Showing papers on "Magnetoresistance published in 1976"

Tunneling of Spin-Polarized Electrons and Magnetoresistance in Granular Ni Films

Abstract: Films consisting of fine Ni grains (\ensuremath{\sim}50 \AA{}) dispersed in ${\mathrm{SiO}}_{2}$ exhibit a large negative magnetoresistance. The effect is accounted for by the field dependence of the magnetic exchange energy, associated with tunneling of electrons between neighboring grains whose magnetic moments are not parallel.

Transport properties of bismuth in quantising magnetic fields

Abstract: The amplitude of the magnetotransport components for Bi (magnetoresistance, Hall, magneto-Seebeck and Nernst) in the phonon-drag region have been studied. Measurements were extended to the quantum regime where only a few electron levels were occupied. The amplitudes of the oscillatory components are found to correlate quite well with predictions of a recent quantum transport theory (J. Phys. Soc., Japan, vol.35, p.1280 (1973)).

Effect of isolated inhomogeneities on the galvanomagnetic properties of solids

Abstract: A general expression is obtained for the magnetoresistance tensor of a solid containing a small number of macroscopic inhomogeneities. The result is used to show that a wide variety of inclusions will generate a linear transverse magnetoresistance in a free-electron metal at high fields. Voids or other nonconducting defects are found to produce a linear longitudinal as well as a linear transverse magnetoresistance. The applicability of the theory to the properties of real metals is briefly discussed.

Transient Thermal Compensation for Quartz Resonators

Abstract: Thermal shock induced into ATand BT-cut temperaturecompensated quartz resonators results in transient frequency excursions that are not predicted by simple elastic theory. Compensation for thermally induced transients in quartz resonators was predicted recently by Holland. The experimental work presented here c o n f i i s the theory and predicts an orientation of @J = 21.93\", e = 33.93' for which full static and dynamic compensation for rapidly changing ambient temperatures should be achieved around 65°C. Experimentally, the predicted orientation showsperformance with respect o thermal transients that is two orders of magnitude better than comparable AT resonators.

Effect of nonparabolicity on Ohmic magnetoresistance in semiconductors

Abstract: The effect of nonparabolicity of the conduction band of $n\ensuremath{-}\mathrm{InSb}$ type semiconductors is studied in the framework of the Arora-Peterson density-matrix formalism. To exhibit clearly the effect of nonparabolicity, only the case of elastic electron-acoustic-phonon scattering is considered. Numerical results are presented both for parabolic and nonparabolic models. The nonparabolicity enhances the magnetoresistance, the effect being larger for larger magnetic fields. The Hall coefficient decreases slightly with the increasing magnetic field.

The nonequilibrium hall effect and related transport phenomena in semiconductors under inhomogeneous excitation by a laser pulse

Abstract: The nonequilibrium Hall effect (photo-Hall effect), conductivity, and magnetoresistance are analysed for an inhomogeneous excited semiconductor. A method for determining carrier concentration and mobility from experimental data is suggested. The influence of electron-hole scattering on the mentioned above effects is evaluated for germanium. Photoconductivity and photoluminescence are compared according to the solution of the diffusion equation when the surface recombination and a light pulse of Gaussian form are taken into account. [Russian Text Ignored.]

Magnetoresistive magnetic head

Abstract: A magnetic head having a magnetoresistive element. The element comprises a center tap, while on the left-hand half a pattern of equipotential strips is provided at an angle with the longitudinal direction of approximately 45° and on the right-hand half a pattern of equipotential strips is provided at an angle of approximately 135° with the longitudinal direction. The halves are each controlled with a current source and their output signals are subtracted from each other. The relation between the resistance of the total element and the value of external magnetic fields is more linear than in known magnetic heads having a magnetoresistive element and thermal noise is eliminated.

Impurity conduction and magnetoresistance in lightly dopedn-type GaAs

Abstract: Measurements of the resistivity and magnetoresistance of epitaxially grownn-type GaAs (N D ~ 1–2 × 1015 cm−3) at 4.2 K and below are reported. The hopping resistivityρ 3 depends onN D according toρ 3=ρ 0 exp (1.88/N 1/3 a) in agreement with predictions of percolation theory, wherea is the Bohr radius of the impurity ground state. The experimentally obtained preexponential factorρ 0 is very close to a recent theoretical prediction, which was derived from the computation of the topology of an infinite cluster. In magnetic fields below 1.3T the resistivity is found to be proportional to exp $$\left( {\frac{{ta}}{{\lambda ^4 N_D }}} \right)$$ ,t having a value of 0.06 in disagreement with a recent theoretical calculation oft=0.04. In the high magnetic field region above 3T the data are described byρ ∝ exp [q (λ 2 a B N D)−1/2], whereλ is the characteristic magnetic length anda B is the magnetic field dependent Bohr radius. From the experiments a value ofq=0.68 is deduced, while the theory predictsq=0.98.

