Particle size dependence of exchange-bias and coercivity in CuO nanoparticles

For CuO nanocrystals of size 6.6–37 nm, the exchange bias Heb and coercivity Hc are measured at 5 K in zero-field-cooled (ZFC) and field-cooled (FC at 50 kOe) samples and their variations investigated as a function of particle size D. The similar 1/D variations observed for the difference coercivity ΔHc=Hc(FC)−Hc(ZFC) and the interfacial exchange energy Δσ=HebMfD are discussed in terms of the ferromagnetic magnetization Mf being produced by the uncompensated surface Cu2+ spins in the otherwise antiferromagnetically ordered CuO nanoparticles. This leads to the observation that the experimentally measured ΔHc provides a good measure of Δσ in nanoparticle systems, with Heb/ΔHc varying as 1/MfD.