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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.

Voltage controlled magnetic components, which consist in dynamically controllable inductances and transformers, are a promising yet understudied technology of growing interest. In fact, these components offer circuit designers an additional degree of freedom to achieve multi-objective optimization with improved Pareto fronts. This article provides a review of some technologies that can be used to create controlled magnetics, including emerging technologies with high potential. Furthermore, a list of possible applications is proposed, where these components can provide a significant advantage in terms of efficiency, size reduction, or controllability. Special emphasis is laid on a use case: a 20 kW multiport DC/DC converter in which power flow control is achieved using voltage-controlled inductors based on partially saturable magnetic cores.

The simultaneous separational control of motion of individual objects is vital to achieve high efficiency separation for biological analytes in biomedical applications. Here, we show the selective and directed movement of different populations of microbeads depending on their size in a flowless environment by means of a hexagonally structured soft-magnetic microchip platform. By adjusting strength and asymmetry of a modulated in-plane magnetic field, discrete and switchable movement patterns of two different types of beads above a magnetic surface structure are achieved. Starting from a heterogeneous mixture of bead populations and depending on the type of field sequences, directional forward transport of one type of beads is achieved, while the other bead population is immobilized. Despite significant size and magnetic content distributions within each population of microbeads, high separation efficiencies are demonstrated. The selection and movement processes are supported by full-scale magnetofluidic numerical simulations. The magnetic platform allowing multidirectional and selective microbead movement can greatly contribute to the progress of functional lab-on-chip and future diagnostics devices.

Mechanically driven magnetoelectric (ME) antennas have been demonstrated to be one of the most effective methods to miniaturise antennas compared to state-of-the-art compact antennas. However, the nanoelectromechanical systems (NEMS) ME antennas are fragile due to their suspended thin-film heterostructure, and have very low power handling capabilities. Here we show that solidly mounted resonator (SMR)-based NEMS ME antennas on a Bragg acoustic resonator, which have a circular resonating disk of 200 μm diameters and operate at 1.75 GHz, show a high antenna gain of -18.8 dBi and 1dB compression point (P1dB) of 30.4 dBm. Compared to same-size thin-film bulk acoustic resonator (FBAR) ME antennas with a free-standing membrane, the SMR-based antennas are much more structurally stable with 23.3 dB higher power handling capability and easier fabrication steps. These SMR-based ME antennas are fabricated with processes compatible with complementary metal-oxide-semiconductor (CMOS), exhibiting dramatic size miniaturisation, high power handling, high mechanical robustness, simple fabrication processes, and much higher antenna radiation gain compared to same-size state-of-the-art antennas.

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.

Focused ion beam (FIB) irradiation was applied to an ultrathin Pt/Co/Pt film to create micrometer-sized, square-shaped regions with perpendicular orientation of magnetization embedded in an in-plane magnetized environment. The FIB parameters like ion fluence and scanning direction are used to control the magnetic properties of the irradiated areas. We have studied the magnetic and magnetooptical properties in the irradiated areas as a function of ion fluence by means of component selective magnetooptical Kerr microscopy that measures separately pure polar and pure longitudinal components of magnetization. Two fluence ranges that induce perpendicular magnetization components were observed. In the higher fluence range, four regions with independently inclined out-of-plane magnetization, depending on FIB fast scan direction, and an unexpected magnetic domain structure were distinguished inside the squares irradiated by FIB.

Magneto-optical microscopy, prominently represented by Kerr microscopy, is the most versatile in-lab imaging technique for magnetic microstructure analysis on any kind of magnetic material being microscopically ordered in magnetic domains. Domains can be studied on a wide range of lateral scales reaching from centimetres down to 200 nanometres, under static conditions as well as under dynamic magnetic field excitation ranging between quasistatic and beyond the microwave regime, at temperatures between 4 K and 800 K, and in applied fields exceeding the Tesla range in arbitrary directions. The surface magnetisation vector field of soft magnetic materials can be quantitatively measured, and the magnetic layers in superlattices may be imaged separately. By plotting the image intensity as a function of magnetic field, local magnetisation curves are obtained that can be interpreted straightforwardly as the underlying magnetisation process is imaged simultaneously. In this chapter the physical and technical basics of magneto-optical microscopy are reviewed together with examples that demonstrate the benefits of the technique.

