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We theoretically study the magnetic proximity effect in the three-dimensional (3D) topological insulator/ ferromagnetic insulator (TI/FMI) structures in the context of possibility to manage the Dirac helical state in TI. Within a continual approach based on the k . p Hamiltonian, we predict that, when a 3D TI is brought into contact with a 3D FMI, the ordinary bound state arising at the TI/FMI interface becomes spin polarized due to the orbital mixing at the boundary. Whereas the wave function of FMI decays into the TI bulk on the atomic scale, the induced exchange field, which is proportional to the FMI magnetization, builds up at the scale of the penetration depth of the ordinary interface state. Such an exchange field opens the gap at the Dirac point in the energy spectrum of the topological bound state existing on the TI side of the interface. We estimate the dependence of the gap size on the material parameters of the TI/FMI contact [ABSTRACT FROM AUTHOR]

We describe the occurrence of a spin-polarized ferromagnetic state on the surface of a topological insulator with implanted 3d transition-metal atoms, within a model which takes into account the hybridization between the (s, p) orbitals of the insulator and the d orbitals of the metal, entailing the delocalized character of magnetic moments. Depending on the position of the Fermi level, the energy spectrum of this state displays both metallic and semimetallic, or semiconducting characteristics. [ABSTRACT FROM AUTHOR]

We discuss a model which is apt to describe the appearance of a spin-polarized half-metallic state around a single δ layer of a magnetic transition metal embedded into a nonmagnetic semiconductor host. We show that ferromagnetism in this system can be attributed to the intrinsic physical properties of the δ layer. The relevant physical effects described by our model are the hybridization of the electron states of the metal and semiconductor atoms, the charge redistribution around the δ layer, and the electron-electron correlation on a metal atom, which is the driving force of ferromagnetism. We obtain the mean-field phase diagram of the model at zero and finite temperature, both in the case when the chemical potential is fixed, and when the density of particles on the δ layer is fixed. The relevance of our results in connection with numerical and experimental results on the so-called digital magnetic heterostructures, in the absence and in the presence of a quantum-well carrier channel, is eventually discussed. [ABSTRACT FROM AUTHOR]

In the frame of k·p method and variational approach for the effective energy functional of a contact between a three-dimensional topological insulator (TI) and normal insulator (NI), we analytically describe the formation of interfacial bound electron states of two types (ordinary and topological) having different spatial distributions and energy spectra. We show that these states appear as a result of the interplay of two factors: hybridization and band bending of the TI and NI electron states near the TI/NI boundary. These results are corroborated by the density functional theory calculations for the exemplar Bi2Se3/ZnSe system. [ABSTRACT FROM AUTHOR]

The magnetic proximity effect is a fundamental feature of heterostructures composed of layers of topological insulators and magnetic materials since it underlies many potential applications in devices with novel quantum functionality. Within density functional theory we study magnetic proximity effect at the three-dimensional topological insulator/magnetic insulator (TI/MI) interface in the Bi2Se3/MnSe(111) system as an example. We demonstrate that a gapped ordinary bound state caused by the interface potential arises in the immediate region of the interface. The gapped topological Dirac state also arises in the system owing to relocation to deeper atomic layers of topological insulator. The gap in the Dirac cone originates from an overlapping of the topological and ordinary interfacial states. This result being also corroborated by the analytic model, is a key aspect of the magnetic proximity effect mechanism in the TI/MI structures. [ABSTRACT FROM AUTHOR]

