Coherent ac spin current transmission across an antiferromagnetic CoO insulator

TL;DR

This study demonstrates that a coherent GHz ac spin current can transmit through an antiferromagnetic CoO insulator, driven by thermal magnons, enabling coherent spin precession in an adjacent ferromagnetic layer.

Contribution

It provides experimental evidence that coherent GHz ac spin currents can pass through an AFM insulator via thermal magnons, resolving a key question in spintronics.

Findings

Coherent GHz ac spin current transmits across CoO insulator.

Thermal magnons, not evanescent spin waves, mediate the transmission.

Coherent spin precession is driven in the ferromagnetic layer after transmission.

Abstract

The recent discovery of spin-current transmission through antiferromagnetic (AFM) insulating materials opens up unprecedented opportunities for fundamental physics and spintronics applications. The great mystery currently surrounding this topic is: how could THz AFM magnons mediate a GHz spin current? This mis-match of frequencies becomes particularly critical for the case of coherent ac spin-current, raising the fundamental question of whether a GHz ac spin-current can ever keep its coherence inside an AFM insulator and so drive the spin precession of another FM layer coherently? Utilizing element- and time-resolved x-ray pump-probe measurements on Py/Ag/CoO/Ag/Fe75Co25/MgO(001) heterostructures, we demonstrate that a coherent GHz ac spin current pumped by the permalloy (Py) ferromagnetic resonance (FMR) can transmit coherently across an antiferromagnetic CoO insulating layer to drive a coherent spin precession of the FM Fe75Co25 layer. Further measurement results favor thermal magnons rather than evanescent spin waves as the mediator of the coherent ac spin current in CoO.