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The electronic and optical spectra of the Bi3Se2 topological insulator digitally doped with V, Cr, Mn, Fe, and Co atoms are calculated using density functional theory. It is shown that the introduction of magnetic atoms into separate delta layers (one per 2–9 Bi2Se3 quintuple layers) multiply enhances magnetic effects. A special emphasis is put on Mn doping, which gives rise to the ferromagnetic spin ordering. The sensitivity of the spin order to the concentration and location of magnetic atoms is revealed. The study of an analytical model describing the resonant scattering of electrons in Bi2Se3 by atomic layers of transition metals also suggests the existence of spin-polarized states within the Bi2Se3 band gap. Our ab initio calculations show that transitions between the nested branches of the electronic spectrum, which exist near the Fermi level, are responsible for features of the optical conductivity at -ω ≈ 0.15−0.3eV, the infrared plasmon, and the Kerr angle θK > 12° in the infrared spectral range. [ABSTRACT FROM AUTHOR]

The properties of (CoFeB)x(LiNbOy)100-x nanocomposite films with a ferromagnetic alloy content x = 6-48 at % are comprehensively studied. The films are shown to consist of ensembles of CoFe granules 2-4 nm in size, which are strongly elongated (up to 10-15 nm) in the nanocomposite growth direction and are located in an LiNbOy matrix with a high content of Fe2+ and Co2+ magnetic ions (up to 3 × 1022 cm-3). At T ≤ 25 K, a paramagnetic component of the magnetization of nanocomposites is detected along with a ferromagnetic component, and the contribution of the former component is threefold that of the latter. A hysteresis of the magnetization is observed below the percolation threshold up to x ≈ 33 at %, which indicates the appearance of a superferromagnetic order in the nanocomposites. The temperature dependence of the electrical conductivity of the nanocomposites in the range T ≈ 10-200 K on the metallic side of the metal-insulator transition (44 at % < x < 48 at %) is described by a logarithmic law σ(T) ∝ lnT. This law changes into the law of “1/2” at x ≤ 40 at %. The tunneling anomalous Hall effect is strongly suppressed and the longitudinal conductivity turns out to be lower than in a (CoFeB)x(AlOy)100-x composite material by an order of magnitude. The capacitor structures based on (CoFeB)x(LiNbOy)100-x films exhibit resistive switching effects. They are related to (i) the formation of isolated chains of elongated granules and an anomalously strong decrease in the resistance in fields E > 104 V/cm because of the suppression of Coulomb blockage effects and the generation of oxygen vacancies VO and (ii) the injection (or extraction) of VO vacancies (depending on the sign of voltage) into a strongly oxidized layer in the nanocomposites, which is located near an electrode of the structure and controls its resistance. The number of stable resistive switchings exceeds 105 at a resistance ratio Roff/Ron ~ 50. [ABSTRACT FROM AUTHOR]

The electronic spectra and relative permittivity of ultrathin (1-3 QL) films of BiSe topological insulator have been calculated by the density functional theory. The calculated spectra exhibit a characteristic feature: the range of 0.0-0.9 eV below the Fermi level contains two doubly degenerate valence bands ('U-bands'), which are geometrically congruent to low-lying spectral branches in the conduction band. It has been shown that the saturation of optical absorption can result in a significant rearrangement of the electronic structure and properties in the near infrared spectral range in the considered film. In particular, the semiconductor (in the absence of interaction with light) type of conductivity of the film can be changed to the metallic type of conductivity strongly nonlinear in the intensity of light. [ABSTRACT FROM AUTHOR]

