Ferromagnetic-antiferromagnetic coexisting ground state and exchange bias effects in MnBi₄Te₇ and MnBi₆Te₁₀

Natural superlattice structures MnBi₂Te₄(Bi₂Te₃)_n (n = 1, 2, ...), in which magnetic MnBi₂Te₄ layers are separated by nonmagnetic Bi₂Te₃ 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 MnBi₄Te₇ (n = 1) and MnBi₆Te₁₀ (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 MnBi₂Te₄(Bi₂Te₃)_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 MnBi₂Te₄(Bi₂Te₃)_n (n = 1, 2) as well as their magnetic applications.