Rau, I. G.Baumann, S.Rusponi, S.Donati, F.Stepanow, S.Gragnaniello, L.Dreiser, J.Piamonteze, C.Nolting, F.Gangopadhyay, S.Albertini, O. R.Macfarlane, R. M.Lutz, C. P.Jones, B. A.Gambardella, P.Heinrich, A. J.Brune, H.2014-06-172014-06-172014-06-17201410.1126/science.1252841https://infoscience.epfl.ch/handle/20.500.14299/104458WOS:000336495800035Designing systems with large magnetic anisotropy is critical to realize nanoscopic magnets. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. We realized the maximum magnetic anisotropy for a 3d transition metal atom by coordinating a single Co atom to the O site of an MgO(100) surface. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts as well as slow relaxation of the Co atom's magnetization. This striking behavior originates from the dominating axial ligand field at the O adsorption site, which leads to out-of-plane uniaxial anisotropy while preserving the gas-phase orbital moment of Co, as observed with x-ray magnetic circular dichroism.text::journal::journal article::research article