Unusual magnetism ofNpO2:A study with resonant x-ray scattering

Abstract

The ground state of ${\mathrm{NpO}}_2$ has remained unresolved since the first evidence for a transition at low temperature was found in specific-heat experiments 45 years ago. Recently, muon spin relaxation measurements have shown the presence of magnetic ordering with an estimated ordered moment of $<~0.1{\mu }_B/\mathrm{Np}.$ Using a small $(<\mathrm{}1\mathrm{}$ mg) single crystal and photons tuned to the $M_4$ absorption edge of Np, we have observed the Bragg scattering from the ordered antiferromagnetic moments in ${\mathrm{NpO}}_2$ below $T_N=25\mathrm{}$ K. The magnetic structure is similar to that found in ${\mathrm{UO}}_2,$ except that the moments in ${\mathrm{NpO}}_2$ are aligned parallel to the magnetic wave vector, rather than perpendicular to it as in ${\mathrm{UO}}_2.$ No tetragonal distortion of the cubic crystal structure is found at $T_N,$ suggesting that the magnetic structure is triple q. The significance for our understanding of the magnetic ground state in ${\mathrm{NpO}}_2$ is discussed. An additional feature of these experiments, on an almost-perfect single crystal using a third-generation undulator beamline, is that we see strong effects arising from the interplay of sample absorption and the finite coherence of the incident photons that limits the full widths at half maxima of the diffraction peaks and modifies their integrated intensities. The observations are consistent with a recent theoretical treatment.