Exchange biased anisotropic magnetoresistance in Co/CoO bilayer*

We measured the anisotropic magnetoresistance of a Co(11nm)/CoO bilayer in exchange biased and unbiased states. The bilayer was fabricated on a-Al$_{2}$O$_{3}$ substrate maintained at 300$^{o}$C by molecular beam epitaxy at a base pressure of 10$^{-11}$mbar. $\theta $-2$\theta $ X-ray diffraction scans reveal hcp (001) texture of the Co film. No peak associated with the naturally formed CoO top layer was identified. Small angle X-ray reflectivity scans yield the Co and CoO thicknesses as 11 and 2.4nm, respectively. Exchange bias was obtained from field cooled magnetoresistance measurements at various temperatures carried out in a closed cycle cryostat. Exchange bias varies quasi linearly with temperature and vanishes at the blocking temperature, $T_{B}$=97K. The latter is less than 2/3 of the bulk N\'{e}el temperature allowing to estimate the T=0 antiferromagnetic correlation length of CoO to be 1.84 nm in accordance with the geometrical confinement.