Search

The ac magnetic susceptibility, the thermal expansion and the magnetostriction were measured using a single crystalline sample of UGe2 under pressure. We find that as the field exceeds a transition field, where the system transforms from a high pressure phase ( P > P X ) into a low pressure phase ( P P X ), the superconductivity steeply collapses within a transient region arising from the pressure inhomogeneity. We also estimate an accurate value of P X in zero temperature limit and a pressure distribution around it. Using thus estimated quantity, we argue that the superconductivity (in zero magnetic field) below the critical pressure is not intrinsic but extrinsic due to the pressure inhomogeneity.

PrPb 3 exhibits an antiferroquadrupolar transition at T Q = 0.4 K . In the present work, in order to reveal the order parameters of the ordered states in much higher fields than 5 T applied along the [1 1 ¯ 0] direction, we performed neutron diffraction experiments in the temperature range between 50 mK and 0.8 K in magnetic fields up to 14.5 T. Above 7 T, we observed magnetic reflections at q = ( δ δ 0.5 ) ( δ ∼ 0.02 ) and its equivalents. The field-induced antiferromagnetic components were normal to the applied field, indicating that the quadrupolar order parameter in the high field phase should be of O 2 2 -type.

We investigated ground state properties of a heavy-fermion antiferromagnet Ce 7 Ni 3 under hydrostatic pressures and magnetic fields. This compound undergoes two antiferromagnetic phase transitions at T N1 = 1.9 K and T N2 = 0.7 K. Below T N1 , a spin-density-wave (SDW) develops. Upon applying rather weak pressure 0.39 GPa = P c , both T N1 and T N2 vanish, and non-Fermi liquid behavior appears in the specific heat and magnetic susceptibility. The enhancement of residual resistivity along the a axis near P c is attributed to the increased spin fluctuations along the a axis. By applying fields B along the c axis, T N1 is suppressed and vanishes at 0.3 T. Magnetoresistance, specific-heat, and magnetization measurements revealed another field-induced magnetic (FIM) phase in the region B || c > 0.7 T and T < 0.5 K. Neutron diffraction experiments indicate that the magnetic unit cell in the c-plane for the FIM phase is treble that of the chemical unit cell. Moreover, this magnetic reflection intensity remains even in the region between the FIM phase and SDW phase. This observation indicates the presence of large spin fluctuations in the c-plane associated with the magnetic frustration, which should be responsible for the magnetic instability of Ce 7 Ni 3 .

We discuss recent results obtained for the heavy-fermion metals UPd2Al3 and YbRh2Si2. UPd2Al3 is the first among all superconductors for which tunneling and inelastic neutron-scattering data highlight a non-phononic, i.e., magnetic-exciton mediated, pair state. YbRh2Si2 represents a model system exhibiting pronounced non-Fermi liquid effects above a weak antiferromagnetic phase transition at T N =70 mK . Upon approaching the quantum critical point (TN→0), by low doping with Ge, one observes for T K disparate behavior in the temperature dependences of both the electrical resistivity and the electronic specific heat as well as a Curie–Weiss law in the uniform magnetic susceptibility, implying uncompensated large 4f moments. These observations indicate a break up of the composite quasiparticles into their local f-spin and itinerant conduction-electron parts.

We have measured magnetization curves and powder neutron diffraction of double-layered Ruddlesden–Popper type ruthenate Sr 3− x Ca x Ru 2 O 7 ( x =1.5, 2.0 and 3.0). The field dependence of the magnetization revealed that the transition field of metamagnetic transition along the b -axis shifted to lower fields and that the transition became broad with increasing Sr content. The slope of the magnetization curve also increased with increasing Sr content below the metamagnetic transition. These results indicate that an itinerant component is partly introduced by the Sr substitution. From the magnetic reflection, on cooling below T N , an additional reflection was observed at (0 0 1) for each x , and the amplitude increased with decreasing temperature. The observed diffraction patterns are very similar to those of Ca 3 Ru 2 O 7 . We conclude that the magnetic structure of the antiferromagnetic ordered phase is basically the same structure with that of Ca 3 Ru 2 O 7 .

We present the results of inelastic neutron scattering experiments on quantum spin systems KCuCl 3 and SrCu 2 (BO 3 ) 2 . Both compounds show non-magnetic singlet ground state behaviour at low temperature. KCuCl 3 turns out to be a weakly coupled dimer system without any strong frustrating interdimer interactions. SrCu 2 (BO 3 ) 2 , on the other hand, should be regarded as a strongly coupled dimer system with a strong frustration originating from the orthogonal arrangement of the neighbouring dimers. The characteristics of the spin excitations in these two dimer systems are compared and discussed.

