Nonreciprocal magnetoacoustic waves in synthetic antiferromagnets with Dzyaloshinskii-Moriya interaction

Abstract

The interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a piezoelectric/magnetic thin film heterostructure yields potential for the realization of novel microwave devices and applications in magnonics. In the present work, we investigate the SAW-SW interaction in a Pt/Co(2 nm)/Ru(0.85 nm)/Co(4 nm)/Pt synthetic antiferromagnet (SAF) composed of two ferromagnetic layers with different thicknesses separated by a thin nonmagnetic Ru spacer layer. Because of the combined presence of interfacial Dzyaloshinskii-Moriya interaction (iDMI) and interlayer dipolar coupling fields, the optical SW mode shows a large nondegenerate dispersion relation for oppositely propagating SWs. Due to SAW-SW interaction, we observe nonreciprocal SAW transmission in the piezoelectric/SAF hybrid device. The equilibrium magnetization directions of both Co layers are manipulated by an external magnetic field to set a ferromagnetic, canted, or antiferromagnetic configuration. This has a strong impact on the SW dispersion, its nonreciprocity, and SAW-SW interaction. The experimental results are in agreement with a phenomenological SAW-SW interaction model, which considers the interlayer exchange coupling, iDMI, and interlayer dipolar coupling fields of the SWs.

Giant Surface Acoustic Wave Nonreciprocity with Low Magnetoacoustic Insertion Loss in CoFeB/Ru/CoFeB Synthetic Antiferromagnets

“…The magnetoacoustic SAW transmission Δ S ij and SW resonance frequency f are simulated with the model described in refs and . The model is based on calculating the equilibrium macrospin directions ϕ 0 l , magnetoacoustic driving fields, and magnetization dynamics defined by coupled linearized Landau–Lifshitz–Gilbert equations.…”

“…We expect a large Δ S ± if, in addition to a large SAW-SW coupling, the SAW and SW dispersion match and the SW dispersion shows a large nonreciprocity. Because the magnetoelastic coupling efficiency between a Rayleigh SAW and optical SW mode in the antiferromagnetic (AFM) configuration of the SAF is at maximum at |ϕ 0 l | = 45, 135°, , we use for the simulations ϕ H = 45°. The optical SW mode in the Damon–Eshbach geometry (ϕ 0 l = ±90°) and AFM configuration of the SAF is illustrated in Figure a.…”

“…Simulations of Δ S ± in dependence of the magnetic layer thicknesses d A and d B show a large nonreciprocity Δ S ± / l f > 100 dB/mm for (i) symmetrical and (ii) asymmetrical SAFs, as depicted in Figure c. The simulations are based on prior studies. , Here, we fabricated a CoFeB(16)/Ru(0.55)/CoFeB(5) asymmetrical SAF (all thicknesses are given in nm) since the large Δ S ± is not sensitive against moderate variations in the magnetic properties for this thickness configuration. We observe indeed a giant surface acoustic wave transmission nonreciprocity of Δ S ± in conjunction with a very low magnetoacoustic insertion loss IL Δ in the experiment.…”

“…Recently, nonreciprocal SAW transmission has been demonstrated for acoustic delay lines loaded with magnetic thin-film systems in several reports. − The basic nonreciprocal magnetoacoustic device is an acoustic isolator, which shows for one external magnetic field magnitude μ 0 H Δ S ± in the ideal case a low insertion loss IL = IL 0 + IL Δ in the forward direction and a large transmission nonreciprocity Δ S ± = |Δ S 21 – Δ S 12 |, resulting in a low transmission in the reverse direction. , As depicted in Figure b, IL 0 denotes the insertion loss of the acoustic delay line in the absence of SAW-SW interaction (large magnetic fields), IL Δ is the insertion loss due to magnetoacoustic interaction in the direction of propagation, which is intended to be low loss, and Δ S ij =21,12 is the background-corrected SAW transmission magnitude Δ S ij (see eq ) of SAWs with wave vectors k + and k – . Two different mechanisms are causing nonreciprocal SAW transmission.…”

“…First, the SAW-SW coupling mechanism itself is already nonreciprocal, because of a helicity mismatch between the SAW-induced magnetoacoustic driving fields and the fixed chirality of the magnetization precession. ,− We name this mechanism the SAW-SW helicity mismatch effect (HME). Second, interfacial Dzyaloshinskii–Moriya interaction (iDMI) and interlayer dipolar coupling (IDC) in magnetic multilayers have been exploited to obtain a nonreciprocal SW dispersion f ( k ) which cause a nonreciprocal SAW transmission. − ,, Magnetic multilayers are at the moment the most promising candidates for efficient magnetoacoustic isolators because large Δ S ± and low IL Δ are not restricted to the thin-film limit and can be obtained for configurations of maximum SAW-SW interaction. Nevertheless, a giant nonreciprocity Δ S ± / l f > 100 dB/mm in conjunction with a low insertion loss IL Δ / l f < 1 dB/mm has so far not been demonstrated experimentally.…”