Large Positive Magnetoresistance Research Articles

From negative to positive magnetoresistance in the intrinsic magnetic topological insulator MnBi2Te4

We report the magnetotransport properties of $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ thin flakes through gate modulation at low temperatures. Under in-plane magnetic field, a large negative magnetoresistance (MR) maintains up to 10 T, which is related to the suppression of spin scattering when the magnetic order is gradually forced into the ferromagnetic (FM) state. Under perpendicular magnetic field, a steep resistance decrease is observed around \ensuremath{\sim}3 T, corresponding to the transition from an antiferromagnetic (AFM) to a canted AFM (CAFM) state. Due to the net Berry curvature, a notable anomalous Hall effect is observed and can be effectively tuned by gate voltages. The enhanced Hall coefficient would emerge under high magnetic fields when the Fermi level is close to the charge neutral point. Moreover, a transition from negative to positive MR is obtained when increasing the magnetic field. A large linear positive MR occurs around $\ensuremath{\sim}8\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, corresponding to the CAFM-FM transition. The nonsaturated positive MR here may have a similar mechanism to the one in Weyl semimetals, revealing the strong combination between topology and magnetism in $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$.

Crossover from weak-antilocalization transport to quantum magnetoresistance of Dirac states in quenched Fe0.01Bi2Te3 single crystals with large magnetoresistance and high Hall mobility

Magnetotransport properties with a large positive magnetoresistance (MR) and a high carrier mobility for applications have been achieved and probed for quenched Fe0.01Bi2Te3 single crystals. Large positive MR of ∼470% with a Hall mobility of ∼44 000 cm2 V−1 s−1 at 5 K and 6 T has been observed on a quenched Fe0.01Bi2Te3 sample, in which the electrical parameters can be tuned by the quenching temperature Tq. The MR behaviors for the quenched samples show a crossover from a weak antilocalization-dominant MR to a linear and non-saturating MR at temperatures of T* ≈ 58−100 K, where the large MR at low temperatures possibly originates from the mechanism of topologically protected backscattering. On the contrary, the MR behaviors for the strain-released sample do not show such a distinct crossover, where only linear-like and non-saturating MR behaviors can be observed. Different electrical transports between the quenched and strain-released samples indicate that the band structure, as well as the surface Dirac electrons in Fe0.01Bi2Te3, can be modified by the lattice strain. Furthermore, it is found that the low-temperature magnetoconductivity can be well described by the weak-antilocalization transport formula, while the high-field linear-like MR at T > T* can be explained in terms of Abrikosov’s quantum transport of Dirac-cone states in quenched Fe0.01Bi2Te3 single crystals.

Low-temperature electrical and magnetic properties of La0.6Sr0.4Co0.2Fe0.8O3-δ nanofibers prepared by electrospinning

The microstructural, magnetic, and electrical properties of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCFO) nanofibers prepared by electrospinning and calcination were systematically investigated. It was found that the as-prepared nanofibers manifested ferromagnetism at low temperatures. Moreover, the resistance and magnetoresistance (MR) data conformed to the one-dimensional variable range hopping (1D VRH) model and revealed that LSCFO nanofibers had different electronic conduction behavior at different temperatures and magnetic fields. LSCFO nanofibers had small negative MR at low temperatures and large positive MR at high temperatures, and the transition temperature ranged between 240 and 260 K. The negative MR and the positive MR of LSCFO nanofibers manifested a fit to the 1D weak localization model and the 1D VRH model, respectively. Therefore, the proposed work can provide an important reference to the design of solid oxide fuel cells (SOFCs) based on the LSCFO cathode.

Anomalous Angle-Dependent Magnetotransport Properties of Single InAs Nanowires.

We study the magnetotransport properties of single InAs nanowires grown by selective-area metal-organic vapor-phase epitaxy. The semiconducting InAs nanowires exhibit a large positive ordinary magnetoresistance effect. However, a deviation from the corresponding quadratic behavior is observed for an orientation of the applied magnetic field perpendicular to the nanowire axis. This additional contribution to the magnetoresistance can be explained by diffuse boundary scattering of free carriers in the InAs nanowire and results in a reduction of the charge carrier mobility. As a consequence, angle-dependent magnetotransport measurements reveal a highly anomalous behavior. Numerical simulations have been conducted to further investigate the effect of classical boundary scattering in the nanowires. On the basis of the numerical simulations, an empirical description is derived, which yields excellent agreement with the experimental data and allows one to quantify the contribution of boundary scattering to the magnetoresistance effect.