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.

Highly sensitive magnetic field sensors using magnetoelectric (ME) bulk and thin-film composites consisting of magnetostrictive and piezoelectric phases are discussed. Examples include PZT (Pb(ZrxTi1–x)O3) fibers and AlN as the piezoelectric component and amorphous magnetostrictive material, respectively, or their multilayers. Additionally, self-organized ME composites are discussed. These ME sensors offer a passive (consuming little to no power) nature, high sensitivities, large effect enhancements at mechanical resonances, and large linear dynamic ranges. At mechanical resonance, limits of detection in the fT/Hz1/2 range can be achieved. Below the mechanical resonance frequency, the sensitivity can be enhanced through frequency conversion using alternating current magnetic or electric fields or by using magnetic field-induced changes of the elastic properties, the delta-E effect, where E represents Young’s modulus. Noise floors of about 1–100 pT/Hz1/2 at a frequency of f = 1 Hz can be obtained depending on the sensor size and the operational mode. For applications in unshielded environments, approaches to suppress acoustic and vibrational cross-sensitivities are presented.

Sensors based on the Delta-E effect can detecting low-frequency and low-amplitude magnetic fields, which makes them potentially interesting for applications in biomagnetism, which require high sensitivity and low noise levels. This work analyzes the high magnetic sensitivity for different resonance modes of cantilever sensors. It is found that the Delta-E effect significantly depends on the resonance, with second-order bending modes increasing the magnetic sensitivity compared to first-order modes. The weighting of local properties by mode shape is not only key to understanding and modeling the Delta-E effect, but also an important factor in improving sensitivity.

We explore the generation of propagating dipolar spin waves by homogeneous magnetic field excitation in the proximity of the boundaries of magnetic microstructures. Domain wall motion, precessional dynamics, and propagating spin waves are directly imaged by time-resolved wide-field magneto-optical Kerr effect microscopy. The aspects of spin wave generation are clarified by micromagnetic calculations matching the experimental results. The region of dipolar spin wave formation is confined to the local resonant excitation due to non-uniform internal demagnetization fields at the edges of the patterned sample. Magnetic domain walls act as a border for the propagation of plane and low damped spin waves, thus restraining the spin waves within the individual magnetic domains. The findings are of significance for the general understanding of structural and configurational magnetic boundaries for the creation, the propagation, and elimination of spin waves.

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/ $\sqrt {\text {Hz}}$ at 10 Hz.

Domain wall dynamics in highly magnetostrictive microwires is remarkably sensitive to thermal treatment. One of the reasons is a partial release of the internal stresses that originate during the production process. Another reason is an interaction of the domain wall with local pinning sites, such as surface inhomogeneities, free volumes, or clusters of ferromagnetic atoms, present in as-cast microwires. However, the observation of domain wall shape variations after thermal treatment or following an interaction with a pinning site is challenging. In this paper, we utilize the magneto-optical Kerr effect (MOKE) to locally map the domain wall shape and velocity in cylindrical microwires. The domain wall is trapped in a magnetic potential well, moving back and forth with limited speed. Two complementary techniques are employed in the study: a laser-based setup and time-resolved MOKE microscopy. Different kinds of domain wall propagation are observed. While a planar domain wall propagates synchronously with the potential well, surface pinning gives rise to local jumps and distortions of the domain wall. We relate the first kind of motion to the pinning-free propagation of the domain wall and the second to the propagation of a distorted wall. Finally, local speed and the shapes of the domain wall before and after the introduction of an artificial pinning site are compared.

