Evolution of 4f heavy fermion state from 5f heavy fermion state in U1−xCexPd2Al3 system

We report the effect of Ce substitution for U in the heavy-electron antiferromagnetic superconductor UPd₂Al₃. CePd₂Al₃ is an established non-superconducting heavy-electron system which undergoes antiferromagnetic ordering below 2.8 K. Thus studies on U_1−xCe_xPd₂Al₃ system provide a unique opportunity to observe the evolution from a 4f heavy-electron state to a 5f heavy-electron state. We have measured the resistivity and magnetic susceptibility from 1.5 to 300 K and the heat capacity from 2 to 20 K for several U_1−xCe_xPd₂Al₃ samples. Our studies show that the antiferromagnetic (AF) ordering temperature (T_N) of U_1−xCe_xPd₂Al₃ does not decrease monotonically from T_N = 14 K for UPd₂Al₃ to T_N = 2.8 K for CePd₂Al₃ but rather shows a local maximum of 5 K near x = 0.4.     

First direct evidence for a magnetic phase transition in the heavy-electron system UCd11

Muon-spin-rotation and relaxation measurements performed on polycrystalline UCd₁₁ give first direct evidence for an antiferromagnetic phase transition at 5 K. As far as probed by the positive muon the behaviour close to the phase transition is clearly contrasted to that in other magnetic heavy-electron materials investigated by μ⁺SR, e.g. U₂Zn₁₇ and UCu₅, because in UCd₁₁ longitudinal field measurements reveal dynamic relaxation effects which are attributed to the critical slowing down of the uranium 5f-moments. No signal could be observed in the ordered regime because of a large dipolar field spread.     

A comparative study of the magnetic heavy-electron materials U2Zn17 and UCu5 by μ+SR

A systematic μ⁺SR study of the magnetic heavy-electron systems U₂Zn₁₇ and UCu₅ in the paramagnetic and in the magnetically ordered state is presented. In both systems the antiferromagnetic nature of the low-temperature phase could be at least partially confirmed, but the muon reveals significant differences with regard to the phase transition itself. UCu₅ behaves like a model-antiferromagnet showing a drastic increase of the relaxation rate both below and above T_N, two spontaneous frequencies in the ordered phase, and a Knight shift above T_N which scales with the bulk susceptibility. In contrast U₂Zn₁₇ shows a loss of μ⁺ asymmetry by 20% below T_N, which is independent of the external field but can be quenched in sufficiently strong longitudinal fields. No scaling of the Knight shift and the susceptibility was observed and no critical increase of the relaxation rate λ. Most astonishing is the strong and nonlinear field dependence of λ above and below T_N in both compounds. The absence of longitudinal relaxation demonstrates the static origin of λ.     

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.     

Nuclear quadrupole resonance study of heavy fermion superconductors

By using nuclear quadrupole resonance (NQR), nuclear spin-lattice relaxation rate 1 /T₁ of the heavy fermion superconductors (URu₂Si₂, UPd₂Al₃, CeRu₂) has been measured. The NQR measurement requires no external field, and is especially suitable for ¹⁰⁵Pd and ¹⁰¹Ru, which have very small nuclear gyromagnetic ratios and large electric quadrupole moments. In URu₂Si₂ and UPd₂Al₃, the absence of the Hebel–Slichter coherence peak just below the superconducting transition temperature T_C and the power law temperature dependence (T³) in the superconducting state has shown appearance of anisotropic non-s-wave superconductivity. On the contrary, an exponential temperature dependence of 1/T₁ was observed in CeRu₂, indicating the superconductivity to be conventional s-wave. This revised version was published online in August 2006 with corrections to the Cover Date.     

μSR studies of heavy fermion systems

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe₁₃, URu₂Si₂ and U₆Fe. The magnetic field penetration depth in the superconducting state of UBe₁₃ is greater than 10000 Å, as no increase in the transverse field relaxation rate is observed belowT _c . A sharp increase in the precession frequency is seen, starting atT _c . This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U₆Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atT _c=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.     

High magnetic field behavior of strongly correlated uranium-based compounds

Magnetic fields are now available to 100?T (pulsed), 45?T (static) at temperatures below 0.3?K. Such technical developments allow the study and tuning of (quantum) phase transitions, unusual magnetic structures and (high-temperature) superconductors in a variety of quantum materials. An especially important class of strongly correlated electron materials is the heavy Fermi liquids (HFLs) displaying numerous reduced-moment antiferromagnets, quantum critical points, unconventional superconductivity, hidden order (HO) and other mysterious ground states. Among the ‘heavy fermions’, the duality of 5f electrons in uranium-based compounds introduces interesting behavior that can be affected by large magnetic fields. I list a few such heavy fermion materials to be considered: URu₂Si₂ and its tunable hidden state, UBe₁₃ and UPt₃ as very HFL paramagnets that become superconducting, the magnetic superconductors UPd₂Al₃ and UNi₂Al₃, and the ferromagnetic s UGe₂, URhGe and UCoGe. There are also the suggested metamagnetic Fermi-surface reconstructed intermetallic compounds such as UPt₂Si₂ and UCo₂Si₂. Present research attention focuses on the high-field behavior (30–40?T) of URu₂Si₂ and its destruction of HO. These and other U-based systems, e.g. UAu₂Si₂, UIrGe, etc., expand the opportunities of high magnetic field studies far into the future.     

