Showing papers on "Magnetoresistance published in 1989"

Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange

Abstract: The electrical resistivity of Fe-Cr-Fe layers with antiferromagnetic interlayer exchange increases when the magnetizations of the Fe layers are aligned antiparallel. The effect is much stronger than the usual anisotropic magnetoresistance and further increases in structures with more than two Fe layers. It can be explained in terms of spin-flip scattering of conduction electrons caused by the antiparallel alignment of the magnetization.

Theory of giant magnetoresistance effects in magnetic layered structures with antiferromagnetic coupling.

Abstract: We present a simple theoretical description of recently measured giant magnetoresistance effects in Fe/Cr layered structures. The resistivity is calculated by solving the Boltzmann transport equation with spin-dependent scattering at the interfaces. The magnitude of the effect depends on the ratio of the layer thickness to the mean free path and on the asymmetry in scattering for spin-up and spin-down electrons. Good agreement with experiment is found for both sandwich structures and superlattices.

Magnetoresistance measurements on the magnetic semiconductor Nd0.5Pb0.5MnO3

Abstract: We present magnetoresistance data up to 20 T and magnetisation measurements on the mixed-valence cubic perovskite semiconductor Nd 05 Pb 05 MnO 3 , which demonstrate that the material orders ferromagnetically below 184 K, and that around and above this temperature the conductivity is dominated by hopping of localized magnetic polarons, with an activation energy ∼95 meV

Electrical Characterization of GaAs Materials and Devices

Abstract: Part 1 The hall effect and magnetoresistance: practical considerations conductivity and hall coefficient scattering theory - mobility semiconductor statistics - carrier concentration. Part 2 Magnetoresistance in ungated structures - contact resistance patterns and gunn diodes: one-layer TLM and magneto-TLM analysis two-layer TLM and magneto-TLM analysis gunn diodes, and geometry corrections. Part 3 Capacitance and magnetoresistance in gated structures - field-effect transistors: gated capacitance - concentration profiling in MESFETs gated capacitance and conductance - mobility profiling in MESFETs gated magnetoresistance - mobility profiling in MESFETs gate current and parasitic-resistance effects capacitance and magnetoresistance in MODETs. Part 4 Capacitance and conductance effects from deep traps: steady-state effects transient behaviour - emission and capture temperature spectrosocpy - DLTS optical excitation in ungated structures - PICTS and HETS. Appendices: solutions to differential equations multi-band conduction semiconductor statistics commercial hall effect instruments.

Magnetoresistance in metals

Abstract: Preface 1. Survey of basic principles 2. Measurement 3. Real metals 4. Quantum effects 5. Inhomogeneous materials 6. Size effects.

Magnetoresistance oscillations in a two-dimensional electron gas induced by a submicrometer periodic potential

Abstract: A new type of magnetoresistance oscillation periodic in 1/B is observed when the carrier density Ns of a two-dimensional electron gas is weakly modulated with a period smaller than the mean free path of the electrons. Experiments with high mobility AlGaAs-GaAs heterojunctions where Ns is modulated by holographic illumination at T ≤ 4.2 K show that the period of the additional quantum oscillation is determined by the separation a of the interference fringes. This period corresponds to Shubnikov-de Haas oscillations where only the electrons within the first reduced Brillouin zone with |k| < π/a contribute.

Novel magnetoresistance oscillations in a periodically modulated two-dimensional electron gas.

Abstract: A novel type of magnetoresistance oscillations is observed in high-mobility AlGaAs/GaAs heterojunctions with a holographically induced lateral periodic modulation in one direction. Theoretically, the effect is shown to result from an oscillatory dependence of the bandwidth of the modulation-broadened Landau levels of the two-dimensional electron system on the band index, which leads in high-mobility samples to a strongly anisotropic oscillating contribution to the conductivity tensor. The oscillations, periodic in ${\mathrm{B}}^{\mathrm{\ensuremath{-}}1}$, reflect the commensurability of cyclotron diameter at the Fermi energy and modulation period.

On the Angle Dependence of the Magnetoresistance in Quasi-Two-Dimensional Organic Superconductors

Abstract: The dependence of the title is shown to originate in a singular angle dependence of the distribution width of the area of the semiclassical closed orbits under the magnetic field in the reciprocal space. In quasi-two-dimensional metals with a cylindrical Fermi surface, the distribution width of the area of the orbits on the Fermi surface vanishes at certain inclination angles of the magnetic field, entailing the appearance of remarkable effects. The observed peaks of the coarse-grained magnetoresistance are found to correspond to these special angles.

Landau band conductivity in a two-dimensional electron system modulated by an artificial one-dimensional superlattice potential.

