Magnetic anisotropy controlled FeCoSiB thin films for surface acoustic wave magnetic field sensors

Abstract

Surface acoustic wave magnetic field sensors based on guided Love waves using the ΔE effect of a magnetostrictive thin film have been shown to be promising candidates for the measurement of weak fields at low frequencies as required for biomagnetic applications or as current sensors benefitting from the large dynamic range and bandwidth. The deposition of soft magnetic films with high magnetostriction is, however, more challenging on piezoelectric substrates such as quartz than on silicon. Thermally induced anisotropic expansion during the deposition process or during post-deposition magnetic field annealing leads to uniaxial stresses acting on the films, which makes the precise control of magnetic anisotropy difficult. Accordingly, this work analyzes the influence of the deposition process and heat treatment on the performance of Love wave devices. ST-cut quartz based delay line surface acoustic wave sensors with a SiO2 guiding layer are employed, and a 200 nm layer of amorphous magnetostrictive (Fe90Co10)78Si12B10 is used as the sensitive element. Magneto-optical imaging is performed for magnetic domain characterization, and the sensor performance is characterized in terms of bias field dependent phase sensitivity and frequency dependent phase noise. By performing a low temperature deposition in an external magnetic field, considerable improvement in limits of detection at biomagnetic relevant frequencies down to 70 pT/Hz at 10 Hz and 25 pT/Hz at 100 Hz is achieved.

Multi-Mode Love-Wave SAW Magnetic-Field Sensors

“…The sensitivity of the fundamental mode measured here is only outperformed by the sensors described by Schell et al [ 11 ] with 2000 / using a magnetic anisotropy controlled magnetostrictive layer or sensors with higher MS layer thicknesses [ 24 ]. Potentially, using these more enhanced layers, the overall sensitivity of the sensor geometry in this study could even surpass the one described there due to its enhanced magnetic sensitivity.…”

“…To promote adhesion and prevent oxidation, 10 Ta layers on both sides of the FeCoSiB layer were deposited. During deposition, a magnetic field is applied along the y -axis to saturate the film and introduce an easy axis of magnetization [ 11 ]. The total stack thicknesses for IDTs and MS layer (Cr/Au/Cr and Ta/FeCoSiB/Ta) are controlled by profilometer after structuring.…”

“…These sensors use shear-wave surface modes (Love-waves) supported by a guiding layer. Recently, Love-wave SAW sensors have been developed for highly sensitive magnetic-field sensing [ 4 , 5 , 6 ] using either resonant [ 7 , 8 ] or delay-line structures [ 5 , 9 , 10 , 11 ]. In Love-wave magnetic-field sensors, the delay line of the SAW device is coated with a magnetostrictive (MS) layer as shown in Figure 1 .…”

Magnetoelastic Coupling and Delta-E Effect in Magnetoelectric Torsion Mode Resonators

“…It has been used for a different kind of delta-E effect sensors where shear waves, traveling through the magnetoelastic material, are influenced by the delta-E effect. This concept was realized with bulk acoustic shear waves in amorphous ribbons [ 38 ] and recently with surface acoustic shear waves in magnetic thin film devices [ 10 , 39 , 40 , 41 , 42 ]. Only very few studies investigate torsion modes in beam structures [ 43 , 44 ], either with electrostatically actuated cantilevers [ 43 ] or double-clamped beams [ 44 ].…”

Fundamental Noise Limits and Sensitivity of Piezoelectrically Driven Magnetoelastic Cantilevers

“…II-A) have been presented, whose mechanical properties depend on an external magnetic field through interaction with a magnetostrictive layer. Although realizations in the form of highly sensitive magnetoelastic surface acoustic wave delay lines were also presented [3]- [9], magnetoelastic sensors are most commonly based on resonant structures [10]- [18], especially cantilevers [19]- [23], with resonance frequencies in the range between 550 Hz and 226 MHz.…”