Showing papers on "Magnetoresistance published in 1979"

Negative Magnetoresistance in the Anderson Localized States

Abstract: It is shown that the mechanism of the electronic conduction due to variable-range hopping in the Anderson localized states gives rise to a large negative magnetoresistance, particularly in the vicinity of the metal-nonmetal transition. This result is mainly caused by an exponential dependence of the hopping rate on the highest occupied energy levels. The same mechanism also leads to a strong electric field-dependence of the conductivity.

The metallic state of the organic conductor TMTSF-DMTCNQ at low temperature under pressure

Abstract: We present experimental evidences for a high conductivity state (σ > 105 (Ω.cm)-1) stabilized by high pressure (P > 10 kbar) at low temperature in the quasi one-dimensional organic conductor TMTSF-DMTCNQ. The magnetoresistance investigations suggest among several possible interpretations of the metallic state, the existence of a quasi 1-D BCS-like superconductivity remaining up to ~ 30 K.

Negative magnetoresistance of pregraphitic carbons

Abstract: A theory of the magnetoresistance of disordered carbons is presented and applied to experimental results on carbon fibers and glassy carbon. The important features of the model include (a) a two-dimensional band structure and density of states, (b) a shallow acceptor level due to defects, (c) collision-broadened Landau levels, and (d) an extra density of states in the $m=0$ Landau level to account for the effects of partial three-dimensional ordering of the graphitelike layers. The negative magnetoresistance results from field-induced changes in the density of states which lead to an increase in the carrier concentration with field. Four structure-dependent parameters are used to fit the resistivity as a function of temperature, field strength, and orientation of the sample in the field. Approaches to improving the model are discussed.

Magnetic and transport properties of nickel-rare earth amorphous alloys

Abstract: The authors have studied the inter-related magnetic and transport properties of several amorphous Ni3RE alloys prepared by sputtering. The resistivity and magnetoresistance measurements reveal new aspects of the influence of the amorphous structure on the transport properties: in particular an increase of resistivity on magnetic ordering and an associated positive magnetoresistance. They analyse the magnetic and transport properties in the random anisotropy model and evaluate the crystal field acting on the rare earth moments.

Negative Magnetoresistance in the Anderson Localized States. II. Effect of Electron Correlations

Abstract: We recently proposed that the Zeeman shift of the occupied states leads to a large negative magnetoresistance in the Anderson localized states. In this paper we considered the effect of electron correlations in the same Anderson localized state and found an apparent linear dependence on the magnetic field of negative magnetoresistance at low temperatures, which explains the experimental data of 1 T -TaS 2 by Kobayashi and Muto.

Transport properties of magnetic amorphous alloys

Abstract: We discuss several aspects of the transport properties of magnetic amorphous alloys : contributions from magnetic ordering and from Kondo effect to the resistivity, resistivity minima of non‐magnetic origin, magnetoresistance, resistivity anisotropy, extraordinary Hall effect.

Magnetoresistance and Hall coefficient of inhomogeneous metals

Abstract: In strong magnetic fields, most metals have highly anisotropic transport coefficients, and these have long been known to be much influenced by sample inhomogeneities. This paper reports a detailed theoretical study of such effects. Various approximations for calculating the effective transport coefficients of inhomogeneous solids are rederived from a unified point of view; these are then used for a variety of model calculations appropriate to metals with open Fermi surfaces. A small concentration of crystallites with open orbits embedded in a free-electron metal is shown to give rise to a strictly linear transverse magnetoresistance (TMR) at strong magnetic fields. The linear coefficient is strongly dependent on the orientation of the open orbit in the plane perpendicular to the magnetic field $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$. Extended-orbit crystallites in a free-electron metal produce a TMR which is initially linear, but saturates at sufficiently strong field. The Hall coefficient ${R}_{H}$ is unchanged from its free-electron value to first order in the concentration of defects. A striking geometrical effect is predicted, the TMR from open-orbit crystallites saturating in geometries such that current distortions are unable to propagate parallel to $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$. The TMR and Hall coefficient of a free-electron metal containing a larger concentration of open-orbit crystallites is calculated in the effective-medium theory (EMT). The TMR is found to saturate at strong fields, in agreement with previous results of Stachowiak, while the Hall coefficient falls off as 1/${\mathrm{H}}^{2}$ at strong fields, except in flat-plate samples with $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$ perpendicular to the plate, in which case it is predicted to saturate at its free-electron value for a sufficiently large concentration of open-orbit crystallites, but to fall off quadratically for lower concentrations. In contrast, calculations within a non-self-consistent approximation give a strictly linear TMR and a Hall coefficient which saturates at a value below the free-electron coefficient. Possible explanations for the discrepancy are discussed. Calculations in the EMT for a model polycrystal with extended-orbit crystallites reveal a broad field region of quasi-linear magnetoresistance, as found previously by Stachowiak, and a reduced Hall coefficient, as well as a conspicuous geometrical effect. The possible relation of these model calculations to experiments of polycrystalline noble metals is examined, but no quantitative theory for these metals is given.

