Strong Magnetoresistance Research Articles

Phase coherence and size effects in double quantum well mesoscopic wires

Conductance fluctuations (CF) have been observed when an in-plane magnetic field is applied to 800 nm-wide mesoscopic wires fabricated from GaAs/(AlGa)As double quantum well structures. This shows that the inter-well tunnelling transition preserves electron phase coherence. The CF disappear when the magnetic field becomes large enough to suppress tunnelling between the quantum wells. The CF amplitude is smaller when the tunnelling rate is reduced either by unbalancing the double well potential, or by increasing the thickness of the tunnel barriers. The wires display a strong magnetoresistance feature as the wells are decoupled by the in-plane magnetic field. This has been seen in macroscopic Hall bars, but is modified due to diffuse edge scattering in the wires.

Strong Magnetoresistance Induced by Long-Range Disorder

We calculate the semiclassical magnetoresistivity $\rho_{xx}(B)$ of non-interacting fermions in two dimensions moving in a weak and smoothly varying random potential or random magnetic field. We demonstrate that in a broad range of magnetic fields the non-Markovian character of the transport leads to a strong positive magnetoresistance. The effect is especially pronounced in the case of a random magnetic field where $\rho_{xx}(B)$ becomes parametrically much larger than its B=0 value.

Positive Magnetoresistance of Composite Fermion Systems with a Weak One-Dimensional Density Modulation

Adopting the mean-field composite fermion picture, we describe the magnetotransport properties around filling factor $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\frac{1}{2}$ of a two-dimensional electron gas with a weak one-dimensional density modulation. The occurrence of a strong positive magnetoresistance at low effective magnetic fields as well as Weiss oscillations, which were observed in recent experiments in such systems, can be explained within a semiclassical Boltzmann equation approach, provided one goes beyond a second order approximation in the modulation strength.

Current–voltage characteristics and electrical transport properties of grain boundaries in La1−x(Sr/Ca)xMnO3

Grain boundaries in doped lanthanum manganites have been shown to have a large low-field magnetoresistance. However, the mechanism of electrical transport across grain boundaries, and the origin of the low-field magnetoresistance, are not well understood. Models based on scattering at domain walls, spin-polarized tunneling and depression of the Curie temperature due to strain near the grain boundary have all been proposed. This article reports detailed studies of the transport properties of artificial grain boundaries formed in a variety of thin films grown on bicrystal substrates. Resistance versus field sweeps on all grain boundary devices showed strong low-field magnetoresistance and the effect of individual domain motion at the grain boundary has been observed in single grain boundaries. In all cases, current versus voltage characteristics were highly non-ohmic, and reminiscent of an electron tunneling process. However, the magnetic dependence of the current–voltage characteristics implies that the magnetoresistance may be unrelated to tunneling.

Phase coherence in double quantum well mesoscopic wires

We have observed conductance fluctuations (CF) on application of an in-plane magnetic field to 800 nm wide mesoscopic wires fabricated from modulation-doped GaAs/(AlGa)As double quantum well structures. This demonstrates the preservation of electron phase coherence in the presence of inter-well tunnelling and intra-well scattering. The CF disappear when the magnetic field becomes large enough to suppress tunnelling between the quantum wells, and the CF amplitude is lower in samples with thicker tunnel barriers. The wires also display a strong magnetoresistance feature as the quantum wells are decoupled by the in-plane magnetic field. This has been seen previously in macroscopic Hall bars, but is modified due to size effects in the wires.

Strong localization of electrons in quasi-one-dimensional conductors

We report on an experimental study of electron transport in submicrometer-wide ``wires'' fabricated from Si \ensuremath{\delta}-doped GaAs. These quasi-one-dimensional (Q1D) conductors demonstrate the crossover from weak to strong localization with decreasing temperature. On the insulating side of the crossover, the resistance has been measured as a function of temperature, magnetic field, and applied voltage for different values of the electron concentration, which was varied by applying the gate voltage. The activation temperature dependence of the resistance has been observed with the activation energy close to the mean energy spacing of electron states within the localization domain. The study of nonlinearity of the current-voltage characteristics provides information on the distance between the critical hops that govern the resistance of Q1D conductors in the strong localization (SL) regime. We observe the exponentially strong negative magnetoresistance; this orbital magnetoresistance is due to the universal magnetic-field dependence of the localization length in Q1D conductors. The method of measuring the single-particle density of states (DOS) in the SL regime has been suggested. Our data indicate that there is a minimum of DOS at the Fermi level due to the long-range Coulomb interaction.