Temperature dependent switching of magnetoresistance in multiwall carbon nanotube-polypyrrole composite fibrils

Carbon nanotubes (CNT) are considered one of the most significant materials in nanoelectronic device applications because they can be used in the fabrication of both CNT-inorganic hybrid structures and CNT-organic composite materials. Also, the study of the electrical properties of these materials has its own fundamental and technological significance. Here, we report on low temperature charge transport characteristics (down to 4.2 K in the magnetic fields up to 11 T) of multiwall carbon nanotube (MWCNT)-polypyrrole (PPy) coaxial composite fibrils synthesized by a facile electrochemical polymerization method. Two types of samples were synthesized by carrying out electrochemical polymerization at room temperature (RT) for different durations of 90 and 45 min, respectively. Scanning electron microscopy studies indicated that the diameters of as-prepared MWCNT-PPy fibril samples were ∼1.5 μm and 0.5 μm, respectively. The dc electrical resistance of the two samples was ∼103 and 102 Ω at RT and exhibited a pronounced temperature dependence, which is indicative of the hopping process being dominant. Furthermore, a large positive magnetoresistance (MR) of ∼29% and ∼18% is displayed at 4.2 K, which switched to negative MR with a maximum magnitude of ∼11% and ∼15% at 10 K for the two samples, respectively. The switching of MR as a function of temperature showed the dominance of two important competing phenomena, namely, wave function shrinkage and forward interference of electron waves.

Asymmetric spin dependent scattering at the interfaces of Si/La0.7Sr0.3MnO3/ZnO heterostructures

A ferromagnetic 120 Å thick La0.7Sr0.3MnO3 (LSMO) film grown on (001)Si using the sputtering deposition technique demonstrates a large positive in-plane magnetoresistance (MR) at 10 K, in the field window of ±0.084 kG to±0.405 kG, although the bulk LSMO exhibits negative MR. Around the coercive field (∼179 G), the positive MR becomes ∼ 11%. The positive MR of the LSMO thin film is explained by the charge transfer driven localized strong antiferromagnetic coupling at the Si−LSMO interface, which favors the reduction of the Curie temperature TC of LSMO compared to that of its bulk value. The construction of the interface on the top surface of LSMO with ZnO thin films further reduces TC ∼ 30 K and the positive MR decreases to ∼ 1% for 45° oriented in-plane current with the in-plane field. The coupling through Mn−O−Zn at the LSMO−ZnO interface preserves the charge state, and the weak exchange coupling at the (La/Sr)O−ZnO interface reduces the spin-dependent scattering process under the field and thereby, the negative MR. The reduced TC and in-plane low-field MR at 10 K of a series of Si/LSMO/ZnO are the same irrespective of the ZnO thickness, which confirms their interfacial origin. The presence of interfacial spin disorder at the Si−LSMO interface is further confirmed from the increase in resistance at low temperatures, which is explained by the Kondo like effect and quantum interference effect. Our investigations show that the technologically important interfacial magnetic coupling and magnetoresistance could be achieved and manipulated by the selective interfacial exchange coupling.

Selective-area growth and transport properties of MnAs/InAs heterojunction nanowires

Abstract

All-Semiconducting Spin Filter Prepared by Low-Energy Proton Irradiation

We report on a spin filter effect that emerges at potential barriers between a ≃10 nm thick magnetic surface layer and nonmagnetic regions prepared at the surface of Li-doped ZnO microwires by low-energy H+-ion implantation. The spin filter effect manifests in a large positive magnetoresistance that persists to room temperature, is linear in magnetic field, shows a maximum around 30 K, scales linearly with the number of successive potential barriers created along the microwires, and vanishes when the potential barrier is absent. The specific features of the observed effect can be reproduced with a model that describes a spin-polarized transport through a potential barrier acting as a minority spin filter with an efficiency Psf > 10% at room temperature and Psf = 100% at 10 K.

