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We study the effects a strong Coulomb disorder on the transverse magnetoresistance in Weyl semimetals at low temperatures. Using the diagrammatic technique and the Keldysh model to sum up the leading terms in the diagrammatic expansion, we find that the linear magnetoresistance exhibits a strong renormalization due to the long-range nature of the Coulomb interaction $\rho_{xx} \propto H\ln(eH\hbar v^2/cT^2_{\rm imp}),\ \ \Omega\alpha^{-1/6}\ll T_{\rm imp}\ll \Omega/\alpha^{-3/4}$, where $\Omega = v\sqrt{2eH\hbar/c}$ is the distance between the zeroth and the first Landau levels, $T_{\rm imp}=\hbar vn^{1/3}_{\rm imp}$ measures the strength of the impurity potential in terms of the impurity concentration $n$ and the Fermi velocity $v$, and $\alpha = e^2/\hbar v$ is the effective fine structure constant of the material. As disorder becomes even stronger (but still in the parametric range, where the Coulomb interaction can be treated as a long-range one), we find that the magnetoresistivity becomes quadratic in the magnetic field $\rho_{xx}\propto H^2$.

Thin films of Dirac semimetal Cd3As2 about 100 nm thick made of single crystals of cadmium arsenide, prepared by direct fusion of the elements by vacuum–ampoule method, by vacuum–thermal sputtering/depostion have been studied. Temperature and magnetic field dependences of electrical resistance have been measured, which indicate the presence of a superconducting transition at a temperature from 0.2 to 0.6 K.

Magnetic properties of two-phase granular semiconductor–ferromagnet GaSb–MnSb films obtained by pulsed laser deposition as a function of growth temperature and postgrowth annealing were stu-died.

Thin films of solid solutions based on the three-dimensional Dirac semimetal Cd3As2 with the addition of manganese are investigated. Cd3 –xMnxAs2 films (x = 0, 0.05, and 0.1) 50–70 nm in thickness are formed on a glassceramic substrate using vacuum-thermal deposition from cadmium arsenide ingots doped by Mn and fabricated by direct alloying elements by the vacuum-cell method. The temperature and magnetic-field dependences of the resistance are measured and the transport parameters of the films under study are determined. Positive magnetoresistance of the characteristic shape corresponding to the contribution of the weak antilocalization effect is observed for films with x = 0 and 0.05. The contribution from the weak localization effect is observed at a higher Mn content (x = 0.1). This change in the quantum correction type as applied to topological semimetals points to reconstruction of the band structure and transition from the Dirac semimetal state into a trivial semiconductor phase, which corresponds to the critical Mn content xc ~ 0.07 in this case.

Thin (~50 nm) cadmium arsenide films have been grown by magnetron sputtering on single-crystal silicon and sapphire substrates. Using X-ray diffraction, scanning electron microscopy, atomic force microscopy, and Raman spectroscopy, the composition of the films has been shown to correspond to the Cd3As2 stoichiometry. Along with the α-Cd3As2 phase, the films contained trace levels of the α'-Cd3As2 phase. Annealing at 520 K led to recrystallization and the formation of [112] textured films on single-crystal silicon substrates. In the annealed films, the crystallite size evaluated using the Debye–Scherrer equation was ~30 nm.

The vapor phase growth of Cd3As2—Zn3As2 (in the following (Cd1−x Znx)3As2 solid solutions process is described. The (Cd0,993 Zn0,007)3As2 solid solution single crystals were synthesized. Scanning electron microscopy and electron diffraction data suggest high crystalline quality of studied sample. Its structure and surface morphology, indicating the presence of growth nuclei and cleavage planes, were investigated. Giant anisotropic magnetoresistance and Shubnikov — de Haas oscillations were observed at low temperatures. Obtained results suggests that peculiarities of Dirac semimetal phase persist in (Cd1−x Znx)3As2 solid solution at low zinc content. At the same time, there are indications of some differences with initial Cd3As2 properties.

We observed a temperature-controlled increase in the magnetic anisotropy and its dispersion in thin GaMnSb films with MnSb nanoinclusions obtained by pulsed laser deposition. The data of transmission electron microscopy indicate that in the samples, a transition of the crystalline structure of magnetic MnSb nanoinclusions from hexagonal (spatial group (sp. gr.) P63/mmc) to cubic (sp. gr. F-43m) takes place. Analysis of the temperature dependences of the magnetic moment m(T), measured using a SQUID magnetometer, obtained for both unannealed and annealed samples cooled in a zero magnetic field and a magnetic field of 10 kOe, indicates that this mechanism is not unique. In unannealed samples, the distribution of the magnetic anisotropy of MnSb nanoinclusions, determined from the dependences of m(T), is unimodal. In the annealed samples, the same dependence becomes multimodal. This means that several thermally activated processes occur in the samples during annealing, resulting in several “populations” of nanoinclusions present in the annealed thin films. The contribution to the increase in the magnetic anisotropy during annealing can result in the structural phase transition, the mismatch of the crystal lattices between MnSb and GaSb, an increase in the average volume of MnSb nanoinclusions, and a change in their stoichiometry.

