Thin films of topological insulator Bi2Se3 were deposited directly on insulating ferromagnetic EuS. Unusual negative magnetoresistance was observed near the zero field below the Curie temperature (TC), resembling the weak localization effect; whereas the usual positive magnetoresistance was recovered above TC. Such negative magnetoresistance was only observed for Bi2Se3 layers thinner than t ∼ 4nm, when its top and bottom surfaces are coupled. These results provide evidence for a proximity effect between a topological insulator and an insulating ferromagnet, laying the foundation for future realization of the half-integer quantized anomalous Hall effect in three-dimensional topological insulators.A topological insulator (TI) has a full energy gap in the bulk, and contains gapless surface states that cannot be destroyed by any non-magnetic impurities. Because of time reversal symmetry, the surface states cannot be back-scattered by non-magnetic impurities. 1,2 When a thin magnetic layer is applied on the surface, a full insulating gap is opened, and an electric charge close to the surface is predicted to induce an image magnetic monopole. 3,4 Probably the most extensively studied threedimensional TI (3D-TI) has been bismuth-selenide (Bi 2 Se 3 ), 5-7 exhibiting crystal structure that consists of atomic quintuple layers (QLs), with three QLs forming a unit cell. As made, uncompensated samples typically have a Fermi level above the Dirac point and intersecting the bulk conduction band. 8,9 In particular, low temperature transport measurements on ungated and uncompensated TI films show positive magnetoresistance (MR) at low magnetic fields and in a wide range of film thicknesses. [10][11][12] This was explained in terms of weak antilocalization (WAL) that results from spin-momentum locking on the surface state Dirac cone. 10,13 While the inability to account for the bulk bands (presumably because of their low mobility) has challenged this simple assignment, the discovery of weak localization (WL) effects at higher fields 12,14 and the ability to accurately separate quantum oscillation effects 15 in high-quality films may provide a first step towards a more comprehensive understanding of transport in these systems.By adopting topologically non-trivial Hamiltonians to describe these materials, various recent theories predict WL or negative MR near the zero field in a TI as a result of gap-opening at its surface state Dirac point. The negative MR may arise from the surface state when the Fermi level is sufficiently close to the top of the gap. 16 Alternatively, it can also be produced by bulk conduction and can only be observed when the surface conduction is sufficiently suppressed. 17,18 Such a phenomenon was reported in cases of gated ultra-thin Bi 2 Te 3 films 19 and magnetically doped Bi 2 Se 3 films. 20 To further elucidate the uniqueness of transport in the surface state of TI materials, and as an initial step towards realizing half-integer quantized anomalous Hall effect (QAHE) and other applications, we studied ...

Strong spin-orbit coupling in topological insulators results in the ubiquitously observed weak antilocalization feature in their magnetoresistance. Here we present magnetoresistance measurements in ultra thin films of the topological insulator Bi2Se3, and show that in the 2D quantum limit, in which the topological insulator bulk becomes quantized, an additional negative magnetoresistance feature appears. Detailed analysis associates this feature with weak localization of the quantized bulk channels, providing thus evidence for this quantization. Examination of the dephasing fields at different temperatures indicates different scattering mechanism in the bulk vs the surface states.A three-dimensions (3D) topological insulator (TI) is fully gapped in the bulk, but exhibits an odd number of surface 2D massless cones of helical Dirac fermions, which are protected by time reversal symmetry (TRS) and thus cannot be destroyed by any non-magnetic impurities [1,2]. The Dirac fermions are helical in the sense that the electron spin points perpendicularly to the momentum, forming a lefthanded helical texture in momentum space. This strong coupling of spin and momentum leads to a range of new phenomena, especially when the TI is brought in contact with either a superconductor or an insulating ferromagnet, giving rise to possible observations of Majorana fermions [3,4], the topological magneto-electric effect [5], and quantum anomalous Hall effect [6].An important consequence of spin-momentum locking is the full suppression of backscattering resulting in a relative π Berry phase acquired by electrons executing time-reversed paths. As a consequence, at low temperatures, when the dephasing length (ℓ φ ) of the surface state electrons is long, this results in destructive interference, which give rise to positive quantum corrections ∆σ > 0, to the Drude conductivity. The result of these quantum corrections to the conductivity are called weak antilocalization (WAL) effects, and are expected to occur in general when spin-orbit (SO) interaction is strong [7][8][9]. When weak magnetic field H H φ ≡ Φ 0 /(8πℓ 2 φ ) (Φ 0 = hc/e is a flux quantum) is applied, these interference effects are reduced giving rise to positive magnetoresistance (MR) which is therefore a hallmark of WAL.While an ideal TI is a true bulk insulator, this property has proven to be very difficult to achieve. Interstitials, vacancies, and antisite doping, are only a few of the common issues that give rise to a substantial bulk carrier density causing the chemical potential to be pinned at the bulk conduction band. Focusing on the Bi 2 Se n Te 3−n system (where n=0,1,2 and 3), partial mitigation of the problem is commonly achieved by compensation (e.g. doping Bi 2 Se 3 with Sb) [10,11], or by controlling the Fermi energy via a gate bias [12][13][14]. However, unless the Fermi energy is tuned to be much below the bottom of the bulk conduction band, transport in such TI systems has been shown to be a complicated combination of surface and bulk states, which in principl...

