Magnetic skyrmions are topologically protected local magnetic solitons that are promising for storage, logic or general computing applications. In this work, we demonstrate that we can use a skyrmion device based on [W/CoFeB/MgO] 1 0 multilayers for three-dimensional magnetic field sensing enabled by spin-orbit torques (SOT). We stabilize isolated chiral skyrmions and stripe domains in the multilayers, as shown by magnetic force microscopy images and micromagnetic simulations. We perform magnetic transport measurements to show that we can sense both in-plane and out-of-plane magnetic fields by means of a differential measurement scheme in which the symmetry of the SOT leads to cancelation of the DC offset. With the magnetic parameters obtained by vibrating sample magnetometry and ferromagnetic resonance measurements, we perform finite-temperature micromagnetic simulations, where we investigate the fundamental origin of the sensing signal. We identify the topological transformation between skyrmions, stripes and type-II bubbles that leads to a change in the resistance that is read-out by the anomalous Hall effect. Our study presents a novel application for skyrmions, where a differential measurement sensing concept is applied to quantify external magnetic fields paving the way towards more energy efficient applications in skyrmionics based spintronics.

We propose a novel device concept using spin-orbit-torques to realize a magnetic field sensor, where we eliminate the sensor offset using a differential measurement concept. We derive a simple analytical formulation for the sensor signal and demonstrate its validity with numerical investigations using macrospin simulations. The sensitivity and the measurable linear sensing range in the proposed concept can be tuned by either varying the effective magnetic anisotropy or by varying the magnitude of the injected currents. We show that undesired perturbation fields normal to the sensitive direction preserve the zero-offset property and only slightly modulate the sensitivity of the proposed sensor. Higher-harmonics voltage analysis on a Hall cross experimentally confirms the linearity and tunability via current strength. Additionally, the sensor exhibits a non-vanishing offset in the experiment which we attribute to the anomalous Nernst effect.

Magnetoresistive sensors based on giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) play a major role towards the miniaturization in the industrial society. Typically, spin-valve-type magnetoresistive sensors are embedded in a Wheatstone bridge configuration with rectangular, meander-like or elliptically shaped thin film elements. Such elements usually switch via multi-domain, C- or S-shaped magnetization states and, therefore, often exhibit an open non-linear hysteresis curve. Linearity and hysteretic effects are key features in the improvement of such sensors. We will present a different approach by using circularly shaped elements exhibiting a different magnetization state of a magnetic vortex [1]. This is one of the fundamental magnetization ground states occurring in disk-shaped thin film elements and is characterized by minimization of the demagnetizing energy at the expense of exchange energy. Experimental data were generated on electrically contacted GMR and TMR disks which were fabricated by optical lithography. The following advantages will be discussed and compared to standard elliptical sensor elements. (a) The vortex state shows essentially no hysteresis in the minor loop. (b) Since the vortex nucleation happens prior to the zero field, the M(H=0)=0 crossing is independent of history. (c) The critical fields can be easily controlled by the element geometry. (d) The noise is low. All characteristic experimental values have been determined in dependence of free layer thickness, disk diameter and temperature. These findings are discussed in the frame of the semi-analytical rigid-vortex-model [2] and micromagnetic simulations. The financial support by the Austrian Federal Ministry of Science, Research and Economy and the Christian Doppler Research Association in Austria is gratefully acknowledged. [1] D. Suess, A. Bachleitner-Hofmann, A. Satz, H. Weitensfelder, C. Vogler, F. Bruckner, C. Abert, K. Prügl, J. Zimmer, C. Huber, S. Luber, W. Raberg, T. Schrefl, H. Brückl, „Topologically Protected Vortex Structures to Realize Low-Noise Magnetic Sensors with High Linear Range”, Nature Electronics 1, 362 (2018) [2] K. Y. Guslienko et al., “Magnetization reversal due to vortex nucleation, displacement, and annihilation in submicron ferromagnetic dot arrays”, Phys. Rev. B 65 (2001)

Automotive magnetic field sensing applications require a robust sensing concept. One way to meet the corresponding sensor requirements, such as a negligible hysteresis and a large linear working range, is to employ the vortex state. Consequently, the nucleation field Hn of the vortex state becomes a highly important sensor parameter. In this study, we examine different factors that affect Hn. Tunneling magnetoresistance spin-valve sensors with disk-shaped CoFeB free layers, which energetically favor the nucleation of the vortex state, are electrically characterized and compared with micromagnetic simulations. Phase transitions into intermediate magnetic states, such as various buckling states, the S-state, or the double vortex state, are extracted from hysteresis loops. The resulting phase diagrams show that the formation of the S-state only occurs below a thickness of approximately 25 nm, whereas the double vortex state nucleates frequently only above approximately 35 nm. Both the S- and double vortex states lower the nucleation field of the single vortex state compared to higher order buckling states. Understanding both the origin and the influence of the intermediate phases opens the way to designing a robust and reliable vortex sensor concept.

