Anisotropic Magnetoresistance Research Articles

Dual mode spin to charge conversion using inverse spin Hall effect in NiFe/FeMn/NiFe multilayer thin films

Microwave devices with more than one operating frequency and their ease of tunability are of great importance for high-frequency information processing. Magnetic thin films offer an unparalleled advantage of engineering different microwave bands that can be precisely tailored and reconfigured externally. Here, novel trilayer structures consisting of NiFe/FeMn/NiFe with varying antiferromagnetic FeMn layer thickness have been investigated by exploring their ferromagnetic resonance (FMR) properties and inverse spin Hall effect (ISHE) responses. Two-step magnetic hysteresis loops are observed for higher FeMn thickness (t ⩾ 12 nm), where the bottom NiFe layer shows a comparatively more significant shift due to the presence of strong interfacial exchange coupling. FMR study reveals two resonant modes associated with the two ferromagnetic layers, which are distinguishable for higher thicknesses of FeMn or at high excitation frequencies. The choice of FeMn thickness determines the operating frequencies, which can be finely tailored by optimizing the FeMn thickness. Gilbert damping parameter is found to be in the range of 0.009–0.012 where the presence of exchange bias adds to the the scattering mechanisms. Prominent ISHE responses are obtained from the bottom NiFe layer as compared to the top NiFe layer. Variation of FeMn thickness also shows a strong influence on the spin pumping ( Vsp ), and perpendicular anisotropic magnetoresistance ( VAMR⊥ ) components. The results are correlated with the efficiency of spin flow and spin-to-charge conversion of the FeMn layer. Our systems can be used as an emerging alternative for microwave detectors and microwave energy harvesters.

Current orientation dependence of anisotropic magnetoresistance in SmCo[formula omitted]

Anisotropic magnetoresistance in a highly oriented SmCo5 device was systematically investigated at various temperatures. Currents were applied along the magnetic easy axis and the magnetic hard axis of SmCo5, and the corresponding anisotropic magnetoresistances were measublack, respectively. In both these instances, the anisotropic magnetoresistances exponentially decrease as the temperature increases. Notably, the anisotropic magnetoresistance magnitudes for the two instances intersect at 220 K. This intersection may relate to the compensation between the spin and orbital magnetic moments of Sm, which is also consideblack to be a hint of a ferromagnetic and ferrimagnetic transition in SmCo5. The results exhibit the capability to monitor magnetic configurations through magnetotransport properties, potentially facilitating the development of spintronic devices based on SmCo5.

Investigation on PECVD-deposited SiO2 underlayer in permalloy-based magnetoelectronic devices

This study presents a comprehensive investigation into the dependence among processing conditions, surface properties of the PECVD-deposited SiO2 underlayer, and their consequential impact on the magnetic and film properties of permalloy. The key exploration parameters involve variations in deposition temperature and annealing temperature. By varying the parameters, microstructure changes within SiO2 were firstly analyzed. A thorough characterization of in-plane magnetic features of the Ni81Fe19/SiO2 multilayer film was then undertaken. The results reveal a significant reliance of magnetic anisotropy and coercivity on PECVD SiO2 deposition temperatures, particularly evident for deposition temperatures below 350°C. This effect is attributed to the formation of a suboxide SiOx film at a lower deposition temperature, which typically possesses more dangling bond and lattice defects, resulting in the formation of surface granular structures after annealing. These characterizations may lead to the identification of optimized processing conditions, which may be used for enhancing the performance of anisotropic magnetoresistance devices with SiO2 as an underlayer. Furthermore, the research findings extend beyond anisotropic magnetoresistance systems, with implications for a broad utilization of magnetoelectronic devices. Overall, the study provides a foundation for informed decision-making in the design and optimization of magnetoelectronic devices, in particular for integrated circuit incorporated devices, highlighting the potential for enhanced functionality through manipulation of processing conditions.