We theoretically study how magnetic modulation can be used to manipulate the transport properties of heterostructures formed by a thin film of a three-dimensional topological insulator sandwiched between slabs of a normal insulator. Employing the k ∙ p scheme, in the framework of a continual approach, we argue that electron states of the system are spin-polarized when ultrathin magnetic insertions are incorporated into the film. We demonstrate that (i) the spin-polarization magnitude depends strongly on the magnetic insertion position in the film and (ii) there is the optimal insertion position to realize quantum anomalous Hall effect, which is a function of the material parameters, the film thickness and the topological insulator/normal insulator interface potential. For the heterostructure with a pair of symmetrically placed magnetic insertions, we calculate a phase diagram that shows a series of transitions between distinct quantum regimes of transverse conductivity. We provide consistent interpretation of recent experimental findings in the context of our results. [ABSTRACT FROM AUTHOR]

The structural, transport, and magnetic characteristics of polycrystalline MnSi ( x ≈ 0.51-0.52) films grown by pulsed laser deposition onto AlO(0001) substrates when the low-energy components are deposited owing to collisions with the atoms of the buffer gas have been studied in the 'shadow' geometry. The magnetization of these films is determined by two ferromagnetic phases-the high-temperature phase with the Curie temperature T ≈ 370 K and the low-temperature one with T ≈ 46 K. The anomalous Hall effect changes sign from positive to negative with a decrease in temperature. The sign change occurs in the temperature range of 30-50 K; the specific value of this temperature depends on the thickness of the MnSi film. The results can be interpreted in terms of the structural self-organization related to the formation of two layers in the course of film growth. These layers have nearly the same chemical composition but significantly differ in the shapes and sizes of crystallites. This leads to a drastic difference in the values of T and in the value and the sign of the anomalous Hall effect for such layers. [ABSTRACT FROM AUTHOR]

Results from a comparative SIMS study of the fragmentation of metal clusters sputtered with atomic Xe and molecular SF ions are presented. It is shown that the average decay rate constants and, hence, the excitation energies of clusters of the same stoichiometry do not depend on the type of bombarding ions. [ABSTRACT FROM AUTHOR]

The results of a comprehensive study of magnetic, magneto-transport and structural properties of nonstoichiometric MnxSi1-x (x≈0.51-0.52) films grown by the Pulsed Laser Deposition (PLD) technique onto Al2O3(0001) single crystal substrates at T = 340°C are present. A highlight of used PLD method is the non-conventional ("shadow") geometry with Kr as a scattering gas during the sample growth. It is found that the films exhibit high-temperature (HT) ferromagnetism (FM) with the Curie temperature TC ~ 370 K accompanied by positive sign anomalous Hall effect (AHE); they also reveal the polycrystalline structure with unusual distribution of grains in size and shape. It is established that HT FM order is originated from the bottom interfacial self-organizing nanocrystalline layer. The upper layer adopted columnar structure with the lateral grain size ≥ 50 nm, possesses low temperature (LT) type of FM order with Tc ≈ 46 K and contributes essentially to the magnetization at T ≤ 50 K. Under these conditions, AHE changes its sign from positive to negative at T ≤ 30K. We attribute observed properties to the synergy of distribution of MnxSi1-x crystallites in size and shape as well as peculiarities of defect-induced FM order in shadow geometry grown polycrystalline MnxSi1-x (x~0.5) films. [ABSTRACT FROM AUTHOR]

A generator of high-power high-voltage nanosecond pulses is intended for electrical discharge disintegration of mineral quartz and other nonconducting minerals. It includes a 320 kV Marx pulsed voltage generator, a high-voltage glycerin-insulated coaxial peaking capacitor, and an output gas spark switch followed by a load, an electric discharge disintegration chamber. The main parameters of the generator are as follows: a voltage pulse amplitude of up to 300 kV, an output impedance of ≈10 Ω, a discharge current amplitude of up to 25 kA for a half-period of 80-90 ns, and a pulse repetition rate of up to 16 Hz. [ABSTRACT FROM AUTHOR]

