotropy in Nickel Films on Surface Acoustic Wave Devices

The combination of magnetostrictive thin films with surface acoustic wave (SAW) devices enables the magnetic sensors that exploit the <inline-formula> <tex-math notation="LaTeX">$\Delta E$ </tex-math></inline-formula>-effect (magnetically induced change in modulus) without the need for free-standing structures that are susceptible to vibration and damage. The performance of these sensors is strongly influenced by magnetic anisotropy and, therefore, the state of stress in the magnetostrictive film. We report the fabrication and characterization of magnetic SAW devices comprised of a magnetostrictive layer and interdigital transducers on a piezoelectric substrate. The state of stress in the magnetostrictive layer is varied by annealing. Vibrating-sample magnetometer measurements indicate that the annealed devices show a strong uniaxial anisotropy with the easy axis parallel to the direction of acoustic propagation. The induced magnetic anisotropy is attributed to the anisotropic thermal expansion in the substrate. The change in frequency of one-port resonators and two-port SAW oscillators is measured as a function of dc bias field and ambient temperature. The normalized change in frequency (<inline-formula> <tex-math notation="LaTeX">$\Delta f/f_{\mathrm {sat}}$ </tex-math></inline-formula>) is on the order of <inline-formula> <tex-math notation="LaTeX">$10^{-4}$ </tex-math></inline-formula>. These results demonstrate that the film stress can be used to optimize the performance of magnetic SAW devices and the SAW devices can be used to characterize the state of stress and magnetic anisotropy of magnetostrictive films.