Conductivity, magnetoresistance, and the Hall effect in granular Fe/SiO2 films

We have investigated the conductance, magnetoresistance, and Hall effect in granular Fe/SiO₂ films with size of the iron grains around 40 Å, whose volume fraction x lies in the range 0.3–0.7. The conduction activation regime has been established for x<0.6. On the insulator side of the transition we observed a giant negative magnetoresistance, falling off sharply as the metal volume fraction decreases. For x<0.4 we observed a large positive magnetoresistance of premagnetized samples, showing up in fields; ～100 Oe and characterized by large response times. The field dependence of the Hall effect in the dielectric samples, as in the metallic samples, correlates with their magnetization. We found that the Hall resistance is proportional to the square root of the longitudinal resistance, which cannot be explained by known models of the anomalous Hall effect.     

Structural, magnetic and transport properties of Ni-doped ZnO films

In this work, Ni-doped ZnO (Zn_1−xNi_xO, x=0, 0.03, 0.06, 0.11) films were prepared using magnetron sputtering. X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), temperature dependence electrical resistance, Hall and magnetic measurements were utilized in order to study the properties of the Ni-doped ZnO films. XRD and XAS results indicate that all the samples have a ZnO wurtzite structure and Ni atoms incorporated into ZnO host matrix without forming any secondary phase. The Hall and electrical resistance measurements revealed that the resistivity increased by Ni doping, and all the Ni-doped ZnO films exhibited n-type semiconducting behavior. The magnetic measurements showed that for the samples with x=0.06 and 0.11 are room-temperature ferromagnetic having a saturation magnetization of 0.33 and 0.39 μ_B/Ni, respectively. The bound-magnetic-polaron mediated exchange is proposed to be the possible mechanism for the room-temperature ferromagnetism in this work.     

Anisotropy of electrical conductivity and hall effect in MnAs0.95P0.05

The resistivity tensor ? and the anisotropic Hall resistance ?_H are measured for 70 K ? T ? 320K. ? is assigned to the anisotropy of the Fermi surface while ?_H is related to the magnetization ellipsoid for T < T_N = 228 K (anomalous Hall effect).     

Fractional quantization of Hall resistance as a consequence of mesoscopic feedback

A nonlinear single-particle model is introduced, which captures the characteristic of systems in the quantum Hall regime. The model involves the magnetic Schrödinger equation with spatially variable magnetic flux density. The distribution of flux is prescribed via the postulates of the mesoscopic mechanics (MeM) introduced in my previous articles (cf. [9, 10]). The model is found to imply exact integer and fractional quantitzation of the Hall conductance. In fact, Hall resistance is found to be R _H = (h/e ²)(M/N) at the filling factor value N/M. The assumed geometry of the Hall plate is rectangular. Special properties of the magnetic Schrödinger equation with the mesoscopic feedback loop allow us to demonstrate quantization of Hall resistance as a direct consequence of charge and flux quantization. I believe results presented here shed light at the overall status of the MeM in quantum physics, confirming its validity.     

Classical and quantum hall effect measurements in GaInNAs/GaAs quantum wells

We have performed magneto-transport experiments in modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells with nitrogen mole fractions 0.4%, 1.0% and 1.5%. Classical magnetotransport (resistivity and low-field Hall effect) measurements have been performed in the temperatures between 1.8 and 275 K, while quantum Hall effect measurements in the temperatures between 1.8 and 47 K and magnetic fields up to 11 T.The variations of Hall mobility and Hall carrier density with nitrogen mole fractions and temperature have been obtained from the classical magnetotransport measurements. The results are used to investigate the scattering mechanisms of electrons in the modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells. It is shown that the alloy disorder scattering is the major scattering mechanism at investigated temperatures.The quantum oscillations in Hall resistance have been used to determine the carrier density, effective mass, transport mobility, quantum mobility and Fermi energy of two-dimensional (2D) electrons in the modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells. The carrier density, in-plane effective mass and Fermi energy of the 2D electrons increases when the nitrogen mole fraction is increased from y=0.004 to 0.015. The results found for these parameters are in good agreement with those determined from the Shubnikov-de Haas effect in magnetoresistance.     

Combined breakdown in bismuth—Antimony semiconductor alloys

S-shaped current-voltage characteristics for Bi_1?xSb_x alloys are studied theoretically and experimentally. The phenomenon is shown to occur due to the combined interband breakdown, the impact ionization being caused both by the external electric field and the Hall field. The latter is governed by the proper magnetic field of the plasma current. The negative differential resistance (NDR) occurs for the range of currents where the impact ionization is growing intensively but the pinch effect is not yet developed to full extent. The phenomenon is enhanced if the impact ionization rate in the Hall field is greater than in the applied one.     

