Tuning Fermi level in ferromagnetic MnSb₂Te₄ by hydrogenation.

Recent realizations of quantum anomalous Hall (QAH) state made it apparent that when long-range ferromagnetism and band topology combine the problem of bulk conduction can become particularly acute. In a newly discovered van der Waals topological magnets of the MnBi₂Te₄ class consisting of antiferromagetically (AFM) coupled septuple layers (SL) of atoms with ferromagnetic (FM) coupling (with T_C ~ 13 K) within each SL the overall order is antiferromagnetic with Neél temperature T_N ~ 25 K.  Recently, a FM MnSb₂Te₄ (MST) with a much higher T_C was realized, where the overall ferrimagnetic coupling is thought to be induced by the abundance of Mn_Sb antisites. Here we investigate flux-grown MST single crystals with Tc ~ 30 K. In addition to high hole density (10²⁰ cm^-3 range) this system also features a non-equilibrium magnetic relaxation promoted by the randomly distributed antisites, with two distinct relaxation rates. We show that ionic hydrogen [1] can move the Fermi level into the bulk gap to access the surface/edge states. We will discuss how the magnetic relaxation is affected by hydrogen tuning and how it can be stabilized towards QAH.