Electron transport and magnetotransport in graphene films grown on iron thin film catalyst

Graphene films were grown by the low-pressure chemical vapor deposition with a single injection of acetylene on an iron film catalyst deposited on oxidized silicon substrate. After treatment of the graphene on the iron film with aqueous solution of iron nitrate the structures consisting of quasi-suspended graphene on reaction products of the iron film with iron nitrate were obtained. The electron transport and magnetotransport properties of the films were investigated. The films have a low resistance of 80 Ohm sq−1 and a high sheet carrier density (8 × 1013 cm−2 at room temperature). At temperatures less than 200 K, the dependence of the Hall resistance on the magnetic field is like the abnormal Hall effect. Large positive linear magnetoresistance at a room temperature (60–100%) was observed in the films in a field of 0.6 T, which is attractive for creating magnetoresistive sensors. It was found that the critical magnetic field at which the MR becomes linear is very small (116–650 Oe) and linearly dependent on a temperature. The MR is proportional to the average mobility 〈µ〉. At low temperatures, the magnetoresistance increases with increasing temperature. At higher temperatures the MR decreases with increasing temperature.

Extremely Large Magnetic-Field-Effects on the Impedance Response of TiO2 Quantum Dots

Here, we report large magnetoresistance and magnetocapacitance response of undoped TiO2 quantum dots weighting the contribution of both grain and grain boundaries by means of impedance spectroscopy. We also performed a complete characterization of the TiO2 quantum dots (~5 nm) prepared by sol-gel via water vapor diffusion method, using X-ray diffraction, small angle X-ray scattering, transmission electron microscopy and Raman spectroscopy. In addition, we showed a complete theoretical study on the electronic properties of TiO2 surface and subsurface oxygen and titanium vacancies to shed some light in their electronic and magnetic properties. Based in our study, we can conclude that the presence of defects, mainly at the grain boundary of these undoped TiO2 quantum dots, could be responsible for the large positive magnetoresistance (+1200%) and negative magnetocapacitance (−115%) responses at low applied magnetic fields (1.8 kOe) and room temperature.

Magneto-transport property of an AlInN/AlN/GaN heterostructure

This paper reports a research of the magneto-transport property of an AlInN/AlN/GaN heterostructure. A large positive magnetoresistance (MR) up to 79% under 9 T is observed, which is indicative of potential sensor application. The observed positive MR is attributed to the presence of the parallel conduction. Meanwhile, the Shubnikov-de Hass (SdH) oscillations of the longitudinal MR appear up at high magnetic field. From the SdH oscillations, beating patterns are observed in the AlInN/AlN/GaN heterostructure. Neither the magneto-intersubband oscillation nor the zero-field spin splitting can explain the observed beating patterns, which can be attributed to the sample inhomogeneity.

Large magnetoresistance and large magnetothermopower effect in the Dirac material EuMn0.8Sb2

In this article, the structure, transport and magnetic properties were studied in details for EuMn0.8Sb2 crystals with the orthorhombic structure and Mn deficiencies. It was found that the temperature dependence of the resistivity exhibits a metallic behavior in the whole measuring temperature range, different from that in the crystals without Mn deficiencies. A large positive magnetoresistance (MR) (∼127% at 2 K and ∼25% at 300 K, in 9 T field) was observed, which can be ascribed to the combination of semiclassical MR and quantum limit MR of Dirac electrons. We also observed the high mobility of the carriers and large magnetothermopower effect at low temperatures, and two magnetic transitions emerging at ∼24 K and ∼10 K, respectively, corresponding to the antiferromagnetic ordering and canted arrangement of the Eu moments. Our findings shed new light on the intrinsic properties of EuMn0.8Sb2 and demonstrate the existence of Dirac fermions.

Extremely large magnetoresistance induced by hidden three-dimensional Dirac bands in nonmagnetic semimetal InBi

Extremely large positive magnetoresistance (XMR) was found in a nonmagnetic semimetal InBi. Using several single crystals with different residual resistivity ratios ($RRR\mathrm{s}$), we revealed that the XMR strongly depended on the $RRR$ (sample quality). Assuming that there were no changes in effective mass ${m}^{*}$ and carrier concentrations in these single crystals, this dependence was explained by a semiclassical two-carrier model. First-principle calculations including the spin-orbit interactions (SOIs) unveiled that InBi had a compensated carrier balance and SOI-induced ``hidden'' three-dimensional (3D) Dirac bands at the $M$ and $R$ points. Because the small ${m}^{*}$ and the large carrier mobilities will be realized, these hidden 3D Dirac bands should play an important role for the XMR in InBi. We suggest that this feature can be employed as a novel strategy for the creation of XMR semimetals.

