Nonreciprocal Transmission of Surface Acoustic Waves in CoFeB/Si₃N₄/CoFeB Heterostructures with Noncollinear Layer Magnetizations

We explore an innovative approach to create an acoustic isolator, which is based on the interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a hybrid made out of a piezoelectric crystal and a ferromagnetic/nonmagnetic/ferromagnetic thin film. Our experimental study confirms the predictions of a recent theoretical work [L. Ushii, Phys. Rev. Appl. 22, 034046 (2024)] in which the noncollinear magnetization configuration has been identified as the state of the potentially largest SAW-SW coupling with a sufficiently large nonreciprocal SW dispersion giving rise to a large nonreciprocal SAW transmission. The noncollinear configuration of the two magnetic layers is stabilized by magnetic in-plane anisotropies that are aligned perpendicular to each other and were induced by oblique sputter deposition. For our samples with a 150 μm long CoFeB (22 nm)/Si 3 N 4 (5 nm)/CoFeB (22 nm) thin film, we demonstrate a large isolation of more than 25 dB at 2–3 GHz and reasonable robustness in the working characteristics against deviations in the magnetic anisotropy and external field directions.