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A van der Waals compound MnBi₄Te₇ belongs to the family of (Bi₂Te₃)_n(MnBi₂Te₄), (n = 0, 1, 2) heterostructures and is a candidate magnetic topological insulator [1]. It is the first magnetic material that features both, the intrinsic net magnetization and a band inversion. Static magnetic susceptibility (χ) and magnetization (M) measurements as a function of the applied field (H) on MnBi₄Te₇ single-crystals show an antiferromagnetic state at T_N = 13 K and a ferromagnetic-like hysteresis occurring upon cooling below 5 K [1]. We performed electron spin resonance (ESR) spectroscopy measurements in wide frequency and temperature ranges to explore the dynamic magnetic properties of MnBi₄Te₇. From high-frequency ESR measurements, we obtain evidence that MnBi₄Te₇ is an easy-axis type ferromagnet and ferromagnetic spin correlations persist up to T = 30 K on the time scale of an ESR experiment (10^-10 - 10^-11 s).

[1] Raphael C. Vidal et. al, Topological electronic structure and intrinsic magnetization in MnBi₄Te₇: a Bi₂Te₃-derivative with a periodic Mn sublattice, arXiv:1906.08394.