Growth and characterization of hybrid insulating ferromagnet-topological insulator heterostructure devices

Abstract

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...

Breaking time reversal symmetry in topological insulators

“…27 With similar objectives, Yang et al 28 and Kandala et al 29 at H C coincides with the occurrence of resistance minima (or higher electrical conduction). This unconventional behavior was attributed to the isotropic distribution of domains and hence the observed isotropic planar MR. Interestingly, the Dirac SSs along the domain wall were theoretically predicted to have zero mass carrying chiral conduction modes similar to the quantum Hall edge channels, 1,2 in agreement with the experimental observations of the MR dips (increased conduction) at H C .…”

“…27 Generally, all the FIs have an in-plane easy magnetization axis, although canted magnetization or small perpendicular magnetic hysteresis has been reported. [27][28][29] However, the perpendicular magnetic anisotropy (PMA) can be engineered by choosing a proper interface layer, which is attributed to the hybridization of the electronic states. 34 Strong PMA in transition metal/rare-earth multilayers has been reported.…”

“…In this case, the allimportant advantage is that the TRS breaking happens mostly at the interface between the TI and the FI, rather than affecting the majority bulk states, while also not introducing defects ( Figure 1b). However, in general, thin films of FIs show in-plane easy axis of magnetization, [27][28][29] which hinders the observation of the QAHE or the zero-field half-integer QHE. 7 We provide an overview of the behavior of FI/TI heterostructures and that of magnetically doped Se/Te-based TI compounds as well as bring out their differences.…”

Proximity effect between a topological insulator and a magnetic insulator with large perpendicular anisotropy

“…Such a quadratic field dependence has never been observed in either perpendicular or parallel fields in previous studies of TI/MI heterostructures, such as Bi 2 Se 3 /EuS, Bi 2 Se 3 /GdN and Bi 2 Se 3 /YIG. 16,17,19,45 The other is the strong T -dependence of both types of parabolic MR. This is further illustrated in Fig.…”

“…Au thin films) with magnetic adatoms. 19,46 In case of very strong magnetic scatterings, there is a crossover from the symplectic limit (α=1/2) to the unitary limit (α=0).…”