Cooperatively tuning magnetic anisotropy and colossal magnetoresistance via atomic-scale buffer layers in highly strained La0.7Sr0.3MnO3 films

Achieving simultaneous control over multiple functional properties, such as magnetic anisotropy, magnetoresistance, and metal-insulator transition, with atomic precision remains a major challenge for realizing advanced spintronic functionalities. Here, we demonstrate a unique approach to cooperatively tune these multiple functional properties in highly strained La0.7Sr0.3MnO3 (LSMO) thin films. By inserting varying perovskite buffer layers, compressively strained LSMO films transition from a ferromagnetic insulator with out-of-plane magnetic anisotropy to a metallic state with in-plane anisotropy. Remarkably, atomic-scale control of the buffer layer thickness enables precise tuning of this magnetic and electronic phase transformation. We achieve a colossal magnetoresistance tuning of 10,000% and an exceptionally sharp transition from out-of-plane to in-plane magnetic anisotropy within just a few atomic layers. These results demonstrate an unprecedented level of control over multiple functional properties, paving the way for the rational design of multifunctional oxide spintronic devices.

A theory of magnetoresistance of non-magnetic metal on magnon valves

One recent exciting development in the field of magnonics is the discovery of universal unusual anisotropic magnetoresistance (UAMR) in nanometer-thick non-magnetic (NM) metallic bars that are deposited on magnon valves of two ferromagnetic insulators (FIs) sandwiching an NM metal. This UAMR has the same angular dependencies as various bilayers consisting of at least one magnetic layer and at least one metallic layer. This suggests that the UAMR of different systems may originate from the same physics, which is yet to be fully understood. Here, we reveal the common feature shared by all these systems: two-vector dependencies of tensor quantities. Specifically, the resistivity of an NM metallic bar depends on the magnetization of its adjacent FI due to the quantum penetration of itinerant electrons of the metallic bar into the FI and on a perpendicular field at the interface of the bar and the FI. We demonstrate that the two-vector dependence of the resistivity tensor is responsible for the observed universal UAMR of metallic bars on magnon valves, independent of the details of the microscopic interactions in different materials. We also propose experiments that can test this theory.

2D Air-Stable Nonlayered Ferrimagnetic FeCr2S4 Crystals Synthesized via Chemical Vapor Deposition.

The discovery of intrinsic 2D magnetic materials has opened up new opportunities for exploring magnetic properties at atomic layer thicknesses, presenting potential applications in spintronic devices. Here a new 2D ferrimagnetic crystal of nonlayered FeCr2S4 is synthesized with high phase purity using chemical vapor deposition. The obtained 2D FeCr2S4 exhibits perpendicular magnetic anisotropy, as evidenced by the out-of-plane/in-plane Hall effect and anisotropic magnetoresistance. Theoretical calculations further elucidate that the observed magnetic anisotropy can be attributed to its surface termination structure. By combining temperature-dependent magneto-transport and polarized Raman spectroscopy characterizations, it is discovered that both the measured Curie temperature and the critical temperature at which a low energy magnon peak disappeared remains constant, regardless of its thickness. Magnetic force microscopy measurements show the flipping process of magnetic domains. The exceptional air-stability of the 2D FeCr2S4 is also confirmed via Raman spectroscopy and Hall hysteresis loops. The robust anisotropic ferrimagnetism, the thickness-independent of Curie temperature, coupled with excellent air-stability, make 2D FeCr2S4 crystals highly attractive for future spintronic devices.

Electrical-Controllable Antiferromagnet-Based Tunnel Junction.

An electrical-controllable antiferromagnet tunnel junction is a key goal in spintronics, holding immense promise for ultradense and ultrastable antiferromagnetic memory with high processing speed for modern information technology. Here, we have advanced toward this goal by achieving an electrical-controllable antiferromagnet-based tunnel junction of Pt/Co/Pt/Co/IrMn/MgO/Pt. The exchange coupling between antiferromagnetic IrMn and Co/Pt perpendicular magnetic multilayers results in the formation of an interfacial exchange bias and exchange spring in IrMn. Encoding information states "0" and "1" is realized through the exchange spring in IrMn, which can be electrically written by spin-orbit torque switching with high cyclability and electrically read by antiferromagnetic tunneling anisotropic magnetoresistance. Combining spin-orbit torque switching of both exchange spring and exchange bias, a 16 Boolean logic operation is successfully demonstrated. With both memory and logic functionalities integrated into our electrically controllable antiferromagnetic-based tunnel junction, we chart the course toward high-performance antiferromagnetic logic-in-memory.

