The discovery of the quantum Hall (QH) effect led to the realization of a topological electronic state with dissipationless currents circulating in one direction along the edge of a two dimensional electron layer under a strong magnetic field. 1,2 The quantum anomalous Hall (QAH) effect shares a similar physical phenomenon as the QH effect, whereas its physical origin relies on the intrinsic spin-orbit coupling and ferromagnetism.

Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin-orbit interaction and the spin-momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.

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 ...