Anomalous Magnetoresistance Research Articles

Anomalous magnetoresistance in magnetized topological insulator cylinders

The close coupling between the spin and momentum degrees of freedom in topological insulators (TIs) presents the opportunity for the control of one to manipulate the other. The momentum can, for example, be confined on a curved surface and the spin influenced by applying a magnetic field. In this work, we study the surface states of a cylindrical TI magnetized in the x direction perpendicular to the cylindrical axis lying along the z direction. We show that a large magnetization leads to an upwards bending of the energy bands at small |kz|. The bending leads to an anomalous magnetoresistance where the transmission between two cylinders magnetized in opposite directions is higher than when the cylinders are magnetized at intermediate angles with respect to each other.

Heterojunction metal-oxide-metal Au-Fe3O4-Au single nanowire device for spintronics

In this report, we present the synthesis of heterojunction magnetite nanowires in alumina template and describe magnetic and electrical properties from a single nanowire device for spintronics applications. Heterojunction Au-Fe-Au nanowire arrays were electrodeposited in porous aluminum oxide templates, and an extensive and controlled heat treatment process converted Fe segment to nanocrystalline cubic magnetite phase with well-defined Au-Fe3O4 interfaces as confirmed by the transmission electron microscopy. Magnetic measurements revealed Verwey transition shoulder around 120 K and a room temperature coercive field of 90 Oe. Current–voltage (I-V) characteristics of a single Au-Fe3O4-Au nanowire have exhibited Ohmic behavior. Anomalous positive magnetoresistance of about 0.5% is observed on a single nanowire, which is attributed to the high spin polarization in nanowire device with pure Fe3O4 phase and nanocontact barrier. This work demonstrates the ability to preserve the pristine Fe3O4 and well defined electrode contact metal (Au)–magnetite interface, which helps in attaining high spin polarized current.

Anomalous room temperature magnetoresistance in brownmillerite Ca2Fe2O5

Anomalous magnetoresistance in wet chemical synthesized bulk polycrystalline Ca2Fe2O5and its comparative magnetic features with nanostructured Ca2Fe2O5.

Anomalous anisotropic magnetoresistance effects in graphene

We investigate the effect of external stimulus (temperature, magnetic field, and gases adsorptions) on anisotropic magnetoresistance (AMR) in multilayer graphene. The graphene sample shows superlinear magnetoresistance when magnetic field is perpendicular to the plane of graphene. A non-saturated AMR with a value of −33% is found at 10 K under a magnetic field of 7 T. It is surprisingly to observe that a two-fold symmetric AMR at high temperature is changed into a one-fold one at low temperature for a sample with an irregular shape. The anomalous AMR behaviors may be understood by considering the anisotropic scattering of carriers from two asymmetric edges and the boundaries of V+(V-) electrodes which serve as active adsorption sites for gas molecules at low temperature. Our results indicate that AMR in graphene can be optimized by tuning the adsorptions, sample shape and electrode distribution in the future application.

Anomalous organic magnetoresistance from competing carrier-spin-dependent interactions with localized electronic and nuclear spins

We describe a new regime for low-field magnetoresistance in organic semiconductors, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere. The regime is studied by the controlled addition of localized electronic spins to a material that exhibits substantial room-temperature magnetoresistance ($\sim 20$\%). Although initially the magnetoresistance is suppressed by the doping, at intermediate doping there is a regime where the magnetoresistance is insensitive to the doping level. For much greater doping concentrations the magnetoresistance is fully suppressed. The behavior is described within a theoretical model describing the effect of carrier spin dynamics on the current.

Colossal anisotropic resistivity and oriented magnetic domains in strained La0.325Pr0.3Ca0.375MnO3 films

Magnetic and resistive anisotropies have been studied for the La0.325Pr0.3Ca0.375MnO3 films with different thicknesses grown on low symmetric (011)-oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 substrates. In the magnetic and electronic phase separation region, a colossal anisotropic resistivity (AR) of ∼105% and an anomalous large anisotropic magnetoresistance can be observed for 30 nm film. However, for 120 nm film, the maximum AR decreases significantly (∼2 × 103%) due to strain relaxation. The colossal AR is strongly associated with the oriented formation of magnetic domains, and the features of the strain effects are believed to be useful for the design of artificial materials and devices.

Anomalous giant positive magnetoresistance and heavy fermion like behaviour in Mn11Ge8

Mn11Ge8 undergoes long range ferromagnetic ordering at 274 K followed by non-collinear antiferromagnetic structure below 150 K. Our investigations indicate the existence of large positive magnetoresistance which changes sign depending upon the applied field and temperature. In the temperature variation of resistivity and heat capacity, the sample shows very large coefficients of quadratic and linear terms, respectively. This indicates the existence of giant spin fluctuation despite the fact that the sample is in a magnetically ordered state. The Mn-moments possibly have both localized and itinerant character and the anomaly appears to be associated with the local moment spin fluctuations mediated via itinerant electrons. The large positive magnetoresistance is believed to be originated from the possible development of short range antiferromagnetic clusters on application of magnetic field.