Electrical transport properties of low-stage AsF5-intercalated graphite

Abstract: The in-plane resistivity of stage 1 and stage 2 AsF5-graphite intercalation compounds was measured using a contactless r.f. eddy current technique from 1.6 to 290 K. The magnetoresistance of a stage 1 compound was similarly measured from 4.2 K to 290 K. The low temperature stage 2 resistivity data show a well-defined intermediate∞ ∝T2 region in addition to the usual∞ ∝T high temperature region, in qualitative agreement with the Kukkonen theory and indicative of a small, elongated cylindrical Fermi surface. Stage 2 resistivity data also show, for the first time in a graphite-acceptor compound, an apparent low temperature phase transition at ≈ 21 K. Magnetoresistance data were used to determine a stage 1 carrier concentration of ≈ 9×1020 holes cm−3. Resistive anomalies were observed at ≈ 200 K and ≈ 220 K for stage 1 and stage 2 compounds, respectively.

Electronic properties of PbMo6S8 and CuxMo6S8

Abstract: Normal-state properties of sputtered, evaporated, and sintered PbMo6S8 and CuxMo6S8 (where x is between 16 and 25) samples are reported These include the temperature dependence of resistivity, magnetoresistance, and Hall effect When combined with superconducting properties (measured on the same samples) and theory, a number of Fermi-surface and superconducting parameters are derived Fits to the temperature dependence of the resistivity are compared with similar fits for A-15 structure superconductors

Magnetic transduction device with magnetoresistances

Abstract: A magnetic transduction device for reading and/or writing of data contained on a magnetic support such as a disc or magnetic tape comprises a magnetic circuit formed by two thin superposed magnetic layers magnetically coupled at one end and forming an air gap at the other end, and a coil between said thin magnetic layers formed by thin conductive layers separated by thin insulating layers. At least one of the thin conducting layers comprises at least one magnetoresistance.

Theory of the Anomalous Magnetoresistance in Amorphous Si and Ge

Abstract: Anomalous magnetoresistance in amorphous Si and Ge is calculated by taking into account the hopping motion with spin flip of localized electrons. Magnetoresistance is deduced to be the results of the modification of the spin-flip relaxation time of the electrons by external magnetic field. The simplified theory can account for experiments.

Magnetoresistive reading head

Abstract: A magnetic reading head having a magnetoresistive element which is connected to a reading amplifier. In order to reduce the modulation noise (Barkhausen effect) when making the relationship between the resistance variation and the strength of the signal field linear in a negative feedback loop of the reading amplifier an electric turn is present which turn is positioned relative to the magnetoresistive element in such manner that a negative feedback field (Ht) can be generated with it which causes a magnetic flux in the element which is directed oppositely to the magnetic flux caused in the element by a magnetic field (H y ) to be detected.

Magnetophonon Resonance and Fourier Analysis in n-GaAs

Abstract: With the use of magnetic field modulation technique up to eighteen magneto-phonon peaks are observed in the transverse and longitudinal magnetoresistance of high purity epitaxial n-GaAs. Fourier analysis reveals the second harmonic component resulting in a sharpening of the oscillatory structure in the transverse magnetoresistance and extrema associated with the two LO phonon process in the longitudinal magnetoresistance in addition to the ordinary magnetophonon series. Two methods are described to obtain the damping factor \barγ which is found to be 0.63 from the Fourier analysis and 0.60 from the method described in this paper for the specimen with the highest mobility (1.7 ×10 5 cm 2 /V ·s) at 77 K. The temperature dependence of \barγ for the high purity n-GaAs exhibits a variation T 0.25 at low temperatures, which indicates an importance of the band failing effect for the damping process. The effective mass is determined to be m * =0.0682 m 0 at 77 K.

