Magnetoresistance In Graphene Research Articles

Crossover between Positive and Negative Magnetoresistance in Graphene: Roles of Absence of Backscattering

The magnetoresistance in weak magnetic fields is calculated in graphene containing “short-range” scatterers causing isotropic intra- and inter-valley scattering and “long-range” scatterers characte...

Magnetophonon spectroscopy of Dirac fermion scattering by transverse and longitudinal acoustic phonons in graphene

Recently observed magnetophonon resonances in the magnetoresistance of graphene are investigated using the Kubo formalism. This analysis provides a quantitative fit to the experimental data over a wide range of carrier densities. It demonstrates the predominance of carrier scattering by low energy transverse acoustic (TA) mode phonons: the magnetophonon resonance amplitude is significantly stronger for the TA modes than for the longitudinal acoustic (LA) modes. We demonstrate that the LA and TA phonon speeds and the electron-phonon coupling strengths determined from the magnetophonon resonance measurements also provide an excellent fit to the measured dependence of the resistivity at zero magnetic field over a temperature range of 4-150 K. A semiclassical description of magnetophonon resonance in graphene is shown to provide a simple physical explanation for the dependence of the magneto-oscillation period on carrier density. The correspondence between the quantum calculation and the semiclassical model is discussed.

Proximity magnetoresistance in graphene induced by magnetic insulators

We demonstrate the existence of the giant proximity magnetoresistance (PMR) effect in a graphene spin valve where spin polarization is induced by a nearby magnetic insulator. PMR calculations were performed for yttrium iron garnet (YIG), cobalt ferrite (CFO), and two europium chalcogenides EuO and EuS. We find significant PMR (up to 100%) values defined as a relative change of graphene conductance with respect to parallel and antiparallel alignment of two proximity-induced magnetic regions within graphene. Namely, for high Curie temperature (${T}_{C}$) CFO and YIG insulators, which are particularly important for applications, we obtain 22% and 77% at room temperature, respectively. For low ${T}_{C}$ chalcogenides, EuO and EuS, the PMR is 100% in both cases. Furthermore, the PMR is robust with respect to system dimensions and edge-type termination, and it even maintains significant values (around 50% for YIG) in the presence of considerable spin-orbit coupling strength. Our findings show that it is possible to induce spin-polarized currents in graphene with no direct injection through magnetic materials.

Weak-Field Magnetoresistance in Graphene with Long-Range Scatterers

The magnetoconductivity proportional to B2 with B the strength of applied magnetic field is calculated in monolayer graphene in a self-consistent Born approximation for scatterers with long-range p...

Formula of Weak-Field Magnetoresistance in Graphene

The formula of weak-field magnetoconductivity Δσ(B) proportional to B2, with B the strength of a magnetic field, is derived for systems corresponding to monolayer and bilayer graphenes. It is repre...

Large unsaturated room temperature negative magnetoresistance in graphene foam composite for wearable and flexible magnetoelectronics

Room temperature positive magnetoresistance (PMR) in graphene is a conventional phenomenon but we observed large negative magnetoresistance (NMR) in GF/polydimethylsiloxane (GF/PDMS) at room temperature for the first time. The largest NMR ~ 35% was detected at 250 K, while PMR is observed below 200 K. Furthermore, PMR at all temperatures is observed in regular GF specimens, hence, NMR is the result of the infiltration with the electrically insulating polymer. Forward interference and wavefunction shrinkage model has been employed to understand the transport mechanism in GF/PDMS. A critical temperature ~ 224 K for switching between NMR and PMR is observed at the crystallization temperature of PDMS, suggesting a change in polymer chain conformation may be a major reason leading to NMR in GF/PDMS specimens thus role of mechanical properties of PDMS in NMR cannot be ignored and observed locally via specially resolved atomic force microscopy. In addition, storage modulus and heat flow study shows similar transition temperature (~ 200 K) of NMR to PMR and provide an evidence of mechanical stable specimens. As is known, large, tunable, and unsaturated NMR at room temperature is very useful for future facile practical shapeable magnetoelectronic devices.

Oscillating Magnetoresistance in Graphene p-n Junctions at Intermediate Magnetic Fields.

We report on the observation of magnetoresistance oscillations in graphene p-n junctions. The oscillations have been observed for six samples, consisting of single-layer and bilayer graphene, and persist up to temperatures of 30 K, where standard Shubnikov-de Haas oscillations are no longer discernible. The oscillatory magnetoresistance can be reproduced by tight-binding simulations. We attribute this phenomenon to the modulated densities of states in the n- and p-regions.

Giant tunnel magneto-resistance in graphene based molecular tunneling junction.

We propose and theoretically investigate a class of stable zigzag graphene nanoribbon (ZGNR) based molecular magnetic tunneling junctions (MTJs). For those junctions having pentagon-connecting formations, huge tunnel magneto-resistance (TMR) is found. Different from most of the other proposed molecular junctions, the huge TMR in our structures is generic, and is not significantly affected by external parameters such as bias voltage, gate voltage, length of the molecule and width of the ZGNRs. The double pentagon-connecting formation between the molecule and ZGNRs is critical for the remarkable TMR ratio, which is as large as ∼2 × 10(5). These molecular MTJs behave as almost perfect spin filters and spin valve devices. Other connecting formations of the ZGNR based MTJs lead to much smaller TMR. By first principles analysis, we reveal the microscopic physics responsible for this phenomenon.

