High Curie temperature ferromagnetic structures of (Sb₂Te₃)_1-x(MnSb₂Te₄)_x with x = 0.7-0.8

Magnetic topological materials are promising for realizing novel quantum physical phenomena. Among these, bulk Mn-rich MnSb₂Te₄ is ferromagnetic due to Mn_Sb antisites and has relatively high Curie temperatures (T_C), which is attractive for technological applications. We have previously reported the growth of materials with the formula (Sb₂Te₃)_1-x(MnSb₂Te₄)_x, where x varies between 0 and 1. Here we report on their magnetic and transport properties. We show that the samples are divided into three groups based on the value of x (or the percent septuple layers within the crystals) and their corresponding T_C values. Samples that contain x < 0.7 or x > 0.9 have a single T_C value of 15-20 K and 20-30 K, respectively, while samples with 0.7 < x < 0.8 exhibit two T_C values, one (T_C1) at ~ 25 K and the second (T_C2) reaching values above 80 K, almost twice as high as any reported value to date for these types of materials. Structural analysis shows that samples with 0.7 < x < 0.8 have large regions of only SLs, while other regions have isolated QLs embedded within the SL lattice. We propose that the SL regions give rise to a T_C1 of ~ 20 to 30 K, and regions with isolated QLs are responsible for the higher T_C2 values. Our results have important implications for the design of magnetic topological materials having enhanced properties.