Topological insulators based on the semi-metallic HgCdTe

The review of peculiarity of growth and experimental results of the magneto-transport measurements (longitudinal magneto-resistance R_xx and the Hall resistance R_xy) over a wide interval of temperatures for several samples of Hg_1?xCd_xTe (x ≈ 0.13–0.15) grown by MBE is presented in this paper. An amazing temperature stability of the SdH-oscillation period and amplitude is observed in the entire temperature interval of measurements up to 50 K. Moreover, the quantum Hall effect (QHE) behaviour of the Hall resistance was shown in the same temperature interval. These peculiarities of the R_xx and R_xy for strained thin layers are interpreted using quantum Hall conductivity (QHC) on topologically protected surface states (TPSS). In the case of not strained layers it is assumed that the QHC on the TPSS contributes also to the conductance of the bulk samples. The experimental results on magneto-transport (QHC and SdH) obtained for the strained 100 nm thickness Hg_1?xCd_xTe layer are interpreted on the basis of the 8 × 8 kp model and an advantage of the Hg_1?xCd_xTe as topological insulators is shown. This article is an expanded version of the scientific reports presented at the International Conference on Semiconductor Nanostructures for Optoelectronics and Biosensors 2016 ICSeNOB2016, May 22–25, 2016, Rzeszow, Poland.     

Nonmonotonic temperature dependence of the Hall resistance of a 2D electron system in silicon

For a 2D electron system in silicon, the temperature dependence of the Hall resistance ρ_xy(T) is measured in a weak magnetic field in the range of temperatures (1–35 K) and carrier concentrations n where the diagonal resistance component exhibits a metallic-type behavior. The temperature dependences ρ_xy(T) obtained for different n values are nonmonotonic and have a maximum at T_max ～ 0.16T_F. At lower temperatures T < T_max, the change δρ_xy(T) in the Hall resistance noticeably exceeds the interaction quantum correction and qualitatively agrees with the semiclassical model, where only the broadening of the Fermi distribution is taken into account. At higher temperatures T > T_max, the dependence ρ_xy(T) can be qualitatively explained by both the temperature dependence of the scattering time and the thermal activation of carriers from the band of localized states.     

Multi-channel magnetotransport in Co2FeSi/(Al,Ga)As spin-LEDs

We have performed Hall effect measurements on Co₂FeSi/(Al,Ga)As spin light emitting diodes and have found unique field dependencies that differ strongly from the expected behaviors for both the ferromagnetic Co₂FeSi layer and the underlying semiconductor structure. To understand such unique field dependencies, we have developed a multi-channel transport model for parallel transport through a ferromagnet and a semiconductor. By applying this model to our data for the Hall and sheet resistance, we extract values for the carrier density and mobility in the semiconductor layer. We find that these values decrease with increasing growth temperature of the Co₂FeSi layer, presumably due to stronger in-diffusion of Co and Fe impurities, which compensate the n-type dopants in the underlying n-(Al, Ga) As layer. Despite such compensation, spin-LEDs with the Co₂FeSi layer grown at the relatively high temperature of 280 °C exhibit the highest spin injection efficiencies of more than 50%, hence calling into question the requirement of electron tunneling through the ferromagnet/semiconductor Schottky barrier for efficient spin injection.     

Field dependent Hall coefficient of the Kondo-system (La, Ce)B6

The Hall coefficient R_H of dilute (La, Ce)B₆ single crystals has been measured in magnetic fields up to 7 T in the temperature range 10 mK- 4.2 K. The field dependence of R_H shows pronounced maxima, in accordance with theoretical predictions. From the size and position of the Hall maximum we obtain a consistent value for the s-wave phase shift δ = 15° of the Ce impurities in LaB₆.     

Large spontaneous Hall angle in [Co,CoFe/Pt] multilayer

We have quantitatively investigated the Hall effect in [Co, CoFe/Pt] multilayer films. The [Co, CoFe/Pt] multilayers exhibit large spontaneous Hall resistivity (ρ_H) and Hall angle (ρ_H/ρ). Even though the Hall resistivity in [Co, CoFe/Pt] multilayer films (2.7–4 × 10⁻⁷ Ω cm) is smaller than that of amorphous RE–TM alloy films which show large spontaneous Hall resistivity (<2 × 10⁻⁶ Ω cm), the Hall angle of multilayer (6–8%) is almost twice than that in amorphous rare earth–transition metal alloy films (∼3%). The Hall angle provides evidence of the effects of the exchange interaction of the Hall scattering. The exchange is between conduction electron spins and the localized spins of the transition metal. The large Hall angle of [Co, CoFe/Pt] multilayer can be considered due to the high spin polarization and high Curie temperature of Co and CoFe transition metal layers. Even though the role of interfaces and surfaces in the magnetic properties of multilayer films may dominate that of the bulk, the Hall effects in [Co, CoFe/Pt] multilayer may be mainly dominated by the bulk effect.     

Studies on Co-Sn-Te thin films

Electrical and galvanomagnetic properties of Co _x Sn_1?x Te thin films have been studied. On the basis of small positive temperature coefficient of resistance, small and constant resistivity, low Hall coefficient and negligible magnetoresistance, it was concluded that CoTe-SnTe system is metallic. Discussion employing the Sondheimer’s model further corroborated the metallic nature of the thin films.     

Zeros of the transverse magneto-resistance in N-type silicon (100) inversion layers

The transverse and Hall resistance are investigated under quantizing electric and magnetic fields in n-type silicon (100) MOSFET inversion layers. The transverse resistance ρ_xx vanishes within finite ranges of the gate voltage where the concentration of channel conduction electrons is constant. The variation of the oscillatory period towards low gate voltages is not compatible with the concept of carrier localization and therefore can be understood by charge transfer into states outside the surface channel.     

