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

Abstract: 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$\mathrm{2}$Te$\mathrm{3}$, due to the proximity of FePS$\mathrm{3}$ (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.