Author(s): Urdiroz, U; Muller, C; Gomez, A; Magaz, MT; Granados, D; Sanchez Agudo, M; Rubio-Zuazo, J; Castro, GR; Stan, C; Tamura, N; Padmore, HA; Cebollada, F; Palomares, FJ; McCord, J; Gonzalez, JM | Abstract: We report on the measurement of the local magnetization dynamics occurring, at units of GHz, in large aspect ratio stripes lithographed from reduced damping amorphous Fe80B20 films. The stripes were submitted to local anisotropy modifications by micrometric beam synchrotron X-ray irradiation. Our results include data on the dispersion relationships and group velocities corresponding to spin waves excited at both the non-irradiated and the irradiated regions. Whereas in the former case we observed standing spin waves with transverse-to-the stripe axis wave vector, in the latter one, for which the wave vector of the spin waves was parallel-to-the stripe axis, propagating spin waves were excited. In both regions, we measured the effective propagation distance of the spin waves, which resulted to be independent of the wave vector orientation. In the spin waves excited at the irradiated region, we also measured the decay time and effective damping coefficient, which was in good agreement with previously reported values obtained from FMR measurements in amorphous Fe80B20 continuous films. We show that the interaction of the non-irradiated and irradiated zones results, at the stripe transverse saturation remanence and under an exciting field frequency of 4 GHz, in the introduction of a π phase shift between the standing spin waves excited at both sides of the irradiated region. This result opens the possibility of using the local, transverse to the stripe axis, magnetic anisotropy easy axis induced by the X-ray irradiation as a crucial constituent of a zero-applied field spin wave phase-shifter.

Miniaturized piezoelectric/magnetostrictive contour-mode resonators have been shown to be effective magnetometers by exploiting the ΔE effect. With dimensions of ~100–200 μm across and

Precise movement control is a key feature for the use of superparamagnetic microbeads in medical and biological lab-on-chip applications. We demonstrate the unidirectional transport of magnetic and biological carriers along a chain of oval shaped magnetic thin film elements by in-plane rotating magnetic fields, enabling controllable manipulation and separation schemes. The same fundamental unidirectional movement is realized independent of the sense of magnetic field rotation and orientation of the magnetic pathway. The flowless directional transport of magnetically labeled rat embryonic fibroblasts is presented, validating the applicability of the structures for biological purposes. The lined up ferromagnetic structures are a critical building block for the construction of flexible pathways for biological lab-on-a-chip applications.

We report on a component-selective and vectorial magneto-optical imaging setup for the visualization of magnetization processes with picosecond temporal resolution. The stable imaging setup is suitable for the investigation of high excitation frequency magnetization dynamics, including domain wall motion, precession of magnetization, and spin-waves. The orthogonally aligned in-plane and out-of-plane components of magnetization are separated by untangling the superpositioned longitudinal and polar magneto-optical effects. Combining images obtained with varying plane and angle of incidence of illumination allow for the quantitative and time-dependent extraction of the spatial magnetization response. The capabilities of the setup are demonstrated with the phase-locked imaging of spin precession and spin-waves in a structured Co 40 Fe 40 B 20 thin film at the precessional frequency of the magnetic microstructure at 1.9 GHz.

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.

Fabrication of a multistrip magnetic/nonmagnetic structure in a thin sandwiched Ni layer [Si(5 nm)/Ni(10 nm)/Si] by a focused ion beam (FIB) irradiation has been attempted. A control experiment was initially performed by irradiation with a standard 30 keV Ga ion beam at various fluences. Analyses were carried out by Rutherford backscattering spectrometry, X-ray reflectivity, magnetooptical Kerr effect (MOKE) measurements and MOKE microscopy. With increasing ion fluence, the coercivity as well as Kerr rotation decreases. A threshold ion fluence has been identified, where ferromagnetism of the Ni layer is lost at room temperature and due to Si incorporation into the Ni layer, a Ni0.68Si0.32 alloy layer is formed. This fluence was used in FIB irradiation of parallel 50 nm wide stripes, leaving 1 micrometer wide unirradiated stripes in between. MOKE microscopy on this FIB-patterned sample has revealed interacting magnetic domains across several stripes. Considering shape anisotropy effects, which would favor an alignment of magnetization parallel to the stripe axis, the opposite behavior is observed. Magneto-elastic effects introducing a stress-induced anisotropy component oriented perpendicular to the stripe axis are the most plausible explanation for the observed behavior., 12 pages, 12 figures (Figure numbers up to 7)

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)78Si12B10. 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.

While the magnetoelectric (ME) resonator is promising for magnetometers, very low-frequency and miniaturized antennas, and energy harvesters, devices based on the converse ME effect in amorphous Metglas alloy face frequency-response nonlinearity issues under high vibrational amplitude. This study aims to resolve this nonlinearity issue, by addressing the dependence of nonlinearity on bias field, driving voltage, mechanical quality factor, and frequency-sweep direction. With magnetic field as a control variable, the effects of nonlinear behavior on a ME antenna's communication distance, bandwidth, and modulation are also presented.

