Quantum Field Theory Model for Spin-Based Devices Using 2-D van der Waals Materials

We explore the effects of layered geometries of two-dimensional (2D) quantum spin systems as a method to tune and control material properties for spintronic devices. We analyze the dispersion relation of a 2D quantum spin system with a shifted bilayer square lattice through the Linear Spin Wave (LSW) approximation of Quantum Field Theory (QFT). Inspired by recent interest in 2D van der Waals (vdW) magnetic materials, we consider both short-range interactions given by nearest-neighbor intralayer ferromagnetic and interlayer antiferromagnetic spin-exchange interactions and long-range dipole-dipole interactions. This case, along with lattice shifts, has not been treated in the literature via LSW theory. Computations show the gap in the frequencies for the two lowest-energy magnon modes at zero wavenumber that depends on the horizontal and vertical layer shifts. The frequency gap is attributed to the long-range dipole interaction with the gap becoming more sensitive to the horizontal layer shifts for small layer separation. Our general framework is suitable for numerical computations of a wide collection of 2D lattice models and presents an essential first step towards a comprehensive model that can incorporate quantum effects leading to quantifiable predictions of additional physical properties, e.g. spin coherence length and damping, of interest for engineering of spintronic devices.