Longitudinal magnetoresistance of platinum: The influence of lattice vacancies

The longitudinal magnetoresistance of polycrystalline platinum wires has been measured in magnetic fields up to 45 kOe at liquid helium temperatures. The deviations from Kohler's rule found depend on the concentration of quenched-in vacancies, and are interpreted as due to the contribution of single vacancies in platinum to the small angle scattering of electrons. The small-angle scattering may also be responsible for the linear magnetoresistance of platinum in high magnetic fields.     

Giant magnetoresistance of granular microwires: Spin-dependent scattering in integranular spacers

The anomalous behavior of magnetoresistance has been revealed in a number of granular microwires. In contrast to the giant magnetoresistance of granular alloys, which is associated with the spin-dependent scattering in the bulk of grains and at their surface, is linear in the square of the magnetization, and decreases with an increase in temperature, the magnetoresistance, for example, in Co₁₀Cu₉₀ microwires is negative, increases with an increase in temperature below the Curie temperature, and does not reach saturation in the field dependence in the high-field range. A simple mechanism of negative giant magnetoresistance due to scattering of spin-polarized charge carriers by impurity magnetic moments localized in the nonmagnetic intergranular spacers has been proposed taking into account that a considerable part of magnetic ions in microwires exhibiting this behavior is dissolved in the intergranular spacers. It has been shown that the corresponding contribution to magnetoresistance can reach 10–20%.     

Description of the Colossal Magnetoresistance of La 1.2 Sr 1.8 Mn 2 O 7 Based on the Spin-Polaron Conduction Mechanism in the Ferromagnetic Temperature Range

We have analyzed the resistance of La1.2Sr1.8Mn2(1 – z)O7 single crystal in magnetic fields from 0 to 90 kOe in the ferromagnetic temperature range. The observed magnetoresistance of La1.2Sr1.8Mn2O7 is described based on the spin-polaron conduction mechanism. The magnetoresistance is determined by the change in the sizes and magnetic moment directions of magnetic inhomogeneities (polarons). It is shown that the colossal magnetoresistance is ensured by an increase (along the magnetic field) of the polaron linear size. It is found using the method for separating the contributions of different conduction mechanisms to the magnetoresistance that the contribution to the magnetoresistance from the orientation mechanism at 80 K in low magnetic fields is close to 50%. With increasing magnetic field, this contribution decreases and becomes small in fields exceeding 30 kOe. The comparable contributions to the conductivity from the orientational and spin-polaron mechanisms unambiguously necessitate the inclusion of both conduction mechanisms in the magnetoresistance calculations. We have calculated the temperature variation of the polaron size (in relative units) in zero magnetic field and in a magnetic field of 90 kOe.     

Magnetoresistance of the compound CeRu2Ge2

In this work we present a magnetoresistance study on the CeRu₂Ge₂ compound. We analyze the ρ(T) curves for several applied magnetic fields using the electron–magnon scattering model for a ferromagnetic spin arrangement. From this analysis, the field dependence of the energy gap of the magnon spectrum is obtained. The magnetoresistance ρ(H) at various temperatures arises from a normal metal contribution with an additional scattering mechanism due to electron–magnon interaction.     

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.     

Structural, magnetic and transport properties of NiFeχAg(1−χ) heterogeneous alloys

We carried out a comprehensive study of structural, magnetic and electrotransport properties of as-deposited and annealed (Ni₈₀Fe₂₀)_χAg(_1−χ) heterogenous alloys prepared by sputtering. The NiFe atomic concentration was varied between 15% and 40%. These alloys consist of small magnetic particles (Ni₈₀Fe₂₀) embedded in a nonmagnetic matrix (Ag). The structures of these alloys were investigated by X-ray diffraction, scanning electron microscopy and high-resolution cross-section transmission electron microscopy. The magnetic measurements were made using SQUID magnetometry and ferromagnetic resonance. Magnetoresistance was measured with a conventional four-point probe between 1.5 K and room temperature in field range 0–6T. Three contributions to the magnetoresistance of these granular alloys have been clearly identified: the spin-valve (or giant) magnetoresistance as in multilayers, scattering on magnetic fluctuations (as in any ferromagnetic metal around its magnetic ordering temperature), and anisotropic magnetoresistance. These three contributions have their own dependences on the size of the magnetic particles, on the degree of intermixing between Ni₈₀Fe₂₀ and Ag, and on temperature. We discuss the different shapes and amplitudes of magnetoresistance versus Ni₈₀Fe₂₀ concentration or temperature and their evolution upon annealing in terms of the relative roles of these three contributions. The magnetoresistance in multilayers (current in-plane or perpendicular to the plane) and granular alloys are also compared.     

