Anomalous Negative Magnetoresistance Research Articles

Anomalous negative magnetoresistance in quantum dot Josephson junctions with Kondo correlations

Anomalous negative magnetoresistance in quantum dot Josephson junctions with Kondo correlations

Robust negative longitudinal magnetoresistance and spin–orbit torque in sputtered Pt3Sn and Pt3SnxFe1-x topological semimetal

Contrary to topological insulators, topological semimetals possess a nontrivial chiral anomaly that leads to negative magnetoresistance and are hosts to both conductive bulk states and topological surface states with intriguing transport properties for spintronics. Here, we fabricate highly-ordered metallic Pt3Sn and Pt3SnxFe1-x thin films via sputtering technology. Systematic angular dependence (both in-plane and out-of-plane) study of magnetoresistance presents surprisingly robust quadratic and linear negative longitudinal magnetoresistance features for Pt3Sn and Pt3SnxFe1-x, respectively. We attribute the anomalous negative longitudinal magnetoresistance to the type-II Dirac semimetal phase (pristine Pt3Sn) and/or the formation of tunable Weyl semimetal phases through symmetry breaking processes, such as magnetic-atom doping, as confirmed by first-principles calculations. Furthermore, Pt3Sn and Pt3SnxFe1-x show the promising performance for facilitating the development of advanced spin-orbit torque devices. These results extend our understanding of chiral anomaly of topological semimetals and can pave the way for exploring novel topological materials for spintronic devices.

Tunable Berry curvature and transport crossover in topological Dirac semimetal KZnBi

Topological Dirac semimetals have emerged as a platform to engineer Berry curvature with time-reversal symmetry breaking, which allows to access diverse quantum states in a single material system. It is of interest to realize such diversity in Dirac semimetals that provides insight on correlation between Berry curvature and quantum transport phenomena. Here, we report the transition between anomalous Hall and chiral fermion states in three-dimensional topological Dirac semimetal KZnBi, which is demonstrated by tuning the direction and flux of Berry curvature. Angle-dependent magneto-transport measurements show that both anomalous Hall resistance and positive magnetoresistance are maximized at 0° between net Berry curvature and rotational axis. We find that the unexpected crossover of anomalous Hall resistance and negative magnetoresistance suddenly occurs when the angle reaches to ~70°, indicating that Berry curvature strongly correlates with quantum transports of Dirac and chiral fermions. It would be interesting to tune Berry curvature within other quantum phases such as topological superconductivity.

Normal-state negative longitudinal magnetoresistance and Dirac-cone-like dispersion in PtPb4 single crystals: a potential Weyl-semimetal superconductor candidate

Magnetotransport measurements of PtPb4 single crystals with the T c of ∼2.80 K reveal pronounced exotic topological nature. An anomalous negative longitudinal magnetoresistance (MR) related to the chiral anomaly and a linear-like transverse MR examined within the framework of quantum MR theory were studied. Results indicate the presence of chiral-anomaly-induced quantum transport associated with the Weyl states and show a Dirac-cone-like band dispersion in PtPb4. This work reveals the drastic impact of the concept that the surface electrons in a Weyl fermion state may dominate the normal-state magnetotransport in PtPb4, leading to the conclusion that PtPb4 can be a Weyl-semimetal superconductor candidate.

Anomalous negative longitudinal magnetoresistance and violation of Ohm's law\xa0deep in the topological insulating regime in Bi_{1-x}Sb_x