We report a highly anisotropic magnetoresistance of (Cd0.993Zn0.007)3As2 single crystals, which have been synthesized by the vapor-phase growth. Scanning electron microscopy and electron diffraction data confirm the high crystalline quality of obtained samples. Studied samples exhibit specific features such as octahedral nuclei growth and step-like morphology of the surface formed by {112} planes. The giant anisotropic magnetoresistance and Shubnikov-de Haas oscillations have been observed at low temperatures. The results suggest the existence of the Dirac semimetal phase in (Cd1 −xZnx)3As2 solid solution with low zinc content. Thus, the observed magnetoresistance anisotropy is partially attributed to the chiral anomaly.

We study the effect of random potential created by different types of impurities on the transverse magnetoresistance of Weyl semimetals. It is shown that the magnetic field and temperature dependence of magnetoresistance is strongly affected by the type of impurity potential. Two limiting cases are analyzed in detail: ($i$) the ultra-quantum limit, when the applied magnetic field is so high that only the zeroth and first Landau levels contribute to the magnetotransport, and ($ii$) the semiclassical situation, for which a large number of Landau levels comes into play. A formal diagrammatic approach allowed us to obtain expressions for the components of the electrical conductivity tensor in both limits. In contrast to the oversimplified case of the $\delta$-correlated disorder, the long-range impurity potential (including that of Coulomb impurities) introduces an additional length scale, which changes the geometry and physics of the problem. It is shown that the magnetoresistance can deviate from the linear behavior as a function of magnetic field for a certain class of impurity potentials., Comment: 13 pages, 8 figures

The structure and conductivity of hybrid nanocomposite poly(p-xylylene) films containing iron nanoparticles are studied. The films are obtained by the method of polymerization on a surface from the gas phase, and the concentration of the filler is 4 and 11 vol %. According to the analysis of microphotographs of nanostructures obtained by transmission electron microscopy, the particles are distributed evenly (with a size in the nanometer range and a rather narrow distribution). Studies of the frequency dependence of conductivity show that the transport of the charge carriers is realized by the hopping mechanism with a jump length of ~3.5 nm.

Conditions and mechanisms of controlled variation of the magnetic anisotropy of GaMnSb films containing magnetic MnSb nanoinclusions by means of heat treatment have been determined. For this purpose, the temperature and magnetic-field dependences of the magnetic moments of samples before and after thermal annealing were measured using a SQUID magnetometer. It is established that the heat treatment of GaMnSb films leads to a significant increase in the values of characteristics determined by the magnetic anisotropy, including the growth of blocking temperature (from 95 to 390 K) and the magnetic anisotropy field (from 330 to 630 Oe). Results of transmission electron microscopy investigation indicate that a change in the magnetic anisotropy of GaMnSb films as a result of their thermal annealing can be related to a transition of the crystalline structure of magnetic MnSb nanoinclusions from hexagonal (space group P62/mmc) to cubic (space group F-43m).

We have studied the properties of relatively thick (about 120 nm) magnetic composite films grown by pulsed laser deposition using the eutectic compound (GaSb)0.59(MnSb)0.41 as target for sputtering. For the studied films we have observed ferromagnetism and an anomalous Hall effect above room temperature, confirming the presence of spin-polarized carriers. Electron microscopy, atomic and magnetic force microscopy results suggest that the films under study have a homogenous columnar structure in the bulk while MnSb inclusions accumulate near the surface. This is in good agreement with the high mobility values of charge carriers. Based on our data we conclude that the magnetic and magnetotransport properties of the films at room temperature are defined by the MnSb inclusions.

