Resonant elastic X-ray scattering from 5f systems

Abstract

Abstract. The first REXS experiments on a uranium compound at the U M 4 edge (3.728 keV) took place at BNL twenty years ago. An enormous enhancement of the scattering intensity was found. Since that time many other systems have been examined. This paper reviews some of the highlights of resonant scattering from actinide systems, and attempts to extrapolate what might be the future of this field.Soon after resonant scattering was discovered at the L edges in Ho [1], it was realised that the strongest effects in resonant elastic X-ray scattering (REXS) could be observed if the transitions involve partially occupied shells with strongly spin-polarised states. Thus the largest resonant effects are found when the L 2,3 edges are used for transition metals of the 3d series, and at M 4,5 (or N 4,5 ) edges for the 4f and 5f series. In the case of the M 4,5 edges of the actinides (5f ) the energies are between 3.5 and 5 keV, so within reach of many diffractometers, without the complications of soft X-rays. Hence the first experiment on UAs [2] at BNL in 1989 found the resonant enhancement to be about 6 orders of magnitude.In the 20 years since this experiment a great many 5f systems have been examined. Some noteworthy highlights are briefly mentioned in this extended abstract.(1) Extension to transuranium materials. At BNL studies were done with NpAs to show both critical scattering and a full examination of the magnetic structure [3,4]. Later, after some difficulties with the crystal surfaces [5], experiments were successfully done on a series of (U 1−x Pu x )Sb single crystals [6,7]. This latter study found an interesting effect at the Pu M 5 edge [7], but this has not been confirmed by XMCD experiments, so its origin remains in doubt. (2) In Ref. [6,7] the element specific nature of REXS was well demonstrated. Another study at ID20 (ESRF) examined alloys of (U 1−x Np x )Ru 2 Si 2 , where the parent compound with x = 0 is the famous heavy-fermion "hidden-order" material, which is still not understood. In REXS study [8] scaling gave a magnetic moment of ∼0.4µ B on the U atom. This was attributed to the large molecular field provided by the ordered Np moments (of 1.5µ B ). It is now known that the large moment state of URu 2 Si 2 does indeed have a moment of this magnitude.

Probing a crystal's short‐range structure and local orbitals by Resonant X‐ray Diffraction methods

“…. In 2012, further reviews appeared on the occasion of the Resonant Elastic X‐ray Scattering workshop 2011 concerning the development within the last 20 years in general and with specific focus on actinide systems , theoretical aspects as well as polarization analysis .…”

Resonant elastic X-ray scattering: Where from? where to?

“…The strength of REXS is to complement neutron diffraction experiments with a much superior resolution in q-space and a wide variety of 'geometrical configurations' (polarisation effects, pure scattering geometry) that enables disentangling equivalent solutions for magnetic structures [83]. Selected resonant edges have moved from the 'easy' 7-20 keV range that was exploited in the early days of REXS (with the notable exception of [14] where K edge resonant of Ni metal was used) to actinide range [79][80][81][82][83][84] but also to the soft X-ray range [85]. Furthermore, the REXS scattering amplitude contains terms that allow the uncovering of multi-q structures; clear signatures of couplings of Fourier components of the ordered magnetic moments with equivalent propagation vectors have been observed [84], leading to complete and unambiguous determinations of magnetic structures.…”

“…Selected resonant edges have moved from the 'easy' 7-20 keV range that was exploited in the early days of REXS (with the notable exception of [14] where K edge resonant of Ni metal was used) to actinide range [79][80][81][82][83][84] but also to the soft X-ray range [85]. Furthermore, the REXS scattering amplitude contains terms that allow the uncovering of multi-q structures; clear signatures of couplings of Fourier components of the ordered magnetic moments with equivalent propagation vectors have been observed [84], leading to complete and unambiguous determinations of magnetic structures. It should be also noted that magnetic correlations at and near surfaces have been detected thanks to REXS methods [86][87][88][89].…”

“…Finally, the element specificity of REXS has been exploited to reveal contributions to magnetic scattering arising from 'non-magnetic' anions [84,102,103], although neutron scattering studies have indicated the absence of long-range ordered magnetic moments on these anions. These observations have provided pertinent information about the hybridization of electronic wavefunctions.…”

REXS contribution to electronic ordering investigation in solids

“…On resonance, atomic scattering factor can be strongly enhanced (up to orders of magnitude in the heavy elements [15]), resulting in selective contrast (in compounds) and promoting the appearance of forbidden reflections in the case of non-symmorphic Space Groups (SGs) [16][17][18][19][20][21][22]. Indeed, the resonant contribution originates from the outer electronic shells (less than spherically symmetric) and appears as tensorial properties of the scattering factor (Anisotropic Tensor of Susceptibility, ATS).…”