structures grown by molecular beam epitaxy (MBE)*

     It has been shown that structural disorder in 3D topological insulators (TIs) has considerable effects on their properties. Addition of magnetic ions like Mn opens a gap in the Dirac point and changes the crystal structure from the quintuple layers (QLs) of non-magnetic TIs into septuple layers (SLs). Growth by MBE results in self-assembled structures of QLs and SLs.

     We reported the magnetic properties of (Sb_2-yMn_yTe₃)_1-x(Mn_1+ySb_2-yTe₄)_x structures grown at slow growth rates (~0.5 nm/min) with high Mn content. Bulk EDS studies showed Mn incorporation into Sb and Te sites and possible Sb_2-yMn_yTe₃ alloy formation, which gave T_C values of up to 100K. Analysis revealed the presence of two T_C components and led us to propose that excess Mn in SLs and QLs contributes to the T_C1 and T_C2, respectively.

     Magnetization studies of these materials were undertaken to understand their unique behavior. Initial work showed T_C independence from carrier density, suggesting long range magnetic interactions via Mn-Te orbital hybridization are dominant. For x ≤ 0.7, Arrott plots gave T_C1 of ~26K, consistent with published data for similar compositions, following Mean-Field theory. However, for 0.7 ≤ x ≤ 0.85, where there is significant disorder and excess Mn, deviation from Mean-Field theory was observed. Regarding T_C2, behavior consistent with predicted values for Sb_2-yMn_yTe₃ as a function of y was seen for y ≤ 0.6, while T_C2 deviated from the predictions at higher Mn content. Independent assessment of ‘y’ in the QLs is being pursued.