In the framework of an effective functional approach based on the k · p method, we study the combined effect of an interface potential and a thickness of a three-dimensional (3D) topological insulator (TI) thin film on the spin Hall conductivity in layered heterostructures comprising TI and normal insulator (NI) materials. We derive an effective two-dimensional (2D) Hamiltonian of a 3D TI thin film sandwiched between two NI slabs and define the applicability limits of approximations used. The energy gap and mass dispersion in the 2D Hamiltonian, originated from the hybridization between TI/NI interfacial bound electron states at the opposite boundaries of a TI film, are demonstrated to change sign with the TI film thickness and the interface potential strength. Finally, we argue that the spin Hall conductivity can efficiently be tuned varying the interface potential characteristics and TI film thickness. [ABSTRACT FROM AUTHOR]

Using the continual model of a semi-infinite three dimensional (3D) topological insulator (TI) we study the effect of the surface potential (SP) on the formation of helical topological states near the surface. The results reveal that spatial profile and spectrum of these states strongly depend on the SP type and strength. We pay special attention to the 3D TI substrate/non-magnetic insulating overlayer system to illustrate the principles of the topological near-surface states engineering. [ABSTRACT FROM AUTHOR]

We present a comparative study of the anomalous Hall effect (AHE) and of the transverse Kerr effect (TKE) in the nonstoichiometric alloys. The data on AHE and TKE are consistent with each other and clearly indicate the intrinsic above room-temperature ferromagnetic order in the studied samples. We argue that this order is not produced by the phase segregation effects, but rather has a global character, while even a small level of the nonstoichiometry in alloys drastically changes their magnetic, electrical, optical and magneto-optical properties as compared to those of the stoichiometric manganese monosilicide MnSi. We propose a qualitative explanation of the obtained experimental results in the frame of the model of defect-induced ferromagnetic order using the first-principles calculations of the electronic and magnetic structure of the system. [ABSTRACT FROM AUTHOR]

We show that, when a three-dimensional (3D) narrow-gap semiconductor with inverted band gap ('topological insulator,' TI) is attached to a 3D wide-gap semiconductor with non-inverted band gap ('normal insulator,' NI), two types of bound electron states having different spatial distributions and spin textures arise at the TI/NI interface. Namely, the gapless ('topological') bound state can be accompanied by the emergence of the gapped ('ordinary') bound state. We describe these states in the framework of the envelope function method using a variational approach for the energy functional; their existence hinges on the ambivalent character of the constraint for the envelope functions that correspond to the 'open' or 'natural' boundary conditions at the interface. The properties of the ordinary state strongly depend on the effective interface potential, while the topological state is insensitive to the interface potential variation. [ABSTRACT FROM AUTHOR]

We report on theoretical study of the bound electron states induced by a ferromagnetic delta-layer embedded into a narrow-band-gap semiconductor of the BiSe-type which is a three-dimensional topological insulator with large spin-orbit coupling. We make use of an effective Hamiltonian taking into account the inverted band structure of the semiconductor host at the Γ point and describe the properties of the in-gap bound states: energy spectrum, characteristic length and spin polarization. We highlight a role of these states for a magnetic proximity effect in digital magnetic heterostructures based on the BiSe-type semiconductors. [ABSTRACT FROM AUTHOR]

It has been found that the Curie temperature ( T ≈ 300 K) in nonstoichiometric SiMn 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 ∼ 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 SiMn 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. [ABSTRACT FROM AUTHOR]

We study the effect of magnetic doping at the surface of a three dimensional topological insulator (TI) on emergence of ferromagnetic ordering at the TI-surface assuming the exchange coupling between the Dirac fermions and the dilute magnetic ions. We show that this coupling results in an uniaxial magnetic anisotropy with out-of-plane magnetization direction. It is found that the system under consideration is unstable with respect to a spontaneous uniform magnetization along the easy axis, which is accompanied by opening a gap in a spectrum of the Dirac surface states. In the framework of a mean-field approach, we study the possibility of ferromagnetic order on the magnetically doped surface of TI at different temperatures and positions of the chemical potential. [ABSTRACT FROM AUTHOR]

