Controlling exchange bias in Co–CoOx nanoparticles by oxygen content

Abstract

We report on the occurrence of exchange bias on laser-ablated granular thin filmscomposed of Co nanoparticles embedded in an amorphous zirconia matrix.The deposition method allows one to control the degree of oxidation of theCo particles by tuning the oxygen pressure at the vacuum chamber (from2 × 10−5 to10−1 mbar). The nature of the nanoparticles embedded in the nonmagnetic matrix ismonitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM)CoOx, with a FM/AFM intermediate regime for which the percentage of the AFM phasecan be increased at the expense of the FM phase, leading to the occurrence ofexchange bias in particles of about 2 nm in size. For an oxygen pressure of about10−3 mbar the ratio between the FM and AFM phases is optimum with an exchange bias field ofabout 900 Oe at 1.8 K. The mutual exchange coupling between the AFM and FM is also atthe origin of the induced exchange anisotropy on the FM leading to high irreversiblehysteresis loops, and the blocking of the AFM clusters due to proximity to the FM phase.