Anomalous Magneto-Transport Behavior in Transition Metal Pentatelluride HfTe_5

There is a long-standing confusion concerning the physical origin of the anomalous resistivity peak in transition metal pentatelluride HfTe_5.Several mechanisms,such as the formation of charge density wave or polaron,have been proposed,but so far no conclusive evidence has been presented.In this work,we investigate the unusual temperature dependence of magneto-transport properties in HfTe_5.It is found that a three-dimensional topological Dirac semimetal state emerges only at around T_p(at which the resistivity shows a pronounced peak),as manifested by a large negative magnetoresistance.This accidental Dirac semimetal state mediates the topological quantum phase transition between the two distinct weak and strong topological insulator phases in HfTe_5.Our work not only provides the first evidence of a temperature-induced critical topological phase transition in HfTe_5 but also gives a reasonable explanation on the long-lasting question.     

Spin Transport in Multi Wall Carbon Nanotubes with Co Electrodes

The magnetic field dependent transport behaviour of Co contacted multi-wall nanotubes is investigated. A sample with three Co electrodes has been measured by two-channel method with an in-plane magnetic field. When the in-plane magnetic field is perpendicular to the tube, high positive magnetoresistance up to 30% is obtained at low temperature from 3 K to 25K and with field parallel to the tube, negative magnetoresistance up to 15% is observed only from the high resistance junction. The detailed positive and negative magnetoresistance behaviour also changes with temperature.     

Magnetoresistance effect in vertical NiFe/graphene/NiFe junctions

For convenient and efficient verification of the magnetoresistance effect in graphene spintronic devices, vertical magnetic junctions with monolayer graphene sandwiched between two NiFe electrodes are fabricated by a relatively simple way of transferring CVD graphene onto the bottom ferromagnetic stripes. The anisotropic magnetoresistance contribution is excluded by the experimental result of magnetoresistance (MR) ratio dependence on the magnetic field direction. The spin-dependent transport measurement reveals two distinct resistance states switching under an in-plane sweeping magnetic field. A magnetoresistance ratio of about 0.17 % is obtained at room temperature and it shows a typical monotonic downward trend with the bias current increasing. This bias dependence of MR further verifies that the spin transport signal in our device is not from the anisotropic magnetoresistance. Meanwhile, the I—V curve is found to manifest a linear behavior, which demonstrates the Ohmic contacts at the interface and the metallic transport characteristic of vertical graphene junction.     

Itinerant electron transport in microscopically inhomogeneous magnetic fields

We report on magnetoresistance measurements in thin nickel films modulated by a periodic magnetic field emanating from micromagnetic arrays fabricated at the film surface. By increasing the strength of the magnetic potential using nickel and dysprosium micromagnets, we are able to quench the anisotropic magnetoresistance (AMR) in the film.     

The magnetization model of multilayered composite thin films: Beyond the effective-medium theories

Multilayered composites consisting of many thin ferromagnetic films with in-plane magnetic anisotropy separated by non-magnetic dielectric layers of different sizes are experimentally and theoretically investigated. Thin samples as well samples with transverse sizes comparable with longitudinal ones are used. The measured static magnetic properties of the bulk sample are found to be different from the properties of constituent thin films. This is an evidence for strong interactions between the magnetic layers in the sample, which interact at distances exceeding greatly the distance between adjacent magnetic layers. A theoretic model is developed taking into account magneto-dipole interactions between iron films in a multi-layer system. The model explains the anomalously high demagnetization field of the sample observed in the measurements.     

Recent observations of negative longitudinal magnetoresistance in semimetal

The discovery of Dirac semimetal and Weyl semimetal has motivated a growing passion for investigating the unique magneto-transport properties in the topological materials.A Weyl semimetal can host Weyl fermions as its low-energy quasi-particle excitations,and therefore perform exotic features analogous to those in high-energy physics,such as the violation of the chiral charge conservation known as the chiral anomaly.One of the electrical transport signatures of the chiral anomaly is the Adler-Bell-Jackiw(ABJ) anomaly which presents as a negative magnetoresistance when the magnetic field and the current are parallel.Very recently,numerous experiments reported negative longitudinal magnetoresistance(NLMR) in topological materials,but the details of the measurement results are various.Here the materials and the corresponding experiment results are briefly reviewed.Besides the plausible explanation of the ABJ anomaly,some other origins of the NLMR are also discussed.     

