Ferromagnetism driven by cation vacancy in GaN thin films and nanowires

The first-principles calculations have been performed to understand the origin of magnetism in undoped GaN thin films. The results show that Ga vacancy, rather than that of N contributes the observed magnetism, and the magnetic moments mainly come from the unpaired 2p electrons at nearest-neighbor N atoms of the Ga vacancy. Calculations and discussions are also extended to bare and passivated GaN nanowires, We find that per Ga vacancy on the surface sites products the total magnetic moment of 1.0  while that inside of the nanowires can lead to the formation of a net moment of 3.0 . The coupling between two Ga vacancies is also studied and we found that the coupling is ferromagnetic coupling. The surface passivation with hydrogen is shown to strongly enhance the ferromagnetism. Our theoretical study not only demonstrates that GaN nanowire can be magnetic even without transition-metal doping, but also suggests that introducing Ga vacancy is a natural and an effective way to fabricate low-dimensional magnetic GaN nanostructures.     

Microwave relaxation and phenomenological damping in thin films

The relation between relaxation timeT, frequency swept resonance linewidth , and phenomenological damping is given by =2/T=(_x+_y), where _x,y = (H ₀+(N _x,y –N _z ) 4M _s ).N _x,y,z are sample demagnetizing factors,H ₀ is the effectivez-directed static field, 4M _s is the saturation induction, and is the gyromagnetic ratio. This fairly simple but general relation shows that the numerical relation between damping and relaxation at a given frequency can be quite different for in-plane and normally magnetized thin films. For thesame loss processes, so thatT andT are equal, is larger than . For permalloy films at 1 GHz, =15 . In addition, the conventional field swept linewidth, H=/, is simply related to only forN _x =N _y . Both and H are geometry dependent and do not provide an intrinsic measure of the relaxation. These results are confirmed by both resonance and transient response experiments. The large values of for large angle switching may also be partially explained by this analysis because the relevant magnetization motion is due to a demagnetizing field normal to the film plane.Visiting scientist on leave fromRaytheon Company, U.S.A. Supported by the Japan Society for the Promotion of Science.     

Confinement-induced relaxation process in thin films of cis-polyisoprene

Broadband dielectric spectroscopy is employed to investigate the molecular dynamics of cis-1,4-polyisoprene (PI) in thin layers down to thicknesses comparable with the chain extension. The segmental and the normal mode dynamics of the PI chains is found not to depend on the layer thickness. Additionally, a novel confinement-induced relaxation process is detected. It is assigned to fluctuations of terminal subchains which are formed due to the immobilization of chain segments at an interface. Most of the experimental findings can be well described by simulations of the chains as ideal random walks.     

Intrinsic nonlinear ferromagnetic relaxation in thin metallic films

We propose an intrinsic mechanism for ferromagnetic relaxation in thin films that can dominate competing linear mechanisms even for rapidly relaxing metals. In particular, we use an analytic theory of four-magnon scattering to demonstrate rapid decay for technologically important systems involving high moment materials subject to large rotations. A micromagnetic simulation is used to verify the results.     

Glass transitions and dynamics in thin polymer films: dielectric relaxation of thin films of polystyrene

The glass transition temperature T(g) and the temperature T(alpha) corresponding to the peak in the dielectric loss due to the alpha process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in T(g) was observed with decreasing film thickness, while T(alpha) was found to remain almost constant for d>d(c) and to decrease drastically with decreasing d for d相似文献   

Identification of A- and B-site cation vacancy defects in perovskite oxide thin films

Cation vacancies on both sublattices (V(Ti), V(Sr)) have been identified in homoepitaxial pulsed laser deposited SrTiO3 films using high intensity variable energy positron annihilation lifetime spectroscopy (PALS) measurements. Film nonstoichiometry was varied by varying laser fluence. PALS showed that on increasing the fluence above the Ti/Sr～1 value, the concentration ratio [V(Sr)]/[V(Ti)] systematically increased. Reducing the fluence into the Ti-poor region below resulted in additional vacancy cluster defect formation. Vacancy concentrations greater than ～50 ppm were observed in all films.     

Glass transition and relaxation following PhotoPerturbation in thin polymeric films

In this letter we employ null ellipsometry on Langmuir Blodgett multilayers of a glass forming photosensitive side chain polymer to investigate both the structural changes and the related time relaxation as a function of temperature and sample thickness following isothermal optical perturbation. Below a thickness of a few layers the glassy multilayer system collapses to a smecticlike crystal. This effect is discussed in the context of the glass transition in restricted dimensionality.     

