Resonance Raman Study of Spin rotational excitations in hexagonal RMnO3 (R = rare earths, Y)

DIPC Seminars

In-Sang Yang
Department of Physics, Ewha Womans University
Donostia International Physics Center (In person seminar)
Deung-Jang Choi
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Resonance Raman Study of Spin rotational excitations in hexagonal RMnO3 (R = rare earths, Y)

We have observed spin rotational excitations in RMnO3 (R=Lu, Y) below the Neel ordering temperatures through resonant Raman scattering. We could assign all the spin excitation peaks in terms of Heisenberg spin interaction, and found that the spin rotational angles are predominantly 60, 120, 180 degrees commensurate with the triangular lattice of the Mn-ion spins. Unlike usual spin-wave excitations, this spin rotational excitations are limited to one triangular unit cell, thus costing relatively high excitation energy (~ 0.1 eV). Optically pumped and optically detected spin rotational excitations confined in a triangular cell can be a good candidate for future spin logic/memory devices. Temperature-dependent Raman spectra of the hexagonal LuMnO3 single crystal in cross polarization scattering geometry show broad Raman peaks near 197, 580, and 805 cm-1 of non-phononic in origin, which get stronger at lower temperatures below the Neel ordering temperature. Similar peaks, but at different wavenumbers were observed in singlecrystal YMnO3. These broad peaks below TN are found to be excited through the resonance with the Mn d-d transition by the incident red laser (~1.85 eV). Along with theoretical analyses of the spin-spin interaction Hamiltonian and the Raman selection rules, we could conclude that the spin excitation peaks are due to simultaneous rotation of all three Mn spins in one Mn-trimer in hexagonal RMnO3 at the antiferromagnetic state. Only special-angle rotations of the three Mn spins are allowed to preserve the triangular symmetry with the antiferromagnetic ordering, thus the spin rotation angles are quantized due to the triangular symmetry of the hexagonal RMnO3.