Visualizing ferromagnetic domains in magnetic topological insulators

Wang, Wenbo; Fu, Yang; Gao, Chunlei; Jia, Jin‐Feng; Gu, Genda; Wu, Weida

doi:10.1063/1.4921093

Cited by 20 publications

(22 citation statements)

References 35 publications

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“…In these materials, ordering of the dopant magnetic moments breaks time reversal symmetry, combining with the strongly spin-orbit coupled electronic structure to produce topologically nontrivial Chern bands [11]. However, the performance of these materials is limited by the inhomogeneous distribution of the magnetic dopants, which leads to microscopic structural, charge, and magnetic disorder [12,13,15,16]. As a result, sub-kelvin temperatures are typically required to observe quantization, despite magnetic ordering temperatures at least one order of magnitude larger.…”

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confidence: 99%

Intrinsic quantized anomalous Hall effect in a moiré heterostructure

Serlin

Tschirhart

Polshyn

et al. 2020

Science

1,217

967

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We report the observation of a quantum anomalous Hall effect in twisted bilayer graphene showing Hall resistance quantized to within .1% of the von Klitzing constant h/e 2 at zero magnetic field. The effect is driven by intrinsic strong correlations, which polarize the electron system into a single spin and valley resolved moiré miniband with Chern number C = 1. In contrast to extrinsic, magnetically doped systems, the measured transport energy gap ∆/kB ≈ 27 K is larger than the Curie temperature for magnetic ordering TC ≈ 9 K, and Hall quantization persists to temperatures of several Kelvin. Remarkably, we find that electrical currents as small as 1 nA can be used to controllably switch the magnetic order between states of opposite polarization, forming an electrically rewritable magnetic memory.

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confidence: 99%

Intrinsic quantized anomalous Hall effect in a moiré heterostructure

Serlin

Tschirhart

Polshyn

et al. 2020

Science

1,217

967

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“…11,18,32,33 In this context, it is important to note that none of the previous studies of doped TI films have directly probed the microscopic magnetic structure within the QAH state. [19][20][21] The FM state was reported to be present in films that either do not show QAH at all due to non-optimal doping or composition, or at temperatures substantially higher than those required for the formation of the QAH. 19,20 Similarly, the SPM was demonstrated locally in films that do show QAH but at elevated temperatures at which no quantization was present.…”

Section: Introductionmentioning

confidence: 99%

“…[19][20][21] The FM state was reported to be present in films that either do not show QAH at all due to non-optimal doping or composition, or at temperatures substantially higher than those required for the formation of the QAH. 19,20 Similarly, the SPM was demonstrated locally in films that do show QAH but at elevated temperatures at which no quantization was present. 21,24 Here we report scanning SQUID-on-tip (SOT) magnetic imaging of modulation-doped (Bi,Sb) 2 Te 3 films performed concurrently with transport measurements that show full QAH quantization.…”

Section: Introductionmentioning

confidence: 99%

“…[13][14][15] In order to establish a TRSB topological state on a macroscopic scale the magnetic order should be long ranged and therefore a robust ferromagnetic (FM) state is believed to be a necessary condition for the experimental observation of the QAH effect with dissipationless chiral edge states. 2,13,16 Indeed, a number of studies using global magnetization measurements 14,17,18 and magnetic force microscopy 19,20 have observed FM-like response in magnetically doped TI thin films. Surprisingly, however, recent scanning SQUID magnetic imaging 21 and transport studies, 22,23 as well as analysis of Kerr microscopy imaging, 24 have suggested the presence of a superparamagnetic (SPM) state in Cr-doped (Bi, Sb) 2 Te 3 films in which instead of a long-range FM order the magnetism is comprised of single-domain magnetic islands.…”

Section: Introductionmentioning

confidence: 99%

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Observation of superparamagnetism in coexistence with quantum anomalous Hall C = ±1 and C = 0 Chern states

et al. 2017

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Simultaneous transport and scanning nanoSQUID-on-tip magnetic imaging studies in Cr-(Bi,Sb) 2 Te 3 modulation-doped films reveal the presence of superparamagnetic order within the quantum anomalous Hall regime. In contrast to the expectation that a longrange ferromagnetic order is required for establishing the quantum anomalous Hall state, superparamagnetic dynamics of weakly interacting nanoscale magnetic islands is observed both in the plateau transition regions, as well as within the fully quantized C = ±1 Chern plateaus. Modulation doping of the topological insulator films is found to give rise to significantly larger superparamagnetic islands as compared to uniform magnetic doping, evidently leading to enhanced robustness of the quantum anomalous Hall effect. Nonetheless, even in this more robust quantum state, attaining full quantization of transport coefficients requires magnetic alignment of at least 95% of the superparamagnetic islands. The superparamagnetic order is also found within the incipient C = 0 zero Hall plateau, which may host an axion state if the top and bottom magnetic layers are magnetized in opposite directions. In this regime, however, a significantly lower level of island alignment is found in our samples, hindering the formation of the axion state. Comprehension and control of superparamagnetic dynamics is thus a key factor in apprehending the fragility of the quantum anomalous Hall state and in enhancing the endurance of the different quantized states to higher temperatures for utilization of robust topological protection in novel devices. 1-3 In the QH state the TRSB is induced by high magnetic field applied perpendicular to a high-mobility 2D electron gas. The QAH state, in contrast, can be formed in thin films of topological insulators (TI) even in the absence of magnetic field.3-10 In this case the TRSB is induced by magnetic order attained by doping the TIs with transition metal elements like Cr, V, or Mn,

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“…The MFM experiments were carried out in a homemade cryogenic atomic force microscope (AFM) using commercial piezoresistive cantilevers (spring constant k ≈ 3 N/m, resonant frequency f 0 ≈ 42 kHz). The homemade AFM is interfaced with a Nanonis SPM Controller and a commercial phase-lock loop (SPECS) 40,41 . MFM tips were prepared by depositing a nominally 100 nm thick Co film onto bare tips using e-beam evaporation.…”

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confidence: 99%