G. L. Whittle

The electronic band structure of a B2 intermetallic compound: localised states in CoGa

The results of a band structure calculation for the equiatomic B2 (CsCl) CoGa alloy are presented and discussed with reference to data obtained from electrical resistivity, magnetoresistance and Hall effect measurements on CoxGa100-x (40<or=x<65) alloys. The band structure calculations reveal a deep pseudo-gap in the density of states near the Fermi energy, around which the electronic wavefunctions are found to contain about 75% d tight-binding character. It is suggested that this partial localisation is responsible for the anomalously high resistivity (about 250 mu Omega cm) and an exponential rise in the electrical resistivity towards low temperatures found in these alloys. The high resistivity in turn necessitates a modification to the general expression for resistivity in which Mattheissens rule is multiplied by an average temperature-dependent Debye-Waller factor. This modification adequately reproduces the overall negative temperature coefficient found for CoxGa100-x alloys.

Ultra short pulse laser annealing of FeB metallic glasses

Abstract We present the results of picosecond laser annealing of as-quenched Fe 85 B 15 and Fe 82 B 18 metallic glasses. The influence of the laser radiation on the surface and bulk properties are studied using CrKα X-ray diffraction and transmission Mossbauer spectroscopy. The X-ray data show that the amorphous nature of the surface of the samples can be improved with laser treatment. The mat (cooling) surfaces of the ribbons appear to be more affected by the laser treatment, and show a higher stability. The Mossbauer data reveal that laser annealing of ribbon surfaces also affects the bulk properties of these materials due to induced stresses from the surface layer. The magnetic properties of these materials can be modified by laser annealing.

A Mössbauer study of rapidly quenchedCuFe alloys — Antiferromagnetic γ-Fe

A series ofCuFe alloys containing 5, 10 and 16 at% Fe has been prepared using standard ribbon spinning techniques. It is found that samples containing significant fractions (∼ 5%) of Fe in the form of r-Fe can be obtained readily on quenching from the (r + liquid) phase, applied field spectra (0–5.2 T) on antiferromagnetic r-Fe at 4. 2 K indicate that the anisotropy energy is small and that spins tilt from the minimum energy configuration (spin axes perpendicular to Bappl) for Bappl ≥ 2.5T.

Interplay of atomic order and magnetic behaviour in AuFe

Abstract Room temperature Mossbauer spectra of 5 and 10% Au Fe alloys reveal a tendency for clustering of iron atoms for samples in the as-rolled state, whereas the opposite tendency towards atomic short range order is observed after heat treatment. The effect of atomic order is also reflected in the spectra of magnetically ordered samples.

A Mossbauer study of local atomic order in AuFe alloys

The first atomic order parameter, alpha 1, has been determined for a series of Au100-xFex alloys (x=5, 10, 15 and 20) from an analysis of room-temperature Mossbauer spectra. All alloys in the as-rolled state exhibit positive values of alpha 1, indicative of clustering of Fe atoms within the Au matrix. Changes in atomic order are introduced with heat treatment. At low annealing temperatures, negligible change is observed in the values of alpha 1, due to the slow diffusion rates, but with increasing aging temperature the value of alpha 1 decreases and finally becomes negative, indicative of atomic short-range order. The lack of change of alpha 1 with heat treatment at aging temperatures greater than about 200 degrees C and about 330 degrees C, respectively, in Au95Fe5 and Au90Fe10 is due to the limitation imposed by the quench time with substantial reordering occurring during the quench. The metallurgical state of AuFe alloys annealed at high temperatures is therefore primarily dependent on the quench time. The diffusion process in AuFe alloys is shown to be predominantly via a single-vacancy diffusion mechanism.

On the determination of the magnetic hyperfine field distribution in an amorphous alloy

Abstract The magnetic hyperfine field distribution P(H) in Fe32Ni36Cr14P12B6 has been determined by several methods. The P(H) curves obtained over the range 1.5 K to the ordering temperature Tc ≈ 233 K, confirm the existence of two distinct regions of strong and weak magnetic interactions.

Magnetic field effects on Mössbauer spectra ofAuFe

Zero and applied field Mössbauer spectra onAuFe alloys with 5, 10 and 20 at % Fe have provided information on the spin glass, cluster glass and long range ferromagnetic phases which are present in this system. The mean hyperfine parameters for these phases are presented.

A Mössbauer effect study of the structural and magnetic properties of Co-Ga alloys

Abstract Mossbauer spectra have been recorded from Co x Ga 100− x (45⩽ x ⩽65) alloys at room temperature (RT) and 4.2 K in order to study the structural and magnetic properties of this system. The nonmagnetically split RT spectra are analyzed with the use of a lattice simulation model that predicts the effect of the antistructure Co atoms. The measured vacancy concentration ((ϱ x − ϱ) ϱ x ) and corresponding calculated number of AS Co atoms ([(100− x )( F −1)+50] where F =fraction of vacancies) were used in establishing the near neighbour atomic distributions around a Co atom. It was established that the large number of vacancies present (as high as 8% of total sites) exist solely on the Co sublattice. The effect of heat treatment on the vacancy concentration and atomic distributions is also studied. Spectra recorded at 4.2 K proved to be in agreement with previous magnetic studies on this system and on the Fe-substituted Co-Ga alloys, in which slow cooled alloys exhibit spin glass characteristics while the quenched alloys are best explained by a superparamagnetic model. Slow cooled alloys display smaller magnetic clusters and lower spontaneous magnetisation compared to quenched alloys of the same concentration. As the concentration is increased to near the limit of the phase boundary, the heat treatment was found to become unimportant.

Mössbauer isomer shifts and electric field grandients in Cr1−xFex Alloys

Room temperature Mössbauer spectra of Cr1−xFex alloys (x=0.05–0.26) have been fitted using a pseudo-lorentzian lineshape; this leads to the widths of gaussian processes which contribute to the line broadening. The variation of isomer shift with composition can be described well by an extension to the cellular model of Miedema and co-workers. Estimations of the contributions of the s electron charge transfer and the s-d electron conversions to the isomer shift are presented.

Effect of Cr and Ni substitution on the hyperfine parameters of Fe-B based metallic glasses

Some features of the variation of isomer shift, quadrupole splitting and magnetic hyperfine field distribution in Fe85−xCrxB15 (x=5–30), Fe85−yCr15By (y=10–20) and Fe85−xNixB15 (x=5–30) metallic glasses are reported. Changes in the isomer shift and quadrupole splitting for the Crx and By series are linked with a precursor stage for the onset of ferromagnetism.