Muon spin relaxation study of exchange biased Co/CoO

The local magnetic properties of an exchange biased Co/CoO nanoparticle system have been examined using zero-field muon spin relaxation (ZF-μSR) spectroscopy. A direct measure of magnetic domains experiencing the exchange coupling due to the ferromagnetic core and antiferromagnetic shell of the Co nanoparticles is given by the observation of an oscillating signal in the field-cooled ZF-μSR spectra that is a clear and unambiguous signature of static magnetic correlations. Excellent agreement between the local field determined from the μSR oscillation and the bulk exchange bias loop shift confirms this interpretation. Furthermore, the unique line shape and increase in moment fluctuation rate on cooling the exchange biased sample are strong indications that spin-density-wave-like behavior is associated with setting the exchange bias in this system. This static field oscillation is likely due to the twisted moment directions at the interface.