W.-H. Li

Structural defects and microstrain in GaN induced by Mg ion implantation

The optical and structural characteristics of GaN films implanted with Mg and Be ions, grown by low-pressure metalorganic chemical vapor deposition were studied. The low temperature (20 K) photoluminescence (PL) spectra of annealed Mg implanted GaN show a 356 nm near band edge emission, a 378 nm donor-acceptor (D-A) transition with phonon replicas, and a 528 nm green band deep level emission. The origin of the 528 nm green band emission and the 378 nm D-A emission might be attributed, respectively, to the Mg implantation induced clustering defect and the vacancy defect in GaN film. Observations of in-plane and out-of-plane x-ray diffraction spectra for as-grown undoped, Mg implanted and rapid thermal annealed GaN suggest that ion implantation induced anisotropic strain may be responsible for the observed PL emission characteristics.

Magnetic structure and spin reorientation of the Mn ions in NdMnO3

The crystal structure and magnetic ordering of the Mn spins in polycrystalline NdMnO3 have been investigated by means of neutron diffraction and ac magnetic susceptibility measurements. The compound crystallizes into an orthorhombic symmetry of space group Pbnm. Three peaks were observed in the temperature dependence of the in-phase component of the ac susceptibility, χ′(T). Neutron diffraction measurements show that all three peaks observed in χ′(T) are of magnetic origin, and are associated with the ordering and the reorientation of the Mn spins. Both ferromagnetic and antiferromagnetic couplings between the Mn spins were observed, resulting in a noncollinear magnetic structure. The Mn spins order at Tm≈75 K, and the moment saturates at ∼20 K at 〈μz〉=2.21 μB.

Optimum massthickness of Ag-nanoparticle film for surface enhanced Raman scattering

Abstract Raman scattering from a few monolayers of benzoic acid (BA) enhanced by a thin layer of Ag-nanoparticles (AGNs) was studied, using a series of AGN films having different massthickness covering a range from 5 to 50 nm. Surface images taken using an atomic force microscope clearly reveal each individual AGN of average size ∼25 nm. Extinction spectra of the AGN films exhibit a surface-plasma resonance peak at ∼380 nm with a long tail extending to the visible region. When emphasizing the electromagnetic effect, the attenuated total reflection configuration was adopted to obtain an enhanced Raman spectrum. An optimum massthickness for enhancing Raman signal was found to be ∼15 nm. A semi-empirical formula was also developed to estimate the Raman signals from a few monolayers of BA, hence the enhancement factor. The estimated enhancement factor is in the order of 104 for the optimum case. The enhancement is believed to originate from the excitation of surface plasmons (SPs).

Short-range magnetic correlations in spinel LiMn2O4

Neutron diffraction, inelastic neutron scattering, and ac magnetic susceptibility measurements were performed to study the magnetic correlations of the Mn spins in a fully oxygenated lithium manganospinel Li0.96Mn2O4. The compound crystallizes into a cubic structure of the Fd3m symmetry at 300 K and an orthorhombic Fddd symmetry at 150 K. Two separate types of magnetic phase transitions were found, signifying that the Mn3+ and Mn4+ spins order separately at two different temperatures. The magnetic correlations were antiferromagnetic in nature, and were observed to develop below 80 and 55 K, respectively, for the two components. Surprisingly, for both components the correlation lengths reached their maxima at 45 and 30 K, but were still short-range with correlation lengths at ∼120 and ∼40 A, respectively.

Two‐dimensional magnetic order in Pb2Sr2TbCu3O8

Neutron diffraction techniques have been used to investigate the magnetic ordering of the Tb ions in polycrystalline Pb2Sr2TbCu3O8. Significant magnetic correlations are found to develop below ∼10 K, and these correlations are two dimensional in nature, representative of the strongly anisotropic magnetic interactions in these layered materials. The correlation length increases with decreasing temperature, and long‐range order is observed to develop at TN≊5.5 K. However, the sawtooth Bragg profile for the scattering demonstrates that the long‐range order is two dimensional, rather than three dimensional, with a spin configuration where nearest‐neighbor spins are antiparallel. The scattering profile can be explained quantitatively by assuming long‐range order within the ab plane, with no significant correlations along the c axis to the lowest temperatures measured (1.36 K). Our results suggest that the system is well described by a two‐dimensional Ising model. The only other (pure) system where a crossover ...