Bi_{1-x}Sb_x is a topological insulator (TI) for x approx 0.03–0.20. Close to the Topological phase transition at x = 0.03, a magnetic field induced Weyl semi-metal (WSM) state is stabilized due to the splitting of the Dirac cone into two Weyl cones of opposite chirality. A signature of the Weyl state is the observation of a Chiral anomaly [negative longitudinal magnetoresistance (LMR)] and a violation of the Ohm’s law (non-linear I{-}V). We report the unexpected discovery of Chiral anomaly-like features in the whole range (x = 0.032, 0.072, 0.16) of the TI state. This points to a field induced WSM state in an extended x range and not just near the topological transition at x = 0.03. Surprisingly, the strongest Weyl phase is found at x = 0.16 with a non-saturating negative LMR much larger than observed for x = 0.03. The negative LMR vanishes rapidly with increasing angle between B and I. Additionally, non-linear I–V is found for x = 0.16 indicating a violation of Ohm’s law. This unexpected observation of a strong Weyl state in the whole TI regime in Bi_{1-x}Sb_x points to a gap in our understanding of the detailed crystal and electronic structure evolution in this alloy system.

Anisotropic character of the metal-to-metal transition in Pr4Ni3O10

As a member of the Ruddlesden-Popper Ln$_{n+1}$Ni$_n$O$_{3n+1}$ series rare-earth-nickelates, the Pr4Ni$_3$O$_{10}$ consists of infinite quasi-two-dimensional perovskite-like Ni-O based layers. Although a metal-to-metal phase transition at Tpt = 157 K has been revealed by previous studies, a comprehensive study of physical properties associated with this transition has not yet been performed. We have grown single crystals of Pr4Ni3O10 at high oxygen pressure, and report on the physical properties around that phase transition, such as heat-capacity, electric-transport and magnetization. We observe a distinctly anisotropic behavior between in-plane and out-of-plane properties: a metal-to-metal transition at Tpt within the a-b plane, and a metal-to-insulator-like transition along the c-axis with decreasing temperature. Moreover, an anisotropic and anomalous negative magneto-resistance is observed at Tpt that we attribute to a slight suppression of the first-order transition with magnetic field. The magnetic-susceptibility can be well described by a Curie-Weiss law, with different Curie-constants and Pauli-spin susceptibilities between the high-temperature and the low-temperature phases. The single crystal X-ray diffraction measurements show a shape variation of the different NiO6 octahedra from the high-temperature phase to the low-temperature phase. This subtle change of the environment of the Ni sites is likely responsible for the different physical properties at high and low temperatures.

Large Magnetoresistance in Fe3O4/4,4′-Bipyridine/Fe3O4 Organic Magnetic Tunnel Junctions

Organic magnetic tunnel junctions (OMTJs) are promising systems thanks to their chemically tunable electronic property, long spin lifetime and easy functionalizations. Here, the spin-dependent electronic transport properties in Fe3O4/4,4’-bipyridine/Fe3O4 OMTJs are investigated by first-principles quantum transport calculations. Since the transport properties of junctions are sensitive to the device details, two types of terminations of Fe3O4 electrodes are considered. The device with tetrahedral Fe termination shows an anomalous negative tunnel magnetoresistance (TMR), i.e., which has a higher and lower junction resistance in the parallel and anti-parallel magnetization configurations, respectively. When the contact termination is octahedral Fe, a large positive TMR of 180% appears. The difference in TMR sign of two OMTJs originates from the electrons transmission mediated by frontier molecular levels coupled differently to Fe d states. Furthermore, TMR can be effectively controlled by applied electrical...

Chiral anomaly factory: Creating Weyl fermions with a magnetic field

Weyl fermions can be created in materials with both time reversal and inversion symmetry by applying a magnetic field, as evidenced by recent measurements of anomalous negative magnetoresistance. Here, we do a thorough analysis of the Weyl points in these materials: by enforcing crystal symmetries, we classify the location and monopole charges of Weyl points created by fields aligned with high-symmetry axes. The analysis applies generally to materials with band inversion in the $T_d$, $D_{4h}$ and $D_{6h}$ point groups. For the $T_d$ point group, we find that Weyl nodes persist for all directions of the magnetic field. Further, we compute the anomalous magnetoresistance of field-created Weyl fermions in the semiclassical regime. We find that the magnetoresistance can scale non-quadratically with magnetic field, in contrast to materials with intrinsic Weyl nodes. Our results are relevant to future experiments in the semi-classical regime.

