Negative thermal expansion and antiferromagnetism in the actinide oxypnictide NpFeAsO

T. Klimczuk, H. C. Walker, R. Springell, A. B. Shick, A. H. Hill, P. Gaczyński, K. Gofryk, S. A.J. Kimber, C. Ritter, E. Colineau, J. C. Griveau, D. Bouëxière, R. Eloirdi, R. J. Cava, R. Caciuffo

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A neptunium analog of the LaFeAsO tetragonal layered compound has been synthesized and characterized by a variety of experimental techniques. The occurrence of long-range magnetic order below a critical temperature T N = 57 K is suggested by anomalies in the temperature-dependent magnetic susceptibility, electrical resistivity, Hall coefficient, and specific-heat curves. Below T N, powder neutron diffraction measurements reveal an antiferromagnetic structure of the Np sublattice, with an ordered magnetic moment of 1.70 ± 0.07μ B aligned along the crystallographic c axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3μ B for the ordered magnetic moment on the iron. High-resolution x-ray powder diffraction measurements exclude the occurrence of lattice transformations down to 5 K, in sharp contrast to the observation of a tetragonal-to-orthorhombic distortion in the rare-earth analogs, which has been associated with the stabilization of a spin-density wave on the iron sublattice. Instead, a significant expansion of the NpFeAsO lattice parameters is observed with decreasing temperature below T N, corresponding to a relative volume change of about 0.2% and to an Invar behavior between 5 and 20 K. First-principles electronic structure calculations based on the local spin density plus Coulomb interaction and the local density plus Hubbard-I approximations provide results in good agreement with the experimental findings.

Original languageEnglish (US)
Article number174506
JournalPhysical Review B - Condensed Matter and Materials Physics
Issue number17
StatePublished - May 9 2012

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics


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