Ensuring a reliable reference in magnetoresistive devices requires robust exchange biased antiferromagnetic/ferromagnetic (AF/FM) bilayers with strong resilience to high temperatures. In this work, we studied the thermal stability of MnPt and MnIr by investigating the exchange bias effect in AF/CoFe bilayers and tunneling magneto resistance (TMR) full stacks. In bilayers, the obtained blocking temperature (T-b) of 380 degrees C for MnPt is higher than Tb of 340 degrees C for MnIr, indicating a better thermal stability. Based on Malozemoff's model, we developed a comprehensive model to describe the exchange bias field (H-ex) behavior at high temperature by considering the T-b distribution and temperature dependence of FM magnetization, giving a good agreement with the experimental data. With increasing temperature, a coercivity (H-c) enhancement peak was observed in MnIr bilayer, which shows a good agreement with Tb distribution. This evidences the H-c peak is dominated by the thermal activation of AF grains, indicating that the Hc peak only appears at T-b(center), which is lower than overall T-b. The structural properties and grain size of AF layers were studied by X-ray diffraction (XRD), where a mean grain size of 10.6 and 12.6 nm were obtained for MnPt and MnIr, respectively. Compared to bilayers, a more stable thermal behavior was observed in full stacks. In conclusions, our study compared the thermal stability of exchange bias coupling using MnPt and MnIr and developed a comprehensive model to study their temperature dependence. This proposed approach can be used for theoretical studies and experimental designs of AF-based materials for spintronic applications, such as magnetoresistive devices and high density memories.

Magnetoresistive spin valve sensors based on the giant- (GMR) and tunnelling- (TMR) magnetoresisitve effect with a flux-closed vortex state free layer design are compared by means of sensitivity and low frequency noise. The vortex state free layer enables high saturation fields with negligible hysteresis, making it attractive for applications with a high dynamic range. The measured GMR devices comprise lower pink noise and better linearity in resistance but are less sensitive to external magnetic fields than TMR sensors. The results show a comparable detectivity at low frequencies and a better performance of the TMR minimum detectable field at frequencies in the white noise limit.

Micromagnetic sensors play a key role in a variety of industries, including the automotive industry, where they are used, for example, for speed and position detection. The adoption of emerging magnetoresistive sensor technology such as anisotropic magnetoresistance, giant magnetoresistance and tunnel magnetoresistance sensors is driven principally by their enhanced sensitivity and improved integration capabilities compared with conventional Hall effect sensors. At the heart of such sensors is a microstructured ferromagnetic thin-film element that transduces the magnetic signal, but these elements often exhibit a nonlinear hysteresis curve and the performance of the sensors is limited by magnetic noise. Here, we examine the origin of magnetic noise in magnetoresistive sensors and show that a topologically protected magnetic vortex state in the transducer element can be used to overcome these limitations. Using analytic and micromagnetic models, we find that the noise is due mainly to irreproducible magnetic switching of the transducer element at external fields that are close to the Stoner–Wohlfarth switching field. Then, using a flux-closed vortex configuration, we develop a giant magnetoresistance sensor layout that, compared to existing state-of-the-art sensors, has lower magnetic noise, a linear regime that is around an order of magnitude higher and negligible hysteresis.

Summary form only given. In this paper, we present a different approach by using circularly shaped elements exhibiting a different magnetization state. The magnetic vortex is one of the fundamental magnetization ground states occurring in disk-shaped thin film elements and is characterized by minimization of the demagnetizing energy at the expense of exchange energy. The state is described as free of hysteresis and planar stray-fields, and discussed as potential oscillator in spin-torque systems, memory bits and magnetic field sensors. In order to compare important parameters like resistance, magnetoresistance amplitude, sensitivity, hysteresis, critical fields like nucleation and annihilation fields in dependence on the free layer thickness and disk radius, single and arrays (10 and 1750 in number) of GMR disks have been fabricated by optical lithography and contacted.

