Showing papers on "Magnetoresistance published in 1980"

Spin-Orbit Interaction and Magnetoresistance in the Two Dimensional Random System

Abstract: Effect of the spin-orbit interaction is studied for the random potential scattering in two dimensions by the renormalization group method. It is shown that the localization behaviors are classified in the three different types depending on the symmetry. The recent observation of the negative magnetoresistance of MOSFET is discussed. In recent experiments on MOSFET by Kawaguchi et al.,u it was found that electrons confined in the MOS inversion layer exhibit the negative magnetoresist­ ance. This effect is closely related to the localization problem in a random potential. In two dimensions, the quantum inter­ ference is important and, if the impurity scattering is spin-independent, the con­ ductivity vanishes at zero temperature even when the scattering is very weak. 2>

Magnetoresistance and Hall effect in a disordered two-dimensional electron gas

Abstract: Two models of disorder in two dimensions are discussed. The first is a localization theory that treats noninteracting particles by perturbation theory in the weak scattering limit. A weak magnetic field is found to have strong effects on the previously predicted logarithmic rise in resistivity at low temperatures. No logarithmic divergence is found for the Hall constant. A second model treats the disorder scattering by conventional diagramatic technique but includes the effects of interactions. In a short communication it has previously been reported that the resistivity and Hall constant both show a logarithmic increase at low temperatures. The details of the calculation are reported here, together with an extension to thin wires which shows a ${T}^{\ensuremath{-}\frac{1}{2}}$ divergence in the resistivity.

Theory of Negative Magnetoresistance I. Application to Heavily Doped Semiconductors

Abstract: Theory of negative magnetoresistance proposed recently by the author is presented in details and is applied to heavily doped semiconductors. Experiments on n-type GaAs are semi-quantitatively explained. The theory is extended to the case of anisotropic systems and is applied to Ge and Si. Quantitative agreements with experiment are obtained on the anisotropy of magnetoresistance Δ ρ in Ge. Square root dependence of Δ ρ out the magnitude of the magnetic field observed in Ge agrees with the prediction of the theory. As for the magnitude of Δ ρ in Ge and Si, discrepancies between the theory and experiments are rather large.

Negative Magnetoresistance in Silicon (100) MOS Inversion Layers

Abstract: The observation of the negative magnetoresistance in silicon (100) n-channel inversion layers is well reproduced by the recent theory for the random potential scattering in two dimension by Hikami, Larkin and Nagaoka. The parameter α is 0.30±0.05 at an areal concentration of electrons of 5.0×10 12 cm -2 .

Magnetoresistive displacement sensor arrangement

Abstract: A magnetic sensor device utilizes the principle that the resistance of anisotropic magnetoresistive material is different in the case where the direction of current flowing in the material and the direction of a saturating magnetic field are parallel from the case where the direction of current and the direction of the saturating magnetic field are perpendicular to each other. The sensor device may be used to sense displacement and includes a ferromagnetic magnetoresistive element having at least one current path portion, a source applying electric current to each current path portion, and a magnetic field source producing first and second magnetic fields, occurring in first and second regions, and lying in first and second different mutually intersecting directions, respectively, with respect to the current path portion. The magnetoresistive element and the magnetic field source are displaceable in a predetermined direction relative to one another so that the relative amounts of the current path portion that are influenced by the first and second magnetic fields vary in accordance with such displacement. The magnetoresistive element can be connected to a constant current source, so that the voltage across the element indicates displacement. Alternatively, the magnetoresistive element can be formed of two series-connected current path portions, with a constant voltage source connected across both portions, so that the voltage appearing at the junction of the two current path portions indicates displacement.

Resistivity anomaly in thin Bi wires: Possibility of a one-dimensional quantum size effect

Abstract: Thin Bi single-crystal wires down to ~0.2 Μm in diameter were prepared by pressing liquid Bi into fine Pyrex capillaries. Electron microphotographs were taken to determine the diameter of the wires. Anisotropic magnetoresistance together with Shubnikov-de Haas oscillations were observed at helium temperatures. An anomalous drop in resistivity ϱ(T) was found at T < 50 K. Possible explanations are considered, including one based on the quantum size effect.

