NAMBE 2022 Session NM-WeM2: Topological Insulators

Magnetic topological insulators such as MnBi₂Te₄ and MnSb₂Te₄ are of great interest due to their promise for realizing exotic physical phenomena and for potential applications in quantum science and quantum computing. However, the control and understanding of the materials properties are still far from adequate. Molecular beam epitaxy (MBE) promises higher control and improved materials properties due to its far from equilibrium conditions and its layer-by-layer growth. Addition of Mn during growth of Sb₂Te₃ results in the formation MnSb₂Te₄ septuple layers (SL). Although the individual SLs are ferromagnetic (FM), when SLs stack to form a crystal, their spins align opposite each other, resulting in net antiferromagnetic properties, not conducive to the desired exotic phenomena. It has also been shown that depending on the amount of Mn added, the structure self-assembles into layered QL:SL stacks of the form (Sb₂Te₃)_1-x(MnSb₂Te₄)_x.

Our group is investigating the MBE growth of the MnSb₂Te₄ system. We recently reported the growth of layered QL:SL structures ranging in composition from 100% QL to 100% SLs.We showed that modification of the growth conditions by incorporation of a preannealing step and increased growth temperatures leads to samples with varying amounts of SL for the same Mn flux ratios. This suggests that Mn incorporates both as a structural component in SLs and as antisite defects, affecting the magnetic properties of the resulting structures.

Here we present the detailed magnetic properties of the materials using Hall Effect and remanent magnetization measurements. We show that under our MBE growth conditions, all layers with at least a few SLs are FM. Furthermore, their Curie temperatures (T_C ) vary as a function of the %SLs in the structure. Results show that: 1) Samples with low %SLs (< 70%) exhibit a single T_C value of 15-20K. 2) Samples with nearly 100% SLs (>90%) also exhibit a single Tc, with a higher value of 35-40K. 3) Samples with intermediate values of 70-85% SLs exhibit unusual T_C behavior, with evidence for two T_C components in the structure (T_C1 and T_C2). For these, the T_C1 value is ~40K, similar to the value for samples with >90% SLs, while the T_C2 values are as high as 80K, higher than values reported to date for these materials. We interpret these results on the basis of the distribution of SL and QL layers within the structure, and the excess Mn contained in the layers. We propose that the structures with two T_C values consist of two distinct regions: regions of mostly SLs, that behave as materials with >90%SLs, and regions of SLs surrounding isolated QLs which lead to the higher T_C2 values.

The interaction between magnetic impurities and topological electronic states of 3D topological insulators (TIs) has attracted many studies of predicted exotic phenomena, which may lead to the observance of quantum anomalous hall effect (QAHE) and realizations of quantum computing, among others.¹

Mixing of Mn, Sb and Te during crystal growth by bulk growth or epitaxial techniques forms a new crystal phase, MnSb₂Te₄, with septuple layer (SL) units rather than typical quintuple layer (QL) units of the undoped TIs. These magnetic TIs display antiferromagnetic (AFM) behavior in bulk, which is not conducive for QAHE, and typically have high bulk conductivity, which limits the ability to detect the surface states and their possible applications. Our group has shown that controlling the Mn incorporation into crystal led to the design of QL:SL structures.² These structures exhibit ferromagnetic (FM) behavior and a reduced bulk conductivity depending on the %SLs in the structure. Our group has also recently observed that, for some QL:SL layer ratios, samples exhibit mixed magnetic behaviors, with two distinct Curie temperature (Tc) components (Tc1 and Tc2), and Tc2 values as high as 75-80K.

Here we report the further enhancement of the Tc2 values by modification of the MBE growth conditions. In particular, growth rates and the Mn beam equivalent pressure (BEP) ratios were varied in this study. Samples with a lower growth rate of about 0.5-0.6 nm/min, compared to ~0.9-1.0 nm/min used previously, and with Mn BEP ratios between 0.07 and 0.09, resulted in structures having 79-89% SLs that also exhibit two distinct Tc values, one Tc value (Tc1) at ~40-50K and a high Tc value (Tc2) of 100K and above. (Fig. 1) These high Tc values are significantly higher than any values reported to date for these materials. Reducing the Mn BEP ratio to levels of 0.04-0.06 led to the observation of a single Tc value of ~30K and evidence for the coexistence of FM and AFM phases, suggesting the approach to stoichiometric MnSb₂Te₄ growth, with little excess Mn. (Fig. 2) We will present these results as they relate to the (MnSb₂Te₄)_x(Sb₂Te₃)_1-x structural properties and the details of the growth mechanism. We will perform first-principle calculations using the Ising model to describe and understand the possible magnetic interactions that lead to samples with such high Curie temperatures.

1. R. Yu, et al., Science 2010, 329, 61−64.
2. I. Levy et al., Crystal Growth & Design 2022 https://doi.org/10.1021/acs.cgd.1c01453 (Published Online)

We report the results from the exploration of the MBE growth and characterization of ternary p-type (Bi,Sb)₂Te₃ and n-type Bi₂(Te,Se)₃. Comprehensive temperature, field and angular dependent magnetotransport measurements of these ternary (Bi,Sb)₂(Te,Se)₃films show band-insulator behavior and display low carrier density on the order of 10¹⁸ cm^–3 and a record high mobility exceeding 10⁴ cm² V^–1 s^–1 at 2 K. The remarkable manifestation of strong Shubnikov–de Haas (SdH) quantum oscillation under 9 T at liquid helium temperatures, as well as the analyses therein, has allowed direct experimental investigation of the epitaxial layer electronic structure. Thermal phenomena including the Seebeck and Nernst coefficients are found to be consistent with the understanding obtained from magneto-transport measurements.The primary significance of these results is a path forward for topological spintronics with low bulk carriers and unprecedented magnetoelectric functionalities.