Extraordinary Magnetoresistance Research Articles

Extreme magnetoresistance at high-mobility oxide heterointerfaces with dynamic defect tunability

Magnetic field-induced changes in the electrical resistance of materials reveal insights into the fundamental properties governing their electronic and magnetic behavior. Various classes of magnetoresistance have been realized, including giant, colossal, and extraordinary magnetoresistance, each with distinct physical origins. In recent years, extreme magnetoresistance (XMR) has been observed in topological and non-topological materials displaying a non-saturating magnetoresistance reaching 103−108% in magnetic fields up to 60 T. XMR is often intimately linked to a gapless band structure with steep bands and charge compensation. Here, we show that a linear XMR of 80,000% at 15 T and 2 K emerges at the high-mobility interface between the large band-gap oxides γ-Al2O3 and SrTiO3. Despite the chemically and electronically very dissimilar environment, the temperature/field phase diagrams of γ-Al2O3/SrTiO3 bear a striking resemblance to XMR semimetals. By comparing magnetotransport, microscopic current imaging, and momentum-resolved band structures, we conclude that the XMR in γ-Al2O3/SrTiO3 is not strongly linked to the band structure, but arises from weak disorder enforcing a squeezed guiding center motion of electrons. We also present a dynamic XMR self-enhancement through an autonomous redistribution of quasi-mobile oxygen vacancies. Our findings shed new light on XMR and introduce tunability using dynamic defect engineering.

Magnetic-Field Response and Giant Electric-Field Modulation of Cu2S.

Cu2S likely plays an important role in the sharp resistivity transition of LK-99. Nevertheless, this immediately arouses an intriguing question of whether the extraordinary room-temperature colossal magnetoresistance in the initial reports, which has been less focused, originates from Cu2S as well. To resolve this issue, we have systematically investigated the electrical transport and magnetotransport properties of near-stoichiometric Cu2S pellets and thin films. Neither Cu2S nor LK-99 containing Cu2S in this study was found to exhibit the remarkable magnetoresistance effect implied by Lee et al. This implies that Cu2S could not account for all of the intriguing transport properties of the initially reported LK-99, and the initially reported LK-99 samples might contain magnetic impurities. Moreover, based on the crystal-structure-sensitive electrical properties of Cu2S, we have constructed a piezoelectric-strain-controlled device and obtained a giant and reversible resistance modulation of 2 orders of magnitude at room temperature, yielding a huge gauge factor of 160,000.

Universal material trends in extraordinary magnetoresistive devices

Extraordinary magnetoresistance (EMR) is a geometric magnetoresistance emerging in hybrid systems typically comprising a high-mobility material and a metal. Due to a field-dependent redistribution of electrical currents in these devices, the electrical resistance at room temperature can increase by 107% when applying a magnetic field of 5 T. Although EMR holds considerable potential for realizing sensitive, all-electronic magnetometers, this potential is largely unmet. A key challenge is that the performance of EMR devices depends very sensitively on variations in a vast parameter space where changes in the device geometry and material properties produce widely different EMR performances. The challenge of navigating in the large parameter space is further amplified by the poor understanding of the interplay between the device geometry and material properties. By systematically varying the material parameters in four key EMR geometries using diffusive transport simulations, we here elucidate this interplay with the aim of finding universal guidelines for designing EMR devices. Common to all geometries, we find that the sensitivity scales inversely with the carrier density, while the MR reaches saturation at low carrier densities. Increasing the mobility beyond 20 000 cm2 Vs−1 is required to observe strong EMR effects at 1 T with the optimal magnetoresistance observed for mobilities between 100 000–500 000 cm2 Vs−1. An interface resistance below cm2 between the constituent materials in the hybrid devices was also found to be a prerequisite for very high magnetoresistances in all geometries. By further simulating several high-mobility materials at room and cryogenic temperatures, we conclude that encapsulated graphene and InSb are amongst the most promising candidates for EMR devices showing high magnetoresistance exceeding 107% below 1 T at room temperature. This study paves the way for understanding how to realize EMR devices with record-high magnetoresistance and high sensitivity for detecting magnetic fields.