We investigated static and dynamical properties in the three-dimensional percolating Heisenberg antiferromagnets, RbMn c Mg 1 - c F 3 , with the magnetic concentration close to the percolation threshold, c P = 0.312 , around the superlattice point well below T N . In neutron diffraction experiment, the wave number dependence of the elastic scattering component was well fitted to q - x . Magnetic fractons were also studied using inelastic neutron scattering, and the observed fractons showed the dispersion relation of q z . The determined exponents, x = 2.43 ± 0.05 and z = 2.5 ± 0.1 , were in good agreement with the fractal dimension ( D f = 2.48 ) .

We report a superconducting phase diagram of the ferromagnetic superconductor UGe 2 investigated by AC magnetic susceptibility measurements. In contrast to previous phase diagrams, we found that the superconducting transition temperature and volume fraction show a “M-shaped” structure as a function of pressure. From this observation, we suggest that both of two critical points will play a crucial role in the occurrence of superconductivity in UGe 2 .

Neutron diffraction experiment was performed to investigate a nature of the ordered phase below 0.9 K of the filled skutterudite Kondo semiconductor CeOs 4 Sb 12 . We observed antiferromagnetic ordering with a tiny magnetic moment of around 0.03 μ B /f.u. characterized by the wave vector q =(1 0 0). The magnetic ordered state can be interpreted as a spin density wave formation due to the nesting of the small Fermi surface within the hybridization gap.

Neutron diffraction experiments were performed to investigate a nature of the antiferromagnetic ordered phase below T N = 1.6 K of the pressure-induced superconductor CeRhSi 3 . We observed incommensurate magnetic reflections characterized by the wave vector Q = ( ± δ , 0 , 0.5 ) ( δ ∼ 0.215 ) . The antiferromagnetic ordered state can be interpreted as a spin-density wave formation by taking account of the results on de Haas–van Alphen (dHvA) signals in CeRhSi 3 where the 4f-electrons of CeRhSi 3 are itinerant.

Inelastic neutron scattering experiments were performed on U ( Ru 1 - x Rh x ) 2 Si 2 with x = 0 and 0.02 in order to investigate the variations of the antiferromagnetic (AF) fluctuations with doping Rh. We have found that the energy of the magnetic excitation observed at Q = ( 1 , 0 , 0 ) in the hidden order (HO) phase markedly decreases from ∼ 2.6 meV ( x = 0 ) to ∼ 0 ( x = 0.02 ) with increasing x . In addition, the staggered susceptibility χ Q estimated from the inelastic peak develops in the HO phase at x = 0.02 , while it is suppressed at x = 0 . These results indicate that the AF fluctuation in the HO phase is significantly enhanced with doping Rh.

In the high temperature superconductor La 1.85 Sr 0.15 Cu 1 - y Zn y O 4 , we directly observed modulated antiferromagnetic correlations of Cu spin orientations via elastic ( y = 0.02 ) and low energy inelastic ( y = 0.004 ) magnetically scattered neutron intensity. A magnetic field was applied perpendicular to the CuO 2 plane to change these correlations. For y = 0.02 we confirmed an earlier observation of elastic scattered intensity at 2 K and 0 T. The component of the magnetic moment perpendicular to the scattering vector is μ ⊥ Q 0.065 μ B , and is enhanced by 32% when a 5.5 T field is applied. For y = 0.004 , we observed a 145% enhancement of 3 meV spin excitations when a 14.5 T magnetic field is applied at 1.6 K. However, we observed no elastic scattered intensity at either 0 or 14.5 T.

We have performed neutron scattering experiments to investigate the static magnetic structure and the magnetic critical scattering in the three-dimensional percolating Heisenberg antiferromagnet, RbMn 0.31 Mg 0.69 F 3 , whose magnetic concentration is just at the percolation concentration for a cubic lattice, c P = 0.312 . Magnetic scattering was observed around the superlattice point, ( 1 2 , 1 2 , 1 2 ) . We determined the Neel temperature of this system to be T N = 4.0 ± 0.5 K , and found that the critical scattering above T N is similar to that observed in the homogeneous system. At T = 1.6 K well below T N , the scattering function is described by a sum of the critical scattering and elastic scattering components, and we demonstrated that the q -variation of the intensity of the elastic scattering component can be described using the fractal dimension.

