Anisotropic Magnetoresistance Effect Research Articles

Anomalous Hall Effect, Robust Negative Magnetoresistance, and Memory Devices Based on a Noncollinear Antiferromagnetic Metal.

We report the successful fabrication of noncollinear antiferromagnetic D019 Mn3Ge thin films on insulating oxide substrates. The anomalous Hall effect and the large parallel negative magnetoresistance that is robust up to 53 T are observed in the thin films, which may provide evidence for the recent theoretical prediction of the existence of Weyl fermions in antiferromagnetic Mn3Ge. More importantly, we integrate the Mn3Ge thin films onto ferroelectric PMN-PT substrates and manipulate the longitudinal resistance reversibly by electric fields at room temperature, demonstrating the anisotropic magnetoresistance effect in noncollinear antiferromagnets, which thus illustrates the potential of antiferromagnetic Mn3Ge for information storage applications.

Theoretical Study on Four-fold Symmetric Anisotropic Magnetoresistance Effect in Cubic Single-crystal Ferromagnetic Model

In this study, we present a theoretical interpretation of the experimental results that the anisotropic magnetoresistance (AMR) effect has a four-fold symmetric component, $c_4$, in cubic ferromagnetic metals. The theoretical model that we employ is based on the Anderson impurity model that includes a four-fold symmetric crystalline electric field, and we assume that the impurities have 3d electron orbitals and spin--orbit interaction (SOI). We describe the DC conductivity on the basis of the Kubo formula, and we investigate $c_4$ by analyzing the magnetization direction dependence of the resultant AMR ratio. Analytical and numerical calculations are performed; the analytical calculation reveals that $c_4$ arises from the fourth-order contribution of the SOI, and the numerical calculation provides the parameter dependencies of $c_{4}$ in our model. From the calculation results, we observe that the splitting of impurity 3d levels due to SOI is responsible for the existence of $c_{4}$ in cubic ferromagnetic metals.

Electric field-controlled anisotropic magnetoresistance effect with four-fold-symmetric magnetic anisotropy

The use of electric fields to control magnetism based on the multiferroic effect provides the possibility to optimize the performance of magnetoresistive sensors. In this work, the electric field of a Ni80Co20/PMN-PT multiferroic heterostructure was used to control the anisotropic field of a Ni80Co20 film with four-fold-symmetric magnetic anisotropy and its anisotropic magnetoresistance. The electric field can be used to control the magnetic field response characteristics of the Ni80Co20 film and change the four-fold-symmetric anisotropy to non-volatile uniaxial anisotropy. Application of an electric field of 6 kV/cm increases the magnetic field response range to 300 Oe when the magnetic field is parallel to the current direction. Once the applied electric field is removed, the uniaxial anisotropy has good stability that can be maintained for 90 days. These results lay the foundation for the research and development of anisotropic magnetoresistive sensors with a stable and wide magnetic field measurement range.

Theoretical study on anisotropic magnetoresistance effect for weak ferromagnets with a crystal field of tetragonal symmetry

We derive an expression of the anisotropic magnetoresistance (AMR) ratio for weak ferromagnets with a crystal field of tetragonal symmetry using the electron scattering theory. This theory is based on the two-current model consisting of a conduction state and localized d states with a spin-orbit interaction and the crystal field. The expression of the AMR ratio is related to the partial density of states of each orbital at the Fermi energy. On the basis of this expression, we investigate the sign of the AMR ratio. In addition, we analyze the experimental result of the negative AMR ratio at low temperature for a typical weak ferromagnet, bcc Fe.

Signs of anisotropic magnetoresistance in Co2MnGa Heusler alloy epitaxial thin films based on current direction

Anisotropic magnetoresistance (AMR) effects in Cox(Mn0.44Ga0.56)100−x epitaxial thin films were investigated in the temperature range of 5–300 K by changing the current (I) direction to the crystal axis and the Co content (x). The AMR ratios were mostly positive for I // Co2MnGa[100] and negative for I // Co2MnGa[110] and showed peak values at x = 49.7 at% for both current directions. AMR ratios calculated from s–d scattering theory, including crystal field effects with density of states information from first-principles calculations, well reproduced experimental features.

