Session L31: Focus Session: Topological Insulators: Synthesis and Transport - Magnetism

Thin films of magnetically doped topological insulators

The interplay between the Dirac surface state and ferromagnetic order in topological insulators can lead to a number of very exotic quantum phenomena. To observe the quantum phenomena such as quantized anomalous Hall (QAH) effect in magnetically doped topological insulators, the materials are required to be in the form of thin film with tunable chemical potential and carrier-independent ferromagnetism. In this talk, I will report our recent progress in molecular beam epitaxy growth, chemical potential tuning and electronic properties of the magnetically doped topological insulator thin films. By Cr doping, we have realized both n-type and p-type conductivity in (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ thin films. Remarkably their ferromagnetism was found independent of the type and concentration of carriers. Moreover, the anomalous Hall effect is significantly enhanced at low carrier concentration regime, with the anomalous Hall angle reaching an unusually large value of 0.2 and the zero field Hall resistance reaching one quarter of the quantum resistance (h/e2). These findings pave the way to ultimately observing the QAH effect and other quantum effects in magnetic topological insulators.   

Proximity induced ferromagnetism at the interface between a topological insulator (TI) Bi$_{2}$Se$_{3}$ and a ferromagnetic insulator (FI)

The ferromagnetic phase of the surface states of a TI is predicted to carry many exotic properties, for example quantum anomalous Hall effect, magnetic monopole, and magneto-electric effects etc. In our study, we explore this novel phase utilizing the proximity induced exchange splitting to introduce ferromagnetism close to the surface of the Bi$_{2}$Se$_{3}$ film. High quality Bi$_{2}$Se$_{3}$ thin films were grown using molecular-beam-epitaxy, and in-situ deposited the ferromagnetic insulator (FI) EuS over this film. Magnetization measurements demonstrated a magnetic moment of more than 7$\mu _{B}$ per Eu$^{2+}$ ion (bulk value), and reaching up to 11$\mu _{B}$ per Eu$^{2+}$ ion for 1nm thick EuS film, showing the unambiguous existence of excess ferromagnetism. The transport studies of these TI/FI bilayers unveiled a clear switching behavior of the magnetoresistance in the Bi$_{2}$Se$_{3}$ film. There was significant temperature dependence seen in both MR and the coercivity. Due to the near range nature of the exchange interactions, these extra magnetic moments and the MR results are attributed to come from the induced ferromagnetism at the Bi$_{2}$Se$_{3}$ surface.   

Possible transport evidence for a surface state gap in a magnetically doped topological insulator

We report magnetoresistance measurements in thin films of a magnetically doped topological insulator Bi$_{2-x}$Mn$_x$Se$_3$ synthesized by molecular beam epitaxy. We observe a crossover from positive magnetoresistance to negative magnetoresistance at low temperature ($T \la 15$ K), accompanied by onset of ferromagnetic signatures (hysteresis and anisotropic magnetoresistance). The observations are consistent with the prediction of a transition of diffusive quantum transport from the symplectic to the unitary class due to a magnetically induced surface state gap. This interpretation is supported by the observation of strongly suppressed surface states at the Dirac point in angle-resolved photoemission spectroscopy. We use the magneto-optical Kerr effect, anomalous Hall effect, SQUID magnetometry, electron microscopy and scanning tunneling microscopy to clarify the source of the ferromagnetism in these samples. Supported by DARPA, ONR and NSF-MRSEC.   

Magnetoconductance crossover and non-linear Hall effect in MBE-grown Cr-doped Bi$_{2}$Te$_{2}$Se

Recent predictions have shown that the magnetoconductance (MC) of a topological insulator should show a crossover from negative MC to positive MC when time-reversal symmetry is broken [1]. Observations of this crossover have been reported in 3 QL thick Cr-Bi$_{2}$Se$_{3 }$[2] in the quasi-2D regime where surface hybridization is not negligible. We report magneto-transport results on 15 QL thick Cr-doped and undoped Bi$_{2}$Te$_{2}$Se films grown on Si (111) substrates by MBE. The undoped film exhibits weak anti-localization at low temperatures. In the Cr-doped film, a crossover from negative to positive MC is observed versus temperature at T=12 K. SQUID measurements show that the sample is ferromagnetic in this temperature range. In addition, a non-linear Hall voltage is observed despite the large bulk carrier concentration. This paves the way for further experiments on (Bi,Cr)$_{2}$Te$_{2}$Se in our search for the quantum anomalous Hall effect. [1] H.Z. Lu et al. Phys. Rev. Lett. \textbf{107}, 076801(2011). [2] M.Liu et al. arXiv 1103.3353(2011).   

