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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.

Magnetoelectric resonators have been studied for the detection of small amplitude and low frequency magnetic fields via the delta-E effect, mainly in fundamental bending or bulk resonance modes. Here, we present an experimental and theoretical investigation of magnetoelectric thin-film cantilevers that can be operated in bending modes (BMs) and torsion modes (TMs) as a magnetic field sensor. A magnetoelastic macrospin model is combined with an electromechanical finite element model and a general description of the delta-E effect of all stiffness tensor components Cij is derived. Simulations confirm quantitatively that the delta-E effect of the C66 component has the promising potential of significantly increasing the magnetic sensitivity and the maximum normalized frequency change ∆fr. However, the electrical excitation of TMs remains challenging and is found to significantly diminish the gain in sensitivity. Experiments reveal the dependency of the sensitivity and ∆fr of TMs on the mode number, which differs fundamentally from BMs and is well explained by our model. Because the contribution of C11 to the TMs increases with the mode number, the first-order TM yields the highest magnetic sensitivity. Overall, general insights are gained for the design of high-sensitivity delta-E effect sensors, as well as for frequency tunable devices based on the delta-E effect.

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.

International audience; Magnetoelastic sensors for the detection of low-frequency and low-amplitude magnetic fields are in the focus of research for more than 30 years. In order to minimize the limit of detection (LOD) of such sensor systems, it is of high importance to understand and to be able to quantify the relevant noise sources. In this contribution, cantilever-type electromechanical and magnetoelastic resonators, respectively, are comprehensively investigated and mathematically described not only with regard to their phase sensitivity but especially to the extent of the sensor-intrinsic phase noise. Both measurements and calculations reveal that the fundamental LOD is limited by additive phase noise due to thermal-mechanical noise of the resonator, i.e. by thermally induced random vibrations of the cantilever, and by thermal-electrical noise of the piezoelectric material. However, due to losses in the magnetic material parametric flicker phase noise arises, limiting the overall performance. In particular, it is shown that the LOD is virtually independent of the magnetic sensitivity but is solely determined by the magnetic losses. Instead of the sensitivity, the magnetic losses, represented by the material's effective complex permeability, should be considered as the most important parameter for the further improvement of such sensors in the future. This implication is not only valid for magnetoelastic cantilevers but also applies to any type of magnetoelastic resonator.

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.

We report on the direct observation of spin wave and elastic wave emission from magnetic domain walls in ferromagnetic thin films. Driven by alternating homogeneous magnetic fields the magnetic domain walls act as coherent magnetisation wave sources. Directional and low damped elastic waves below and above the ferromagnetic resonance are excited. The wave vector of the magnetoelastically induced acoustic waves is tuned by varying the excitation frequency. The occurrence of elastic wave emission is proved by a combination of micromagnetic and mechanical finite element simulations. Domain wall emitted magnetostatic surface spin waves occur at higher frequencies, which characteristics are confirmed by micromagnetic simulations. The distinct modes of magnetisation wave excitation from micromagnetic objects are a general physical phenomenon relevant for dynamic magnetisation processes in structured magnetic films. Magnetic domain walls can act as reconfigurable antennas for spin wave and elastic wave generation. The wave orientation can be controlled separately via the domain wall orientation for elastic waves and via magnetization orientation for magnetostatic surface spin waves.

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.

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.

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.

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.

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.

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.

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 influence of homogeneous magnetic fields with flux density up to 1 T superimposed during the deposition of CoFe thin films on their properties has been studied. It has been clearly demonstrated that the superimposition of magnetic fields influences the resulting layer properties significantly. A pronounced impact on the layer morphology has been observed. The layers deposited under the influence of the parallel-to-electrode magnetic field appear denser and more homogenous than those obtained without a magnetic field. On the contrary, the layers deposited in the perpendicular-to-electrode magnetic field appeared more diverse. A scaling analysis revealed a smoothing effect of a parallel- and a roughening effect of a perpendicular-to-electrode magnetic field. No influence of magnetic fields neither on the deposited layers chemical composition nor the structure and texture has been found, whereas the internal stress state of the layer is affected by the superimposition. The effects are discussed with respect to the Lorentz force driven convection, which increases the electrochemical reaction's rates and improves desorption of hydrogen from the electrode surface. The alterations of magnetic properties of the CoFe thin films correlate well with the observed microstructural changes. Moreover, an in-plane magnetic anisotropy is induced by a parallel magnetic field superimposition. This phenomenon origins from a preferential next neighbour atomic pair-ordering in the direction of the magnetic field, e.g. magnetization, during deposition of the ferromagnetic alloy.