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.

This paper investigates magnetic field and stress-induced magnetic domain reorientation and its correlation with Magnetic Barkhausen Noise (MBN). The magnetic domain dynamics and MBN of grain-oriented transverse and longitudinal electrical steels are studied by using time-resolved magneto-optical imaging. Time and frequency-domain characteristics of MBN signal are investigated for evaluation of stress state and magnetocrystalline anisotropy. Transitions between magnetic domain states with differing tensile stress and magnetic anisotropy directly influence the MBN signal. Its shown that 180° domain wall (DW) reorganization and DW-type conversion generate peaks in the MBN measurement for longitudinal and transverse electrical steel, respectively. The magnetic domain imaging reveals complicated domain arrangement processes in transverse electrical steel. The results provide a physical insight into bulk micromagnetic phenomena.

Temperature measurements are vital in laboratory and industry settings. We demonstrate magnetooptical measurement schemes to relate magnetic and micromagnetic features to temperature sensing using magnetooptical active iron garnet films. A particular focus is on a temperature measurement scheme based on changes in the magnetic domain wall susceptibility, analyzing the resulting magnetooptical signal amplitude in a magnetic modulation field. The range of application is limited by the Neel temperature of the sensing garnet material. Temperatures are perceived with an accuracy of below 0.1 °C from the changes in the magnetooptical domain wall response. The thin film sensor element features additionally high sensitivity magnetic field and electrical current sensing capabilities, allowing for multi-functional sensor applications.

Highlights X-ray irradiation of amorphous stripes allows to stabilize a transverse 180o wall. An inhomogeneous resonance can be excited with a 180° phase-shift across the wall. That phase-shift can be exploited into the design of magnonic devices. We report on the induction, through X-ray irradiation, of local anisotropy modifications in amorphous Fe80B20 stripes prepared by laser lithography from 15 nm thick films deposited by pulsed laser ablation. Those anisotropy modifications, taking place in a zone having a dimension of 15 μm around the X-ray beam (2 µm width) incidence area center point, lead to an effective transverse-to-the-stripe long axis local magnetic easy axis that can be exploited to stably localize a 180° domain wall, in remanence after saturation with a transverse-to the stripe easy axis dc field. Upon stabilizing that wall it is possible to excite spatially inhomogeneous ferromagnetic resonances nearby the irradiated region (IR). The time dependencies of the magnetization precession at both sides of the IR exhibit a uniform phase shift of 180° over distances of the order of 20 μm from the stabilized domain wall. This work has been developed with funds corresponding to project MAT2016-80394-R financed by the Spanish Research Agency (AEI). We also acknowledge the Spanish Ministerio de Ciencia e Innovaci ' on and Consejo Superior de Investigaciones Cientificas for financial support and for provision of synchrotron radiation at beamline BM25-SpLine (ESRF). This research used resources of the Advanced Light Source, a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. FC and JMG would like to dedicate this work to the memory of Dr. Dominique Givord with gratitude for the many things they were able to learn from him Peerreview

The influence of the size of anti-notches on the domain wall propagation in Permalloy nanowires with edge taper is investigated. The critical magnetic fields and current pulses required for a domain wall to pass a symmetrical circular anti-notch obstacle were estimated by high-resolution in-situ Kerr microscopy experiments and by micromagnetic simulations. The nanowires, made using electron beam lithography and ion beam etching, had an average width of 220 nm and the anti-notches consisted of circular features that increased the wire width from 5% to 35%. The critical magnetic flux densities for domain walls to pass the obstacles increased with anti-notch diameter, from 0.6 mT to 3.4 mT in the simulations and 0.3 mT to 1.5 mT in the experiment. The critical current densities ranged from $0.5 \times {10^{12}}\ \text{A/m}^{2}$ to $10 \times {10^{12}}\ \text{A/m}^{2}$ in the simulations, with a strong dependence on the domain wall type, but the experiment yielded higher critical current densities of $6 \times {10^{12}}\ \text{A/m}^{2}$ to $25 \times {10^{12}}\ \text{A/m}^{2}$ .

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.