Spin waves parametrically excited via three-magnon scattering in narrow NiFe strips

Three-magnon scattering can be used to evaluate the lowest frequency of the magnon band in a ferromagnet. Both the ferromagnetic-resonant (FMR) frequency ${f}_{\mathrm{FMR}}$ and the lowest frequency of the magnon band ${f}_{\mathrm{min}}$ in narrow-shaped NiFe strips were electrically measured using the anisotropic-magnetoresistance effect. The comparison with a micromagnetic simulation shows that ${f}_{\mathrm{min}}$ of the magnon band can be controlled independent of ${f}_{\mathrm{FMR}}$ by varying the width $w$ and thickness $t$ of the NiFe strip while maintaining a constant $t/w$ ratio. In addition, we found that the frequency difference, ${f}_{\mathrm{FMR}}\ensuremath{-}{f}_{\mathrm{min}}$, can be greatly increased in thicker NiFe strips. Our results show that narrow-shaped ferromagnets allow us to tune the magnon-band structures by varying their $w$ and $t$. This ability is important for designing magnon circuits in integrated magnonic devices and for improving the quantitative study on the Bose-Einstein condensation of magnons.

A low-temperature Kerr effect microscope for the simultaneous magneto-optic and magneto-transport study of magnetic topological insulators

Magneto-optic Kerr effect (MOKE) microscopy involves the surface-sensitive probing of magnetisation at micron-sized lateral resolution. Here, we present a low-temperature, focused polar MOKE microscope for simultaneous magneto-optic and magneto-transport measurements which has a temperature range of 1.6–300 K and is equipped with a magnet capable of delivering a field of up to 9 T. In this microscope, all optical components are integrated in a free-standing probe, allowing for straightforward incorporation into many non-optical cryostat systems. Two-dimensional magnetisation scans on patterned ferromagnetic [CoFeB/Pt]n films demonstrate a magnetisation sensitivity of 10 µrad (Kerr angle) and a spatial resolution of 2.2 µm. The combination of optical and electrical measurements provides complementary temperature-dependent information, as demonstrated by the study of magnetic topological insulator thin films with out-of-plane magnetic anisotropy. Using this complementary approach, we study the effects of a secondary phase in Cr and V co-doped Sb2Te3 thin films, which show a combination of weak antilocalisation and anisotropic magnetoresistance effects above 70 K. Our results highlight the virtue of MOKE and electrical transport to optimise exotic topological magnetic materials, paving the way for energy-efficient spintronic devices.

Tunneling Anisotropic Magnetoresistance in Ferroelectric Tunnel Junctions

Using a simple quantum-mechanical model, we explore a tunneling anisotropic magnetoresistance (TAMR) effect in ferroelectric tunnel junctions (FTJs) with a ferromagnetic electrode and a ferroelectric barrier layer, which spontaneous polarization gives rise to the Rashba and Dresselhaus spin-orbit coupling (SOC). For realistic parameters of the model, we predict sizable TAMR measurable experimentally. For asymmetric FTJs, which electrodes have different work functions, the built-in electric field affects the SOC parameters and leads to TAMR dependent on ferroelectric polarization direction. The SOC change with polarization switching affects tunneling conductance, revealing a new mechanism of tunneling electroresistance (TER). These results demonstrate new functionalities of FTJs which can be explored experimentally and used in electronic devices.

Anisotropic magnetoresistive sensors for control of additive manufacturing machines

Abstract Magnetic sensors are employed for dimensional measurements by detection of sensor motion relative to a small magnet. This is widely used everywhere in industrial automation, car industry and in many home appliances. The use of magnetic sensors in machines for additive manufacturing improves control and long term reliability by non contact position measurements. Magnetic sensors with linearized characteristic based on the anisotropic magnetoresistance (AMR) effect can replace mechanical switches, while specialized AMR angle sensors are preferred for the measurement of rotational motions. Both are easy to use and can be integrated with help of 3D printed holders at low cost. In this work, appropriate sensors are selected, integrated and discussed regarding magnetic disturbance signals apparent in low-cost 3D printers.