The relationship between structure, electrical and magnetic properties of thin poly( p -xylylene) - nickel nanocomposite films with Ni concentrations from 5 to 30 vol% was studied. It was found that metal concentration strongly affects size and oxidation state of the nanoparticles and composites morphology. At nickel concentration below 5 vol% the nanoparticles are oxidized to NiO and homogeneously distributed within fine-grained polymer matrix. An increase of Ni concentration up to 10 vol% results in the development of coarse-grained morphology with preferable localization of the nanoparticles at the boundaries of polymeric grains. And finally, in the composite films with nickel concentration above 20 vol%, the fine-grained morphology is observed again, but the nanoparticles are mainly metallic. Effect of the filler content on electrical and magnetic properties of the nanocomposites was elucidated showing that they are determined by percolation phenomenon with the threshold value of about 10 vol%. The well-pronounced magnetic hysteresis as well as ferromagnetic ordering were observed at Ni content above the percolation threshold. The diagrams of magnetic properties of these composites as a function of composition and temperature were elaborated. It was demonstrated that film annealing can be used to control magnetic properties of the composites and strongly enhance magnetoresistance.

The magnetotransport in Bi2Se3 thin films with magnetic Eu dopants is studied within the range of low applied magnetic fields. With the increase in the doping level, the saturation of the dephasing length on cooling is observed. This can be related to the existence of magnetic inclusions. The observed anisotropy of the magnetoresistance is qualitatively similar to the anisotropy characteristic of nonmagnetic Bi2Se3 films. Numerous similarities to the properties of pure Bi2Se3 films and some observed differences can be interpreted under the assumption of the local interaction of topologically nontrivial interface states with Eu-rich inclusions.

Heterostructures with a GaAs/InGaAs/GaAs quantum well and aMn magnetic impurity layer separated from it, which have different conductivity types, are studied. At a Mn content not exceeding the amount corresponding to 0.5 monolayer of MnAs, a percolation cluster formed in the quantum well plane is not simply connected, but consists of metal drops separated by low-conductivity interspaces. Despite the absence of the simply connected conducting channel, Shubnikov–de Haas oscillations are observed in all studied systems, which are controlled by carrier properties in conducting drops, independent of Mn content. The estimate of drop sizes corresponds to theoretical values.

The magnetoconductivity of thin Bi2Se3 films covered by a protective Se layer and grown at (111) BaF2 substrates is studied. It is shown that the negative magnetoconductivity observed at low magnetic fields and caused by the effect of weak antilocalization, as well as the Shubnikov−de Haas oscillations at higher fields, is determined only by the magnetic field component perpendicular to the film plane. The obtained experimental results can be reasonably interpreted under the assumption that the studied films exhibit two-dimensional topologically protected electron states. Moreover, the contribution of these states to the total conductivity turns out to be the dominant one.

We report an experimental observation of superconductivity in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ thin films without application of external pressure. The films under study were synthesized by magnetron sputtering. Surface studies suggest that the observed transport characteristics are related to the polycrystalline continuous part of the investigated films with a homogeneous distribution of elements and the Cd-to-As ratio close to stoichiometric ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$. The latter is also supported by Raman spectra of the studied films where two pronounced peaks inherent to ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ were observed. The obtained x-ray diffraction patterns for studied films also correspond to the ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ lattice. The formation of a superconducting phase in the films under study is confirmed by the characteristic behavior of the temperature and the magnetic field dependence of the sample resistivity, as well as by the presence of pronounced zero-resistance plateaus in the $dV/dI$ characteristics. The corresponding ${H}_{c}\text{\ensuremath{-}}{T}_{c}$ plots reveal a clearly pronounced linear behavior within the intermediate temperature range, similar to that observed for bulk ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ and ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ films under pressure, suggesting the possibility of a nontrivial pairing in the films under investigation. We discuss a possible role of the sample inhomogeneities and crystal strains in the observed phenomena.

We report the results of experimental and theoretical studies of Eu-doped Bi

Effect of charge carriers concentration on magnetic parameters of the ferromagnetic MnSb nanoclusters as well as opposite effect of cluster magnetization on spin polarization of holes and magnetoresistance were observed in GaSbMn thin films. Microwave magneto-resistance effect is evidence of the charge carriers spin filtering by ferromagnetic clusters.

Transport and magnetic properties of δ-Mn doped GaAs/InGaAs/GaAs quantum wells (QW) with various In content were studied at temperatures 4.2K≤T≤300K. Fluctuation potential (FP) appeared to be crucial for transport characteristics of structures under investigation. The magnetic percolation transition was observed at temperatureTpin the range 20 - 40K. TheTpdependence on the In content is nonmonotonic due to the peculiarities of free-carrier mediated exchange interaction mechanisms. The change of the anomalous Hall effect (AHE) sign with decreasing temperature was detected at temperatures close to theTp. The main reason of the AHE sign change is the variation of contributions of different AHE mechanisms (intrinsic and side-jump) caused by the reduction of spin-dependent scattering intensity with temperature decrease. We believe that our results are the experimental observation of the AHE intrinsic mechanism in 2D.