Abstract: We investigate the magnetoresistance of a quasi-two-dimensional electron system subject to a one-dimensional superlattice potential created by field effect in gated AlGaAs/GaAs heterojunctions. At low temperatures this potential gives rise to a new type of magneto-resistance oscillations with a period governed by the ratio of the classical cyclotron diameter 2${\mathrm{R}}_{\mathrm{c}}$ to the superlattice period a. The oscillations are quantitatively explained in period, phase, and magnitude by the formation of Landau bands in the two-dimensional electron system under the influence of the periodic potential.

Magnetic surface anisotropy of transition metal ultrathin films

Abstract: In order to study the magnetic anisotropy of transition metal ultrathin films, we have performed tight-binding calculations including spin-orbit coupling. Beside the anisotropy energy these calculations also yield the orbital moment, which turns out to be much more anisotropic than in bulk materials. The effects of interfacial mismatch and roughness are discussed within phenomenological models. We also briefly review experimental results on the magnetic surface anisotropy (MSA) in transition metal ultrathin films. In some cases such as Au/Co/Au(111) sandwiches the MSA wins the competition with the shape anisotropy arising from the magnetostatic energy: below a critical thickness this leads to aperpendicular spontaneous magnetization. We show the effects of this crossover on the hysteresis loops and on the magnetoresistance, and the effects of interface roughness on the critical thickness.

Quantized Hall effect and a new field-induced phase transition in the organic superconductor (TMTSF)2PF6.

Abstract: We report a Hall effect and magnetoresistance study of sin- gle crystals of the organic superconductor tetramethyl-tetraselenafulvalinium hexafluorophosphate, (TMTSF${)}_{2}$${\mathrm{PF}}_{6}$, under hydrostatic pressure at temperatures down to 0.5 K in magnetic fields up to 25 T. There is clear evidence for a quantized Hall effect, with plateaus labeled by the integers n=5 to 1, up to 17 T. A new field-induced phase transition to a very resistive state occurs at 18 T. We believe that this is the ``n=0'' spin-density-wave state predicted by the quantized nesting theory.

Electronic Properities of the Single-Grained Icosahedral Phase of Al-Li-Cu

Abstract: The low-temperature specific heat (1.5 K to 6 K), the electrical resistivity (1 K to room temperature) and the magnetoresistance at 1.4, 4.2 and 10 K have been measured for the single-grained I-phase of Al–Li–Cu with near 6:4:1 composition grown by the Bridgman method. An etching method, which allows us to dissolve only the fcc Al second phase completely, was developed. The density of states at the Fermi energy obtained from the electronic specific heat coefficient is extremely small compared with the corresponding free-electron value. The resistivity at 4.2 K is as high as 870 µΩcm and decreases as \({-}{\sqrt{T}}\) below 20 K and as - T above 20 K, indicating the development of the weak localization of electrons. Magnetic field dependence of the magnetoresistance is consistent with the theory of the localization without involving electron-electron interaction.

Theory of unusual anisotropy of magnetoresistance in organic superconductors.

Abstract: We suggest that weak electron tunneling between layers is responsible for the experimentally observed dramatic deviations from two-dimensional behavior in the magnetoresistance in the metallic phase of the quasi-two-dimensional conductors (TMTSF${)}_{2}$${\mathrm{ClO}}_{4}$ and (TMTSF${)}_{2}$${\mathrm{PF}}_{6}$. We predict a fine structure due to resonances in open orbits between motion parallel and perpendicular to the planes.

Zeeman effect on magnetoresistance in high-temperature superconductors.

Abstract: The Zeeman effect on the magnetoresistance in high-temperature superconductors is investigated above the transition temperature. It is found that this effect is dominant for the positive magnetoresistance in a parallel magnetic field and the phase relaxation time ${\ensuremath{\tau}}_{\ensuremath{\phi}}$ can be estimated by a comparison of this term with experimental data for single-crystal films. This value is consistent with an estimate from the orbital effect of the Maki-Thompson term in a perpendicular magnetic field, and a temperature dependence of ${\ensuremath{\tau}}_{\ensuremath{\phi}}$ proportional to $\frac{1}{T}$ is obtained from the experimental data.

Transport phenomena in polymer-graphite composite materials

Abstract: The dc electrical resistance of a polymer-graphite composite has been studied in high magnetic fields (l10 T) and at low temperatures for different concentrations of graphite. The resistivity of the composites for the high graphite concentrations was much greater (${g10}^{4}$) than that of pure graphite, and above a critical concentration of graphite a percolative behavior occurred. In addition to the ordinary positive magnetoresistance, an anomalous negative magnetoresistance at low fields was observed in the composite, but not in pure graphite. The data are explained in terms of contact resistances between adjacent graphite particles, with a special emphasis on the bidimensional aspect of graphite.

