Zhao-Hua Cheng

High Tunability of the Surface-Enhanced Raman Scattering Response with a Metal-Multiferroic Composite

We demonstrate active control of the plasmonic response from Au nanostructures by the use of a novel multiferroic substrate-LuFe(2)O(4) (LFO)-to tune the surface-enhanced Raman scattering (SERS) response in real time. From both experiments and numerical simulations based on the finite-difference time-domain method, a threshold field is observed, above which the optical response of the metal nanostructure can be strongly altered through changes in the dielectric properties of LFO. This offers the potential of optimizing the SERS detection sensitivity in real time as well as the unique functionality of detecting multiple species of Raman active molecules with the same template.

Surface morphology and magnetic anisotropy of obliquely deposited Co/Si(111) films

We report an investigation on magnetic anisotropy of Co/Si(111) films deposited at oblique incidence. An in-plane uniaxial magnetic anisotropy (UMA) with the easy axis perpendicular to the incident flux plane was observed to superimpose on sixfold magnetocrystalline anisotropy of Co films. We built a total energy model to investigate the magnetization reversal mechanism around hard axis. The simulated value of UMA is Ku=1.7×105u2002erg/cm3, which is consistent with Kshape=1.1×105u2002erg/cm3 calculated from scanning tunneling microscope image. This good agreement suggests the in-plane UMA is mainly originated from the shape of the oblique deposited Co stripes.

Magnetic properties of R2Fe14M3 compounds with M=Ga and Si; R=Y, Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm

Samples with composition R2Fe14M3 (M = Ga and Si; R = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm) have been prepared by are-melting. Crystal-structure studies have shown that the prepared samples are single phase with the rhombohedral Th2Zn17 and/or the hexagonal Th2Ni17 structure. Substitution of Ga for Fe in R2Fe17 leads to an increase of the unit-cell volume, whereas Si substitution reduces the unit-cell volume. The Curie temperatures of the R2Fe14Ga3 and R2Fe14Si3 compounds are much larger than those of the corresponding R2Fe17 compounds. The Fe moment decreases upon Ga or Si substitution. The compounds R2Fe14Ga3 and R2Fe14Si3 With R = Er and Tm exhibit a spin-reorientation transition at low temperature

Uniaxial magnetic anisotropy of quasi-one-dimensional Fe chains on Pb∕Si

We fabricated quasi-one-dimensional Fe chains on a 4° miscut Si (111) substrate with a Pb film as a buffer layer. The magnetic properties and morphology of Fe chains were investigated by means of scanning tunneling microscope (STM) and surface magneto-optical Kerr effect (SMOKE). STM images show that Fe chains are formed by Fe random islands along the steps of the Pb film due to step decoration. SMOKE data indicate that the Fe chains exhibit in-plane uniaxial magnetic anisotropy along the step direction. The effective in-plane uniaxial anisotropy constant at room temperature was determined by means of electron spin resonance.

Observation of a Griffiths-like phase in bilayered manganites

The authors report the observation of a Griffiths-like phase in La2−2xSr1+2xMn2O7 (x=0.30, 0.33, and 0.40) and (La0.8Eu0.2)2−2xSr1+2xMn2O7 (x=0.33) single crystals by means of electron spin resonance, magnetic susceptibility, and magnetoresistance measurements. In the temperature region TC<T<350K, the resonance signal consists of a ferromagnetic resonance line and a paramagnetic resonance line, which suggests that the system is not in a pure paramagnetic phase. The ferromagnetic resonance signal disappears and the magnetic susceptibility starts to deviate from the Curie-Weiss law at the same temperature T*≈350K, independent of doping level and anisotropy. These results indicate that a Griffiths-like phase exists at TC<T<T* in bilayered manganites, which is due to intrinsic inhomogeneity caused by quenched disorder.

Phase development in titanium by mechanical alloying under hydrogen atmosphere

Titanium hydride was successfully synthesized at room temperature by ball milling. X-ray diffraction patterns demonstrated that a transformation of elemental Ti powders into TiH2 could be induced at room temperature by ball milling under a hydrogen atmosphere. Almost no hydrogen absorption was observed during the initial few minutes of the milling process on the basis of x-ray diffraction patterns and hydrogen pressure in the reservoir system, although the grain size of Ti decreased significantly. A non-stoichiometric titanium hydride with an approximate composition of TiH1.9 was first formed after about 7 min of milling and existed together with unhydrided Ti flakes. The stoichiometric hydride phase TiH2 was developed after further milling at room temperature. Due to the trapping of hydrogen by numerous defects and grain boundaries, the dilute solution Ti(H) does not exist in the nanograin milled metal powders.

Preferential arrangement of uniform Mn nanodots on Si(111)-7 ? 7 surface

Under proper growth conditions, ordered and uniform Mn nanodots were fabricated on a Si(111)-7 ? 7 surface without the presence of a wetting layer. The Mn nanodots deposited onto the elevated substrates were observed to occupy preferentially on the faulted half unit cells (FHUCs) of the Si(111)-7 ? 7 surface. This phenomenon implies that the Mn dots adsorbed on the FHUCs are more stable than those adsorbed on the unfaulted half unit cells (UFHUCs). Within the framework of quasi-equilibrium thermodynamics, the energy difference between adsorption on the UFHUCs and the FHUCs was estimated to be 0.05? 0.01?eV. The intrinsic attractive potential wells on the FHUCs effectively trap the outdiffusion of Mn atoms and consequently result in a preferential arrangement of islands with well-defined sizes.

Mössbauer effect probe of local Jahn-Teller distortion in Fe-doped colossal magnetoresistive manganites

The local structure of the Fe-doped La1−xCaxMnO3u200a(x=0.00–1.00) compounds has been investigated by means of Mossbauer spectroscopy. 57Fe Mossbauer spectra provide direct evidence of Jahn–Teller distortion in these manganites. On the basis of the Mossbauer results, the Jahn–Teller coupling was estimated. It is noteworthy that the Ca-concentration dependence of the Jahn–Teller coupling strength is very consistent with the magnetic phase diagram. Our results reveal that Mossbauer spectroscopy cannot only detect the local structural distortion, but also provide a technique to investigate the Jahn–Teller coupling of Fe-doped La1−xCaxMnO3 colossal magnetoresistive perovskites.

Preparation of nanocrystalline metal hydrides by ball milling

The diffusion of hydrogen into metals during ball milling has been investigated. A system is described that enables the milling to be done under approximately constant hydrogen pressure and allows for the continuous measurement of the quantity of hydrogen absorbed. This method has been used to prepare nanocrystalline hydrides of a variety of early transition metals and alloys. X-ray diffraction studies are used to identify phases present before and after milling and the width of diffraction peaks is used to estimate average grain size. Changes in the microstructure of Fe-containing samples during hydrogen absorption have been investigated by Mössbauer effect. Studies of the influence of milling conditions and the effect of milling time are also presented.

Monte Carlo simulation of magnetic properties of irregular Fe islands on Pb/Si(111) substrate based on the scanning tunneling microscopy image

Magnetic properties of irregular Fe islands grown on Si(111) substrate with Pb buffer layer by molecular beam epitaxy have been investigated by means of Monte Carlo simulation. During the simulation, the dipolar interaction energy among islands and the demagnetization energy of each island were accurately calculated in the Fourier space with the aid of cluster multiple labeling technique. The simulated results, i.e., the magnetic hysteresis loops and the temperature dependence of remanent magnetization, are in good agreement with the experimental ones measured by in situ surface magneto-optical Kerr effect (SMOKE).