Magnetization Vector Rotation Reservoir Computing Operated by Redox Mechanism.

Physical reservoir computing is a promising way to develop efficient artificial intelligence using physical devices exhibiting nonlinear dynamics. Although magnetic materials have advantages in miniaturization, the need for a magnetic field and large electric current results in high electric power consumption and a complex device structure. To resolve these issues, we propose a redox-based physical reservoir utilizing the planar Hall effect and anisotropic magnetoresistance, which are phenomena described by different nonlinear functions of the magnetization vector that do not need a magnetic field to be applied. The expressive power of this reservoir based on a compact all-solid-state redox transistor is higher than the previous physical reservoir. The normalized mean square error of the reservoir on a second-order nonlinear equation task was 1.69 × 10-3, which is lower than that of a memristor array (3.13 × 10-3) even though the number of reservoir nodes was fewer than half that of the memristor array.

Anisotropic magnetoresistance probed by non-resonant microwave absorption in exchange-biased NiFe/IrMn structures

Sharp change of non-resonant microwave absorption was found in exchange-biased NiFe/IrMn and NiFe/Cu/IrMn structures possessing anisotropic magnetoresistance (AMR) and planar Hall effect (PHE). Angular dependence of the microwave detected AMR and contribution of the domain expansion to AMR microwave signal were analyzed. Decrease in the microwave absorption signal caused by Cu spacer between ferromagnetic NiFe and antiferromagnetic IrMn layers was observed, when NiFe and IrMn layers become exchange decoupled. Additional characterization of the electric transport was proved by asymmetry of resonant line. Sensitivity of the microwave AMR method is enough high to detect magnetic fields of microTesla level, that provides remote detection with a sensitivity corresponding to standard galvanic sensors. Developed technique is promising for wireless AMR sensor technology providing coordinate and angular field measurements through remote microwave absorption.

Unexpected versatile electrical transport behaviors of ferromagnetic nickel films

Perpendicular magnetic anisotropy (PMA) of magnets is paramount for electrically controlled spintronics due to their intrinsic potentials for higher memory density, scalability, thermal stability and endurance, surpassing an in-plane magnetic anisotropy (IMA). Nickel film is a long-lived fundamental element ferromagnet, yet its electrical transport behavior associated with magnetism has not been comprehensively studied, hindering corresponding spintronic applications exploiting nickel-based compounds. Here, we systematically investigate the highly versatile magnetism and corresponding transport behavior of nickel films. As the thickness reduces within the general thickness regime of a magnet layer for a memory device, the hardness of nickel films’ ferromagnetic loop of anomalous Hall effect increases and then decreases, reflecting the magnetic transitions from IMA to PMA and back to IMA. Additionally, the square ferromagnetic loop changes from a hard to a soft one at rising temperatures, indicating a shift from PMA to IMA. Furthermore, we observe a butterfly magnetoresistance resulting from the anisotropic magnetoresistance effect, which evolves in conjunction with the thickness and temperature-dependent magnetic transformations as a complementary support. Our findings unveil the rich magnetic dynamics and most importantly settle down the most useful guiding information for current-driven spintronic applications based on nickel film: The hysteresis loop is squarest for the ∼8 nm-thick nickel film, of highest hardness with Rxy r /Rxy s ∼ 1 and minimum Hs −Hc , up to 125 K; otherwise, extra care should be taken for a different thickness or at a higher temperature.

