Tunable magnetism, charge carrier type and density in Mn(Bi,Sb)₄Te₇

Intrinsic antiferromagnetic (AFM) topological insulators MnBi_2nTe_3n+1 (n=1, 2, 3) have attracted intense interest since they can support a variety of topological quantum states. The n=2 member, MnBi₄Te₇ was recently predicted to be a Weyl semimetal in its ferromagnetic phase [1,2]. To observe this state, tuning its magnetism from an AFM to ferromagnetic (FM) state is necessary and its chemical potential m is also required to be tuned to the charge neutrality point (CNP). In this talk, we will show these desired features are accessible in the Mn(Bi_1-xSb_x)₄Te₇ alloy system. We find both the charge carrier type and concentration can be systematically tuned by the Sb concentration, with m being close to the CNP at x ~ 0.27. Our established phase diagram also shows the FM phase is accessible in a wide range of x, but is suppressed when the carrier density is low, implying the interlayer FM coupling is mediated by the RKKY interaction. Furthermore, we also observed strong quantum oscillations in the samples with low carrier density, from which evidence of Fermi surface reconstruction caused by the FM transition is also observed. This result, together with the negative longitudinal magnetoresistance, suggests a possible FM Weyl state in Mn(Bi_1-xSb_x)₄Te₇ with x~0.27.