Electronic Properties of Graphite-Potassium intercalation Compounds. II. Resistivity, Hall Effect and Magnetoresistance

Abstract: Resistivity (ρ), Hall coefficient ( R H ) and transverse magnetoresistance (tMR) of C 8 K(1st) and C 12 N K( N -th stage, N =2, 3 and 4) at temperatures of 1.2 to 300 K have been investigated in connection with the study of the Fermi surfaces (Part I). Resistivities in the N ≧2 stage compounds show some anomalies which are associated with the order-disorder phase transitions. In C 8 K, ρ has a temperature derendence of a + b T + c T 2 . The temperature dependence of R H indicates the one-electron conduction for the N ≧2 compounds and the two-carriers one for C 8 K. The tMR of each compound is represented by a linear junction of magnetic field at about 4 to 15 T. The angular dependence of tMR indicates the cylindrical Fermi surface in a whole. The charge transfer rate from potassium to the graphite band is evaluated as 0.1 to 0.4 electron per atom for the 1st to 4th stage compounds.

Quantum Localization in Amorphous W-Re Alloys

Abstract: The resistance of amorphous W-Re wires between 2 and 20\ifmmode^\circ\else\textdegree\fi{} K has been measured. Wires with a resistance of approximately 10 k\ensuremath{\Omega} show a different temperature dependence from that of wires with a resistance of over 200 k\ensuremath{\Omega}. The latter show a relative increase in resistance as the temperature is decreased or the magnetic field is increased. These wires also show an anomalous dependence of resistance on width and length. These results are discussed in the perspective of Thouless's suggestion of maximum metallic resistance.

Graphite intercalation compounds: A simple model of Fermi surface and transport properties

Abstract: A parametrized model of the $\ensuremath{\pi}$ bands of $n$-layer films of graphite is developed for the purpose of studying the Fermi-level properties of graphite intercalation compounds. Specifically, the charge transfer, the density of states, and the conductivity tensor as a function of magnetic field are considered. The first two quantities are proportional to the de Haas-van Alphen frequency and cyclotron mass, respectively. A numerical study is carried out for a single layer of graphite as a function of excess charge. It is shown that the anisotropic dispersion of the energy bands causes unusual galvanomagnetic behavior at small magnetic fields, such as a nonzero magnetoresistance mobility for moderate intercalant concentrations and also a change in the sign of the low-field Hall coefficient at high intercalant concentrations.

Magnetoresistive head employing field feedback

Abstract: An electrically conductive element is employed in proximity to a thin film magnetic structure. The conductive element has current applied to it which is just sufficient to maintain, by field coupling between the thin film structure and the conductive element, the given magnetic orientation of the thin film structure, this occurring despite the application of a signal field tending to reorient the magnetization of the thin film structure. Such current is, therefore, the analog of the applied signal field and, since the magnetization of the thin film structure, by virtue of induced magnetic feedback, does not appreciably change direction, harmonics of any signal field applied to the thin film structure, and noise, are not manifested in the current analog.

Calculation of hopping transport coefficients

Abstract: The calculation of d.c. conductivity, a.c. conductivity, magnetoresistance and Hall mobility from the Miller–Abrahams equivalent circuit is reviewed. Analytical formulae are derived by making simpl...

Galvanomagnetic properties of transition metal-boron films

Abstract: The Hall effect and the anisotropic magnetoresistance have been studied in Fe-B, Co-B and Co-Ni-B films. These are sputtered films with boron content from 5 to 37% and are only amorphous when B is above 15%. In all cases the alloys are ferromagnetic at room temperature. Amorphous Fe 87 B 17 has both the largest Hall angle and magnetoresistance. Co-B and Co-Ni-B have only one third the values found for Fe-B. The Co-Ni-B system is mostly crystalline but the resistivity is nearly as large as the amorphous binary alloys.

Transducer for measuring a current-generated magnetic field

Abstract: A current-measurement transducer comprises a thin-film magnetic coil for passing an alternating premagnetizing current, generating an alternating premagnetizing field, and a magnetic-field comparator which includes first and second magnetic anisotropic films disposed in respective substantially parallel planes. Each film is composed of ferromagnetic and magnetoresistive material, and defines a magnetic easy axis, along which the film is relatively easily magnetizable, and a magnetic hard axis, at right angles to the easy axis, along which the film is harder to magnetize than along the easy magnetic axis. A non-magnetic and electrically at least poorly conducting layer separates the films, and the films are coupled to one another sufficiently tightly so that a current passing in one magnetic film generates an auxiliary magnetic field along the hard magnetic axis of the other magnetic film.

