Defects in CoO in oxidized cobalt nanoparticles dominate exchange biasing and exhibit anomalous magnetic properties

Defects can vastly modify the optical, electrical, mechanical, and magnetic properties of materials. In this paper, we report an investigation of the magnetic properties of moments associated with defects in CoO in partially and fully oxidized colloidal Co nanoparticles. The defect moments freeze at low temperature and have a distribution of melting temperatures, in which most melt below 500.3em0exK, but exchange biasing (EB) generated by the CoO lattice stabilizes the Co core moments in the partially oxidized sample up to 1700.3em0exK [J. B. Tracy, D. N. Weiss, D. P. Dinega, and M. G. Bawendi, Phys. Rev. B 72, 064404 (2005)]. By switching the polarity of the biasing field at intermediate temperatures during cooling, we show that the defect moments dominate EB at low temperature and exhibit a thermal memory effect. Selected cooling sequences show that the exchange shift (HEB) may change sign as the sample is heated and demonstrate tuning of HEB and the coercivity. Thermal remanent magnetization measurements directly show how changes in the orientation of CoO defect moments cause the Co core moments to reorient. An investigation of the temporal stability of the defect moments is also reported.