Extraordinary Magnetoresistance Effect Research Articles

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.

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.

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.

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.

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.

ChemInform Abstract: Extraordinary Magnetoresistance in Semiconductor/Metal Hybrids: A Review

AbstractReview: 51 refs.

Extraordinary Magnetoresistance in Semiconductor/Metal Hybrids: A Review.

The Extraordinary Magnetoresistance (EMR) effect is a change in the resistance of a device upon the application of a magnetic field in hybrid structures, consisting of a semiconductor and a metal. The underlying principle of this phenomenon is a change of the current path in the hybrid structure upon application of a magnetic field, due to the Lorentz force. Specifically, the ratio of current, flowing through the highly conducting metal and the poorly conducting semiconductor, changes. The main factors for the device’s performance are: the device geometry, the conductivity of the metal and semiconductor, and the mobility of carriers in the semiconductor. Since the discovery of the EMR effect, much effort has been devoted to utilize its promising potential. In this review, a comprehensive overview of the research on the EMR effect and EMR devices is provided. Different geometries of EMR devices are compared with respect to MR ratio and output sensitivity, and the criteria of material selection for high-performance devices are discussed.

Influence of Semiconductor/Metal Interface Geometry in an EMR Sensor

The extraordinary magnetoresistance (EMR) is well known to be strongly dependent on geometric parameters. While the influence of the aspect ratios of the metal and semiconductor areas has been thoroughly investigated, the geometry of the semiconductor/metal interface has been neglected so far. However, from a fabrication point of view, this part plays a crucial role. In this paper, the performance of a bar-type hybrid EMR sensor is investigated by means of finite element method and experiments with respect to the hybrid interface geometry. A 3-D model has been developed, which simulates the EMR effect in case of fields in different directions. The semiconductor/metal interface has been investigated in terms of different layer thicknesses and overlaps. The results show that those parameters can cause a change in the output sensitivity of 2%-10%. In order to maintain a high sensitivity and keep the fabrication relatively simple and at low cost, a device with a thin metal shunt having a large overlap on the top of the semiconductor bar would provide the best solution.

Extraordinary magnetoresistance: sensing the future

AbstractSimulations utilising the finite element method (FEM) have been produced in order to investigate aspects of circular extraordinary magnetoresistance (EMR) devices. The effect of three specific features on the resultant magnetoresistance were investigated: the ratio of the metallic to semiconducting conductivities (σ M/σ S); the semiconductor mobility; and the introduction of an intermediate region at the semiconductormetal interface in order to simulate a contact resistance. In order to obtain a large EMR effect the conductivity ratio (σ M/σ S) is required to be larger than two orders of magnitude; below this critical value the resultant magnetoresistance effect is dramatically reduced. Large mobility semiconductors exhibit larger EMR values for a given field (below saturation) and reduce the magnetic field required to produce saturation of the magnetoresistance. This is due to a larger Hall angle produced at a given magnetic field and is consistent with the mechanism of the EMR effect. Since practical magnetic field sensors are required to operate at low magnetic fields, high mobility semiconductors are required in the production of more sensitive EMR sensors. The formation of a Schottky barrier at the semiconductor-metal interface has been modelled with the introduction of a contact resistance at the semiconductor-metal interface. Increasing values of contact resistance are found to reduce the EMR effect with it disappearing altogether for large values. This has been shown explicitly by looking at the current flow in the system and is consistent with the mechanism of the EMR effect. The interface resistance was used to fit the simulated model to existing experimental data. The best fit occurred with an interface with resistivity of 1.55×10−4 m (overestimate). The EMR effect holds great potential with regard to its future application to magnetic field sensors. The design of any such devices should incorporate high mobility materials (such as graphene) along with the specific features presented in this paper in order to produce effective magnetic field sensors.