Enhancing extraordinary magnetoresistance devices through geometric variations of the outer boundary

Magnetometers with a high sensitivity at weak magnetic fields are desirable for a wide range of sensing applications. Devices that operate on the principle of extraordinary magnetoresistance (EMR) are appealing candidates because of their simplicity and ability to operate at room temperature but they suffer from low sensitivity when compared to state-of-the-art magnetometers such as superconducting quantum interference devices. Since the EMR phenomenon is principally a geometric effect, the shapes of the various parts of the device represent additional degrees-of-freedom which can be manipulated in order to modify the performance of the devices. While previous studies have mostly focused on the inner part of the sensor, in this work, we study the effect of systematically manipulating the shape of the outer boundary. We show that the maximum sensitivity of the device can be increased by 70% by placing a constriction between the voltage or current probes and by 300% if the shape of the boundary is shifted from circular to elliptical. We also show that a finite zero-field sensitivity can be obtained if the horizontal symmetry of the device is broken. These results demonstrate that the outer boundary can have a significant effect on device performance, a finding which paves the way for using shape optimization on the outer boundary for designing sensitive magnetometers.

Nontrivial spin textures induced remarkable topological Hall effect and extraordinary magnetoresistance in kagome magnet TmMn6Sn6

Kagome magnets RMn6Sn6 (R = lanthanides) manifest novel topological phases and field-induced transitions, owing to the interplay between strong correlations and geometrical frustration among the corner-sharing-triangle intralayers. Here we report the magnetometry and magnetoelectric transport properties of TmMn6Sn6 single crystal, which coexists with Tm and Mn magnetic sublattices. We reveal that this compound displays multiple magnetic structures with the increase of in-plane magnetic field. The combination of non-collinear magnetic structure and real-space Berry curvature leads to a remarkable topological Hall effect over an extensive temperature range. The in-plane magnetic transition is accompanied by a large magnetoresistance effect of -32 ∼ -17% from 10 K to 300 K at 7 T, and the magnetoresistance manifests two peaks with increasing the in-plane field. These observations suggest a robust Mn - Tm magnetic coupling, which provides a nontrivial ferrimagnetic network for the further investigation of new emergent topological magnetic structures.

Symmetry breaking in magnetoresistive devices

Detecting weak magnetic fields is paramount in areas such as scanning magnetometers and manipulation of magnetic nanoparticles, thus rendering it crucial to increase the weak-field sensitivity for developing next-generation magnetic sensors. The current approaches for high-sensitivity sensors, such as superconducting quantum interference devices, are complex and expensive. By contrast, magnetoresistive sensors and particularly extraordinary magnetoresistive sensors offer a simple operation at room temperature but, to date, at inferior sensitivity. To overcome these challenges, we induce device symmetry breaking to enhance the weak magnetic field sensitivity in semiconductor-metal hybrid structures exhibiting extraordinary magnetoresistance. Retaining the device mirror symmetry yields symmetric magnetoresistance curves with $R(B)=R(\ensuremath{-}B)$, which results in inferior detection of weak magnetic fields as ${[dR/dB]}_{B\ensuremath{\rightarrow}0}=0$. Using finite element modeling, we study the change in device behavior as the symmetry is broken by varying the device geometry by spatially varying the constituent material properties or both. We show that symmetry breaking has three important implications: First, breaking the mirror symmetry causes an asymmetric sensor response with $R(B)\ensuremath{\ne}R(\ensuremath{-}B)$, benefiting from a largely enhanced sensitivity to weak magnetic fields and detection of the magnetic field direction. Second, an interplay with the Hall effect causes a large negative magnetoresistance exceeding 79% at $B=1$ T and room temperature without magnetic constituents or explicit optimization of this property. Third, the asymmetric geometries can be used as a key ingredient toward designing on-demand magnetoresistive characteristics such as linearity at low magnetic fields, step functions, and magnetic switchlike behavior. These implications pave the way for asymmetric topology optimization of magnetoresistive devices with unparalleled performance.

Weyl Semimetal States Generated Extraordinary Quasi‐Linear Magnetoresistance and Nernst Thermoelectric Power Factor in Polycrystalline NbP