Magnetic properties of highly magnetostrictive amorphous glass-coated microwires are strongly correlated to the presence of a glass coating that introduces a spatially inhomogeneous stress field distribution. We investigate the influence of mechanical stresses on the inclination of magnetic domain walls in magnetic microwires. Magneto-optical Kerr effect imaging is used to compare the tilted orientation of the domain wall shape in as-cast and annealed microwires. Angular dependencies of magnetization loops measured by alternating gradient field magnetometry reveal that the change of domain wall tilting with annealing is related to the decrease of magnetic anisotropy with axial orientation. Finally, micromagnetic simulations are used to show that sufficiently high uniaxial magnetic anisotropy gives rise to the presence of observed charged domain walls with tilted orientation.

Surface acoustic wave magnetic field sensors based on guided Love waves using the DE 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/SRHz at 10 Hz and 25 pT/SRHz at 100 Hz is achieved.

Magnetic nanoparticles (MNPs) are a hot topic in the field of medical life sciences, as they are highly relevant in diagnostic applications. In this regard, a large variety of novel imaging methods for MNP in biological systems have been invented. In this proof-of-concept study, a new and novel technique is explored, called Magnetic Particle Mapping (MPM), using resonant magnetoelectric (ME) sensors for the detection of MNPs that could prove to be a cheap and efficient way to localize the magnetic nanoparticles. The simple and straightforward setup and measurement procedure includes the detection of higher harmonic excitations of MNP ensembles. We show the feasibility of this approach by building a measurement setup particularly suited to exploit the inherent sensor properties. We measure the magnetic response from 2D MNP distributions and reconstruct the distribution by solving the inverse problem. Furthermore, biological samples with magnetically labeled cells were measured and reconstruction of the distribution was compared with light microscope images. Measurement results suggest that the approach presented here is promising for MNP localization.

We study the correlation between the magnetic reversal and the anisotropic magnetoresistance (AMR) response in magnetic hybrid structures that were created by local modification of magnetic properties induced by ion implantation. The stripe pattern has been investigated simultaneously by dual-wavelength Kerr microscopy and magnetoresistance measurements. We observe that the switching of the stripe pattern introduces an additional AMR maximum. The domain wall in between the stripes provides a positive resistance contribution, whereas domains at the stripe edges lead to an asymmetric AMR response. A method for calculating the AMR response from the quantitative Kerr micrographs is demonstrated that allows the reconstruction of the AMR value within a region of interest only.

A new method of imaging of temperature distributions is presented. The method is based on the use of a BiLu2 Fe4 GaO12 ferrimagnetic garnet layer as a pyro-magneto-optical detection layer. Thermal imaging with high sensitivity, with micrometer spatial and with millisecond temporal resolution is demonstrated in a contactless read-out scheme.

We present a comprehensive study of a magnetic sensor system that benefits from a new technique to substantially increase the magnetoelastic coupling of surface acoustic waves (SAW). The device uses shear horizontal acoustic surface waves that are guided by a fused silica layer with an amorphous magnetostrictive FeCoSiB thin film on top. The velocity of these so-called Love waves follows the magnetoelastically-induced changes of the shear modulus according to the magnetic field present. The SAW sensor is operated in a delay line configuration at approximately 150 MHz and translates the magnetic field to a time delay and a related phase shift. The fundamentals of this sensor concept are motivated by magnetic and mechanical simulations. They are experimentally verified using customized low-noise readout electronics. With an extremely low magnetic noise level of ≈100 pT/$$\sqrt{{\rm{Hz}}}$$ Hz , a bandwidth of 50 kHz and a dynamic range of 120 dB, this magnetic field sensor system shows outstanding characteristics. A range of additional measures to further increase the sensitivity are investigated with simulations.

We report the structural properties of thin magnetic Permalloy films treated by two different methods: broad beam Ga+ ion implantation at an energy of 30 keV as well as annealing at different temperatures under ultrahigh vacuum. Transmission electron microscopy imaging and X-ray diffraction measurements have demonstrated that both ion implantation and annealing (above 300 °C) lead to further material crystallization and crystallite growth. Whereas annealing (above 400 °C) leads to a strain-free state with an almost constant lattice parameter and to a further enhancement of the initial (111) texture, ion beam implantation boosts the growth of small, arbitrarily oriented crystallites and leads to an linear increase in the lattice parameter, introducing microstrain into the sample. The observed decrease in the saturation magnetization for the implanted samples is mainly attributed to the presence of the non-magnetic Ga atoms incorporated in the Permalloy film itself. The increase in the saturation magnetization for the samples annealed at temperatures above 500 °C is explained by an arising dewetting effect since no ordered Ni3Fe phase was detected with anomalous X-ray diffraction.

