Reverse Doppler effect in backward spin waves scattered on acoustic waves

TL;DR

This study demonstrates the reverse Doppler effect in backward spin waves in a YIG film, caused by acoustic wave-induced periodic magnetic anisotropy modulation, showing frequency shifts opposite to conventional Doppler behavior.

Contribution

The paper reports the first observation of reverse Doppler effect in backward spin waves scattered by acoustic waves in a magnetostrictive material.

Findings

Backward spin waves exhibit frequency downshift when reflecting off approaching acoustic waves.

Frequency upshift occurs when reflecting from receding acoustic waves.

A traveling Bragg grating is formed by acoustic waves modulating magnetic anisotropy.

Abstract

We report on the observation of reverse Doppler effect in backward spin waves reflected off of surface acoustic waves. The spin waves are excited in a yttrium iron garnet (YIG) film. Simultaneously, acoustic waves are also generated. The strain induced by the acoustic waves in the magnetostrictive YIG film results in the periodic modulation of the magnetic anisotropy in the film. Thus, in effect, a travelling Bragg grating for the spin waves is produced. The backward spin waves reflecting off of this grating exhibit a reverse Doppler shift: shifting down rather than up in frequency when reflecting off of an approaching acoustic wave. Similarly, the spin waves are shifted up in frequency when reflecting from receding acoustic waves.