Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling

Abstract

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 magneto-rotation coupling application.

Nonreciprocal surface magnetoelastic dynamics

“…1, focusing on both the magnetoelastic and magnetorotation couplings between them. The dielectric substrate is assumed to be semi-infinite; it is usually not magnetic, since nonmagnetic substrates are used in the recent experiments, which can be GGG [13], MgO [24], or Pt [25]. We assume the thickness d [O(10 nm)] is much smaller than the decay length or wavelength λ of the SAWs (>100 nm), while the width w is comparable to λ. Experimentally, such a geometry with thin cobalt or nickel nanowires on top of a thin YIG film was used to realize the pumping of short-wavelength spin waves [30,32,53].…”

“…We assume the thickness d [O(10 nm)] is much smaller than the decay length or wavelength λ of the SAWs (>100 nm), while the width w is comparable to λ. Experimentally, such a geometry with thin cobalt or nickel nanowires on top of a thin YIG film was used to realize the pumping of short-wavelength spin waves [30,32,53]. We restrict the magnetization to be parallel to the substrate surface but allow an angle between it and the nanowireẑ direction [24]. To this end, we assume a sufficiently strong magnetic field −H 0ẑ is applied, with an angle ϕ between z and the wire z direction, to saturate and control the direction of the wire magnetization with an equilibrium component ∼ − M sẑ and transverse components m x x + m y y (see the Appendix).…”

“…The magnetoelastic coupling Hamiltonian generally depends on the crystal symmetry of the material [54]. Here we adopt the simplest form that describes a wide class of material [11,24,25,55,56], which may be written as [11,54,55]…”

“…Very recently, the nonreciprocal surface phonon transmissions were observed when the phonons pass through the ultrathin extended ferromagnetic films in proximity to the piezoelectric substrate [24,25] and are explained by the magnetorotation or/and magnetoelastic couplings [26]. These indicate that the interaction between magnons and surface phonons is nonreciprocal (or chiral when emphasizing the symmetry), i.e., the magnons in the magnets can dominantly couple the traveling surface phonon prop-agating in one direction.…”

“…In this work we study the surface magnetoelastic coupling in the spin mechanical system and formulate the nonreciprocal dynamics [31,36,42] via the Green function method [49][50][51][52]. Rather than considering the extended magnetic film in which the edge effect is marginal [24][25][26], we focus on the thin nanomagnets with dominant edge effect. We show the uniform magnetization dynamics exerts rotating forces at the edges of the nanomagnet that causes the nonreciprocal interaction by the rotation-momentum locking of surface phonons.…”