Concentration and heat treatment effects on magnetoresistive properties of Ag-added Ni80Fe20 film systems

• The film phased state does not depend on the Ag concentration. • Different types of magnetoresistive effect was observed. • The maximum value of isotropic MR observed at the addition of Ag with c = 60 at.%. • Temperature dependences of LMR and TMR in the range from 300 to 900 K are presented. Comprehensive investigation of the phase state and the magnetoresistive properties of Ag-added Ni 80 Fe 20 film systems with a total thickness of 55 nm in a wide range of component concentration under the heat treatment within the temperature range 300–900 K was presented. Thin films based on permalloy Ni 80 Fe 20 with addition of Ag atoms were prepared by the electron-beam co-evaporation method at room temperature. Their composition and phase state were studied using X-ray spectrometry and electron diffraction methods. The film phased state remained unchanged when adding Ag within the entire range of investigated compositions. The process of the high-temperature treatment also did not affect their structure. Different types of magnetoresistive effect were observed. The anisotropic magnetoresistive effect appeared in systems with low concentration of Ag atoms. With increasing concentration of the non-magnetic atoms the shape of magnetoresistive field dependences transformed to the linear-like drop curves without hysteresis that led to the appearance of the isotropic magnetoresistive effect in the film plane. The maximum isotropic magnetoresistance was observed at the Ag concentration of about 60 at.%. The magnitude of this maximum value increased after annealing at 500 K and significantly reduced after high-temperature annealing at 900 K.

Calibration of Off-the-Shelf Anisotropic Magnetoresistance Magnetometers.

Magnetometers based on the anisotropic magnetoresistive effect are used in many applications for orientation determination, by measuring the magnetic field of the Earth. As sensors of this type are commercial, off-the-shelf components, manufacturers provide limited information on their measurement performance. Therefore, we present a (to date) unprecedented comprehensive calibration study on three state-of-the-art digital anisotropic magnetoresistance magnetometers, to precisely determine various performance parameters and stability across different sensors of the same model. With the evaluation of sensitivity, noise, offset, and orientation determination, as well as considering dependencies on temperature and frequency, the performance of each sensor can be improved significantly, enabling their implementation in demanding fields of application (such as in satellites). Different measurement and calibration techniques, specifically aimed at the characteristics of the examined magnetometers, were utilized, using a sophisticated magnetic laboratory that has served as a calibration facility for several interplanetary space missions. Our study allows operators to decide whether to consider anisotropic magnetoresitance magnetometers for their application and, more importantly, to be able to (at least partially) skip a time-intensive and complicated calibration by using the sensor parameters given in this paper. To that end, the most promising sensor is recommended. The sensor examination suggests a good comparability of different sensors of the same model, and shows the importance of noise regarding the sensor performance with a noise floor up to 124 nT/ at 1 Hz. Additionally, depending on the sensor model, the sensitivity is 14 nT at best, and the attitude determination error can be reduced to about 0.3° with the given calibration.

Imaging of current induced Néel vector switching in antiferromagnetic Mn2Au

The effects of current induced N\'eel spin-orbit torques on the antiferromagnetic domain structure of epitaxial Mn$_2$Au thin films were investigated by X-ray magnetic linear dichroism - photoemission electron microscopy (XMLD-PEEM). We observed current induced switching of AFM domains essentially corresponding to morphological features of the samples. Reversible as well as irreversible N\'eel vector reorientation was obtained in different parts of the samples and the switching of up to 30 % of all domains in the field of view of 10 $\mu$m is demonstrated. Our direct microscopical observations are compared to and fully consistent with anisotropic magnetoresistance effects previously attributed to current induced N\'eel vector switching in Mn$_2$Au.

High-throughput screening for spin-gapless semiconductors in quaternary Heusler compounds

Based on high throughput density functional theory calculations, we performed systematic screening for spin-gapless semiconductors (SGSs) in quaternary Heusler alloys XX 0 YZ (X, X 0 , and Y are transition metal elements without Tc, and Z is one of B, Al, Ga, In, Si, Ge, Sn, Pb, P, As, Sb, and Bi). Following the empirical rule, we focused on compounds with 21, 26, or 28 valence electrons, resulting in 12, 000 possible chemical compositions. After systematically evaluating the thermodynamic, mechanical, and dynamical stabilities, we successfully identified 70 stable SGSs, confirmed by explicit electronic structure calculations with proper magnetic ground states. It is demonstrated that all four types of SGSs can be realized, defined based on the spin characters of the bands around the Fermi energy, and the type-II SGSs show promising transport properties for spintronic applications. The effect of spin-orbit coupling is investigated, resulting in large anisotropic magnetoresistance and anomalous Nernst effects.

