Ferromagnetic-antiferromagnetic coexisting ground state and exchange bias effects in MnBi 4 Te 7 and MnBi 6 Te 10 .

Natural superlattice structures MnBi 2 Te 4 (Bi 2 Te 3 ) n (n = 1, 2, ...), in which magnetic MnBi 2 Te 4 layers are separated by nonmagnetic Bi 2 Te 3 layers, hold band topology, magnetism and reduced interlayer coupling, providing a promising platform for the realization of exotic topological quantum states. However, their magnetism in the two-dimensional limit, which is crucial for further exploration of quantum phenomena, remains elusive. Here, complex ferromagnetic-antiferromagnetic coexisting ground states that persist down to the 2-septuple layers limit are observed and comprehensively investigated in MnBi 4 Te 7 (n = 1) and MnBi 6 Te 10 (n = 2). The ubiquitous Mn-Bi site mixing modifies or even changes the sign of the subtle interlayer magnetic interactions, yielding a spatially inhomogeneous interlayer coupling. Further, a tunable exchange bias effect, arising from the coupling between the ferromagnetic and antiferromagnetic components in the ground state, is observed in MnBi 2 Te 4 (Bi 2 Te 3 ) n (n = 1, 2), which provides design principles and material platforms for future spintronic devices. Our work highlights a new approach toward the fine-tuning of magnetism and paves the way for further study of quantum phenomena in MnBi 2 Te 4 (Bi 2 Te 3 ) n (n = 1, 2) as well as their magnetic applications.