AbstractUnsaturated and extremely large magnetoresistance (MR), as well as the giant Nernst effect, are intriguing transport phenomena in Weyl semimetals, which are technically appealing for potential applications in magneto‐electric sensors and transverse thermoelectric conversion. The prominent properties are originated from Weyl semimetal states, i.e., the coexistence of electron and hole pockets combined with linear band dispersion. However, previous studies have been focused on small‐sized single crystals, rendering the practical applications of Weyl semimetals. Here, it is reported an unsaturated, quasi‐linear MR as well as a very large Nernst power factor PFxy in the prepared centimeter‐sized and polycrystalline Weyl semimetal NbP. An extraordinary MR of ≈2 × 104% is observed below 60 K with a magnetic field up to 55 T and persists to elevated temperatures. The unusual quasi‐linear MR behavior is explained by the theory of classical linear MR arising from structural disorder. The polycrystalline NbP exhibits state‐of‐the‐art PFxy that reaches a maximum value of 74.81 μW cm–1 K–2 at 9 T and 220 K, which is 1.5 times larger than its longitudinal thermoelectric power factor PFxx. Given that polycrystalline Weyl semimetal, NbP is suitable for large‐scale production, the results pave the way for its practical applications in magneto‐electric sensors and transverse thermoelectric conversion.

Magnetic sensors and geometrical magnetoresistance: A review

Magnetic sensing devices are of the extremely significant kind of detectors, that are used several important and useful applications. Geometrical extraordinary magnetoresistance (EMR) is the geometrical kind of magnetoresistance associated with the non-magnetic semiconductor-metal hybrid structure and influenced by geometrical shape. As a result of Lorentz force, the current path change from metal (in absence of magnetic field) to semiconductor (under the subjection of the magnetic field) in semiconductor-metal hybrid structure is the key of EMR phenomena, i.e. once the metal is placed in a semiconductor, it works as a short circuit with the majority of applied current moving through metallic inhomogeneity and the almost whole resistance of semiconductor-metal hybrid structure drops to value smaller than that of homogeneous semiconductor in absence of magnetic field, in other hands, applying of magnetic field alters the current route to be around the metallic inhomogeneity where it works as an open circuit and the whole resistance turns into a quite high magnitude relies on the geometrical form of a device. The variables govern these phenomena are metal and semiconductor conductivity, semiconductor charge carriers mobility, and device geometry. Within this review, EMR phenomena history, variables governed it, materials, and applications of EMR devices are overviewed.

Two Intermediate States in Exchange Biased [Co/Pt]2/Co layers and Possible Applications of These States

Two intermediate states in the [Co(1.0 nm)/Pt(tPt)/]2/Co(1.0 nm)/IrMn(12.0 nm) multilayer are investigated as a function of the Pt layer thickness, tPt. Ferromagnetic coupling between the Co layers was dominant when the Pt layer thickness was smaller than 1.6 nm. Because of the exchange bias from IrMn layer and interlayer coupling between the Co layers, symmetric magnetization curves with two intermediate state are revealed when the Pt layer is thicker than 2.0 nm. The spin configurations of the intermediate states are demonstrated by measuring the extraordinary Hall effect and magnetoresistance changes depending on the external magnetic field. We will discuss the details of the structures and the measurements and give possible applications, such as memory, logic, and switching devices.

Exploration of the duality between generalized geometry and extraordinary magnetoresistance

We outline the duality between the extraordinary magnetoresistance (EMR), observed in semiconductor-metal hybrids, and non-symmetric gravity coupled to a diffusive $U(1)$ gauge field. The corresponding gravity theory may be interpreted as the generalized complex geometry of the semi-direct product of the symmetric metric and the antisymmetric Kalb-Ramond field: ($g_{\mu\nu}+\beta_{\mu\nu}$). We construct the four dimensional covariant field theory and compute the resulting equations of motion. The equations encode the most general form of EMR within a well defined variational principle, for specific lower dimensional embedded geometric scenarios. Our formalism also reveals the emergence of additional diffusive pseudo currents for a completely dynamic field theory of EMR. The proposed equations of motion now include terms that induce geometrical deformations in the device geometry in order to optimize the EMR. This bottom-up dual description between EMR and generalized geometry/gravity lends itself to a deeper insight into the EMR effect with the promise of potentially new physical phenomena and properties.

Extraordinary magnetoresistance in encapsulated monolayer graphene devices

We report a proof-of-concept study of extraordinary magnetoresistance (EMR) in devices of monolayer graphene encapsulated in hexagonal boron nitride having metallic edge contacts and a central metal shunt. Extremely large EMR values, MR=(R(B)−R0)/R0∼105, are achieved in part because R0 approaches or crosses zero as a function of the gate voltage, exceeding that achieved in high mobility bulk semiconductor devices. We highlight the sensitivity, dR/dB, which in two-terminal measurements is the highest yet reported for EMR devices and in particular exceeds previous results in graphene-based devices by a factor of 20. An asymmetry in the zero-field transport is traced to the presence of pn-junctions at the graphene-metal shunt interface.

