We investigated the magnetic anisotropy energy of monatomic surface-step atoms in antiferromagnetic/ferromagnetic (AF/FM) epitaxial $\mathrm{Mn}/\mathrm{Co}$ bilayers grown on vicinal Cu(001) surfaces. The step-induced anisotropy of the $\mathrm{Co}/\mathrm{Cu}(001)$ films was quenched upon submonolayer Mn deposition, but a reentrant uniaxial surface anisotropy was observed for Mn thickness ($t_{\mathrm{Mn}}$) between 1 and 2 monolayers, which disappears for Mn thickness above 2 monolayers. In the $\mathrm{Mn}/\mathrm{Co}/\mathrm{Cu}(001)$ system, Mn films undergo a $t_{\mathrm{Mn}}$-dependent transition from FM to AF in the 1–2 Mn monolayer thickness range, which entails the coexistence of FM and AF Mn phases in the film. The observation of a sizeable uniaxial anisotropy exclusively in the Mn-thickness range of coexistence of the FM and AF phases points out the crucial role of the boundaries between FM and AF regions within the Mn film. A symmetry-breaking mechanism of a magnetic type, rather than a purely geometric one, is therefore proposed as the origin of the reentrant anisotropy.

• Received 1 August 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.037201