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High-quality thin films of the ferromagnetic insulator europium(II) sulfide (EuS) were fabricated by pulsed laser deposition on Al 2 O 3 (0001) and Si (100) substrates. A single orientation was obtained with the [100] planes parallel to the substrates, with atomic-scale smoothness indicates a near-ideal surface topography. The films exhibit uniform ferromagnetism below 15.9 K, with a substantial component of the magnetization perpendicular to the plane of the films. Optimization of the growth condition also yielded truly insulating films with immeasurably large resistance. This combination of magnetic and electric properties open the gate for novel devices that require a true ferromagnetic insulator.Over more than 50 years a wealth of new effects and properties have been discovered in binary lanthanide compounds. In particular, compounds of europium with elements of the sixth group (O,S,Se,Te) exhibit a rocksalt (NaCl)-type crystal structure with ordered magnetic states at low temperatures. As the lattice parameter increases from EuO to EuTe, a ferromagnetic ordered state of moments localized on Eu ions appear in EuO (T C ≈ 69 K) and in EuS (T C ≈ 16.7 K), 1,2 while EuSe and EuTe show collinear antiferromagnetic ordering with T N ≈ 4.2 K, T N ≈ 9.8 K respectively. 3,4 In these chalcogenide compounds, the S ground state of Eu 2+ ions and their simple face centered cubic (FCC) magnetic lattice facilitate testings of the Heisenberg model of ferromagnetism and theories of critical phenomena. 5-9 A variety of applications were proposed or implemented utilizing these magnetic semiconductors. 10-12 A class of magnetoelectric applications, such as π-Josephson junctions for quantum qubits 13-15 and recently proposed topological magnetoelectric effect associated with the surface state of topological insulators, 16-20 require fabrication of highquality insulating ferromagnet thin films with robust magnetic properties.Here we focus on EuS, which is a semiconductor with an indirect energy gap between the 4f 7 Eu states and the conduction band minimum at 300 K is 1.65 eV. [21][22][23] The lattice parameter of bulk crystals of EuS is a 0 = 5.967Å, with a ferromagnetic Curie temperature T C ≈ 16.7 K. When strained, the lattice constant change is accompanied by a change in Curie temperature, e.g. thin films of EuS grown on KCl show an increase in T C by as much as 2 K, due to compression induced by differential expana) Electronic mail: qiyang@stanford.edu sions of the film and substrate. 24 At the same time, very thin films will exhibit slightly lower T C due to dimensionality reduction. 25 However, although good electric insulation (ρ ∼ 10 4 Ω·cm) was obtained in high-quality single crystals, difficulties in material fabrication lead to disorder and unintentional doping, which may drastically reduce the resistivity to as low as ρ ∼ 10 −2 Ω·cm. 2,26 Such reduction in resistivity was found to be accompanied by increased Curie temperatures due to interactions between charge carriers and the Eu 2+ ions. 1,26-28 Particularly for t...