Localized Singlets and Ferromagnetic Fluctuations in the Dilute Magnetic Topological Insulator Sn${}_{0.95}$Mn${}_{0.05}$Te

Abstract

The development of long-range ferromagnetic (FM) order in dilute magnetic topological insulators can induce dissipationless electronic surface transport via the quantum anomalous Hall effect. We measure the magnetic excitations in a prototypical magnetic topological crystalline insulator, Sn${}_{0.95}$Mn${}_{0.05}$Te, using inelastic neutron scattering. Neutron diffraction and magnetization data indicate that our Sn${}_{0.95}$Mn${}_{0.05}$Te sample has no FM long-range order above a temperature of 2 K. However, we observe slow, collective FM fluctuations ($<$~70 $\mu$eV), indicating proximity to FM order. We also find a series of sharp peaks originating from local excitations of antiferromagnetically (AF) coupled and isolated Mn-Mn dimers with $J_{\mathrm{AF}}=460$~$\mu$eV\@. The simultaneous presence of collective and localized components in the magnetic spectra highlight different roles for substituted Mn ions, with competition between FM order and the formation of AF-coupled Mn-Mn dimers.