In recent years the delta-E effect has been used for detecting low frequency and low amplitude magnetic fields. Delta-E effect sensors utilize a forced mechanical resonator that is detuned by the delta-E effect upon application of a magnetic field. Typical frequencies of operation are from several kHz to the upper MHz regime. Different models have been used to describe the delta-E effect in those devices, but the frequency dependency has mainly been neglected. With this work we present a simple description of the delta-E effect as a function of the differential magnetic susceptibility &chi, of the magnetic material. We derive an analytical expression for &chi, that permits describing the frequency dependency of the delta-E effect of the Young&rsquo, s modulus and the magnetic sensitivity. Calculations are compared with measurements on soft-magnetic (Fe90Co10)78Si12B10 thin films. We show that the frequency of operation can have a strong influence on the delta-E effect and the magnetic sensitivity of delta-E effect sensors. Overall, the delta-E effect reduces with increasing frequency and results in a stiffening of the Young&rsquo, s modulus above the ferromagnetic resonance frequency. The details depend on the Gilbert damping. Whereas for large Gilbert damping the sensitivity continuously decreases with frequency, typical damping values result in an amplification close to the ferromagnetic resonance frequency.

The stochasticity of magnetic domain wall positioning after applying rotating magnetic fields to polygonal ferromagnetic nanowire-type magnetic field sensor structures is investigated. High-resolution magneto-optical Kerr effect microscopy is utilized to determine the travelled domain wall distances. The impact of the applied magnetic field amplitude, of field orientation, and of the polygon angle on domain wall positioning is systematically investigated to identify conditions for a deterministic response. Under certain conditions of magnetic field angle and amplitude, a highly predictable behaviour of domain wall positions with standard deviations below 30 nm is achievable. The presented magneto-optical method enables high throughput detection of domain wall positions with nanometer resolution.

High sensitivity magnetoelectric (ME) thin film composite sensors, which enable the detection of picotesla magnetic fields, are improved in terms of magnetic noise performance by the elimination of magnetic domain (MD) activity. Using an antiparallel (AP) exchange biasing scheme, suppression of magnetic noise is obtained. Postsetting of AP biased ferromagnetic/antiferromagnetic multilayers is accomplished by magnetic field free annealing with in-situ MD control. Overcoming the shape and demagnetization effects, stable single MD configurations in the magnetic sensing layers of magnetic multilayers are formed. Magnetic noise contributions are undetectable. The achieved single domain field stability opens the path to ultralow noise ME sensor applications. The demonstrated AP biasing scheme is applicable to other magnetic layer-based field sensing devices.High sensitivity magnetoelectric (ME) thin film composite sensors, which enable the detection of picotesla magnetic fields, are improved in terms of magnetic noise performance by the elimination of magnetic domain (MD) activity. Using an antiparallel (AP) exchange biasing scheme, suppression of magnetic noise is obtained. Postsetting of AP biased ferromagnetic/antiferromagnetic multilayers is accomplished by magnetic field free annealing with in-situ MD control. Overcoming the shape and demagnetization effects, stable single MD configurations in the magnetic sensing layers of magnetic multilayers are formed. Magnetic noise contributions are undetectable. The achieved single domain field stability opens the path to ultralow noise ME sensor applications. The demonstrated AP biasing scheme is applicable to other magnetic layer-based field sensing devices.

There is a need in industry to supply safe, effective and reliable technique to characterize the stress of steel components and structures, both at the manufacturing stage and in service. Bridging the correlation between micro and macro magnetic properties and the applied tensile stress is the first conceptual step to come up with a new method of non-destructive material testing. We investigate the stress-associated changes in domain wall dynamics in grain-oriented electrical steel by in-situ magnetic imaging using magneto-optical indicator films. The 180° domain walls velocity distribution is used as a parameter for applied stress determination. Additionally, the in-plane magnetic stray field above the surface of the sample is synchronously measured for stress evaluation. The variations in magnetic stray field outside the sample under different loading are investigated for the analysis of the domain wall dynamics. From this, an interrelation of the domain wall dynamics and magnetic stray fields with varied tensile stress is derived. The results provide substantial microscopic and macroscopic insight for the interplay of domain wall dynamics and stress-induced demagnetizing effect.