Transport, magnetotransport and magnetic properties of structures GaAs/InAs/GaAs with a InAs quantum dot (QD) layer have been investigated in the temperature interval 4.2p-type conductivity. The ferromagnetic phase up to 400 K was detected by SQUID magnetometer. Anomalous Hall-effect was observed at low T. The role of additional disorder in conducting channel due to the QD layer was investigated. It is shown a principal role of a fluctuation potential of Mn layer separated from conducting QD layer by a spacer in anomalous transport properties of structures. The negative magnetoresistance was observed at low T due to the reduction of the spin-flip scattering by aligning spins by magnetic field.

The influence of the charge carrier concentration on the magnetic properties of GaSb-MnSb alloys is studied. The ferromagnetism of GaSb-MnSb films is caused by the presence of MnSb granules and manifests itself in both magnetometric measurements and the presence of an anisotropic magnetoresistance and the anomalous Hall effect. Electric conduction is executed by charge carriers (holes) in a GaSb matrix. The magnetization of clusters depends on stoichiometry and the concentration of Mn2+ and Mn3+ ions, which is specified by the film growth conditions. At high film growth temperatures, ferromagnetic clusters containing Mn2+ ions mainly form. At low growth temperatures, an antiferromagnetic phase containing Mn3+ ions forms.

We have studied magnetoresistance and Hall effect of GaAs/InxGa1−xAs quantum wells with remote Mn impurity. Temperature and magnetic field dependencies of samples resistivity indicate several effects related to the magnetic subsystem. Shubnikov - de Haas oscillations indicate the presence of several types of regions in conduction channel with significantly different hole mobilities. We discussed the impact of magnetic impurities on quantum corrections to conductivity by comparing our results with the data for similar non-magnetic structures. Our results suggest that the presence of Mn atoms leads to the damping of quantum corrections in in the investigated structures.

We study InSb:Mn polycrystals with different values of the Mn content. In these samples, two ferromagnetic phases have been found: MnSb nanoinclusions with TC ≈ 600 K and an InMnSb magnetic host with TC below 10 K. The magnetic field dependence obtained for the measured Hall resistance exhibits a nonlinear behavior within a wide temperature range. At high temperatures, such a behavior can be attributed to the existence of two types of charge carriers, namely, light and heavy holes. At temperatures below the Curie point of the InMnSb host, the anomalous Hall effect contributing to the nonlinearity of the Hall resistance has been observed. Ferromagnetic MnSb inclusions do not contribute to the anomalous Hall effect. They do not lead to any spin polarization of charge carriers owing to the Schottky barrier, which surrounds these inclusions and prevents their interaction with charge carriers. A method has been proposed for distinguishing the anomalous Hall component in the case where the Hall resistance includes a nonlinear contribution of a different nature.

For heterostructures with a GaAs/InGaAs/GaAs quantum well and a magnetic (Mn) δ-layer spatially separated from it (remote Mn δ-layer), the magnetic field and temperature dependences of the anomalous component of the Hall resistivity have been analyzed. The comparison with the temperature dependence of the longitudinal electrical resistance reveals three temperature ranges where three different mechanisms of the anomalous Hall effect are manifested. The reported results can be treated as experimental evidence of the essential role of the intrinsic mechanism of the anomalous Hall effect in a two-dimensional system.

Structural, magnetic and magnetotransport properties of (Bi$_{1-x}$Eu$_x$)$_2$Se$_3$ thin films have been studied experimentally as a function of Eu content. The films were synthesized by MBE. It is demonstrated that Eu distribution is not uniform, it enter quint-layers forming inside them plain (pancake-like) areas containing Eu atoms, which sizes and concentration increase with the growth of Eu content. Positive magnetoresistance related to the weak antilocalization was observed up to 15K. The antilocalization was not followed by weak localization as theory predicts for nontrivial topological states. Surprisingly, the features of antilocalization were seen even at Eu content $x$ $=$ 0.21. With the increase of Eu content the transition to ferromagnetic state occurs at $x$ about 0.1 and with the Curie temperature $\approx$ 8K, that rises up to 64K for $x$ $=$ 0.21. At temperatures above 1-2 K, the dephasing length is proportional to $T^{-1/2}$ indicating the dominant contribution of inelastic $e-e$ scattering into electron phase breaking. However, at low temperatures the dephasing length saturates, that could be due to the scattering on magnetic ions., 7 pages, 4 figuers