Here we study how interface and edge perturbations as well as a size effect can be used to manipulate the transport properties in semiconductor heterostructures where a thin film of three-dimensional topological insulator (TI) is sandwiched by normal insulator (NI) slabs. Within the framework of the NI/TI/NI trilayer model based on a continual scheme, we argue that characteristics of electron states in the TI film (energy spectrum, envelope function profile, the Berry curvature, etc.) are controlled by the film thickness and TI/NI interface potential whose variation can lead to the modification of topological properties of the system. Calculating a spin Hall response for the NI/TI/NI trilayer infinite in the interface plane, we find that a series of quantum transitions between topological insulating phase and trivial band insulator phase can be induced by tuning both the film thickness and the interface potential. We draw in detail the corresponding phase diagram of the NI/TI/NI trilayer, which is controlled by change of the sign of either the hybridization gap or the dispersion parameter. To quantify the edge effect, we formulate a model of the half-infinite in the interface plane NI/TI/NI trilayer, which describes evanescent edge states and provides the necessary conditions under which they exist. It is found that the presence of the in-gap edge states is ambiguously determined by the phase of the TI film. Our findings provide a useful guide in choosing the relevant material parameters to facilitate the observation of quantum spin Hall effect in the TI/NI heterostructures. [ABSTRACT FROM AUTHOR]

We study theoretically and experimentally the frequency and temperature dependences of resistivity noise in semiconductor heterostructures 3-doped by Mn. The resistivity noise is observed to be nonmonotonous as a function of frequency. As a function of temperature, the noise increases by two orders of magnitude for a resistivity increase of about 50%. We study two possible sources of resistivity noise, dynamic spin fluctuations and charge fluctuations, and find that dynamic spin fluctuations are more relevant for the observed noise data. The frequency and temperature dependences of resistivity noise provide important information on the nature of the magnetic interactions. In particular, we show how noise measurements can help resolve a long-standing debate on whether the Mn-doped GaAs is a p-d Zener/Ruderman-Kittel-Kasuya-Yosida (RKKY) or double-exchange ferromagnet. Our analysis includes the effect of different kinds of disorder such as spin-glass type of interactions and a site-dilution type of disorder. We find that the resistivity noise in these structures is well described by a disordered RKKY ferromagnet model dynamics with a conserved order parameter. [ABSTRACT FROM AUTHOR]

A detailed study of the magnetic and transport properties of Si1-xMnx (x≈0.35) films is presented. We observe the anomalous Hall effect in these films up to room temperature. The results of the magnetic measurements and the anomalous Hall effect data are consistent and demonstrate the existence of long-range ferromagnetic order in the systems under investigation. A correlation of the anomalous Hall effect and the magnetic properties of the samples with their conductivity and substrate type is shown. A theoretical model based on the idea of a two-phase magnetic material, in which molecular clusters with localized magnetic moments are embedded in the matrix of a weak itinerant ferromagnet, is discussed and used to explain experimental results. The long-range ferromagnetic order at high temperatures is mainly due to the Stoner enhancement of the exchange coupling between clusters through thermal spin fluctuations ("paramagnons") in the matrix. Theoretical predictions do not contradict experimental data when model parameters of a plausible order of magnitude are used. [ABSTRACT FROM AUTHOR]

We present results of an experimental study of structural, magnetotransport, and magnetic properties of a disordered system which consists of the strained crystalline CoFe nanogranules with the size of 2-5 nm embedded into the B-Al-O oxide matrix with a large number of dispersed Fe or Co atoms. They act in the matrix as magnetic ions and contribute essentially to the magnetization at T ≼ 25 K. The conductivity of the system follows the lnT law on the metallic side of the metal-insulator transition in the wide range of metal content variation x=49-56at.% that formally corresponds to the conductivity of the array of granules with strong tunnel coupling between them. We found that scaling power laws in the dependence of anomalous Hall effect (AHE) resistivity ρAHE vs longitudinal resistivity ρ strongly differ if temperature T or metal content x are variable parameters. The obtained results are interpreted in terms of the model of two sources of AHE emf arising from metallic nanogranules and insulating tunneling regions, respectively. We suggest that the tunneling AHE component can be caused by the recently predicted scattering assisted mechanism [A. V. Vedyayev et al., Phys. Rev. Lett. 110, 247204 (2013)] and is strongly shunted due to generation of local circular Hall current. [ABSTRACT FROM AUTHOR]