Emergent magnonic singularities in anti parity-time symmetric synthetic antiferromagnets

Abstract

We study the impact of the transition across the anti-parity-time (anti- $\mathcal{PT}$ ) symmetry breaking point on the low-energy spin excitations in a synthetic antiferromagnet (SyAFM) that consists of two magnetic layers coupled antiferromagnetically via the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction. When varying the interlayer (RKKY) coupling strength a degeneracy point (exceptional point) is reached beyond which the system enters the anti- $\mathcal{PT}$ symmetry-broken phase. This phase is marked by the emergence of a finite net magnetization and low-lying excitations beyond the Goldstone modes in the anti- $\mathcal{PT}$ symmetry-preserved phase. For systems hosting an interfacial Dzyaloshinskii–Moriya interaction, we find a bound state in the continuum with a maximal coherent superposition of SyAFM excitations without any radiation. Analytical results for linearized models are corroborated with full numerical micromagnetic simulations endorsing the robustness and generalities of the theory predictions.

Keywords

• spin waves
• synthetic antiferromagnet
• parity-time symmetry
• spontaneous symmetry breaking
• magnonic singularities