Large Positive Magnetoresistance Research Articles

Transversal magnetotransport in Weyl semimetals: Exact numerical approach

Magnetotransport experiments on Weyl semimetals are essential for investigating the intriguing topological and low-energy properties of Weyl nodes. If the transport direction is perpendicular to the applied magnetic field, experiments have shown a large positive magnetoresistance. In this work, we present a theoretical scattering matrix approach to transversal magnetotransport in a Weyl node. Our numerical method confirms and goes beyond the existing perturbative analytical approach by treating disorder exactly. It is formulated in real space and is applicable to mesoscopic samples as well as in the bulk limit. In particular, we study the case of clean and strongly disordered samples.

Large positive magnetoresistance of graphene at room temperature in magnetic fields up to 0.5 T

Graphene films were grown on an iron film catalyst deposited on oxidized silicon substrate, using the low-pressure chemical vapor deposition. Subsequently the iron film was dissolved with an aqueous solution of iron nitrate. The structures consisting of quasi-suspended graphene on reaction products of an iron film with iron nitrate were obtained. Magnetoresistance properties of the structures were investigated at a room temperature, and the positive magnetoresistance was observed. The maximum magnetoresistance value was 100% in the magnetic field of 0.5T. The dependence of magnetoresistance on the magnetic field was quasi-linear in the range of 0.07–0.5T.

Large positive magnetoresistance in semiconducting single-walled carbon nanotubes at room temperature.

We have investigated the effect of a magnetic field on the resistance (magnetoresistance, MR) of single-walled carbon nanotube (SWNT) arrays. The SWNT devices consist of a mixture of metallic and semiconducting SWNTs between palladium electrodes. The MR of the devices is studied at room temperature and in the presence of perpendicular magnetic fields up to 0.24 tesla. The resistance increases as the external magnetic field becomes higher, suggesting a positive MR of SWNTs. After etching the metallic SWNTs by electrical breakdown, the MR can be further enhanced. Large positive MR values about 15%, 20% and 25% were found in three different devices at 0.24 tesla for semiconducting SWNTs at room temperature. Our results show potential for the development of magneto-electronic devices that are operable at room temperature.

High field (up to 140 kOe) angle dependent magneto transport of Bi2Te3 single crystals

We report the angle dependent high field (up to 140 kOe) magneto transport of Bi2Te3 single crystals, a well-known topological insulator. The crystals were grown from melt of constituent elements via solid state reaction route by self-flux method. Details of crystal growth along with their brief characterisation up to 5 Tesla applied field was reported by some of us recently (Sultana et al 2017 J. Magn. Magn. Mater. 428 213). The angle dependence of the magneto-resistance (MR) of Bi2Te3 follows the cos (θ) function i.e., MR is responsive, when the applied field is perpendicular (tilt angle θ = 0° and/or 180°) to the transport current. The low field (±10 kOe) MR showed the signatures of weak anti localisation character with typical ν-type cusp near origin at 5 K. Further, the MR is linear right up to highest applied field of 140 kOe. The large positive MR are observed up to high temperatures and are above 250% and 150% at 140 kOe in perpendicular fields at 50 K and 100 K respectively. Heat capacity CP(T) measurements revealed the value of Debye temperature (ѲD) to be 135 K. Angle resolved photoemission spectroscopy data clearly showed that the bulk Bi2Te3 single crystal consists of a single Dirac cone.

Magnetoresistance and robust resistivity plateau in MoAs2

We have grown the MoAs2 single crystal which crystallizes in a monoclinic structure with C2/m space group. Transport measurements show that MoAs2 displays a metallic behavior at zero field and undergoes a metal-to-semiconductor crossover at low temperatures when the applied magnetic field is over 5 T. A robust resistivity plateau appears below 18 K and persists for the field up to 9 T. A large positive magnetoresistance (MR), reaching about 2600% at 2 K and 9 T, is observed when the field is perpendicular to the current. The MR becomes negative below 40 K when the field is rotated to be parallel to the current. The Hall resistivity shows the non-linear field-dependence below 70 K. The analysis using two-band model indicates a compensated electron-hole carrier density at low temperatures. A combination of the breakdown of Kohler’s rule, the abnormal drop and the cross point in Hall data implies that a possible Lifshitz transition has occurred between 30 K and 60 K, likely driving the compensated electron-hole density, the large MR as well as the metal-semiconductor transition in MoAs2. Our results indicate that the family of centrosymmetric transition-metal dipnictides has rich transport behavior which can in general exhibit variable metallic and topological features.

