An exchange gap in the Dirac surface states of a topological insulator (TI) is necessary for observing the predicted unique features such as the topological magnetoelectric effect as well as to confine Majorana fermions. We experimentally demonstrate proximity-induced ferromagnetism in a TI, combining a ferromagnetic insulator EuS layer with Bi 2 Se 3 , without introducing defects. By magnetic and magnetotransport studies, including anomalous Hall effect and magnetoresistance measurements, we show the emergence of a ferromagnetic phase in TI, a step forward in unveiling their exotic properties.(Dated: December 16, 2012) *e-mail: pwei@mit.edu; moodera@MIT.EDU Three-dimensional topological insulators (TIs) are materials carrying surface states protected by time reversal symmetry.1,2 The short-range nature of magnetic proximity coupling with a ferromagnetic insulator (FI) allows the TI surface states to experience the ferromagnetic interactions, where the symmetry breaking happens right at the interface, [3][4][5][6] rather than affecting the majority bulk states or introducing defects. The well behaved Heisenberg FI such as EuS, is an excellent candidate to isolate the magnetic response of the surface states from the parallel conduction of the TI bulk material. Furthermore, the local time-reversal symmetry breaking is essential for inducing a quantized topological magnetoelectric response. 7 This may be used to investigate interesting emergent phenomena, such as the zero-field half-integer quantum Hall effect, 7 the topological magnetoelectric effect, 7,8 and the magnetic monopole, 9 to name a few.Experimentally the most common method of introducing ferromagnetic order in TI is by doping with specific elements; in this case, it is hard to separate the surface and the bulk phases.10-14 Although a surface ferromagnetic order is shown achievable by uniformly depositing magnetic atoms, i.e. Fe, over the TI surface, 15,16 the transport properties of a TI can be influenced by the metallic ferromagnetic overlayer/atoms. Besides, the doping of magnetic elements inevitably introduces crystal defects, magnetic scattering centers, as well as impurity states in the insulating gap, which are detrimental to mobility and the transport of spin-momentum locked surface electrons in TIs.1 From the point of view of confining Majorana fermions in topological superconductors, 17,18 the exchange field of an FI is capable of lifting the spin degeneracy without destroying the superconductivity pairing. 5,6 This is in contrast to the adverse effects resulting from the introduction of magnetic impurities. In combination with an FI, the Majorana bound states can be well established on the top surface of a superconducting TI 19 or superconducting proximity-coupled TI. 20Here, we introduce ferromagnetic order onto the surface of Bi 2 Se 3 thin films by using the FI EuS (Fig. 1a) forming Bi 2 Se 3 /EuS heterostructures. Ultra-thin FI EuS layers are stable with good growth characteristics and clean interface on a variety of materials; they form ...

Magnetic exchange field in magnetic multilayers can potentially reach tens or even hundreds of Tesla. 6 The single-atomic-layer (2D) materials, such as graphene, mono-layer WS 2 etc., is expected to experience the strongest MEF in heterostructures with magnetic insulators due to the short-range nature of magnetic exchange coupling. 4 2D material/magnetic insulator heterostructures enable local spin modulation by magnetic gates, 4,5,7 and the realization of efficient spin generation for spintronic applications. 8,9 As a proof of concept, here we demonstrate substantial MEF and spin polarization in CVD graphene/EuS heterostructures. We have chosen EuS as a model magnetic insulator because of its wide band-gap (1.65 eV), large exchange coupling J~10 meV, and large magnetic moment per Eu ion ௭~7 , 10 yielding large estimated exchange splitting ௭ in graphene. 4,5 EuS has also been shown to spin-polarize quasiparticles in materials including superconductors and topological insulators. 6,11 The strength of the MEF depends critically on the interface and EuS quality, 12,13 which we optimize with an in-situ cleaning and synthesis process (Methods and Fig. 1a). In contrast to other means, such as defect-or adatom-induced spin polarization, 14,15 depositing insulating EuS well preserves graphene's chemical bonding, confirmed by Raman spectroscopy (Fig. 1b) (Fig. S5-1), indicative of high graphene quality and well-preserved Dirac band structure.We utilize Zeeman spin-Hall effect (ZSHE) to probe the MEF in graphene which splits the Dirac cone via Zeeman effect and generates electron-and hole-like carriers with opposite 4 spins near the Dirac point ( Fig. 2a right panel). 8,9 Under a Lorentz force, these electrons and holes propagate in opposite directions, giving rise to a pure spin current and non-local voltage ( Fig. 2a left panel). We measure the non-local resistance of ZSHE using the device configuration in Fig. 2a where ௫ is the MEF. We further define the parameter :where ௭ denotes the Zeeman energy at the reference field . Given , deriving of graphene/AlO x is straightforward because ௭ is solely determined by . The inset of Fig. 3(b) shows the calculated using T, a proper reference field as we will explain below.To derive of graphene/EuS, we note that according to the theory of ZSHE, 9,17 depends on sample mobility, while other sample-dependents terms (including spin relaxation length, density of thermally activated carriers and Fermi velocity) cancel out (see S3 in SI). The mobility difference between our graphene/EuS and graphene/AlO x samples is~25% (see S1 in SI), which would only yield a~10% correction to  (see S3 in SI). Since~10% difference is 6 small, for an order-of-magnitude estimate of the MEF, we adopt the value of graphene/AlO x for graphene/EuS as an approximation. We then evaluate E Z in graphene/EuS usingTo obtain the lower bound of , we approximate , ignoring the ௫contribution. This constrains us to use a small such that ௫ is small. Meanwhile, should be high enough to ensure that , is much large...

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In addition to the weak antilocalization cusp observed in the magnetoresistance (MR) Thin films of Bi 2 Te 2 Se were grown on Si (111) at 200 °C using MBE. The base pressure in the MBE chamber was ~ 7×10 -10 torr prior to the growth. Flux monitors in the MBE system allow the evaporated thickness to be monitored in real time during the growth. 14 The stochiometry was calibrated prior to the growth and monitored during the growth to achieve a 2:2:1 ratio of Bi:Te:Se. This ratio was later confirmed by energy dispersive X-ray spectrometry. X-ray diffraction data showed good c-axis alignment for all films, having (000ℓ)diffractionpeaksuptoℓ=21asshownin Fig. 1. We confirmed the formation of a single phase and found no evidence of peak splitting. The inset of Fig. 1 shows the (0006) reflection peak with Kiessig fringes in the vicinity of the Bragg peak corresponding to the 15 nm thickness of the film. Magnetotransport measurements were performed on photolithography-patterned Hall bars (1 mm long and 0.3 mm wide). The samples were measured at temperatures down to T = 7 K and in magnetic fields up to B = 14 T. The Hall voltage was found to be linear in field and the electron density was ~ 5×10 19 cm -3 .

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