Bernard Enrico Watts

Oriented PbFe12O19 thin films prepared by pulsed laser deposition on sapphire substrate

Abstract PbFe12O19 hexaferrite thin films with a high degree of orientation in the perpendicular direction have been deposited by pulsed laser ablation on 1×1-cm2 (0001) sapphire substrate at 700°C under 3.0 mbar partial pressure of high purity oxygen. The anisotropic character of the films was demonstrated by a remanence ratio of 86% in the perpendicular direction in contrast with to 20% in the parallel direction to the film plane. The X-ray diffraction analysis confirmed this orientation, showing only the (0001) lines of PbFe12O19 compound. The hexagonal lattice parameters determined were a=5.885 A and c=23.066 A. Moderate values of coercive field (2.5 kOe) and saturation magnetisation (165 emu/cm3) were obtained.

Ferromagnetic resonance studies of amorphous and nanocrystalline FeCuNbSiB alloys

Alloys with composition Fe73.5Cu1Nb3Si22.5−xBx, where x=6 and 9, have been studied from the as-cast state, through various stages of crystallization, in annealing range of 300–650 °C, by the technique of ferromagnetic resonance (FMR). The annealing was performed isothermally at preset temperatures to produce nanocrystalline structures in an amorphous matrix. Both the nanocrystalline structures and the surrounding amorphous matrix are ferromagnetic, and will therefore contribute to the FMR spectra. The spectral features, resonance field, intensity, and linewidth have been used to characterize the structurally related changes in the sample during the crystallization process. The major changes in the spectra are observed to occur in the region of the crystallization peak in the differential thermal analysis curves for these samples. The FMR spectra exhibit a complex in-plane angular variation, which is understood in terms of preferential orientation of the magnetization vector in the direction of the ribbon,...

Characterisation of amorphous Fe–Cr–Si–B alloys

Abstract The Fe80–xCrxSi10B10 amorphous alloy system has been investigated for values of x between 0 and 14. The substitution of Cr for Fe caused a decrease in the Curie temperature, T C , of the alloy. Low-temperature annealing was found to increase T C by up to 16°C. Magnetic measurements were also performed on selected alloys. This series of alloys, which have T C markedly below the crystallisation temperature, have potential to be used in magnetic temperature sensors.

The influence of ambient gas pressure on the surface quality and magnetic properties of nanocrystalline NdFeB based melt spun ribbons

Abstract The melt spinning of a NdFeB alloy of stoichiometric composition was investigated under various ambient gas pressures, from atmospheric pressure, 100 kPa (760 Torr) to vacuum, 0.13 Pa (10 −3 Torr). Gas pockets were observed on the roll contact surface of ribbons produced at pressures close to atmospheric, whilst a reduction in pressure, to 50 kPa (380 Torr) caused the disappearance of the gas pockets. The absence of the gas pockets led to more consistent magnetic properties for the nanocrystalline ribbon.

Magnetic and Mössbauer investigation of PbFe12−xGaxO19 hexagonal ferrites

Magnetic and Mö ssbauer measurements have been performed on polycrystalline samples of Ga substituted M-type hexagonal ferrites having compositions PbFe12−xGaxO19 (0 < x < 11). The saturation magnetisation (σS) at 0 K falls with increasing x, from 20 μB for x = 0 to 4.32 μB for x = 6. The Curie temperature tends to zero at x ≈ 9, indicating that the compounds with x > 9 are paramagnetic at all temperatures. The magnetic moments of the non substituted Fe3+ ions in the spin-up predominant sublattice do not have the same temperature dependence, because of the presence of non magnetic Ga3+ ions in the neighbouring lattice sites. For x < 3, the Mössbauer data show that there is no significant deviation from collinear magnetic order. The decrease of σS(0) for x < 3 agrees with the collinear model and the distribution of gallium in all the five available sublattices. At x > 3, the faster decline of the saturation magnetisation can be attributed to the onset of non collinear magnetic order.

Mössbauer and magnetic investigation of scandium and indium substituted PbFe12O19 hexagonal ferrite

Magnetic and Mössbauer measurements have been performed on polycrystalline substituted M-type hexagonal ferrites of compositions PbFe12−xScxO19 (x = 0, 0.5, 1, 1.5) and PbFe12−xInxO19 (x = 0, 1, 2). The substitution of the Fe3+ by Sc3+ and In3+ ions perturbs the axial magnetic order giving rise to angled magnetic structures. From the values of the quadrupole splitting of the Fe57 14.4 keV level, measured above and below the Curie temperatures, the angle between the magnetic moment of the Fe3+ ions in various sublattices and the hexagonal c-axis have been deduced. At 77 K the measured angles do not exceed 20° and depending on x are different for different sublattices. The dependence of x on the saturation magnetisation at 0 K, expressed in Bohr magnetons, differs markedly in Sc and In containing compounds, partly due to the different substitution probability of iron in the spin up and spin down sublattices, and also due to the different spin canting angles.

Test structures for dielectric spectroscopy of thin films at microwave frequencies

This work describes the application of two different test structures to execute broadband microwave measurements of the dielectric constant of ceramic thin films. Coplanar waveguide probes and vector network analyzer were used to measure the dielectric constant versus frequency of thin films of lead zirconate titanate and zirconium titanate, fabricated by sol gel methods. One-step lithography was used to produce planar metal-insulator-metal and interdigitated capacitor test patterns. The two test structures are compared for zirconium titanate films. The metal-insulator-metal method has been applied also to a lead zirconate titanate film and to show the capability of computing the dielectric tunability.

Scanning force and friction microscopy of ferroelectric Pb(Zr, Ti)O3 thin films

Abstract Scanning force and friction microscopy (SFFM) has, for the first time, been applied to investigate the morphology and phase structure of Pb(Zr, Ti)O3 ferroelectric thin films, deposited by sol-gel. The topographic characterization (roughness 20–50 nm, grain size 150–500 nm) reveals the presence of clusters of grains (rosettes) in zirconium rich films. The friction measurements show that the rosettes and the surrounding material are composed of different phases. Quantitative comparisons of the friction forces are performed.

A Comparative Study of the Morphology of 3C-SiC Grown at Different C/Si ratios

A comparative study of the morphology of 3C-SiC films prepared with different C:Si ratios is presented. The silane precursor controls the growth rate at all values of C:Si ratio but combined of observations using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) indicates that the C:Si ratio is critical in determining the grain size and at values of C:Si close to 1 texturing and faceting become evident. Makyoh Topography reveals various surface defects, a slight mesoscale roughness and bending of the epiwafers.

Optimal target–substrate distance in the growth of oxides thin films by pulsed laser deposition

Abstract A simple model to investigate the formation of the oxide phase during the growth of ternary oxides films by laser ablation deposition has been proposed. Since the crystallinity properties of the oxide films are subject to the oxygen stoichiometry, the model is based on the balance between the density of the oxygen and a characteristic density of the system, δ =3 N 0 / L 0 3 , in order to scale the optimal target–substrate distance, L 0 . The interaction plume–laser is not considered. The model was applied for growing PbFe 12 O 19 hexaferrite thin film, and it shows that the theoretical conditions found are near to the optimal ones for the film growth. According to the model, for a fixed oxygen pressure of 1.0 mbar and a substrate temperature of 700 °C, a target–substrate distance, L 0 ≈3.1 cm, corresponding to a critical density of oxygen, n g =2.74×10 15 cm −3 , has been determined. The films show a high degree of c -axis orientation, high crystallographic quality and a proper stoichiometry.