Weak Delocalization in Graphene on a Ferromagnetic Insulating Film.

Graphene has been predicted to develop a magnetic moment by proximity effect when placed on a ferromagnetic film, a promise that could open exciting possibilities in the fields of spintronics and magnetic data recording. In this work, the interplay between the magnetoresistance of graphene and the magnetization of an underlying ferromagnetic insulating film is studied in detail. A clear correlation between both magnitudes is observed but through a careful modeling of the magnetization and the weak localization measurements, that such correspondence can be explained by the effects of the magnetic stray fields arising from the ferromagnetic insulator is found. The results emphasize the complexity arising at the interface between magnetic and 2D materials.

Nondiffusion theory of weak localization magnetoresistance in graphene

We put forward a theory of the weak localization in two dimensional graphene layers which explains experimentally observable transition between positive and negative magnetoresistance. Calculations are performed for the whole range of classically weak magnetic field with account on intervalley transitions. Contribution to the quantum correction which stems from closed trajectories with few scatterers is carefully taken into account. We show that intervalley transitions lead not only to the transition from weak antilocalization to the weak localization, but also to the non-monotonous dependence of the conductivity on the magnetic field.

Nonlinear magnetotransport theory and Hall induced resistance oscillations in graphene

The quantum oscillations of nonlinear magnetoresistance in graphene that occur in response to a dc current bias are investigated. We present a theoretical model for the nonlinear magnetotransport of graphene carriers. The model is based on the exact solution of the effective Dirac equation in crossed electric and magnetic fields, while the effects of randomly distributed impurities are perturbatively added. To compute the nonlinear current effects, we develop a covariant formulation of the migration center theory. The current is calculated for short- and large-range scatterers. The analysis of the differential resistivity in the large magnetic field region, shows that the extrema of the Shubnikov de Hass oscillations invert when the dc currents exceed a threshold value. These results are in good agreement with experimental observations. In the small magnetic field regime, corresponding to large filling factors, the existence of Hall induced resistance oscillations are predicted for ultra clean graphene samples. These oscillations originate from Landau–Zener tunneling between Landau levels, that are tilted by the strong electric Hall field.

Magnetoresistance in graphene under quantum limit regime

We report the magnetoresistance (MR) in single-layer graphene field-effect transistors at different gate voltages. At low temperatures, the MR exhibits fluctuations at Dirac point, which is attributed to quantum interference effect. Apart from the Dirac point, the MR shows a linear behavior under the extreme quantum limit that is identified by Shubnikov-de Haas oscillations and quantum Hall effect. Our experimental results give a clear manifestation of the quantum linear MR.

Electron-Electron Interaction in the Magnetoresistance of Graphene

We investigate the magnetotransport in large area graphene Hall bars epitaxially grown on silicon carbide. In the intermediate field regime between weak localization and Landau quantization, the observed temperature-dependent parabolic magnetoresistivity is a manifestation of the electron-electron interaction. We can consistently describe the data with a model for diffusive (magneto)transport that also includes magnetic-field-dependent effects originating from ballistic time scales. We find an excellent agreement between the experimentally observed temperature dependence of magnetoresistivity and the theory of electron-electron interaction in the diffusive regime. We can further assign a temperature-driven crossover to the reduction of the multiplet modes contributing to electron-electron interaction from 7 to 3 due to intervalley scattering. In addition, we find a temperature-independent ballistic contribution to the magnetoresistivity in classically strong magnetic fields.

Tunnel anisotropic magnetoresistance in graphene with Rashba spin–orbit interaction

Ballistic transport in a graphene-based normal/ferromagnetic barrier/normal junction in the presence of Rashba-type spin–orbit interaction (RSOI) is investigated by the non-equilibrium Green’s function approach. It is found that due to the interplay between ferromagnetic exchange coupling and RSOI, the energy dispersion in the ferromagnetic barrier depends on the magnetization direction. The conductance changes by varying the magnetization direction, resulting in a tunnel anisotropic magnetoresistance (TAMR). The predicted TAMR effect oscillates with the RSOI strength or on-site energy, which is efficiently controllable by the gate voltage, making this junction very promising in spintronics applications.

From positive to negative magnetoresistance in graphene with increasing disorder

Artificial disorder was introduced gradually into monolayer graphene by controlling Ga+ ion irradiation and the corresponding electronic transport properties regulated by gate voltage, source-drain voltage, temperature, and magnetic field were studied experimentally. An unsaturated positive magnetoresistance (MR) up to 100% at ∼5 T was observed in as-fabricated graphene, while there is significant negative MR in disordered graphene. This phenomenon was attributed to the monocrystalline breaking and crystallite-boundary scattering in disordered graphene.

Theory of huge tunneling magnetoresistance in graphene

We investigate theoretically the spin-independent tunneling magnetoresistance effect in a graphene monolayer modulated by two parallel ferromagnets deposited on a dielectric layer. For the parallel magnetization configuration, Klein tunneling can be observed in the transmission spectrum but at specific oblique incident angles. For the antiparallel magnetization configuration, the transmission can be blocked by the magnetic-electric barrier provided by the ferromagnets. Such a transmission discrepancy results in a tremendous magnetoresistance ratio and can be tuned by the inclusion of an electric barrier.