The mechanism exploration for zero-field ferromagnetism in intrinsic topological insulator MnBi2Te4 by Bi2Te3 intercalations

Abstract

Recent research on intrinsic magnetic topological insulators (MTIs), MnBi2Te4, sheds new light on the observation of a long-expected high-temperature quantum anomalous Hall effect (QAHE). However, the strong interlayered anti-ferromagnetic (AFM) coupling hinders the practical applications without applying a magnetic field. Thus, how to adjust the magnetism of this compound under zero field is essential. Here, we theoretically and experimentally study the magnetic properties of two new promising intrinsic MTI candidates MnBi4Te7 and MnBi6Te10, formed by intercalating the Bi2Te3 layer into MnBi2Te4. The first-principles calculations reveal that the relative energy between ferromagnetic (FM) and AFM states is greatly reduced by Bi2Te3 intercalations. The calculated energy barriers for the spin flipping process also point out that the metastable FM state is more easily retained by intercalation. Meanwhile, we also experimentally carry out magnetic and transport measurements on these materials. By increasing Bi2Te3 intercalations, the AFM coupling becomes weaker, and an almost fully polarized FM state can be preserved in MnBi6Te10 at low temperatures, which are consistent with our calculations. We believe that the demonstration of the intrinsic MTI preserving zero-field FM state and the in-depth investigation for the mechanism behind pave the way for investigating the high-temperature QAHE and the related physics.

Epitaxial Growth and Structural Characterizations of MnBi2Te4 Thin Films in Nanoscale

“…Under magnetic field 3 T, the spin-flip transition occurred as indicated by red arrows in Fig- ure 3b. Similar large-field magnetic behavior was observed in MnBi2Te4 flakes [42,[48][49][50]. Figure 3c shows the field dependence out of plane of the magnetization curve of a 7 SL MnBi2Te4 sample in the temperature range from 10 K to 30 K. The anomalous magnetic hysteresis loop dwindled with increasing temperature and disappeared at 30 K, which indicates that a magnetic phase transition from an antiferromagnetic state to a paramagnetic state occurred with increasing temperature, consistent with the observation of M-T curves in Figure 3a.…”

“…The M-H loop out of plane exhibits a non-linear ferromagnetic shape, whereas the M-H loop in plane is much flatter, indicating a perpendicular magnetic anisotropy with the easy magnetization axis along the c-axis [42,49]. For H//c, the M-H curve exhibits an anomalous magnetic hysteresis loop centered at the regime of a small magnetic field, indicating the spin-flip of individual layers and a ferromagnetic feature, which differs from the antiferromagnetic characteristics of MnBi2Te4 bulk and flakes [42,49,50]. In bulk and exfoliated flakes of MnBi2Te4, the M-H curves are nominally flat in the regime of a small magnetic field.…”

Coexistence of ferromagnetism and topology by charge carrier engineering in the intrinsic magnetic topological insulator MnBi4Te7

“…MnBi2Te4 SL, the energy difference between the AFM and FM is relatively small in MnBi4Te7 18,19,34 . Weak interlayer AFM coupling paves the way for the magnetism modulation by various approaches.…”

Ferromagnetic-antiferromagnetic coexisting ground states and exchange bias effects in $\bf{MnBi_4Te_7}$ and $\bf{MnBi_6Te_{10}}$

“…Therefore, there is an urgent need to develop a magnetic topological insulator of the MBT family that is magnetically insensitive to the number of layers and has a small saturation field, in which engineering of the interlayer magnetic interaction is the key. Now, the natural superlattice structures (MnBi 2 Te 4 )(Bi 2 Te 3 ) n (n = 1, 2, ...) provides an opportunity to modify the interlayer exchange coupling using non-magnetic Bi 2 Te 3 (BT) quintuple layer (QL) as spacer layers [14][15][16][17][18][19]. With the increase of n, the interlayer AFM coupling gradually weakens, while the global spin-orbit coupling strength gradually increases as the increase of Bi content [15][16][17][18][19][20].…”

“…Now, the natural superlattice structures (MnBi 2 Te 4 )(Bi 2 Te 3 ) n (n = 1, 2, ...) provides an opportunity to modify the interlayer exchange coupling using non-magnetic Bi 2 Te 3 (BT) quintuple layer (QL) as spacer layers [14][15][16][17][18][19]. With the increase of n, the interlayer AFM coupling gradually weakens, while the global spin-orbit coupling strength gradually increases as the increase of Bi content [15][16][17][18][19][20]. For MnBi 4 Te 7 (n = 1) and MnBi 6 Te 10 (n = 2), neutron scattering experiments and theoretical calculations show that they possess weak interlayer magnetic coupling, but still maintain the A-type AFM structure [18,19,[21][22][23], that is, in each MnBi 2 Te 4 septuple layer (SL), the Mn magnetic moments are ferromagnetically aligned, while the adjacent SLs are antiferromagnetically aligned.…”