Changfei Zhu

LATTICE VIBRATION FUNDAMENTALS IN NANOCRYSTALLINE ANATASE INVESTIGATED WITH RAMAN SCATTERING

A detailed nano-size and temperature-dependent Raman scattering study of the Eg(1) mode was carried out from 83 to 723 K for nanocrystalline anatase with sizes of 5.6, 8.6, and 19.4 nm. The Eg(1) Raman spectra were fitted and calculated on the basis of a combined model of both anharmonic coupling and phonon confinement. The temperature-dependent lattice vibration fundamentals are essentially the same for all these three nanocrystallites. Both the three- and four-phonon processes are needed to obtain excellent fitting, and the main contribution comes from the three-phonon processes. The anharmonic-decay-related phonon lifetime increases with decreasing nanocrystallite size, and the smaller nanoparticles have slower anharmonic decay. The phonon confinement has a comparative effect on both the linewidth broadening and the peak frequency shift, and it is responsible for a much shorter phonon lifetime at low temperatures for the 5.6 nm nanocrystallite.

Ultrasonic anomalies in La0.67Ca0.33Mn1-xZnxO3 perovskites

Both the longitudinal and transverse ultrasonic sound velocities and attenuations in the single-phase polycrystalline compound La0.67Ca0.33Mn1-xZnxO3 (x = 0.00, 0.05, 0.10, 0.15) have been carefully measured by a conventional pulsed echo technique at a frequency of 10 MHz, between 20 and 300 K. A dramatic increase in sound velocity, accompanied by a sharp peak in attenuation, is observed near the ferromagnetic transition temperature TC for both longitudinal and transverse modes in all samples. This feature implies extremely strong spin-phonon coupling in this system and gives direct evidence for the spontaneous linear magnetostriction effect. At temperatures below 60 K, a dramatic softening in sound velocity accompanied by a sharp increase in ultrasonic attenuation for both longitudinal and transverse modes is observed. The analysis of the results suggests that the softening of the sound velocity for both longitudinal and transverse modes may correspond to the formation of a spin-glass state.

Ultrasonic behaviours near different phase transitions in La1-xCaxMnO3

The ultrasonic longitudinal and transverse sound velocities in the single-phase polycrystalline compound La1-x Cax MnO3 (x = 0.33, 0.63, 0.73, 0.83) have been measured by a conventional pulse-echo-overlap technique at a frequency of 10 MHz, between 50 K and 300 K. Dramatic anomalies in sound velocity for both longitudinal and transverse modes were observed near the temperature of the ferromagnetic transition (TC ), charge-ordering transition (TCO ), and antiferromagnetic transition (TN ). The detailed form of the sound velocity anomalies is somewhat different near different phase transitions. The results imply a strong spin-phonon coupling.

Transport and ultrasonic properties in La0.67Sr0.33Mn0.87Fe0.13O3 perovskite

The longitudinal and transverse ultrasonic sound velocity and attenuation, as well as electric resistance have been carefully measured in single-phase polycrystalline giant magnetoresistance perovskite La 0.67 Sr 0.3 Mn 0.87 Fe 0.13 O 3 at a frequency of 10 MHz, from 20 to 300 K. A big electric resistance peak was observed at 85 K (T C ). At the temperature above T C , the resistivity can be fitted well by Motts law ρ=exp(T o /T) 1/4 and both the longitudinal and transverse sound velocities show a lattice softening, which was accompanied by an attenuation peak. This simultaneous occurrence of electron and lattice softening implies electron-phonon coupling, known to exist for the octahedrally coordinated d 4 ion, originating in the Jahn-Teller distortion. Below 55 K, pronounced sound-velocity softening for both longitudinal and transverse waves was observed; this may correspond to the formation of a spin-glass state.