Candice Forrester1,2,Christophe Testelin3,Kaushini Wickramasinghe4,Ido Levy5,Xiaxin Ding4,Lia Krusin-Elbaum4,1,Gustavo Lopez1,2,Maria Tamargo1,4
The Graduate Center, CUNY1,Lehman College, CUNY2,Sorbonne Université, CNRS, Institut des NanoSciences de Paris3,The City College of New York4,New York University5
Candice Forrester1,2,Christophe Testelin3,Kaushini Wickramasinghe4,Ido Levy5,Xiaxin Ding4,Lia Krusin-Elbaum4,1,Gustavo Lopez1,2,Maria Tamargo1,4
The Graduate Center, CUNY1,Lehman College, CUNY2,Sorbonne Université, CNRS, Institut des NanoSciences de Paris3,The City College of New York4,New York University5
Recently it has been shown that structural disorder in 3D Topological Insulators (TIs) has considerable effects on the properties of the materials. The addition of magnetic ions like Mn breaks time reversal symmetry and opens a gap in the Dirac point.<sup>1</sup> This addition also changes the crystal structure from the typical quintuple layer (QLs) structure of non-magnetic TIs to a septuple layer (SLs) structure.<sup>2</sup> Furthermore, addition of a Mn flux during MBE growth results in self-assembled structures of mixed QLs and SLs.<sup>2</sup><br/>Previously we reported the MBE growth of self-assembled structures of (MnSb<sub>2</sub>Te<sub>4</sub>)<sub>x</sub>(Sb<sub>2</sub>Te<sub>3</sub>)<sub>1-x</sub> magnetic topological materials,<sup>3</sup> and showed that their Curie temperature (T<sub>C</sub>) is dependent on the composition x (or %SL).<sup>4</sup> Samples with 0.7 < x < 0.85 exhibit very high T<sub>C</sub> values. Additionally, it was observed that as the Mn beam equivalent pressure (BEP) ratio used was increased, there was a corresponding increase in T<sub>C</sub>. Decreasing the growth rate further increased T<sub>C</sub> to >100 K, the highest values reported for this material system. An understanding of how the changes in growth conditions lead to the T<sub>C</sub> enhancement is not well-established.<br/>Here we investigate the structural properties of the materials as they relate to the growth conditions, specifically Mn flux ratio during growth and reduced growth rate (GR). Samples grown at slow GRs (0.4 – 0.6 nm/min) were compared to samples grown at fast GRs (0.8 – 1.0 nm/min). Samples were investigated using several characterization techniques including X-ray diffraction, Energy Dispersion X-Ray Spectroscopy (EDS), Hall effect and scanning transmission electron microscopy (STEM).<br/>We found that for the same Mn BEP ratio, low GR yields similar composition, x, as fast GRs. On the other hand, EDS showed that for x > 0.7, there was increased intermixing between Sb and Mn in both the fast and the slow GR samples. Furthermore, the samples grown with slow GR showed much greater Mn and Sb intermixing, as well as Mn and Te intermixing, suggesting increased Mn incorporation at the slow GR, as well as more disorder. Cross sectional EDS studies reveal a high Mn content in the QLs, consistent with (Sb<sub>1-y</sub>Mn<sub>y</sub>)<sub>2</sub>Te<sub>3</sub> alloy formation. Hall effect measurements show that GR does not significantly affect the electrical doping in (MnSb<sub>2</sub>Te<sub>4</sub>)<sub>x(</sub>Sb<sub>2</sub>Te<sub>3</sub>)<sub>1-x</sub> supporting the proposal that a super-exchange magnetic mechanism is likely at play. Other techniques, such as magnetic force microscopy (MFM) are being explored to better understand the magnetic mechanisms leading to high T<sub>C</sub> values and Raman spectroscopy is being explored to understand the disorder in these materials. Our results provide insight as to how to achieve on-demand magnetic TIs with enhanced properties.<br/><br/><sup>1 </sup>Y. Tokura et al, <i>Nature Reviews Physics </i><b>1</b>, 126 (2019)<br/><sup>2 </sup>J.A. Hagmann et al, <i>New Journal of Physics </i><b>19</b>, 085002 (2017)<br/><sup>3 </sup>I. Levy et al, <i>Crystal Growth & Design </i><b>22</b>, 3007 (2022)<br/><sup>4</sup> I. Levy et al, Science Reports 13, 7381(2023)