Charge carriers parameters on a 2D-layer surface for (Cd1−x−yZnxMny)3As2 (y = 0.08) (the concentration n 2 D = 1.9 × 1012 cm–2, the effective value of the 2D-layer d 2 D = n 2 D / n 3 D = 14.5 nm, the wave vector k F = 0.1 nm–1, the charge carriers relaxation time due to dispersion τ D = 1.8 × 10–13 s, the velocity of charge carriers on Fermi surface v F = ℏ k F / m c = 2.65 × 105 m s−1, the mean free path l F = v F τ D = 47.7 nm) were determined. It was found that the dependence of the cyclotron mass m с (0)/m0 on Fermi wave vector k F for (Cd1−x−yZnxMny)3As2 (y = 0.08) is in compliance with a theoretical linear dependence, that describes mass-less Dirac fermions.

Eutectic GaSb + MnSb films ranging in thickness from 80 to 130 nm have been grown on sapphire substrates by pulsed laser deposition using mechanical droplet separation. The films were similar in composition to the ablation target, consisting of the eutectic GaSb-MnSb alloy. According to atomic force and electron microscopy data, the films were homogeneous, with p-type conductivity. Their electrical properties depended significantly on deposition conditions. The best films had a resistivity of 7 × 10−3 Ω cm, carrier concentration of 8.1 × 1019 cm−3, and carrier mobility of 102 cm2/(V s). Characteristically, the films had a negative magnetoresistance. Their magnetization curves showed saturation in a magnetic field of ∼1 × 10−1 T. According to the magnetic-field dependences, the coercive force in the films was within 3 × 10−2 T; that is, the films were soft magnets with a small domain size.

The study is concerned with the structural, optical, magnetic, and transport properties of Zn1 − xCoxO (x = 0.05–0.45) films produced on Al2O3 (0001) substrates at a temperature of Ts = 500°C by pulsed laser deposition with fast particle separation. The film thickness is d = 60–300 nm. It is found that the Zn1 − xCoxO ternary alloy retains its wurtzite-type crystal structure up to x = 0.35, if the films are produced at low buffer-oxygen pressures (∼10−6 Torr). It is established that, in these conditions, the electron concentration is higher than 1020 cm−3 because of the high density of oxygen donor vacancies. In this case, the films start to exhibit ferromagnetism in the magnetization and the anomalous Hall effect at temperatures above 100 K. The sign of the anomalous Hall effect is found to be positive and opposite to the sign of the normal Hall effect, as occurs in Co metal layers. This is indicative of the cluster nature of ferromagnetism of the Zn1 − xCoxO films. For thin Zn1 − xCoxO layers (d = 60 nm, x = 0.2) in a transverse magnetic field, profound hysteresis of the magnetoresistance is observed, which is indicative of the out-of-plain easy axis of magnetization of the films. The magnetic anisotropy is attributed to the structuring of the layers (elongation of magnetic clusters along the growth axis of the films). The structuring can lead to noticeable strengthening of the layer ferromagnetism.

Diluted magnetic semiconductor InSb:Mn exhibits a ferromagnetic behavior up to T ∼ 600 K due to presence of nanosize MnSb precipitates [Kochura et al., J. Appl. Phys. 113, 083905 (2013)]. Transport properties of InSb:Mn, including the resistivity, the magnetoresistance (MR), and the Hall effect, are investigated between T ∼ 1.6 and 300 K in magnetic fields B up to 15 T. The resistivity, ρ(T), displays an upturn with lowering the temperature below T ∼ 10–20 K attributable to the Kondo effect, where the universal Kondo behavior is observed. The Hall resistivity, ρH, demonstrates a nonlinear dependence on B up to T ∼ 300 K, suggesting an anomalous contribution due to the effect of the MnSb nanoprecipitates. The relative MR, Δρ(B)/ρ(0), is positive (pMR) above T ∼ 10 K and transforms into a negative one (nMR) with lowering temperature. The Hall effect and pMR are interpreted simultaneously with the two-band model, addressed to presence of the two types of holes with quite different concentrations and mobilities. The dependences of nMR on B and T follow those of the Khosla–Fischer model, taking into account damping of the spin-dependent scattering of charge carriers in magnetic field.

