Magnetoelectric control of topological phases in graphene

Abstract

Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the Néel vector. The control of these states is envisioned through manipulation of the Néel vector by spin-orbit torques driven by electric currents. Here we propose a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the Néel vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry protected boundary magnetization and the induced Rashba-type spin-orbit coupling at the interface between graphene and chromia, the emergent topological phases in graphene can be controlled by the Néel vector. Using density functional theory and tightbinding Hamiltonian approaches, we model a graphene/Cr2O3 (0001) interface and demonstrate non-trivial band gap openings in the graphene Dirac bands asymmetric between the K and K′ valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) with a quantized value of 2e 2 /h and an additional step-like feature at a value close to e 2 /2h, and the emergence of the spin-polarized valley Hall effect (VHE). Furthermore, depending on the Néel vector orientation, we predict the appearance and transformation of different topological phases in graphene across the 180° AFM domain wall, involving the QAHE, the valley-polarized QAHE and the quantum VHE (QVHE), and the emergence of the chiral edge state along the domain wall. These topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques. A. Optimization of the graphene/Cr 2 O 3 (0001) interface structureThe in-plane position of graphene on the Cr2O3 (0001) surface was optimized by considering three different interface structures: (1) a graphene C atom is atop the Cr atom of the Cr2O3 surface (Figures 1a,b in the main text), (2) a graphene C atom is atop the O atom in the first O monolayer from the Cr2O3 surface ( Figure S1a), and (3) the Cr surface atom is below the center of a hexagonal ring of graphene ( Figure S1b). Starting from these initial configurations, the atomic structure of the whole supercell is relaxed. We find that the lowest total energy structure forms C located atop Cr (Figures 1a,b in the main text). The structure with C atop O atom ( Figure S1a) and the structure with Cr under the graphene hollow site, respectively, have the total energy 36 meV and 55 meV higher. In the main text, we focus on the most stable atomic structure to investigate the electric, magnetic, and spin transport of the on the Cr2O3 (0001) interface.

Graphene on Chromia: A System for Beyond‐Room‐Temperature Spintronics

“…The spin signal is also found to persist well beyond T N , where any boundary magnetism should be lost, remaining visible at the highest studied temperature of ≈450 K. These observations are consistent with the results of prior theoretical work, which predict that chromia breaks the crystal symmetry of graphene (from C 6v to C 3v ) and generates an effective SOC term of ≈40 meV. [ 25,26,41 ] As such, our observations point to the suitability of chromia/graphene heterostructures for use in spintronic devices, capable of operation at room temperature and beyond. [ 26,42 ] This should be contrasted with prior studies of proximity effects in graphene/AFM heterolayers; [ 24,43–45 ] while some of these works [ 24,45 ] have established the presence of very large exchange coupling between the layers, this situation is only realized for temperatures below T N .…”

“…This is precisely the behavior expected for the spin-Hall signal, whose magnitude is inversely proportional to the charge conductivity of graphene, [52] and points to the presence of extrinsic SOC that arises from the contact with chromia. [41] To allow comparison with the magnitude of R nl , in Figure 2a we also plot the corresponding variation of the ohmic resistance (R Ohmic ). This latter quantity is essentially the resistance that should be measured at the non-local probes due to leakage of classical current lines from the local measurement setup [20,24,[53][54][55] ln cosh 1…”

Quantum Hall phase in graphene engineered by interfacial charge coupling

“…Interfacial coupling is known to affect the QHE in graphene, usually in two different ways: charge impurities that cause a reduced mobility yet a wider quantum Hall (QH) plateaux in some circumstances 6 , and charge transfer that, to some extent, shifts the effective doping [11][12][13][14][15][16] . Theories predict that the interplay between an antiferromagnetic insulator and graphene can give rise to topological quantum ground states, such as quantum anomalous Hall phases [17][18][19] . Experimentally, RuCl 3 /graphene is, indeed, spotted with a strong charge transfer, which is sometimes possibly coupled to the magnetism 20 and sometimes not fully evidenced so 21 .…”

Insulator-to-conductor transition driven by the Rashba–Zeeman effect

“…The exchange coupling between Cr 2 O 3 and Ag 2 Te across the interface in the Ag 2 Te/Cr 2 O 3 structure is mediated by the surface magnetization through the proximity effect. The recent work has shown that a topological phase of graphene can be tuned by magnetization orientation in a graphene/Cr 2 O 3 system 38 . Figure 3a shows the atomic structure (produced using the VESTA software 39 ) of the Ag 2 Te/Cr 2 O 3 (0001) system consisting of monolayer Ag 2 Te and Cr 2 O 3 substrate composed of 6 and 12 atomic layers of O and Cr, respectively.…”