Magnetic and transport properties of sputtered Fe/Si multilayers

The structural, magnetic and transport properties of sputtered Fe/Si multilayers were studied. The analyses of the data of the X-ray diffraction, resistance and magnetic measurements show that heavy atomic interdiffusion between Fe and Si occurs, resulting in multilayers of different complicated structures according to different sublayer thicknesses. The nominal Fe layers in the multilayers generally consist of Fe layers doped with Si, ferromagnetic Fe-Si silicide layers and nonmagnetic Fe-Si silicide interface layers, while the nominal Si spacers turn out to be Fe-Si compound layers with additional amorphous Si sublayers only under the condition either \(\) for the series \(\) or \(\) for the series \(\) multilayers. A strong antiferromagnetic (AFM) coupling and negative magnetoresistance (MR) effect, about 1%, were observed only in multilayers with iron silicide spacers and disappeared when \(\)-Si layers appear in the spacers. The dependences of MR on \(\) and on bilayer numbers N resemble the dependence of AFM coupling. The increase of MR ratio with increasing N is mainly attributed to the improvement of AFM coupling for multilayers with N. The \(\) dependence of MR ratio is similar to that in metal/metal system with predominant bulk spin dependent scattering and is fitted by a phenomenological formula for GMR. At 77 K both the MR effect and saturation field \(\) increase. All these facts suggest that the mechanisms of the AFM coupling and MR effect in sputtered Fe/Si multilayers are similar to those in metal/metal system.

Effect of crystallinity on the magnetoresistive properties of La0.8MnO3−δ thin films grown by chemical vapor deposition

We report here a study on the role of crystallinity on the magnetoresistive properties of self-doped La0.8MnO3−δ thin films. Films have been grown by a liquid-source metalorganic chemical vapor deposition technique on two different single crystals: SrTiO3 (001) and Al2O3 (012). Epitaxial films are obtained on strontium titanate and show a high magnetoresistance peak at low fields: maximum magnetoresistive effect in the vicinity of the magnetic transition temperature, and negligible 50 K below. Polycrystalline films are observed on sapphire; they exhibit a lower magnetoresistance of several percent which remains nearly constant over a wide temperature range. Two magnetoresistive regimes can be distinguished in these films: in low fields (up to 5 mT), a strong magnetoresistance is obtained with a sensitivity of 0.28%/mT at 78 K; in higher fields, the magnetoresistance is linear and lower than this obtained in epitaxial films. The role of grain boundaries on the magnetoresistive effect is discussed.

High magnetic field studies of quantum transport and cyclotron resonance on 2D gas in GaN/GaAlN heterojunction

Magnetotransport and cyclotron resonance measurements were performed on Si-doped GaN/GaAlN heterostructure under pulsed magnetic field up to 35 T. The existence of 2D gas in the structure has been confirmed by cyclotron absorption and Shubnikov–de-Haas oscillations measurements under tilted magnetic field. In addition, a strong positive and temperature-dependent magnetoresistance is observed. The transport measurements are analyzed by a model based on two parallel conductive channels.

Duality Transformation in a Three Dimensional Conducting Medium with Two Dimensional Heterogeneity and an In-Plane Magnetic Field

The classical duality transformation of two dimensional continuum conductors is extended to three dimensional conductors with a two dimensional heterogeneity. This is used to discuss the classical magnetotransport of a periodic array of parallel cylindrical inclusions, which are either perfect insulators or superconductors, embedded in a free electron conducting host. A detailed understanding of the local field and current distributions is thereby achieved, and closed form expressions are obtained for the strong field magnetoresistance in some important configurations.