Finite Element Analysis on the Influence of Contact Resistivity in an Extraordinary Magnetoresistance Magnetic Field Micro Sensor

In this paper, an extraordinary magnetoresistance (EMR) device made of an InSb/Au hybrid structure was investigated. Those devices have a large potential in becoming a new generation of highly sensitive and cheap magnetic micro sensors. A crucial factor for the performance is the interface between the InSb and Au, which suffers from a certain contact resistivity. The Finite Element Method (FEM) was employed to simulate the current redistribution in the device, under an applied magnetic field. Specifically, the influence of the contact resistivity between the InSb bulk and Au shunt was studied. In a device with optimized geometry and without contact resistivity between the layers of InSb and Au, the EMR effect and the sensitivity show values of 1.89×104% and 0.02%/(10−4 T), respectively, at 1 Tesla. For values of contact resistivity up to 10−8 Ω cm2 the EMR effect is almost constant, while for higher values the EMR effect decreases exponentially. However, the sensitivity of the device does not decrease until 5×10−6 Ω cm2 of contact resistivity. Only beyond this value the sensitivity, which in most cases is associated with the performance of the device, will deteriorate.

Extraordinary magnetoresistance in shunted chemical vapor deposition grown graphene devices

We report gate tunable linear magnetoresistances (MRs) of ∼600% at 12 T in metal-shunted devices fabricated on chemical vapor deposition (CVD) grown graphene. The effect occurs due to decreasing conduction through the shunt as the magnetic field increases (known as the extraordinary magnetoresistance effect) and yields an MR that is at least an order-of-magnitude higher than in un-shunted graphene devices.

Extraordinary magnetoresistance in nonmagnetic semiconductors: The effective-medium approximation

The self-consistent effective-medium approximation is used to study the extraordinary magnetoresistance effect observed in nonmagnetic semiconductors. The inhomogeneous materials are treated as a three-dimensional resistor network of binary disorder, where the receptivity of each component is a tensor describing both the zero-field resistance and the Hall effect. The effective conductivity tensor of the total system is calculated with applied magnetic field. The resulting transversal magnetoresistance, longitudinal magnetoresistance and effective Hall coefficient are shown for different component concentrations and magnetic fields. When the components have two different types of charge carriers, and the mismatch between the zero-resistivity is enlarged, the macroscopic magnetoresisance exhibits complex behaviors which are related closely with the formation of the percolation structure in the inhomogeneous system.

Ordinary and extraordinary Coulomb blockade magnetoresistance in a (Ga, Mn)As single electron transistor

In the conventional Ohmic regime, magnetoresistance effects comprise the ordinary responses to the external magnetic field and extraordinary responses to the internal magnetization. Here we study magnetoresistance effects in the Coulomb blockade regime using a ferromagnetic (Ga, Mn)As single electron transistor. We report measurements of the magneto-Coulomb blockade effect due to the direct coupling of high external magnetic fields and the Coulomb blockade anisotropic magnetoresistance associated with magnetization rotations in the ferromagnet. The latter, extraordinary magnetoresistance effect is characterized by low-field hysteretic magnetoresistance which can exceed three orders of magnitude. The sign and size of this magnetoresistance signal is controlled by the gate voltage, and the data are interpreted in terms of anisotropic electrochemical shifts induced by magnetization reorientations. Non-volatile transistor-like applications of the Coulomb blockade anisotropic magnetoresistance are briefly discussed.

Enhanced extraordinary magnetoresistance in the semiconductor-metal hybrid structure with three current leads

Earlier researches show that nonmagnetic semiconductor-metal hybrid structures exhibit a very large magnetoresistance effect, the so-called extraordinary magnetoresistance effect. Here, we designed a modified semiconductor-metal hybrid device with IVIVI configuration, where I and V represent current lead and voltage probe, respectively. In this device, applied magnetic field can lead to the current redistribution between the two output current leads. The change of the output currents reaches 62.4% under magnetic field of 5T. As a result, the magnetoresistance value is 2.4–3.7 times higher than that of the traditional semiconductor-metal hybrid device with IVVI configuration. The sensitive position dependence of magnetic bit on output current shows that this device could be potentially used as magnetic sensor.