Planar configuration of extraordinary magnetoresistance for 2D-material-based magnetic field sensors

Nanoscale-resolution magnetic field sensing plays a significant role in fundamental research in a wide range of fields, from material science to biology. Magnetoresistors in particular offer simplicity of construction and easy adaptation for fabrication based on two-dimensional materials, together with the ability to detect small and medium magnetic fields. Among the wide range of magnetoresistance effects, extraordinary magnetoresistance is unique because it does not require a magnetic material as part of its structure, which may be advantageous in certain applications. In this paper we demonstrate that a recently proposed planar configuration of extraordinary magnetoresistance is naturally suited to the fabrication of devices based on two-dimensional materials. Several operating magnetoresistors have been made for this purpose, using epitaxial mono- and bilayer graphene as well as Bi2Se3 microflakes. Besides the implementation of the extraordinary magnetoresistance in a planar configuration, we demonstrate experimentally the consequences of geometry optimization for future applications. Using the finite element method to simulate the extraordinary magnetoresistance effect in a specific hybrid structure containing a 2D sheet, our research goes beyond the experimental framework, and the efficiency of the applied optimization method can be compared to the best results achieved so far in the field.

Extraordinary Magnetoresistance in Janus Monolayer MoTeB2: A Theoretical Prediction

By means of first-principles calculations, we investigated the geometric structure, dynamic and thermal stabilities, and electronic properties of the two-dimensional (2D) Janus group III chalcogenide monolayer MoTeB2. The MoTeB2 monolayer exhibits a stable sandwiched structure, and its semimetal electronic structure features the perfect electron–hole compensation. The 1:1 electron–hole carrier ratio and high carrier mobility endow the MoTeB2 monolayer with large and nonsaturating magnetoresistance. Its electronic properties are easily adjustable by minute charge doping and small tensile stains; in particular, the switch of carrier polarity and metal–semiconductor phase transformation can be achieved. This study not only leads to the finding of the Janus MoTeB2 monolayer as a promising 2D material with extraordinary magnetoresistance but also provides a general route to adjust the magnetoresistance effect by compressive stain and charge doping.

Raman scattering study of large magnetoresistance semimetalsTaAs2andNbAs2

We have performed polarized and temperature-dependent Raman scattering measurements on extremely large magnetoresitance compounds TaAs$_2$ and NbAs$_2$. In both crystals, all the Raman active modes, including six A$_g$ modes and three B$_g$ modes, are clearly observed and well assigned with the combination of symmetry analysis and first-principles calculations. The well-resolved periodic intensity modulations of the observed modes with rotating crystal orientations, verify the symmetry of each assigned mode and are fitted to experimentally determine the elements of Raman tensor matrixes. The broadening of two A$_g$ modes seen in both compounds allows us to estimate electron-phonon coupling constant, which suggests a relatively small electron-phonon coupling in the semimetals TaAs$_2$ and NbAs$_2$. The present study provides the fundamental lattice dynamics information on TaAs$_2$ and NbAs$_2$ and may shed light on the understanding of their extraordinary large magnetoresistance.

High-sensitivity two-terminal magnetoresistance devices using InGaAs/AlGaAs two-dimensional channel on GaAs substrate

We demonstrate experimentally the two-terminal magnetic sensors exhibiting an extraordinary magneto-resistance effect by using an InGaAs quantum well channel with a metal-shunting structure. A high magneto-resistance of 17.3% and a sensitivity of 488.1 Ω/T have been obtained at 1 T and room temperature with our geometrical design. The two-contact configuration and the high-mobility electron transistor-compatible epitaxy structure make the devices promising for high-sensitivity magnetic sensing integration and applications.

