Magnetoresistance and open orbits domain in cobalt

A detailed study of the transverse magnetoresistance of cobalt monocrystals is presented. Among other features the angular variations of the magneto-resistance for J 6 〈10$\text{1}$0〉 shows a maximum with a volcano-like structure. This is interpreted as produced by open orbits around 〈0001〉 direction. The 5° angular domain of these open orbits is found to be in excellent agreement with the 5.5° angular width of the Γ neck in the spin ↑ Fermi surface previously obtained from de Haas-van Alphen experiments.     

Giant magnetoresistance of PbSnTe:In films in the space-charge-limited current regime: Angular features and effect of the surface

In the space-charge-limited current regime at T = 4.2 K, the magnetoresistance of PbSnTe:In/(111)BaF₂ films has been studied at various mutual orientation of the magnetic field B (up to 4 T), electric field E (up to ~10³ V/cm), and normal to the surface n. At B ‖ n, the reduction of the current reaches a factor of ~10⁵, whereas at B ‖ E, the current increases by a factor of ~10³. The angular dependences of the magnetoresistance have been studied at the “rotation” of B in three different planes. The angular dependences of the magnetoresistance for the plane corresponding to the orientation B ⊥ E exhibit local maxima near the orientations B ⊥ n, at which charge carriers are deflected by the magnetic field to one of the boundaries of the film. At the deviation to the free surface, the half-width of maxima is several degrees. At the deviation to the interface with the substrate, the half-width of maxima is about an order of magnitude larger and their amplitude is one or two orders of magnitude smaller. Possible mechanisms of giant positive and negative magnetoresistance, as well as the effect of the boundaries of the film on the angular dependences of the magnetoresistance, have been discussed.     

Field Confinement Energy at a Nonlinear Interface between Nonlinear Defocusing Media

The dependences of the resistance of the layered quasi-one-dimensional semiconductor TiS₃ on the direction and magnitude of the magnetic field B have been measured. The anisotropy and angular dependences of the magnetoresistance indicate the two-dimensional character of the conductivity at T < 100 K. Below T₀ ≈ 50 K, the magnetoresistance for the directions of the field in the plane of the layers (ab plane) increases sharply, whereas the transverse magnetoresistance (B ∥ c) becomes negative. The results confirm the possibility of an electron phase transition to a collective state at T₀. The negative magnetoresistance (at B ∥ c) below T₀ is explained by the magnetic-field-induced suppression of two-dimensional weak localization. The positive magnetoresistance (at B ∥ ab) is explained by the effect of the magnetic field on the spectrum of electronic states.     

The microwave absorption of high-T c superconducting thin film samples

The experimental and theoretical investigations of microwave losses (ML) in HTSC thin films are carried out. It is shown theoretically that ML in the maximum of the magnetic componentB ₁ are essentially larger than those in the maximum of the electric componentE ₁. This is because eddy currents make much more substantial contribution to ML as compared to conventional conductivity currents. The consequence of this is the angular dependence of ML with respect to theB ₁ field direction which was experimentally observed. The angular dependences of ML with respect to theB ₀ field direction for both low and highB ₀ values were also investigated. The majority of experimental data can be well explained within the mixed model which predicts the existence of a critical state in inter- and intragranular Josephson medium.     

Luminescence Microscopy of Single Quantum Dot Pairs with Nanometer Spatial Resolution

The transverse magnetoresistance of Ho_0.8Lu_0.2B₁₂ dodecaboride with a cage glass structure is studied at low (2–10 K) temperatures. It is demonstrated that the isotropic negative magnetoresistance in this antiferromagnet is dominant within the broad temperature range near T_N ≈ K. This contribution to the total magnetoresistance is due to the scattering of charge carriers by nanoclusters formed by Но³⁺ ions, and it can be scaled in the ρ = f(μ²_effH²/T² ) representation. It is found that the magnetoresistance anisotropy above (about 15% at 80 kOe) is due to the positive contribution, which achieves maximum values at the magnetic field direction close to H ║ [001]. The anisotropy of the charge carrier scattering is interpreted in terms of the cooperative dynamic Jahn?Teller effect at В12 clusters.     

