S. Tsutsui

238U Mössbauer study on the magnetic properties of uranium-based heavy fermion superconductors

We have performed 238U Mössbauer spectroscopy of uranium-based heavy fermion superconductors, UPd2Al3 and URu2Si2, in order to investigate their physical properties, mainly their magnetic properties. The slow relaxation of magnetic hyperfine interaction in a paramagnetic state and the static hyperfine field has been observed in an antiferromagnetic ordered state for each compound. The line-widths have maximum at their characteristic temperatures where their magnetic susceptibilities have maximum values.

238U and 57Fe Mössbauer Spectroscopic Study of UFe2

We have performed 57Fe and 238U Mössbauer measurements of UFe2 in order to investigate its magnetic properties. From the results of 57Fe Mössbauer spectroscopy, the ferrromagnetic ordering at 167 K is caused by the iron 3d-electrons, which hybridize strongly with the uranium 5f-conduction electrons. It is also clarified from the results of 238U Mössbauer spectroscopy that there are no magnetic moments on the uranium atoms.

238U Mössbauer spectroscopic study of UPd2Al3 and URu2Si2

Abstract The magnetic properties of heavy-fermion superconductors, UPd2Al3 and URu2Si2, were studied by 238U Mossbauer spectroscopy to elucidate the local electronic states of uranium atoms directly. In the paramagnetic states of UPd2Al3 and URu2Si2, we have observed an appreciable change in line width caused by the paramagnetic relaxation, which is the evidence for the formation of the heavy fermion.

Magnetostriction in the ferromagnetic state of UGa2

Abstract X-ray diffraction study of ferromagnetic UGa2 indicated that a lattice distortion occurs below a Curie temperature of TC=123xa0K. We also observed the distortion in the sound velocity and thermal expansion measurements in the ferromagnetic state. We performed the thermal expansion in the magnetic field of 20xa0kOe along the magnetic easy axis and X-ray diffraction using a single crystal, and 238 U Mossbauer measurements using its powder in order to investigate the relationships between the ferromagnetic order and the lattice distortion.

238U Mössbauer Study on the 5f-Quadrupoles in Uranium-Based Intermetallics

We have determined the distribution of the 5f-quadrupoles and detected the 5f-quadrupole order in the uranium intermetallics, uranium dipnictides and UPd3, with 238U Mössbauer spectroscopy. We have declared that the distribution of the 5f-qaudrupoles depends on the magnetic sequence in uranium dipnictides. The distribution of the 5f-quadrupole is spherical in UBi2, while those are oblate within c-plane in UP2, UAs2 and USb2. We have observed the sudden drop of the recoil-free fraction above the transition temperature of the quadrupole order in UPd3. The present result suggests the change of the phonon states due to the quadrupole order in this compound.

238U Mössbauer study of UPt3

Abstract Using Mossbauer spectroscopy, the magnetic properties of UPt 3 have been investigated. The Mossbauer spectra in UPt 3 shows that the paramagnetic hyperfine interaction is similar to that in UPd 2 Al 3 and URu 2 Si 2 . The quadrupole interaction has been observed in the measured temperature range from 2.8 to 125xa0K. Around T χ max at which the magnetic susceptibility shows a maximum, the spectrum is the sharpest. Although the temperature dependence of the spectra in UPt 3 is different from those in UPd 2 Al 3 and URu 2 Si 2 , the obtained results suggest that the paramagnetic hyperfine interaction is related to the formation of the heavy fermion state in the 5f-electron system.

Phase transition of NpO2 at 25 K

Abstract We have measured the 237 Np Mossbauer spectra of NpO2 whose specimens have been stored for 12 years before and after annealing at 773, 973, 1273xa0K. The Mossbauer spectrum of NpO2 after annealing consists of a symmetrical absorption line, while the line shape of the sample before annealing is broad and asymmetric below 25xa0K. The change of line shape is attributed to the phase transition at 25xa0K which might be enhanced by the existence of the self-irradiation-induced defects. Irradiation-induced damage can be recovered by annealing above 973xa0K.

235U NMR study of antiferromagnet UO2

A typical hyperne structure of U4‘ ions is obtained by the 235U NMR in the antiferromagnetic state of UO 2 . The nuclear electric quadrupole interaction indicates that the 5f-wave function in the cubic crystalline eld is reduced to the axial symmetry below „ N . The nuclear spin-lattice relaxation rate at both uranium and oxygen sites shows a „7-like behavior in the antiferromagnetic state, which indicates the presence of a gapless spin-wave excitation. ( 2000 Elsevier Science B.V. All rights reserved.