Magnetoelastic composites which use surface acoustic waves show great potential as sensors of low frequency and very low amplitude magnetic fields. While these sensors already provide adequate frequency bandwidth for most applications, their detectability has found its limitation in the low frequency noise generated by the magnetoelastic film. Amongst other contributions, this noise is closely connected to domain wall activity evoked by the strain from the acoustic waves propagating through the film. A successful method to reduce the presence of domain walls is to couple the ferromagnetic material with an antiferromagnetic material across their interface and therefore induce an exchange bias. In this work we demonstrate the application of a top pinning exchange bias stack consisting of ferromagnetic layers of (Fe90Co10)78Si12B10 and Ni81Fe19 coupled to an antiferromagnetic Mn80Ir20 layer. Stray field closure and hence prevention of magnetic edge domain formation is achieved by an antiparallel biasing of two consecutive exchange bias stacks. The set antiparallel alignment of magnetization provides single domain states over the complete films. This results in a reduction of magnetic phase noise and therefore provides limits of detection as low as 28 pT/Hz1/2 at 10 Hz and 10 pT/Hz1/2 at 100 Hz.

Some methods of magnetic flux modulation are used to overcome flicker phase noise, low-frequency acoustical distortions, and movement artifacts. This work proposes employing a chopping flux modulation technique controlling a high permeability toroid together with a surface acoustic wave sensor inside. In this primary proof-of-concept study, an external magnetic field is generated to estimate quantitative signal parameters and the effect of the toroid shielding factor. Finally, the limitations of this approach should be identified and how low-frequency magnetic signals are influenced. The achievable sensitivity was empirically evaluated, and a quantitative signal quality value was calculated by estimating the signal power spectrum and noise power spectrum. Thus, the study compares the signal-plus-noise to noise ratio with and without magnetic flux modulation of a reproducible excitation magnetic signal generated by a solenoid coil. The experimental results show that the noise floor of this magnetic sensor system is improved. However, the signal-plus-noise to noise ratio without the modulation is 17 dB, and with the modulation, this parameter becomes 13 dB for a given mono-frequency signal of $20\ \mu\mathrm{T}$. In perspective, this method exhibits disadvantages in reducing the sensitivity because, with the toroid inside, the calibration factor of the solenoid is not the same anymore, and the shielding factor reduces the field strength of the alternative-current field. Furthermore, the results show that the chopping flux modulation technique requires exploring how to compensate for the losses and setup issues that affect the magnetic field to define how suitable it is for surface acoustic waves magnetic sensors.

The complex behavior of horizontally polarized surface shear waves in magnetoelectric surface acoustic wave based magnetic field sensor devices is revealed by time-resolved magnetooptical microscopy with picosecond temporal and submicron spatial resolution. The imaging of the propagating waves in the magnetoelectric composites is realized through the functional soft-magnetic layer by coupled magnetoelastic interactions. Partial surface wave reflections, wave front dephasing, and secondary wave generation occur, which originate from structures and magnetic domain walls. Closure domain structures bend and reflect the magnetic surface waves. Strain stimulated magnetic domain walls display dynamic periodic expansions, which propagate along the domain walls and change the magnetomechanical response also in the surrounding regions. The revealed spatial and temporally varying nondeterministic response restricts the noise performance of the surface acoustic wave based magnetic field sensors and thus confines the sensor's limit of detection. Magnetic time-resolved optical imaging is shown to be a powerful method for the operando characterization of magnetoelectric devices and in-plane displacement surface acoustic wave fields that are not accessible by other methods.

Typical demands for magnetic field sensor applications are high dynamic magnetic field ranges, ambient temperature operation, small dimensions, necessary for high spatial resolution, and low energy consumption. In the case of sensors for biomagnetic sensing very high requirements in case of sensitivity and limit of detection (LOD) in the pT/Hz1/2 to fT/Hz1/2 range arise. Surface acoustic wave (SAW) sensors based on the delta E effect combine the advantages of small form factor, ambient temperature operation and high sensitivity. The SAW sensor consists of a piezoelectric quartz substrate, a silicon oxide layer and a magnetoelastic (Fe90Co10)78Si12B10 layer, deposited on top of the oxide layer. Between interdigital transducers (IDTs) at the ports of the sensor, horizontal shear (Love) waves propagate. The oxide layer serves as a guiding layer. If an external magnetic field is applied, the magnetization alterations in the FeCoSiB layer are accompanied by changes of the shear modulus G and the wave propagation changes. The phase change of a transmitted signal serves as a measure of a magnetic field. Very low phase noise read out electronics is required to detect the phase changes. Heterodyne and homodyne electronic read out circuits are equivalent in performance while homodyne systems are advantages in terms of monolithic integration. The presented SAW sensors reach a high sensitivity with an LOD of 152 pT/Hz1/2 at 10 Hz and 52 pT/Hz1/2 at 100 Hz. ed signal serves as a measure of a magnetic field. Very low phase noise read out electronics is required to detect the phase changes.

