The breaking of time-reversal symmetry by ferromagnetism is predicted to yield profound changes to the electronic surface states of a topological insulator. Here, we report on a concerted set of structural, magnetic, electrical and spectroscopic measurements of Mn-Bi2Se3 thin films wherein photoemission and x-ray magnetic circular dichroism studies have recently shown surface ferromagnetism in the temperature range 15 K ≤ T ≤ 100 K, accompanied by a suppressed density of surface states at the Dirac point. Secondary ion mass spectroscopy and scanning tunneling microscopy reveal an inhomogeneous distribution of Mn atoms, with a tendency to segregate towards the sample surface. Magnetometry and anisotropic magnetoresistance measurements are insensitive to the high temperature ferromagnetism seen in surface studies, revealing instead a low temperature ferromagnetic phase at T 5 K. The absence of both a magneto-optical Kerr effect and anomalous Hall effect suggests that this low temperature ferromagnetism is unlikely to be a homogeneous bulk phase but likely originates in nanoscale near-surface regions of the bulk where magnetic atoms segregate during sample growth. Although the samples are not ideal, with both bulk and surface contributions to electron transport, we measure a magnetoconductance whose behavior is qualitatively consistent with predictions that the opening of a gap in the Dirac spectrum drives quantum corrections to the conductance in topological insulators from the symplectic to the orthogonal class.

Circularly polarized photons are known to generate a directional helicity-dependent photocurrent in three-dimensional topological insulators at room temperature. Surprisingly, the phenomenon is readily observed at photon energies that excite electrons to states far above the spin-momentum locked Dirac cone and the underlying mechanism for the helicity-dependent photocurrent is still not understood. Here we show a comprehensive study of the helicity-dependent photocurrent in (Bi1−xSbx)2Te3 thin films as a function of the incidence angle of the optical excitation, its wavelength and the gate-tuned chemical potential. Our observations allow us to unambiguously identify the circular photo-galvanic effect as the dominant mechanism for the helicity-dependent photocurrent. Additionally, we use an analytical model to relate the directional nature of the photocurrent to asymmetric optical transitions between the topological surface states and bulk bands. The insights we obtain are important for engineering opto-spintronic devices that rely on optical steering of spin and charge currents.

We report the integration of the insulating ferromagnet GdN with epitaxial films of the topological insulator Bi2Se3 and present detailed structural, magnetic and transport characterization of the heterostructures. Fabrication of multi-channel Hall bars with bare and GdN-capped sections enable direct comparison of magnetotransport properties. We show that the presence of the magnetic overlayer results in suppression of weak anti-localization at the top surface.Combining three dimensional (3D) topological insulators (TIs) with magnetism is of great current interest because of the unique effects predicted when the time reversal protected topological surface states are modified by symmetry breaking magnetic perturbations [1,2]. Experiments aimed at observing such phenomena have primarily focused on magnetically doped TIs [3][4][5][6][7]. Complementary to the direct magnetic doping of TIs, several theoretical schemes have proposed explorations of a different sample geometry wherein a patterned ferromagnet (FM) is interfaced with a TI [8][9][10][11]. When the magnetic easy axis of the FM overlayer is out-of-plane, a gap opens in the surface states of the vicinal TI and this massive Dirac Hamiltonian should lead to chiral 1D edge states along FM domain walls where the mass changes sign. If the easy axis of the FM lies in-plane, no gap is expected to first order but the moment can be levered outof-plane by an external field, thereby opening or closing a gap. Calculations[12] also show that hexagonal warping effects can lead to a gap even with an in-plane FM. Unlike magnetically doped films [3], such a geometry leaves the bulk band structure unaffected. However, electrical transport experiments require an insulating ferromagnet to ensure current paths that flow solely through the TI. Recent advances in this context have exploited the synthesis of EuS/Bi 2 Se 3 heterostructures [13,14]. In this Letter, we demonstrate an alternative scheme towards such "magnetic gating" proposals by creating hybrid electrical transport devices wherein we interface the insulating FM GdN with a TI (Bi 2 Se 3 ).GdN is an insulating FM that has elicited interest for low temperature spintronic devices, particularly since it can be deposited by reactive sputtering at ambient temperature [15,16]. This is of prime importance in studies of magnetic exchange coupling effects, as it minimizes thermal diffusion of magnetic species. Changing the nitrogen composition allows access to metallic [17] Bi 2 Se 3 thin films were grown by molecular beam epitaxy (MBE) on InP (111)A substrates under typical growth conditions described elsewhere [19]. Following a brief exposure to ambient atmosphere, the films were transferred to a Kurt Lesker CMS-18 system. After an in-situ Ar + surface clean, GdN was deposited by reactive rf sputtering of Gd in an Ar:N 2 environment at ambient temperature. The GdN films were deposited at a rate of ∼ 0.1−0.2Å/sec, with a sputtering power of 4.93 W/cm 2 in a 15% N 2 :Ar gas environment at 5 mTorr pressure with a source to substr...