Resistivity Ρxx Research Articles

Method for determining the residual electron- and hole-densities about the neutrality point over the gate-controlled n ↔ p transition in graphene

The Hall effect and the diagonal resistance, which indicates a residual resistivity ρxx≈h/4e2, are experimentally examined over the p ↔ n transition about the nominal neutrality point in chemical vapor deposition (CVD) grown graphene. A distribution of neutrality potentials is invoked in conjunction with multi-carrier conduction to model the experimental observations. From the modeling, we extract the effective residual electron- and hole-densities around the nominal neutrality point. The results indicate mixed transport due to co-existing electrons and holes in large area zero-band gap CVD graphene devices, which indicates domain confined ambipolar currents broadly over the gate-induced n ↔ p transition.

Effect of growth temperature on the electronic transport and anomalous Hall effect response in co-sputtered Co2FeSi thin films

Co-sputtered Co2FeSi thin films are studied by varying the growth temperature (Ts) as a control parameter in terms of the appreciable change in the disorder. The effect of Ts on structural, magnetic, electrical, and magneto-transport properties was investigated. As Ts is increased from room temperature to 400 °C, an improvement in the crystallinity and atomic ordering are observed. These are found to be correlated with the associated reduction in residual resistivity (ρxx0) from 410 to 88 μΩ cm, an increment in residual resistivity ratio (r) from 0.8 to 1.23, and an increase in saturation magnetization from 1074 to 1196 emu/cc. The spin wave stiffness constant in these films is found to increase with Ts, with a reasonably high value of 358 meVÅ2 at the optimum value of Ts of 400 °C. Further, the obtained high carrier concentration and mobility values (at 10 K) of ∼30 e−s/f.u. and ∼0.11 cm2 V−1 s−1 for the films deposited at Ts = 400 °C shows the presence of compensated Fermi surface. The transport properties are investigated qualitatively from the scaling of anomalous Hall resistivity ρxys(T) with the longitudinal resistivity ρxx(T) data, employing the extrinsic (skew- and side-jump scatterings) and intrinsic scattering contributions. The variation in the intrinsic scattering contributions observed via the variation in linear dependence of ρxys on ρxx2 with the change in Ts is found to be associated with the improvement in the crystallinity of these films.

Anomalous Hall effect in monodisperse CoO-coated Co nanocluster-assembled films

We have fabricated the uniform CoO-coated Co nanocluster-assembled films at various oxygen gas flow rates (fO) by using a plasma-gas-condensation method and studied their anomalous Hall effect (AHE). The longitudinal resistivity (ρxx) of all the films exhibits a minimum at a temperature of Tmin. With the increase of fO, Tmin shifts from 150 to 300K and has no longer change when fO is up to 0.10sccm. The saturated AHE resistivity (ρxyA) presents a near linear increase as fO rises. The anomalous Hall coefficient (Rs) at fO=0.20sccm is 4.9×10−9ΩcmG−1 at 300K, which is almost three orders of magnitude larger than bulk Co. Moreover, at fO=0 and 0.05sccm, the scaling exponents γ=1.2 and 1.24 in ρxyA∝ρxxγ are obtained in the region of 325–375K; At fO=0.10, 0.15 and 0.20sccm, ρxyA decreases with the increase of ρxx on a double-logarithmic scale, following a new scaling relation of log(ρxyA/ρxx)=a0+b0logρxx in two temperature ranges of 5–300K and 325–375K.

Anomalous Hall Effect in (Co<sub>41</sub>Fe<sub>39</sub>B<sub>20</sub>)<sub>x</sub>(Al–O)<sub>100-x</sub> Nanocomposites: Temperature Dependence

We present our last results on anomalous Hall effect (AHE) in (Co41Fe39B20)x(Al–O)100-x nanocomposites focusing on the possible correlation between temperature dependence of AHE and resistivity. It is shown that the temperature dependence of conductivity G=1/Rxx, where Rxx is resistivity, for compositions with x=49-56% at 10K<T<Tk follows the relation, where the parameters A, , Tk depend on x. For x=47% this relation changes to the exponential law “1/2” Rxx ∝ exp (Т0/T)1/2. The correlation between AHE resistivity RH (T) and resistivity Rxx (T) can be described as RH ∝ (Rxx)m , where m increases from 0.38 to 0.58 with an increase of x from 49 to 56 %.