Asymmetry in magnetic behavior caused by superposition of unidirectional and four-fold magnetic anisotropies in CoPt/FeMn bilayers

We investigated magnetic anisotropy and magnetization reversal process in CoPt/FeMn bilayers by four-probe measurements of anisotropic magnetoresistance (AMR) effect. Here, a single crystal-CoPt/polycrystal-FeMn interface has been established, where A1-CoPt layer with four-fold magnetic anisotropy was epitaxially grown on MgO(100) substrate, while the top FeMn layer was 111 textured showing in-plane polycrystalline feature. Due to the noncollinear 3Q AFM spin structure and existence of uncompensated spins in ultrathin FeMn, the magnetic anisotropy of the bilayer can be tuned through interfacial exchange coupling, controlled by field cooling treatment. As a result, it gives rise to asymmetric AMR curves. We show that this asymmetry strongly relies on the direction of induced unidirectional magnetic anisotropy with respect to the easy axis of CoPt layer, leading to a modulated magnetization reversal process.

Electric Field Tuning of Anisotropic Magnetoresistance in Ni-Co/PMN-PT Multiferroic Heterostructure

Electric field tuning of anisotropic magnetoresistance (AMR) effect is investigated in Ni-Co/PMN-PT multiferroic heterostructure. The AMR ratio increases with the electric field when an external magnetic field is perpendicular to the applied current and decreases when the magnetic field is parallel to the current, showing anisotropic tunability of AMR due to the change of magnetization orientation induced by strain-mediated magnetoelectric coupling. A magnetic sensor with hall bar geometry is developed based on AMR effect in Ni-Co/PMN-PT heterostructure. The linear output range of the AMR sensor is enhanced by 250% with the applied electric field, and stable electrical response to the external magnetic field is demonstrated. The electrically tunable AMR effect has great potential for applications in magnetic field sensors with enhanced measurement range and sensitivity.

A flexible way to modulate the detection range of anisotropic magnetoresistance by exchange bias field

Abstract A series of Ta/FeMn/[NiFe/FeMn]n/Ta films was grown through DC-sputtering. They acted as a new structure of multilayer anisotropic magnetoresistance (AMR). Based on the exchange bias (EB) field producing at the NiFe/FeMn interface, the linear range and saturation field along the multilayer’s hard axis enlarged when compared with a single NiFe layer. The linear range and saturation field decided the detection range of the AMR. Therefore, by fixing the total thickness of magnetic NiFe layer and selecting different number of n in multilayer, each thickness of NiFe layer was changed to tune EB field. Then, the detection range was modulated by varying exchange bias fields. Adopting this method, the detection range could be easy to control and adjustable from 20.5 Gs to 116 Gs on the basis of AMR responses. The proposed method promotes the application of the AMR effect over a wide magnetic detection area.

Andreev reflection spectroscopy of ferromagnetic Fe<sub>0.26</sub>TaS<sub>2</sub> with layered structure

<sec>An elementary mission of spintronics research is to prevent the interface reacting in spin device and extract spin polarization of ferromagnetic material reliably. Layered transition metal sulfide has very strong anisotropic magnetism, magnetoresistance, and unique Hall effect. It provides a good platform for studying the magnetic order related physical phenomena and may lay a foundation for spintronic applications. In this work, the magnetism, electronic transport and Andreev reflection spectrum of a novel ferromagnetic material Fe<sub>0.26</sub>TaS<sub>2</sub> with a layers-stacked structure are measured. Strong magnetic anisotropy, double-peak magnetoresistance and anomalous Hall effect are found. In the magnetic measurement, the strong magnetic anisotropy behavior in Fe<sub>0.26</sub>TaS<sub>2</sub> single crystal is observed. Curie temperature <i>T</i><sub>C</sub> of the Fe<sub>0.26</sub>TaS<sub>2</sub> single crystal is confirmed by zero field cooling, field cooling and Arrot plot. The electronic transport in the Fe<sub>0.26</sub>TaS<sub>2</sub> single crystal also reveals strong anisotropic behaviors, such as butterfly-like magnetoresistance and obvious anomalous hall effect below <i>T</i><sub>C</sub>.</sec><sec>To obtain the spin polarization of Fe<sub><i>x</i></sub>TaS<sub>2</sub>, we fabricate an Fe<sub><i>x</i></sub>TaS<sub>2</sub>/superconductor Andreev junction to measure the spin polarization that is fitted by the modified Blonder-Tinkham-Klapwijk (BTK) theory. Perhaps the diffusion of Pb can form an alloy structure, creating another superconductor behavior. The two-gap BTK theory confirms our hypothesis, and the result spin polarization can reach 26%. To avoid the interference from Pb alloy superconductor, we also fabricate an Fe<sub>0.26</sub>TaS<sub>2</sub>/Al/Pb superconductor junction by evaporating Al and then Pb film on the surface of Fe<sub>0.26</sub>TaS<sub>2</sub> in sequence. The results of BTK fit show that the spin polarization from the first technical route cannot be reliable due to the tunneling layer on the Al interface. In order to obtain a clean interface, Fe<sub>0.26</sub>TaS<sub>2</sub>/NbSe<sub>2</sub> junction is fabricated through mechanical-exfoliation and dry-transfer method. Through the Andreev reflection spectrum of this junction, the spin polarization of Fe<sub>0.26</sub>TaS<sub>2</sub> is extracted to be 47% ± 7%. For various two-dimensional ferromagnetic materials, our work suggests that the dry-transfer method is well applicable in spin polarization extraction. The results of spin polarization indicate that the Fe<sub>0.26</sub>TaS<sub>2</sub> is a promising candidate of next-generation material of spintronics.</sec>