Large Magnetoresistance and Electrical Transport Properties in Reduced Graphene Oxide Thin Film

We report a systematic study of room temperature large positive and negative magnetoresistance (MR) in the reduced graphene oxide (RGO) thin-film devices grown by pulsed-laser deposition (PLD) at high and low applied magnetic fields, respectively. Raman spectroscopy, X-ray photoelectron spectroscopy, and electrical measurements on the RGO films help to explain the observed MR properties in the device. The temperature-dependent (5–400 K) electrical characterization of the thin films shows two distinct transport regimes: at low temperature, it follows 2-D Efros-Shoklovoski variable range hopping (VRH) transport mechanism and above 200 K, the device shows Arrhenius-like transport behavior. The crossover from VRH transport to Arrhenius transport is due to shortening in the characteristic lengths in the disordered 2-D system. We interpret the source of negative MR by vacancy and disorder-induced magnetic moments and the diffuse scattering at crystallite boundaries. At the high applied magnetic field, the lifting in degeneracy due to the Lorentz force explains the large positive MR effect. The highest value of the measured MR (160%) is surprisingly high for a non-magnetic material at room temperature, which can be attributed to the greater inhomogeneity in the PLD grown wafer-scale RGO thin films.

Pressure-induced charge density wave phase in Ag2−δTe

Considerable excitement was generated by the observation of large and linear positive magnetoresistance in nonmagnetic silver chalcogenides. Renewed interest in these materials was kindled by the discovery that $\mathrm{A}{\mathrm{g}}_{2}\mathrm{Te}$ in particular is a topological insulator with gapless linear Dirac-type surface states. High-pressure x-ray-diffraction studies, combined with first-principles electronic structure calculations, have identified three phase transitions as the pressure is increased: an isostructural transition identified with an electronic topological transition followed by two structural phase transitions. These recent studies were carried out on nominally stoichiometric $\mathrm{A}{\mathrm{g}}_{2}\mathrm{Te}$. For the present work we have prepared single-phase self-doped $\mathrm{A}{\mathrm{g}}_{2\text{\ensuremath{-}}\ensuremath{\delta}}\mathrm{Te}$ samples with a well-characterized silver deficit ($\ensuremath{\delta}=2.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$) for structural and electrical transport measurements over extended ranges of pressure (0--43 GPa), temperature (2--300 K), and magnetic field (0--9 T). The temperature dependence of the resistivity exhibits anomalous behavior at 2.3 GPa, slightly above the isostructural transition, which we postulate is due to Fermi surface reconstruction associated with a charge density wave (CDW) phase. The anomaly is enhanced by the application of a 9-T magnetic field and shifted to higher temperature, implying that the electronic Zeeman energy is sufficient to alter the gapping of the Fermi surface. A peak in the pressure dependence of the resistivity and a sudden drop in the pressure dependence of the mobility, occurring at 2.3 GPa, provide additional evidence for a CDW phase at pressures slightly above the isostructural transition.

Extreme magnetoresistance and Shubnikov-de Haas oscillations in ferromagnetic DySb

The electronic structures of a representative rare earth monopnictide (i.e., DySb) under high magnetic field (i.e., in the ferromagnetic state) are studied from both experimental and theoretical aspects. A non-saturated extremely large positive magnetoresistance (XMR) is observed (as large as 3.7 × 104% at 1.8 K and 38.7 T), along with the Shubnikov-de Haas oscillations that are well reproduced by our first principles calculations. Three possible origins of XMR are examined. Although a band inversion is found theoretically, suggesting that DySb might be topologically nontrivial, it is deeply underneath the Fermi level, which rules out a topological nature of the XMR. The total densities of electron-like and hole-like carriers are not fully compensated, showing that compensation is unlikely to account for the XMR. The XMR is eventually understood in terms of high mobility that is associated with the steep linear bands. This discovery is important to the intensive studies on the XMR of rare earth monopnictides.