Magnetoresistance of a Two-Dimensional TbTe<Subscript>3</Subscript> Conductor in the Sliding Charge-Density Wave Regime

The magnetoresistance of a TbTe₃ two-dimensional conductor with a charge-density wave (CDW) has been measured in a wide temperature range and in magnetic fields of up to 17 T. At temperatures well below the Peierls transition temperature and in high magnetic fields, the magnetoresistance exhibits a linear dependence on the magnetic field caused by the scattering of normal charge carriers by “hot” spots of the Fermi surface. In the sliding CDW regime in low magnetic fields, a qualitative change in the magnetoresistance has been observed associated with the strong scattering of carriers by the sliding CDW.     

In0.53Ga0.47As/In0.52Al0.48As量子阱中的正磁电阻效应

在低温强磁场条件下,对In_0.53Ga_0.47As/In_0.52Al_0.48As量子阱中的二维电子气进行了磁输运测试.在低磁场范围内观察到正磁电阻效应,在高磁场下这一正磁电阻趋于饱和,分析表明这一现象与二维电子气中的电子占据两个子带有关.在考虑了两个子带之间的散射效应后,通过分析低磁场下的正磁电阻,得到了每个子带电子的迁移率,结果表明第二子带电子的迁移率高于第一子带电子的迁移率.进一步分析表明,这主要是由两个子带之间的 关键词： 二维电子气 正磁电阻 子带散射     

Fe-Ag金属颗粒膜的巨磁电阻

采用量子唯象模型研究金属颗粒膜巨磁电阻的特性．在计算散射截面时，考虑了颗粒内各杂质原子之间的相位相干性．依据实验报告提供的颗粒尺寸分布的数据，推导了巨磁电阻依赖于颗粒尺寸及其分布和外磁场的函数表示式．对样品Fe-Ag的计算结果表明，其磁电阻随外磁场的变化和颗粒尺寸的分布有关；在某个颗粒尺寸标度D₀，磁电阻出现极大值，尺寸标度D₀的大小与外加磁场、样品材料和制备条件有关 关键词：     

Contribution of “unusual” domain walls to the magnetoresistance of multilayer magnetic structures

The contribution to electrical resistance due to scattering of charge carriers by domain walls is analyzed. It is revealed that “unusual” domain walls are created by frustrations in ferromagnet-antiferromagnet multilayer magnetic structures. The thickness of an unusual domain wall is substantially less than that of a usual domain wall. It is shown that scattering of charge carriers by unusual domain walls can contribute significantly to the magnetoresistance of ferromagnet-antiferromagnet multilayer magnetic structures. An analysis of the contribution made by the Levy-Zhang mechanism to the magnetoresistance demonstrates that the initial estimate obtained for this contribution is considerably exaggerated.     

Magnetoresistance of dilute magnetic alloys

The magnetoresistance of dilute magnetic alloys is calculated. The difficulties appearing in the calculation of scattering times within the density wave model are discussed and overcome semi-empirically.     

Large magnetothermoelectric power in Co/Cu, Fe/Cu and Fe/Cr multilayers

We report and discuss experimental data on the thermoelectric power of magnetic multilayers. Measurements of the thermoelectric power of Fe/Cr, Co/Cu and Fe/Cu multilayers have been carried out in the temperature range 4K < T < 150 K magnetic fields perpendicular to the layers. All specimens were found to exhibit pronounced magnetothermoelectric power (MTEP) effects correlating with their giant negative magnetoresistance. The main difference between the MTEP and the magnetoresistance is in their temperature dependence. Whereas the magnetoresistance is a decreasing function of temperature, the MTEP, at least in Co/Cu and Fe/Cu multilayers, is very small at low temperature and increases rapidly above 30–40 K. We ascribe this high temperature part of the MTEP to spin-dependent electron-magnon scattering and we propose a theoretical model.     

Semiclassical negative magnetoresistance of a 2D electron gas caused by scattering by short-range and long-range potentials

The magnetoresistance of a 2D electron gas confined in narrow GaAs quantum wells with AlAs/GaAs superlattice barriers is studied in the classical range of magnetic fields. It is shown that the negative magnetoresistance observed in this kind of structures with nonplanar heterointerfaces is semiclassical and qualitatively agrees with the model of negative magnetoresistance due to the scattering of charge carriers by two types of random potential, namely, the short-range and long-range ones.     

Magnetoresistive effect in RCu<Subscript>3</Subscript>Mn<Subscript>4</Subscript>O<Subscript>12</Subscript> perovskite-like oxides

The electrical properties of and the magnetoresistive effect in RCu₃Mn₄O₁₂ (R=rare-earth ion or Th) are studied. In all compounds of this series, the magnetoresistive effect amounts to 20% at liquid nitrogen temperature in the presence of a field of 0.9 T. An increase in the magnetoresistance with decreasing temperature and a high sensitivity to weak magnetic fields at low temperatures point to the intergranular nature of the effect. The magnetoresistance shows a peak in the vicinity of the Curie temperature T_C. Based on the dependences of the magnetoresistance on an external magnetic field, it is assumed that the magnetoresistance peak near T_C is related to the charge carrier scattering by magnetic inhomogeneities as in substituted orthomanganites. We believe that the magnetoresistance value near the magnetic ordering temperature depends on the synthesis conditions and the effect of the intergranular spacer on the transport properties of these compounds.     