Extraordinary Hall resistance and unconventional magnetoresistance inPt/LaCoO3hybrids

We report an investigation of transverse Hall resistance and longitudinal resistance on Pt thin films sputtered on epitaxial LaCoO$_3$ (LCO) ferromagnetic insulator films. The LaCoO$_3$ films were deposited on several single crystalline substrates [LaAlO$_3$ (LAO), (La,Sr)(Al,Ta)O$_3$ (LSAT), and SrTiO$_3$ (STO)] with (001) orientation. The physical properties of LaCoO$_3$ films were characterized by the measurements of magnetic and transport properties. The LaCoO$_3$ films undergo a paramagnetic to ferromagnetic (FM) transition at Curie temperatures ranging from 40 K to 85 K, below which the Pt/LCO hybrids exhibit significant extraordinary Hall resistance (EHR) up to 50 m$\Omega$ and unconventional magnetoresistance (UCMR) ratio $\Delta$$\rho$/$\rho_0$ about $1.2 \times 10^{-4}$, accompanied by the conventional magnetoresistance (CMR). The observed spin transport properties share some common features as well as some unique characteristics when compared with well-studied Y$_3$Fe$_5$O$_{12}$-based Pt thin films. Our findings call for new theories since the extraordinary Hall resistance and magnetoresistance cannot be consistently explained by the existing theories.

Superconducting magnetoresistance in ferromagnet/superconductor/ferromagnet trilayers.

Magnetoresistance is a multifaceted effect reflecting the diverse transport mechanisms exhibited by different kinds of plain materials and hybrid nanostructures; among other, giant, colossal, and extraordinary magnetoresistance versions exist, with the notation indicative of the intensity. Here we report on the superconducting magnetoresistance observed in ferromagnet/superconductor/ferromagnet trilayers, namely Co/Nb/Co trilayers, subjected to a parallel external magnetic field equal to the coercive field. By manipulating the transverse stray dipolar fields that originate from the out-of-plane magnetic domains of the outer layers that develop at coercivity, we can suppress the supercurrent of the interlayer. We experimentally demonstrate a scaling of the magnetoresistance magnitude that we reproduce with a closed-form phenomenological formula that incorporates relevant macroscopic parameters and microscopic length scales of the superconducting and ferromagnetic structural units. The generic approach introduced here can be used to design novel cryogenic devices that completely switch the supercurrent ‘on’ and ‘off’, thus exhibiting the ultimate magnetoresistance magnitude 100% on a regular basis.

Perfect charge compensation in WTe2 for the extraordinary magnetoresistance: From bulk to monolayer

The electronic structures of the WTe2 bulk and layers are investigated by using the first-principles calculations. The perfect electron-hole charge compensation and high carrier mobilities are found in the WTe2 bulk, which may result in the large and non-saturating magnetoresistance (MR) observed very recently in the experiment (Ali M. N. et al, Nature, 514 (2014) 205). The monolayer and bilayer of WTe2 preserve the semimetallic property, with equal hole and electron carrier concentrations. Moreover, very high carrier mobilities are also found in WTe2 monolayer, indicating that the WTe2 monolayer would have the same extraordinary MR effect as the bulk, which could have promising applications in nanostructured magnetic devices.

Modeling the planar configuration of extraordinary magnetoresistance

Recently the planar version of the extraordinary magnetoresistance (EMR) magnetic field sensor has been constructed and verified in practice. Planar configuration of the EMR device gives many technological advantages, it is simpler than the classic and allows one to build the sensor using electric materials of the new type (such as graphene or topological insulators) much easier. In this work the planar configuration of the EMR sensor is investigated by performing computational simulations using the finite element method (FEM). The computational comparison of the planar and classic configurations of EMR is presented using three-dimensional models. Various variants of the geometry of EMR sensor components are pondered and compared in the planar and classic version. Size of the metal overlap is considered for sensor optimization as well as various semiconductor-metal contact resistance dependences of the EMR signal. Based on computational simulations, a method for optimal placement of electric terminals in a planar EMR device is proposed.

Planar Structure Optimization of Extraordinary Magnetoresistance in Semiconductor–Metal Hybrids

The performance of extraordinary magnetoresistance (EMR) depends on the material parameters, hybrid structure design, contacts configuration, and manufacturing technology. In this paper, we pay close attention to the hybrid structure of the EMR device. A semiconductor–metal hybrid model based on the finite element method (FEM) is constructed to study the EMR effect, and the results show good agreement with the experimental data. The analysis of the van der Pauw plate structure indicates that the relationship between the two voltage probe contacts and the different EMR structures is the key factor to the design optimization. Accordingly, we find that the elliptic inclusion configurations improve the performance of the van der Pauw structure of EMR devices within a wide range of applied magnetic field (0–5 T). The bar-type and multibranched inclusion structures are subsequently optimized based on this principle. The new structures show excellent performance; more specifically, the modified multibranched inclusion structure displays a 2-fold increase in the magnetoresistance at 0.1 T and more than 2-order-of-magnitude increase at 5 T when compared with the original structure.