B. K. Tanner

The effect of plastic deformation and residual stress on the permeability and magnetostriction of steels

We report the results of magnetic measurements performed on specially prepared steel samples. These measurements were made after the samples had been uniaxially plastically deformed and the applied stress removed. The observed magnetic behaviour included characteristic changes in the shape of the hysteresis loops and total differential permeability curve, a linear response of the coercive field to the prestress and a lack of continuous changes in the magnetostrictive behaviour with increasing prestress. The results of the experiments indicated that residual stress did not play an important role in the observed variation of these magnetic properties. The changes in the magnetic behaviour were attributed instead to the generation of pinning sites during the plastic deformation process and the different effects of these pinning sites on 180° and 90° domain walls and magnetisation vector rotations. It was also found that the prestress applied to the samples before measurement, rather than the resulting strain, was the factor that directly affected the magnetic parameters.

Bonding and mechanical properties of ultrathin diamond-like carbon films

Ultrathin 2-nm-thick carbon coatings are needed to increase the storage density in magnetic hard disks. We show how x-ray reflectivity, surface Brillouin scattering, resonant Raman scattering, and electron energy loss spectroscopy can measure consistently the structural and mechanical properties of these thin films. 2 nm films retain a Young’s modulus of 100 GPa.

Determination of bonding in amorphous carbons by electron energy loss spectroscopy, Raman scattering and X-ray reflectivity

X-ray reflectivity (XRR) and Raman scattering are developed as non-destructive methods to find the density and sp 3 content of unhydrogenated and hydrogenated amorphous carbon films. An empirical relationship is found to derive the sp 3 fraction from visible Raman spectra, while ultraviolet (UV) Raman is able to directly detect sp 3 sites. The sp 3 fraction and density are linearly correlated in amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) films. ” 2000 Elsevier Science B.V. All rights reserved.

The magnetic properties of specially prepared pearlitic steels of varying carbon content as a function of plastic deformation

Abstract Specially prepared low carbon steels with pearlite fractions varying from 20% to 100% have been used to study the independent effects of pearlite fraction and tensile plastic deformation on the structure sensitive magnetic properties of low carbon pearlitic steels. In addition to bulk magnetic measurements, Barkhausen noise emissions were studied and domain wall motion observed directly using the technique of Lorentz electron microscopy. The magnetostrictive effects of residual stress and pinning effects of dislocation tangles were found to combine to cause the changes in the magnetic properties.

Time-dependent magnetization in fine-particle ferromagnetic systems

Abstract A new theory is presented to describe the time-dependent decay of the isothermal remanent magnetization of a fine-particle ferromagnetic system. Analytical expressions are obtained for a model based on the summation of relaxation times over a very narrow range of the particle size distribution. The model predicts the well established ln t decay of magnetization for small t but a t -1 -dependence for large t . The results are compared with precise experimental data taken on a frozen Co fine-particle magnetic fluid. Excellent agreement between theory and experiment is found over a large measurement time-scale.

Nearly perfect single crystals of layer compounds grown by iodine vapour-transport techniques

Abstract Single crystals of the di-sulphides, di-selenides and di-tellurides of tin, zirconium, titanium and hafnium have been grown by iodine vapour transport techniques to widths of at least 5 mm. Of these, tin di-selenide and tin, zirconium, titanium and hafnium di-sulphides have been examined by X-ray topography and found to be nearly perfect. The dislocation densities were low enough to resolve individual dislocations and in several compounds large areas free from dislocations were observed. The defects in tin di-sulphide are studied in more detail and compared with previous results obtained using the transmission electron microscope.

The in situ measurement of the effect of plastic deformation on the magnetic properties of steel. Part I - Hysteresis loops and magnetostriction

We report measurements of the bulk magnetic properties of pearlitic steels recorded in-situ during plastic deformation. Hysteresis loops were initially found to become squarer but became less so as the stress increased. Magnetostrictive measurements revealed that the low field region showed zero magnetostriction, while the change to magnetisation vector rotations was shown to take place at lower magnetisation values with increasing tension. The coercive field remained constant or decreased with increasing stress at lower values of tension, but higher stress levels produced an increase attributed to new pinning sites generated by the plastic deformation process. This increase, proportional to the square of the stress, was seen to initiate before yield, rather than at the yield point. Remnant magnetisation was found to increase at lower tension levels, but higher stress levels produced a decrease in value that was attributed to stress induced changes in magnetic anisotropy. The high field magnetisation decreased monotonically with stress in both the pre-yield and plastically deformed regions.

The magnetocrystalline anisotropy of cobalt

Abstract Recent high precision measurements of the uniaxial anisotropy constants K 1 and K 2 are reported taken from three samples grown by different techniques. The results vindicate the early work of Succksmith and Thompson and agree well with the recent measurements of Ono. While some variation was observed between results from the three samples we conclude that these are insufficient to account for the spread of results in the literature and suggest inaccurate shearing corrections as the most probable cause of discrepancy. A very good fit to the K 1 data was obtained using a single ion model and the c/a ratio as an adjustable parameter. These c/a values gave a satisfactory fit to K 2 and also lay within the scatter of the experimentally measured c/a values. However, a definitive test of the single ion theory was not possible due to these experimental uncertainties in c/a . Data for the temperature variation of the basal plane anisotropy constant K 4 are reported for the first time. The single ion model predictions are not in very good agreement with the K 4 data.

Measurement of aluminum concentration in epitaxial layers of AlxGa1−xAs on GaAs by double axis x‐ray diffractometry

The composition of a series of AlxGa1−xAs layers grown epitaxially by molecular beam epitaxy (MBE) on GaAs has been measured independently by double axis x‐ray diffractometry and reflection high‐energy electron diffraction. From a quadratic fit to the data, we deduce the lattice parameter mismatch between AlAs and GaAs and the Poisson ratio of AlAs. Asymmetric reflection rocking curves and synchrotron x‐ray topography have been used to show that the anomalously low substrate‐layer peak splitting for the 1‐μm‐thick AlAs layer results from relaxation, which is asymmetric. Use of the AlAs rocking curve peak splitting corrected for relaxation yields a mismatch of 1600 ppm (±1%) between AlAs and GaAs, and 0.28±0.01 for the Poisson ratio of AlAs.

Characterization of nanometer‐scale epitaxial structures by grazing‐incidence x‐ray diffraction and specular reflectivity

X‐ray rocking curve analysis is a powerful and nondestructive technique for the characterization of heteroepitaxial structures. Conventionally, measurements are performed in symmetrical scattering geometry using a double‐crystal x‐ray diffractometer but the technique can be extended to the study of very thin layers (<200 A) by the use of glancing‐incidence scattering geometry and a triple‐crystal diffractometer. These structures can also be studied by the technique of total external x‐ray reflectivity. This is sensitive to the electron‐density profile of the heterostructure as a function of depth. By combining the above techniques we have found it possible to obtain structural information on layers as thin as 20 A. Such measurements permit accurate measurement of individual layer thicknesses and interface roughnesses on the angstrom level. The lattice parameter strain can be obtained by modeling of the intensity distribution of the crystal truncation rod.