Distinguishing Interface Magnetoresistance and Bulk Magnetoresistance through Rectification of Schottky Heterojunctions

High performance of many spintronic devices strongly depends on the spin-polarized electrical transport, especially the magnetoresistance (MR) in magnetic heterojunctions. However, it has been a great challenge to distinguish the bulk MR and interface MR by transport measurements because the bulk resistance and interface resistance formed a series circuit in magnetic heterojunctions. Here, a unique interface-sensitive rectification MR method is proposed to distinguish the interface MR and bulk MR of nonmagnetic In/GeO x/n-Ge and magnetic Co/GeO x/n-Ge diode-like heterojunctions. It is demonstrated that the low-field "butterfly" hysteresis loop observed only in the conventional MR curve originates from the anisotropic MR of ferromagnetic bulk Co layer, whereas the orbit-related large nonsaturating positive MR contains contributions from both the Schottky interface and bulk Ge substrate. This rectification MR method could be extended to magnetic heterojunctions with asymmetric potential barriers to realize a deeper understanding of the fundamental interface-related functionalities.

Large positive linear magnetoresistance in the two-dimensional t2g electron gas at the EuO/SrTiO3 interface

The development of novel nano-oxide spintronic devices would benefit greatly from interfacing with emergent phenomena at oxide interfaces. In this paper, we integrate highly spin-split ferromagnetic semiconductor EuO onto perovskite SrTiO3 (001). A careful deposition of Eu metal by molecular beam epitaxy results in EuO growth via oxygen out-diffusion from SrTiO3. This in turn leaves behind a highly conductive interfacial layer through generation of oxygen vacancies. Below the Curie temperature of 70 K of EuO, this spin-polarized two-dimensional t2g electron gas at the EuO/SrTiO3 interface displays very large positive linear magnetoresistance (MR). Soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) reveals the t2g nature of the carriers. First principles calculations strongly suggest that Zeeman splitting, caused by proximity magnetism and oxygen vacancies in SrTiO3, is responsible for the MR. This system offers an as-yet-unexplored route to pursue proximity-induced effects in the oxide two-dimensional t2g electron gas.

Extreme magnetoresistance and SdH oscillation in compensated semimetals of NbSb2 single crystals

Topological semimetals represent one of the most interesting classes of materials that continue to attract worldwide interest. Here, we report magnetotransport properties of MPn2-type (M = Nb, Ta; Pn = P, As, Sb) NbSb2 single-crystal semimetals with a centrosymmetric C12/m1 space group, paramagnetic ground state, and non-saturation parabolic-like magnetoresistance. The NbSb2 crystals show metallic conductivity down to 2 K and undergo a metal-to-insulator-like transition under a magnetic field B (B ≥ 4 T) and exhibit a resistivity plateau in the low-temperature region (T ≤ 10 K), where the value of resistivity strongly depends on the magnitude and direction of the magnetic field. Upon sweeping the magnetic field from 0 to 14.5 T in the transverse configuration at T = 1.5 K, the NbSb2 crystal shows a large positive magnetoresistance (4.2 × 103% at B = 14.5 T) with Shubnikov–de Haas (SdH) oscillation. Hall measurements reveal that both the carrier compensation between electrons and holes and the high mobility and large mean free path of carriers contribute to the large magnetoresistance. Fast Fourier transform analyses of angle-resolved SdH oscillation indicate that the Fermi surface of the NbSb2 crystal is quasi-two-dimensional with three-dimensional components. These findings, together with the theoretically calculated electronic band structure obtained within the framework of density functional theory, suggest that NbSb2 is a good candidate compensated semimetal for further theoretical and experimental investigation of this family of materials.

First-principles study of electronic structure and Fermi surface in semimetallic YAs

In the course of searching for new systems, which exhibit nonsaturating and extremely large positive magnetoresistance, electronic structure, Fermi surface, and de Haas-van Alphen characteristics of the semimetallic YAs compound were studied using the all-electron full-potential linearized augmented-plane wave (FP–LAPW) approach in the framework of the generalized gradient approximation (GGA). In the scalar-relativistic calculation, the cubic symmetry splits fivefold degenerate Y-d orbital into low-energy threefold-degenerate t2g and twofold degenerate doublet e1g states at Γ point around the Fermi energy. One of them, together with the threefold degenerate t1u character of As-p orbital, render the YAs semimetal with a topologically trivial band order and fairly low density of states at the Fermi level. Including spin–orbit (SO) coupling into the calculation leads to pronounced splitting of the t1u state and shifting the bands in the energy scale. Consequently, the determined four different 3-dimensional Fermi surface sheets of YAs consists of three concentric hole-like bands at Γ and one ellipsoidal electron-like sheet centred at the X points. In full accordance with the previous first-principles calculations for isostructural YSb and YBi, the calculated Fermi surface of YAs originates from fairly compensated multi-band electronic structures.