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McQueeney, R J

Publications and source records attributed to McQueeney, R J.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Crossover from spin waves to diffusive spin excitations in underdoped Ba(Fe1-xCox)2 As2

Using inelastic neutron scattering, we show that the onset of superconductivity in underdoped Ba(Fe1-xCox)2As2 coincides with a crossover from well-defined spin waves to overdamped and diffusive spin excitations. This crossover occurs despite the presence of long-range stripe antiferromagnetic order for samples in a compositional range from x=0.04 to 0.055, and is a consequence of the shrinking spin-density wave gap and a corresponding increase in the particle-hole (Landau) damping. The latter effect is captured by a simple itinerant model relating Co doping to changes in the hot spots of the Fermi surface. We argue that the overdamped spin fluctuations provide a pairing mechanism for superconductivity in these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Relation between Ga ordering and magnetostriction of Fe-Ga alloys studied by x-ray diffuse scattering

Transmission synchrotron diffraction was employed to characterize the Ga ordering in magnetostrictive Fe{sub 100-x}Ga{sub x} alloys with Ga concentrations from 0 to 20.3 at.%. The experiments focused on the development of atomic short-range ordering (SRO) by analysis of the diffuse scattering appearing at superlattice positions of the D0{sub 3} ordered alloy structure. No SRO was found for Ga concentrations less than 4 at. %. Between 13 and 20.3 at. %, D0{sub 3}-type SRO clusters are observed whose size increases with Ga concentration for both slow-cooled and quenched samples. Thermal quenching of the samples suppresses the cluster sizes when compared to slow-cooled samples of the same concentration. For quenched samples with large Ga concentration, Ga pairing is evidenced by growing B2-type correlations. At low concentrations, the presence of SRO appears to have little (if any) effect on the enhanced magnetostriction. However, the dramatic suppression of magnetostriction near {approx} 20 at. % Ga can be linked rapid growth of SRO just before the onset of long-range chemical order.

43 PARTICLE ACCELERATORS↗
DOE OSTI2010

Magnetic structure of Gd[subscript 5]Ge[subscript 4]

Gd{sub 5}Ge{sub 4} crystallizes in the orthorhombic space group Pnma, and orders antiferromagnetically below the Neel temperature T{sub N} {approx} 127 K. We have employed x-ray resonant magnetic scattering to elucidate the details of the magnetic structure. The magnetic unit cell is the same as the chemical unit cell. From azimuth scans and the Q dependence of the magnetic scattering, all three Gd sites in the structure were determined to be in the same magnetic space group Pnma. The magnetic moments are primarily aligned along the c axis and the c components of the magnetic moments at the three different sites are equal. The ferromagnetic Gd-rich slabs are stacked antiferromagnetically along the b direction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