We directly connect specific magnetic domain behaviors in magnetically modulated thin-film magnetoelectric composite sensors to the effective noise levels exhibited. Through simultaneous magnetoelectric response and time-resolved-magneto-optical-microscopy measurements, as well as additional complementary noise and limit of detection measurements, different regimes of magnetic noise with distinct magnetic domain activities are identified. Transitions between magnetic domain states with differing domain-wall densities and local effective magnetic anisotropy perturbations directly influence the magnetoelectric sensor signal and the effective noise level. By this, the limit of detection of the sensor deteriorates by orders of magnitude, depending on the discrete magnetic domain characteristics and interconnecting magnetic losses. We show that the performance of magnetic field sensors is dominated by the physical micromagnetic processes revealed. The underlying physical mechanisms should affect all magnetic-layer-based field-sensing devices.

To enable the measurement of low-frequency magnetic signals with cantilever type thin-film magnetoelectric sensors, magnetic frequency conversion transfers the frequency of the desired signal into the mechanical resonance of the cantilever. The system electronics for the realization of this approach and the approach itself introduce additional noise sources as compared with direct detection, which lowers the limit of detection. In this paper, the magnetic frequency conversion noise sources are reviewed, discussed, and evaluated for our setup. The model for the nonlinear transfer process is implemented in the time domain. This enables the consideration of the pump noise in a noise equivalent circuit. For the sensor type under investigation, the dominant noise near its optimal working point originates from the pump source. If the noise of the pump can be decreased and magnetic excess noise is not dominant, the noise limit is the thermal-mechanical noise of the sensor. The implementation of a filter after the excitation source decreases the limit of detection to 60 pT/root Hz at 10 Hz.

We show the use of nickel-titanium (NiTi) metal thin films as functional substrates of magnetoelectric composites allowing the composites’ mechanical resonance to be tuned towards the frequency range of interest by employing the gradual change of the Young’s modulus during the martensite-austenite phase transformation. Such composites are successfully fabricated by layering functional piezoelectric aluminum nitride and highly magnetostrictive FeCoSiB on sputtered NiTi-films. Subsequent characterizations reveal high magnetoelectric response. Applied as magnetic field sensors a high sensitivity (with a magnetoelectric coefficient αME=290 V/cm Oe) and low limit of detection of approximately 110 pT/Hz1/2 is observed for AC magnetic fields matching the mechanical resonance frequency. This mechanical resonance of the sensor can be altered with a Δf of 12% in the temperature interval between room temperature and 130 °C. This offers the possibility to adjust the sensor’s mechanical resonance frequency featuring the highest sensitivity to the magnetic field frequency to be measured.

Thin-film magnetoelectric sensors reach a sensitivity in the picotesla range around the resonance frequency of the mechanical structure. Using magnetic frequency conversion, a magnetic low-frequency signal can be transferred to the sensor's resonance frequency. However, the required additional large carrier signal leaks to the sensor's output with a large amplitude, requiring a wide dynamic range of the sensor electronics. In this paper, it is shown that the unbalance of the magnetostriction curve is responsible for this leakage and that a suppression approach is devised. After theoretical analysis of the nonlinear magnetostriction characteristic, a carrier suppression is achieved through balancing by an altered signal excitation. A suppression of the carrier signal of about three orders of magnitude is measured. Thus, the requirements regarding analog-to-digital conversion can be reduced.

Magnetic frequency conversion is a promising technique to enhance the limit of detection of magnetoelectric sensors detecting low-frequency magnetic signals. In comparison with the direct detection in the mechanical resonance of the sensor, this method shows a limit of detection increased, i.e., worsened, by approximately 2.5 decades. For the detection of bio-magnetic signal, frequencies ranging from 0.1 Hz up to approximately 100 Hz though the method yield a better limit of detection than direct detection. Still, it is worse than theoretically expected. The cause of the deterioration of the signal-to-noise ratio during magnetic frequency conversion is investigated. Besides the conversion loss, it is due to the arising magnetic noise during excitation of a magnetostrictive material with a pumping signal, which is also in the order of approximately 2.5 decades. The noise can be reduced by applying an additional dc-bias field, which simultaneously results in less output signal. Measurements are confirmed by a numerical model. An existing equivalent noise model for magnetoelectric sensors is extended accordingly.