Nonlinear Anomalous Hall Effect and Negative Magnetoresistance in a System with Random Rashba Field

We predict two spin-dependent transport phenomena in two-dimensional electron systems, which are induced by a spatially fluctuating Rashba spin-orbit interaction. When the electron gas is magnetized, the random Rashba interaction leads to the anomalous Hall effect. An example of such a system is a narrow-gap magnetic semiconductor-based symmetric quantum well. We show that the anomalous Hall conductivity reveals a strongly nonlinear dependence on the magnetization, decreasing exponentially at large spin density. We also show that electron scattering from a fluctuating Rashba field in a two-dimensional nonmagnetic electron system leads to a negative magnetoresistance arising solely due to spin-dependent effects.

Magnetic impurity behavior in the type II broken-gap p-GaInAsSb/p-InAs:Mn heterostructures with 2D-semimetal channel at the interface

Abstract Magnetic impurity behavior have been studied in type II p-GaInAsSb/p-InAs heterostructures grown by liquid-phase epitaxy on InAs substrate heavily doped with manganese ( p =(5−7)×10 18 cm −3 ). Ferromagnetic properties of these structures become apparent during observation of anomalous Hall effect and negative magnetoresistance (Δ ρ ⊥ /ρ ) in low magnetic fields H =3–20 kOe at T =77–200 K. High value of negative magnetoresistance up to 30% and its quadratic dependence on a magnetic field were found. Effective magnetic moment M *=200 μ B and Curie temperature T C =200 K were evaluated. Contribution of Mn δ-layer placed on GaInAsSb epilayer was manifested in vertical quantum magnetotransport in high magnetic fields ( B >8 T) and low temperatures (1.5–4.2 K). Presence of the narrow layer of Mn can control over spin-oriented tunneling of electrons from a semimetal channel across the type II broken-gap heterointerface.

Transport properties of the two‐dimensional electron gas in GaN/AlGaN heterostructures grown by ammonia molecular‐beam epitaxy

AbstractTransport properties of the two‐dimensional electron gas in AlGaN/GaN heterostructures grown by ammonia molecular‐beam epitaxy are experimentally investigated. Conventional Hall and Shubnikov–de Haas measurements as well as investigations of quantum transport phenomena are reported. It is found that negative magnetoresistance (NMR) caused by weak localization demonstrates an unusual behavior at low temperature (1.8 K). The observed NMR cannot be described by the ordinary theory of quantum corrections to conductivity based on a single phase breaking time τϕ . The anomalous NMR behavior can be explained by the presence of two occupied quantum subbands, characterized by their own phase breaking times τ ϕ 1 and τ ϕ 2. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetotransport properties of type II heterojunctions based on GaInAsSb/InAs and GaInAsSb/GaSb

The results of detailed study of the magnetotransport properties of broken-gap type II heterojunctions in a GaInAsSb/InAs(GaSb) system are reported. An electron channel with a high charge-carrier mobility (as high as 50000–60000 cm2/(V s)) is observed and studied for the first time in an isotype broken-gap p-GaInAsSb/p-InAs heterostructure. The effects of electron-channel depletion and semimetal-semiconductor transition in the case of heavy doping of the quaternary alloy with acceptors are studied. Magnetotransport properties at temperatures of 4.2–200 K are studied in detail. Data on the energy spectrum and parameters of two-dimensional charge carriers at the heteroboundary are obtained. It is ascertained experimentally that, depending on the composition, either staggered (at x = 0.85) or broken-gap (at x = 0.95) heterojunctions can be formed in the Ga1−x InxAsySb1−y /GaSb, which is confirmed by theoretical calculations. The anomalous Hall effect and negative magnetoresistance were observed in GaInAsSb/InAs:Mn grown on substrates doped heavily with Mn magnetic acceptor impurity so that the hole concentration was as high as p > 5 × 1018 cm−3; these phenomena are caused by exchange interaction of Mn ions in InAs with high-mobility charge carriers in the electron channel at the heterointerface.