ENHANCED MAGNETORESISTIVE EFFECT IN TRILAYERED FILMS OF LaCaMnO/LaNdCaMnO/LaCaMnO

A series of La0.67Ca0.33MnO3/(La0.35Nd0.65)2/3Ca1/3MnO3/La0.67Ca0.33MnO3(termed as LCMO/LNCMO/LCMO) epitaxial trilayered films, in which LNCMO under zero magnetic field exhibits semiconductor-like conducting behavior down to 4.2K, have been fabricated on SrTiO3(001) substrates by d.c. magnetron sputtering technique. The measurements of resistance show that transport properties of the trilayered films are clearly different from that of the pure LNCMO or LCMO films. In the LCMO(75nm)/LNCMO(450nm)/LCMO(75nm)film, two resistance peaks have been observed. With the measurements of magnetic hysteresis loop, it show that there may exist a magnetic exchange-coupling interaction and an intra-magnetic field in the trilayered film. When the external magnetic field is applied, the resistance decreases and the resistance peak shifts to higher temperature, therefore it appears colossal magnetoresistance effect. Compared with the magnetoresistance response in pure LCMO film, the enhanced magnetoresistance effect have been observed in these trilayed films, and it maintains a stronger magnetoresistance effect in a wide temperature range. We suggest it results from the middle layer LNCMO in the trilayered film.

Magnetoresistance of epitaxial thin films of ferromagnetic metallic oxide SrRuO3 with different domain structures

We have studied the magnetoresistive behavior of epitaxial thin films of the conductive ferromagnetic oxide SrRuO3 with different domain structures grown on both miscut (001) SrTiO3 and exact (001) LaAlO3 substrates. A strong anisotropic magnetoresistance (MR) has been observed in the single domain SrRuO3 thin film on miscut (001) SrTiO3 substrate. In contrast, the SrRuO3 thin film on (001) LaAlO3 substrate shows identical MR behavior in two orthogonal directions on the film due to the presence of 90° domains in the plane. For both the films, large negative magnetoresistance effects (∼10%) were observed when the current and the applied magnetic field are parallel. This is attributed to a reduction in spin fluctuations near Tc and to magnetization rotation leading to a change in the angle between the current and magnetization at low temperatures.

Higher order magnetoresistance commensurability oscillations in low aspect ratio antidot lattice and focusing structures

We observe unusually strong magnetoresistance commensurability oscillations in two different types of artificial arrays of scattering centers, multiparallel focusing slits and a square antidot lattice, both fabricated on a two-dimensional electron gas by electron-beam lithography and damageless wet etching. The strength of the magnetoresistance peaks is attributed to a high reflection specularity and a small effective antidot cross section. The absence of commensurability effects near Landau level filling factor υ=3/2 is attributed to the composite fermion mean free path being smaller than the slit and antidot spacings. The mechanism for the observed commensurability is discussed.

Crossover from Weak to Strong Localization in Quasi-One-Dimensional Conductors

The crossover from weak to strong localization in the resistance of quasi-1D conductors is observed for the first time with decreasing the temperature; it occurs when the phase-breaking length becomes comparable with the localization length. The signature of the strong-localization regime is an activation-type temperature dependence of the resistance and exponentially strong negative magnetoresistance. The magnetoresistance is well described by the theory of doubling of the localization length in quasi-1D conductors in strong fields; this provides a direct measurement of the localization length.

Effect of Domain Structure on the Magnetoresistance of Epitaxial Thin Films of Ferromagnetic Metallic Oxide SrRuO3

ABSTRACTWe have grown epitaxial ferromagnetic metallic oxide SrRuO3 thin films with different domain structures on (001) LaAlO3 and miscut (001) SrTiO3 substrates. The effect of crystallographic domain structures on the magnetization and magnetoresistive behavior of epitaxial SrRuO3 thin films has been studied. Magnetization measurements on the single domain film on 2° miscut (001) SrTiO3 substrate showed that the in-plane [110] direction, which is aligned along the miscut direction, is the easier axis for magnetization compared to the [001] direction. This film also showed a strong anisotropie magnetoresistance (AMR) effect of ∼ 8% in magnitude. In contrast, the SrRuO3 thin film on (001) LaAlO3 substrate shows identical magnetization and magnetoresistance behavior in two orthogonal directions on the film due to the presence of 90 domains in the plane. For both the films, large negative magnetoresistance effects (-10%) were observed when the current and the applied magnetic field are parallel. The magnetoresistance behavior is explained in terms of suppression of spin fluctuations near Tc and the AMR effect.

