Showing papers on "Magnetoresistance published in 1972"

Conductivity studies in europium oxide.

Abstract: The electrical and optical properties of EuO are studied in order to help determine conduction mechanisms. A strong relationship between growth parameters and electrical behavior is noted. Free-carrier absorption below the band edge in energy is observed in moderately conducting crystals. By comparing optical and electrical results, an experimental determination is made of carrier-density and scattering-time contributions to resistivity behavior. The carrier-density variations are explained in terms of an earlier proposed model which is developed here. The model consists of a donor-trap level, believed to be caused by an oxygen vacancy, which is above the conduction-band edge at low temperature, but crosses below it (near 50 \ifmmode^\circ\else\textdegree\fi{}K) with increasing temperature. Magnetoresistance and pressure results are presented which strongly support the model.

Galvanomagnetic Studies of Bismuth Films in the Quantum-Size-Effect Region

Abstract: Bismuth films (200-1400 \AA{}) were grown epitaxially on freshly cleaved mica substrates. These films consisted of a mosaic of equally oriented crystallites averaging several microns in diameter. The plane of the films coincided with the trigonal plane of Bi. We have studied the thickness dependence of the resistivity, the Hall coefficient, and the transverse magneto-resistance, by gradually varying the thickness of a single film which was kept under high vacuum during the entire experiment. The resistivity at 360 and 77 \ifmmode^\circ\else\textdegree\fi{}K is a smooth monotonic function of the thickness. At 12 \ifmmode^\circ\else\textdegree\fi{}K, we observed small oscillations in the resistivity and in the magnetoresistance. These oscillations are regarded as probable manifestations of the quantum size effect (QSE). The thickness dependence of the Hall coefficient is in striking disagreement with the predictions of the infinite-potential-well model. Better agreement between the theory and experimental results is obtained when we assume a less rigid boundary condition. Also for several films we have investigated the temperature dependence of these three transport coefficients and found it to be quite different from that of bulk bismuth. We have attempted to explain these results in terms of the behavior of the carrier concentration and of the different scattering mechanisms that can come into play in these films.

Low-Temperature Non-Ohmic Galvanomagnetic Effects in Degenerate n -Type InAs

Abstract: The dependence of the mobility of $n$-InAs on temperature and electric field was measured between 4.2 and 30 K and 0.05 and 10 V/cm, respectively. Furthermore, the variation of the nonoscillatory as well as the oscillatory part of the magnetoresistance (Shubnikov-de Haas effect) with applied electric field was studied experimentally. It was found that the decrease of the amplitudes of the Shubnikov-de Haas (SdH) effect depends on the time after application of the electric field and allows a direct time-resolved observation of the increasing electron temperature. The non-Ohmic transport is interpreted with the aid of an electron-temperature model. At electric fields below 0.3 V/cm, where the electron gas is strongly degenerate, electron temperatures are deduced from the decreases of the SdH amplitudes. At higher fields the degeneracy decreases gradually and electron temperatures are obtained from a comparison of the field-dependent non-Ohmic mobility and the temperature-dependent Ohmic mobility. From energy-balance considerations, the dependences of the electron temperature and the mobility on the electric field strength are calculated up to 10 V/cm assuming that ionized-impurity scattering is the dominant mechanism for momentum relaxation. The energy loss was assumed to involve scattering by acoustic phonons via the screened deformation potential and the screened piezoelectric interaction, and also scattering by polar optical phonons. A value of 4.05 eV for the deformation potential constant yielded good agreement between the experimental and the calculated dependence of the energy-loss rate on the electron temperature up to 18 K. Above 18 K the energy loss because of polar optical phonons, which is \ifmmode \mbox{\c{c}}\else \c{c}\fi{}alculated for a degenerate electron gas, dominates the increase of the electron temperature and leads to a kink in the mobility-field characteristic. The nonoscillatory positive magnetoresistance is shown to be dependent on the electric field. The negative magnetoresistance of the samples under investigation was studied between 2.4 and 4.2 K. A semiempirical relation of the form $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{{\ensuremath{\rho}}_{0}}=\ensuremath{-}{B}_{1} \mathrm{ln}[1+{B}_{2}(T){B}^{2}]$ was used to analyze the data.

Internally biased magnetoresistive magnetic transducer

Abstract: A magnetic transducer exhibiting the magnetoresistive (MR) effect includes at least two thin film layers. A three-legged MR film in electrical contact with a higher resistivity layer is magnetically biased by a portion of the MR sense current shunted through the nonmagnetic layer. The need for an accurate and stable bias to reduce distortion is eliminated by subjecting similar adjacent MR elements to opposite currents. Each element is then symmetrically and oppositely biased so that a differential resistance sensing circuit provides an output signal relatively immune to distortion.

