Magnetization studies of high Curie Temperature (Sb_2-yMn_yTe₃)_1-x(Mn_1+ySb_2-yTe₄)_x 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.