Electron magnetotransport in carbon nanotubes and negative magnetoresistance: The density matrix method

A solution (stationary and linear in electric field) to the kinetic equation for a one-electron density matrix in an arbitrary magnetic field is found for deformation-potential scattering in the approximation of a small deviation of the electron gas from equilibrium. An expression for the nanotube conductivity is obtained in the form of a sum over magnetic-quantization states. In the absence of a magnetic field, this expression coincides with the corresponding classical relations. In weak magnetic fields, the magnetoresistance of a multilayer nanotube is positive for high electron mobilities and negative for low mobilities. In intermediate fields, it reverses sign with increasing field strength. The magnetoresistance of a one-layer nanotube is always positive.     

Hall effect and negative magnetoresistance in thin crystals of NbSe<Subscript>3</Subscript>

We have measured the Hall effect and the transverse magnetoresistance in NbSe₃ single crystals. In the liquid helium temperature range we observed an absolute negative magnetoresistance (NMR) — the value of the resistance under magnetic field being much lower than that at zero field — in NbSe₃ single crystals with a thickness less than 5 μm with the magnetic field oriented in the (b, c) plane. We show that this NMR effect is observed in the magnetic field range in which the Hall constant changes its sign. The results are qualitatively explained by the change of the surface scattering contribution to the magnetoconductance in the magnetic field range near the Hall voltage zero crossing.     

Quantum transport in topological semimetals under magnetic fields

Topological semimetals are three-dimensional topological states of matter, in which the conduction and valence bands touch at a finite number of points, i.e., the Weyl nodes. Topological semimetals host paired monopoles and antimonopoles of Berry curvature at the Weyl nodes and topologically protected Fermi arcs at certain surfaces. We review our recent works on quantum transport in topological semimetals, according to the strength of the magnetic field. At weak magnetic fields, there are competitions between the positive magnetoresistivity induced by the weak anti-localization effect and negative magnetoresistivity related to the nontrivial Berry curvature. We propose a fitting formula for the magnetoconductivity of the weak anti-localization. We expect that the weak localization may be induced by inter-valley effects and interaction effect, and occur in double-Weyl semimetals. For the negative magnetoresistance induced by the nontrivial Berry curvature in topological semimetals, we show the dependence of the negative magnetoresistance on the carrier density. At strong magnetic fields, specifically, in the quantum limit, the magnetoconductivity depends on the type and range of the scattering potential of disorder. The high-field positive magnetoconductivity may not be a compelling signature of the chiral anomaly. For long-range Gaussian scattering potential and half filling, the magnetoconductivity can be linear in the quantum limit. A minimal conductivity is found at the Weyl nodes although the density of states vanishes there.     

Magnetothermo-emf and Wiedemann-Franz law for tungsten single crystals under the conditions of static skin effect

Results of the experimental investigation of the thermal conductivity, the thermo-emf, the Nernst effect, and the magnetoresistance of thin monocrystalline tungsten films under conditions of static skin effect in the temperature range T=2–18 K in magnetic fields of up to 28 kOe are presented. It is shown that the field dependence of the magnetothermo-emf follows the predictions of the theory of galvano-and thermomagnetic phenomena in strong fields and exhibits anisotropy associated with both the shape of the sample and reflecting properties of the crystal surface. The temperature dependence of the magnetothermo-emf is investigated and a contribution from the phonon drag is detected. The surface contribution to the electrical and thermal conductivity under conditions of the static skin effect is singled out, and the temperature dependence of the Lorentz number for surface scattering is traced.     

Non-fermi-liquid scaling in Ce(Ru0.5Rh)2Si2

We study the temperature and field dependence of the magnetic and transport properties of the non-Fermi-liquid (NFL) compound Ce(Ru0.5Rh0.5)2Si2. For fields H less, similar0.1 T the results suggest that the observed NFL behavior is disorder driven. For higher fields, however, magnetic and transport properties are dominated by the coupling of the conduction electrons to critical spin fluctuations. The temperature dependence of the susceptibility as well as the scaling properties of the magnetoresistance are in very good agreement with the predictions of recent dynamical mean-field theories of Kondo alloys close to a spin-glass quantum critical point.     

Enhanced granular magnetoresistance due to ferromagnetic layers

Giant magnetoresistance (GMR) of sequentially evaporated Fe-Ag structures has been investigated. Direct experimental evidence is given, showing that inserting ferromagnetic layers into a granular structure significantly enhances the magnetoresistance. The increase in the GMR effect is attributed to spin polarization effects. The large enhancement (up to more than a fourfold value) and the linear variation of the GMR in low magnetic fields are explained by scattering of the spin polarized conduction electrons on paramagnetic grains.