Scaling of anomalous Hall effect in amorphous CoFeB films with accompanying quantum correction

Abstract Scaling of anomalous Hall effect in amorphous CoFeB films with thickness ranging from 2 to 160 nm has been investigated. We have found that the scaling relationship between longitudinal (ρxx) and anomalous Hall (ρAH) resistivity is distinctly different in the Bloch and localization regions. For ultrathin CoFeB films, the sheet resistance (Rxx) and anomalous Hall conductance (GAH) received quantum correction from electron localization showing two different scaling relationships at different temperature regions. In contrast, the thicker films show a metallic conductance, which have only one scaling relationship in the entire temperature range. Furthermore, in the dirty regime of localization regions, an unconventional scaling relationship σ A H ∝ σ x x α with α=1.99 is found, rather than α=1.60 predicted by the unified theory.

Giant planar Hall effect in reactive sputtered epitaxial CrxFe3–xO4 films

The epitaxial CrxFe3-xO4 films were fabricated by dc reactive sputtering on MgO (001) substrates. The longitudinal resistivity ρxx was enhanced 3 orders of magnitude with the increase of Cr content x from 0 to 0.87. The tunneling magnetoresistance like planar Hall effect in the CrxFe3-xO4 films was observed while the angle between ⟨100⟩ in (001) oriented films and the direction of magnetic field is 45° and 135°, respectively. Both the phase relation of angular dependent planar Hall resistivity ρxy and the numeral relation of ρxy with longitudinal resistivity difference ρ//−ρ⊥ cannot be understood by the planar Hall effect expression in isotropic magnetic medium. The largest planar Hall resistivity was ∼105 μΩ cm for x = 0.71, which is one, two, and six orders of magnitude larger than that in Fe3O4, GaMnAs, and ferromagnetic metals, respectively. The giant planar Hall resistivity ρxy is weak saturated at high fields and increases with the magnetic field. This giant planar Hall effect in the highly resistive CrxFe3-xO4 films is closely correlated to the longitudinal resistivity and antiphase boundaries.

Enhanced anomalous Hall effect in Fe nanocluster assembled thin films

An enhanced anomalous Hall effect is observed in heterogeneous uniform Fe cluster assembled films with different film thicknesses (ta = 160-1200 nm) fabricated by a plasma-gas-condensation method. The anomalous Hall coefficient (Rs) at ta = 1200 nm reaches its maximum of 2.4 × 10(-8) Ω cm G(-1) at 300 K, which is almost four orders of magnitude larger than bulk Fe. The saturated Hall resistivity (ρ(A)xy) first increases and then decreases with the increase of temperature accompanied by a sign change from positive to negative. Analysis of the results revealed that ρ(A)xy decreases with increasing longitudinal resistivity (ρxx) on a double-logarithmic scale and obeys a new scaling relation of log(ρ(A)xy/ρxx) = a0 + b0 log ρxx.

Interaction of microwave radiation with the high mobility two-dimensional electron system in GaAs/AlGaAs heterostructures

The influence of microwave excitation on the magnetotransport properties of the high mobility two-dimensional electron system (2DES) in the GaAs/AlGaAs heterostructure system is investigated by exploring (a) the dependence of the amplitude of the microwave-induced magnetoresistance-oscillations on the polarization direction of the linearly polarized microwaves and (b) the microwave reflection from the 2DES. The polarization study indicates that the amplitude of the magnetoresistance oscillations is remarkably responsive to the relative orientation between the linearly polarized microwaves and the current-axis in the specimen. At low microwave power, P, experiments indicate a strong sinusoidal variation in the diagonal resistance Rxx vs. θ at the oscillatory extrema of the microwave-induced magnetoresistance oscillations. The reflection study indicates strong correlations between the microwave induced magnetoresistance oscillations and oscillatory features in the microwave reflection in a concurrent measurement of the magnetoresistance and the microwave magnetoreflection from the 2DES. The correlations are followed as a function of the microwave frequency and the microwave power, and the results are reported.

Nonlinear magnetotransport in a two-dimensional electron system with anisotropic mobility

Nonlinear magnetotransport of two-dimensional electrons in modulation-doped GaAs/AlAs heterostructures with anisotropic mobility is investigated. The mobility attains its maximum and minimum values in the $$\left[ {1\bar 10} \right]$$ and [110] directions, respectively. It is found that, upon an increase in the direct electrical current I dc though Hall bars oriented along the [110] direction, the transition of the two-dimensional system to the state with differential resistance r xx ≈ 0 in a magnetic field occurs at a lower value of I dc and is accompanied by a more pronounced “plunge” into the region of negative r xx values as compared to bars oriented along the $$\left[ {1\bar 10} \right]$$ direction. The results are explained by the impact of mobility on the spectral diffusion of nonequilibrium charge carriers.

