Anisotropic Magnetoresistance Ratio Research Articles

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.

High-field magnetoresistance measurements on Ni75Co25 and Ni40Co60 alloys at 3 K and 300 K

In the present work, a detailed field dependence of the resistivity of Ni75Co25 and Ni40Co60 bulk alloys was measured at T = 3 K and 300 K up to high magnetic fields. The focus of the study was to determine the anisotropic magnetoresistance (AMR) and to get a quantitative description of the field-induced resistivity change, the latter not yet being available for Ni-Co alloys. The AMR parameters were derived from the resistivity data in the magnetically saturated (monodomain) state by using the Kohler analysis. The values of the AMR ratio were found to be close to the relevant previous data both at low and high temperatures. Due to the measurement precision and careful data evaluation, our AMR data obtained on well-characterized samples can be considered as reference values for the bulk state of the investigated compositions. In addition, also the resistivity anisotropy splitting was determined. The experimentally found field dependence of the resistivity at both T = 3 K and 300 K turned out to be at variance with the current theoretical descriptions both for the resistivity increase due to the ordinary magnetoresistance effect being significant at T = 3 K and for the resistivity decrease due to the magnon suppression process at T = 300 K, invoking for a refinement of theory in both cases.

Combinatorial optimization for high spin polarization in Heusler alloy composition-spread thin films by anisotropic magnetoresistance effect

Half-metallic Heusler alloys are promising candidates for spintronic applications due to their high spin polarization. However, the spin polarization strongly depends on the atomic composition, which is time-consuming to optimize from various compositional combinations. Here, we demonstrate a high-throughput compositional optimization method for high spin polarization in Co2(Mn, Fe)Ge Heusler alloys by combining composition-spread films and anisotropic magnetoresistance (AMR) measurement. Two types of composition-spread films of polycrystalline Co2(Mn1−xFex)Ge and (Co2Mn0.5Fe0.5)1−yGey are fabricated on SiO2/Si substrates by combinatorial sputtering deposition, followed by post-annealing. The compositional dependence of AMR shows the largest negative AMR ratio of −0.13% and the smallest temperature dependence of the resistance change of AMR for y = 0.25 in the (Co2Mn0.5Fe0.5)1−yGey composition-spread film, suggesting the highest spin polarization and the closest nature to the ideal half-metal at this composition ratio. To verify this, we also develop a new technique to measure the compositional dependence of spin polarization by measuring the spin accumulation signals of nonlocal spin-valve devices fabricated on the composition-spread films and observe the highest spin polarization of 82% for y = 0.24. This confirms a clear qualitative correlation between the large negative AMR ratio and high spin polarization. Our combinatorial method using the composition-spread films and the AMR measurement proves to be a facile way for optimizing the fabrication conditions of half-metallic Heusler alloys with high spin polarization.

Anisotropic magnetic and magnetotransport properties in morphologically distinct Nd0.6Sr0.4MnO3 thin films

We investigate the magnetic and magnetotransport properties of nanostructured Nd0.6Sr0.4MnO3 (NSMO) thin films grown on (100) oriented SrTiO3 (STO) substrates. The thin films of 100 nm thickness fabricated using the pulsed laser deposition technique possess two distinct surface morphologies—granular and nano-rod type. The morphological change present in the system significantly affects the magnetic and magnetotransport properties of the thin films. Magnetization measurements revealed that the films with rod-type morphology exhibit improved in-plane magnetic anisotropy. The colossal magnetoresistance (∆R/R(H = 0)) of the granular sample is ∼91 %, and the rod morphology sample is ∼97 % at 3 T magnetic field. Additionally, magnetotransport studies revealed that the granular thin films display a characteristic butterfly-shaped low-field magneto-resistive (LFMR) behavior with the value of LFMR of up to ∼10 %. Furthermore, it is observed that the thin film’s morphology has a significant effect on the anisotropic magnetoresistance ratio (AMR). Thin films with rod-type morphology show an enhanced AMR of ∼30 % around its metal-insulator transition temperature. Such morphology-dependent tunability in magnetoresistance properties over a wide temperature range is potentially interesting for developing oxide-based sensors and devices.

