Yoshichika Ōnuki

Magnetic and Fermi Surface Properties of UCoGa5 and URhGa5

We succeeded in growing high-quality single crystals of UCoGa5 and URhGa5 by the self-flux method and measured the electrical resistivity, magnetic susceptibility, and de Hass-van Alphen(dHvA) oscillation. Three kinds of dHvA branches were observed, ranging from 8.6x 105 to 8.1 x 106Oe, which correspond to closed Fermi surfaces. UCoGa5 and URhGas are concluded to be semimetals. The cyclotron mass is small, ranging from 0.6 m0 to 2.9 m0.

Spin-glass behavior with short-range antiferromagnetic order in Nd2AgIn3

We present experimental results of complex susceptibility, static magnetization, magnetic relaxation, specific heat, and electrical resistivity measurements on an intermetallic compound Nd2AgIn3. The results indicated that Nd2AgIn3 undergoes a spin-glass transition at a freezing temperature Tf=12.6u200aK accompanied with the formation of short-range antiferromagnetic order. The mechanism of formation of spin-glass state in Nd2AgIn3 is different from that in diluted metallic spin-glass or uranium intermetallic compound. The existences of frustrated moments due to the triangles of nearest neighbors in Nd atomic layers and randomly distributed Ruderman–Kittle–Kasuya–Yosida interactions may be responsible for the spin glass state formed in Nd2AgIn3.

Magnetic and Electrical Properties of UIr

We succeeded in growing a high-quality single crystal of UIr with the monoclinic PbBi-type structure by the Czochralski pulling method in a tetra-arc furnace. The magnetization measurement indicates that UIr is an Ising-like ferromagnet with the Curie temperature TC= 46 K. The magnetic moment is found.to be oriented along the [101] direction in the (010) plane. The saturated moment is 0.5 μB/U. The magnetic property, reflected in the magnetic susceptibility, electrical resistivity, Hall coefficient and thermoelectric power, is highly anisotropic. We also observed the de Haas-van Alphen (dHvA) oscillation. All the detected branches have large cyclotron effective masses ranging from 9.6 to 32 m0. They are consistent with the electronic specific heat coefficient γ=49 mJ/K2·mol.

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.

Transport properties in UCoAl under uniaxial pressure

Abstract We report the first study of uniaxial pressure (Pu) effect on the transport properties in the unique 5f electron metamagnet UCoAl. A metamagnetic transition around a critical magnetic field B M ∼0.7 T applied along the c-axis at ambient pressure shifts to higher fields with increasing Pu applied along the a-axis. This increase is in contrast to the decrease of BM under Pu applied along the c-axis. The result may be interpreted most likely as an enhancement of 3d–5f hybridization in the basal plane under Pu along the a-axis.

Neutron diffraction study of 5f itinerant antiferromagnet UPtGa5 and UNiGa5

Abstract Magneto-striction and magnetic form factors in 5f itinerant antiferromagnets UNiGa5 and UPtGa5 are studied by means of neutron scattering. Remarkable magneto-striction was observed around TN, indicating large spin–orbit coupling in the itinerant system. The orbital magnetic moment is found to be strongly suppressed due to the hybridization of uranium 5f with Ga-4p electron.

Magnetic Properties of UNiGa5 and UPtGa5

In order to investigate the magnetic properties in UNiGa5 and UPtGa5, we have performed the magnetic susceptibility, electrical resistivity and neutron scattering experiments. Small magnetic susceptibility and its weak temperature dependence demonstrate the itinerant character of the 5f electrons. The resistivity exhibits a hump at the Néel temperature, which is characteristic of the spin-density wave state. We determined the antiferromagnetic structures in UNiGa5 and UPtGa5. Uranium spins in UNiGa5 has the Néel-type structure with the magnetic moment 0.9 μB/U along the [0 0 1] direction. On the other hand, the magnetic moments of uranium atoms in UPtGa5 are aligned ferromagnetically in the basal plane and are directed along the [0 0 1] direction in an antiferromagnetic sequence. The magnetic moment was estimated to be 0.24 μB/U.

Single Crystal Growth and the High-pressure Effect of an Electronic State in UNiGa5

We studied the electronic state of UNiGa5 by measuring the electrical resistivity under pressure. When pressure is applied to UNiGa5, the Néel temperature TN = 86.5 K decreases steeply and becomes zero around 4.5 GPa. Correspondingly, a coeffiecient A in the electrical resistivity of ρ = ρ0 + AT2, which is a characteristic nature of Fermi liquid, increases steeply with increasing pressure. Around a quantum critical region of about 4.5 GPa, a non-Fermi liquid nature is found, not showing a T2-dependence of the resistivity. With further increasing pressure above 6 GPa, the Fermi liquid nature recovers completely and the A-value becomes small.

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.