Magnetism of rare-earth elements, alloys, and compounds

Abstract: The magnetic properties of the rare-earth metals and a number of the alloy and compound systems formed with rare earths are discussed. Emphasis is placed on current experimental results of bulk magnetic measurements and related transport phenomena and their bearing on theoretical studies. Topics covered include exchange interactions, magnetization processes, crystal field effects and anisotropy, magnetostriction, spin structures and their field dependence, specific heat, optical effects, resistivity, magnetoresistivity, and other transport properties affected by the magnetic order. Examples are drawn from a number of material systems including ionic compounds, solid solution compounds (e.g., rare-earth magnetic semiconductors), intermetallic compounds (permanent magnet materials, etc.), and bubble domain materials in addition to the rare-earth elements and intra-rare-earth alloys. Tables are presented giving the spin structures, ordering temperatures, saturation and effective moments, and other properties of the elements and specific compounds.

Linear Magnetoresistance and Anisotropic Quantum Fluctuations

Abstract: We propose a theory which accounts for the linear magnetoresistance found in the transverse and longitudinal components in various metals. The theory is based on the existence of quasiperiodic static fluctuations, which have the anisotropy of the lattice and may be due to electron-phonon and/or electron-electron interactions. The fluctuations produce no noticeable change in the static properties of the metal but produce the dominant linear increase of the resistance at high magnetic fields.

Low-Temperature Magnetoresistance in Degenerate n-Type Si

Abstract: Transverse magnetoresistance of degenerate $n$-type Si has been measured in the liquid-helium temperature range. A characteristic negative magnetoresistance is observed. The data are analyzed by a semiempirical expression originally used for CdS by us, thus providing further evidence for the general validity of our expression and for the localized-magnetic-moment model.

Some electronic properties of polyacrylonitrile-based carbon fibres

Abstract: The resistivity, thermoelectric power and magnetoresistance of carbon fibres derived from a polyacrylonitrile-based precursor heat treated to between 1000°C and 2800°C have been examined. The resistivity ratio as a function of heat treatment temperature exhibits a broad minimum roughly at about a heat treatment temperature (HTT) of 1750°C, and both the thermoelectric power and the magnetoresistance undergo a change of sign at that temperature. The importance of the temperature 1750°C, at which fundamental structural changes are believed to take place, is examined in the light of existing band models in the carbon and amorphous semiconductor fields.

Magnetoresistive devices and transducers

Abstract: A magnetoresistive device responsive to the value and direction of an external magnetic field generated near an edge thereof by a localized source by a corresponding variation of an electrical current applied thereto comprises at least one magnetoresistive layer of anisotropic material having its easy axis of magnetization orientated at an angle which lies between 0* and 90* and preferably approximately 45* with respect to the direction of flow of electrical current through the device. The magnetoresistive layer is inserted between a pair of thicker high permeability magnetic layers when a more accurate localization of the source of the external magnetic field is required.

Zero-Bias-Conductance-Peak Anomaly of Ta- I -Al Tunnel Junctions at 0.3 K and 90 kG

Abstract: We have studied the zero-bias-conductance-peak anomaly in a Ta-$I$-Al tunnel junction over a magnetic field range of 1 to 90 kG at a temperature of 0.3 K, and have made a detailed comparison between experiment and the $s\ensuremath{-}d$ exchange theory of electron tunneling. Good agreement between theory and experiment was obtained for the line shape of the conductance peak and its zero-bias temperature dependence. The line shape of the conductance curves found experimentally at high magnetic fields agreed reasonably well with the theory if magnetic-field-induced lifetime broadening was included. There was, however, a serious discrepancy between theory and experiment for the zero-bias magnetoresistance.

Geometrical effects in measurements of magnetoresistance

Abstract: A duality theorem for a two dimensional conductor in a magnetic field is proved by relating the current and field distribution for a specimen bounded by perfect conductors and insulators to that of a dual system defined by the interchange of conducting and insulating boundaries and reversal of the magnetic field. The theorem is used to consider Hall effect and magnetoresistance of rectangular plates. The authors discuss recent attempts to explain the high-field linear magnetoresistance of potassium and other alkali metals. Simple expressions for the Lippmann-Kuhrt magnetoresistance valid for all magnetic fields are derived. The associated linear magnetoresistance requires much higher magnetic fields for its observation than the experimental fields for which a linear magnetoresistance is actually observed.

Electric-Field-Dependent Magnetoresistance in Ferromagnetic Semiconductors

Abstract: Changes in the sign of the longitudinal magnetoresistance as a function of applied magnetic and electric fields are observed in single crystals of $p$-type Cd${\mathrm{Cr}}_{2}$${\mathrm{Se}}_{4}$. A phenomenological theory for the non-Ohmic behavior associated with spin-wave-carrier interaction is developed, and its comparison with the experimental results indicates the existence of spin-wave amplification by the drifting carriers. All the experimental results can be explained in terms of spin-wave-carrier interaction and spin-disorder scattering.