A phenomenological model of high-temperature ferromagnetism in silicon-manganese alloys has been proposed taking into account phase separation in these alloys, where manganese-rich particles of the secondary phase (precipitate MnSi with z ≈ 0.25-0.30) are formed inside a manganese-depleted matrix of almost pure silicon. Precipitate MnSi is considered as the silicide MnSi containing a certain number of magnetic defects whose origin is due to the presence of weakly hybridized 3 d orbitals of manganese. The silicide MnSi is a weak band ferromagnet in which strong fluctuations of the spin density (paramagnons) are present at a temperature much higher than its Curie temperature. It has been shown that the ferromagnetic exchange interactions between the magnetic moments of defects in precipitate exists due to thermal excitations of the spin density and the ferromagnetic order can appear at a temperature much higher than the Curie temperature of the silicide. The spatial structures and characteristics of this order have been described in the framework of the proposed approach for both homogeneous bulk precipitate and precipitate particles of various shapes and sizes. The short-range magnetic order near the bulk phase transition has been analyzed taking into account inhomogeneities in the distribution of magnetic defects in precipitate. The experimental data on the magnetic properties of silicon-manganese alloys have been interpreted in terms of the theoretical results obtained in this work. [ABSTRACT FROM AUTHOR]

The ab initio investigation of the magnetic ordering in digital alloys consisting of monolayers of 3 d-transition metals Ti, V, Cr, Mn, Fe, Co, and Ni introduced into the Si, Ge, and SiGe semiconductor hosts is reported. The calculations of the parameters of the exchange interactions and total-energy calculations indicate that the ferromagnetic order appears only in the manganese monolayers, whereas the antiferromagnetic order is more probable in V, Cr, and Fe monolayers, and Ti, Co, and Ni monolayers are nonmagnetic. The stability of the ferromagnetic phase in digital alloys containing manganese monolayers has been analyzed using the calculations of magnon spectra. [ABSTRACT FROM AUTHOR]

We discuss a possible route to explain high-temperature ferromagnetism in Si:Mn dilute magnetic semiconductors. We argue that most Mn atoms are segregated within nanometer-sized regions of magnetic precipitate and form the alloy, or compound, MnSi with z ≈ (0.25 $\div$0.30), whereas a small minority of Mn atoms forms Ångström-sized magnetic defects embedded in the host. Assuming that MnSi is a weak itinerant ferromagnet which supports sizable spin fluctuations ( paramagnons) far above the intrinsic Curie temperature, we show that the Stoner enhancement of the exchange interaction between the local magnetic moments of the defects occurs. As a result, a significant increase of the temperature of global ferromagnetic order in the system is achieved. We develop a phenomenological approach, to qualitatively describe this effect. [ABSTRACT FROM AUTHOR]

The transport and magnetic properties of Mn xSi1 − 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. [ABSTRACT FROM AUTHOR]

The magnetic properties of a magnetic-metal δ layer placed in a nonmagnetic nondegenerate semiconductor matrix are studied theoretically. The diffusion-induced spread of the δ layer, which is inevitable during δ doping, is taken into account, and a model is proposed in which this layer consists of a thin core enriched in metal atoms and a smeared periphery depleted in metal atoms. The exchange and potential scattering of carriers by the core causes confinement states in the form of two-dimensional spin-polarized sub-bands inside the energy gap of the semiconductor. The mechanism of the indirect exchange between impurity spins located in the strongly dilute peripheral region of the δ layer through partly filled confinement states is analyzed. In the case of a ferromagnetic core, impurity spins are oriented along (or opposite to) the core magnetization owing to carrier polarization on the confinement states. The magnetic configuration of impurity spins at the periphery of the δ layer is phenomenologically studied with allowance for the confinement mechanism of the interaction of impurity spins and the superexchange through the deep states in the semiconductor matrix. [ABSTRACT FROM AUTHOR]