Weak antilocalization effect due to topological surface states in Bi2Se2.1Te0.9

We have investigated the weak antilocalization (WAL) effect in the p-type Bi2Se2.1Te0.9 topological system. The magnetoconductance shows a cusp-like feature at low magnetic fields, indicating the presence of the WAL effect. The WAL curves measured at different tilt angles merge together when they are plotted as a function of the normal field components, showing that surface states dominate the magnetoconductance in the Bi2Se2.1Te0.9 crystal. We have calculated magnetoconductance per conduction channel and applied the Hikami-Larkin-Nagaoka formula to determine the physical parameters that characterize the WAL effect. The number of conduction channels and the phase coherence length do not change with temperature up to T = 5 K. In addition, the sample shows a large positive magnetoresistance that reaches 1900% under a magnetic field of 35 T at T = 0.33 K with no sign of saturation. The magnetoresistance value decreases with both increasing temperature and tilt angle of the sample surface with respect to the magnetic field. The large magnetoresistance of topological insulators can be utilized in future technology such as sensors and memory devices.

Enhancement in power factor due to anti-correlation between electrical conductivity and thermoelectric power and induced magnetic ordering in high mobility Zn doped Bi2Te3 topological insulator

Electrical resistivity, thermoelectric power, magnetotransport and magnetization of Zn doped Bi2Te3 Topological Insulator were studied. Electrical conductivity is enhanced at higher Zn concentration, and the carrier mobility estimated from Hall data reaches a remarkable value of ∼7200 cm2 V-1S−1. Large positive magnetoresistance (MR∼400%) is observed in high mobility samples. Interestingly it is found that the coupling between electrical conductivity and Seebeck coefficient is broken for higher Zn doped Bi2Te3 samples which effectively enhances the thermoelectric power factor (from 2.1mW/K2m for Bi2Te3 to 4.64mW/K2m for Zn doped Bi2Te3).

Anisotropic large magnetoresistance in TaTe4 single crystals

Strong anisotropic magnetotransport is reported in high-quality TaTe4 single crystals synthesized by flux methods. Large positive magnetoresistance (MR) and field-induced metal-semiconductor-like transition are observed at low temperatures with B perpendicular to c axis. The MR value reaches 3200% in 9 T at 2 K with B parallel to a axis, contrast to 79% for B along c axis. Angle dependent magnetoresistance with B rotated within ab plane displays eightfold symmetry and pronounced Shubnikov-de Haas (SdH) oscillations at low temperatures. The analysis of angle dependent resistivity, Hall effect and observed SdH oscillations suggest the high mobile electron and anisotropic Fermi surface responsible for the large anisotropic MR in TaTe4.

Tuning magnetoresistance in molybdenum disulphide and graphene using a molecular spin transition

Coupling spins of molecular magnets to two-dimensional (2D) materials provides a framework to manipulate the magneto-conductance of 2D materials. However, with most molecules, the spin coupling is usually weak and devices fabricated from these require operation at low temperatures, which prevents practical applications. Here, we demonstrate field-effect transistors based on the coupling of a magnetic molecule quinoidal dithienyl perylenequinodimethane (QDTP) to 2D materials. Uniquely, QDTP switches from a spin-singlet state at low temperature to a spin-triplet state above 370 K, and the spin transition can be electrically transduced by both graphene and molybdenum disulphide. Graphene-QDTP shows hole-doping and a large positive magnetoresistance ( ~ 50%), while molybdenum disulphide-QDTP demonstrates electron-doping and a switch to large negative magnetoresistance ( ~ 100%) above the magnetic transition. Our work shows the promise of spin detection at high temperature by coupling 2D materials and molecular magnets.

Effect of spin fluctuations in magnetocaloric and magnetoresistance properties of Dy10Co20Si70 alloy

Systematic investigations on the structure, magnetic, thermodynamic, magnetocaloric and magnetoresistance (MR) properties of the arc melted Dy10Co20Si70 alloy are presented. The Dy10Co20Si70 alloy crystallizes in tetragonal BaNiSn3-type DyCoSi3 (space group = I4mm; No. 107) as a major phase and CaF2-type CoSi2 (space group = Fm-3m; No. 225) and C-type Si (space group = Fd-3m; No. 227) as minor phases. The title compound exhibits multiple magnetic transitions having antiferromagnetic ordering at temperatures, viz., T1 = 10.8 K, T2 = 8.8 K and T3 = 3.3 K. The magnetic and thermodynamic studies confirm these magnetic anomalies in the compound. The large value of maximum magnetic entropy change, −ΔSMMax = 16.4 and 26.6 J/kg K for the field change ΔH of 50 and 90 kOe, respectively, observed in the compound is associated with field induced magnetic transitions. Asymmetric broadening of the magnetic entropy change peaks above the ordering temperatures resulting in significant refrigerant capacities of 361 and 868 J/kg for ΔH = 50 and 90 kOe, respectively, in the compound is due to the spin fluctuation effect. The sign reversal in MR measurements is attributed to the field induced antiferromagnetic to ferromagnetic transition. A large positive MR (42% in 90 kOe) is observed at 2 K. The H2 dependence of both the magnetocaloric effect (MCE) and MR in the paramagnetic regime indicates the role of the applied magnetic field in suppressing the spin fluctuations. The large MCE and MR together with no thermal or magnetic hysteresis establish this new compound as an attractive multifunctional magnetic material.

