  Optical control of the spin degree of freedom is often desired in application
of the spin technology. Here we report spin-rotational excitations observed
through inelastic light scattering of the hexagonal LuMnO3 in the
antiferromagnetically (AFM) ordered state. We propose a model based on the
spin-spin interaction Hamiltonian associated with the spin rotation of the Mn
ions, and find that the spin rotations are angularly quantized by 60, 120, and
180 degrees. Angular quantization is considered to be a consequence of the
symmetry of the triangular lattice of the Mn-ion plane in the hexagonal LuMnO3.
These angularly-quantized spin excitations may be pictured as isolated flat
bubbles in the sea of the ground state, which may lead to high-density
information storage if applied to spin devices. Optically pumped and detected
spin-excitation bubbles would bring about the advanced technology of optical
control of the spin degree of freedom in multiferroic materials.