Electrical transport properties of natural and synthetic graphite

Abstract: Resistivity and magnetoresistance measurements were performed on a series of natural graphite crystals and highly oriented pyrolytic graphite (HOPG) samples as a function of temperature from 4.2 K to 293 K using a contactless r.f. technique. Resistance ratios of the natural graphite between 4.2 K and 293 K ranged from about 30 to 49. Carrier mobility of natural graphite was observed to obey at T−3/2 behaviour up to about 35 K. An anomalously high power dependence of μ versus T was observed below 35 K. A new model describing the dispersion of mobility of electrons and holes is presented which gives exact agreement with magnetoresistance results in the low field regime.

Magnetic field-induced semimetal-to-semiconductor transition in InAs-GaSb superlattices

Abstract: With the application of a magnetic field, we have observed a semimetal‐to‐semiconductor transition in InAs‐GaSb superlattices with closely overlapped subbands of electrons and holes. The transition is manifested in a sharp increase in the magnetoresistance in the quantum limit, where the ground Landau levels associated with the subbands are crossed at the Fermi level, resulting in carrier depletion.

New information on the spin-glass state of AuFe from transport measurements

Abstract: The author has investigated the influence of the state of remanent magnetisation on the transverse and longitudinal magnetoresistances of AuFe spin-glass alloys. Large hysteresis effects have been observed for the first time. Whatever the magnetic history of the sample, the magnetoresistance depends only on the macroscopic magnetisation m= chi H+mr where chi H and mr are the reversible and the irreversible magnetisations. This dependence is very simple and quadratic in m: Delta rho / rho 0=- alpha ( chi H+mr)2. The coefficient alpha is found to be almost independent of the concentration when this concentration is varied from 200 PPM to 70000 PPM (at least). These data contradict a cluster model for AuFe spin-glass alloys but are consistent with a homogeneous picture for both the reversible and the irreversible magnetisations. The anisotropy field concept is not found to be useful in the case of AuFe spin-glass alloys.

Galvanomagnetic Effect of CoS2 Single Crystal

Abstract: Making use of a single crystal of ferromagnetic CoS 2 , the resistivity, the transverse and longitudinal magnetoresistances and the Hall effect were measured in the temperature range from 4.2 K to 130 K under magnetic field strength up to 15.3 kOe. The ordinary and extraordinary Hall coefficients were determined. By assuming an electron correlation effect on the effective mass and the relaxation time for the majority and minority spin band electrons, the anomalies of resistivity and magnetoresistance were explained. The electron configuration having dγ 1 was confirmed from the ordinary Hall coefficient. The extraordinary Hall coefficient resembles that of Fe. In appendix, determinations of the Curie point and the critical exponents of this crystal were shown.

Theory of the resonant and non-resonant luminescence changes in amorphous silicon

Abstract: The resonant and non-resonant luminescence changes in a-silicon are considered. It is suggested, that the effect is closely related to the anomalous spin-dependent magnetoresistance and photoconductivity changes observed in evaporated and glow-discharge-deposited silicon. The novel feature here is the existence of stable quenching lines. This considerably reduces the choice of recombination models to either a geminate radiative process with singlet-spin memory or to the usual localized-localized recombination process with a spin-dependent, non-radiative transition. The steady-state and time-dependent features of the latter model are analysed. The results are in good agreement with experiment if the exchange interaction between the spins is assumed to be negligibly small. Some interesting and novel time-dependent effects are found and related to recent observations. In particular, it is found that microwave chopping can turn a quenching line into an enhancing one. The problems associated with exch...

Theory of the resonant and non-resonant photoconductivity changes in amorphous silicon

Abstract: The resonant and non-resonant photoconductivity changes due to a magnetic field and an additional microwave field are ocnsidered. The effect is shown to bear a close relationship to the anomalous spin-dependent magnetoresistance observed in amorphous semiconductors. It is suggested that a spin-dependent, non-radiative recombination process is responsible for the resonant and non-resonant photo-conductivity changes. Interesting time-dependent effects are found; in particular microwave chopping can turn a quenching line into an enhancing one. The problems associated with exchange-coupled spins are discussed. We compare our results with our recent luminescence results and conclude that the same model can account for both experiments.