The results of a detailed study of magnetic properties and of the microstructure of SiMn films with a small deviation from stoichiometry are presented. The aim was to reveal the origin of the high temperature ferromagnetic ordering in such compounds. Unlike SiMn single crystals with the Curie temperature ~30 K, considered Si

The p-(InSb + MnSb)/n-InSb diode structures have been produced by the pulse laser deposition method on the n-InSb single-crystal substrates from the targets in which positive giant magnetoresistance at 298 K in forward bias and at the application of the magnetic field of 0.15 T was observed. The I–V characteristics of the diode structure changed under the application of a magnetic field both in the plane of the structure, and perpendicular to it at the room temperature. The current magnitude of diode at a voltage of 1 V in the magnetic field 0.15 T, parallel and perpendicular to the plane of the diode structure, decreased by more than by 3 times from 28 to 8.2 mA and than by 10 times from 28 to 2.7 mA accordingly The values of parasitic resistance, the size of magneto-resistive effect for the p-(InSb + MnSb)/n-InSb diode have been defined without application of a field and in the presence of the 0.15 T magnetic field in the plane and perpendicular to the transition plane. The spin polarization of carriers has been calculated.

The transport and magnetic properties of Si1 − x Mn x films of thickness 55–70 nm with various Mn content (x = 0.44–0.6) are studied in the temperature range of 5–400 K and in magnetic fields up to 2 T. The films are grown by pulsed laser deposition on Al2O3 (0001) substrates at a temperature of 340°C using velocity separation of deposited particles. The films exhibit metal conductivity and the resistivity ρ = (2−8) × 10−4 Ω cm, typical of highly degenerate semiconductors. It is found that the anomalous component of the Hall effect dominates over the normal component at T = 300 K for the Si1 − x Mn x alloy with x ≈ 0.5, and that the Curie temperature significantly exceeds room temperature and is estimated as ∼500 K from magnetization measurements (for MnSi silicide the Curie temperature is T C = 30 K). It is shown that the anomalous component of the Hall conductivity at low temperatures is controlled by “side-jump” and (or) “intrinsic” mechanisms independent on the carrier scattering time. The results are explained by features of the formation of defects with localized magnetic moments in the case of Si1 − x Mn x films with x ≈ 0.5 and by the significant role of matrix spin fluctuations in the exchange between these defects.

It has been found that the Curie temperature (T C ≈ 300 K) in nonstoichiometric Si1 − x Mn x alloys slightly enriched in Mn (x ≈ 0.52–0.55) in comparison to the stoichiometric manganese monosilicide MnSi becomes about an order of magnitude higher than that in MnSi (T C ∼ 30 K). Deviations from stoichiometry lead to a drastic decrease in the density of charge carries (holes), whereas their mobility at about 100 K becomes an order of magnitude higher than the value characteristic of MnSi. The high-temperature ferromagnetism is ascribed to the formation of defects with the localized magnetic moments and by their indirect exchange interaction mediated by the paramagnetic fluctuations of the hole spin density. The existence of defects with the localized magnetic moments in Si1 − x Mn x alloys with x ≈ 0.52–0.55 is supported by the results of numerical calculations performed within the framework of the local-density-functional approximation. The increase in the hole mobility in the nonstoichiometric material is attributed to the decay of the Kondo (or spin-polaron) resonances presumably existing in MnSi.

We report the results of experimental and theoretical studies of Eu-doped Bi2Se3 thin films with extremely inhomogeneous distribution of magnetic component. The obtained electron microscopy images suggest that Eu atoms are concentrated within platelet-like nanoinclusions. The number of inclusions grows with the increase in Eu content, x. Moreover, at relatively high x values, the stacks of platelets (inclusions located one under another) become rather frequent. A comparative analysis of magnetic properties of the films under study reveals no pronounced changes of their temperature dependence with the increase in x, which, however, leads to the decrease in the average magnetic moment [Formula: see text] per Eu atom. A theoretical analysis of different mechanisms contributing to a possible magnetic ordering in the Eu-doped films demonstrates that at small distances (i.e. within a platelet) a dominant contribution is related to the RKKY interaction via electrons in the bulk, while the ordering at inter-platelet distances is governed by magnetic dipole-dipole interaction. The latter implies the antiferromagnetic ordering within the stacks of platelets explaining a drop of [Formula: see text] per Eu atom. We employ the model of a metallic spin glass to estimate the transition temperature, characterising the interaction within the ensemble of randomly distributed magnetic platelets. This estimate gives satisfactory agreement with the experiment, even if we take into account a finite film thickness, thus, neglecting the interaction anisotropy and including only the antiferromagnetism related to the stacking. While the overall contribution of interface Dirac electrons is damped in the systems under study, we argue that the obtained results can be used for the investigation of ultrathin films with analogous impurity profile, where this contribution should be clearly pronounced.