Guidelines for the evaluation of magnetotransport parameters from measurements on thin strip-shaped samples of bulk metallic ferromagnets with finite residual resistivity

In the present paper, the formulae are summarized for the evaluation of magnetoresistance data from the measured field dependence of the resistivity obtained on thin strip-shaped samples of ferromagnetic metals and alloys with a finite residual resistivity for various relative orientations of the magnetic field and the measuring current. This sample shape simplifies the problem in that the specimen shape can be approximated with a general ellipsoid for which the demagnetizing fields can be estimated with sufficient accuracy from tabulated data. First, the various experimental configurations are defined for the magnetotransport measurements. Then, the formulae for describing the field-dependence of the resistivity in metallic ferromagnets with finite residual resistivity are discussed, by treating separately the case of low and high temperatures, low and high being defined with respect to the Curie temperature of the ferromagnet under study. The extraction of some magnetotransport parameters (anisotropic magnetoresistance and Hall effect) often requires an extrapolation of the measured data to zero magnetic field or zero magnetic induction and this will also be discussed. Finally, a summary of the relations between the experimentally measured magnetotransport parameters and those derived from theoretical calculations will be given.

Anisotropic giant magnetoresistive effect in the sandwich based FexNi1−x (x ≈ 0.5) and Cu

The anisotropic effect of a giant magnetoresistance (GMR) in the three-layer FexNi1−x/Cu/FexNi1−x/Sub (x ≈ 0.5, Sub is the substrate) magnetically ordered film is analyzed experimentally and theoretically using the phenomenological approach. It is shown that in the case when the direction of the current density vector j coincides with the direction of the local magnetization vector M (j∥M) in magnetic layers, the consideration of the resistance anisotropy results in a decrease in the magnitude of the GMR effect; and while the vectors are mutually perpendicular in the film plane (j⊥M), the GMR value increases. The analysis of the size dependence (dependence of the top magnetic layer on the thickness dm2) of the magnetoresistance ratio (MR ratio) shows that, in the case of the isotropic GMR effect, when inequalities dm2 > dm1) (dm1 is the thickness of the bottom magnetic layer) hold, the indicated effect is negligible due to shunting of the top layer resistance by the base layer resistance (shunting of the base layer resistance by the top-layer resistance). In the case when the magnetic layer thicknesses are comparable in size (dm1 ~ dm2), the magnitude of the anisotropic giant magnetoresistance (AGMR) acquires its maximum (amplitude) value because of the absence of the shunting effect. The method for calculating the asymmetry parameter αlj=l0j+∕l0j– (l0js is the spin-dependent free path of charge carriers in the s = ±th spin channel of the jth magnetic layer), which characterizes the difference in the free paths of electrons in the spin conduction channels, is proposed for the first time.

Giant anisotropic magnetoresistance and planar Hall effect in Sr0.06Bi2Se3

We report the observation of giant negative anisotropic magnetoresistance and planar Hall effect in superconducting topological insulator Sr0.06Bi2Se3. It is found that giant anisotropic magnetoresistance and planar Hall effect with non-zero transverse voltage are developed by tilting the in-plane magnetic field away from the direction of the electrical current. Quantitative analyses of the measured data suggest that the observed anisotropic magnetoresistance and planar Hall effect originate from the chiral anomaly behavior of the material. The large anisotropic magnetoresistance and planar Hall effect demonstrate that this material has potential to be utilized in magnetoresistive devices with low power consumption.