Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling

A fundamental form of magnon-phonon interaction is an intrinsic property of magnetic materials, the "magnetoelastic coupling."This form of interaction has been the basis for describing magnetostrictive materials and their applications, where strain induces changes of internal magnetic fields. Different from the magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may induce surface magnons via rotational motion of the lattice in anisotropic magnets. However, a signature of this magnon-phonon coupling mechanism, termed magneto-rotation coupling, has been elusive. Here, we report the first observation and theoretical framework of the magneto-rotation coupling in a perpendicularly anisotropic film Ta/CoFeB(1.6 nanometers)/MgO, which consequently induces nonreciprocal acoustic wave attenuation with an unprecedented ratio of up to 100% rectification at a theoretically predicted optimized condition. Our work not only experimentally demonstrates a fundamentally new path for investigating magnon-phonon coupling but also justifies the feasibility of the magnetorotation coupling application.

磁振子 - 声子相互作用的一种基本形式是磁性材料的一种固有特性，即“磁弹耦合”。这种相互作用形式一直是描述磁致伸缩材料及其应用的基础，在这些应用中，应变会引起内部磁场的变化。与磁弹耦合不同，40多年前就有人提出，在各向异性磁体中，表面声波可能通过晶格的旋转运动诱导表面磁振子。然而，这种被称为磁旋耦合的磁振子 - 声子耦合机制的特征一直难以捉摸。在此，我们报道了在垂直各向异性薄膜Ta/CoFeB（1.6纳米）/MgO中磁旋耦合的首次观测结果和理论框架，这进而在理论预测的优化条件下诱导出非互易声波衰减，其整流比高达前所未有的100%。我们的工作不仅通过实验展示了研究磁振子 - 声子耦合的一种全新的基本途径，还证明了磁旋耦合应用的可行性。