Concentration dependence of the anomalous Hall effect in Fe/SiO2 granular films below the percolation threshold

The anomalous Hall effect is studied on Fe_x(SiO₂)_1?x nanocomposite films with x<0.7 in the vicinity of the percolation transition (x _c ≈0.6). It is found that, as the transition is approached from the side of metallic conduction, the Hall angle nonmonotonically varies, passing through a minimum. A qualitative model for describing the concentration dependence of the anomalous Hall effect is proposed. The model is based on that of the conductivity of a two-phase system near the percolation threshold [9, 10]. The anomalous Hall effect is governed by two conduction channels: one of them (a conducting network) is formed by large metal clusters that are separated by narrow dielectric interlayers below the percolation threshold, and the other is represented by the dielectric part of the medium containing Fe grains; in this part of the medium, the anomalous Hall effect occurs through the interference of amplitudes from the tunneling junctions in a set of three grains. It is shown that, at x<x _c , the network may give rise to a “shunting” effect, which makes the effective Hall voltage even less than the Hall voltage of the dielectric component.     

Planar Hall effect and anisotropic magnetoresistance in layered structures Co0.45Fe0.45Zr0.1/a-Si with percolation conduction

Magnetic and magnetotransport properties of multilayered nanostructures Co_0.45Fe_0.45Zr_0.1/a-Si obtained by ion-beam sputtering are investigated. The temperature dependence of the resistance obeys a law of the form R _xx ∝-logT, which is typical of metal-insulator nanocomposites on the metal side of the percolation transition. The magnetoresistance anisotropy effect, as well as the planar Hall effect, is observed for the first time for this type of nanocomposites in the vicinity of the percolation transition. The correlation of these two effects with the transverse (between Hall probes) magnetoresistive effect, which may reach 6–9%, is revealed. A weak negative magnetoresistance of the order of 0.15%, which is observed for subnanometer amorphous silicon layer thicknesses, is attributed to spin-dependent electron transitions between adjacent ferromagnetic layers in the case when the exchange interaction between these layers is of the antiferromagnetic type.     

The Nernst-Ettingshausen, Seebeck, and Hall effects in Sb2Te3 single crystals

This paper reports on measurement of the temperature dependences of the following transport coefficients: electrical conductivity in the σ₁₁ cleavage plane, the Seebeck coefficients S ₁₁ and S ₃₃ (axis 3 is along the trigonal crystal axis), the Hall coefficients R ₁₂₃ and R ₃₂₁, and the Nernst-Ettingshausen constant Q ₁₂₃; all measurements were made on high-quality Czochralski-grown Sb₂Te₃ single crystals. The results obtained are analyzed in terms of phenomenological theory. It is shown that the main features of the experimental data, including the anisotropy of the Hall and Seebeck effects, can be explained within a two-band model with notice-ably different anisotropy of the mobilities of holes of two types in the cleavage-plane and trigonal-axis directions. Estimates are made of the band-gap width (?_g?0.3 eV), as well as of the energy gap between the main and additional valence-band extrema (Δ?_v～0.1 eV).     

Enhancement of the anomalous Hall effect and spin glass behavior in the bilayered manganite La2−2xSr1+2xMn2O7

The Hall resistivity and magnetization have been investigated in the ferromagnetic state of the bilayered manganite La_2−2xSr_1+2xMn₂O₇ (x=0.36). The Hall resistivity shows an increase in both the ordinary and anomalous Hall coefficients at low temperatures below 50 K, a region in which experimental evidence for the spin glass state has been found in a low magnetic field of 1 mT. The origin of the anomalous behavior of the Hall resistivity relevant to magnetic states may lie in the intrinsic microscopic inhomogeneity in a quasi-two-dimensional electron system.     

Edge current switch of two-dimensional electron gas using carrier density control

We have investigated the effects of electron density discontinuity on the transports of edge currents of two-dimensional electron gas (2DEG). The electric field applied to a gate, which covers the 2DEG partially, gives rise to change in the carrier density and results in a density gradient, which deforms the edge currents. The transverse and longitudinal resistances were measured as functions of gate voltage V_G in the quantum Hall regime. The deviations of the longitudinal resistances from the normal quantum Hall resistances are attributed to the reflections of the edge currents under the influence of the abrupt density discontinuity. A switching behavior of the transverse resistance by controlling the gate voltage was observed when V_G=−2.2 and −2.0 V for magnetic field H=5 and 7.2 T, respectively.     

Experimental observation of a striking similarity between quantum hall transport coefficients

The derivative of the Quantum Hall resistance, ρ_xy, with respect to the carrier density, n, has been measured for a two-dimensional electron gas in a GaAs-Al_xGa_1?xAs heterostructure, as a function of magnetic field. dρ_xy/dn exhibits a remarkable similarity to the diagonal resistivity, ρ_xy, to the extent that one is almost directly proportional to the other. Our result suggests the possibility of a fundamental connection between the two quantities.