Geometry-enhanced magnetoresistance of narrow Au∕InAs hybrid structures incorporating a two-dimensional electron system

We have investigated the magnetoresistance of metal-semiconductor hybrid structures at 4.2 K. The devices consisted of polycrystalline Au and an InAs-based heterostructure that hosted a high-mobility two-dimensional electron system. We have varied the electrical parameters and, in particular, the width-to-length ratio W∕L of the linear hybrid structures. The recently discovered extraordinary magnetoresistance effect is most pronounced for narrow devices with W∕L⩽0.05, consistent with our theoretical prediction. Relative resistance changes with a factor of 1000 are observed at 1 T. To achieve this an interface resistance of only 10−8Ωcm2 is a prerequisite. This can be prepared routinely by in situ cleaved edge overgrowth.

Semiconductor-metal hybrid structures as local magnetic-field probes: Magnetoresistance and spatial sensitivity profile

Tailored nonmagnetic semiconductor-metal hybrid structures exhibit a large magnetoresistance effect in a homogeneous magnetic field. This is the so-called extraordinary magnetoresistance effect. Here, we study numerically the magnetoresistance of such hybrid structures in the inhomogeneous field of a magnetic dot. Surprisingly, the four-point resistance R versus magnetic field B changes its symmetry if compared to the case of a homogeneous field and is strongly dependent on the position of the local magnetic field. Interestingly, the active device area is not defined by the voltage probe separation, but by the positioning of voltage probes and current leads. We find a magnetoresistance effect as large as 18% although only 1∕60 of the device area is subject to a small magnetic field of ±50mT. These results are promising for sensing magnetic-field distributions in the nanoscale regime such as the stray fields of magnetic recording media.

Geometry and material optimization of the extraordinary magnetoresistance in the semiconductor–metal hybrid structure

The effects of geometry and material parameters on the extraordinary magnetoresistance (EMR) of the rectangular semiconductor–metal hybrid structure have been studied systematically using the finite-element method (FEM). We find that the EMR depends sensitively on the placement of the voltage probes and explain the origin of the EMR enhancement in the asymmetric voltage probe configuration. The width of the metal is important for the EMR effect as well as the width of the semiconductor. The low-field EMR shows an approximate quadratic with the mobility of semiconductor, while the high-field EMR gradually saturates with the increase of mobility due to the little change of hall angle. To obtain significant EMR effect, the ration of conductivity of metal and semiconductor should be larger than 10 4 .

Low-noise magnetic-flux sensors based on the extraordinary magnetoresistance effect

We report noise measurements on Au–InAs hybrid structures involving a high-mobility two-dimensional electron system. Such structures show the extraordinary magnetoresistance (EMR) effect. We find excellent noise performance at room temperature close to the Johnson noise, which is in particular important for a technical application. At 4.2 K and in a magnetic field of about 1 T the nonoptimized EMR device is found to exhibit a low magnetic flux noise, offering the perspective of sensor applications also in a high magnetic field and at cryogenic temperature.

Optimization of the extraordinary magnetoresistance in semiconductor–metal hybrid structures for magnetic-field sensor applications

Semiconductor–metal hybrid structures can exhibit a very large geometrical magnetoresistance effect, the so-called extraordinary magnetoresistance (EMR) effect. In this paper, we analyze this effect by means of a model based on the finite element method and compare our results with the experimental data. In particular, we investigate the important effect of the contact resistance ρ c between the semiconductor and the metal on the EMR effect. Introducing a realistic ρ c =3.5×10 −7 Ω cm 2 in our model we find that at room temperature this reduces the EMR by 30% if compared to an analysis where ρ c is not considered.

Optimization of semiconductor–metal hybrid structures for application in magnetic-field sensors and read heads

Semiconductor–metal hybrid structures can exhibit a very large geometrical magnetoresistance effect, the so-called extraordinary magnetoresistance (EMR) effect. Using the finite element method, we study the EMR effect in rectangular semiconductor–metal hybrid structures and investigate the effects of material parameters and of device geometry. We find that the EMR device exhibits inverse scalability, i.e., the output characteristics improve with decreasing device width. This is promising for miniaturized magnetic-field sensors like, e.g., read heads. Using realistic device parameters, we predict an optimized performance as a sensor for a width-to-length ratio of 0.025.