Hu An

Training Effect and Hysteretic Behaviour of Angular Dependence of Exchange Bias in Co/IrMn Bilayers

The training effect and the hysteresis behaviour of the angular dependence of exchange bias are extensively investigated upon the variation of the IrMn layer thickness tIrMn in a series of Co/IrMn bilayers. When tIrMn is very small, both of them are negligible. Then they increase very sharply with increasing tIrMn and then reach maxima at almost the same value of tIrMn. Finally they both decrease when tIrMn is further increased. The similar variation trends suggest that these phenomena arise from irreversible change of antiferromagnet spin orientations, according to the thermal activation model.

Raman scattering in Fibonacci metallic superlattices

Raman scattering experiments from acoustic phonons in Fibonacci metallic Nb-Cu superlattices were performed. The main features of the experimental results show that the Raman spectrum is dominated by triplet peaks which originate from the density of states of longitudinal-acoustic (LA) phonons in the subbands of phonon dispersion. The expected rich structure of gaps in the phonon spectrum has been labeled by strongest diffraction wave vector factor nτp.

Effect of Barrier Width on Bias-Dependent Tunnelling in Ferromagnetic Junctions

We present a finite temperature theory for bias-dependent tunnelling in ferromagnetic tunnelling junctions. The effects of the barrier width d on the tunnelling magnetoresistance (TMR) and its sign change behaviour are discussed with this theory. Numerical results show that both the zero-bias TMR and the critical voltage Vc at which the TMR changes its sign decrease with the increasing barrier width for a considerably thick barrier junction. Furthermore, it is found that a minimum exists in the curve of Vc versus d if a composite junction is under oxidized.

Dynamical Model in Low-Frequency-Pulsed Electron-Stimulated Desorption

A dynamical model of low-frequency-pulsed electron-stimulated desorption is developed. The characteristic of desorbed gas flow is taken as an exponential function, and can be degenerated to a triangular and square wave. The transient pressure is given according to the gas flow of desorbing gas and vacuum system parameters, including the pumping speed and the system volume. Although the mathematical model is deduced from the electron-stimulated desorption, it can be applied to other similar processes of intermittent desorption.

BRILLOUIN LIGHT-SCATTERING INVESTIGATION OF ELASTIC ANOMALY FROM SUPERLATTICES

In this paper we report some new experimental results concerning the elastic properties of periodic and quasiperiodic superlattices studied by means of Brillouin spectroscopy. The observation of both bulk and surface acoustic phonons as well as the fitting using effective medium approximation on the phonon spectrum enables us to extract accurately some relevant effective elastic constants in both structures. We have verified that the elastic anomaly which occurs in the periodic structures is also present in quasiperiodic ones. These results imply that the interaction mechanism of Fermi electrons with reduced Brillouin zone boundary is not responsible for the observed elastic anomalies. The comparative study of these superlattices proves that the elastic anomaly is relevant directly to interfacial effects.

Brillouin Scattering from Metallic Ta/Al Fibonacci Multilayers

Surface phonon spectra from Ta/Al Fibonacci multilayers are studied via Brillouin scattering. Determination of the frequency positions of the Rayleigh wave and the intensity dip corresponding to the longitudinal threshold has enabled us to extract two effective elastic constants C44 and C11 of the whole structure. The 22.6% enhancement of the shear modulus is found in our experiment.

Double-Crystal x-Ray Diffraction Study on Ta/Al Multilayers

Double-crystal x-ray diffraction has been used to measure the rocking curve of Ta/Al multilayers. The diffraction profiles of the multilayers are different. Besides the normal profile, two kinds of distorted profiles have been found. The simulations show that the deformed profiles of the peaks are caused by the imperfect periodicity (e.g. systematic period deviation and coexistence of two slightly different periods) in the multilayers.

X-ray diffraction of Cu/Ti fibonacci superlattices

1-D quasiperiodic (Fibonacci) Cu/Ti metallic superlattices have been prepared in a dual magnetron sputter deposition system. X-ray scattering measurements of these samples are presented. The x-ray diffraction peaks at low angles can be indexed by the projection method from the high-dimensional periodic structure. In the high angle region, there is one broad peak, which indicates that the samples are amorphous.