Even–Odd Layer-Dependent Anomalous Hall Effect in Topological Magnet MnBi₂Te₄ Thin Films

Abstract

Recently, MnBi 2 Te 4 has been demonstrated to be an intrinsic magnetic topological insulator and the quantum anomalous Hall (QAH) effect was observed in exfoliated MnBi 2 Te 4 flakes. Here, we used molecular beam epitaxy (MBE) to grow MnBi 2 Te 4 films with thickness down to 1 septuple layer (SL) and performed thickness-dependent transport measurements. We observed a nonsquare hysteresis loop in the antiferromagnetic state for films with thickness greater than 2 SL. The hysteresis loop can be separated into two AH components. We demonstrated that one AH component with the larger coercive field is from the dominant MnBi 2 Te 4 phase, whereas the other AH component with the smaller coercive field is from the minor Mn-doped Bi 2 Te 3 phase. The extracted AH component of the MnBi 2 Te 4 phase shows a clear even− odd layer-dependent behavior. Our studies reveal insights on how to optimize the MBE growth conditions to improve the quality of MnBi 2 Te 4 films.

Surface‐Driven Evolution of the Anomalous Hall Effect in Magnetic Topological Insulator MnBi2Te4 Thin Films

“…Similarly, the surface term also increases, albeit less dramatically. For Te capped samples R xy , F increases for reduced thickness, [ 48 ] and suggests the sample's effective magnetic thicknesses becomes thinner, agreeing with the above results of the MR. H C is nominally the same for both components and exhibits a small increase for times up to 40 min. Surprisingly, at ≈40–60 min there emerges an additional component with negative amplitude, R xy , F3 .…”

“…For finite thickness MnBi 2 Te 4 it has been empirically found that the hysteretic loops of the anomalous Hall effect can be decomposed into multiple individual loops. [ 48 ] Specifically, this can be quantified by fitting the data to a function of the form: $$\[ \begin{array}{*{20}{c}}{{R_{xy,A}}\left( H \right) = \mathop \sum \limits_i {R_{xy,Fi}}\tanh {w_i}\left( {H - {H_{C,i}}} \right)}\end{array} \]$$where i indexes the individual anomalous Hall component, which is found to be either 2 or 3 in this work, R xy , Fi is the magnitude of the response, w i is the width of the transition, and H C , i is the coercive field (where the anomalous Hall component vanishes). The hyperbolic tangent function is derived from a Langevin‐type analysis for the net magnetization in a magnetic field.…”

“…First, at short times ( t = 10–40 min, as well as the Te capped samples from Ref. [ 48 ] ) it is seen that the magnitude of the impurity‐like term, R xy , F2 , steadily increases. Similarly, the surface term also increases, albeit less dramatically.…”

“…The first scenario would have the near surface layer being magnetically split from the remaining bottom portion of the film, driven possibly by exchange striction to the magnetic surface oxide (see below); this is consistent with the large magnitude of R xy,F which is similar to several SL thick capped samples. [ 48 ] Alternatively, a qualitatively similar response could come from a non‐homogeneous oxide thickness across the film that varies from 1 to 2 SL, and, thereby, leads to portions of the film which are effectively an even number of layers and portions of the film with an effective odd number of layers. For example, if the films were effectively 5/6 SL or 4/5 SL the magnitude R xy,F would likely be significantly smaller, which makes the scenario of the magnetic split across thickness perhaps more likely.…”

Nonreciprocal Transport in a Bilayer of MnBi2Te4 and Pt

“…surface collapse and reconstruction 12 , temperature-independent bandgap 1,13 and even the gapless surface Dirac cone 14 , revealed that magnetic and topological properties of MBT are sensitive to the surface structure. Inevitable Mn-doped Bi2Te 3 phase exists in the MBT, which originates from the Te vacancy 12 or the FM island on some areas 15 , can induce the magnetic moment at the surface 13 and weaken the interaction between magnetism and topological surface state.…”