Browsing by Author "McQueeney, RJ"

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    Anisotropic three-dimensional magnetism in CaFe2As2
    (American Physical Society, 2008-11-28) McQueeney, RJ; Diallo, SO; Antropov, VP; Samolyuk, GD; Broholm, C; Ni, N; Nandi, S; Yethiraj, M; Zarestky, JL; Pulikkotil, JJ; Kreyssig, A; Lumsden, MD; Harmon, BN; Canfield, PC; Goldman, AI
    Inelastic neutron scattering measurements of the magnetic excitations in CaFe2As2 indicate that the spin wave velocity in the Fe layers is exceptionally large and similar in magnitude to the cuprates. However, the spin wave velocity perpendicular to the layers is at least half as large that in the layer, so that the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Exchange constants derived from band structure calculations predict spin wave velocities that are consistent with the experimental data. © 2008, American Physical Society
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    Zener double exchange from local valence fluctuations in magnetite
    (American Physical Society, 2007-12-14) McQueeney, RJ; Yethiraj, M; Chang, S; Montfrooij, W; Perring, TG; Honig, JM; Metcalf, P
    Magnetite (Fe3O4) is a mixed valent system where electronic conductivity occurs on the B site (octahedral) iron sublattice of the spinel structure. Below Tv=123 K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B site sublattice (similar to 80 meV) are shifted upwards in energy above Tv due to the occurrence of B-B ferromagnetic double exchange in the mixed valent phase. The double exchange interaction affects only spin waves of Δ5 symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by short-range electron correlations above Tv. © 2007, American Physical Society