Spin Seebeck effect and ballistic transport of quasi-acoustic magnons in room-temperature yttrium iron garnet films

TL;DR

This study demonstrates that quasi-acoustic magnons in room-temperature yttrium iron garnet films propagate ballistically over a fixed length, significantly advancing understanding of thermal magnon transport in magnetic insulators.

Contribution

It provides experimental evidence for ballistic transport of quasi-acoustic magnons in YIG films at room temperature, highlighting their linear dispersion and fixed propagation length.

Findings

Magnons move ballistically with constant group velocity.

Propagation length is independent of film thickness.

Ballistic transport supports quasi-acoustic magnon behavior.

Abstract

We studied the transient behavior of the spin current generated by the longitudinal spin Seebeck effect (LSSE) in a set of platinum-coated yttrium iron garnet (YIG) films of different thicknesses. The LSSE was induced by means of pulsed microwave heating of the Pt layer and the spin currents were measured electrically using the inverse spin Hall effect in the same layer. We demonstrate that the time evolution of the LSSE is determined by the evolution of the thermal gradient triggering the flux of thermal magnons in the vicinity of the YIG/Pt interface. These magnons move ballistically within the YIG film with a constant group velocity, while their number decays exponentially within an effective propagation length. The ballistic flight of the magnons with energies above 20K is a result of their almost linear dispersion law, similar to that of acoustic phonons. By fitting the time-dependent LSSE signal for different film thicknesses varying by almost an order of magnitude, we found that the effective propagation length is practically independent of the YIG film thickness. We consider this fact as strong support of a ballistic transport scenario - the ballistic propagation of quasi-acoustic magnons in room temperature YIG.