The Origin of Local Strain in Highly Epitaxial Oxide Thin Films

The ability to control the microstructures and physical properties of hetero-epitaxial functional oxide thin films and artificial structures is a long-sought goal in functional materials research. Normally, only the lattice misfit between the film and the substrate is considered to govern the physical properties of the epitaxial films. In fact, the mismatch of film unit cell arrangement and the Surface-Step-Terrace (SST) dimension of the substrate, named as “SST residual matching”, is another key factor that significantly influence the properties of the epitaxial film. The nature of strong local strain induced from both lattice mismatch and the SST residual matching on ferroelectric (Ba,Sr)TiO3 and ferromagnetic (La,Ca)MnO3 thin films are systematically investigated and it is demonstrated that this combined effect has a dramatic impact on the physical properties of highly epitaxial oxide thin films. A giant anomalous magnetoresistance effect (~1010) was achieved from the as-designed vicinal surfaces.

Effect of partial magnetic order on resistivity and thermopower of Ho(Co1−x Al x )2 alloys

The electrical resistivity ρ and the thermopower S of Ho(Co1−xAlx)2 alloys (x = 0 to 0.2) were measured at temperatures from 2 K to 300 K in magnetic fields up to 10 T. While ρ(T) and S(T) of pure HoCo2 reveal abrupt changes at the magnetic ordering temperature TC, indicating a firstorder transition, the temperature variations of both properties across the ordering temperature in Ho(Co1−xAlx)2 alloys clearly show that the type of magnetic transition changes under substitution of Al for Co from first order to second order around x ≈ 0.06. ρ(T) of the Al-substituted samples (x = 0.15 and 0.2) has a very unusual variation at temperatures below TC: in contrast to the expected and usually-observed decrease of ρ(T), the resistivity of Ho(Co1−xAlx)2 alloys (x = 0.15 and 0.2) increases at temperatures below TC. Moreover, the magnetoresistance of these alloys is positive around TC; this is also in a sharp contrast to the usually observed negative magnetoresistance. We show that the anomalous resistivity and magnetoresistance in these alloys are related to the metamagnetic instability of the Co 3d subsystem and to the fluctuating local magnetic susceptibility due to atomic substitution. Due to this fluctuating local magnetic susceptibility, the uniform exchange field of 4f moments induces a non-uniform polarization of the 3d system, with static spatial fluctuations of the local magnetization. This static magnetic disorder gives an additional contribution (ρm) to the electrical resistance in the alloys at temperatures below TC. The degree of this disorder changes with decreasing temperature, resulting in specific variation of ρm with temperature.

Anomalous magnetoresistance of an array of GeSi nanowires

The magnetoresistance (MR) of a GeSi nanowires (NWs) array is studied as a function of magnetic field, B, and temperature. We observe an asymmetrical MR versus B with a minimum MR at Bmin ≠ 0. Moreover, Bmin increases with temperature, and is larger for a current perpendicular to the NWs than one parallel. A model is proposed to explain this anomalous MR based on the spin-orbit coupling of holes, which is associated with the band structure and morphology of GeSi NWs. By analyzing the MR around Bmin, considerably different mobilities for holes of different spin orientations are derived.

Exchange-Coupling-Induced Symmetry Breaking in Topological Insulators

An exchange gap in the Dirac surface states of a topological insulator (TI) is necessary for observing the predicted unique features such as the topological magnetoelectric effect as well as to confine Majorana fermions. We experimentally demonstrate proximity-induced ferromagnetism in a TI, combining a ferromagnetic insulator EuS layer with Bi(2)Se(3), without introducing defects. By magnetic and magnetotransport studies, including anomalous Hall effect and magnetoresistance measurements, we show the emergence of a ferromagnetic phase in TI, a step forward in unveiling their exotic properties.

Investigation of anomalous magnetoresistance in topological insulator Bi2Te3 at the onset of superconductivity in indium contacts

Presented here is evidence that suggests induced superconductivity in topological insulator Bi2Te3 occurring at its interface with indium (In) contacts. Thin Bi2Te3 films of several thicknesses were grown by molecular beam epitaxy and were studied by magnetotransport measurements using In as contacts. Magnetotransport measurements at temperatures below the superconducting transition of In reveal an anomalous abrupt drop in resistance of the Bi2Te3 samples suggesting the formation of a superconducting region at the In/Bi2Te3 interface induced by the In contacts.

Observation of crossover from weak localization to antilocalization in the temperature dependence of the resistance of a two-dimensional system with spin-orbit interaction

A nonmonotonic temperature dependence of the resistance with a maximum in the temperature range of 2–4 K whose position depends on the hole density has been observed in hole channels of silicon field-effect transistors. The spin-orbit hole relaxation time and the temperature dependences of the phase relaxation time of the electron wave have been obtained from the measurements of the alternating sign anomalous magnetoresistance. The nonmonotonic temperature dependence of the resistance can be described by the formulas of weak-localization theory with these parameters. The maximum appears owing to a temperature-induced change in the relation between the measured times. As a result, the localization behavior of the conductivity at high temperatures is changed to the antilocalization behavior at low temperatures. The inclusion of quantum corrections to the conductivity caused by the electron-electron interaction improves quantitative agreement between the experiment and calculation. Thus, it has been demonstrated that, in contrast to the widely accepted concept, there is a region of the parameters where the electron-electron interaction does not change the antilocalization (metallic) type of the temperature dependence of the resistance.

