# Exceptional points in classical spin dynamics

## TL;DR

This paper explores how exceptional points in non-Hermitian spin systems can be used to achieve non-reciprocal, chiral spin transmission, advancing potential applications in spintronics and magnonics.

## Contribution

It demonstrates the use of topological properties of exceptional points to realize non-reciprocal spin transport in non-Hermitian spin systems, a novel approach in spintronics.

## Key findings

- Encircling EPs leads to non-reciprocal spin dynamics.
- Identified optimal parameters for high-efficiency asymmetric spin filtering.
- Proposed a platform for non-reciprocal spin devices.

## Abstract

Non-conservative physical systems admit a special kind of spectral degeneracy, known as exceptional point (EP), at which eigenvalues and eigenvectors of the corresponding non-Hermitian Hamiltonian coalesce. Dynamical parametric encircling of the EP can lead to non-adiabatic evolution associated with a state flip, a sharp transition between the resonant modes. Physical consequences of the dynamical encircling of EPs in open dissipative systems have been explored in optics and photonics. Building on the recent progress in understanding the parity-time (PT)-symmetric dynamics in spin systems, we use topological properties of EPs to implement chiral non-reciprocal transmission of a spin through the material with non-uniform magnetization, like helical magnet. We consider an exemplary system, spin-torque-driven single spin described by the time-dependent non-Hermitian Hamiltonian. We show that encircling individual EPs in parameter space results in non-reciprocal spin dynamics and find the range of optimal protocol parameters for high-efficiency asymmetric spin filter based on this effect. Our findings offer a platform for non-reciprocal spin devices for spintronics and magnonics.

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Source: https://tomesphere.com/paper/1903.09729