Negative Thermal Expansion and Antiferromagnetism in the Actinide Oxypnictide NpFeAsO

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 TN = 57 K is suggested by anomalies in the temperature-dependent magnetic susceptibility, electrical resistivity, Hall coefficient, and specific-heat curves. Below TN, powder neutron diffraction measurements reveal an antiferromagnetic structure of the Np sublattice, with an ordered magnetic moment of 1.70 ± 0.07μB aligned along the crystallographic c axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3μB for the ordered magnetic moment on the iron. High-resolution x-ray powder diffraction measurements exclude the occurrence of lattice transformations down to 5K, 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 TN, 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.

KLIMCZUK Tomasz;  WALKER H.C.;  SPRINGELL R.;  SHIK Alexander;  HILL A. H.;  GACZYNSKI Piotr;  GOFRYK Krzysztof;  KIMBER S. A. J.;  RITTER C.;  COLINEAU Eric;  GRIVEAU Jean-Christophe;  BOUEXIERE Daniel;  ELOIRDI Rachel;  CAVA R. J.;  CACIUFFO Roberto; 

2012-05-16

AMER PHYSICAL SOC

JRC70538

1098-0121,   

http://prb.aps.org/abstract/PRB/v85/i17/e174506,    https://publications.jrc.ec.europa.eu/repository/handle/JRC70538,   

10.1103/PhysRevB.85.174506,