The actuation performance and twin-boundary velocity in bulk NiMnGa magnetic shape memory single crystals at various frequencies are investigated. A dynamic actuation experimental setup with the ability to apply mechanical stress and a high-frequency alternating magnetic field is developed in tandem with time-resolved optical microscopy. Reversible twin-boundary motion activated in the frequency range up to 600 Hz is directly observed. The maximum field-induced strain increases with actuation frequency. Strain hysteresis curves measured at different frequencies show similar onset fields of strain. The data imply that twin-boundary velocity is not the limiting factor on actuation performance. The results provide significant information on twin-boundary performance during high-frequency actuation.

Magnetostrictive amorphous FeCoBSi thin films for sensor applications have been investigated with regard to their magnetic properties dependent on strain. With special test jigs, allowing measurements in a vibrating sample magnetometer, Kerr-Effect microscope and high frequency permeameter, the magnetic dc and high frequency properties as a function of applied strain were investigated and correlated to the domain behavior. Under the condition that the applied strain stabilizes the magnetic easy axis in an orientation perpendicular to the high frequency magnetic field direction these magnetostrictive thin films can be used as cores of microinductors monitoring the strain via inductance changes. With FeCoBSi films a material quality factor of 10 and a high gauge factor (Δμ/μ 0 /Δe) of 2400 is obtained at 250 MHz.

Investigations on amorphous FeCoSiB/SiO/sub 2//FeCoSiB thin-film samples with crossed anisotropies are presented in this paper. The samples consist of two magnetic layers, 40 nm thick each, where the second magnetic layer's uniaxial anisotropy is rotated by an angle of 90/spl deg/ with respect to the anisotropy of the first layer. The individual layers were separated by a 20 nm SiO/sub 2/ thin film. The angular dependent magnetic properties of the samples have been characterized by vibrating sample and inductive magnetometry. The high-frequency (HF) response was determined by HF-permeameter measurements and pulsed inductive microwave magnetometry (PIMM). Good HF-response independent of the magnetic field excitation angle is achieved. The results are compared to an 80 nm FeCoSiB reference film with uniaxial anisotropy. Our results provide new insights important for the design of HF magnetic devices.

The high-frequency response of amorphous CoZrTa thin films was measured by using a pulsed inductive microwave magnetometer. The anisotropy of the magnetic films was varied by magnetic field annealing. Static anisotropy field values ranging from Hk=100 to 1920 A/m were obtained. The dynamically determined anisotropy field is shifted to higher values compared to the static anisotropy by an additional isotropic internal field Hadd. This internal field is independent of the strength of the static anisotropy field. We determined a value of about Hadd=510 A/m.

Sputtered polycrystalline iron gratings with a varying stripe width between 0.5 and 3.7 /spl mu/m at a constant periodicity of 5 /spl mu/m were studied by Kerr microscopy. The magnetic anisotropy direction is aligned transverse to the stripe length axis. The observed domain processes are directly compared with hysteresis curve measurements. Domain nucleation and the resulting domain structure depend on the external magnetic field direction. Complicated multidomain states are observed down to a stripe width of 1.7 /spl mu/m. The most narrowly spaced stripes display a simple domain configuration with the average magnetization vector perpendicular to the stripes, along the full film anisotropy axis, indicating dipolar coupling between the elements. With decreasing stripe width and increasing stripe distance, a change to a Landau-Lifshitz-like domain pattern is found. For very narrow stripes, it transforms into a ripple modulated domain structure. Altogether, a continuous transition from full-film-like to single strip-like magnetization behavior with increasing stripe spacing is found. The results are explained by the existing uniaxial magnetic anisotropy together with the shape anisotropy increasing with reduced stripe width. The strong influence of coercivity on the magnetization patterns is shown.

The magnetodynamics of lens-shaped thin-film elements are studied using vector network analyzer ferromagnetic resonance. A strong increase in the frequency at resonance is found when approaching the switching field. From magnetic force microscopy imaging the increase in resonance frequency is ascribed to the formation and evolution of a buckling domain state. The experimental data are qualitatively reproduced by micromagnetic simulations of a model element. Thereby, the roles of the external magnetic field and the buckling wavelength are extracted separately. Magnetic domain modes with dynamic magnetization modulations parallel and perpendicular to the static magnetization are identified. Based on magnetostatic energy considerations qualitative arguments are derived that allow for an interpretation of the dynamic response in such low-symmetry magnetization distributions.

