Nanomechanical Measurements of an Antiferromagnetic Topological Insulator*

The antiferromagnetic topological insulator Mn(Bi_1-xSb_x)₂Te₄ exhibits an ideal platform to study exotic topological phenomena and magnetic properties. The transport signatures of the magnetic phase transitions in the MnBi₂Te₄ family material have been well studied. However, their mechanical properties and magneto-mechanical coupling, in particular, have not been explored in the magnetic topological insulator system. In this work, we use nanoelectromechanical systems to study the intrinsic magnetism in Mn(Bi_1-xSb_x)₂Te₄ thin flakes via their magnetostrictive coupling. We investigate mechanical resonance signatures of the magnetic phase transitions from antiferromagnetic (AFM) to canted antiferromagnetic (cAFM) to ferromagnetic phases versus magnetic field and temperature. The spin-flip transitions in MBST are revealed by frequency shifts of mechanical resonance. With temperatures going above T_N (~25 K) the transitions disappear in the resonance frequency map, consistent with transport measurements. We develop a model to correlate the mechanical frequency shifts with the spin canting states. Our study shows a quadratic relationship between resonance frequency and magnetization of the material and reveals a technique to study the phase transitions and magnetization of the magnetic topological insulator.