Anomalous Hall effect in magnetic disordered alloys: Effects of spin orbital coupling

For disordered ternary Fe0.5(Pd1−xPtx)0.5 alloy films, the anomalous Hall effect obeys the conventional scaling law ρAH=aρxx+bρxx2 with the longitudinal resistivity ρxx and anomalous Hall resistivity ρAH. Contributed by the intrinsic term and the extrinsic side-jump one, the scattering-independent anomalous Hall conductivity b increases with increasing Pt/Pd concentration. In contrast, the skew scattering parameter a is mainly influenced by the residual resistivity. The present results will facilitate the theoretical studies of the anomalous Hall effect in magnetic disordered alloys.

The anomalous Hall effect in highly disordered SmCo5 thin films

The anomalous Hall effects in highly disordered SmCo5 films 14–204nmthick have been studied systematically. Owing to the weak localization effect, in the samples thicker than 14nm, we find a logarithmic temperature dependence in both the longitudinal resistivity ρxx(T) and the hall resistivity ρAH(T) at low temperatures. We calculate the weak localization correction to the conductance.

WIDTH OF QUANTUM HALL TRANSITION REGION IN THE PRESENCE OF LANDAU LEVEL MIXING

In this paper, we have provided a systematic measurement of the longitudinal resistance Rxx of quantum Hall transition in a two-subband two-dimensional electron system. The data shows that the standard scaling behavior with single critical point remains unchanged even with strong Landau level degeneracy. There is no definite evidence to support the recent prediction that the width of transition region becomes finite when extrapolated to zero temperature, resulting from Landau level mixing.

Dirac fermion heating, current scaling, and direct insulator-quantum Hall transition in multilayer epitaxial graphene

We have performed magnetotransport measurements on multilayer epitaxial graphene. By increasing the driving current I through our graphene devices while keeping the bath temperature fixed, we are able to study Dirac fermion heating and current scaling in such devices. Using zero-field resistivity as a self thermometer, we are able to determine the effective Dirac fermion temperature (TDF) at various driving currents. At zero field, it is found that TDF ∝ I≈1/2. Such results are consistent with electron heating in conventional two-dimensional systems in the plateau-plateau transition regime. With increasing magnetic field B, we observe an I-independent point in the measured longitudinal resistivity ρxx which is equivalent to the direct insulator-quantum Hall (I-QH) transition characterized by a temperature-independent point in ρxx. Together with recent experimental evidence for direct I-QH transition, our new data suggest that such a transition is a universal effect in graphene, albeit further studies are required to obtain a thorough understanding of such an effect.

Quantum resistance standard accuracy close to the zero-dissipation state

We report on a comparison of four GaAs/AlGaAs-based quantum resistance standards using an original technique adapted from the well-known Wheatstone bridge. This work shows that the quantized Hall resistance at Landau level filling factor ν=2 can be reproducible with a relative uncertainty of 32×10−12 in the dissipationless limit of the quantum Hall effect regime. In the presence of a very small dissipation characterized by a mean macroscopic longitudinal resistivity Rxx(B)¯ of a few μΩ, the discrepancy ΔRH(B) between quantum Hall resistors measured on the Hall plateau at magnetic induction B turns out to follow the so-called resistivity rule Rxx(B)¯=αB×d(ΔRH(B))/dB. While the dissipation increases with the measurement current value, the coefficient α stays constant in the range investigated (40−120 μA). This result enlightens the impact of the dissipation emergence in the two-dimensional electron gas on the Hall resistance quantization, which is of major interest for the resistance metrology. The quantum Hall effect is used to realize a universal resistance standard only linked to the electron charge e and the Planck constant h and it is known to play a central role in the upcoming revised Système International of units. There are therefore fundamental and practical benefits in testing the reproducibility property of the quantum Hall effect with better and better accuracy.

Structural ordering driven anisotropic magnetoresistance, anomalous Hall resistance, and its topological overtones in full-Heusler Co2MnSi thin films

We report the evolution of crystallographic structure, magnetic ordering, and electronic transport in thin films of full-Heusler alloy Co2MnSi deposited on (001) MgO with annealing temperatures (TA). By increasing the TA from 300 °C to 600 °C, the film goes from a disordered nanocrystalline phase to B2 ordered and finally to the L21 ordered alloy. The saturation magnetic moment improves with structural ordering and approaches the Slater-Pauling value of ≈5.0μB per formula unit for TA=600 °C. At this stage, the films are soft magnets with coercive and saturation fields as low as ≈7 mT and 350 mT, respectively. Remarkable effects of improved structural order are also seen in longitudinal resistivity (ρxx) and residual resistivity ratio. A model based upon electronic transparency of grain boundaries illucidates the transition from a state of negative dρ/dT to positive dρ/dT with improved structural order. The Hall resistivity (ρxy) derives contribution from the normal scattering of charge carriers in external magnetic field, the anomalous effect originating from built-in magnetization, and a small but distinct topological Hall effect in the disordered phase. The carrier concentration (n) and mobility (μ) have been extracted from the high field ρxy data. The highly ordered films are characterized by n and μ of 1.19 × 1029 m−3 and 0.4 cm2V−1s−1 at room temperature. The dependence of ρxy on ρxx indicates the dominance of skew scattering in our films, which shows a monotonic drop on raising the TA. The topological Hall effect is analyzed for the films annealed at 300 °C. We find maximum topological contribution to Hall resistivity around 0.5 T while it approach to zero with increasing magnetic field. The anisotropic magnetoresistance changes its sign from positive to negative as we go from as deposited to 600 °C annealed film, suggesting a gradual increase in the half-metallic character.

