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

Abstract

In this study, we optimize the nonreciprocal transmission of surface acoustic waves (SAWs) in a piezoelectric/ magnetic heterostructure loaded with a CoFeB(d A )/Ru(0.55 nm)/ CoFeB(d B ) synthetic antiferromagnet (SAF). The SAF is composed of two ferromagnetic layers separated by a thin nonmagnetic Ru spacer layer, providing strong antiferromagnetic interlayer exchange coupling. The optical spin wave (SW) mode shows a large nonreciprocal dispersion relation in conjunction with a narrow line width. Therefore, for oppositely propagating SAWs, the SAW-SW resonances can become fully separated. For the optimized CoFeB(16 nm)/Ru(0.55 nm)/CoFeB(5 nm) SAF, we demonstrate both, a giant nonreciprocal SAW transmission of ΔS ± /l f > 250 dB/mm and a very low magnetoacoustic insertion loss of IL Δ /l f < 1 dB/mm per length l f of the SAF in the acoustic path at frequencies above 5 GHz. These results yield the potential for the realization of novel microwave devices with nonreciprocal signal transmission, such as acoustic isolators. In addition, the experimental results are in excellent agreement with a phenomenological model that considers the dipolar fields of the SWs and takes the bilinear and biquadratic interlayer exchange coupling into account.

Wide-Band Nonreciprocal Transmission of Surface Acoustic Waves in Synthetic Antiferromagnets

“…1,24 Note that this mode was named out-of-plane polarized or optical SW mode in previous publications. 1,[24][25][26]36 Wide-band nonreciprocal SAW transmission requires a magnetic system for which the nonreciprocal SW dispersion matches the linear dispersion of the SAW…”

“…Note that piezoelectric LiNbO 3 is well-established for SAW applications 3 and CoFeB is well-suited for magnetoacoustic devices because of its low magnetic damping and reasonable large magnetoelastic coupling constants. 26 Moreover, since CoFeB has a relatively large M s ≈ 1280 kA m −1 , large c SW and large Δf ± ≠ 0 can be achieved for moderate magnetic layer thicknesses d. In Figure 2 For the optimization of a large wide-band SAW transmission nonreciprocity, further aspects must be taken into consideration. The coupling efficiency between the Rayleigh-type SAW and low-frequency SW mode in the AFM configuration of the SAF has a symmetry ∝ sin (ϕ 0…”

“…More details about the deposition parameters are presented in Section 1 in the Supporting Information. Because of the expected large SAW−SW interaction, 26 the SAF film has a low length of l f = 100 μm along the SAW propagation direction.…”

“…So far, the nonreciprocal transmission of SAWs has been limited to a relatively narrow frequency range (see Figure 1c), which depends on the line width of the SAW−SW resonances and is typically in the range of 10−1000 MHz. 1,20,26 This restriction results in limitations for possible technological applications, such as a limited operation band and the necessity to fabricate different structures for different frequency ranges or their fine-tuning by an external magnetic field. 1 However, in a recent theoretical study, Verba et al proposed wide-band nonreciprocal transmission of SAWs in SAFs with a tailored SW dispersion.…”

Magneto-acoustic coupling: Physics, materials, and devices

“…CoFeB/Pt [33] 磁弹耦合 6.77 界面DMI诱导的非互易, 隔离度27.9 dB/mm FeGaB/Al 2 O 3 /FeGaB [34] 1.435 6.87 偶极耦合诱导的非互易, 隔离度74 dB/mm CoFeB/Ru/CoFeB [36] 1.4 RKKY耦合诱导的非互易, 隔离度37 dB/mm Pt/Co/Ru/Co/Pt [37] 6.77 RKKY耦合和DMI诱导的非互易, 隔离度 3 dB/mm CoFeB/Ru/CoFeB [38] 5.08 RKKY耦合诱导的非互易, 隔离度250 dB/mm 磁传感器 FeCoSiB [39] 磁电耦合 0.148 SAW延迟线结构激发勒夫波, 10 Hz下 70 pT/Hz 1/2 的探测极限 FeCoSiB [40] 0.477 SAW谐振器结构激发勒夫波, 灵敏度 630.4 kHz/Oe 磁电天线 AlN/FeGaB [41] 磁电耦合 2.53 FBAR结构, 首次实验验证可行性, 增益 -18 dBi, 辐射效率0.4%…”