Session A13: Focus Session: Topological Materials - Magnetic Topological Insulators

Ab initio study of topological surface states of Sb (111) surface with magnetic impurities

We study effects of magnetic impurities on topological surface state of Sb (111) surface by using an ab-initio pseudopotential density-functional method. We have implemented the spin-orbit interaction into the SIESTA code in a form of additional fully non-local projectors. To calculate surface band structures, we use a slab of Sb using a 4x4 supercell containing 20 atomic layers. In particular, we compare Fe impurities with Mn impurities, whose atoms have larger magnetic moments, and compare interstitial impurities with substitutional impurities for each atom. To understand the impurity effects on the topological surface states, we simulate ARPES spectra and calculate projected density of states of impurity near Fermi level. This work was supported by NRF of KOREA (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2012-C2-14).   

Manipulating Surface-induced Ferromagnetism in Modulation-doped Topological Insulators

The manipulation of topological surface states is a key to realize applicable devices of topological insulators. In addition to the direct engineering of time-reversal-symmetry protected surface states, recent work suggests that various physical responses can be obtained from surface helical states by integrating additional ferromagnetism or superconductivity to the original topological order. Here, we report the coexistence and tunability of bulk carrier density-independent and surface-mediated electrically controllable ferromagnetisms in modulation-doped Crx(BiySb1-y)2Te3 epitaxial thin films. We demonstrate for the first time a dramatic enhancement of surface-induced magnetization on TI / Cr-TI bilayer devices. The surface magneto-electric effects can be either enhanced significantly or completely switched-off, by tuning the separation of the surface from the magnetic impurities. The electric-field-modulated ferromagnetism in our modulation-doped TI hetero-structures is fundamentally important for the realization of the quantum anomalous Hall Effect as well as the axion electromagnetic dynamics, and thus provides a new approach for spintronics applications.   

Effect of Magnetic Doping on Electrical and Thermal Conductivities and Seebeck Coefficient of Suspended Bismuth Telluride Nanoplates

Bismuth telluride has been investigated intensively as a model system for topological insulators. In this work, we have studied electrical and thermal transport properties of suspended bismuth telluride nanoplates grown by the vapor-solid method. The thin crystals were transferred onto micro-fabricated suspended structures with built-in electrodes and thermometers, which allowed us to measure electrical ($\sigma )$ and thermal ($\kappa )$ conductivities as well as the Seebeck coefficient ($S)$. The through-etched hole in the devices enabled us to evaporate Cr layers on both surfaces of the crystal. After H$_{2}$ annealing at 500 K, we measured enhanced $\sigma $, $\kappa $, and $S$ values by 40, 10, and 20{\%}, respectively. In comparison, H$_{2}$ annealing without Cr evaporation resulted in 10, 10, and -8{\%} changes of $\sigma $, $\kappa $, and $S$ values, respectively. The effect of magnetic doping by Cr will be discussed. Additionally, magneto-transport measurements were performed on the samples to resolve the transport properties of the surface states. We observed a pronounced weak anti-localization feature in undoped samples. Changes in this feature after Cr doping will be presented.   

Electrical control of the ferromagnetism in Sb$_{\mathrm{2-x}}$Cr$_{\mathrm{x}}$Te$_3$ magnetic topological insulators

The spin helical Dirac fermions living on the surface of three-dimensional topological insulators (TIs) provide a platform for exploring the coupling between the charge and spin degrees of freedom. In particular, breaking the time reversal symmetry in TIs is expected to create exotic topological magnetoelectric effects. To realize these phenomena and apply them in TI-based spintronic devices, it is desirable to achieve in situ manipulation of the magnetism in TIs via an electrical field. In this talk we present the fabrication and transport studies of Cr doped Sb$_2$Te$_3$ magnetic TI thin films. By applying a gate voltage in a field effect transistor device, we can control the coercive force and Curie temperature. The ferromagnetic order is found to be enhanced when more hole-type carriers are injected into the sample. This trend suggests the itinerant bulk holes in TIs can mediate ferromagnetic ordering of local moments in a similar manner as that in the diluted magnetic semiconductors. The electrical control of the ferromagnetism in TIs demonstrated here paves the road for realizing the TI-based devices.   

Ferromagnetism in Mn-doped Bi$_2$Te$_3$ Thin Films by Molecular Beam Epitaxy

We demonstrate the ferromagnetic properties of Mn-doped thin films of the topological insulator Bi$_2$Te$_3$ grown by molecular beam epitaxy. Films with Mn concentrations up to 10\% and thickness up to 60 nm were studied. The electrical transport measurements reveal a strong anomalous Hall effect (AHE) with a coercive field of 3000 Oe at 500 mK. The onset (10 - 16 K) of the AHE is at about the same temperature with the Tc obtained by the superconducting quantum interference device (SQUID) measurements. The magneto-conductivity shows hysteresis and a crossover from weak antilocalization to weak localization when going below Tc. The carrier type and the carrier concentration are modified by varying the Mn doping and the film thickness. Most of films are n-type, but some films thicker than 50 nm at a certain Mn concentration are p-type. Shifts in x-ray diffraction indicate that the n-type films have Mn atoms between quintuple layers, but the p-type films are substitutional. Funded by ONR and DARPA.   