Weak Localization in Pregraphitic Carbon-fibers

Abstract: High-resolution resistivity and magnetoresistance measurements have been performed as a function of temperature on pregraphitic carbon fibers. The resistivity exhibits a negative temperature coefficient (d\ensuremath{\rho}/dTl0) from room temperature down to 1.5 K. It is shown that such behavior can hardly be explained by a thermal excitation of carriers in the liquid-helium temperature range within the framework of the simple two-band model, usually applied to graphitic and pregraphitic structures. The magnetoresistance present a negative component more pronounced at low temperature. It is found that the magnetic field acts to restore the expected behavior of the resistivity in the low-temperature range, in the sense that the resistivity becomes gradually temperature independent. These observations as well as the experimental results obtained at higher temperature are consistent with the weak-localization theory for two-dimensional systems.

Negative magnetoresistance in the variable-range-hopping regime in n-type GaAs

Abstract: Low-field magnetoresistance measurements for compensated, n-type three-dimensional GaAs with net donor concentration just below the metal-insulator transition show a quadratic field dependence for values of B less than 750 G. Temperature-dependent measurements in zero field show that transport is by variable-range hopping, and are consistent with the presence of a Coulomb gap which narrows close to the transition. It is found that the temperature dependence of the effective area in which the flux is enclosed is not related to the temperature dependence of the optimum hopping length. The relevant dephasing time appears not to be the hopping time. Rather, phase coherence may be lost after successive hops assisted by long-wavelength acoustic phonons or as a result of electron-electron interactions.

Anomalous field dependence of magnetoresistance in Fe/Gd multilayered ferrimagnets

Abstract: The magnetoresistance of Fe/Gd ferrimagnetic multilayered films prepared by sputtering was investigated. The longitudinal magnetoresistance ρl increases first, followed by a deep decrease, and then increases again, with an increasing applied field. The transverse magnetoresistance ρt behaves in an opposite manner such that ρl and ρt play a crossover at an intermediate field. This new type of field dependence of ρl and ρt can be explained in terms of the spin flop of artificial ferrimagnetism which gives rise to an anomalous up‐turn curve in magnetization versus field.

Critical current and magnetoresistance hysteresis in polycrystalline YBa2Cu3O7−x

Abstract: We have observed an irreversible behavior on critical current and magnetoresistance for sintered bulk YBa 2 Cu 3 O 7− x in low fields. The hysteresis shows up at a field of about 15 Oe, a value smaller than H c1 of the oxide crystal. We attribute the hysteresis to flux repelling and trapping in the loops comprised by superconducting grains and weak links in between. An I c − H curve was simulated on the basis of a one loop model, which shows the irreversible behavior clearly.

Perpendicular magnetic anisotropy of Pd-Co alloy films and related multilayers

Abstract: with the easy axis perpendicular to the film plane . The anisotropy is usually considered to originate from the interface between CO and Pd layers or from CO moment which located at the interface. Magneto-strictive efffyt and alloying at the interface were not considered to be the origin ' . However, very few experiments have been done for Pd-Co alloy films and their properties are still not clarified. Furthermore, it has been reported-4 @at PdCo alloys exhibit very large negative magnetostriction up to -2r10 . This suggests that the alloy films are very sensitive to the stress due to the substrate constraint or lattice misfits. The large magnetostriction also suggests that PdCo alloys are very sensitive to the crystal field which may be related to so called interface anisotropy in Pd/Co multilayer films. In this paper, we report that vacuum deposited Pd-Co alloy films exhibits extremely large perpendicular anisotropy. We further report preliminary results for multilayer films'consists of PdCo alloy layers. Pd/Co multilayer films are known to exhibit 1affS)magnetlc

The oscillatory magnetoresistance of electrons in a square superlattice potential

Abstract: The transverse magnetoresistance of a two-dimensional electron gas in an n-type GaAs/(AlGa)As heterostructure subjected to a square superlattice potential is investigated. Magneto-oscillations are observed at low field (B ≤ 0.4 T) with period Δ(1/B) = ea/2kF, where a = 145 nm is the superlattice constant. At higher fields the magneto-resistance is dominated by Shubnikov-de Haas oscillations. A comparison is made with experiments on a one-dimensional superlattice.

