Superconductivity and magnetism in rapidly solidified perovskites

The report is divided into six parts, reflecting major thrusts of our work since 1987. The six areas are: molecular orbital theory of high {Tc} superconductivity; rapid solidification processing of oxide superconductors; time dependent magnetic and superconducting properties of these inhomogeneous materials; excess Gd in Gd{sub 1+x}Ba{sub 2-x}Cu{sub 3}O{sub 7-{delta}} perovskites; rapid solidification and directional annealing to achieve high Jc; and Mossbauer studies of T = Fe, Co and Ni site selection in YBa{sub 2}(CuT){sub 3}O{sub 7-{delta}} and GdBa{sub 2}(CuT){sub 3}O{sub 7-{delta}}

Thermodynamic consideration of the effect of alloying elements on martensitic transformation in Fe-Mn-Si based alloys

The critical driving force for martensitic transformation, the stacking fault energy of parent phase at the start martensite start temperature Ms and the influence of anti-ferromagnetic transition on the Gibbs energy of Fe-Mn-Si alloys have been reviewed. The Gibbs energies of the fcc($\gamma$) and hcp($\varepsilon$) phases as a function of temperature have also been evaluated for Fe-Mn-Si-Cr quaternary alloys. The critical driving force for martensitic transformation in a representative quaternary alloy (Fe-26.4Mn-6.2Si-5.2Cr) was determined to be-145.25 J/mol, significantly lower than the value for Fe-30Mn-6Si, implying that the substitution of Cr for Mn decreases the stacking fault energy of the austenite

STRUCTURAL ASPECTS OF REVERSIBLE TRANSFORMATIONS IN Co-Nb-B AMORPHOUS ALLOYS

Des mesures de dilatométric sur des rubans amorphes de Co80 Nb14B6 sont présentées. Un modèle est proposé pour expliquer la transformation structurale du premier ordre observée.Results of dilatometric measurements on amorphous Co80Nb14B6 ribbons are presented. A model is proposed for the first-order structural transformation observed in this system. Measurements were made on samples prepared to have different initial local strain configurations induced by tensile stress treatment

Micromagnetics and micromechanics of Ni-Mn-Ga actuation

An analytic model is described that accounts for thc nanoscale strain field that defines the twin-boundary width and energy density as well as the interaction of a 90° domain wall with such a twin boundary. These parameters are being used as input to micromagnetic calculations of the domain-wall profile perturbed by the twin-boundary strain field. Further, application of a magnetic field can displace the domain wall from a pinned twin boundary with the Zeeman energy being stored elastically in the domain-wall anisotropy energy

Ni-Mn-Ga AC engineering properties

Magnetic-field-induced strains in Ni-Mn-Ga alloys have shown promise since their recent discovery. Large, free strains and moderate AC strains that provide mechanical work have already been demonstrated by this author and others. If the development of this ferromagnetic shape memory alloy (FSMA) is to continue towards application, its preparation and processing need to routinized so that its properties are as consistent as possible. Here, a first look into the AC stiffness, susceptibility, magneto-mechanical coefficient and coupling coefficient properties will be shown and their dependencies interpreted. Close to the austenite start temperature, results shown here may indicate better AC actuation performance 10–20°C below the martensite transformation temperature due to lattice softening. Through these results, an engineer will start to understand some of the technical issues necessary for designing FSMAs into applications

Antiferromagnetic coupling in perpendicularly magnetized Ni/Cu/Ni epitaxial trilayers

The magnetic properties of perpendicularly magnetized Ni/Cu/Ni epitaxial trilayers grown on Si(0 0 1) substrates have been studied. The thickness is 3.3nm for both the Ni layers while the Cu interlayer thickness ranges from 1.83 to 3.66 nm. The M–H loops measured with the magnetic field applied along the axis perpendicular to the film show plateaus characterized by an almost constant value of the magnetization. For specific values of the Cu interlayer thickness, evidence is given regarding an appreciable magnetoresistance effect, which can be explained in terms of an antiferromagnetic coupling between the two Ni films