Magnetotransport properties of undoped amorphous carbon films

Anomalous positive magnetoresistance (MR) up to 36% was observed at 2K and 12T in the undoped amorphous carbon (a-C) film deposited on glass substrate by pulsed laser deposition at 500°C. There is no tendency of saturation of MR with increase of the magnetic field. The MR decreases as the measurement temperature increases from 2 to 80K, and could hardly be observed above 80K. As the deposition temperature grows from 300 to 600°C, the disorder degree of the a-C film decreases, and the value of MR also decreases, indicating that the lower disorder degree results in a smaller MR. The mechanism of this MR could be ascribed to the wave function shrinkage. It could help to understand the MR phenomenon in amorphous material systems.

Anomalous Hall effect and magnetoresistance behavior in Co/Pd1−xAgx multilayers

In this paper, we report anomalous Hall effect (AHE) correlated with the magnetoresistance behavior in [Co/Pd1−xAgx]n multilayers. For the multilayers with n = 6, the increase in Ag content from x = 0 to 0.52 induces the change in AHE sign from negative surface scattering-dominated AHE to positive interface scattering-dominated AHE, which is accompanied with the transition from anisotropy magnetoresistance (AMR) dominated transport to giant magnetoresistance (GMR) dominated transport. For n = 80, scaling analysis with Rs∝ρxxγ yields γ ∼ 3.44 for x = 0.52 which presents GMR-type transport, in contrast to γ ∼ 5.7 for x = 0 which presents AMR-type transport.

Electron transport in a ferromagnetic/normal/ferromagnetic tunnel junction based on the surface of a topological insulator

We theoretically study the electron transport properties in a ferromagnetic/normal/ferromagnetic tunnel junction, which is deposited on the top of a topological surface. The conductance at the parallel (P) configuration can be much bigger than that at the antiparallel (AP) configuration. Compared P with AP configuration, there exists a shift of phase which can be tuned by gate voltage. We find that the exchange field weakly affects the conductance of carriers for P configuration but can dramatically suppress the conductance of carriers for AP configuration. This controllable electron transport implies anomalous magnetoresistance in this topological spin valve, which may contribute to the development of spintronics. In addition, there shows an existence of Fabry-Perot-like electron interference in our model based on the topological insulator, which does not appear in the same model based on the two dimensional electron gas.

Weak localization of Dirac fermions in HgTe quantum wells

Weak localization in a system of gapless two-dimensional Dirac fermions in HgTe quantum wells with thickness d = 6.6 nm, which corresponds to the transition from a normal to an inverted spectrum, has been investigated experimentally. A negative logarithmic correction to the conductivity of the system has been observed both at the Dirac point and in the vicinity of this point. The anomalous magnetoresistance of two-dimensional Dirac fermions is positive. This indicates that weak localization in the system of two-dimensional Dirac fermions occurs owing to localization and interaction effects in the presence of rapid spin relaxation.

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.

Large magnetoresistance and Jahn-Teller effect in Sr2FeCoO6

Neutron diffraction measurement on the spin glass double perovskite Sr$_2$FeCoO$_6$ reveals site disorder as well as Co$^{3+}$ intermediate spin state. In addition, multiple valence states of Fe and Co are confirmed through M\"{o}ssbauer and X-ray photoelectron spectroscopy. The structural disorder and multiple valence lead to competing ferromagnetic and antiferromagnetic interactions and subsequently to a spin glass state, which is reflected in the form of an additional $T$-linear contribution at low temperatures in specific heat. A clear evidence of Jahn-Teller distortion at the Co$^{3+}$-O$_6$ complex is observed and incorporating the physics of Jahn-Teller effect, the presence of localized magnetic moment is shown. A large, negative and anomalous magnetoresistance of $\approx$ 63% at 14K in 12T applied field is observed for Sr$_2$FeCoO$_6$. The observed magnetoresistance could be explained by applying a semi-empirical fit consisting of a negative and a positive contribution and show that the negative magnetoresistance is due to spin scattering of carriers by localized magnetic moments in the spin glass phase.

Magnetoresistance oscillations arising from edge-localized electrons in low-defect graphene antidot-lattices

The so-called zigzag edge of graphenes has localized and strongly spin-polarized electrons. However, magnetoresistance (MR) behavior associated with the edge electrons has not been reported in graphenes. Here, we measure MR of graphene antidot-lattices, honeycomb-like arrays of hexagonal antidots with a large ensemble of hydrogen-terminated and low-defect antidot edges, prepared by a nonlithographic method using nanoporous alumina templates. We find anomalous MR oscillations arising from localized electron spins existing at the antidot edges. These are promising for realization of spintronic devices.