Low-field magnetoresistance in metals

Abstract: Low-field magnetoresistance and Hall coefficient are calculated in the anisotropic-relaxation-time approximation. Simple results are obtained for Fermi-surface models which can be composed of spherical, cylindrical, and planar surfaces. Influences of the Fermi-surface geometry and the scattering anisotropy on the low-field magnetoresistance are discussed. With small modifications the method is applied to polyvalent metals with nearly-free-electron Fermi-surfaces. Simultaneous magnetoresistance and Hall-coefficient measurements combined with a three-group model calculation for the electronic mean free path are suggested as a means to determine the anisotropy of the electronic scattering in nearly-free-electron-like polyvalent metals. In an Appendix the results are extended to the longitudinal magnetoresistance.

Influence of Fermi surface and defect structure on the lowfield galvanomagnetic properties of Al

Abstract: Low-field Hall coefficient R and transverse magnetoresistance Delta rho / rho 0 were measured in Al single crystals. A definite defect structure (statistically distributed Frenkel pairs) was created in the specimens by low-temperature electron irradiation. This defect structure could then be changed in a controlled way by annealing at different temperatures. Eliminating the influence of defect concentration with the aid of the Kohler representation thus allowed the dependence of R and Delta rho / rho 0 on the defect structure to be investigated. Various effects of the Fermi surface geometry and of the scattering mechanism on R and Delta rho / rho 0 are discussed on the basis of simple equations for the low-field Hall coefficient and magnetoresistance.

Electrical resistivity of Mn-doped SnTe crystals at low temperature

Abstract: The temperature dependence of the resistivity over the range 1.8–20 K has been measured on the narrow-gap semiconductor SnTe with various Mn contents (<2.2at.%) and carrier concentrations [p = (1.2–8) × 1020 cm−3]. The resistivity shows an anomaly at some magnetic ordering temperature T m, which depends sublinearly on the Mn content c, but not linearly. However, together with a negative magnetoresistance, we have confirmed that the carrier scattering in this crystal is due to the s-d interaction as in dilute magnetic alloys.

Strain gauges for the measurement of magnetostriction in the range 4K to 300K

Abstract: The temperature dependence of gauge factor and magnetoresistance for two types of foil strain gauge commonly used in magnetostriction measurements has been investigated. Although the temperature variation of gauge factor for either type does not change significantly from one batch to another, the magnetoresistance in Ni-Cr alloy gauges is less predictable. The relative merits of the two types are discussed.

Anisotropic size effects in semiconductors and semimetals

Abstract: Large differences between the characteristic times of various electronic relaxation processes (momentum relaxation, energy relaxation, intervalley relaxation, electron-hole recombination) make it possible to divide carriers into groups between which relaxation is relatively slow. Each of the "long" relaxation times can be matched by a characteristic diffusion length which is much greater than the usual mean free path. Transport coefficients of such groups are generally anisotropic even in cubic crystals and the anisotropy varies from group to group (this anisotropy may be natural or it may be induced by pressure, magnetic field, etc.). Therefore, the passage of a current produces nonequilibrium carrier densities in such groups. The density gradients are oriented at right-angles to the current and they decay over distances of the order of the diffusion length. The effects associated with the formation of nonequilibrium carriers and the influence of their diffusion on the transport coefficients are referred to in the paper as the anisotropic size effects. The paper reviews experimental and theoretical investigations of various manifestations of such effects. An analysis is made of the size dependences of the electrical conductivity and magnetoresistance manifested in "thick" samples (thickness of the order of the diffusion length). Other topics considered include nonlinearity of the electrical conductivity in relatively weak fields, redistribution of carriers in "strong" fields (accompanied by giant changes in the total number of carriers and by formation of domains, depletion layers, and accumulation layers), influence of the anisotropic size effects on the skin effect (which changes the surface impedance of semimetals by an order of magnitude), and electromagnetic excitation of sound in semimetals.