The cubic intermetallic compound PrPb 3 is considered to exhibit an antiferro-quadrupolar (AFQ) ordering at T Q = 0.4 K . We have performed neutron diffraction experiments on PrPb 3 in order to detect a possible induced antiferromagnetic (AFM) moment in the AFQ phase. At T = 40 mK , a small magnetic reflection q = ( 1 2 δ δ ) with δ = 0.062 is observed above 3 T. The emergence of the magnetic reflection is accompanied by a large hysteresis, implying the transition to be of first order. The direction of the AFM component ( | | [ 1 1 0 ] ) suggests an underlying AFQ order of O 2 2 type.

Elastic neutron scattering experiments on the pressure-induced ferromagnetic superconductor UGe 2 have been carried out under pressures P up to 1.6 GPa at temperatures T down to 60 mK, to study the correlation between superconductivity and ferromagnetism. No change was observed in ferromagnetic Bragg peak intensities nor a line width at Q = ( 0 , 0 , 1 ) through the superconducting transition temperature T SC under P ∼ 1.10 and 1.25 GPa, suggesting that the ferromagnetism is not affected by the onset of superconductivity under these two pressures.

The magnetic form factor in the pressure-induced ferromagnetic superconductor UGe2 has been measured with polarized neutrons. The experiments were performed on single crystal samples at 2 K by applying a magnetic field of 2 T along the easy magnetization axis a under pressures of 0 and 1.4 GPa . In spite of the itinerant nature of the 5f electrons, the shapes of the observed magnetic form factors at both 0 and 1.4 GPa are very close to the theoretical 5f form factor of the free U3+ or U4+ ions calculated by the dipole approximation. The effect of pressure on the 5f form factor seems to be small in spite of the reduction of the magnetic moment.

The static and dynamic magnetic properties of the heavy fermion superconductor UNi 2 Al 3 have been studied using spherical neutron polarimetry (SNP) and three-axis spectroscopy (TAS), respectively. The SNP measurements give strong evidence that, in the incommensurate antiferromagnetic state and without applied external field, the magnetic moments lie in the basal plane with a dominant component along the a ∗ -direction. The form of the structure is a longitudinally modulated spin-density wave. The TAS experiments reveal quasi-elastic magnetic scattering in the vicinity of the ordering wave vector. These results are compared to those determined for the isostructural compound UPd 2 Al 3 .

The heavy fermion compound UPd 2 Al 3 has attracted much interest on account of the coexistence of antiferromagnetism and superconductivity at temperatures below 2 K. The antiferromagnetic fluctuations provide a window on the low-frequency dynamics in this material. By an analysis of neutron scattering data, and taking into consideration results from other experimental probes, it is suggested which sheet(s) of the f-electron Fermi surface play an active role in forming the superconducting state in UPd 2 Al 3 .

Recently, in phase IV of Ce x La 1 - x B 6 , weak but distinct superlattice reflections from the order parameter of phase IV have been detected by our unpolarized neutron scattering experiment [K. Kuwahara, K. Iwasa, M. Kohgi, N. Aso, M. Sera, F. Iga, J. Phys. Soc. Japan 76 (2007) 093702]. The scattering vector dependence of the intensity of superlattice reflections is quite unusual; the intensity is stronger for high scattering vectors. This result strongly indicates that the order parameter of phase IV is the magnetic octupole. However, the possibility that the observed superlattice reflections are due to lattice distortions could not be completely ruled out only on the basis of the unpolarized neutron scattering experiment. To confirm that the superlattice reflections are magnetic, therefore, we have performed a single crystal polarized neutron diffraction experiment on Ce 0.7 La 0.3 B 6 . The obtained result has clearly shown that the time reversal symmetry is broken by the order parameter of phase IV. This is further evidence for the magnetic octupole order in Ce x La 1 - x B 6 .

We have performed inelastic neutron scattering experiments to examine the magnetic ordering in U3Pd20Si6, which exhibits antiferromagnetic ordering at T-N =19 K and ferromagnetic ordering at T-C = 2 K. We observed that both anti ferromagnetic spin wave excitations due to the magnetic moments at the 8c site and ferromagnetic ones due to the magnetic moments at the 4a site coexisted below TC = 2 K. Our results indicate direct experimental evidence that U3Pd20Si6 is a rare localized 5f electron system in uranium intermetalic compounds, with coexistence of both anti ferromagnetic and ferromagnetic ordering. The anomolous line width around the zone center is also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

We report on recent neutron inelastic scattering results on NaV2O5 in wide ω– Q space supporting the realization of zigzag-type charge ordering in the low-temperature singlet state of NaV2O5, as has been already indicated by our earlier neutron scattering studies.