The transport properties of GaAs/Mn/GaAs/InxGa1 − xAs/GaAs structures with a layer that is separated from the quantum well and contains Mn impurities in the concentration range 4–10 at % corresponding to the reentrant metal-insulator transition observed in the bulk GaMnAs material [17] have been investigated. The hole mobility in the objects under investigation is more than two orders of magnitude higher than the known values for the GaMnAs semiconductor and GaMnAs-based magnetic heterostructures. This makes it possible to observe Shubnikov-de Haas oscillations, which confirm a two-dimensional character of the hole energy spectrum. The calculated Curie temperature for heterostructures with indirect exchange interaction through a two-dimensional hole channel is in good agreement with the position of the maximum (at 25–40 K) in the temperature dependences of the electrical resistance of the channel. This suggests that two-dimensional holes play an important role in ferromagnetic ordering of the Mn layer under these conditions. The observations of a negative spin-dependent magnetoresistance and an anomalous Hall effect, whose magnitude correlates well with the results of theoretical calculations for two-dimensional ferromagnetic systems based on III-Mn-V, also indicate a significant role of the two-dimensional channel in ferromagnetic ordering.

The transport and magnetic properties of MnxSi1 − x films with a high (x ≈ 0.35) content of Mn produced by laser deposition at growth temperatures of 300–350°C have been studied in a temperature range of 5–300 K in magnetic fields of up to 2.5 T. The films exhibit a hole-type metallic conductivity and a relatively weak change of magnetization in a temperature range of 50–200 K. An anomalous Hall effect with an essentially hysteretic behavior from 50 K up to ≈230 K has been discovered. The properties of the films are explained by the two-phase model, in which ferromagnetic clusters containing interstitial Mn ions with a localized magnetic moment are embedded in the matrix of a weak band MnSi2 − x (x ≈ 0.3) type ferromagnet with delocalized spin density.

The paper reports on the results of a study of the synthesis conditions effects on magnetic and transport properties of nanosized layers of high- T c diluted magnetic semiconductors (DMS), such as Ge:Mn, Si:Mn and Si:Fe, fabricated by laser-plasma deposition over a wide range of the growth temperature, T g =(20–550) °C on single-crystal GaAs or Al 2 O 3 substrates. Ferromagnetism of the layers was detected by measurement data of the magneto-optical Kerr effect, anomalous Hall effect, negative magnetoresistance and ferromagnetic resonance (FMR) at 5–500 K. The optimum growth temperature, T g , for Si:Mn/GaAs layers with T c ≈400 K is shown to be about 400 °C. The Si:Mn/Al 2 O 3 layers with 35% of Mn have the metal-type of conductivity with manifestation of magnetization up to room temperature. Different types of uniformly doped structures and digital alloys have been investigated. In contrast to GaSb:Mn films, Si-based ferromagnetic layers have strongly different magnetic and electric properties in case of uniformly doped structures and digital alloys. Positive results of the Fermi level variation effect on the improvement of Si- and Ge-based DMS layers have been gained on the use of additional doping with shallow acceptor Al impurity which contributes to the increase of the hole concentration and the RKKY exchange interaction of 3d-ions. The Ge:(Mn, Al)/GaAs or Ge (Mn, Al)/Si layers grown at 20 °C feature surprising extraordinary angular dependence of FMR.

We have studied the magnetotransport properties of p-InMnAs layers in strong (up to 30 T) pulsed magnetic fields. The p-InMnAs layers were obtained by laser plasma deposition with subsequent annealing by radiation of a pulsed ruby laser. Under anomalous Hall effect conditions in a strong magnetic field (above 20 T), the Hall resistance in the paramagnetic region of temperatures is greater than that in the ferromagnetic region (below 40 K). It has also been established that, at helium temperatures, the negative magnetoresistance exhibits saturation in a field of about 10 T, whereas the anomalous Hall effect is saturated at about 2 T. At T ≈ 4 K, the resistance in a field of 10 T exhibits a more than tenfold decrease. The results are explained by a mesoscopically inhomogeneous distribution of the acceptor (Mn) impurity, a local ferromagnetic transition, and a percolation character of the conductivity of p-InMnAs films in a state close to the insulator-metal phase transition. The characteristic scale of magnetoelectric inhomogeneity in the system is evaluated based on an analysis of mesoscopic fluctuations of the nondiagonal component of the magnetoresistance tensor.