Magnetoresonance EMF in thin manganite films

The effect of magnetoresonance emf (MREMF) in thin epitaxial films of rare-earth manganites La_0.67Sr_0.33MnO₃ is detected and investigated. The effect is manifested in the occurrence of a constant voltage under the action of microwave pumping in magnetic fields corresponding to ferromagnetic resonance conditions. The MREMF signal includes symmetric and antisymmetric components and changes its polarity upon switching of the external magnetic field. The temperature dependence of the effect (including the range in the neighborhood of the phase transition) is analyzed. The experimental data including the shape of the signal and its dependence of the field orientation are in good agreement with the results obtained in the theoretical model based on the mechanism of anisotropic magnetoresistance. It is shown that the magnetoresistance anisotropy in the manganite under investigation is negative and sharply attenuates as the temperature approaches the Curie point, almost vanishing in the paramagnetic phase.     

Magnetization reversal process in thin Co nanowires

The magnetoresistance of single Co nanowires of various widths is investigated at low temperatures applying magnetic fields μ₀H up to 4.5 T. The in-plane longitudinal magnetoresistance shows pronounced features at coercive fields H_c explained by the anisotropic magnetoresistance indicating the magnetization reversal process. Monte Carlo simulations present the magnetization distribution during the reversal process, revealing different mechanisms depending on the wire width.     

Magnetoresistance of Fe3O4/Au/Fe3O4 and Fe3O4/Au/Fe spin-valve structures

A detailed study of the in-plane magnetotransport properties of spin valves with one and two Fe₃O₄ electrodes is presented. Fe₃O₄/Au/Fe₃O₄ spin valves exhibit a clear anisotropic magnetoresistance in small magnetic fields but no giant magnetoresistance (GMR). The absence of GMR in these structures is due to simultaneous magnetization reversal in the two Fe₃O₄ layers. By contrast, a negative GMR effect is measured on Fe₃O₄/Au/Fe spin valves. The negative GMR is attributed to an electron spin scattering asymmetry at the Fe₃O₄/Au interface or an induced spin scattering asymmetry in the Au interfacial layers.     

Strongly anisotropic electronic transport in higher Landau levels

Low-temperature, electronic transport in higher Landau levels (N>1) in a two-dimensional electron system is strongly anisotropic. At half-filling of either spin level of such Landau levels ( etc.) the magnetoresistance either collapses to form a deep minimum or is peaked in a sharp maximum, depending on the in-plane current direction. The anisotropic axis can be reoriented by applying an in-plane magnetic field of 1–2 T strength. The magnetoresistance at and (N=1) is initially isotropic but an in-plane field induces a strong anisotropy. Our observations are strong evidence for a new many-electron phase in higher Landau levels, which forms spontaneously or can be induced by an in-plane field.     

Rippled graphene in an in-plane magnetic field: effects of a random vector potential

We report measurements of the effects of a random vector potential generated by applying an in-plane magnetic field to a graphene flake. Magnetic flux through the ripples cause orbital effects: Phase-coherent weak localization is suppressed, while quasirandom Lorentz forces lead to anisotropic magnetoresistance. Distinct signatures of these two effects enable the ripple size to be characterized.     

Hopping conductivity and Coulomb correlations in 2D arrays of Ge/Si quantum dots

The temperature and magnetic-field dependences of the conductivity associated with hopping transport of holes over a 2D array of Ge/Si(001) quantum dots with various filling factors are studied experimentally. A transition from the Éfros-Shklovski? law for the temperature dependence of hopping conductivity to the Arrhenius law with an activation energy equal to 1.0–1.2 meV is observed upon a decrease in temperature. The activation energy for the low-temperature conductivity increases with the magnetic field and attains saturation in fields exceeding 4 T. It is found that the magnetoresistance in layers of quantum dots is essentially anisotropic: the conductivity decreases in an increasing magnetic field oriented perpendicularly to a quantum dot layer and increases in a magnetic field whose vector lies in the plane of the sample. The absolute values of magnetoresistance for transverse and longitudinal field orientations differ by two orders of magnitude. The experimental results are interpreted using the model of many-particle correlations of holes localized in quantum dots, which lead to the formation of electron polarons in a 2D disordered system.     

High-field magnetoresistance and Hall effect in Bi2Sr2CuO x single crystals

We investigated the in-plane magnetoresistance and the Hall effect of high-quality Bi₂Sr₂CuO_x single crystals with T _c (midpoint) = 3.7–9.6 K in dc magnetic fields up to 23 T. For T < 10 K, the crystals show the classical positive magnetoresistance. Starting at T ≈ 14 K, an anomalous negative magnetoresistance appears at low magnetic fields; for T ≥ 40 K, the magnetoresistance is negative in the whole studied range of magnetic fields. Temperature and magnetic field dependences of the negative-magnetoresistance single crystals are qualitatively consistent with the electron interaction theory developed for simple semiconductors and disordered metals. As is observed in other cuprate superconductors, the Hall resistivity is negative in the mixed state and changes its sign with increasing field. The linear T-dependence of cotθ_H for the Hall angle in the normal state closely resembles that of the normal-state resistivity as expected for a Fermi liquid picture.     