Epitaxial strain effect in tetragonal SrRuO3 thin films

We report on our characterization of the structural, electrical, and magnetic properties of tetragonal SrRuO3 (SRO) thin films stabilized under both compressive and tensile strain. These tetragonal films consisting of the deformed RuO6 octahedra without rotations were coherently grown on (110)ortho NdGaO3 and (110)ortho GdScO3 substrates, which provide compressive (−1.7%) and tensile (+1.0%) strains, respectively. The ferromagnetic transition temperature TC for the compressively strained film is found to be as high as 155 K, while TC of the film under tensile strain is only 100 K. The longitudinal resistivity ρxx of the compressively strained films is lower than that of the films under the tensile strain. This is attributed to the enhanced mobility for the compressive-strain case. The magnetic anisotropy also exhibits strong dependence on the substrate-induced epitaxial strain. The film under the compressive strain has the uniaxial magnetic easy axis along the out-of-plane direction, while the easy axis of the film under the tensile strain lies along the in-plane direction parallel to the [1-10]GSO one. The results demonstrate that the electrical and magnetic properties of the tetragonal SRO thin films are closely related to the RuO6 octahedral deformations due to the substrate-induced strain.

Experimental evidence for direct insulator-quantum Hall transition in multi-layer graphene

We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity ρxx, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility μq of our device. It is found that at the direct I-QH transition, μqBc ≈ 0.37 which is considerably smaller than 1. In contrast, at Bc, ρxx is close to the Hall resistivity ρxy, i.e., the classical mobility μBc is ≈ 1. Therefore, our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.

The anomalous Hall effect in the perpendicular Ta/CoFeB/MgO thin films

The anomalous Hall effect (AHE) in the perpendicular Ta/CoFeB/MgO thin film has been investigated. Between the AHE coefficient (RS) and longitudinal resistivity (ρxx), a linear behavior of RS/ρxx versus ρxx can be found. Moreover, the conductivity of the film is about 5 × 103 S/cm, which suggests that AHE in the Ta/CoFeB/MgO film be dominated by the intrinsic or scattering-independent mechanism.

Linear Polarization Rotation Study of the Microwave-Induced Magnetoresistance Oscillations in the GaAs/AlGaAs System

ABSTRACTMicrowave-induced zero-resistance states appear when the associated B-1-periodic magnetoresistance oscillations grow in amplitude and become comparable to the dark resistance of the two-dimensional electron system (2DES). Existing theories have made differing predictions regarding the influence of the microwave polarization in this phenomenon. We have investigated the effect of rotating, in-situ, the polarization of linearly polarized microwaves relative to long-axis of Hall bars. The results indicate that the amplitude of the magnetoresistance oscillations is remarkably responsive to the relative orientation between the linearly polarized microwave electric field and the current-axis in the specimen. At low microwave power, P, experiments indicate a strong sinusoidal variation in the diagonal resistance Rxx vs. θ at the oscillatory extrema of the microwave-induced magnetoresistance oscillations. Interestingly, the phase shift θ0 for maximal oscillatory Rxx response under photoexcitation is a strong function of the magnetic field, the extremum in question, and the magnetic field orientation.

Design of Spin Polarization Analyzer using Transverse-Longitudinal Correlation in Resistivities Induced by Spin–Orbit Interaction

We have theoretically studied a methodology for the measurement of the degree of spin polarization (P) in metals as well as semiconductors. Our principle is based on the correlation existing between transverse resistivity (ρyx) and longitudinal resistivity (ρxx), both influenced by transverse scattering due to a spin–orbit interaction (SOI) as well as longitudinal scattering due to usual mechanisms. Our spin polarization analyzer employs an unknown polarization conductor as a source electrode from which spin-polarized electrons are injected into a nonmagnetic (NM) channel region. The channel length is set to be much smaller than its spin diffusion length so that ρyx and ρxx in the NM region, both complementarily influenced by carrier spin polarization, would be measured to obtain the P value. Also, application to OR and XOR logic gates are discussed on the basis of our spin polarization analyzer.