Magnetic signal field sensor that is substantially immune to angular displacement relative to the signal field

Abstract: Apparatus is provided for sensing an external magnetic field. This apparatus is comprised of first and second coplanar magnetoresistive elements, each magnetoresistive element having a main current conducting path and also having anisotropic resistance as a function of the direction of the resultant magnetic field applied thereto. First and second equal bias magnetic fields are supplied to the first and second magnetoresistive elements, respectively, these bias magnetic fields being supplied at different angles with respect to their associated magnetoresistive elements. In one embodiment, the current conducting paths of the magnetoresistive elements are perpendicular to each other, and the respective bias magnetic fields extend in opposite directions, these directions being parallel to one of the main current conducting paths. In another embodiment, the main current conducting paths of the respective magnetoresistive elements extend in the same direction, and the respective bias magnetic fields are at an angle of, for example ±45° to these main current conducting paths. The first and second magnetoresistive elements are connected in series with each other, and a d.c. current is supplied to this series connection. An output signal representing the magnitude of an external magnetic field as supplied to the magnetoresistive elements is derived from the junction formed by the series-connected elements.

Evidence for light and heavy conduction electrons at the zone center in KTa O 3

Abstract: The Shubnikov-de Haas oscillation of the magnetoresistance was observed for the $n$-type KTa${\mathrm{O}}_{3}$ in the magnetic field up to 105 kOe at liquid-helium temperatures. Angular variation of the extremal areas of the Fermi surface supports the zone-center-warped-band model, and rules out the many-valley model. However, contrary to the theoretical expectation, the $〈100〉$ bulges of the warped Fermi surface of the heavy electron are found to be absent or strongly depressed by some unknown mechanism.

Electrical Resistivity and Thermal Expansion Anomalies of Ferromagnetic CoS2

Abstract: The resistivity of CoS 2 having the pyrite structure was measured between 4.2 K and 300 K under pressure up to 18 kbar and under magnetic field up to 45 kOe. The hump anomaly just below the Curie temperature ( T c ) was found to become sharper with decreasing T c by pressure and to become sluggish with increasing T c by field. The magnetization ( M ) dependence of both the hump anomaly below T c and a positive magnetoresistance above T c was empirically analyzed, which resulted in M 2 dependent effective mass. The spontaneous and induced volume-strictions were also measured near T c up to 45 kOe, the latter of which was found to vary as M 2 for small M . These anomalies can be qualitatively understood through the variations of M and T c in a unified way including the effect of impurity-doping.

The effects of irradiation on the copper normal metal of a composite superconductor

Abstract: This report presents a new body of magnetoresistance data for copper that were obtained in neutron irradiation experiments at 4K and 330K. These data are combined with previously obtained results on initial damage rates and saturation effects to yield a projection of total resistivity vs neutron dose (expressed in displacements per atom) for copper in service at < 10K in a magnetic field.

A review of the magneto-impurity effect in semiconductors

Abstract: Recent observations of the magneto-impurity effect in n-GaAs, n-InP, p-Te and n- and p-Ge are reviewed. The effect is due to resonant inelastic scattering of free carriers by shallow donors or acceptors in the presence of a quantising magnetic field and gives rise to oscillatory structure in the magnetoresistance or photoconductivity.

Read/write magnetic head assembly with magnetoresistive sensor

Abstract: A thin film magnetic head assembly incorporates a nonconductive magnetic substrate which serves as a first pole piece and a thin film magnetic layer for the second pole piece. A magnetoresistive (MR) element is disposed between the pole pieces. During the write mode, the MR element serves as a partial write turn to enhance the write signal current applied to a conductor means. During the read mode, the conductor means serves as a magnetic shield, and the transducing gap is effectively narrowed.

Hall effect and magnetoresistance in Fe1−xCuxCr2S4

Abstract: The Hall effect and the magnetoresistance effect are studied in Fe1−xCuxCr2S4. FeCr2S4 is a ferrimagnetic semiconductor and CuCr2S4 is a ferromagnetic metal. The number and the mobility of charge carriers are determined independently. The mobility decreases as the temperature increases toward the Curie temperature. This shows that the anomalous increase in the electrical resistivity around the Curie temperature is caused by the decrease in the mobility by the spin‐disorder scattering. The temperature dependence of the number of charge carriers indicates that this system is still in a semiconductor state at x=0.65. The negative magnetoresistance effect is observed below the Curie temperature, and is discussed in terms of the spin‐disorder scattering. The compositional dependences of the magnetoresistance effect and the Curie temperature suggest the semiconductive‐to‐metallic transition near x=0.85.

