Manipulating Spin-Lattice Coupling in Layered Magnetic Topological Insulator Heterostructure $via$ Interface Engineering

Induced magnetic order in a topological insulator (TI) can be realized either by depositing magnetic adatoms on the surface of a TI or engineering the interface with epitaxial thin film or stacked assembly of two-dimensional (2D) van der Waals (vdW) materials. Herein, we report the observation of spin-phonon coupling in the otherwise non-magnetic TI Bi${}_2$Te${}_3$, due to the proximity of FePS${}_3$ (an antiferromagnet (AFM), $T_{\mathrm{N}}$ $\sim$ 120 K), in a vdW heterostructure framework. Temperature-dependent Raman spectroscopic studies reveal deviation from the usual phonon anharmonicity originated from spin-lattice coupling at the Bi${}_2$Te${}_3$/FePS${}_3$ interface at/below 60 K in the peak position (self-energy) and linewidth (lifetime) of the characteristic phonon modes of Bi${}_2$Te${}_3$ (106 cm${}^{-1}$ and 138 cm${}^{-1}$) in the stacked heterostructure. The Ginzburg-Landau (GL) formalism, where the respective phonon frequencies of Bi${}_2$Te${}_3$ couple to phonons of similar frequencies of FePS${}_3$ in the AFM phase, has been adopted to understand the origin of the hybrid magneto-elastic modes. At the same time, the reduction of characteristic $T_{\mathrm{N}}$ of FePS${}_3$ from 120 K in isolated flakes to 65 K in the heterostructure, possibly due to the interfacial strain, which leads to smaller Fe-S-Fe bond angles as corroborated by computational studies using density functional theory (DFT). Besides, inserting hexagonal boron nitride within Bi${}_2$Te${}_3$/FePS${}_3$ stacking regains the anharmonicity in Bi${}_2$Te${}_3$. Controlling interfacial spin-phonon coupling in stacked heterostructure can have potential application in surface code spin logic devices.