Critical size for exchange bias in ferromagnetic-antiferromagnetic particles

3 pags. ; 3 figs.We present a study of the magnetic properties of oxidized Co nanoparticles with an average grain size of 3 nm, embedded in an amorphous Al₂O₃ matrix. These nanoparticles can be considered as imperfect Co-core CoO-shell systems. Magnetization measurements after magnetic field cooling show a vertical shift of the hysteresis loop, while no exchange bias is observed. With a simple model, we show that there is a critical grain size for hybrid ferromagnetic-antiferromagnetic particles, below which exchange bias is absent for any ratio of ferromagnetic and antiferromagnetic constituents. The reason is that the interfacial exchange energy dominates over other energies in the system due to a large surface-to-volume ratio in the nanoparticles.One of the authors (J.M.)acknowledges an I3P fellowship from the CSIC and the European social fund. This work is supported by the Fund for Scientific Research—Flanders (FWO), the Flemish Concerted Action (GOA/2004/02), the Belgian Interuniversity Poles of Attraction (IAP/P5/01), and the European Community’s Human Potential NanoCluster (HPRN-CT-2002-00328) programs. K.T. is a Postdoctoral Fellow of the FWO.Peer reviewe