Unidirectional Pumping of Phonons by Magnetization Dynamics

Abstract

We propose a method to control surface phonon transport by weak magnetic fields based on the pumping of surface acoustic waves (SAWs) by magnetostriction. We predict that the magnetization dynamics of a nanowire on top of a dielectric films injects SAWs with opposite angular momenta into opposite directions. Two parallel nanowires form a phononic cavity that at magnetic resonances pump a unidirectional SAW current into half of the substrate.

Nonreciprocal surface magnetoelastic dynamics

“…Although they are not circularly polarized even when the magnetization are, the elliptically polarized forces bring chirality in the mechanics. When ϕ = π/2 (ϕ = 0) with the magnetization perpendicular (parallel) to the wire, the force becomes linearly polarized (vanish), recovering our previous results [27]. Since the rotation direction of the SAWs is locked to their momenta [see Eq.…”

“…The coupling constant depends on the sign of momentum and generally show the nonreciprocity with |g |k| | = |g −|k| |. We see that the coupling tends to vanish when ϕ = 0, while when ϕ = π/2, there is no chirality as |g |k| | = |g −|k| | [27]. Considering ϕ ∈ (0, π/2), the complete chirality arises whenB ⊥ = cos ϕ c B (1 + b 2 )/a such that g |k| = 0, implying the critical angles ϕ c satisfy…”

“…Here we have assumed that the nanowire is sufficiently thin such that the displacements at its top and bottom surface are identical and hence have no net contribution to the magnetoelastic coupling. Classically we obtain the forces by F x,y (r) = δH c /δu x,y (r) [11,27], of the lth nanowire at the right edge (y = w/2 + y l )…”

“…(2) and the total forces depend on the length of the wire L, the excited SAW amplitude is independent of L since only the force density, i.e., the stress, plays a role [11]. A detailed description of the forces and how they excite the SAWs, for the perpendicular configuration, are given in our previous work [27]. We showed there that the forces arise at the left and right boundaries of a wire and exert the stress on the dielectric substrate that excite the SAWs.…”

“…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. The inverse process of acoustic pumping-the nonreciprocal pumping of phonon by magnetization dynamics-has not yet been experimentally reported and was theoretically considered by us [27]. There, interference with dynamical phase shift π between two remote magnetic nanowires that couples with the phonon reciprocally is responsible rather than a direct nonreciprocal magnonphonon coupling in the presence of one magnet [27].…”