Chiral Pumping of Spin Waves

Abstract

We report a theory for the coherent and incoherent chiral pumping of spin waves into thin magnetic films through the dipolar coupling with a local magnetic transducer, such as a nanowire. The ferromagnetic resonance of the nanowire is broadened by the injection of unidirectional spin waves that generates a non-equilibrium magnetization in only half of the film. A temperature gradient between the local magnet and film leads to a unidirectional flow of incoherent magnons, i.e., a chiral spin Seebeck effect.

Unidirectional Pumping of Phonons by Magnetization Dynamics

“…It can be measured by Brillouin light scattering [18], the spin-rotation coupling by fabricating a conductor on top of the acoustic medium [15,16], and other techniques [52]. The generation of unidirectionality by interference does not require a nonreciprocal coupling mechanism [25][26][27][28] but only an out-of-phase relation of the two fields at resonance. The phenomenon should be universal for many field propagation phenomena, such as exchange coupled magnetic nanowires and films [25] and reciprocally coupled magnons and waveguide photons [28].…”

“…When exciting a single magnetic nanowire by microwaves, we predict a passive wire induces the unidirectional phonon current and formation of standing waves in the region between two magnetic nanowires. This mechanism should also lead to a unidirectional spin Seebeck effect [26] generated by a temperature gradient between the magnetic and acoustic insulators [59]. Unidirectionality emerges here from a dynamical phase shift, rather than the purely geometrical interference employed by electrical unidirectional SAW generators [29,30].…”

“…1. Similar configurations on magnetic substrates led to the electrical detection of diffuse magnon transport [22,23] and discovery of nonreciprocal magnon propagation [24], i.e., the generation of a unidirectional spin current in half space [25,26]. Magnetic stray fields of the magnetization dynamics also generate chiral electron [27] and waveguide photon [28] transport.…”

Magnon trap by chiral spin pumping

“…Magnons, the elementary excitations of the magnetic order, carry an intrinsic angular momentum that can be utilized to transport information [7][8][9][10]. Dynamic dipolar stray fields emitted by ferromagnetic nanostructures can generate a unidirectional magnon current in a ferromagnetic film or conductors in its proximity by "chiral spin pumping" [11][12][13][14][15]. Magnons can propagate over centimeters [16] in magnetic insulators such as yttrium iron garnet (YIG) without Joule heating.…”

“…In this work, we theoretically demonstrate trapping of waves on short length scales by the unique functionalities of chiral pumping [3,4,14,23]. We first focus on a device consisting of two magnetic transducers in the form of nanowires on top of a high-quality thin film of a magnetic insulator such as YIG ( Fig.…”