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

For CuO nanocrystals of size 6.6-37 nm, the exchange bias H-eb and coercivity H-c are measured at 5 K in zero- field-cooled (ZFC) and field-cooled (FC at 50 kOe) samples and their variation,, investigated as a function of particle size D. The similar 1/D variations observed for the difference coercivity DeltaH(c) = H-c(FC) - H-c(ZFC) and the interfacial exchange energy Deltasigma = HebMfD are discussed in terms of the ferromagnetic magnetization M-f being produced by the uncompensated surface Cu2+ spins in the otherwise antiferromagnetically ordered CuO nanoparticles. This leads to the observation that the experimentally measured DeltaH(c) provides a good measure of Deltasigma in nanoparticle systems, with H-eb/DeltaH(c) varying as 1/MfD. (C) 2003 Elsevier Ltd. All rights reserved.