Ferromagnetism in chromium doped topological insulator thin films and nanoplate crystals

The surface states of topological insulators are protected by time-reversal symmetry. Introducing magnetic impurities should break this symmetry and open a gap in the otherwise gapless surface states. Recent first-principle calculations predict that when topological insulators are doped with transition metal elements, such as Cr or Fe, a \emph{magnetically ordered} insulating state will form -- a state that in thin (quasi-2D) samples may support a quantized Hall conductance. Here we report on electrical and magnetic characterization of thin Cr doped topological insulators: Sb$_2$Te$_3$ nanoplate crystals and $\sim 50$ nm thin films of Bi$_2$Te$_3$. Electrical contacts to samples were lithographically defined, with \textit{rf} sputtered films grown on pre-patterned substrates. Low-temperature in-plane resistivity, Hall, and magnetization measurements were performed in up to 5 T magnetic fields. For 5 at\% Cr content, a distinct ferromagnetic hysteretic response is observed at temperatures below 10 K. Hysteretic loops, also observed in Hall resistivity, indicate low-$T$ coercive fields of the order of 0.5 T. Correlation of transport and magnetic measurements indicating anomalous Hall effect, and strong dependence on dopant concentration and sample thickness will be presented.   

Ferromagnetism in vanadium doped thin films of a topological insulator Bi$_2$Te$_3$

Recent first-principle calculations predict a new class of ferromagnetic systems that are distinctly different from the conventional dilute magnetic semiconductors. A novel ferromagnetic topological insulator (ferro-TI) state can be obtained when topological insulator are doped with certain transition metal elements. In the the quasi-2D limit these ferro-TIs are expected to support a quantized anomalous Hall effect. Here we report on electrical and magnetic characterization of vanadium doped thin ($\sim 50$ nm) films of a topological insulator Bi$_2$Te$_3$. Films were grown by \textit{rf} sputtering on S$_3$N$_4$/Si substrates with lithographically pre-patterned contact pads. Low-temperature in-plane and Hall resistivity measurements were performed in magnetic fields up to 5 T fields. We find that below 100 K, V-doped films display \textit{negative linear magnetoresistance}, which at lower temperatures becomes hysteretic. Hall resistivity is also hysteretic, suggesting an unusual ferromagnetic ordering below 10 K. Moreover, V-doping turns the \textit{p}-type conduction in as-grown films into \textit{n}-type. The doping and thickness dependence of these effects will be discussed.   

Controllable ferromagnetism of iron doped topological insulator

The higher than room temperature ferromagnetism was found in iron doped Bi2Se3. Samples generated by different processes have different magnetic characters. The Curie temperature is independent on iron concentration which against all discovered dilute magnetic systems. EXAFS observations show that the local structure of iron in samples with paramagnetic character is complex. On the contrary, that with ferromagnetic character is very simple that the iron atoms make up small single atom, dimer or trimer structures and these structures randomly distributed in Bi2Se3 crystal. The ferromagnetism can be enhanced or suppressed by the shift of Fermi edge by co-doping of Mg and Fe to Bi2Se3 crystal. The less than 3 atoms small structure cannot have room temperature ferromagnetism, so we believe that the higher than room temperature controllable ferromagnetism is intrinsic character of iron doped topological insulator.   

In-plane anisotropy of Fe atoms on Bi$_2$Se$_3$(111)

Topological insulators exhibit a linearly dispersing gapless topological surface state where both the spin and momentum degrees of freedom are locked. The topological nature of this state results in interesting effects such as suppression of back-scattering. Recently, the robustness of these surface states against magnetic order has come under heavy investigation. Here, we explore the magnetic properties of single Fe adatoms on the Bi2Se3 surface, in the coverage range $< 1$\% , with combined non-local x-ray magnetic circular dichroism techniques and local low temperature scanning tunneling spectroscopy. We reveal that the adatoms heavily relax into the surface and exhibit a magnetic easy axis within the surface-plane, contrary to recent reports. Furthermore, we demonstrate, using $ab-initio$ approaches, how the easy axis can reorient from out-of-plane to in-plane when considering the interplay of Coulomb interactions, spin-orbit coupling, and dynamic hybridization effects.   

Topological insulators with magnetic impurities in the bulk

We show that a three dimensional topological insulator with magnetic impurities could have a regime where the surface is magnetically ordered but the bulk is not. This is in contrast to conventional materials where bulk ordered phases are typically more robust than surface ordered phases. This difference originates from the topologically protected gapless surface states characteristic of topological insulators. We study the problem using a mean field approach, using two concrete models that give the same qualitative result, with some interesting differences. This work could help explain recent experimental results showing the emergence of a spectral gap in the surface state of Bi$_2$Se$_3$ doped with Mn or Fe atoms, but with no measurable bulk magnetism. \\[4pt] [1] Massive Dirac Fermion on the Surface of a Magnetically Doped Topological Insulator, Y. L.~Chen {\em et al.}, Science { \bf 329}, 659-662 (2010).   