Anisotropic resistivity and anisotropic magnetoresistance of the magnetic semiconductor FeCr2S4

Transport measurements were performed on FeCr₂S₄ parallelepipeds with {100} faces, cut from vapour grown crystals. This made possible the separation of the spontaneous effect from the field dependent effect. The anisotropy of the latter effect only depends on the direction of B relative to the crystallographic axes. The anisotropy of the magnetoresistance has been analysed in terms of spin disorder scattering.     

Magnetoresistance of 2H−NbSe2

The magnetoresistance anisotropy of 2H?NbSe₂ was studied at 9.3 K and 35 K. Data indicate that the Fermi surface above and below the charge-density-wave onset temperature includes one or more sheets that are highly elongated (and possibly open) along the [0001] direction. The magnetoresistance at 9.3 K exhibits an anomalous linear dependence on magnetic field for all field directions studied, similar to that reported previously for $\text{B}$[0001]     

Hall effect in dilute magnetic alloys

We review the theory and the experimental results on the Hall effect in noble metals containing magnetic impurities of transition metals. In order to illustrate the various types of observed effects, we focus succesively on selected systems: $\text{Cu}$Mn, with only enhancement of the ordinary Hall effect due to the existance of different spin-up and spin-down currents; $\text{Au}$Fe and $\text{Au}$Cr, with skew scattering by magnetic impurities; $\text{Cu}$MnT ternary alloys (where T is a non-magnetic impurity), with skew scattering effects due to combined spin—orbit scattering by non-magnetic impurities and spin scattering by Mn impurities. The skew scattering in $\text{Au}$Fe and $\text{Au}$Cr can be ascribed to the orbital character of the impurity moments and accounted for in an orbitally degenerate virtual bound state model. However, the anomalous temperature dependence of the skew scattering in Kondo alloys at low temperature is not well understood. We also present some magnetoresistance data in order to describe the links between the Hall effect and the magnetoresistance in magnetic alloys. In particular, we relate the skew scattering and the magnetoresistance anisotropy observed in $\text{Au}$Cr alloys.     

The metal-insulator transition in extrinsic semiconductors

The main purpose of this article is to describe the electrical and magnetic properties of extrinsic semiconductors when the concentration of donors varies, particularly for concentrations near that for which a metal-insulator transition takes place. Since the donor centres are distributed at random in space, the combined effects of correlation and Anderson localization have to be considered. As an introduction, in § 1 we give an outline of our present understanding of the transition in some crystalline materials, particularly V₂O₃. For this the metallic phase, a highly correlated electron gas, is discussed, as is also the question of a discontinuous change in the number of current carriers. In § 2 we discuss Si : P and similar materials. For uncompensated samples, the material near the transition is to be described by overlapping Hubbard bands, and the metal-insulator transition occurs when the states at the Fermi energy show Anderson localization. Just on the insulator side of the transition conduction is by variable-range hopping for uncompensated as for compensated materials, and a variation of log (resistivity) as T ^?1/4 is observed. The existence or otherwise of moments in the metallic phase is discussed, as is also the magnetoresistance, Hall effect and Knight shift. In § 3 the transition is described for narrow-band semiconductors, particularly La_1?x Sr _x VO₃. In § 4 some compounds of rare earths are treated, including Sm_1?x Nd _x Se.     

Collective modes in quasi-two-dimensional conductors under strong spatial dispersion

Propagation of electromagnetic and spin waves in layered conductors with a quasi-two-dimensional dispersion law of charge carriers is investigated theoretically in the presence of an external magnetic field with induction B₀. In layered conductors, the drift velocity v_D of electrons along B₀ is an oscillatory function of the angle between the magnetic field direction and the normal to the layers. For certain orientations of the magnetic field with respect to the layers of the conductor, v_D is close to zero. In these directions, there is no collision-free absorption, and weakly damped waves may propagate even under strong spatial dispersion. In the short-wave-length limit, there may exist collective modes with frequencies in the neighborhood of resonances for arbitrary orientation of the wavevector k relative to B₀. Similar types of excitations in quasi-isotropic metals are possible only when k is perpendicular to the direction of the external magnetic field.     

