Spectroscopic characterization of 2D van der Waals antiferromagnetic topological insulators MnBi₂Te₄(Bi₂Te₃)_n

The recent discovery of 2D van der Walls (vdW) magnetic topological insulators (TIs) has opened new avenues towards spintronics research from combining unique electronic properties arising from nontrivial band structures and the magnetic degree of freedom when magnetic ordering also establishes in the same material. MnBi₂Te₄(Bi₂Te₃)_n  represents an excellent example of this, being identified as a vdW TI with antiferromagnetic ordering for n=0,1,2. While emphasis has been placed into studying its electronic and magnetic properties using techniques like ARPES, transport, and magnetometry, a comprehensive detailed experimental spectroscopic characterization of these materials at low frequencies is missing. In this study we perform a series of low frequency measurements at varied temperatures and magnetic fields strengths and orientations that show clear antiferromagnetic modes in compounds with n=1 and n=2. The motion of the antiferromagnetic modes with the direction of application of the magnetic field reveal the anisotropic nature of the system, characterized by an easy magnetic symmetry axis and a corrugated hard plane that change slightly with the stoichiometry variation. Our results show how the transition temperature also varies between n=1 and n=2 compounds, indicating that the exchange interaction originating the antiferromagnetic order changes with the interlayer configuration.