Specific features of the Hall effect in Cr/Co bilayer films

The Hall effect and magnetoresistance in Cr(50 Å)/Co(200 Å) bilayer films prepared by ion sputtering on a silicon substrate are investigated at room temperature. The planar Hall effect revealed in the bilayer films differs from the planar Hall effect observed usually in that it is symmetric with respect to the sign of the change in the rotation angle of the magnetic moment in the film plane. Under conditions where the symmetric planar Hall effect is realized, the change in the Hall resistance is more than 10% and exceeds the anisotropic magnetoresistance by two orders of magnitude. The hysteresis loops are measured at different orientations of magnetic fields. The planar Hall effect is studied in a weak longitudinal magnetic field. The results obtained demonstrate that the symmetric planar Hall effect is associated with the multidomain structure of the cobalt film in Cr/Co bilayer composites.     

Sensing domain wall pinning in the longitudinal magnetoresistance of a two-dimensional electron gas

We investigate the sensing of domain wall pinning in thin Co wires positioned on top of a two-dimensional electron gas (2DEG) heterostructure by measuring the longitudinal resistance of the 2DEG as the magnetic field is swept, in an analogy to the Barkhausen effect. For comparison, we also measure the magnetoresistance of the ferromagnetic film in the same device in a subsequent sweep. Compared to the Hall measurements, the longitudinal measurement has the advantage of sensing magnetic activity over longer lengths, while compared to the measurement of the magnetoresistance in the ferromagnetic wire, it offers complementary information related to the pinning and unpinning of the domain wall, due to its sensitivity only to the out-of-plane magnetic field component.     

Anisotropic interface magnetoresistances in Pt(111)/Co n /Pt(111)

It is shown in terms of a fully relativistic spin-polarized ab initio-type approach that in Pt/Co/Pt trilayers two types of anisotropic magnetoresistance (AMR) have to be distinguished: an in-plane and an out-of-plane AMR. The obtained results, namely the magnetic field dependence as well as the thickness dependence of both AMR types are in very good agreement with a very recent experimental study, in which the in-plane as well as the out-of-plane AMR was reported for this system. The difference between the two types of AMR is visualized in terms of layer-resolved resistivities. In particular, it is confirmed that the anisotropic interface magnetoresistance (AIMR) introduced in the recent publication mainly originates in the vicinity of the Co/Pt interfaces.     

Fermi surface topology effect on interlayer magnetoresistance with in-plane magnetic field in layered multiband system: Application to FeAs-based superconductors

We propose interlayer magnetoresistance experiments which provide information about Fermi surface topology in layered multi-band systems. The interlayer magnetoresistance shows an oscillating behavior with respect to the azimuthal angle of the applied in-plane magnetic field if the Fermi surface is anisotropic. We discuss applications to LaFeAsO, a parent compound of FeAs-based superconductors. We show the results on the paramagnetic state and the antiferromagnetic state based on a mean field calculation.     

Unexpected negative nonmonotonic magnetoresistance of the two-dimensional electrons in Si in a parallel magnetic field

We report observation of the unexpected negative and nonmonotonic magnetoresistance of 2D electrons in Si-MOSFET subjected to a varying in-plane magnetic field superimposed on a constant perpendicular field component. We show that this nonmonotonic magnetoresistance is irrelevant to the energy spectrum of mobile 2D electrons. We also observed variations of the density of mobile electrons with the in-plane field. We argue that both variations of the negative magnetoresistance and of the density of mobile electrons originate from the band of localized states. The latter coexist and interact with mobile electrons even at relatively high density, a factor of 1.5 higher than the critical density of the apparent metal-insulator transition.     

Angular dependent negative magnetoresistance in Si-MOS (111) inversion layers

The negative transverse magnetoresistance effect was observed in n-inversion layers in Si-MOS (111) surfaces at temperatures between 1.5 and 8.3 K. The negative magnetoresistance depends only on the normal component of the magnetic field to the surface and has a saturation value at high fields. The difference between the resistivity at zero field and that at saturation field increases logarithmically with decreasing temperature such as the effect due to the s-d scattering (Kondo effect).