The mechanism of the appearance of the magnetic order is considered for the GeMn alloys inside which ferromagnetic “nanocolumns” highly enriched with manganese are formed during the process of low-temperature growth. To describe this ordering, a model of effective exchange between ferromagnetic strings arranged inside a highly compensated nondegenerate semiconductor is suggested. [ABSTRACT FROM AUTHOR]

The interlayer exchange coupling in ferromagnet-semiconductor digital magnetic alloys in which monolyers (submonolayers) of transition metals are embedded into a semiconductor matrix is studied theoretically. A mechanism of an indirect exchange between ferromagnetic δ layers is proposed; it is based on the confinement of carriers in two-dimensional spin-polarized states inside the energy gap of the semiconductor. These appear due to strong potential and exchange carrier scattering by the δ layers. The interlayer exchange coupling is shown to occur through a nondegenerate semiconductor interlayer because of virtual electron excitations through an energy barrier separating these partly filled two-dimensional spin-polarized states and the edge of the bulk semiconductor band. The interlayer coupling intensity decreases exponentially with increasing distance between neighboring δ layers, and the type of this coupling can change from ferromagnetic into antiferromagnetic or vice versa as the interlayer thickness or the degree of filling the two-dimensional states increases. [ABSTRACT FROM AUTHOR]

The field and temperature dependences of the magnetization of GaAs/δ〈Mn〉/GaAs/In x Ga1 − x As/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. [ABSTRACT FROM AUTHOR]

The X-ray K spectra of Mn absorption were used for Mn ion distribution in (Ga, Mn)N. According to the performed calculations, two threshold lines were observed in the K spectra of (Ga, Mn)N crystals with uniform Mn ion distribution. If Mn ions form chains, these two threshold lines merge into one broad line. [ABSTRACT FROM AUTHOR]

Magnetic and magnetotransport properties of multilayered nanostructures Co0.45Fe0.45Zr0.1/ a-Si obtained by ion-beam sputtering are investigated. The temperature dependence of the resistance obeys a law of the form R xx ∝-log T, which is typical of metal-insulator nanocomposites on the metal side of the percolation transition. The magnetoresistance anisotropy effect, as well as the planar Hall effect, is observed for the first time for this type of nanocomposites in the vicinity of the percolation transition. The correlation of these two effects with the transverse (between Hall probes) magnetoresistive effect, which may reach 6–9%, is revealed. A weak negative magnetoresistance of the order of 0.15%, which is observed for subnanometer amorphous silicon layer thicknesses, is attributed to spin-dependent electron transitions between adjacent ferromagnetic layers in the case when the exchange interaction between these layers is of the antiferromagnetic type. [ABSTRACT FROM AUTHOR]

A superexchange mechanism is considered for the interlayer exchange coupling in Fe/Si nanostructures. It is assumed that the most important role in this mechanism is played by iron-silicon interfaces, in whose immediate vicinity the contact-induced ferromagnetic phase of body-centered iron monosilicide forms and spin-polarized interface states arise. The magnetic coupling between iron layers is accomplished by means of the interaction of interface states through the normal-semiconductor spacer layer and involves both intraband and interband processes. The nonmonotonic character of the dependence of the interband exchange coupling on the spacer thickness and composition (the occurrence of a maximu, the change in shape, the possible sign reversal of the bilinear component) is caused by competition between the ferro-and antiferromagnetic super-exchange components and by the complex character of the spacer electronic spectrum (in particular, by the occurrence of several equivalent extrema in the semiconductor bands). This model qualitatively reproduces the main features of the mechanism of interlayer exchange coupling in real Fe/Si nanostructures. [ABSTRACT FROM AUTHOR]

