Nonreciprocal Magnetoacoustic Waves in Dipolar-Coupled Ferromagnetic Bilayers

Abstract

We study the interaction of surface acoustic waves (SAWs) with spin waves (SWs) in a Co 40 Fe 40 B 20 /Au/Ni 81 Fe 19 system composed of two ferromagnetic layers separated by a nonmagnetic Au spacer layer. Because of interlayer magnetic dipolar coupling between the two ferromagnetic layers, a symmetric and an antisymmetric SW mode form, which both show a highly nondegenerate dispersion relation for oppositely propagating SWs. Due to magnetoacoustic SAW-SW interaction, we observe highly nonreciprocal SAW transmission in the piezoelectric-ferromagnetic hybrid device. We experimentally and theoretically characterize the magnetoacoustic wave propagation as a function of frequency, wave vector, and external magnetic field magnitude and orientation. Additionally, we demonstrate that the nonreciprocal SW dispersion of a coupled magnetic bilayer is highly tuneable and not limited to ultrathin magnetic films, in contrast to the nonreciprocity induced by the interfacial Dzyaloshinskii-Moriya interaction. Therefore, magnetoacoustic coupling in ferromagnetic multilayers provides a promising route towards building efficient acoustic isolators.

Forward volume magnetoacoustic spin wave excitation with micron-scale spatial resolution

“…For the sake of simplicity, we neglect non-magnetoelastic interaction, like magnetorotation coupling 12,22,44 , spin-rotation coupling [45][46][47] or gyromagnetic coupling 48 . In contrast to previous magnetoacoustic studies 10,12,20,22,23,42,49 where the equilibrium magnetization direction was aligned in the plane of the magnetic film (θ 0 = 90°), the strain component ε zz results in a modified driving field for geometries with θ 0 = 90°.…”

“…In contrast to previous magneotoacoustic studies performed with conventional IDTs 10,12,20,22,23,31,42,49 , here we use "tapered" or "slanted" interdigital transducers (TIDTs) [52][53][54][55] to characterize SAW-SW interaction in three different magnetic thin micro-stripes in one run. Although the fingers of the TIDT are slanted, the SAW propagates dominantly parallel to the x-axis in Fig.…”

“…High flexibility in the design of these devices is possible since the properties of the SWs can be varied in a wide range of parameters. For instance, the SW dispersion can be reprogrammed by external magnetic fields or electrical currents 15,16 and more complex design of the magnet geometry 17,18 or use of multilayers 14,[19][20][21] allow for multiple dispersion branches with potentially large nonreciprocal behavior. Vice versa, SAW-SW interaction can be also used as an alternative method to characterize magnetic thin films, SWs, and SAWs 12,20,22,23 .…”

“…For instance, the SW dispersion can be reprogrammed by external magnetic fields or electrical currents 15,16 and more complex design of the magnet geometry 17,18 or use of multilayers 14,[19][20][21] allow for multiple dispersion branches with potentially large nonreciprocal behavior. Vice versa, SAW-SW interaction can be also used as an alternative method to characterize magnetic thin films, SWs, and SAWs 12,20,22,23 . Design of future magnetoacoustic devices can benefit from the fact that SAW technology is well-developed and already employed in manifold ways in our daily life [24][25][26][27] .…”

Progress on nonreciprocity of acoustic metamaterials

“…(b) Dispersion curve of the multilayer structure. (b) Close-ups of the codirectional anticrossing points 最近，自旋波的非互易性被大量研究 [66,67] 。自旋波是序磁性体中相互作用的自旋体系由于各 种激发作用引起的集体运动。由于自旋进动总是沿一个方向，因此在施加磁场的情况下，自旋波 是非互易的。那么，能否可以利用自旋波的非互易性和磁弹性耦合作用实现声波的非互易性呢？ Verba 等人 [68] [33] 通过选择合适的多层结构材料(LiNbO3-FeGaB-Al2O3-FeGaB)，使得系统 的隔离度达到 48.4-dB ， 实现了声波较强的非互易性。最近，Küß 团队 [34] exhibiting the rectifying effect on thermal energy are achieved in nonlinear lattices. Similarly, the nonreciprocity in photonic systems can be realized by many mechanisms, such as the nonlinearity of optical medium and the mode conversion.…”