This study addresses the experimental and theoretical investigations of guided elastic waves propagation in piezo-magnetic multi-layered structure. The structure is composed of a 20×TbCo2(5nm)/FeCo(5nm) nanostructured multi-layer deposited between two Aluminum (Al) Inter-Digitals Transducers forming a surface acoustic wave delay line, on a Y-cut LiNbO3 substrate. We compare the calculated and measured phase velocity variation under the action of the external magnetic field orientation and magnitude. We find quantitative agreement between the measured and modeled phase velocity shift for all external magnetic field configurations (hard axis and easy axis) and for different shape modes of elastic waves at their first and third harmonic operation frequencies. The shear horizontal mode exhibits a maximum phase velocity shift close to 20% for a ratio close to 1 between magneto-elastic film thickness and wavelength.

Multilayer magnetostrictive structure based surface acoustic wave devices

A surface-acoustic-wave (SAW) magnetic-field sensor utilizing fundamental, first- and second-order Love-wave modes is investigated A 45 μ m SiO2 guiding layer on an ST-cut quartz substrate is coated with a 200 n m (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 ° / m T is measured for the fundamental Love mode at 263 M Hz  For the first-order Love mode a value of 45 ° / m T is obtained at 352 M Hz  This result is compared to finite-element-method (FEM) simulations using one-dimensional (1D) and two-and-a-half-dimensional (25 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 h GL / λ ≈ 15 for an IDT pitch of p = 12 μ m  For this layer configuration, the first three modes are sufficiently far away from cut-off and show good sensitivity

Multi-Mode Love-Wave SAW Magnetic-Field Sensors

Magnetic sensors based on the interaction between the delta-E effect and surface acoustic waves (SAWs) have been widely investigated. However, the influences of the delta-E effect on the SAW parameters have not been demonstrated systematically. Therefore, in this study, we performed a comprehensive analysis of the influences of the delta-E effect on the properties of a SAW resonator. The one-port SAW magnetic sensor had a four-layer structure composed of ST-cut 90° X quartz (piezoelectric substrate), Ta (interdigital electrode), SiO2 (insulating layer), and (Fe90Co10)78Si12B10 (magnetostrictive layer). The parameters of the SAW resonator such as the conductance spectrum, resonance frequency, full-width-at-half-maximum (FWHM) of the resonance frequency, Q-value, electromechanical coupling coefficient, and figure of merit were investigated. The change trends of the parameters followed the delta-E effect, except for the FWHM which exhibited an opposite trend. The magneto-elastic coupling efficiency of the magnetic layer varied with the change of external magnetic fields, which led to variation in the efficiency of electro-acoustic power conversion in the piezoelectric layer. We assume that this variation also contributed to the variation of the SAW parameters. These findings demonstrate that the variation of SAW resonator parameters is related not only to the delta-E effect but also to magneto-elastic coupling.

Influence of the delta-E effect on a surface acoustic wave resonator

In this work surface acoustic Love wave delay line magnetic field sensors with varying magnetostrictive layer thicknesses are discussed. Amorphous FeCoSiB is used as the sensitive layer with a thickness variation in the range of 25–400 nm. Each sensor is analyzed both by magneto-optical methods as well as electrical signal and noise measurements. In accordance with previously reported simulation results, the impact on the acoustically propagating wave's velocity distinctly increased with thicker magnetostrictive layers leading to magnetic sensitivities as high as 35 rad/T for a delay line coated with 400 nm thick FeCoSiB. Measurements of the sensor-intrinsic phase noise revealed distinct flicker phase noise that scales proportionally with the increase in magnetic sensitivity, thus leading to virtually constant limits of detection (LOD). Assuming that the observed flicker noise is caused by magnetic hysteresis losses due to the high-frequency excitation of the magnetic material, the LOD's independence of the magnetic sensitivity can even be shown analytically. However, it also becomes clear that such sensors need a magnetostrictive coating with a minimum thickness of at least 50 nm such that the signal-to-noise ratio is dominated by the magnetic material and not by fundamental noise contributions of the substrate. On the contrary, layers with thickness above 300 nm lead to distinct higher hysteresis losses that outrun the simultaneous increase in sensitivity, thus leading to a worse overall performance.

Sensitivity and noise analysis of SAW magnetic field sensors with varied magnetostrictive layer thicknesses

Optimum geometrical structure of highly magnetostrictive multilayer films for the propagation of horizontally polarized magneto‐surface‐acoustic‐wave (Love‐type MSAW) was determined theoretically. The multilayered medium is composed of highly magnetostrictive alloy films and insulating films which are stacked alternately on a glass substrate. The analysis revealed that outstanding magnetoacoustic quantities comparable to those of an ideal nonconductive medium can be realized with the conductive multilayer films whose geometrical parameters meet the following conditions: DM/DL≥1, DM/DI≥10, and N∼20, where N is the number of insulating layers having the whole thickness of DI, and DM is the whole thickness of magnetic layers. DL is a limiting thickness of the magnetic film with a single‐layer structure, above which the magnetoacoustic quantities are considerably deteriorated due to the micro‐eddy current losses.

Optimum geometrical structure of highly magnetostrictive multilayer films for horizontally polarized high frequency magneto‐surface‐acoustic‐wave propagation

The propagation of Love‐type surface‐acoustic waves (SAWs) in a multilayer structure was investigated. The structures consisted of rf‐sputtered iron‐boron (FeB) amorphous magnetic films stacked alternatively to SiOx insulating layers i.e., FeB‐(SiOx−FeB)×n (where n denotes the stacking number) onto a soft glass substrate. The phase of waves was controlled over a wide range by a magnetic field applied in the film plane, through change in the strength of magnetoelastic coupling between the magnetic moment of the FeB film and SAW propagating in the substrate. The transducers for the Love‐type SAW excitation were constructed by combining the interdigital electrodes and ZnO films, prepared by sputtering in an oblique electrode arrangement. The controllable phase shift (ΔΦ) under a magnetic field increases linearly with n, when the total FeB film thickness DM is kept constant (DM=1.5 μm). The ΔΦ of a device with n=8 is about 2300°/cm within the magnetic field smaller than about 120 Oe.

Love‐type surface‐acoustic waves propagating in amorphous iron‐boron films with multilayer structure

We have constructed Love-type magneto-surface-acoustic-wave (MSAW) devices using amorphous FeB alloy films sputtered on glass substrates. For the Love-type SAW transducers, we employed interdigital electrodes combined with ZnO films rf-sputtered by oblique deposition, where the crystallographic c-axis of ZnO film is grown inclined to the substrate plane. The velocity controllability of a device using monolayer FeB film (operated at 72 MHz) is 1.03%. This controllability can be improved through the suppression of eddy-current accompanying MSAW propagation using multilayer FeB films alternately stacked with thin SiOx layers.

Shin'ichi Tanaka

Papers

Love‐type surface‐acoustic waves propagating in amorphous iron‐boron films with multilayer structure

Propagation Properties of Love-Type Magneto-Surface-Acoustic-Waves in Amorphous FeB Films : SAW and Communication Devices

Propagation Properties of Love-Type Magneto-Surface-Acoustic-Waves in Amorphous FeB Films