Colossal Gilbert Damping Anisotropy in Heusler‐Alloy Thin Films

AbstractManipulating Gilbert damping is of critical importance in spintronic devices. Here, this work reports the damping anisotropy of Heusler‐alloy Co3−xFexAl single crystalline thin films using inductive ferromagnetic resonance. A colossal Gilbert damping anisotropy ratio of up to 1400%, nearly three times higher than the highest value reported among known metallic ferromagnets, is observed in the optimized sample Co1.4Fe1.6Al. The Gilbert damping anisotropy with strong fourfold symmetry is attributed to the variation of the spin–orbit coupling, which is further corroborated by the current‐orientation‐dependent anisotropic magnetoresistance ratio in Co1.4Fe1.6Al thin film. These results provide a practical approach to adjust Gilbert damping continuously and show great promise for future spintronic applications.

Origin of anisotropic magnetoresistance tunable with electric field in Co2FeSi/BaTiO3 multiferroic interfaces

We report a first-principles investigation based on density functional theory with the Hubbard U correction to identify the mechanism behind the electric-field modulation, via a-c domain-wall motion, of the anisotropic magnetoresistance (AMR) ratio in Co2FeSi/BaTiO3 heterostructures. The effects of BaTiO3 (BTO) electric polarization in the [001], [001¯], and [01¯0] directions on the FeSi/TiO2 and CoCo/TiO2 interface terminations are taken into account. We show that the response of the interface geometric and electronic properties to the BTO polarization depends on the interface termination. For instance, the pinning of atoms at the FeSi-terminated interface inhibits the [001] polarization. Through the a-c domain-wall motion, interface hybridized 3dyz states shift in energy and change the minority-spin density of states at the Fermi level, modifying the AMR through the α=ρ↓ρ↑ component. A discussion of the results based on the Campbell–Fert–Jaoul model with s-s and s-d scattering is provided. The electronic states of Co2FeSi inner layers remained mostly unchanged upon the transition between the ferroelectric domains, which indicates that long-range magnetoelastic effects have a negligible influence on the AMR ratio. Hence, the results indicate that interface bonding effects are the origin of the electric-field modulation of the AMR via a-c domain-wall motion in Co2FeSi/BaTiO3 heterostructures.

Compositional dependence of anisotropic magnetoresistance effects in Weyl semimetal Co2MnAl Heusler alloy epitaxial thin films

Anisotropic magnetoresistance (AMR) effect on a Weyl semimetal Co2MnAl Heusler alloy was experimentally and theoretically investigated by changing the Co content, the direction of electric current against the crystal axis, and the measurement temperature. The experimentally fabricated Co2MnAl thin films epitaxially grown on a MgO single-crystal substrate showed the positive AMR ratios independent of the Co content, the direction of electric current, and the measurement temperature. The AMR ratios for the direction of electric current along Co2MnAl[100] were larger than those along Co2MnAl[110]. The maximum AMR ratios for Co2MnAl[100] were 0.576% at 5 K and 0.349% at 300 K for a Co content of 47 at. %, which is close to the stoichiometric composition. Furthermore, the theoretical calculations of AMR ratios based on the s-d scattering process of the electrons at the Fermi energy, including the information on the density of states (DOS) obtained from first-principles calculations, exhibited trends similar to the experimental results. Compared with the AMR results of Co2MnGa[110], we found that the magnitude relation between the partial DOS of the ε and γ orbitals of the d state of Co atoms determined the sign of the AMR ratios. These results suggest that the AMR effect of Weyl semimetal Co2MnAl can be elucidated by the s-d scattering process of the electrons.

Anisotropic magnetoresistance in Mn4−xNixN and the change in the crystalline field

We focus on rare-earth-free anti-perovskite Mn4−xNixN epitaxial films, which can be used for ultrafast current-induced domain wall motion (CIDWM) in magnetic strips. The magneto-transport properties of these materials are very important for a deep understanding of CIDWM. In this study, we investigated the magneto-transport properties of Mn4−xNixN epitaxial films grown on SrTiO3(001) and MgO(001) substrates through anisotropic magnetoresistance (AMR) measurements at temperatures between 2 and 300 K. In samples with a small Ni composition such as x = 0.05−0.1, the AMR ratio of Mn4−xNixN drastically decreased with increasing temperature. We also analyzed the twofold and fourfold symmetries in the AMR curves. Fourfold symmetry is caused by tetragonal crystal fields and is unique to anti-perovskite 3d-metal nitrides such as Mn4N and Fe4N. Only slight fourfold symmetry was observed in Mn4−xNixN. We also performed first-principles calculations with the Vienna ab initio simulation package (VASP) to obtain the projected density of states (PDOS) of d orbitals in Mn4−xNixN, which is responsible for the magnetism of these materials. We conclude that these results are due to the Ni atoms, which function as magnetic impurities and lead to a noticeable change in PDOS, as proved by VASP calculation.

