Multi-Mode Love-Wave SAW Magnetic-Field Sensors

A surface-acoustic-wave (SAW) magnetic-field sensor utilizing fundamental, first- and second-order Love-wave modes is investigated. A 4.5 μm SiO2 guiding layer on an ST-cut quartz substrate is coated with a 200 nm (Fe90Co10)78Si12B10 magnetostrictive layer in a delay-line configuration. Love-waves are excited and detected by two interdigital transducers (IDT). The delta-E effect in the magnetostrictive layer causes a phase change with applied magnetic field. A sensitivity of 1250°/mT is measured for the fundamental Love mode at 263 MHz. For the first-order Love mode a value of 45°/mT is obtained at 352 MHz. This result is compared to finite-element-method (FEM) simulations using one-dimensional (1D) and two-and-a-half-dimensional (2.5 D) models. The FEM simulations confirm the large drop in sensitivity as the first-order mode is close to cut-off. For multi-mode operation, we identify as a suitable geometry a guiding layer to wavelength ratio of hGL/λ≈1.5 for an IDT pitch of p=12 μm. For this layer configuration, the first three modes are sufficiently far away from cut-off and show good sensitivity.

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