The microscopic processes underlying magnetostriction in ferrites were studied for the case of $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ single crystals by high-resolution in situ x-ray diffraction and complementary magnetic microscopy techniques. The data support the reports of Yang and Ren [Phys. Rev. B 77, 014407 (2008)] that magnetostriction in these materials originates from the switching of crystallographic domains, similar to ferroelastic or ferroelectric domain switching, and reveals the presence of two coexisting tetragonal spinel structures, corresponding to domains of high and of low strain. The latter alternate in the crystal, separated by 90\ifmmode^\circ\else\textdegree\fi{} domain boundaries, and can be explained by the effect of internal stress emerging during the transition into the ferrimagnetic phase. During magnetization of the sample two structural transitions are observed: a conversion of the transversal into axial domains at 1.95 kOe and a growth of the high-strain domains at the cost of the low-strain axial domains at 2.8 kOe. These microscopic changes are in good agreement with the macroscopic magnetization and magnetostriction behavior of $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$.

High density magnetic domain wall gratings are imprinted in ferromagnetic-antiferromagnetic thin films by local ion irradiation by which alternating head-to-tail-to-head-to-tail and head-to-head-to-tail-to-tail spatially overlapping domain wall networks are formed. Unique magnetic domain processes result from the interaction of anchored domain walls. Non-linear magnetization response is introduced by the laterally distributed magnetic anisotropy phases. The locally varying magnetic charge distribution gives rise to localized and guided magnetization spin-wave modes directly constrained by the narrow domain wall cores. The exchange coupled multiphase material structure leads to unprecedented static and locally modified dynamic magnetic material properties.

The dynamic magnetic properties of soft-ferromagnetic thin film element arrays are strongly influenced by long range interelement magnetostatic interaction. In order to estimate the effective array dipolar field a quantitative model is presented, which is based on the superposition of stray fields that arise from the neighborhood of a reference element. Kittel's equation, that describes the magnetodynamics, is generalized for magnetically saturated arrays by additional array dipolar field terms. Measurements of the magnetodynamic response of quasi-saturated arrays with a rectangular base agree with theoretical predictions. Thus, our model allows the estimation of the frequency of the uniform precessional mode in mesoscopic thin film arrays with non-negligible magnetostatic interaction.

Magnetic vortices show promise as data storage structures, however the vortex formation process imposes a lower limit on the element’s size. In this article a technique is presented, which application increases the probability of nucleating of magnetic vortices in sub-micrometer sized soft magnetic thin film elements. By tailoring the edge geometry of the elements, the symmetry of their magnetic configuration is broken in a manner which favors vortex nucleation. Micromagnetic simulations are presented, which demonstrate this effect in soft-magnetic disks with beveled edges. The favored edge geometry is realized by applying nanosphere lithography directly on top of a ferromagnetic thin film material. In this process, the film is masked with a self assembled monolayer of SiO2-nanospheres and subsequently ion-etched. The resulting magnetic reversal loops show that in both magnetically isolated as well as in closely packed arrays of beveled disks, vortex formation takes place. The technique presented facilitates the vortex formation even in closely packed and small elements. The lowering of the minimum critical diameter for vortex formation enables a significant increase of data storage density in devices based on magnetic vortices.

The capability of showing large magnetically induced strains (MFIS) up to ∼10% has attracted considerable research interest to magnetic shape memory (MSM) alloys. The prototype MSM alloy is the ternary Ni2MnGa. In this work, a comprehensive study of the local unit cell orientation distribution on single crystalline, polycrystalline and epitaxial thin film of martensitic Ni2MnGa is conducted by electron backscattering diffraction (EBSD). By EBSD, the constraint-dependent twin variant distribution, the corresponding stresses and the three-dimensional orientation of twin planes will be investigated. In polycrystals, the differentiation between twin and grain boundaries as well as proof of twin boundary motion is shown. From the knowledge of the local unit cell orientation at surfaces, it is possible to explain the magnetic domain configuration imaged by magnetic force microscopy.