Magnetic position detection apparatus having two magnetic recording medium tracks with magnetoresistors arranged in a bridge circuit so as to eliminate even order harmonic distortion

Abstract: An apparatus for magnetically detecting a positional relation between first and second members the positions of which change relative to each other comprises a magnetic recording medium carried on the first member and a magnetic sensor substrate supported by the second member. Two or more tracks each including a multiplicity of magnetic poles arranged at a predetermined pitch are provided on the magnetic recording medium, corresponding to the respective phases of two or more detection signals to be detected by the apparatus and in a direction in which the positional relation between the first and second members changes. The magnetic poles in any one of the tracks are shifted with respect to the magnetic poles in the other track by an amount corresponding to a phase difference between the detection signal for the phase which corresponds to the one track and the detection signal for the phase which corresponds to the other track. The magnetic sensor substrate includes a plurality of magnetoresistive elements which are respectively provided corresponding to the tracks and the electrical resistance value of each of which changes in response to a magnetic field originating from the corresponding track. The magnetoresistive elements are combined to generate electric signals as the detection signals. Each of the magnetoresistive elements corresponding to any one of the tracks and the corresponding one of the magnetoresistive elements corresponding to the other track are placed at the same position when viewed in the direction in which the positional relation between the first and second members changes.

Scattering experiments in two-dimensional systems in the presence of quantizing magnetic fields

Abstract: We present magnetoresistance measurements in an ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As/GaAs heterojunction with a gate covering a small region of the sample. With this device the Landauer approach to the resistance can be directly studied. The two-dimensional electron gas preserved in the nondissipative quantum Hall regime acts as the ideal leads necessary in this approach. The behavior of the scattering region formed by the gated part in between the undisturbed parts (ideal leads) of the sample is investigated by application of different voltages to the gate. We measured the dependence of the device resistance on the gate voltage and the number of available channels within the ideal leads. It is found that a theory based on the classical drift of electrons along equipotential lines (trajectories) accompanied by two types of transitions (intra- and inter-Landau-level transitions) between them is able to explain the main features of experiments.

Electrical resistivity, magnetoresistance, magnetisation, hall coefficient and excess conductivity in Pb-doped Bi-Sr-Ca-Cu oxides

Abstract: The electrical resistance, Meissner signal, magnetoresistance and Hall coefficient of Bi1.75Pb0.25Sr2Ca2Cu3Ox and Bi1.5Pb0.5Sr2Ca2Cu3Ox (nominal compositions) have been measured. Resistance of the Bi1.75Pb0.25 sample becomes zero for T≤106 K while the Bi1.5Pb0.5 sample shows a 65% drop in the resistance around 106 K and TR = 0c ≈ 73 K. Powder X-ray diffraction analysis reveals that the major phase in the Bi1.75Pb0.25 sample is the high-Tc phase and in the Bi1.5 Pb0.5 sample the low-Tc phase. The Hall coefficients (RH) of both samples are positive with R−1H> linear in temperature. The temperature dependence of RH is stronger in the Bi1.75Pb0.25 sample than in the Bi1.5Pb0.5 sample. The carrier concentration determined R−1H for the Bi1.75Pb0.25 and the Bi1.5Pb0.5 samples at 300 K are 1.59 × 1021 cm−3 and 3.66 × 1021 cm−3, respectively. The excess conductivity of both samples is analyzed using the Aslamazov-Larkin expression. The critical exponent λ obtained for the Bi1.75Pb0.25 sample is 0.78 and that for the Bi1.5Pb0.5 sample, in the temperature region where the high-Tc phase contributes only, is 0.77.

Magneto-optical and transport studies of ultrahigh mobility films of InAs grown on GaAs by molecular beam epitaxy

Abstract: InAs layers of very high electrical quality are grown on GaAs substrates by molecular beam epitaxy (MBE). The observation of sharp cyclotron resonance and donor lines (linewidths approximately=1 cm-1) in far-infrared magneto-optical studies suggest that the low-temperature mobilities in the bulk of the films are in the range 200 000-300 000 cm2 V-1 s-1 with an electron concentration of approximately=2*1014 cm-3. A strong but broad cyclotron resonance line and the Shubnikov-de Haas effect are observed from a two-dimensional electron gas (2DEG) at the surface or GaAs interface (nS approximately=1*1012 cm-2 and mu s approximately=20 000 cm2 V-1 s-1). As a consequence of parallel conduction from the low mobility layer the Hall mobility measured from a 5 mu m thick sample is 80 000 cm2 V-1 s-1 at 77 K and that in a 2 mu m sample is only 30 000 cm2 V-1 s-1. The width of bulk cyclotron resonance and impurity lines depend only weakly on thickness and consequently scattering from dislocations generated by the misfit at the GaAs/InAs interface is not thought to affect the bulk mobility strongly down to film thicknesses of 1 mu m. The parallel conduction from the 2DEG also produces a large magnetoresistance. Please note - the first author name has been corrected from Homes to Holmes