We report on the response of multilayer spin textures to static magnetic fields. Coupled magnetic vortex pairs in trilayer elements (ferromagnetic/nonmagnetic/ferromagnetic) are imaged directly by means of layer-selective magnetic x-ray microscopy. We observe two different circulation configurations with parallel and opposing senses of magnetization rotation at remanence. Upon application of a field, all of the vortex pairs investigated react with a displacement of their cores. For purely dipolar coupled pairs, the individual core displacements are similar to those of an isolated single-layer vortex, but also a noticeable effect of the mutual stray fields is detected. Vortex pairs that are linked by an additional interlayer exchange coupling (IEC), which is either ferromagnetic or antiferromagnetic, mainly exhibit a layer-congruent response. We find that, apart from a possible decoupling at higher fields, these strict IEC vortex pairs can be described by a single-layer model with effective material parameters. This result implies the possibility to design multilayer spin structures with arbitrary effective magnetization.

Time-resolved metallographic optical microscopy techniques are used together with magnetic domain imaging to clarify the interaction between magnetic domains and twin boundary motion in magnetic shape memory NiMnGa single crystals. The magnetic field and stress induced magnetic domain formation is imaged by a magneto-optical indicator film technique. Reversible twin boundary motion is visualized up to high actuation speeds. From domain observation at adjacent crystal surfaces the fundamental volume magnetic processes during strain and field induced twin boundary motions are derived. For magnetic field induced structural reorientations a concurrent absence of magnetic domain wall motion is found. In contrast, for strain induced reorientations processes, a complete rearrangement of the magnetic domain structure by the moving twin boundary is observed. Dynamic actuation experiments on twin boundary motion reveal non-linear time effects on twin boundary mobility. In addition to training effects, the maximum field induced strain increases with actuation speed. Both effects can be interpreted as the interaction of moving twin boundaries with local non-fixed defects. The summarized results provide key information for the understanding of the connection of magnetic and crystallographic domains in magnetic shape memory alloys and for the optimization of devices for future technical applications.

A method for pinning synthetic antiferromagnet (SAF) based spin valves is presented, which allows pinning to be established perpendicular to the setting field H. As the SAF(AP1/Ru/AP2) magnetizes, its layer moments undergo a spin–flop transition for a magnetic field H=HSF and align with H for H=HSat. The magnetization of layer AP1 is perpendicular to H for a field H=HGold. This field corresponds to the maximum giant magnetoresistance GMR on HSF

Magneto-optical properties of nominally 10, 20, and 30 nm thick ferromagnetic Ni films have been investigated at room temperature by vector-magneto-optical generalized ellipsometry under saturated magnetization conditions in the sample surface plane. The magneto-optical dielectric tensor of Ni has been determined by reflection Mueller matrix ellipsometry in the spectral range from 300 to 1100 nm. Different sets of magnetic field induced Mueller matrix elements enable us to identify the magnetization directions in the sample. The extracted magnetic field and thickness independent magneto-optical coupling constant is useful for modeling the Mueller matrix and complex Kerr angle of magnetized Ni thin films in layered sample systems in dependence of the incident angle of light, wavelength, and magnetization.

International audience; The behavior of minor hysteresis loops in perpendicular anisotropy Co/Pt and Co/Pd multilayers has been investigated. Upon applying a succession of identical magnetic field cycles, we observe a very substantial cumulative growth of the minor loop area. For the Co/Pt multilayers this effect only saturates near complete magnetization reversal while the behavior is slightly more limited for Co/Pd multilayers. We also find this cumulative growth to occur even if the minor loop field cycles are made asymmetric by means of a positive bias field. The cumulative behavior persists up to a sample-dependent threshold value above which this effect disappears. In all samples, the cumulative minor loop growth is correlated with a small reduction in the maximum magnetization value in each cycle. Magneto-optical Kerr microscopy studies correlate the minor loop growth with the memory and cumulative expansion of lateral domain cycling. All experimental observations can be consistently explained as an accumulation of small nucleation domains that aid subsequent reversals and facilitate the cumulative minor loop growth.

