Ferromagnetic MnBi4Te7 obtained with low-concentration Sb doping: A promising platform for exploring topological quantum states

The tuning of the magnetic phase, chemical potential, and structure is crucial to observe diverse exotic topological quantum states in MnBi2Te4(Bi2Te3)m (m=0-3). Here we show a ferromagnetic (FM) phase with a chiral crystal structure in Mn(Bi1-xSbx)4Te7, obtained via tuning the growth conditions and Sb concentration. Unlike previously reported Mn(Bi1-xSbx)4Te7, which exhibits FM transitions only at high Sb doping levels, our samples show FM transitions (TC=13.5K) at 15%-27% doping levels. Furthermore, our single-crystal x-ray-diffraction structure refinements find Sb doping leads to a chiral structure with the space group of P3, contrasted with the centrosymmetric P3¯m1 crystal structure of the parent compound MnBi4Te7. Through angle-resolved photoemission spectroscopy measurements, we also demonstrated that the nontrivial band topology is preserved in the Sb-doped FM samples. Given that the nontrivial band topology of this system remains robust for low Sb doping levels, our success in making FM Mn(Bi1-xSbx)4Te7 with x=0.15, 0.175, 0.2, and 0.27 paves the way for realizing the predicted topological quantum states, such as the axion insulator and Weyl semimetals. Additionally, we also observed magnetic glassy behavior in both antiferromagnetic MnBi4Te7 and FM Mn(Bi1-xSbx)4Te7 samples, which we believe originates from cluster spin-glass phases coexisting with long-range antiferromagnetic/FM orders. We have also discussed how the antisite Mn ions impact the interlayer magnetic coupling and how FM interlayer coupling is stabilized in this system.