We present a theoretical description and electrical conductivity measurements for amorphous (Gd, Y)xSi1 – x alloys with 0.1 < x < 0.2. In our model, we take into account the strong topological disorder in the system, causing the appearance of regions with higher electron density (electron “drops”) around nanoscale structural defects enriched with rare-earth ions (“clusters”). We calculate the local density of electron states in the drops and in the matrix and establish the criterion for local instability to ferromagnetism. In the framework of the “local phase transition” approach, we find that short-range ferromagnetic order is more favorable inside the drops than in the matrix and exists in a wide temperature range. We analyze recent measurements of the temperature and magnetic-field dependence of the electrical conductivity in these systems and show that the spin polarization of the electron states in the drops enhances the tendency towards the metal–insulator transition. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

Direct experimental evidence has been obtained for the existence of short-range ferromagnetic order in GdxSi1 – x alloys. Along with the standard magnetotransport measurements, a “local” experimental method of the elastic scattering of thermal neutrons on the magnetic moment of Gd is applied. The measurement results provide the conclusion that short-range ferromagnetic order is formed in nanoclusters with an increased content of Gd ions. We consider the amorphous alloy GdxSi1 – x as a new magnetic material with unique properties characteristic of both strongly inhomogeneous alloys and nanocomposite compounds. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

The magnetic phase diagram of the Fe/Cr/Fe three-layer structure with almost ideal interlayer boundaries was constructed. The effective interlayer interaction in this structure was described by the “half-angle coupling” model. Various system configurations were analyzed taking into account crystalline anisotropy, and the ground state of the system was determined. The behavior of the structure in an external magnetic field applied along easy and hard magnetic axes was studied. The magnetization curves M(H) characteristic of structures with various interface roughness parameter and interlayer exchange values were described and analyzed. The experimental situation is discussed. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

A model of noncollinear magnetic ordering in Fe/Cr-type multistructures was suggested. The model was based on the idea of charge (and, as a consequence, spin) density redistribution near a metal–metal interface. A peculiar state of the whole structure characterized by strong short-range antiferromagnetic ordering in the interlayer and a pronounced dependence of magnetic characteristics on the properties of the boundary between iron and chromium layers was shown to be formed in a certain temperature range. Inhomogeneous antiferromagnetic structures with a vector order parameter were found, and the effective exchange coupling between neighboring iron layer moments was calculated using the Ginzburg–Landau expansion of the thermodynamic potential. The results were used to discuss the experimental data on Fe/Cr superlattices obtained in neutron scattering and magnetization measurements. © 2002 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

A mechanism of the formation of the short antiferromagnetic order with a spin density wave (SDW) in the vicinity of the interfaces in the Fe/Cr type multilayers is proposed. The main reason behind the emergence of magnetic ordering with SDWs is the redistribution of charge (and, hence, spin) density in the vicinity of Fe/Cr interfaces, which leads to the paramagnetic phase instability at a temperature considerably higher than the Néel temperature in chromium. The Ginzburg-Landau expansion for the free energy of the system is used for determining the inhomogeneous collinear structures of CDWs and for constructing the phase diagram (the dependence of the transition temperature on the thickness of the antiferromagnetic interlayer). The obtained results are used for discussing the experimental data on neutron scattering and tunnel microscopy. [ABSTRACT FROM AUTHOR]

The spin density wave model in a quasi-one-dimensional itinerant antiferromagnet with staggered potential at finite temperature is studied. Only short-range ordering exists in this system above the Néel temperature. The local-band theory of spin fluctuations is developed to calculate the spin density wave amplitude and the effective exchange integral. The one-electron spectrum and magnon spectrum are obtained in the short-range ordering regime. [ABSTRACT FROM AUTHOR]

An Anderson model with N-fold degeneracy in the Kondo regime is considered. It is presumed that the electron-electron correlations in the system off electrons have their maximum strength. A criterion for instability against the formation of a weakly antiferromagnetic phase superposed on the Kondo state is obtained by the auxiliary-boson method using the 1/N expansion. An effective interaction leading to the formation of magnetic ordering appears because of the spin fluctuations in the system of localized electrons. The phase diagram of the system is constructed. [ABSTRACT FROM AUTHOR]