A dynamic phase transition (DPT) with respect to the period $P$ of an applied alternating magnetic field has been observed previously in numerical simulations of magnetic systems. However, experimental evidence for this DPT has thus far been limited to qualitative observations of hysteresis loop collapse in studies of hysteresis-loop area scaling. Here, we present significantly stronger evidence for the experimental observation of this DPT in a ${[\text{Co}(4\text{ }\text{\AA{}})/\text{Pt}(7\text{ }\text{\AA{}})]}_{3}$-multilayer system with strong perpendicular anisotropy. We applied an out-of-plane time-varying (sawtooth) field to the ${[\text{Co}/\text{Pt}]}_{3}$ multilayer in the presence of a small additional constant field, ${H}_{b}$. We then measured the resulting out-of-plane magnetization time series to produce nonequilibrium phase diagrams (NEPDs) of the cycle-averaged magnetization, $Q$, and its variance, ${\ensuremath{\sigma}}^{2}(Q)$, as functions of $P$ and ${H}_{b}$. The experimental NEPDs are found to strongly resemble those calculated from simulations of a kinetic Ising model under analogous conditions. The similarity of the experimental and simulated NEPDs, in particular the presence of a localized peak in the variance ${\ensuremath{\sigma}}^{2}(Q)$ in the experimental results, constitutes strong evidence for the presence of this DPT in our magnetic multilayer samples. Technical challenges related to the hysteretic nature and response time of the electromagnet used to generate the time-varying applied field precluded us from extracting meaningful critical scaling exponents from the current data. However, based on our results, we propose refinements to the experimental procedure which could potentially enable the determination of critical exponents in the future.

International audience; The mechanism of chirality reversal for a planar interface domain wall in a hard/soft magnetic bilayer has been identified by combining magnetoresistance measurements, modeling, and direct magnetic domain observations. The reversal occurs through IDW nucleation and lateral domain wall propagation. Over an unpredicted wide range of applied magnetic fields, the chirality transition takes place by an unwinding followed by a rewinding of the IDW. The chirality transition mechanism of phase transition could be identified from a micromagnetic analysis of the lateral magnetic domain wall orientation. Up to three magnetization phases coexist in the uniaxial material during reversal.

A combination of experimental techniques, e.g. vector-MOKE magnetometry, Kerr microscopy and polarized neutron reflectometry, was applied to study the field induced evolution of the magnetization distribution over a periodic pattern of alternating exchange bias stripes. The lateral structure is imprinted into a continuous ferromagnetic/antiferromagnetic exchange-bias bi-layer via laterally selective exposure to He-ion irradiation in an applied field. This creates an alternating frozen-in interfacial exchange bias field competing with the external field in the course of the re-magnetization. It was found that in a magnetic field applied at an angle with respect to the exchange bias axis parallel to the stripes the re-magnetization process proceeds via a variety of different stages. They include coherent rotation of magnetization towards the exchange bias axis, precipitation of small random (ripple) domains, formation of a stripe-like alternation of the magnetization, and development of a state in which the magnetization forms large hyper-domains comprising a number of stripes. Each of those magnetic states is quantitatively characterized via the comprehensive analysis of data on specular and off-specular polarized neutron reflectivity. The results are discussed within a phenomenological model containing a few parameters which can readily be controlled by designing systems with a desired configuration of magnetic moments of micro- and nano-elements., 11 pages, 7 figures

We present magnetization and Kerr microscopy measurements of the exchange-bias training effect in hard/soft, ${\mathrm{Tb}}_{12}{\mathrm{Fe}}_{88}$/${\mathrm{Gd}}_{40}{\mathrm{Fe}}_{60}$, bilayers. These experimental results, compared with micromagnetic simulations, unambiguously show the role of the soft GdFe layer reversal on irreversible magnetic changes in the hard TbFe layer. After a partial reversal of the GdFe magnetization layer, the next field cycle exhibits a double hysteresis loop which is due to the existence of two types of domains in the sample, and only one of these domains has been subject to the training effect. The antiferromagnetically interfacial coupling and positive exchange-bias shift permit us to exclude any direct contribution from the magnetic field to this mechanism.

The high-frequency behavior of magnetic domain-walls of the cross-tie type is investigated by static and time-resolved Kerr microscopy. By applying a high-frequency (hf) sinusoidal in-plane magnetic field perpendicular to the wall plane, additional vortex-antivortex pairs are created that lead to a shrinking of the cross-tie spacing. This creation of additional Bloch lines happens by a channelized flux transport across the wall plane at the vortex sites until a new metastable narrow-spaced cross-tie state is reached.

