Growth and Magnetism of Van der Waals Magnetic Topological Insulators*

Magnetic topological insulators (MTI) combine a non-trivial topology of the electron band spectrum with a long-range magnetic order. These narrow-gap semiconductors are intensely explored as a promising platform that harbors new physics and enables new technological applications.

The first intrinsic antiferromagnetic MTI MnBi₂Te₄ was discovered in a joint theoretical and experimental pursuit [1]. This layered van der Waals material is a progenitor of an entire family of (MnX₂Te₄)(X₂Te₃)_n, n = 1–4, X = Bi, Sb, structures [2-11]. Their magnetic ground states alter as a function of stacking sequence and Mn/X antisite disorder. This fact inspires our experimental study of the ‘tweaking knobs’ that would stabilize the net spin polarization and higher critical temperatures in these materials.

Various stacking sequences can be generated by interlacing septuple (MnX₂Te₄) and quintuple (X₂Te₃) layers. The magnetic order changes as a result of varying interlayer magnetic exchange coupling and strong magnetocrystalline anisotropy, for instance, from AFM in MnBi₂Te₄ to a ferromagnetic-like behavior in MnBi₆Te₁₀ and MnBi₈Te₁₃. Besides the stacking order, a more subtle factor – antisite Mn/X disorder – influences the long-range magnetic order greatly. This phenomenon is very strong in Mn_1-xSb_2+yTe₄ as it shifts the Curie temperature of a ferrimagnetic-to-paramagnetic transition between 27 and 46 K while x changes only on the scale of 0.05–0.1. We elucidate the Mn/X intermixing by single-crystal X-ray and neutron powder diffraction and link these results to the crystal-growth procedure. We are looking for a greater connection between crystal growth, structure and magnetic properties that would allow to tailor-make an MTI with a desired magnetic order.