Optical activity of a single MnAs cluster: Birefringence or Kerr effect

We have grown In0.54Ga0.46As:Mn/MnAs granular paramagnetic–ferromagnetic hybrid structures by metal–organic vapor-phase epitaxy. The MnAs clusters have a Curie temperature of about 320 K. We have studied the optical activity of individual ferromagnetic MnAs clusters embedded in the paramagnetic In0.54Ga0.46As:Mn matrix at room temperature by far-field depolarization measurements. A scanning near-field optical microscopy set-up in constant height mode (≈100nm above the sample surface) was used to achieve a high spatial resolution. Individual MnAs clusters rotate the linear polarization of the incoming light by almost 2∘2∘ in this reflection geometry. This optical activity was analyzed in terms of birefringence and polar Kerr effect and correlated with the structural and magnetic properties of the MnAs clusters as determined by ferromagnetic resonance measurements. The optical activity of the MnAs clusters turns out to be dominated by linear birefringence caused by the uniaxial symmetry of the hexagonal crystal structure of MnAs. The polar Kerr effect plays a minor role in this experiment.