Magneto-transport properties of amorphous Ge1−Mn thin films

Abstract Amorphous Ge 1− x Mn x thin films were grown in order to expand the solubility limit of Mn. The amorphous Ge 1− x Mn x thin films were grown on (1 0 0)Si substrate at 373 K by using a thermal evaporator. The solubility of Mn in amorphous Ge 1− x Mn x thin films reaches up to 17 at.%. The amorphous Ge 1− x Mn x thin films are ferromagnetic and the T C is ∼150 K. The largest saturation magnetization of amorphous Ge 1− x Mn x thin films is ∼100 emu/cm 3 for x = 0.118 at 5 K. The variation of electrical resistivity with respect to temperature reveals that the amorphous Ge 1− x Mn x thin films have semiconductor characteristics. The in-field electrical resistivity of amorphous Ge 1− x Mn x thin films is lower than the zero-field electrical resistivity when T T C , but the reverse is true when T > T C . However, the in-field electrical resistivity of amorphous Ge 1− x Mn x thin films is always higher than the zero-field electrical resistivity when x > ∼12 at.%. Magneto-transport characteristics of amorphous Ge 1− x Mn x thin films show anomalous Hall phenomenon and negative magnetoresistance when T T C . The results suggest that the Mn atoms in amorphous Ge 1− x Mn x thin films be related to spin dependent scattering depending on magnetization.

Anomalous magnetoresistance in NiMnGa thin films

The origin of anomalous negative magnetoresistance and its temperature dependence in polycrystalline Ni–Mn–Ga films prepared by pulse laser deposition was studied. The investigation of structural, transports, magnetic, and ferromagnetic resonance properties of the films suggests contributions of different mechanisms in magnetotransport. At low magnetic fields the main contribution to magnetoresistance is due to the transport between the areas with different orientation of magnetic moments, while at high fields it is an electron scattering of in spin-disordered areas.

Anomalous negative magnetoresistance caused by non-Markovian effects

A theory of recently discovered anomalous low-field magnetoresistance is developed for the system of two-dimensional electrons scattered by hard disks of radius a, randomly distributed with concentration n. For small magnetic fields the magnetoresistance is found to be parabolic and inversely proportional to the gas parameter, $\ensuremath{\delta}{\ensuremath{\rho}}_{\mathrm{xx}}/\ensuremath{\rho}\ensuremath{\sim}\ensuremath{-}({\ensuremath{\omega}}_{c}\ensuremath{\tau}{)}^{2}{/na}^{2}.$ With increasing field the magnetoresistance becomes linear $\ensuremath{\delta}{\ensuremath{\rho}}_{\mathrm{xx}}/\ensuremath{\rho}\ensuremath{\sim}\ensuremath{-}{\ensuremath{\omega}}_{c}\ensuremath{\tau}$ in a good agreement with the experiment and numerical simulations.

Interaction of charge carriers with the localized magnetic moments of manganese atoms in p-GaInAsSb/p-InAs:Mn heterostructures

Transport properties of p-Ga1−xInxAsySb1−y/p-InAs:Mn heterostructures with undoped layers of solid solutions similar in composition to GaSb (x≲0.22) grown by liquid-phase epitaxy on substrates with a Mn concentration of (5–7)×1018 cm−3 are studied. It is ascertained that there is an electron channel at the interface (from the InAs side). The anomalous Hall effect and negative magnetoresistance are observed at relatively high temperatures (77–200) K. These phenomena can be attributed to the s-d-exchange interaction between Mn ions of the substrate and s electrons of the two-dimensional channel. The effective magnetic moment of Mn ions was evaluated as μ=200µB at T=77 K.