Electric-Field Control of Magnetoresistance Behavior in a Conetic Alloy Thin Film/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Multiferroic Heterostructure

In this work, we investigate the influence of electric fields (E-fields) on the room-temperature magnetotransport behavior of an artificial multiferroic heterostructure, a Conetic alloy (Ni77Fe14Cu5Mo4) thin film/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (011). When the external magnetic field is parallel to the applied current, the switching field increases from 0.8 to 3.3 Oe at 0 and 8 kV/cm, respectively, and the corresponding magnetoresistance (MR) ratio at 20 Oe respectively decreases from 0.14% to 0.03% at 0 and 8 kV/cm. However, when the external magnetic field is perpendicular to the current, the switching field decreases from 10.1 to 1.7 Oe at 0 and 8 kV/cm, and the MR ratio in such a case decreases from −0.001% to −0.10%, respectively. Consequently, under the parallel and perpendicular modes, the tunabilities of the switching field are approximately +313% and −83%, and the MR ratio tunabilities under E-fields are approximately −79% and +9,900%, respectively. Such a large and anisotropic tunability of both the switching field and MR ratio is attributed to the ultrasoft magnetic property of the Conetic alloy thin film and anisotropic in-plane strain-mediated magnetoelectric coupling. However, the anisotropic MR ratio is approximately 0.15% and does not vary with the applied E-fields owing to the intrinsic property of Conetic thin films using transfer and circle transfer curve measurements, rather than the magnetization rotation caused by E-field-induced magnetoelastic anisotropy. This work demonstrates that multiferroic heterostructures with electrically tunable MR show considerable potential in designing energy-efficient electronic and spintronic devices.

High-field magnetoresistance of microcrystalline and nanocrystalline Ni metal at 3 K and 300 K

The longitudinal and transverse magnetoresistance curves MR(H) and the magnetization isotherms M(H) were measured at T = 3 K and 300 K up to high magnetic fields for a microcrystalline (µc) Ni foil with grain sizes above 1 µm (corresponding to bulk Ni) and for a nanocrystalline (nc) Ni foil with an average grain size of about 100 nm. At T = 3 K, the field-induced resistivity change was quite different for the two microstructural states of Ni and the evolution of resistivity with magnetic field was also different which could be explained as arising from their very different electron mean free paths. At T = 300 K, the MR(H) curves of both the μc-Ni and nc-Ni samples were very similar to those known for bulk Ni. The MR(H) data were analyzed at both temperatures with the help of Kohler plots from which the resistivity anisotropy splitting (ΔρAMR) and the anisotropic magnetoresistance (AMR) ratio were derived, the latter values being very similar at both temperatures and for both microstructural states of Ni metal. The present high-precision MR(H) data revealed that the available theoretical models do not accurately describe the suppression of thermally induced magnetic disorder at around room temperature in high magnetic fields.

Anisotropic magnetoresistance (AMR) of cobalt: hcp-Co vs. fcc-Co

In spite of the numerous works devoted to studying the magnetoresistance of Co metal, very diverging results have been reported in the literature on the magnitude of the anisotropic magnetoresistance (AMR) ratio of Co samples mostly without detailed structural characterization. Therefore, the main purpose of the present work was to establish if the crystal structure of Co has an effect on the AMR ratio. With the help of structural studies by X-ray diffraction (XRD) and transmission electron microscopy, fully hcp-Co and predominantly fcc–Co polycrystalline foils were produced by electrodeposition and their magnetoresistance curves MR(H) were measured at room temperature in magnetic fields up to H = 8 kOe. The MR(H) curves indicated much lower saturation fields for fcc-Co than for hcp-Co, in good agreement with the significantly larger magnetocrystalline anisotropy of the hcp phase. These findings were supported also by the measured magnetization isotherms. The coercive field and MR(H) peak position data indicated a magnetically softer behavior of the fcc-Co phase than that of the hcp-Co phase, in agreement with literature findings. Finally, it was established that the AMR ratio is about +1.2 % for hcp-Co whereas it is about +1.9 % for the predominantly fcc-Co samples. By having an estimated volume fraction of the hcp-Co phase in the latter samples from the XRD studies, we could assess an AMR ratio of about 2.2 % for pure fcc-Co. It is the first time that the AMR ratio has been determined separately for the two crystalline phases of Co and the finding that AMR(fcc–Co) ≈ 1.8 AMR(hcp-Co) is a hitherto unknown result. Finally, our AMR results for the two phases of Co metal are discussed in the light of recent progress on the microscopic mechanisms of the AMR effect.