Low-Cost Throttle-by-Wire-System Architecture for Two-Wheeler Vehicles

<div>This article investigates the performance of a low-cost throttle-by-wire-system (TbWS) for two-wheeler applications. Mopeds/scooters are still restricted as environmentally harmful. TbWSs can contribute to environmental protection by replacing conventional restrictors. Its consisting of an anisotropic magnetoresistance (AMR) throttle position sensor and a position-controlled stepper motor-driven throttle valve actuator. The decentralized throttle position sensor is operating contactless and acquires redundant data. Throttle valve actuation is realized through a position-controlled stepper motor, sensing its position feedback by Hall effect. Using a PI controller, the stepper motor position is precisely set. Both units transmit and receive data by a CAN bus. Furthermore, fail-safe functions, plausibility checks, calibration algorithms, and energy-saving modes have been implemented. Both modules have been evaluated through hardware-in-the-loop testing in terms of reliability and measuring/positioning performance before the system was integrated into a <i>Peugeot Kisbee 50 4T</i> (Euro 5/injected). Finally, the sensor unit comes with a measurement deviation of less than 0.16%, whereas the actuator unit can approach throttle valve positions with a deviation of less than 0.37%. The actuators’ settling time does not exceed 0.13 s in the case of stable, step loss free, and noiseless operation.</div>

Exploring anisotropic phases and spin transport in perovskite heterostructures: Insights into 3d/5d interfaces for antiferromagnetic spintronics

Transition metal oxides (TMOs) demonstrate a broad spectrum of properties encompassing electronic correlations, anisotropic transport, magnetism, and optical behavior. The anisotropy arises from both intrinsic crystal symmetry and extrinsic factors like epitaxial strain and structural asymmetry at TMO interfaces. Weiss and Neel's work has elucidated anisotropic magnetic behavior in antiferromagnetic (AFM) materials. AFM TMOs exhibit unique magnetotransport behavior, including weak antilocalization (WAL) and anisotropic magnetoresistance (AMR). Understanding the magnetic structure and band topology in AFM perovskites and their interfaces enables the tailored design of materials for spintronics and energy conversion. In few interfaces lacking inversion symmetry, Rashba spin–orbit coupling (SOC) induces WAL, a quantum correction in conductivity in a two-dimensional electronic system. Electron accumulation and charge transfer across 3d, 5d transition metal-based perovskite interfaces affect WAL and AMR, as observed in 3d/3d and 3d/5d AFM heterostructures, respectively. Advancements in spintronics rely on exploring spin-dependent transport anisotropy. This review focuses on various scattering mechanisms, categorized as extrinsic and intrinsic, in anisotropic transport, particularly in 3d/5d AFM superlattices. The WAL scattering mechanism depends on both intrinsic factors related to Rashba SOC-induced band topology and extrinsic sources like spin impurities and lattice ions. Moreover, the investigation into AMR mechanisms involves the application of impurity-based extrinsic scattering models, which are aligned with the Rashba and Dresselhauss models on Fermi surfaces. This exploration specifically targets the interface of two-band insulators, exemplified by LaAlO3/SrTiO3 and LaVO3/KTaO3. Furthermore, this model achieves comprehensive coverage, extending its applicability to 3d/5d AFM heterostructures like LaMnO3/SrIrO3 and CaMnO3/CaIrO3. Additionally, the intrinsic scattering mechanism tied to Berry phase effects related to band topology is studied, focusing on the CaMnO3/CaIrO3 superlattice. Despite manipulation challenges stemming from reduced stray fields, AFM materials show potential in interface physics and applications within the realm of spintronics.

3D Anisotropic Magnetoresistance sensor for steel health monitoring

Electromagnetic sensors are widely used for safety and maintenance inspection purposes of ferromagnetic materials used in aerospace, construction and transport industry. In this paper, a portable device based on an Anisotropic Magnetoresistance (AMR) sensor for steel health monitoring is presented. The principle of operation of this device is the detection of magnetic anomalies of ferromagnetic materials, such as steel, due to stresses, corrosion, or other kind of defections. For this purpose, a cost-effective, low-consumption and high-sensitivity device has been developed, consisting of a sensor, a magnetic yoke and a microcontroller. The AMR sensor, is placed above the material under test, giving the ability of contactless measurement of the magnetic flux density on 3 axes. Additionally, a yoke, consisting of two iron poles and a permanent magnet between them, is used in this implementation, in order to create a closed magnetic circuit with the steel sample, enhancing the measurements. Then, a microcontroller with the appropriate electronic components receives the acquired data, enabling their real-time analysis and visualization of the measurements through a personal computer, smartphone, etc. Moreover, a 3D-printed enclosure has been designed and constructed, which both protects the sensor and maintains the desired position and alignment with the samples. The device can be portable due to an integrated Li-Po battery and its Wi-Fi capabilities. It can be used to determine the location of the defective spots or zones of the material under inspection, facilitating its treatment through rehabilitation methods that can be applied on the affected area. Both single-point and scanning mode monitoring are possible, making it suitable for many testing scenarios. Furthermore, its ease of use and portability render it accessible to a wide range of users, and appropriate for on-site measurements. The initial measurements using the device on test samples, demonstrate its reliability and accuracy in steel health monitoring.