A comparison is presented between magnetoelectric composite sensors based on AlN and FeCoSiB with and without exchange bias coupling. All layer stacks were fabricated by thin film deposition on Si substrates. Whereas sensors without exchange bias exhibit a low limit of detection in the 1 pT/√Hz regime for resonant AC fields, such sensors fail at the detection of low frequency signals. Accordingly, their detection limit increases to about 10 nT/√Hz for alternating magnetic fields with 10 Hz frequency and an integration time equal to 3 s. A frequency conversion technique based on magnetic modulation of the sensors improves their detection limit by about one order of magnitude. However, frequency conversion can be applied more effectively to magnetoelectric sensors with exchange biased multilayers as a magnetostrictive phase. As a result, their limit of detection is about 180 pT/√Hz for 10 Hz signals and an integration time of 1 s. This is in contrast to the magnetoelectric coefficient αME of the two types o...

The layer magnetization, the saturation magnetization as well as the magnetic anisotropy, and damping behavior of 20nm thick Ni81Fe19 films have been modified by 30keV Ni ion implantation with fluences up to 1×1016Ni∕cm2 (≈5at.%). With increasing ion fluence a magnetic dead layer of increasing thickness is formed which leads to a reduction of the total magnetization. In addition, the saturation magnetization of the residual ferromagnetic film decreases due to, both, a shift in stoichiometry and radiation damage. Accordingly a reduction of the magnetic anisotropy and a strong enhancement of the magnetic damping parameter are observed. Moreover, ion implantation in an applied magnetic field allows the setting of the uniaxial anisotropy direction irrespective of its original orientation. Static and dynamic magnetic properties of Ni81Fe19 films can be tailored over a wide range after film deposition.

Conversion electron Mossbauer spectroscopy is used to determine the magnetic ground state at zero magnetic field of four-monolayer thick amorphous iron layers as part of a CoFeB-Fe multilayer stack. By comparing the intensities of the magnetic hyperfine field, an easy in-plane axis of the amorphous embedded Fe layer is verified, which is collinear to the uniaxial anisotropy axis of the neighboring amorphous CoFeB. Despite the soft magnetic character of the Fe layers, external fields up to 4 T perpendicular to the film plane are insufficient to completely align the embedded Fe moments parallel to the magnetic field due to a local disorder of the magnetic moments of the Fe atoms.

Magneto-optic Kerr effect microscopy and nanofocus X-ray diffraction are combined to investigate the local mapping of the magnetoelectric microcomposite properties of ZnO microrods coated with an amorphous (Fe90Co10)78Si12B10 layer. We follow the magnetic domain behavior and lattice deformation upon applying an external magnetic field. In addition to the expected field induced strain, we observe a local magnetic induced strain in the 10−5 range in the ZnO localized near the (Fe90Co10)78Si12B10 /ZnO interface.

A detailed study of twin boundary motion in NiMnGa single crystals together with in situ magnetic domain observation is presented. Optical polarization microscopy in connection with a magneto-optical indicator film technique was used to investigate the reorganization of the magnetic domains during twin boundary motion over a wide magnetic field range. Images at different field strengths demonstrate that no magnetic domain wall motion within the twins takes place, even during the structural reorientation by twin boundary movement. This absence of interaction of magnetic and structural domains is different from currently proposed models, which assume domain wall movement under an external field.

The magnetization dynamics of a constrained cross-tie wall in a 50-nm-thick Ni81Fe19 element is investigated using static and time-resolved wide-field Kerr microscopy. By applying a series of short magnetic-field pulses to the sample the creation of additional vortex antivortex pairs within the cross-tie wall is observed. The cross-tie spacing decreases by a factor of 2–4 relative to the initial equilibrium state. By using sinusoidal and pulsed magnetic-field excitations, a discrimination between the influence of the repetition rate and the rise time of the applied fields is possible. We found a logarithmic increase of the Bloch-line number with decreasing rise time. A resonant excitation with increased repetition rate decreases the cross-tie spacing even further. From direct stroboscopic observation of the precessional remagnetization process, a mechanism for vortex multiplication is proposed.

The magnetization dynamics of a Ni81Fe19 disk of 16μm diameter and 50nm thickness was measured by time-resolved stroboscopic wide-field Kerr microscopy with a temporal resolution of less than 25ps. Time resolution is obtained by illumination with a frequency-doubled high-power Nd:YVO4 solid-state laser. The ground state of the magnetization is excited with a 700‐ps-wide pulse of about 1.2kA∕m amplitude. After a fast rotation of magnetization and vortex displacement due to the pulsed magnetic field the magnetization slowly relaxes back to the ground state. During relaxation of the vortex we observe in the center of the element a stretched vortex structure that could be a 180° wall or a superposition of two vortices with switched core magnetization.