H Fukumura

Synthesis and Characterization of Mn-Doped BiFeO_3 Nanoparticles

BiFeO3 is a multiferroic material showing antiferromagnetic ordering and ferroelectric behavior simultaneously. Here, Mn-doped BiFeO3 nanoparticles were synthesized up to 10% of Mn composition by a sol-gel process. The samples showed high crystallinity with no secondary phase up to 2% of Mn doping. A phonon peak at 1250 cm−1 in undoped BiFeO3 showed anomalous intensity enhancement in the magnetically ordered phase below TN = 643 K due to a spin-phonon coupling. This behavior was less pronounced in the Mn-doped samples, suggesting a suppression of magnetic ordering between Fe spins by Mn doping.

Optical studies on GaN-based spintronics materials

Structural properties of GaN layers doped with magnetic impurities (Mn, Cr) were investigated by Raman scattering. In the case of Mn-doping, an impurity mode was observed at around 585 cm−1 with concentrations of Mn up to 1–2%. This mode was assigned to a local vibrational mode of Mn substituting the Ga site, and interpreted as spectral evidence of a uniform solid solution in the samples. For Cr-doping, good crystalline quality was also confirmed up to [Cr] = 3–5%. Cr is more miscible in GaN than Mn. Resonance enhancement of LO phonon signal was observed in GaCrN layers when excited by a deep UV laser at 266 nm (4.7 eV). This indicates generation of photo-carriers.

Spin-coupled phonons in multiferroic YbMnO3 epitaxial films by Raman scattering

YbMnO3 epitaxial thin films were investigated by Raman scattering in the temperature range of 15-300K. We successfully observed some phonon modes (5A1+6E2) at 15K with mode assignment based on polarized Raman study. Furthermore, an E2 mode at 253 cm-1 which affected Mn-O-Mn bond angles showed anomalous temperature variation in frequency at temperature below ~80K. It suggested a spin-phonon coupling that occurred below TN.