Metallic states in Topological Insulators with Magnetic Impurities

Topological insulators are characterized by an insulating bulk, and an odd number of Dirac cones in the surface. Their existence are due a band inversion in the bulk phase created by a strong spin orbit coupling. Those metallic states have their spin polarization locked in a plane giving rise to a chiral spin texture, similar to the quantum spin hall effect. Such spin helicity suppresses backscattering processes. Based on first principles calculations, we performed a systematic study of transition metal (TM) impurities (Co, Mn, Ni, Cr and Fe) lying on the topmost layers of the $Bi_2Se_3$ topological insulator. Based upon formation energy results, by considering a number of plausible configurations, we find an energetic preference for the TMs occupying the topmost Bi substitutional site, and the subsurface interstitial sites neighboring Bi atoms. Our simulated scanning tunneling images (STM) show that there is local perturbation on the electronic structure of the surface. Further electronic band structure calculations indicate that (for some systems) the topologically protected surface metallic bands are suppressed, opening a band gap. In those systems the time reversal symmetry has been broken due to the formation net magnetic moment aligned perpendicularly to the surface plane.   

Electrical transport studies in the topological insulator Bi$_{2}$Se$_{3}$ with exchange induced ferromagnetism

The proximity-induced ferromagnetic order in topological insulator (TI)/ferromagnetic insulator (FI) heterostructures induces ferromagnetism in TI, which breaks local time reversal symmetry that can lead to many exotic properties, such as image magnetic monopole, topological magneto-electric effects, etc.[1] We achieved this novel ferromagnetic order in a TI Bi$_{2}$Se$_{3}$ through Bi$_{2}$Se$_{3}$/EuS bi-layer structures. Electric transport studies show a dramatic suppression of the weak anti-localization (WAL) effect in Bi$_{2}$Se$_{3}$/EuS compared to controlled Bi$_{2}$Se$_{3}$ samples. In contrast to the case of surface doping a TI with magnetic atoms (i.e. Fe), here the WAL cannot be quenched even with a full coverage EuS capping layer, which points that its origin can be the opening of a surface gap rather than a reduction of the magnetic scattering length. The results are analyzed with a theoretical model providing a value for the induced surface exchange gap. Other experimental results, such as the anomalous Hall effect that support the proximity induced ferromagnetism in Bi$_{2}$Se$_{3}$ will be discussed.\\[4pt] [1] Qi, X.-L. {\&} Zhang, S.-C., Rev Mod Phys 83, 1057-1110, (2011).   

Magnetotransport in topological insulator-ferromagnetic insulator heterostructure devices

Topological surface states modified by the presence of magnetism are predicted to play host to a number of exotic phenomena and are of great fundamental as well as applied interest. Interfacing topological insulators with magnetic insulators offers a unique opportunity to access these effects by transport, without affecting the bulk band structure. We demonstrate the integration of MBE grown thin films of Bi$_2$Se$_3$ with the insulating ferromagnet GdN. SQUID measurements of the hetero-structure reveal an in-plane easy axis with a ferromagnetic Curie temperature $T_c$ $\sim$ 13 K. The fabrication of hall devices with bare and GdN-capped channels enables direct comparison of magneto-transport properties. While the bare channel displays conventional weak anti-localization (WAL), the capped channel reveals a weakened WAL and a superimposed negative magnetoresistance (MR) associated with weak localization. These observations are discussed in the context of gap-opening in the Dirac surface state. Finally, we discuss the observation of hysteresis in the MR of the capped channel below 2 K.   

Dirac Surface State of Metamagnetic Topological Insulators

We report the observation of metamagnetism in iron-doped bismuth selenide topological insulators. The structural, magnetic, and transport properties of the materials were investigated both computationally and experimentally. First-principles density functional calculations are employed to determine the most favorable site location of the iron atoms in the bismuth selenide lattice and to analyze the magnetic properties of the resulting structures. Magnetization measurements showed the system is anisotropic with a magnetic phase transition at $\sim$ 100 K. However, this magnetic-doped topological insulator did not show an opening of a surface gap in ARPES data at temperatures below the transition temperature. This is due to the antiferromagnetic ground state of the system. With an applied magnetic field greater than 300 Oe, the system becomes ferromagnetic. In addition, Shubnikov-de Haas oscillations were observed in the longitudinal resistivity measurements under the applied magnetic fields up to 9 T.   