Calculation of the performance of a magnetoresistive permalloy magnetic field sensor

Abstract: Design equations have been developed that predict the performance, in a uniform magnetic field, of a saturated Permalloy flat film magnetoresistor with uniaxial anisotropy. The functional dependence on film dimensions has been determined, of parameters characterizing the performance, including optimal operating point, optimal sensitivity, and range of linearity (one percent) around the optimal operating point. These performance parameters were derived from the device transfer or performance function which, in turn, was determined from a model based on a nonuniform demagnetizing field (NDF). We found that in a uniform field 1) the shape of the normalized performance function depends geometrically only on the film thickness-to-width ratio t/w ; 2) the performance parameters were either linear or hyperbolic functions of t/w ; and 3) to avoid serious error in predicting performance, the NDF model should be used for t/w \gsim 1 \times 10^{-4} . The coefficients for the performance parameter (design) equations are given and, with an upper bound error of ±5 percent, are valid for t/w ratios that cover practical device dimensions.

The influence of magnetic ordering on the resistivity of PdMn alloys

Abstract: Measurements of the temperature dependence of the resistivity and the magnetoresistance of PdMn alloys are reported for the concentration range 1%-10% Mn. These alloys order quasiferromagnetically for concentrations below 4% Mn and as spin glasses for concentrations above 4% Mn. Strong negative magnetoresistance associated with changing spin disorder scattering is observed in both regimes. The zero-field temperature dependence of the spin glass alloy resistivities is essentially due to changes in the inelastic scattering rate only. No evidence is found for a spin glass state in a Pd-5% Mn-0.35% Fe alloy.

Magnetic and transport properties of Metglas Fe40Ni38Mo4B18

Abstract: Systematic studies of the magnetic and the transport properties are lacking in Metglas 2826 MB (Fe40Ni38Mo4B18). Low‐field magnetic susceptibility between 300 and 1000 K gives Tc=(591±2) K in the amorphous state. A thermal cycling up to 1000 K yields two crystalline magnetic phases with Tc=(760±10) K and (545±5) K. Differential thermal analysis shows that the crystallization temperature is about 810 K. The ferromagnetic anisotropy of resistivity (FAR) is very small and positive with a typical value of 0.1%. The average slope of magnetoresistivity changes from −(1.1×10−8) G−1 to −(0.55×10−8) G−1 from 300 K to 77 K. The transverse and the longitudinal magnetoresistances could be related to the tangent of the Hall angle for a number of Metglas alloys including 2826 MB by considering the normal anisotropic magnetoresistance as well as the reduction in resistance due to the removal of domain walls in a uniaxial ferromagnet.

A new dc method of measuring the magnetoconductivity tensor of anisotropic crystals

Abstract: A dc method is described for measuring the magnetoconductivity tensor of an anisotropic crystal when the magnetic field is along a crystallographic symmetry axis The method is particularly suited for compounds with large anisotropies, for which conventional techniques are not practical The sample to be measured is a rectangular parallelepiped, each of whose faces is either free or on a highly conducting substrate The electrodes are arbitrarily located point contacts Special attention is given to self‐consistency checks for verifying that the conditions required to apply the method are satisfied A convenient alternative to the usual procedure of separating resistive and Hall voltages by magnetic field reversal is presentedThis new procedure is based on a reciprocity theorem in a magnetic field, proved here apparently for the first time

Apparatus for monitoring and detecting operational defects in the state of vehicle lamps

Abstract: Process for monitoring the instantaneous or momentary state of an electric circuit having to function in a range limited to two states, namely a normal state corresponding to the passage of a nominal intensity in a conductor and a critical or threshold state corresponding to a threshold intensity and in particular a zero intensity in the said conductor, using the detection of possible variations in the resistance of a magnetic layer or "magnetoresistor" of the influence of the magnetic field due to the intensity of the electric current circulating in a conductor and whereof part of the path is located in the vicinity of at least one oriented thin magnetic layer, wherein the said detection is carried out by means of a differential measurement of the magnetoresistance of at least two adjacent and separate magnetic layers, whereof one is subject to the field of the circuit to be monitored and the other is subject to the field of a reference current, the circuits and layers being arranged also in such a way as to be independent of the variations in the magnetoresistance due to ambient variations of temperature and/or magnetic field. The invention also relates to the application of the process to the monitoring of the possible breaking state of the supply circuits of electric lamps of a motor vehicle.