Giant anisotropic magnetoresistance at low magnetic fields in a layered semiconductor

Materials exhibiting a drastic change in electrical resistivity depending on magnetic-field direction is of great interest and is rarely seen in real systems. We report the huge anisotropic magnetoresistance ratio exceeding 7500 at low-magnetic field of 0.4 T at 2 K in the layered magnetic semiconductor ${\mathrm{CeTe}}_{1.83}{\mathrm{Sb}}_{0.17}$ with extremely low carrier density in charge-density-wave state. We found that the electrical resistivity is governed only by the out-of-plane component of magnetization and steeply decreases nearly four orders of magnitude in the magnetic field along the $c$ axis. In contrast, it is quite insensitive to the in-plane component of magnetization and huge anisotropy is preserved even in a much higher magnetic field.

Planar Hall effect and large anisotropic magnetoresistance in a topological superconductor candidate Cu0.05PdTe2

We report the observation of a large anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) in a topological superconducting candidate Cu0.05PdTe2. The AMR and PHE data in Cu0.05PdTe2 can be well explained by the semiclassical theory, confirming that the magneto-transport behaviors of the Cu0.05PdTe2 superconductor are related to its topological nature. The AMR ratio in Cu0.05PdTe2 is one order of magnitude larger than those in traditional ferromagnetic metals. The present results suggest that Cu0.05PdTe2 is a promising material in future magnetoresistive devices with low power consumption.

Orbit-Engineered Anisotropic Magnetoresistive Effect for Constructing a Magnetic Sensor with Ultrahigh Sensitivity.

A strong anisotropic magnetoresistance (AMR) effect induced by spin-orbit coupling is the basis for constructing a highly sensitive and reliable magnetic sensor. Presently, effective AMR enhancement in traditional films focuses on the modulation of the lattice or charge degree of freedom, leading to a general AMR ratio below 4%. Here, we demonstrate a different strategy to strengthen the AMR effect by tuning the orbital degree of freedom. By inserting an oxygen-affinitive Hf layer into a Ta/MgO/NiFe/MgO/Ta multilayer film, Fe-O orbital hybridization at the MgO/NiFe interface was modulated to trigger an effective orbital reconfiguration of Fe. In turn, the number of holes in the in-plane symmetric d orbits of Fe increased substantially, facilitating the s-d electron scattering to enhance the AMR ratio to 4.8%. By further micromachining the film into a Wheatstone bridge, we constructed a sensing element that displayed an ultrahigh sensitivity of 2.7 mV/V/Oe and a low noise detectability of 0.8 nT/√Hz. These findings help to advance the development of orbit-governed AMR sensors and provide an alternative method for tuning other orbit-related physical effects.

Nanoscale-Thick Ni-Based Half-Heusler Alloys with Structural Ordering-Dependent Ultralow Magnetic Damping: Implications for Spintronic Applications

Atomic ordering and its consequence on the crystal structure are fundamental issues in the application of half-metallic Heusler alloys considered as a high spin-polarized electron source. NiMnSb is a promising half-metallic half-Heusler alloy for spintronic devices due to its low magnetic damping, non-centrosymmetric crystal structure, and rich anisotropies. Here, we report a high-quality half-metallic epitaxial nano-scale thick NiMnSb alloy film grown on a Cr/Ag-buffered MgO (001) single crystalline substrate. High temperature deposition is a crucial parameter for crystal growth and magneto-dynamic properties such as damping, anisotropy, magnetization, etc. After investigating the ferromagnetic resonance (FMR) spectroscopy using a time-resolved magneto-optical Kerr (TRMOKE) microscope and calculating the crystal structure using VESTA and the density of states from first-principles calculations, we discuss how the deposition temperature influences the crystal ordering, damping, and magnetization. Ultralow Gilbert damping with high anisotropic field and saturation magnetization were found for the highly ordered C1b structured nano-scale thick NiMnSb film deposited at 573 K. We also found quite nice agreement between theoretical and experimental data of crystal ordering and damping that explains how the ordering affects the magnetic parameter as well as sample quality. Furthermore, the negative anisotropic magnetoresistance ratio implies the robust bulk half-metallicity. Such unique combination of highly ordered half-metallic NiMnSb alloy in the nano-scale regime with ultralow damping can be regarded as an advantage for applications in spintronic devices.