Ferromagnetic resonance and relaxation in evaporated GdCo amorphous thin films

Ferromagnetic resonance of amorphous Gd_1?xCo_x films was studied at 36 GHz in order to obtain the dependence of effective anisotropy field, g-factor and relaxation parameters on chemical composiiton in the interval 0.8<x<1.0.     

Hydrogen binding in vacancy clusters in platinum

The binding of hydrogen in different vacancy complexes in platinum metal was investigated with atomic-scale sensitivity using perturbed angular correlations of gamma rays (PAC). Hydrogen was introduced by cathodic charging. Detrapping was monitored microscopically during desorption at 294 K by changes in site fractions of hydrogen-decorated and undecorated complexes. Analysis of desorption includes effects of retrapping of hydrogen at other sites. Assuming a trap concentration of 10^–3, binding enthalpies of 0.23(2), 0.28(1), 0.24(1) and >0.20 eV are obtained for hydrogen atoms in 1V to 4V complexes, respectively. The small differences between the binding enthalpies demonstrate that hydrogen binding is insensitive to the detailed geometrical structure of small vacancy complexes. However, the magnitudes found here are a factor of two smaller than in the literature.     

Size quantization of electronic states in very thin platinum films

The Quantum Size Effect (QSE) is expected to exist in metal films. It has been discussed in a number of theoretical papers. The experimental verification is difficult because of the very short Fermi wave length of the charge carriers in metals. Surface roughness of the films and diffuse scattering of the charge carriers from the film surface usually prevent the observation of QSE. We succeeded in preparing Pt-films with surface roughness below 0.3 nm, which also exhibited partly specular reflection of the electrons from the film surface. Oscillations of the conductivity with the film thickness, due to QSE, were observed in the thickness range from 0.5 nm to 2.0 nm. Band splitting was observed in the thickness range from 5 nm to 20 nm by means of tunneling spectroscopy. From the results we suggest that electrons as well as holes in closed Fermi surfaces participate in the QSE. Holes in open Fermi surfaces, despite their larger density, could not be observed. The measurements allow the determination of the Fermi energy and the effective masses of both electrons and holes in quasi amorphous films.     

Present status of thin oxide films creation in a microwave plasma

This paper summarizes present knowledge of the creation of thin films in isotropic and magnetoactive plasma. It analyses conditions under which films in the microwave plasma can be created and shows how the growth rate and properties of films depend on microparameters of plasma. On the basis of plasma floating potential measurements it is shown why the creation of thin films in microwave discharges takes place at high electron plasma densities (N 10¹² cm^–3) only. Besides this it describes properties of created films, underlies negative role of fast electrons in forming of good quality films and gives recommendation how to avoid their generation. Considerable attention is devoted also to a comparison of films creation in pulse and continuous plasma. At the end possibility of films creation at low temperatures by a deposition technique utilizing a microwave excitation of molecular gases is given.The authors gratefully acknowledge many stimulating and fruitful discussions with Dr. F. áek of Institute of Plasma Physics of Czechoslovak Academy of Sciences. We also thank Dr. V. Malina of Institute of Radioelectronics of Czechoslovak Academy of Sciences for measurements of C—V curves of SiO₂ films.     

Magnetic force signal of vortex creation in type-II superconducting thin films

The possibility of the formation of a vortex in thin film due to the field of magnetic dipole moment when a point dipole is vertically approaching the surface of a thin film is discussed. Calculations are performed for certain discrete values of the critical position of the point dipole for creating the vortex in the thin film and the equilibrium positions of the vortex, both of which depend on the strength of magnetic dipole moment. The creation of a new single vortex in the thin film causes an abrupt change in the vertical force.     

Strain relaxation of CdTe films growing on lattice-mismatched substrates

We have deposited CdTe films by laser-assisted epitaxy approach and investigated the influence of substrate and film thickness on the film properties. Grown on Si(001), GaAs(001), and quartz substrates; the CdTe films exhibit preferential orientation along the cubic CdTe(111) direction. When the films are thin (<500 nm), a blueshift of the band gap and splitting of valence bands were observed. These results are attributed to the existence of residual strains induced by mismatch of the film lattice constant with that of the substrate, and by their difference in thermal expansion coefficients. The bulk band-gap energy of 1.5 eV was achieved on the surface of thick CdTe films grown on Si(001) substrate, indicating that strain was almost completely relaxed in this case. Our results demonstrate that by a proper selection of substrate and film thickness it is possible to grow film semiconductors with band gap approaching those of bulk crystals.