Interface-driven magnetism in topological insulators Bi2Te3 and Bi2Se3

Topological insulators (TIs) draw great attention due to their unique quantum properties and applications. TIs possess metallic surface states within band gaps induced by spin-orbit coupling (SOC), and they allow only an odd number of Dirac cones in dispersion of the surface states at a given surface. Novel physical phenomena and applications proposed including TIs, critically rely on stability and topological nature of the surface states when TIs are interfaced with other types of materials. Despite many studies on TIs, it still remains unclear how the surface states of TIs behave in contact with other materials. In this talk, we present our study of an effect of interfaces on the surface states of TIs Bi2Te3 and Bi2Se3, using density-functional theory including SOC self-consistently. We simulate interfaces using adsorption layers on TI films in an asymmetric fashion. We discuss unexpected results on stability and topological nature of the surface states, as well as changes in their spin structure and energy gaps at zero momentum. Our findings reveal importance of interfaces and a possibility of engineering new hybrid TI structures using adsorption layers.   

Magnetic structure of magnetic semiconductor NdBiPt studied by elastic neutron diffraction

We report a study of the magnetic structure of the antiferromagnetic (AFM) half-heusler NdBiPt via neutron scattering. NdBiPt exibits an AFM transition at $T_{\mathrm N} = 2.2 \mathrm{K}$ with an ordered moment of 3.6$\mu_{\mathrm B}$ per Nd atom, as determined from magnetization measurement. The insulating state of NdBiPt has been proposed to be a new class of a three dimensional topological insulator, which possesses surfaces states at step edges on facets, with properties that are analogous to those observed in quantum Hall states. However, this would require an orientation of the magnetic moments in the ordered state along the crystallographic direction $[001]$. Triple-axis neutron axis diffraction on a single crystal was used to study the magnetic order. We found that in the ordered state consists of a conventional two sublattice antiferromagnet with the spins oriented along $[001]$ with very little fluctuations present close to T$_{\mathrm N}$.   

Iron impurities on Sb (111) surface and their effects on topological surface state

We study iron impurities on Sb (111) surface and their effects on topological surface state by using an \textit{ab-initio }pseudopotential density-functional method. We implemented the spin-orbit interaction into the SIESTA in a form of additional fully non-local projectors. To calculate electronic structure of topological surface states, we consider a slab of Sb using a supercell containing 20 atomic layers with experimental bulk Sb lattice parameters. We determine atomic positions of Fe impurities on Sb (111) surface by minimizing the total energy, and calculate surface band structures near the Fermi level. To find effects of the impurity on the surface states of Sb (111) surface, we simulate ARPES spectra as a function of impurity density on the surface. From the results, we find that Fe impurity states are present near Fermi level and they strongly interact with the surface states. This work was supported by the NRF of Korea (Grant Nos. 2009-0081204 and 2011-0018306) and KISTI Supercomputing Center (Project No. KSC-2011-C2-04).   

Impurities in Bi2Se3 topological insulator: ab initio calculation

Topological insulators are materials that have a bulk as an ordinary insulator but have protected conducting states on their surface. The surface states with an odd number of Dirac cones are robust against time-reversal invariant perturbation. However the interaction of magnetic impurities with the Dirac fermions can break time reversal symmetry open unp a surface band gap. In this work we investigate the magnetic anisotropy and spin-texture of Co impurities embedded at the interlayer vdW spacings and onto the topmost Se network of the topological insulator Bi$_{2}$Se$_{3}$. The interaction of the magnetic impurity with the surface spin texture break time reversal symmetry, opening up a surface band gap. For Co atom adsorbed onto the surface, the net magnetic moment is aligned perpendicular to the surface plane, with anisotropy energy of 6$\sim $meV. On the other hand for the Co impurity at the vdW interlayers, the net magnetic moment is aligned in-plane. While pristine Bi$_{2}$Se$_{3}$ presents helical spin-texture in the massless surface Dirac cone and states resonant within the valence band, the presence of Co impurity reduces the planar spin helicity of now massive Dirac fermions. On the other hand O impurities do not break the protected surface Dirac cones, but they move the position of the Dirac crossing upwards. Also we observe that there is an energy barrier for the O$_{2}$ molecule when it approximates to the Bi$_{2}$Se$_{3}$ surface, but atomic O inside the Bi$_{2}$Se$_{3}$ is more stable than the formation of O$_{2}$ molecules.