An Anomalous Behavior of the Longitudinal Magnetoresistance in Semimetals

Abstract: An anomalous longitudinal magnetoresistance which decreases with increasing magnetic field in an appropriate specimen geometry has been investigated for semimetals. An intuitive explanation of this phenomenon is given in terms of geometry of inhomogeneous currents under the influence of a magnetic field. Experimental results obtained for bismuth specimens at 77 K are also discussed.

Magnetoresistance of gadolinium

Abstract: The authors have examined the magnetoresistance of a single crystal of gadolinium which had been refined by solid state electrotransport processing. The anisotropy of resistance at 4.2K in a field of 75 kG is consistent with open orbits along (0001) and in the basal plane: this being in agreement with the effect of exchange splitting on the RAPW band structure.

Transverse magnetoresistance and its temperature dependence for high‐purity polycrystalline aluminum

Abstract: Polycrystalline‐aluminum‐tape samples of 0.3×6 mm2 cross section have been used to measure the zero‐field resistivity and the transverse magnetoresistance up to 3.9 T. The main purpose of our investigation was to study the temperature and purity dependence of the magnetoresistance. Measurements were taken continuously in the temperature range 4.2–32 K for six samples differing in purity. The bulk residual resistivity ratio rb chosen as a measure of purity varied from 2400 to 30 000. In the high‐field region the magnetoresistance increases with temperature up to a maximum at about 20 K. The maximum itself increases with purity and leads for the purest sample to a resistivity increase by a factor of 6.8 (B=3.9 T, T=17 K). The unusual temperature behavior of the resistivity with and without magnetic field, which means a violation of the Kohler and Matthiessen rule, is discussed and explained regarding the anisotropic electron‐phonon scattering. Following the theoretical concept of Kagan a qualitative agreeme...

Magnetic recording material with extremely high bit density - using anisotropic alloys of gadolinium with iron or cobalt

Abstract: Array for magnetic data recording and read-out, using a magnetic material (I) with its anisotropy in a direction (a) at 90 degrees to the recording surface. (I) has coercive force (Hc) of 50-400 oersted, with magnetisation, recording and read-out in direction (a) by moving a magnet head w.r.t. a substrate carrying material (2). Pref. (I) is an amorphous layer of transition metal(s), esp. Fe and/or Co; lanthanide(s), esp. Gd, Tb, and/or Ho. The magnetically active part of (II) may be Gd1-xFex, where x = 0.6-0.8; or Gd1-x'Cox' (where x' = 0.6-0.9). (II) is pref. evaporated or sputtered as a film onto a tape or plate substrate. A single inductive head, esp. a magnetoresistive head, is pref. used at a small height above the substrate for recording and read-out, but two separate heads may be used and the active part of the head(s) can be made by thin film technology. A bit density of ca. 10,000 bits/cm can be obtd. which is several times greater than that available by conventional methods.

Magnetoresistance Effect in p -Zn 3 As 2 Single Crystals

Abstract: Magnetoresistance effect in p -Zn 3 As 2 Single crystals with carrier concentration of ∼7×10 15 ∼10 18 /cm 3 was measured at low temperatures. The undoped crystals usually had ∼10 17 /cm 3 carriers and showed negative magnetoresistance which could be explained by the model of metallic impurity conduction. Specimens containing less carriers (∼7×10 15 /cm 3 ) showed positive magnetoresistance which was characterized by the intermediate impurity conduction. Specimens with p ∼3×10 16 /cm 3 showed magnetoresistance which could be explained by the additive contribution of the negative and positive effect. A possible model is proposed to explain these effects consistently.

Hall effect in thin films of gold-silver alloys

Abstract: Measurements on Hall constant and magnetoresistance of thin films (350 A-800 A) of Ag-Au alloys with concentrations up to 20% gold have been made. An effective relaxation time is deduced from the variation of the Hall coefficient with the concentration which is found to be proportional to the mean free path of the electrons. The mean free path of the film decreases by about a factor of 3 from the bulk value for silver on the addition of gold to silver. The results are consistent with those of other workers who have used different methods and also indicate that the relaxation time is anisotropic in thin films of alloy. Size effects have also been discussed.

Theory of the negative photoconductivity in crossed fields

Abstract: The photocurrent in crossed fields is characterized by the feedback of the transverse ambipolar drift of the excess carriers on the particle velocities in the longitudinal direction. In extrinsic semiconductors this reduction of the particle velocities may cause a negative photocurrent which is here discussed for the case of low plasma densities considering the influence of the ambipolar particle motions, the diffusion currents, the surface recombination, the bulk lifetime, the sample thickness, and the physical magnetoresistance. Numerical calculations were performed forn-InSb at 85 K.