Anisotropic magnetoresistance in room temperature ferromagnetic single crystal CrTe flake

Chromium tellurides have triggered tremendous investigations in physics and materials science, where their unique properties offer excellent opportunities for spintronic applications. Here, we report anisotropic magnetoresistance (AMR) of ferromagnetic CrTe single crystal flake grown by chemical vapor deposition. An individual hexagonal flake exhibits an easy magnetization axis along its normal direction, and cooling from room temperature to low temperature, its electrical resistance measured with the magnetization parallel to current geometry is larger than that for the perpendicular to the current geometry, indicating the presence of an AMR behavior. The AMR ratio has been found to increase with decreasing temperature and is 5.2% at 5 K, comparable with general magnetic transition metal and alloy. This suggests CrTe single crystal flake to be a promising magnetic material for spintronics applications.

Magneto-transport properties of cubic NiMnAs film epitaxied on GaAs (110) substrate

The magneto-transport properties of cubic NiMnAs film epitaxied on the GaAs (110) substrate are investigated. The x-ray diffraction measurements reveal that the NiMnAs (111) crystal plane is parallel to the GaAs (110) crystal plane. The temperature dependence of resistivity at high temperature can be described by a thermal activation model, from which the thermal activation energy is obtained and found to be comparable with many other Heusler alloys. By fitting the temperature dependence of resistivity at low temperature, the coefficient of the quadratic temperature term is determined to be 1.34 × 10−3 μΩ cm K−2. This value suggests the possible presence of single-magnon scattering in the NiMnAs film. The negative magnetoresistance is attributed to the suppression of the spin-dependent scattering, which would not take place in a half-metal. The angle dependence of the anisotropic magnetoresistance (AMR) is measured, and the AMR ratios are positive even at low temperature. These magneto-transport properties indicate that the predicted half-metallicity is destroyed in the NiMnAs film. The absence of the half-metallicity may be attributed to the atomic disorder in the NiMnAs lattice, which needs to be confirmed by further experimental and theoretical studies.

Minority-spin conduction in ferromagnetic Mn5Ge3Cx and Mn5Si3Cx films derived from anisotropic magnetoresistance and density functional theory

The anisotropic magnetoresistance (AMR) of ferromagnetic ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}{\mathrm{C}}_{x}\phantom{\rule{4pt}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}1)$ and ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}{\mathrm{C}}_{x}\phantom{\rule{4pt}{0ex}}(0.5\ensuremath{\le}x\ensuremath{\le}1)$ thin films was investigated and compared with density functional theory calculations from which the spin-split electronic density of states at the Fermi level and the spin polarization were obtained. The isostructural compounds exhibit different AMR behavior. While only ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}{\mathrm{C}}_{0.5}$ shows a positive AMR ratio and a positive spin polarization, the negative AMR ratio of all other compounds is due to a negative spin polarization. The correlation between the sign of the AMR and the degree of spin polarization is in agreement with theoretical calculations of the AMR ratio indicating that the magnetoelectronic transport in both compounds is dominated by minority-spin conduction. The dominating role of minority-spin conduction remains unaffected even after incorporation of carbon into the crystalline lattice which weakens both AMR and spin polarization.

The accurate measurement of spin orbit torque by utilizing the harmonic longitudinal voltage with Wheatstone bridge structure

The microscopic mechanism for the current-induced spin–orbit torque (SOT) in magnetic heterostructures is still under debate. The accurate measurement of SOT effective fields and their thickness dependence is the basis for understanding this issue. In this work, we measured the SOT effective fields for Pt/NiFe bilayers by utilizing the harmonic longitudinal voltage (HLV) method with a Wheatstone bridge structure. Benefiting from the elimination of the linear resistance in the bridge structure and the large magnetoresistance difference resulting from the large length–width ratio of the bridge element, we achieved very high measurement accuracy for both field-like and damping-like effective fields in the Pt/NiFe bilayers. On this basis, we demonstrated the possibility of the SOT measurement with a relatively low current density (∼109 A/m2) by utilizing the HLV method with a Wheatstone bridge structure and found that the method we proposed is also applicable to the Pt/CoFeB system with a low anisotropic magnetoresistance ratio.

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.