Structural, electronic, and magnetic properties of single MnAs nanoclusters in GaAs

MnAs nanoclusters in GaAs were investigated with cross-sectional scanning tunneling microscopy. The topographic images reveal that the small clusters have the same zinc-blende crystal structure as the host material, while the larger clusters grow in a hexagonal crystal phase. The initial Mn concentration during molecular beam epitaxy growth has a strong influence on the size of the clusters that form during the annealing step. The local band structure of a single MnAs cluster is probed with scanning tunneling spectroscopy, revealing a Coulomb blockade effect that correlates with the size of the cluster. With a spin-sensitive tip, for the smaller clusters, superparamagnetic switching between two distinct states is observed at T=77 K. The larger clusters do not change their magnetic state at this temperature, i.e., they are superferromagnetic, confirming that they are responsible for the ferromagnetic behavior of this material at room-temperature.