The possibility of tuning the magnetic properties of materials with voltage (converse magnetoelectricity) or generating electric voltage with magnetic fields (direct magnetoelectricity) has opened new avenues in a large variety of technological fields, ranging from information technologies to healthcare devices and including a great number of multifunctional integrated systems, such as mechanical antennas, magnetometers, and radio frequency (RF) tunable inductors, which have been realized due to the strong strain-mediated magnetoelectric (ME) coupling found in ME composites. The development of single-phase multiferroic materials (which exhibit simultaneous ferroelectric and ferromagnetic or antiferromagnetic orders), multiferroic heterostructures, as well as progress in other ME mechanisms, such as electrostatic surface charging or magneto-ionics (voltage-driven ion migration), have a large potential to boost energy efficiency in spintronics and magnetic actuators. This article focuses on existing ME materials and devices and reviews the state of the art in their performance. The most recent progress on different ME devices based on ME heterostructures is presented but with a larger emphasis on ME antennas and sensors due to the significant advances achieved in these applications. The rapid development of mechanically actuated ME antennas has been observed over the past several years, producing ME antennas that are miniaturized by 1-2 orders compared to conventional antenna size. Magnetic sensors based on simple ME composites are potentially promising alternatives to conventional magnetometers due to their very good detectivity (<pT/Hz(1/2)) at low frequencies. Other ME devices reviewed in this article include RF tunable inductors with high inductance tunability and quality (Q) factor; non-reciprocal microelectromechanical system (MEMS) bandpass filters with dual H- and E-field tunability; passive isolators and gyrators in the low-frequency (LF) range; and ME random access memories for low-power data storage. All these compact and lightweight ME devices are also promising for future biomedical and wireless applications. Finally, some open questions and future directions where the community might be headed are provided.

For the best possible limit of detection of any thin film-based magnetic field sensor, the functional magnetic film properties are an essential parameter. For sensors based on magnetostrictive layers, the chemical composition, morphology and intrinsic stresses of the layer have to be controlled during film deposition to further control magnetic influences such as crystallographic effects, pinning effects and stress anisotropies. For the application in magnetic surface acoustic wave sensors, the magnetostrictive layers are deposited on rotated piezoelectric single crystal substrates. The thermomechanical properties of quartz can lead to undesirable layer stresses and associated magnetic anisotropies if the temperature increases during deposition. With this in mind, we compare amorphous, magnetostrictive FeCoSiB films prepared by RF and DC magnetron sputter deposition. The chemical, structural and magnetic properties determined by elastic recoil detection, X-ray diffraction, and magneto-optical magnetometry and magnetic domain analysis are correlated with the resulting surface acoustic wave sensor properties such as phase noise level and limit of detection. To confirm the material properties, SAW sensors with magnetostrictive layers deposited with RF and DC deposition have been prepared and characterized, showing comparable detection limits below 200 pT/Hz(1/2) at 10 Hz. The main benefit of the DC deposition is achieving higher deposition rates while maintaining similar low substrate temperatures.

In this work, the first surface acoustic-wave-based magnetic field sensor using thin-film AlScN as piezoelectric material deposited on a silicon substrate is presented. The fabrication is based on standard semiconductor technology. The acoustically active area consists of an AlScN layer that can be excited with interdigital transducers, a smoothing SiO2 layer, and a magnetostrictive FeCoSiB film. The detection limit of this sensor is 2.4 nT/Hz at 10 Hz and 72 pT/Hz at 10 kHz at an input power of 20 dBm. The dynamic range was found to span from about ±1.7 mT to the corresponding limit of detection, leading to an interval of about 8 orders of magnitude. Fabrication, achieved sensitivity, and noise floor of the sensors are presented.

We report the first study of diverse ferromagnetic thin films via acoustically driven ferromagnetic resonance (ADFMR). Angle and field-dependent ADFMR was performed at room temperature on thin films of FeCo, FeCoGd, FeGaB, and FeCoSiB, which take the place of the ferromagnetic Ni film traditionally used in these experiments, with a few exceptions. Surface acoustic wave (SAW) devices are operated at three harmonics in the 0.8-2 GHz frequency range. Each magnetic material has a unique ADFMR signature: FeCo shows a standard 4-lobe pattern with a broad ~100 mT linewidth; FeCoGd breaks odd symmetry and reveals an additional low-field lobe; and FeGaB and FeCoSiB show extreme asymmetry and narrow linewidths. In FeCoSiB, we observe nonreciprocal SAW propagation. We also perform direct comparison of ADFMR and standard ferromagnetic resonance (FMR) on the same devices, revealing that ADFMR absorption is measurable even when FMR signals are extremely weak, and magnetic anisotropy does not fully explain asymmetry observed in ADFMR. These results demonstrate that strain-driven magnetization dynamics is a rich field; the effects can be observed in a variety of materials with unexpected behavior, motivating further work in the field.

