Magnetic proximity effect at the 3D topological insulator/magnetic insulator interface

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 3D topological insulator/magnetic insulator (TI/MI) interface in Bi$_2$Se$_3$/MnSe(111) system as an example. We demonstrate that a gapped ordinary bound state which spectrum depends on 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 is originated 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.Comment: 10 pages, 3 figure

Proximity-induced spin ordering at the interface between a ferromagnetic metal and a magnetic semiconductor

14 páginas, 5 figuras.-- PACS number8s): 73.40.Sx, 75.70.-iWe carry on a theoretical investigation of the conditions for the appearance and/or modification of spin ordering in a dilute magnetic semiconductor that is in contact with a ferromagnetic metal. We show that the magnetic proximity effect has a rather complex physical nature in this system. Allowing for the hybridization between the ferromagnetic metal and semiconductor electron states, we calculate the spin polarization and spin susceptibility of carriers in the semiconductor layer near the contact. The peculiar mechanism of indirect exchange coupling that occurs between local spins dissolved in the semiconductor host when a dilute magnetic semiconductor is in contact with a ferromagnetic metal is analyzed. The structure of the proximity-induced ordering of local moments in a dilute magnetic semiconductor is qualitatively described within a mean-field approach. On the basis of our results, we interpret the experimental data on Fe/(Ga,Mn)As and Py/(Ga,Mn)As layered structures.The work was partially supported by the University of the Basque Country [Proyecto GV-UPV/EHU under Grant No. IT-366-07), Spanish Ministerio de Ciencia y Tecnología (Grant No. FIS2007-66711-C02-01)] , and by RFBR (Grant No. 10-02-00118). S.C. also acknowledges financial support by PRIN 2007 under Project No. 2007FW3MJX003. V.V.T. acknowledges financial support by Ikerbasque (Basque Foundation for Science).Peer reviewe

High-temperature ferromagnetism in Si:Mn alloys

13 páginas, 3 figuras.-- PACS number(s): 75.50.Pp, 75.50.Cc, 75.75.−cA possible mechanism for high-temperature ferromagnetic order in Si:Mn alloys is proposed. These materials, which are semiconducting or metallic, depending on the Mn content, are suggested to undergo phase separation. In the phase-separated state, again depending on the Mn content, Mn atoms can be gathered within nanometer-sized particles or micrometer-sized islands composed of the MnSi2-z precipitate with z≈(0.25–0.30), which are embedded in the Mn-poor silicon matrix. We consider the MnSi2-z precipitate to be the MnSi1.7 silicide host containing a certain amount of magnetic defects associated with unbound Mn 3d orbitals. The MnSi1.7 silicide is considered to be a weak itinerant ferromagnet, where sizable spin fluctuations (paramagnons) exist far above its intrinsic Curie temperature, leading to a strong enhancement of the exchange coupling between the local moments of the defects. As a result, a significant enhancement of the temperature of onset of long-range order among the local moments may be achieved. We associate this temperature with the global Curie temperature of the precipitate. A phenomenological model is developed to determine the spatial structures and characteristics of ferromagnetic order for the cases of a bulk precipitate and of precipitate particles of various shapes. Moreover, allowing for the presence of strong quenched disorder in the precipitate, we describe short-range ferromagnetic order in the system. Experimental data on Si:Mn alloys are interpreted on the basis of our theoretical results.The work was partially supported by the University of the Basque Country (Proyecto GV UPV/EHU Grant No. IT-366-07), the Spanish Ministerio de Ciencia y Tecnologia (Grant No. FIS2007-66711-C02-01), and the RFBR (Grant No. 10-02-00118). SC also acknowledges financial support by PRIN 2007 under Project No. 2007FW3MJX003. VVT acknowledges financial support by Ikerbasque (Basque Foundation for Science).Peer reviewe

Superexchange coupling in iron/silicon layered structures

We propose the mechanism of interlayer exchange coupling in Fe∕Si structures, based on three principal ideas: (i) Contact induced ferromagnetic phase of body-centered-cubic iron silicide and spin-polarized interfacial states are formed at the Fe∕Si boundaries; (ii) exchange coupling between Fe layers is effectuated by means of the superexchange of spin-polarized interfacial states through the nonmagnetic semiconductor spacer; (iii) the complex character of the dependence of interlayer exchange coupling on the spacer thickness and composition is due to the competition between antiferromagnetic and ferromagnetic components of superexchange. We calculate the bilinear and biquadratic components of an exchange coupling energy in the framework of a simple two-band scheme of electron spectrum inside the spacer at zero temperature. Our model qualitatively explains existing experimental results.The work was partially supported by the UPV/EHU, the Departamento de Educación del Gobierno Vasco, and MCyT -Grant No. FIS 2004-06490-C03-01.Peer reviewe

Spin-fluctuation mediated high-temperature ferromagnetism in Si:Mn dilute magnetic semiconductors

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, MnSi2-z 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 MnSi2-z 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

Quantum anomalous Hall effect in magnetically modulated topological insulator/normal insulator heterostructures

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.This work was supported in part by the St. Petersburg State University (project no. 15.61.202.2015) and by the Russian Foundation of Basic Research (project no. 16-02-00024).Peer reviewe

Carrier mediated ferromagnetism on the surface of a topological insulator

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. © 2011 Pleiades Publishing, Ltd.Peer Reviewe

Anomalous Hall and spin Hall conductivities in three-dimensional ferromagnetic topological insulator/normal insulator heterostructures

In this letter we theoretically demonstrate how an interface perturbation and size effect can be used to manipulate the transport properties of semiconductor heterostructures composed of a thin film of a three-dimensional topological insulator (TI) doped with magnetic impurities and sandwiched between topologically normal insulators. In the framework of a continual scheme, we argue that electron states of the TI film are strongly dominated by its thickness and magnetization as well as by an interface potential whose variation can lead to the modification of topological properties of the heterostructure. This opens diverse possibilities to efficiently tune intrinsic Hall conductivity in the system. We calculate a phase diagram of the heterostructure, which demonstrates a series of quantum transitions between distinct regimes of conductivity. We derive the anomalous Hall conductivity and the spin Hall conductivity dependences on the chemical potential. Applicability conditions of the used approach are also discussed.We acknowledge partial support from Saint Petersburg State University (project No. 15.61.202.2015) and Russian Foundation of Basic Researches (grant No. 16-02-00024).Peer reviewe

Confined electron states in heterostructures based on 3D topological insulators (towards a better understanding of the QSHE)

Resumen del trabajo presentado al New Trends in Topological Insulators (NTTI), celebrado en Donostia-San Sebastián (España) del 6 al10 de julio de 2015.Peer reviewe