Evolution of magnetic anisotropy in La0.67Sr0.33MnO3 thin films due to the appearance of out-of-plane magnetization at low temperature

We have observed quadrupolar angle-dependent anisotropic magnetoresistance (AMR) at low temperature in La0.67Sr0.33MnO3 (LSMO) films grown on SrTiO3 substrate, where the magnetic field was applied while rotating the direction of the applied magnetic field from out-of-plane (OP) to in-plane. At room temperature, the easy axis is in-plane and has a two-fold magnetic anisotropy. As the temperature decreases, it changes to a quadrupolar anisotropy. Along with the AMR, we have measured the magnetoresistance (MR) in the OP direction at various temperatures. Like the AMR, we found the appearance of a new magnetic configuration in the OP direction at low temperatures. We suggest that one of the important origins of the new OP magnetism, which appears at temperature much lower than the Curie temperature (Tc), is the d3z2−r2 orbital at the interface between the LSMO films and SrTiO3 substrate. It has been reported that despite the tensile strain imposed by the SrTiO3 substrate, the d3z2−r2 orbital exists at the interface and causes the OP magnetism. We demonstrate by a polarized x-ray resonant scattering that there is a higher density of d3z2−r2 orbitals at the interface than in the region away from the interface. As other references have proposed, we suggest the orbital reconstructions and the orbital reconstruction-driven magnetic reorientation at the interface as the origin of the appearance of the OP magnetism present in the AMR and MR results.

Proposal for All-Electrical Skyrmion Detection in van der Waals Tunnel Junctions.

A major challenge for magnetic skyrmions in atomically thin van der Waals (vdW) materials is reliable skyrmion detection. Here, based on rigorous first-principles calculations, we show that all-electrical skyrmion detection is feasible in two-dimensional vdW magnets via scanning tunneling microscopy (STM) and in planar tunnel junctions. We use the nonequilibrium Green's function method for quantum transport in planar junctions, including self-energy due to electrodes and working conditions, going beyond the standard Tersoff-Hamann approximation. We obtain a very large tunneling anisotropic magnetoresistance (TAMR) around the Fermi energy for a graphite/Fe3GeTe2/germanene/graphite vdW tunnel junction. For atomic-scale skyrmions, the noncollinear magnetoresistance (NCMR) reaches giant values. We trace the origin of the NCMR to spin mixing between spin-up and -down states of pz and dz2 character at the surface atoms. Both TAMR and NCMR are drastically enhanced in tunnel junctions with respect to STM geometry due to orbital symmetry matching at the interface.

Observation of giant room-temperature anisotropic magnetoresistance in the topological insulator β-Ag2Te

Achieving room-temperature high anisotropic magnetoresistance ratios is highly desirable for magnetic sensors with scaled supply voltages and high sensitivities. However, the ratios in heterojunction-free thin films are currently limited to only a few percent at room temperature. Here, we observe a high anisotropic magnetoresistance ratio of −39% and a giant planar Hall effect (520 μΩ⋅cm) at room temperature under 9 T in β-Ag2Te crystals grown by chemical vapor deposition. We propose a theoretical model of anisotropic scattering — induced by a Dirac cone tilt and modulated by intrinsic properties of effective mass and sound velocity — as a possible origin. Moreover, small-size angle sensors with a Wheatstone bridge configuration were fabricated using the synthesized β-Ag2Te crystals. The sensors exhibited high output response (240 mV/V), high angle sensitivity (4.2 mV/V/°) and small angle error (<1°). Our work translates the developments in topological insulators to a broader impact on practical applications such as high-field magnetic and angle sensors.