Thin-film magnetoelectric sensors, i.e., composites of magnetostrictive and piezoelectric materials, are able to measure very low magnetic flux densities in the picotesla range. In order to further improve the limit of detection it is of high importance to understand and quantify the relevant noise sources. In this paper, a common model for the deflection noise in vibrational structures is applied to the cantilever structure of resonant magnetoelectric sensors. By means of deflection and noise measurements the existence of thermal-mechanical noise even in sensor structures with a size in the centimeter range is proven. Based on these findings a noise equivalent circuit is suggested which allows not only the distinction between the impact of different sensor-intrinsic noise sources and also the involvement of the preamplifier noise. We found that the thermal-mechanical noise is the dominant noise source if direct signal detection is performed at the first bending resonance frequency of the sensor. However, this kind of noise is not the limiting influence when applying magnetic frequency-conversion techniques.

Composite magnetoelectrics implemented as thin film heterostructures are discussed in view of their applicability as highly sensitive magnetic field sensors. Here, either PZT or AlN served as piezoelectric component. The magnetostrictive phase consisted of layer systems based on FeCo or (Fe90Co10)(78)Si12B10. All functional layers were deposited with thicknesses of a few micrometers on Si cantilever structures with typical lateral dimensions of 25 mm by 2.2 mm. Magnetoelectric coefficients as large as 6900 V/cm Oe and a limit of detection as low as 1 pT/(Hz)(1/2) were measured. Currently, the best result demonstrates a detection limit of 500 fT/(Hz)(1/2) at 958 Hz frequency using a set of two sensors for external noise suppression. A frequency conversion technique is proposed to broaden the applicability of resonant magnetoelectric sensors to a wider frequency range. Finally, the achieved sensor performance is evaluated with regard to typical magnetic field amplitudes in medical applications.

We demonstrate the feasibility of investigating periodically driven magnetization dynamics in a scanning electron microscope with polarization analysis based on spin-polarized low-energy electron diffraction. With the present setup, analyzing the time structure of the scattering events, we obtain a temporal resolution of 700 ps, which is demonstrated by means of imaging the field-driven 100 MHz gyration of the vortex in a soft-magnetic FeCoSiB square. Owing to the efficient intrinsic timing scheme, high-quality movies, giving two components of the magnetization simultaneously, can be recorded on the time scale of hours.

Magneto-electric composites constitute a promising class of materials for the application as highly sensitive magnetic field sensors. Their compatibility to miniaturization, their passive nature and their large dynamic range are just some of the numerous advantages of these composites compared to alternative magnetic field sensors, as e.g. flux-gates or magneto-resistive sensors. However, the highest sensitivities are observed in the presence of well-defined magnetic bias fields that are of significant disadvantage in terms of miniaturization and noise performance. Self-biased magneto-electric composites, which are fabricated using the exchange bias effect [1], exhibit an increased total anisotropy in comparison to systems without exchange bias. As a consequence, small exchange bias fields are favourable because of a minor reduction of the magneto-electric voltage coefficient. However, weakly biased magneto-electric composites might loose their self-biasing properties and possibly show an increase of discontinuities in magnetization reversal due to the formation of magnetic domains. By a thickness variation of the ferromagnetic layer a maximum voltage coefficient αME ≈ 430 V/cmOe was found for a magnetostrictive multilayer of 3 x (5 nm Ta / 3 nm Cu / 8 nm Mn-Ir / 333 nm Fe-Co-Si-B). Yet, a stable single domain state indicating a well defined magnetization reversal by coherent magnetization rotation was achieved for layer thicknesses up to 100 nm Fe-Co-Si-B with a slightly reduced magneto-electric cofficient of αME ≈ 340 V/cmOe. This slight reduction is overcompensated by the improved control of the magnetic domain pattern that is highly desirable for magnetic field sensors and of special importance when frequency conversion techniques are applied [2]. [1] Lage, E., Kirchhof, C., Hrkac, V., Kienle, L., Jahns, R., Knöchel, R., Quandt, E. & Meyners, D. Exchange biasing of magnetoelec¬tric composites. Nature materials 11, 523–9 (2012). [2] Jahns, R., Greve H., Woltermann, E., Quandt, E. & Knöchel, R. Sensitivity enhancement of mag¬netoelectric sensors through frequency-conversion. Sensors and Actuators A: Physical 183, 16-21 (2012). This work was funded by the German Research Foundation (DFG) as part of the Collaborative Research Center 855 “Magnetoelectric Composites–Future Biomagnetic Interfaces”