High field Shubnikov-De-Haas effect and magnetoresistance in the organic metal (TMTSF)2 ClO4

The transverse magnetoresistance of (TMTSF)₂ ClO₄ has been investigated in magnetic fields up to 32 T at several temperatures down to 4.2 K and different angles between B and the crystalline axis. Shubnikov-de-Haas oscillations are observed directly on the magnetoresistance at fields higher than 10 T giving a fundamental field of 259± 10 T in a direction close to c¹ axis.     

Observation of magnetic breakdown in double quantum wells

We report on our studies of magnetic breakdown (MB) in coupled GaAs/Al_0.3Ga_0.7As double quantum wells (DQWs) subject to crossed magnetic fields. MB is a failure of semiclassical theory that occurs when a magnetic field causes electrons to tunnel across a gap ink-space from one Fermi surface (FS) branch to another. We study MB in a two-branch FS created by subjecting a DQW to an in-plane magnetic field (B_∥). The principal effect ofB_∥is a distortion in the dispersion curve of the system, yielding a FS consisting of two components, a lens-shaped inner orbit and an hour-glass-shaped outer orbit. The perpendicular field (B_⊥) causes Landau level formation and Shubnikov–de Haas (SdH) oscillations for each branch of the FS. At higher perpendicular fields MB occurs and electrons tunnel throughk-space from one FS orbit to the other. MB is observed by noting which peaks are present in the Fourier power spectrum of the magnetoresistance versus 1/B_⊥at constantB_∥. We observe MB in two DQW samples over a range ofB_∥.     

Magnetic properties and phase transitions in hexagonal DyMnO3 single crystals

DyMnO₃ single crystals of hexagonal modification were grown for the first time by zone melting, and their anisotropic magnetic properties were studied in detail. At T ～ 7 K, a small uncompensated magnetic moment (～1 μ_B per formula unit) is shown to occur along the hexagonal axis. At temperatures 7–45 K, a transition from the purely antiferromagnetic state to a state with uncompensated magnetic moment was revealed; the transition is induced by a magnetic field H parallel to the c axis. This transition seems to be due to a ferrimagnetic ordering of the moments of Dy cations located in nonequivalent crystallographic positions.     

Percolation magnetoresistance in ferromagnetic superconductors

It is shown that the magnetoresistance (as a function of magnetic field H) in polycrystalline magnetic superconductors has the percolation character which is the consequence of the anisotropy of magnetic susceptibility. The magnetoresistance R_m(H), and the upper critical field H_c2(T) of ErRh₄B₄ are evaluated and compared with the experimental data.     

Pair tunneling in the low-temperature conductivity of the Cd-Sb alloy high-resistance state near the superconductor-insulator transition

We report a study of the temperature dependence, down to 30 mK, of the magnetoresistance of Cd-Sb alloy in the insulating phase obtained by annealing the quenched metallic superconducting ( _{T c} ≈4.5 K) phase of the alloy. Even though the sample in this state is no longer superconducting, the observed negative magnetoresistance points to single-particle tunneling in the presence of a superconducting gap in the spectrum. At magnetic fields B<T the ratio α(T,B)=R(T,B)/R(T,B=4 T)is found to be maximum at a temperature of about 0.1 K. This behavior indicates a change of the conductivity mechanism from single-particle tunneling to incoherent two-particle tunneling as the temperature decreases. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 713–718 (25 November 1996) Published in English in the original Russian Journal. Edited by Steve Torstveit.     