Angular Position Sensor Based on Anisotropic Magnetoresistive and Anomalous Nernst Effect

Magnetic position sensors have extensive applications in various industrial sectors and consumer products. However, measuring angles in the full range of 0–360° in a wide field range using a single magnetic sensor remains a challenge. Here, we propose a magnetic position sensor based on a single Wheatstone bridge structure made from a single ferromagnetic layer. By measuring the anisotropic magnetoresistance (AMR) signals from the bridge and two sets of anomalous Nernst effect (ANE) signals from the transverse ports on two perpendicular Wheatstone bridge arms concurrently, we show that it is possible to achieve 0–360° angle detection using a single bridge sensor. The combined use of AMR and ANE signals allows a mean angle error in the range of 0.51–1.05° within a field range of 100 Oe–10,000 Oe to be achieved.

Combining annealing temperature and interface engineering for improving anisotropic magnetoresistance in Ta/NiFe/Ta films

Anisotropic magnetoresistance (AMR) sensors are pivotal in various applications due to their low power consumption, scalability, and cost-effectiveness due to the simple sensor structure, comprising one NiFe film, usually encased in a buffer and cap layer. In this work, we explore the effects of inserting MgO and Pt dusting layers between the NiFe sensing layer and adjacent capping and buffer layers, on the electric, magnetic and structural properties of AMR sensors. We describe results on sensors based on Ta/NiFe/Ta, with an as-processed AMR value of 2.0 %. The insertion of Pt thin films had a positive impact, with AMR values increasing to 2.2 %, contrary to the observed with MgO dusting films. Magnetic annealing up to 370 °C caused an increase of the resistivity and reduction in AMR (with Pt dusting layers), on the contrary, MgO dusting layers improved the sensor performance upon annealing, with AMR increasing to 2.5 % (5 h at 370 °C). In light of the findings, the incorporation of Pt and MgO dusting layers enables tailoring the grain size and resistance of Ta/NiFe/Ta films, while combined with proper annealing, which is relevant for applications where Ta and NiFe are available for AMR sensor fabrication.

Photoelectric properties of two-dimensional electron gas at the (Ga0.5Al0.5)2O3/SrTiO3 heterointerfaces

We report a two-dimensional electron gas created at the (Ga0.5Al0.5)2O3/SrTiO3 heterointerfaces with different deposition temperatures and thicknesses by a pulsed laser deposition method. It is found that the critical deposition temperature for the metal-to-insulator transition is about 400 °C with a thickness of 43.2 nm. The carrier density and mobility of sample at the deposition temperature of 300 °C are 1012 cm−2 and 67.2 cm2 V−1 s−1 at 50 K, respectively. The thickness of the conductive layer is ∼4.0 nm estimated from the anisotropic magnetoresistance. The bandgaps of films deposited at different temperatures are in the range of 5.6–5.9 eV. Furthermore, the heterointerfaces have a strong response to 360 nm light with a relative resistance change of ∼70%. Our results are related to the charge transfer due to the oxygen defect at the interfaces, which provides a way for the development of third-generation semiconductor optoelectronic devices.

Resonant anomalous Hall effect in a ferromagnetic Weyl semimetal

The anomalous Hall effect (AHE) has been widely studied and is well-known in ferromagnetic metals. It is usually investigated in the static regime with the magnetization at equilibrium. In this work, we study the AHE in the dynamic regime where the magnetization is resonantly excited. The microwave-induced ac current and magnetization precession both at GHz frequencies can cooperatively generate a dc voltage. In conventional ferromagnets, this effect, dubbed as resonant AHE (RAHE), is often overwhelmed by other effects such as spin pumping and spin rectification induced by anisotropic magnetoresistance. Here, we observe the RAHE in Co2MnGa, a ferromagnetic Weyl semimetal. In this material, the RAHE dominates over other effects, as observed in angle-dependent measurements. The RAHE in Co2MnGa is an order of magnitude larger than in Permalloy, a conventional ferromagnet. This enhancement is induced by the Berry curvature in the topological band structure of Co2MnGa. The large RAHE demonstrated in this work provides a viable methodology to convert microwave signals into dc voltages for telecommunication applications.