Experimental Realizations of Magnetic Topological Insulator and Topological Crystalline Insulator

Over the past few years the experimental research on three-dimensional topological insulators have emerged as one of the most rapidly developing fields in condensed matter physics. In this talk, we report on two new developments in the field: The first part is on the dynamic interplay between ferromagnetism and the Z$_2$ topological insulator state (leading to a magnetic topological insulator). We present our spin-resolved photoemission and magnetic dichroic experiments on MBE grown films where a hedgehog-like spin texture is revealed on the magnetically ordered surface of Mn-Bi$_2$Se$_3$ revealing a Berry's phase gradient in energy-momentum space of the crystal. A chemically/electrically tunable Berry's phase switch is further demonstrated via the tuning of the spin groundstate in Mn-Bi$_2$Se$_3$ revealed in our data (Nature Physics 8, 616 (2012)). The second part of this talk describes our experimental observation of a new topological phase of matter, namely a topological crystalline insulator where space group symmetries replace the role of time-reversal symmetry in an otherwise Z$_2$ topological insulator predicted in theory. We experimentally investigate the possibility of a mirror symmetry protected topological phase transition in the Pb$_{1-x}$Sn$_x$Te alloy system, which has long been known to contain an even number of band inversions based on band theory. Our experimental results show that at a composition below the theoretically predicted band inversion, the system is fully gapped, whereas in the band-inverted regime, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order (Nature Communications 3, 1192 (2012)) distinct from that observed in Z$_2$ topological insulators. We discuss future experimental possibilities opened up by these new developments in topological insulators research. This work is in collaboration with M. Neupane, C. Liu, N. Alidoust, I. Belopolski, D. Qian, D.M. Zhang, A. Richardella, A. Marcinkova, Q. Gibson, R. Sankar, Y.J. Wang, T. Chang, H. Jeng, H. Lin, L.A. Wray, J.D. Denlinger, M. Leandersson, T. Balasubramanian, J. S\'anchez-Barriga, O. Rader, G. Landolt, B. Slomski, J.H. Dil, F.C. Chou, E. Morosan, N. Samarth, R.J. Cava and M.Z. Hasan.   

Efficient Generation of Spin Current and Spin Transfer Torque by the Topological Insulator Bismuth Selenide

We study the use of topological insulators as a source of spin current for applying spin transfer torque to a ferromagnet. We fabricate bismuth selenide / permalloy bilayers and use the spin-torque FMR technique to make quantitative measurements of the torque applied to the magnetic permalloy layer resulting from an in-plane current. Despite the fact that only a small fraction of the current flows in the bismuth selenide, we still observe large spin torque effects. There is a component of torque in the sample plane with the symmetry expected from the spin Hall effect, with a strength corresponding to a spin Hall angle greater than 1, larger than measured for any other material. There is also an additional out-of-plane, field-like torque several times larger than expected from the Oersted field. We will discuss the dependence of these effects on layer thickness, and attempt to distinguish whether they result from bulk or surface-state effects.   

Surface state driven spin-torque in topological-insulator / ferromagnetic-metal bilayers

A hallmark of surface states in strong 3D topological insulators (TI) is the helical spin texture. While there have been proposals on exploiting this spin texture for spintronics applications, they focused on TI/ferromagnetic-insulator (FI) structures predicting field-like torque due to spin accumulation. Motivated by recent spin-torque experiments on Pt/ferromagnetic-metal(FM) structures, we consider a TI/FM bilayer, where the magnetic moment as well as the current driven through the system are in plane. While existing TIs have a conducting bulk, recent transport experiments showed that the main contribution to the current in Bi$_2$Se$_3$ thin films comes from two distinct surface states: the topological Dirac surface state and an additional 2D electron gas with Rashba spin-orbit coupling. We thus consider spin torque in the TI/FM structure due to these two surface states. We find that each surface state leads to out-of-plane (field-like) torque due to current driven spin accumulation. Moreover, we find an in-plane torque due to spin diffusion into the FM, an effect absent in TI/FI structures. Interestingly, the two surface states contribute with opposite sign to the spin density. This allows for the experimental identification of the dominant state based on its sign.   

Topological Magnetic Heterostructures of Epitaxial Bi$_{2}$Se$_{3}$ on Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As

Topological Insulators (TI) are characterized by conducting surface states with a Dirac-like dispersion protected by time reversal symmetry. A magnetic perturbation that breaks this symmetry, such as placing a ferromagnet in proximity with a TI, can lead to a wide range of unusual effects such as a half integer quantum Hall conductance, magnetic monopoles, or an inverse spin-galvanic effect, among others. Such structures are challenging to create however due to the difficulty in finding insulating magnetic materials that are compatible with topological materials. We demonstrate one approach to this, the epitaxial growth of Bi$_{2}$Se$_{3}$ on the ferromagnetic semiconductor Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As. We discuss the growth and characterization of these heterostructures, where the Mn concentration of the GaMnAs can be tuned from a highly resistive state near the metal-insulator transition, up to a highly doped semiconductor with a T$_{\mathrm{C}}$ well in excess of 100 K. This allows the study of a wide range of regimes and interactions between the two layers. As GaMnAs is a prototypical material for the demonstration of many spintronic devices, and has a highly tunable anisotropy, this opens up the possibility of an exciting range of hybrid spintronic/Topological Insulator structures. Funded by ONR and DARPA.