Growth of the dilute magnetic semiconductor GaMnN by molecular-beam epitaxy

Growth by molecular-beam epitaxy (MBE) of the dilute-magnetic alloy GaMnN is reported. The Mn concentration, as determined by Auger electron spectroscopy (AES), is found to be linear with increasing Mn-cell temperature up to ∼43at.%Mn. No second phases are observed for Mn levels below 9 at.%. The cubic-phase Mn4N is found to be the thermodynamically stable phase at the growth conditions used to produce GaMnN. Hysteresis in M versus H is observed in both GaMnN and GaMnN:C grown on both sapphire and metal-oxide chemical-vapor deposition (MOCVD) GaN at several growth temperatures. Magnetotransport results show the anomalous Hall effect, negative magnetoresistance, and magnetic hysteresis, indicating that Mn is incorporating into the GaN and forming the ferromagnetic-semiconductor GaMNN. Room-temperature hysteresis is obtained in magnetization measurements with an optimum Mn concentration of ∼3 at.%.

Vortex states at low temperature in disordered thin and thick films of a-MoxSi1–x

We have measured the ac complex resistivity in thelinear regime, as well as dc resistivity, for thick (100, 300 nm)amorphous (a-)MoxSi1–x films at low temperatures(T > 0.04 K) in constant fields B. The critical behaviorassociated with the second-order transition has been observedfor both dc and ac resistivities, which is similar to that observedfor granular indium films. This is the first convincing evidencefor the vortex glass transition (VGT) in the homogeneouslydisordered low-TC superconductors containing microscopicpinning centers. We have found that the VGT persists down to T ∼ 0.1TC0 up to B ∼ 0.9BC2(0), where TC0 and BC2(0) are themean-field transition temperature and upper critical field at T = 0, respectively. At T → 0 the VGT line Bg(T) extrapolatesto a field below BC2(0), indicative of the presence of a T = 0quantum-vortex-liquid phase in the region Bg(0) < B < BC2(0).For thin (4 nm) films the (T = 0) field-driven superconductor–insulator transition takes place at BC. We have notobtained evidence of the metallic quantum liquid phase below BC, while in B > BC an anomalous negative magnetoresistance(MR) suggesting the presence of the localized Cooper pairs hasbeen observed. The negative MRis commonly observed for thinfilms; however, for thick films the MR is always positive. Thismeans that the two-dimensionality plays an important role inthe appearance of the negative MR (or localized Cooper pairs).The negative MR is no longer visible as the field is appliedparallel to the film surface, consistent with the view that mobilevortices, as well as localized Cooper pairs, are present in B > BC.

Superconducting transition in nanoscale aluminum structures

Superconducting resistive transition in mesoscopic Al ring, disk, and wire samples is reported. In the ring sample, a periodic sharp peak structure, Little–Parks (LP) oscillation, is clearly observed as a function of magnetic field down to 40mK. The oscillation period of about 56G agrees with the value estimated from the sample size. The peak structure is also found in both of the disk and the wire samples. The peaks in the disk sample are qualitatively explained in the framework of the linearized Ginzburg–Landau theory, but those in the wire sample are not understood by the present theory. In all the samples, anomalous negative magnetoresistance is observed at low fields, which may be ascribed to normal superconducting junctions at the nodes.

Anomalous magnetoresistance behavior of an insulating zirconium oxynitride Cernox TM thermometer

Zirconium oxynitride insulating films make excellent thermometry at 4 K and above. The resistance is almost independent of magnetic fields up to 30 T and exhibits good temperature sensitivity and good reproducibility upon temperature cycling. However below 4 K, this material exhibits an anomalous negative magnetoresistance (MR); for example at 86 mK, the resistance decreases by a factor of 24 in a 17 T field as compared to its zero-field value. The negative magnetoresistnce behavior is explained using a model proposed by Movaghar, Schweitzer and Osaka. Fits to the negative MR data are good above 0.3 K but become poorer at lower temperatures.