Magnetic,electrical, magnetoelectrical,and magnetoelastic properties of La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>MnO<Subscript>3 − <Emphasis Type="Italic">y</Emphasis></Subscript> manganites

This paper reports on a study of the influence of oxygen deficiency on the magnetization, paramagnetic susceptibility, electrical resistivity, magnetoresistance, and volume magnetostriction of the La_0.9Sr_0.1MnO_{3 − y} manganite with y = 0.03, 0.10, and 0.15. The magnetization M(T) behaves in a complex way with temperature; for T < 80 K, it only weakly depends on T, and at 80 ≤ T ≤ 300 K, the M(T) curve shows a falloff. Within the interval 240 K ≤ T ≤ 300 K, the long-range magnetic order breaks up into superparamagnetic clusters. For T < 80 K, the magnetic moment per formula unit is about one-fourth that which should be expected for complete ferromagnetic alignment of Mn ion moments. Although the composition with y = 0.03, in which part of acceptor centers is compensated by donors (oxygen vacancies), the negative magnetoresistance Δρ/ρ and volume magnetostriction ω are observed to pass through maxima near the Curie point, their values are one to two orders of magnitude smaller than those for the y = 0 composition. In compositions with y = 0.10 and 0.15 with electronic doping, the values of Δρ/ρ and ω are smaller by one to two orders of magnitude than those observed for the y = 0.03 composition. They do not display giant magnetoresistance and volume magnetostriction effects, which evidences the absence of ferrons near unionized oxygen vacancies. This allows the conclusion that the part played by both compensated and uncompensated doubly charged donors consists in forming dangling Mn-O-Mn bonds, which lead to a decrease in the Curie temperature with increasing y and to the formation above it of superparamagnetic clusters of the nonferron type.     

Random magnetic field and weak localization effects in a dimpled 2D electron gas

We have studied negative magnetoresistance due to the weak localization effects in a 2D electron gas (2DEG) grown on dimpled substrates. Since the 2DEG is sensitive only to the normal component of B, depending on the orientation of the external magnetic field, electrons will move in a spatially inhomogeneous (B perpendicular to the substrate-B_⊥) or sign alternating, random magnetic field (B parallel to the substrate B_∥). A difference in the magnetoresistance at B_∥ and B_⊥ is seen for the sample with a coherence length larger than the spatial periodicity of magnetic field. We believe that the difference in the magnetic flux through the closed electron trajectories at B_∥ and B_⊥, taken into account random character of B_∥, is responsible for this behaviour. Features connected with Aharonov Bohm flux through the different areas on the dimpled surface were observed.     

Theory of magnetoresistance due to lattice dislocations in face-centred cubic metals

A theoretical model to describe the low temperature magneto-resistivity of high purity copper single and polycrystals containing different density and distribution of dislocations has been developed. In the model, magnetoresistivity tensor is evaluated numerically using the effective medium approximation. The anisotropy of dislocation-induced relaxation time is considered by incorporating two independent energy bands with different relaxation times and the spherical and cylindrical Fermi surfaces representing open, extended and closed electron orbits. The effect of dislocation microstructure is introduced by means of two adjustable parameters corresponding to the length and direction of electron orbits in the momentum space, which permits prediction of magnetoresistance of FCC metals containing different density and distribution of dislocations. The results reveal that dislocation microstructure influences the character of the field-dependent magnetoresistivity. In the orientation of the open orbits, the quadratic variation in magnetoresistivity changes to quasi-linear as the density of dislocations increases. In the closed orbit orientation, dislocations delay the onset of magnetoresistivity saturation. The results indicate that in the open orbit orientations of the crystals, the anisotropic relaxation time due to small-angle dislocation scattering induces the upward deviation from Kohler’s rule. In the closed orbit orientations Kohler’s rule holds, independent of the density of dislocations. The results obtained with the model show good agreement with the experimental measurements of transverse magnetoresistivity in deformed single and polycrystal samples of copper at 2 K.     

Onset of linear field dependence of the transverse magnetoresistance in a polycrystalline metal

The linear field dependence of the transverse magnetoresistance of a polycrystalline uncompensated metal with an open Fermi surface is said to arise from a contrast between extended and truly open orbits. We show that in the intermediate-field regime this is incorrect for Cu at least, where the linear dependence is initiated by the transition to the high-field condition of electrons in closed orbits.