Anomalous magnetoresistance in AuFe spin glass

Abstract: Transverse magnetoresistance has been measured in AuFe polycrystals betweem 2.5 K and 300 K in magnetic fields up to 18 KG. For a sample with 2.9 at. % Fe (T0=16 K) the magnetoresistance at low temperature first increases with the field, peaks at around 1 KG and then decreases. The peak height decreases rapidly with increasing temperature. At temperatures above 6 K there is no measurable positive magnetoresistance even at 50 gauss. The negative component above 2 KG varies roughly as (Δρ/ρ) = −a(T) Bn with n=1.9±0.1. The amplitude a(T) saturates at low temperatures (2.5 K

Electrical conduction in CdSe near the metal-insulator transition

Abstract: Strongly compensated samples of CdSe with 10l7–1018 cm−3 have been investi-gated. Two samples exhibit metallic and three non-metallic behaviour. The various regions of impurity-band conductivity have been identified from Hall and resistivity measurements. The density of states at the Fermi level has been estimated and compared with theory. A negative magnetoresistance has been observed in both metallic and non-metallic samples. A strongly temperature-dependent positive magnetoresistance in the variable-range-hopping regime is explained in terms of a simple extension of the Mikoshiba theory.

Unified weak‐field magnetoresistance phenomenology for cubic and noncubic (001)‐ and (111)‐oriented epitaxial films and surface layers

Abstract: Extensions of the Seitz‐Pearson‐Suhl weak‐field magnetoresistance (WFMR) formula are applied to cubic and noncubic (001) ‐ and (111) ‐oriented epitaxial films and surface layers, including the extreme case of two‐dimensional conductivity in a quantized layer. It is shown that four distinct WFMR measurements may be made on samples in either plane, thus providing a simple and reliable means of distinguishing cubic from noncubic transport environments.

Quantum-limit magnetoresistance for impurity scattering

Abstract: The theoretical formalism for calculating the magnetotransport effects developed by Arora and Peterson is extended to include the effects of anisotropic screened ionized impurity scattering in nondegenerate semiconductors in the quantum limit when ħωc ≫ kBT. Screening is found to be negligible at high magnetic fields for the transverse magnetoresistance. It is found that for point defect scattering, both the longitudinal and transverse magnetoresistance are linear in the magnetic field, whereas the Hall coefficient differs from its high field value 1/encc by a temperature dependent factor which is found to be close to unity. For ionized impurity scattering, the longitudinal magnetoresistance is found to be independent of the magnetic field, whereas the transverse magnetoresistance and the Hall coefficient decrease with magnetic field, the former as B−1, where B is the magnetic field. For combined acoustic phonon and ionized impurity scattering, the acoustic phonon scattering is found to dominate at very high magnetic fields giving rise to the ex-perimentally observed linear magnetoresistance. The combination of acoustic phonon and point defect scattering also yields similar results. Der von Arora und Peterson entwickelte Formalismus zur Berechnung der Magnetotransport-effekte wird erweitert zur Berucksichtigung der Effekte anisotrop-abgeschirmter ionisierter Stor-stellen in nichtentarteten Halbleitern in der Quantengrenze (wenn ħωc ≫ kBT). Fur den transversalen Magnetowiderstand ist der Einflus der Abschirmung bei hohen Magnetfeldern vernach-lassigbar. Fur punktformige Storstellen wird gefunden, das transversaler und longitudinaler Magnetowiderstand linear im Magnetfeld sind, wahrend der Hallkoeffizient von seinem Hochfeld-wert 1/enec um einen nahe bei eins gelegenen temperaturabhangigen Faktor abweicht. Fur die Streuung an ionisierten Storstellen ergibt sich, das der longitudinale Magnetowiderstand magnet-feldunabhangig ist, dagegen der transversale Magnetowiderstand und der Hallkoeffizient mit wachsendem Feld abfallen, letzter wie 1/B. Fur kombinierte Streuung an ionisierten Storstellen und akustischen Phononen resultiert eine Dominanz der Streuung an akustischen Phononen bei sehr hohen Feldern, was zum experimentell beobachteten linearen Magnetowiderstand fuhrt. Die Kombination von punktformigen Storstellen und akustischen Phononen fuhrt zu ahnlichen Ergebnissen.