Galvanomagnetic Measurements of the 2D Electron‐Hole Gas in GaSb/InAs/GaSb Quantum Wells under Pressure up to 2.5 GPa

AbstractGalvanomagnetic measurements at 4.2 K in fields up to 7 T under hydrostatic pressures up to 2.5 GPa are presented for a series of GaSb/InAs/GaSb quantum wells with different cap‐to‐well ratios and interface type. The pressure derivatives of electron and hole concentrations have been obtained from Shubnikov‐de Haas (ShdH) oscillations and from fitting the low field data to the classical two‐carrier model. Above the pressureinduced metal‐insulator transition, a strong negative magnetoresistance occurs which is attributed to localisation associated with the presence of GaSb surface donors.

X z−X xy crossover in a two-dimensional electron gas in AlAs

We have performed magnetoresistance measurements up to 20 T on high-mobility two-dimensional electron gases in single AlAs quantum wells with well widths ranging from 40 to 80Å. We observe a sharp transition from a two-fold valley degenerate X xy occupied state in a 60Åwell to a non-degenerate X z occupied state in a 40Åwell. These two states are distinguished by their different effective masses, which have been determined using the temperature dependence of the Shubnikov-de Haas effect. Using self-consistent calculations, we can explain this transition in relative occupation assuming an X-splitting energy of 24 meV. Furthermore, for the X xy electrons, the effect of the large ratio of spin to Landau splitting is shown by the occurrence of strong magnetoresistance minima at filling factors 2, 6 and 10 in the Hall effect.

Anomalous magnetisation process in UFe4Al8 probed by magnetisation and magnetoresistance

A strong anisotropic magnetoresistance has been measured in a single crystal of UFe4Al8 and has been used to prove the ferromagnetic order of the U lattice. The giant anomaly detected in the magnetoresistance curves is interpreted as two 90° rotations of the magnetisation. This interpretation has been confirmed by magnetisation measurements.

Photogalvanic effect in an asymmetric system of three quantum wells in a strong magnetic field

The photogalvanic effect (PGE) in an asymmetric undoped system of three GaAs/AlGaAs quantum wells illuminated with white light of various intensities is investigated in magnetic fields up to 75 kOe at temperatures ranging from 4.2 K up to 300 K. A maximum of the spontaneous photogalvanic current J PGE as a function of the magnetic field predicted by A. A. Gorbatsevich et al., JETP Lett. 57, 580 (1993), is observed. Analysis of the experimental data shows that the main initial characteristic of the PGE is not the spontaneous current but rather the electromotive force E PGE arising in the direction perpendicular to the applied magnetic field. It is determined that this emf is independent of the intensity of the incident light, increases linearly with the size d of the illuminated region, and decreases slowly with temperature: E max PGE ∼0.8 V at 300 K and ∼0.1 V at 4.2 K for d∼3 mm. The curve E PGE(H) at room temperature is determined with allowance for the strong transverse magnetoresistance of the nanostructure.

Giant-magnetoresistance anomaly associated with a magnetization process in UFe4Al8

Magnetoresistance and magnetization measurements on a U${\mathrm{Fe}}_{4}$${\mathrm{Al}}_{8}$ single crystal are reported. A strong anisotropic magnetoresistance is found in this compound and is used to prove the ferromagnetic order of the U lattice. A strong anomaly in the parallel and perpendicular magnetoresistance is interpreted as an unusual magnetization process in which the magnetization remains blocked perpendicularly to the applied magnetic field. This interpretation is confirmed by the magnetization measurements.