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Register a new userExchange Bias and Quantum Anomalous Hall Effect in the MnBi2Te4-CrI3 Heterostructure
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The layered antiferromagnetic MnBi2Te4 films have been proposed to be an intrinsic quantum anomalous Hall (QAH) insulator with a large gap. To realize this proposal, it is crucial to open a magnetic gap of surface states. However, recent experiments have observed gapless surface states, indicating the absence of out-of-plane surface magnetism, and thus the quantized Hall resistance can only be achieved at the magnetic field above 6 T. In this work, we propose to induce out-of-plane surface magnetism of MnBi2Te4 films via the magnetic proximity with magnetic insulator CrI3. Our calculations have revealed a strong exchange bias ~ 40 meV, originating from the long Cr-eg orbital tails that hybridize strongly with Te p-orbitals. By stabilizing surface magnetism, the QAH effect can be realized in the MnBi2Te4/CrI3 heterostructure. Our calculations also demonstrate the high Chern number QAH state can be achieved by controlling external electric gates. Thus, the MnBi2Te4/CrI3 heterostructure provides a promising platform to realize the electrically tunable zero-field QAH effect.
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The polarity-tunable anomalous Hall effect (AHE) is useful for electronic device applications. Here in a magnetic topological insulator MnBi2Te4 grown by molecular beam epitaxy, we report the polarity change of the AHE by increasing the temperature or tuning the gate bias. This is possible because the anomalous Hall response is composed of two competing contributions with opposite polarities. The negative contribution is intrinsic to MnBi2Te4, follows an ambipolar gate response and has a larger coercivity with increasing thickness. Meanwhile, the positive one has a coercivity that is about one order of magnitude greater than the negative one, dominates the Hall response at higher temperatures, is more tunable by a gate bias and vanishes by increasing the thickness of the thin film. One possible explanation for the additional positive AHE is an extra surface ferromagnetism caused by the surface-state-mediated RKKY interaction among magnetic impurities on the surface. Our work provides the understanding of the AHE of MnBi2Te4, and paves the way for many device applications, e.g. energy-efficient voltage-controlled memory.
A short review paper for the quantum anomalous Hall effect. A substantially extended one is published as Adv. Phys. 64, 227 (2015).
A central theme in condensed matter physics is to create and understand the exotic states of matter by incorporating magnetism into topological materials. One prime example is the quantum anomalous Hall (QAH) state. Recently, MnBi2Te4 has been demonstrated to be an intrinsic magnetic topological insulator and the QAH effect was observed in exfoliated MnBi2Te4 flakes. Here, we used molecular beam epitaxy (MBE) to grow MnBi2Te4 films with thickness down to 1 septuple layer (SL) and performed thickness-dependent transport measurements. We observed a non-square hysteresis loop in the antiferromagnetic state for films with thickness greater than 2 SL. The hysteresis loop can be separated into two AH components. Through careful analysis, we demonstrated that one AH component with the larger coercive field is from the dominant MnBi2Te4 phase, while the other AH component with the smaller coercive field is from the minor Mn-doped Bi2Te3 phase in the samples. The extracted AH component of the MnBi2Te4 phase shows a clear even-odd layer-dependent behavior, a signature of antiferromagnetic thin films. Our studies reveal insights on how to optimize the MBE growth conditions to improve the quality of MnBi2Te4 films, in which the QAH and other exotic states are predicted.
Heterostructures between topological insulators (TI) and magnetic insulators represent a pathway to realize the quantum anomalous Hall effect (QAHE). Using density functional theory based systematic screening and investigation of thermodynamic, magnetic and topological properties of heterostructures, we demonstrate that forming a type-I heterostructure between a wide gap antiferromagnetic insulator Cr$_2$O$_3$ and a TI-film, such as Sb$_2$Te$_3$, can lead to pinning of the Fermi-level at the center of the gap, even when magnetically doped. Cr-doping in the heterostructure increases the gap to $sim$ 64.5 meV, with a large Zeeman energy from the interfacial Cr dopants, thus overcoming potential metallicity due to band bending effects. By fitting the band-structure around the Fermi-level to a 4-band k.p model Hamiltonian, we show that Cr doped Sb$_2$Te$_3$/Cr$_2$O$_3$ is a Chern insulator with a Chern number C = -1. Transport calculations further show chiral edge-modes localized at the top/bottom of the TI-film to be the dominant current carriers in the material. Our predictions of a large interfacial magnetism due to Cr-dopants, that coupled antiferromagnetically to the AFM substrate is confirmed by our polarised neutron reflectometry measurements on MBE grown Cr doped Sb$_2$Te$_3$/Cr$_2$O$_3$ heterostructures, and is consistent with a positive exchange bias measured in such systems recently. Consequently, Cr doped Sb$_2$Te$_3$/Cr$_2$O$_3$ heterostructure represents a promising platform for the development of functional topological magnetic devices, with high tunability.
Intrinsic magnetic topological insulators offer low disorder and large magnetic bandgaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the Quantum Anomalous Hall (QAH) effect and axion insulator phases have been realised. These observations occur at temperatures significantly lower than the Neel temperature of bulk MnBi2Te4, and measurement of the magnetic energy gap at the Dirac point in ultra-thin MnBi2Te4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verifying whether the gap is magnetic in the QAH phase. Here we utilise temperature dependent angle-resolved photoemission spectroscopy to study epitaxial ultra-thin MnBi2Te4. We directly observe a layer dependent crossover from a 2D ferromagnetic insulator with a bandgap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>100 meV) at 8 K in 3 and 5 SL MnBi2Te4. The QAH gap is confirmed to be magnetic in origin, as it abruptly diminishes with increasing temperature above 8 K. The direct observation of a large magnetic energy gap in the QAH phase of few-SL MnBi2Te4 is promising for further increasing the operating temperature of QAH materials.
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