Enhanced Raman scattering from crystal violet and benzoic acid molecules adsorbed on silver nanocrystals

Abstract Enhanced Raman scattering by crystal violet molecules and benzoic acid molecules adsorbed on Ag nanocrystals was measured by employing the attenuated total reflection method. The Ag nanocrystals were deposited directly on a glass substrate by the gas condensation method. Scanning electron microscopy images show that they were evenly distributed over the surface of the substrate. Films composed of Ag nanocrystals were found to be good enhancers of Raman signals. No obvious frequency shift with respect to the “normal” Raman lines was observed in the enhanced Raman spectra, thus indicating that the enhancement effect is mainly electromagnetic in nature. The enhancement factor depends on the laser excitation line used. Under our experimental conditions the enhancement obtained from Ag nanocrystals of mean grain size about 15 nm is an order of magnitude greater than that obtained from a flat Ag thin film of mass thickness about 56 nm. The excitation profile obtained using benzoic acid molecules shows an intensity maximum at about 500 nm, which may be linked to an effect caused by the aggregation of Ag nanocrystals.

Magnetic ordering of Pr in Pb2Sr2PrCu3O8

The magnetic ordering of the Pr ions in Pb2Sr2PrCu3O8 has been studied using neutron‐diffraction and ac‐suspectibility measurements. An imperfect three‐dimensional magnetic ordering of the Pr spins was observed at a temperature well below its Neel temperature of TN≊7 K. The magnetic intensities observed at T=1.4 K can be explained by assuming long‐range order in the ab plane with short‐range correlations, of correlation length ξc=20 A, along the c axis and a moment directed along the c‐axis direction. The magnetic ordering is 2D in nature, and the basic magnetic structure consists of nearest‐neighbor spins that are aligned antiparallel along all three crystallographic directions. The magnetic transition is also evident in the ac‐susceptibility versus temperature measurements, where a cusp that is typical of antiferromagnetic ordering is clearly observed, which matches the TN obtained by neutron diffraction.

Magnetic ordering of Mn in Ca-doped NdMnO3

Abstract We have performed neutron diffraction measurements on polycrystalline Ca-doped NdMnO 3 . Magnetic ordering of the Mn spins and spin reorientation were observed for all the three compounds studied. Both ferromagnetic and antiferromagnetic couplings between the Mn spins were found, that result in a non-collinear magnetic structure for the Mn spins and suggesting the existence of a close competition between the double exchange and superexchange interactions. The ordering temperature shifts to a higher temperature as the Ca concentration is increased, while the size of the moment remains basically unaffected.

Oscillating magnetic behavior in La1+xSr2−xMn2O7

Abstract The AC magnetic susceptibilities in weak magnetic fields, with and without the presence of an applied DC magnetic field, have been performed to study the basic magnetic properties of the layered colossal magnetoresistance (CMR) compounds La1+xSr2−xMn2O7 with x=0–0.4. A great number of peaks are evident in the temperature dependence of the AC susceptibility, and the peaks become better defined as the La concentration x is increased. The observed peaks may be grouped into four types according to their behavior under an applied DC field. Among them, the peaks associated with the quasi-2D bilayered Mn ordering, CMR effect, and 3D Mn ordering are recognized. The fourth one may possibly be due to charge ordering.

Magnetic order and spin reorientation in Nd0.45Ca0.55MnO3

Neutron diffraction and ac magnetic susceptibility measurements have been performed to study the magnetic ordering of the Mn spins in polycrystalline Nd0.45Ca0.55MnO3. Three peaks around 260, 90, and 40 K were observed in the temperature dependence of the ac susceptibility. Structural analysis using high-resolution neutron diffraction patterns shows that the peak at 260 K is associated with the charge ordering of the Mn ions. Neutron magnetic diffraction shows that the peak at 90 K originates from the ordering of the Mn spins. Both ferromagnetic and antiferromagnetic couplings between the Mn spins were found. The spin structure in the basal plane consists of tilted ferromagnetic chains (along the [110] direction) aligned antiferromagnetically, whereas the interlayer coupling is ferromagnetic. A spin reorientation occurs below 15 K, which results in a change in the title angle. The Mn spins order at TN≈120 K, and the moment saturated at 30 K at 〈μZ〉=2.29(4)μB.