Coexistence of different magnetic moments in CeRuSn probed by polarized neutrons

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding clearly documents that distnictly different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a re-distribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions co-exist in different valence states. Localized 4f states that fulfill the third Hunds rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous alpha-gamma transition in elemental cerium is discussed.

Anomalous spin distribution in the superconducting ferromagnet UCoGe studied by polarized neutron diffraction

We report a polarized neutron diffraction study conducted to reveal the nature of the weak ferromagnetic moment in the superconducting ferromagnet UCoGe. We find that the ordered moment in the normal phase in low magnetic fields (B // c) is predominantly located at the U atom and has a magnitude of about 0.1 muB at 3 T, in agreement with bulk magnetization data. By increasing the magnetic field the U moment grows to about 0.3 muB in 12 T and most remarkably, induces a substantial moment (about 0.2 muB) on the Co atom directed antiparallel to the U moment. The anomalous polarizability of the Co 3d orbitals is unique among uranium intermetallics and might reflect the proximity to a magnetic quantum critical point of UCoGe in zero field.

Evidence from neutron diffraction for superconductivity in the stabilized tetragonal phase of CaFe2As2 under uniaxial pressure

CaFe2As2 single crystals under uniaxial pressure applied along the c axis exhibit the coexistence of several structural phases at low temperatures. We show that the room-temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in CaFe2As2 found below 10 K in samples subjected to nonhydrostatic pressure conditions.