Density-functional study of oxidation at the Mn-Co interface

By using first-principles plane-wave pseudopotential code, with the Perdew-Wang 91 form of the density functional, we study the Mn-Co magnetic coupling of ultrathin Mn films on fcc Co(0 0 1) substrates and the effect of oxygen on this Mn-Co coupling. First, in agreement with experiments and recent calculations we find the MnCo (2 × 2) alloy overlayer with ferromagnetic coupling of all spins as the most stable structure among several other natural candidates. The magnetic coupling remains however unchanged after deposition of a full oxygen overlayer, leaving thus unexplained the experimentally observed Mn antiferromagnetic moment alignment. For more complicated structures, however, indications of antiferromagnetic stabilization appear. Particularly, the antiferromagnetic ground state of a model two-layer (Mn,Co)O oxide slab with rock-salt structure on Co(0 0 1) is found more stable by 0.14 eV per elementary cell than the ferromagnetic state. These results display clear indication that superexchange in bimetallic Mn-O-Co groups can stabilize the Mn-Co antiferromagnetic coupling.