Electrically tunable large magnetoresistance in graphene/silicon Schottky junctions

It should be a promising paradigm for graphene application in semiconductor industry by incorporating graphene into silicon to improve the behavior of silicon-based devices or develop high-performance devices with a new physical mechanism. Here we report on a large positive magnetoresistance (MR) over 80% at a magnetic field of 2.2 T and a temperature of 80 K in graphene/Si Schottky junctions by stacking chemical vapor deposition derived monolayer graphene on silicon. The produced MR is anisotropic and dependent on the angle between the direction of the magnetic field and the configuration plane, and can be modulated by the electrical bias across the configuration due to the built-in electric field. The MR effect should be ascribed to the charge carriers scattering and the released silicon magnetic moments in the graphene/Si interface that is suggested by first principles calculations. The study here should be helpful to understand the interface between the graphene and silicon, develop high-performance silicon based devices, complement the extraordinary properties of graphene and open one possible way to exploit their application for magneto-electronics.

Angle-dependent magnetoresistance and quantum oscillations in high-mobility semimetal LuPtBi

The recent discovery of ultrahigh mobility and large positive magnetoresistance in the topologically non-trivial half-Heusler semimetal LuPtBi provides a unique playground for studying exotic physics and significant perspective for device applications. As an fcc-structured electron–hole-compensated semimetal, LuPtBi theoretically exhibits six symmetrically arranged anisotropic electron Fermi pockets and two nearly-spherical hole pockets, offering the opportunity to explore the physics of Fermi surfaces with simple angle-related magnetotransport properties. In this work, through angle-dependent transverse magnetoresistance measurements, in combination with high-field SdH quantum oscillations, we aimed to map out a Fermi surface with six anisotropic pockets in the high-temperature and low-field regime, and furthermore, identify a possible magnetic field driven Fermi surface change at lower temperatures. Reasons account for the Fermi surface change in LuPtBi are discussed in terms of the field-induced electron evacuation due to Landau quantization.

Two-carrier analyses of the transport properties of black phosphorus under pressure

We report on the electronic transport properties of black phosphorus and analyze them using a two-carrier model in a wide range of pressure up to 2.5 GPa. In a semiconducting state at 0.29 GPa, the remarkable nonlinear behavior in the Hall resistance is reasonably reproduced by assuming the coexistence of two kinds of holes with different densities and mobilities. On the other hand, two-carrier analyses of the magnetotransport properties above 1.01 GPa suggest the coexistence of high-mobility electron and hole carriers that have almost the same densities, i.e., nearly compensated semimetallic nature of black phosphorus. In the semimetallic state, analyses of both the two-carrier model and quantum oscillations indicate a systematic increase in the carrier densities as pressure increases. An observed sign inversion of Hall resistivity at low magnetic fields suggests the existence of high-mobility electrons $(\ensuremath{\sim}{10}^{5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$, which is roughly ten times larger than that of holes, in the semimetallic black phosphorus. We conclude that the extremely large positive magnetoresistance that has been observed in the semimetallic state cannot be reproduced by a conventional two-carrier model.

Large positive in-plane magnetoresistance induced by localized states at nanodomain boundaries in graphene

Graphene supports long spin lifetimes and long diffusion lengths at room temperature, making it highly promising for spintronics. However, making graphene magnetic remains a principal challenge despite the many proposed solutions. Among these, graphene with zig-zag edges and ripples are the most promising candidates, as zig-zag edges are predicted to host spin-polarized electronic states, and spin–orbit coupling can be induced by ripples. Here we investigate the magnetoresistance of graphene grown on technologically relevant SiC/Si(001) wafers, where inherent nanodomain boundaries sandwich zig-zag structures between adjacent ripples of large curvature. Localized states at the nanodomain boundaries result in an unprecedented positive in-plane magnetoresistance with a strong temperature dependence. Our work may offer a tantalizing way to add the spin degree of freedom to graphene.

