Noncontact Spin Pumping by Microwave Evanescent Fields

Abstract

The angular momentum of evanescent light fields has been studied in nano-optics and plasmonics but not in the microwave regime. Here we predict noncontact pumping of electron spin currents in conductors by the evanescent stray fields of excited magnetic nanostructures. The coherent transfer of the photon to the electron spin is proportional to the g factor, which is large in narrow gap semiconductors and surface states of topological insulators. The spin pumping current is chiral when the spin susceptibility displays singularities that indicate collective states. However, 1D systems with linear dispersion at the Fermi energy, such as metallic carbon nanotubes, are an exception since spin pumping is chiral even without interactions.

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].…”

“…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. The unidirectional excitation of SAWs is important for acoustic device applications [29], which conventionally is achieved by metal electrodes on a piezoelectric crystal such that reflected SAWs constructively interfere with the source.…”

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.…”

Nonreciprocal surface magnetoelastic dynamics

“…With chiral coupling, the unidirectional traveling waves are excited by the nearby magnet in half-space [30][31][32]. The evanescent dipolar field or microwaves can realize noncontact (and chiral) spin pumping to the nearby conductors [42]. The traveling waves mediate a long-range nonreciprocal interaction between remote magnets and the spin accumulates at the edge of magnets by the non-Hermitian skin effect [43][44][45].…”

“…The surface acoustic waves exhibit rotation-momentum locking as well [23,48], from which their nonreciprocal coupling with magnons may be understood universally. 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.…”