We describe novel features of the induced magnetic anisotropy in Co nanoclusters coupled with a CoO(111) layer. Individual cluster magnetism was studied using new microbridge superconducting quantum interference devices. Intrinsically, the Co clusters are single domains with an effective anisotropy constant $K_F\approx 1.5\times {10}^6\mathrm{erg}·{\mathrm{cm}}^{-3}$. A bistable state of the ferromagnetic-antiferromagnetic coupling is revealed, with a maximum bias systematically observed along $\mathrm{CoO}[10\overline{1}]$ and an interfacial coupling energy of $0.9\mathrm{erg}·{\mathrm{cm}}^{-2}$. The small bias observed in cluster assembly results from an averaging over the two opposite stable states.

• Received 28 February 2011

DOI:https://doi.org/10.1103/PhysRevLett.107.057204