Varied temperature photoemission study is performed to investigate the quantum size effects on the thermal property of atomically flat Pb films grown on Si͑111͒. The binding energies of the quantum well states for the films with thicknesses from 10 to 24 ML exhibit a linear increase with increasing temperature from 75 to 270 K. Under free electron approximation, thermal expansion coefficients of the thin films are determined, which manifest a large enhancement and oscillation behavior. The large enhancement is interpreted by a model based on the quantum confinement along the film normal direction. The oscillation is shown to be closely related to the structural instability and is a result of the formation of the quantized electronic states in thin films.

Based on a quantum spin model and by using the many-body Green's function method, we discuss the mechanism of anomalous hysteretic loops for exchange-coupled ultrathin bilayers by analyzing in detail the spin behavior in all monolayers. Spins in each monolayer can flip abruptly to generate steplike or triplet loops, or they can reverse gradually to generate double loops. Specially, the spins in an interfacial monolayer may be oriented in the opposite direction from those in the other monolayers of a given layer under a strong interfacial antiferromagnetic exchange. This individual monolayer analysis improves the earlier understanding of the anomalous hysteresis loops by phenomenological models.Anomalous hysteresis loops could occur experimentally in ferromagnetic (FM) films. Here we mean the anomalous loops as steplike 1,2 and multiloops. The latter could be either double loops or triplet loops, which also includes the case of inverted loops. 2-7 Phenomenological models have been used to explain the anomalous loops by exchange-coupled bilayers 2,3,6,8,9 or by introducing higher-order anisotropy. 4,10 In exchange-coupled bilayer models, the system consisted of two FM layers. Each FM layer was assigned a magnetization, the magnitude of which was independent of external field, but could rotate under an external field. There was an antiferromagnetic (AFM) exchange between the two magnetizations. The orientations of the magnetizations were not always along the field direction. Hence the magnetizations of both layers had projections on the field direction. The combination of the projections made up the anomalous loops.If one realizes the system from a microscopic view, the physical picture will be different from what the phenomenological model gave. In a microscopic model, each FM layer consists of several monolayers (ML's), and each ML is in turn composed of spins at crystalline sites. Every spin interacts with its neighbors by Heisenberg exchange interaction. As there is a lack of translation invariance along the out-ofplane direction, the behavior of the spins may be different from ML to ML. The interaction between the two FM layers is in fact an AFM interfacial exchange, not just the interaction between the two magnetizations. Thus the behavior of the spins inside each FM layer may not be uniform, and neither can the behavior of each layer be recognized as a single magnetization. Compared with the microscopic model, a phenomenological model seemed too rough. A detailed microscopic explanation based on quantum mechanics should help us to understand the physics of the anomalous loops more clearly. In this paper, we present a monolayer spin-flip or spin-reversal picture of the anomalous loops for ultrathin exchange-coupled bilayer.The Hamiltonian we use includes three parts:H 1 is the Hamiltonian of the first FM layer:The first term is the Heisenberg FM exchange in each ML, where only the nearest-neighbor interaction is considered.Here i and j label sites and the Greek letters label ML's. The second term is the Heisen...

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We determine the zeta potential (ZP) by using electrophoretic laser light scattering and Laser Doppler Velocimetry (LDV). Particle sizes are measured by photon correlation spectroscopy (PCS). We studied the ZP for colloidal suspensions of TiO2 and Ag metal particles in order to determine the kinetic interaction and charge exchange between the particles. We investigated the natural tendency of the particles for aggregation and varied the pH of the solution. It was found that the ZP versus pH curve for the mixed TiO2/Ag did not behave as the average of the individual TiO2 and Ag curves as one would expect, and instead there was a slight horizontal shift towards higher pH values which implies that the particles in the mixed TiO2/Ag colloid are interacting with the result of charge exchange. The average particle size was measured in terms of effective diameter, for the TiO2, Ag and the mixed TiO2/Ag particles. The results indicated an increase of approximately 100 nm in the effective diameter of the mixed TiO2/Ag particles size compared to the size of the individual TiO2 particles. This can be explained as the fact that 50 nm Ag particles are adsorbed on the surface of the TiO2 particles.

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