Surface acoustic wave (SAW) sensors for the detection of magnetic fields are currently being studied scientifically in many ways, especially since both their sensitivity as well as their detectivity could be significantly improved by the utilization of shear horizontal surface acoustic waves, i.e., Love waves, instead of Rayleigh waves. By now, low-frequency limits of detection (LOD) below 100 pT/Hz can be achieved. However, the LOD can only be further improved by gaining a deep understanding of the existing sensor-intrinsic noise sources and their impact on the sensor’s overall performance. This paper reports on a comprehensive study of the inherent noise of SAW delay line magnetic field sensors. In addition to the noise, however, the sensitivity is of importance, since both quantities are equally important for the LOD. Following the necessary explanations of the electrical and magnetic sensor properties, a further focus is on the losses within the sensor, since these are closely linked to the noise. The considered parameters are in particular the ambient magnetic bias field and the input power of the sensor. Depending on the sensor’s operating point, various noise mechanisms contribute to f0 white phase noise, f−1 flicker phase noise, and f−2 random walk of phase. Flicker phase noise due to magnetic hysteresis losses, i.e. random fluctuations of the magnetization, is usually dominant under typical operating conditions. Noise characteristics are related to the overall magnetic and magnetic domain behavior. Both calculations and measurements show that the LOD cannot be further improved by increasing the sensitivity. Instead, the losses occurring in the magnetic material need to be decreased.

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.

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 400nm 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. (C) 2020 Elsevier B.V. All rights reserved.

This article presents the frequency response of a surface acoustic wave (SAW) sensor for current measurements. The SAW sensor exploits the Love waves of a magnetostrictively coated delay line. The focus of this article lies on the frequency response of the sensor, which shows a very wide bandwidth. The delay time tau(D) is the time an acoustic wave needs to traverse the delay line. Since the acoustic wave is modulated during the whole duration of the delay line crossing, tau(D) is comparable to the integration time of a sample-and-hold device. The signal measured with the SAW sensor is averaged over the delay time. Therefore, undersampling behavior occurs for signal frequencies approaching 1/tau(D). A mathematical model of their frequency response up to several megahertz was developed and compared with measurements of two SAW sensors with different delay line lengths.

Magnetoelectric (ME) thin film composites consisting of sputtered piezoelectric (PE) and magnetostrictive (MS) layers enable for measurements of magnetic fields passively, i.e. an AC magnetic field directly generates an ME voltage by mechanical coupling of the MS deformation to the PE phase. In order to achieve high field sensitivities a magnetic bias field is necessary to operate at the maximum piezomagnetic coefficient of the MS phase, harnessing mechanical resonances further enhances this direct ME effect size. Despite being able to detect very small AC field amplitudes, exploiting mechanical resonances directly, implies a limitation to available signal bandwidth along with the inherent inability to detect DC or very low frequency magnetic fields. The presented work demonstrates converse ME modulation of thin film Si cantilever composites of mesoscopic dimensions (25 mm × 2.45 mm × 0.35 mm), employing piezoelectric AlN and magnetostrictive FeCoSiB films of 2 µm thickness each. A high frequency mechanical resonance at about 515 kHz leads to strong induced voltages in a surrounding pickup coil with matched self-resonance, leading to field sensitivities up to 64 kV/T. A DC limit of detection of 210 pT/Hz1/2 as well as about 70 pT/Hz1/2 at 10 Hz, without the need for a magnetic bias field, pave the way towards biomagnetic applications.

Measurements of the converse magnetoelectric effect, observed for mesoscopic cantilever type magnetoelectric composites, are presented. The silicon based samples employ 2 mu m of amorphous (Fe90CO10)(78)Si12B10 film as soft magnetic, magnetostrictive phase. The piezoelectric phase consists of 2 mu m sputter deposited, highly textured aluminum nitride (MN) in a plate capacitor arrangement. Exciting the piezoelectric phase at various frequencies leads to sharp peaks of induced voltage in a surrounding, mechanically decoupled pickup coil, corresponding to several mechanical resonances of the beam. The peak amplitude modulation can be exploited to detect DC magnetic fields. This entirely passive readout strategy is advantageous over other methods of sensitivity enhancement, typically requiring an active source of magnetic fields, thus prohibiting the construction of sensor arrays. Field dependent mechanical quality factors of up to 3800 near magnetic saturation are featured by a strong field dependence of induced voltage, reaching to 2290 V T-1 in the 20 mu T field regime. Vibrational measurements reveal a combination of the 15th flexural with a high order torsional mode as primarily active, at a resonance frequency of 520.7 kHz. This finding is supported by simple analytical estimations and literature. In unbiased operation, a linear resolution of 1.2 nT towards small 200 mHz fields is shown.