The field and temperature dependences of the magnetization of GaAs/δ〈Mn〉/GaAs/InxGa1 − xAs/GaAs quantum wells with the δ〈Mn〉 layer separated from the well by a 3-nm GaAs spacer have been studied in the temperature range of 3–300 K in a magnetic field up to 6 T. An external magnetic-field-induced phase transition to a ferromagnetic state with a magnetization hysteresis loop shifted from a zero magnetic field has been found to occur at a temperature below 40 K. A theoretical model is proposed that implies the coexistence of ferromagnetically and antiferromagnetically ordered regions within the GaAs layers.

We present results of magnetic and transport measurements of GaAs/Ga1–xInxAs/GaAs quantum well structures containing a Mn δ-layer separated from the quantum well by a 3 nm spacer. The kink observed in temperature dependence of the sample resistance and anomalous Hall effect give a hint for carrier spin polarization. Magnetization measurements show an unusual shifted hysteresis loop. The observed results are treated in the frame of long range fluctuation potential model. Parameters of the potential are estimated. It is shown that disorder determines the structural properties in spite of the hole mobility of about 2000 cm2/(V·s) being much higher than in presently known ferromagnetic two-dimensional structures. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In n-Si-based metal-oxide-semiconductor structures with a boron-doped surface layer, at the temperature T = 77 K, mesoscopic fluctuations in nondiagonal resistance-tensor component R xy are discovered under hole depletion of the Si: B layer caused by the field effect. A sharp increase in the fluctuation in R xy is discovered during the study of a shift from the 3D weakly perturbed hole transport to the quasi-2D percolation conduction on the back boundary of the Si: B layer. The fluctuations in R xy are due to the restructuring that occurs in the percolation cluster when the hole shielding of the fluctuation potential of ionized acceptors is weakened. Correlation length L c of the percolation cluster, which determines the boundary of the transition from the macroscopic to the mesoscopic system, has been estimated experimentally. Good agreement between the estimates and the computational results is observed for L c ranging from ≈ 10 nm to ≈ 1 μm.

Samples containing InGaAs quantum wells with p-type conductivity delta-doped by Mn were synthesized and studied. Magnetic moment measurements on a SQUID magnetometer revealed the presence of ferromagnetism at temperatures T from 4.2 to 400 K. Anomalous Hall effect caused by additional sample magnetization was observed at temperatures of from about 30 to 80 K. The Shubnikov-de Haas effect was recorded at 4.2 K. Negative magnetoresistance changed sign for positive at T ≈ 30 K as the temperature increased.

Nontrivial features of the formation of glancing incidence x-ray rocking curves from superlattices are revealed and analyzed for the magnetic digital alloy GaSb/15(Mn/GaSb)/GaAs. The qualitative analysis of the shape of the experimental curve in the framework of specific phase relations in the reflection amplitude makes it possible not only to describe these features, in particular, the two-humped profile of the first Bragg peak and to reconstruct the real structure of the alloys under investigation, but also to develop the general scheme for analyzing x-ray rocking curves from imperfect superlattices.

Magnetic and magnetotransport properties of GaAs(δ〈Mn〉)/In0.17Ga0.83As/GaAs quantum wells with different Mn concentrations are studied. The delta-doped manganese layer has been separated from the GaAs quantum well with a spacer with an optimal thickness (3 nm), which has provided a sufficiently high hole mobility (≥103 cm2V−1 s−1) in the quantum wells and their effective exchange with Mn atoms. It is found that the anomalous Hall effect (AHE) is exhibited only in a restricted temperature range above and below the Curie temperature, while the AHE is not observed in quantum wells with quasi-metallic conductivity. Thus, it is shown that the use of the AHE is inefficient in studying magnetic ordering in semiconductor systems with high-mobility carriers. The features observed in the behavior of the resistance, magnetoresistance, and Hall effect are discussed in terms of the interaction of holes with magnetic Mn ions with regard to fluctuations of their potential, hole transport on the percolation level, and hopping conduction.

A method of formation of two-dimensional structures containing a δ〈Mn〉-doped layer in GaAs and an InxGa1−x As quantum well (QW) separated by a GaAs spacer of thickness d = 4–6 nm is developed using laser evaporation of a metallic target during MOS hydride epitaxy. It is shown that, up to room temperature, these structures have ferromagnetic properties most likely caused by MnAs clusters. At low temperatures (T m ∼ 30 K), the anomalous Hall effect is revealed to occur. This effect is related to hole scattering by Mn ions in GaAs and to the magnetic exchange between these ions and QW holes, which determines the spin polarization of the holes. The behavior of the negative magnetoresistance of these structures at low temperatures indicates the key role of quantum interference effects.