Anisotropic phonon coupling in the magnetophonon resonance in p type tellurium

Abstract: The magnetophonon effect in p-tellurium is extensively studied over the temperature range from 20 to 90K in magnetic fields of up to 300 kOe. High resolution of the peaks is achieved by means of a double differentiation technique. When the magnetic field is applied perpendicular to the c axis, the position of the magnetophonon peaks in the transverse magnetoresistance differs according to whether the current direction (J) is parallel or perpendicular to the c axis. When the magnetic field is applied parallel to the c axis, the oscillations in the longitudinal magnetoresistance are almost the same order of magnitude as those observed in the transverse magnetoresistance. This rather large amplitude in the longitudinal magnetoresistance is ascribed to the double minimum structure of the energy of the Landau levels.

Single wall domain arrangement

Abstract: A single wall domain arrangement comprises channels which are defined in a layer of magnetic material by patterns of magnetically soft elements along which domains move responsive to a magnetic field reorienting in the plane of the layer. The elements are formed by a single photoresist process resulting in the simultaneous formation of a compatible magnetoresistance detector.

Hall Voltage and Magnetoresistance of Bi Measured by a Sum Frequency Method in a Belt Apparatus

Abstract: An alternating magnetic field, 14 kG in amplitude at 5 Hz, was obtained in a belt type of high pressure apparatus by the use of conical pistons made of hardened high speed tool steel, permitting operation up to 50 kilobar A frequency of 12 Hz was used for the current through the sample, and the Hall voltage was synchronously detected at 17 Hz A signal yielding the coefficient of magnetoresistance was extracted at 22 Hz, using another lock‐in amplifier The Ohmic resistance may also be measured in the same experiment, and, by making certain assumptions, the variation of the carrier density and the hole and electron mobilities of a semimetal may be determined Data for polycrystalline Bi at room temperature are given

Martensitic pretransformation phenomena in the Au‐47.5 at.% Cd and the Fe‐29.7 at.% Ni alloys

Abstract: Premartensitic transformation characteristics have been investigated in the Au‐47.5 at.% Cd and the Fe‐29.7 at.% Ni alloys by measuring electrical resistivity and magnetoresistance. Preliminary electron diffraction studies have also been made for the Au–Cd alloy. An anomalous magnetoresistance peak was observed for both of these alloys prior to the martensitic transformation. In the Au–Cd alloy this peak was about 2–3°C above the Ms, and for the Fe–Ni alloy it was about 5–6°C above the Ms. Furthermore, when the Ms was lowered by isothermal stabilization, this peak moved in conjunction with the new Ms temperature. This indicates that the observed peak is truly associated with the premartensitic instability of the parent phase. The electron diffraction pattern of the parent phase of the Au–Cd alloy shows extensive 〈110〉 and 〈112〉 streaking in the temperature range where anomalous magnetoresistance was observed. These experimental results could tentatively be interpreted in terms of anisotropic scattering ce...

Galvanomagnetic Properties of Well-Oriented Graphite in Relation to the Structural Imperfections

Abstract: The Hall coefficient and magnetoresistance in well oriented graphite are measured at 77 K with the field oriented parallel to the c-axis. Various degrees of structural perfections are observed in the samples investigated. The magnetic field is continuously changed from 0 to 10 kG for each run. The concentrations and mobilities of conduction carriers in the samples are estimated by an analysis based on coventional multi-carrier models. Special care is taken in accurate determination of the Hall coefficient and magnetoresistance at low field (H<3 kG), which leads to a conclusion that highly mobile electrons do exist as minority carriers in each sample. With increasing imperfection, the concentrations of majority electrons and holes are found to increase, and mobilities to decrease. The same trend is also observed for minority electrons. These results imply that the introduction of structural imperfections into graphite causes considerable perturbation on the band structure.

Magnetoresistance of Hexagonal FeGe Single Crystals between 4.2 K and 30 K

Abstract: The low field transverse magnetoresistance of hexagonal FeGe has been measured for the field in the basal plane with current both parallel and perpendicular to the c-axis. At a temperature dependen ...

A sensitive method of Hall measurement

Abstract: A novel system is described for measuring very small Hall voltages by reversing the magnetic field with a time period of 1 s or more. The current is supplied from a direct constant current source and the system has a limit of sensitivity at 0.1 mu V Hall signal measured in a sample of total resistance 5.6 M Omega with a field of 0.5 Wb m-2 and a current of 10 mu A. The apparatus described has been used to measure mobilities of 10-3 cm2 V-1 s-1 in amorphous Ge films. The method described does not utilize very expensive equipment or special magnets and there are no calibration problems or difficulties in determining conductivity type. Utilization magnetoresistance and Nernst thermomagnetic effects are discussed.