A. Barla

High-energy-resolution x-ray optics with refractive collimators

We have tested the concept of a high-resolution x-ray monochromator with a refractive collimator as an optical element. Two options were examined, where the refractive collimator was included either instead of, or in addition to the first crystal of the high-resolution monochromator. The first approach offers an easy means of improving the energy resolution of conventional optical schemes by few orders of magnitude while still accepting the entire angular divergence of the primary beam. The second approach improves the performance of existing devices with very high-energy resolution and simplifies the design of future optical schemes.

Transition from the antiferromagnetic to a nonmagnetic state in FeBO3 under high pressure

A 57FeBO3 single crystal is studied by the nuclear forward scattering (NFS) method. The NFS time spectra from 57Fe nuclei are recorded at room temperature under high pressures up to 50 GPa in a diamond anvil cell. In the pressure interval 0<p<44 GPa, the magnetic field HFe at the 57Fe nuclei is found to increase nonlinearly, reaching a maximum value of 48.1 T at p=44 GPa. As the pressure increases further and reaches the point p=46 GPa, the field HFe abruptly drops to zero, indicating that a transition from the antiferromagnetic to a non-magnetic state occurs in the crystal. In the pressure interval 0<p<46 GPa, the magnetic moments of the iron ions lie in the (111) basal plane of the crystal. Several possible mechanisms of magnetic collapse are discussed.

Probing the (p,T ) phase diagram of CeFe2 and SmS using resonant x-ray scattering

Two strongly correlated electron systems CeFe2 and SmS have been studied using x-ray magnetic resonant scattering at low temperatures and high pressures. First, the magnetic ground state of CeFe2 doped with 7% Co has been probed by means of resonant x-ray magnetic scattering across the temperature range 10–95 K at pressures up to 9.5 kbar. A strong increase of the Neel temperature with pressure has been evidenced. Furthermore, a large increase in scattering intensity is observed just before TN at 9.5 kbar. These results reveal that the itinerant character of the 4f electrons which stabilize the antiferromagnetic state has been probed. Secondly, the structural and electronic properties of SmS have been studied under pressure up to 29 kbar and at temperatures down to 4.5 K via absorption and diffraction techniques. The measurements are a direct probe of the valence of Sm in SmS at low temperature from the black insulator phase to the gold metallic phase and furthermore across the magnetic transition. In particular, it is found that Sm in SmS has an intermediate valence (2.81(4)) in the magnetically ordered phase at 29 kbar and 4.5 K.

Delocalization of the U 5f magnetic moments in U(In0.6Sn0.4)3 and UNiSn under high pressure

The pressure–temperature dependence of the electronic and magnetic properties of the compounds U (In1−xSnx)3 and UNiSn has been investigated by means of high pressure x-ray diffraction, 119Sn nuclear forward scattering (NFS) of synchrotron radiation and Mossbauer spectroscopy (MS) measurements. We show that pressure has different effects on these systems: while U(In0.8Sn0.2)3 shows some of the typical properties of nearly localized 5f systems, pressure induces the delocalization of the U 5f electrons in U(In0.6Sn0.4)3 and UNiSn with a pressure dependent interplay between RKKY exchange interaction and the consequent collapse of the magnetic order. These results are discussed in terms of hybridization between U 5f electrons and conduction band electrons such as U 6d, Sn/In 5sp and Ni 3d electrons.