Micromagnetic sensors play a major role towards the miniaturization in the industrial society. The adoption of new and emerging sensor technologies like anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) sensors are mainly driven by their integrability and enhanced sensitivity. At the core of such sensors, a microstructured ferromagnetic thin film element transduces the magnetic signal. Such elements usually switch via multi-domain, C- or S-shaped magnetization states and, therefore, often exhibit an open non-linear hysteresis curve. Linearity and hysteretic effects, as well as magnetic noise are key features in the improvement of such sensors. Here, we report on the physical origin of these disturbing factors and the inherent connection of noise and hysteresis. Critical noise sources are identified by means of analytic and micromagnetic models. The dominant noise source is due to irreproducible magnetic switching of the transducer element at external fields close to the Stoner Wohlfarth switching field. Furthermore, a solution is presented to overcome these limiting factors: a disruptive sensor design is proposed and analyzed which realizes a topologically protected magnetic vortex state in the transducer element. Compared to state of the art sensors the proposed sensor layout has negligible hysteresis, a linear regime about an order of magnitude higher and lower magnetic noise making the sensor ideal candidate for applications ranging from automotive industry to biological application.

Magnetoresistive sensors using a tunnel magnetoresistance (TMR) spin valve structure with CoFe and CoFeB free layer (FL) are investigated. Lateral dimension, thickness, and magnetic properties of the circular-shaped FL energetically favor the nucleation of a magnetic vortex. In the vortex configuration, the sensors show the expected hysteresis-free transfer curve. Distinct differences between CoFe and CoFeB in the vortex typical parameters are observed. The experimental results are compared to micromagnetic simulations. Influence of saturation magnetization Ms on the vortex transfer curve is studied in order to reproduce the experimental data. However, only by adjusting M s iteratively the experimental data cannot be fully reproduced: The vortex annihilation (H an ) in CoFeB occurs at smaller fields than simulated. On the other hand, the change in TMR signal at H an is significantly smaller in the experiment for CoFe. Discrepancies are most pronounced at H an . This investigation focuses on the magnetic and electric properties at the edge of the FL structure. Reduction in H an can be explained for 1.1 μm FL diameters by introducing a magnetically disturbed edge. Reduced change in TMR signal at H an can be understood by introducing an electrically inactive edge area. The analysis shows that for CoFe and CoFeB different edge inhomogeneity effects are present or edge inhomogeneities have different impacts.

A monolithically integrated GMR angle sensor for automotive applications is presented. The vertical integration concept allows a combination of our spin-valve type GMR angle sensors with conventional CMOS or bipolar base technologies providing the signal conditioning. The PtMn based sensor stack exhibits an excellent thermal stability. A combination of temperature and magnetic field, however, leads to a drift of the magnetization direction of the reference system and therefore, to an increased angular error. A reduction of the net magnetic moment as well as the film thickness of the pinned and reference layer of the artificial antiferromagnet leads to a significant improvement of the robustness against thermo-magnetic stress.

Field strength sensors based on the AMR effect require field biasing concept to guarantee a unique signal characteristic. The exchange bias approach using a natural antiferromagnetic material is an advantageous possibility to provide a field bias to the sensor. PtMn exhibits a high thermal robustness and a high blocking temperature i.e. a good exchange bias coupling even at high temperatures. However, PtMn provokes a magnetically hysteretic behavior of the AMR layer. An antiferromagnetic interlayer coupling to an additionally introduced directly exchange bias coupled pinned layer significantly reduces the observed magnetic hysteresis. By adapting the interlayer coupling and the thickness of the pinned layer the hysteresis as well as the linear range of the sensor can be modified. The experimental findings will be qualitatively explained by a Stoner-Wohlfarth model.

Spin valve systems based on the giant magnetoresistive effect as used, for example, in hard disks and automotive applications consist of several functional metallic thin film layers. We have identified by secondary ion mass spectrometry two main degradation mechanisms: one is related to oxygen diffusion through a protective cap layer and the other one is interdiffusion directly at the functional layers of the giant magnetoresistive stack. By choosing a suitable material as cap layer (TaN), the oxidation effect can be suppressed.