Origin of the large positive magnetoresistance of Ge1−xMnx granular thin films

GeMn granular thin films are a unique and promising material for spintronics applications due to large positive magnetoresistance (MR). Previous studies on GeMn have suggested that the large MR is related to nanospinodal decomposition of GeMn into Mn-rich ferromagnetic nanoparticles and Mn-poor paramagnetic matrix. However, its microscopic origin of the MR has not been clarified yet. Here, using X-ray magnetic circular dichroism (XMCD), which is extremely sensitive to the local magnetic state of each atom, we investigate the magnetic properties of the nanoparticles and the matrix in GeMn separately. We find that the MR ratio is proportional to the product of the magnetizations originating from the nanoparticles and the matrix. This result indicates that spin-polarized holes in the nanoparticles penetrate into the matrix and that these holes undergo spin-disorder magnetic scattering by the paramagnetic Mn atoms in the matrix, which induces the large MR.

Large, Linear, and Tunable Positive Magnetoresistance of Mechanically Stable Graphene Foam-Toward High-Performance Magnetic Field Sensors.

Here, we present the first observation of magneto-transport properties of graphene foam (GF) composed of a few layers in a wide temperature range of 2-300 K. Large room-temperature linear positive magnetoresistance (PMR ≈ 171% at B ≈ 9 T) has been detected. The largest PMR (∼213%) has been achieved at 2 K under a magnetic field of 9 T, which can be tuned by the addition of poly(methyl methacrylate) to the porous structure of the foam. This remarkable magnetoresistance may be the result of quadratic magnetoresistance. The excellent magneto-transport properties of GF open a way toward three-dimensional graphene-based magnetoelectronic devices.

Self-magnetism induced large magnetoresistance at room temperature region in graphene nanocrystallited carbon film

We report large positive magnetoresistance (MR) of over 12% at 273 K in graphene nanocrystallited pure carbon film. MR behaviors at different temperatures implied that low temperature MR was from carrier diffusive scattering and room temperature MR was from spin arrangement effect. Temperature dependences of the film resistance and magnetization recognized that as temperature decreased from 300 to 200 K, transitions occurred on the electrical transporting process from conductive mode to semi-conductive mode, and the nanocrystallited structure showed competition of ferromagnetic and antiferromagnetic interactions. The large room temperature MR was ascribed to the ferromagnetic order of spin magnetic moment arrangement at the of graphene layer edges.

Large magnetoresistance of insulating silicon films with superconducting nanoprecipitates

We report on large negative and positive magnetoresistance in inhomogeneous, insulating Si:Ga films below a critical temperature of about 7 K. The magnetoresistance effect exceeds 300 % at temperatures below 3 K and fields of 8 T. The comparison of the transport properties of superconducting samples with that of insulating ones reveals that the large magnetoresistance is associated with the appearance of local superconductivity. A simple phenomenological model based on localized Cooper pairs and hopping quasiparticles is able to describe the temperature and magnetic field dependence of the sheet resistance of such films.

Pressure-induced Td to 1T′ structural phase transition in WTe2

WTe2 is provoking immense interest owing to its extraordinary properties, such as large positive magnetoresistance, pressure-driven superconductivity and possible type-II Weyl semimetal state. Here we report results of high-pressure synchrotron X-ray diffraction (XRD), Raman and electrical transport measurements on WTe2. Both the XRD and Raman results reveal a structural transition upon compression, starting at 6.0 GPa and completing above 15.5 GPa. We have determined that the high-pressure lattice symmetry is monoclinic 1T′ with space group of P21/m. This transition is related to a lateral sliding of adjacent Te-W-Te layers and results in a collapse of the unit cell volume by ∼20.5%. The structural transition also casts a pressure range with the broadened superconducting transition, where the zero resistance disappears.

Positive magnetoresistance in Co40Fe40B20/SiO2/Si heterostructure

Current-perpendicular-to-plane electronic transport properties and magnetoresistance of amorphous Co40Fe40B20/SiO2/Si heterostructures are investigated systematically. A backward diode-like rectifying behavior was observed due to the formation of a Schottky barrier between Co40Fe40B20 and Si. The junction resistance shows a metal-insulator transition with decreasing temperature in both the forward and reverse ranges. A large positive magnetoresistance (MR) of ∼2300% appears at 200 K. The positive MR can be attributed to the magnetic-field–controlled impact ionization process of carriers. MR shows a temperature-peak–type character under a constant bias current, which is related to the spin-dependent barrier in the Si near the interface.