Magnetic properties of UT2Si2 and UT2Ge2 (T = Co,Ni, Cu) intermetallic systems

Neutron diffraction and magnetization measurements indicate that, at low temperatures, long-range magnetic order is present in UCO₂Si₂, UNi₂Si₂, UCu₂Si₂, UNi₂Ge₂, and UCo₂Ge₂. UCo₂Si₂ and UNi₂Ge₂ are simple collinear antiferromagnets of +-+- type, UCu₂Si₂ a simple collinear ferromagnet. In UNi₂Si₂, a magnetic phase transition from a LSW type structure to collinear antiferromagnetism of +-+- type was found, while in UCu₂Ge₂, the antiferromagnetic structure of ++-- transforms into collinear ferromagnetism. Crystal structure and magnetic parameters are given. No magnetic moment on transition metal ions was found within the accuracy of a powder neutron diffraction experiment. The stability of particular magnetic ordering schemes is discussed in terms of an isotropic RKKY mechanism.     

Anomalous hall effect in some uranium intermetallic compounds

We report Hall effect measurements on the uranium intermetallic compounds UAl₂, U_0.97La_0.03Al₂, U_0.93La_0.07Al₂, U_0.85La_0.15Al₂, UPt₃ and UPd₃. We find in all of these substances, a large positive Hall effect at high temperatures and maxima or minima at low temperatures. We have compared our data with the results of magnetic skew scattering theories and found qualitative agreement at high temperatures in the cases of U_1-xLa_xAl₂ and UPt₃, but not in the case of UPd₃.     

High-field magnetism and magnetoacoustics in uranium intermetallic antiferromagnets

We report on studies on the magnetic and magnetoacoustic properties of uranium intermetallic antiferromagnets UCo₂Si₂, UCu_0.95Ge, UIrGe and U₂Ni₂Sn in pulsed magnetic fields up to 60 T, where they undergo metamagnetic transitions. The measurements were performed in the temperature range from 1.4 to 100 K. The magnetic ordering and the metamagnetic transitions are accompanied by pronounced anomalies in the ultrasound velocity and the ultrasound attenuation. All studies were performed on single crystals grown by the Czochralski method in a tri-arc furnace.     

Itinerant and local magnetism,superconductivity and mixed valency phenomena in RM2Si2, (R = rare earth,M = Rh,Ru)°

X-Ray, magnetization and Mossbauer (¹⁵¹Eu, ¹⁵⁵Gd, ¹⁶¹Dy and dilute ⁵⁷Fe) studies of RM₂Si₂ reveal that when R is a magnetic ion the compounds order antiferromagnetically. For M = Rh a second antiferromagnetic phase transition is observed, corresponding to Rh itinerant electron magnetic ordering. In EuRh₂Si₂ the Eu ion is predominantly divalent with a mixed valent component. In EuRu₂Si₂ the Eu is predominantly trivalent. LaRu₂Si₂ and LuRu₂Si₂ display enhanced electron paramagnetism and become superconducting at 3.5 K and 2.4 K respectively. LaRh₂Si₂, YRh₂Si₂ and LuRh₂Si₂ display an itinerant electron magnetic phase transition, T_M (LaRh₂Si₂) = 7 K, and at lower temperatures a superconducting phase transition, T_c(LaRh₂Si₂) = 3.8 K. There is evidence that in the superconducting phase the itinerant magnetic order survives.     

Neutron diffraction study of the heavy fermion superconductors UM2Al3(M=Pd,Ni)

An elastic neutron scattering study was performed on the new superconducting heavy fermion systems UPd₂Al₃ and UNi₂Al₃. The neutron diffraction patterns reveal unambiguously long range antiferromagnetic order in UPd₂Al₃ with an ordered magnetic moment _U = (0.85±0.03) _B , which coexists with the superconducting state. This is by far the largest _U value observed for any heavy fermion superconductor. For UNi₂Al₃, no long-range magnetic order could be observed for temperaturesT1.5 K, yielding an upper limit of the ordered moment of 0.2 _B .     

Superconductor order parameter symmetry in UPd Al

The heavy fermion compound UPd₂Al₃ has attracted much interest on account of the coexistence of antiferromagnetism and superconductivity at temperatures below 2 K. The antiferromagnetic fluctuations provide, principally via inelastic neutron scattering, 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 may play an active role in forming the superconducting state in UPd₂Al₃. The proposed scheme sheds new light on previously reported anomalies in this material. Received 16 July 1999     

Magnetic ordering in NdRh2Si2 and ErRH2Si2

Neutron diffraction and magnetization study of polycrystalline NdRh₂Si₂ and ErRh₂Si₂ was performed in the temperature range from 4.2 to 293 K. Both compounds are of ThCr₂Si₂ type crystal structure and exhibit antiferromagnetic ordering below T_N = 53 K and T_N = 12.8 K respectively. The magnetic structure wave vector is τ = [0, 0, 1].     

Influence of doping admixture and anisotropic tension on behaviour of muon spin polarization in insulators

Zero‐field μSR measurements have been carried out in the paramagnetic and magnetically ordered states of the heavy fermion alloys: Ce_1-xLa_xRu₂Si₂ (x=0.05,0.13,0.25). In Ce_1-xLa_xRu₂Si₂ system, T_K drastically decreases with x from 24 K for x=0 and becomes very close to the magnetic ordering temperature T_N. As functions of T_K and T_N, the magnetic instabilities were studied in this system. For x=0.05,\ 0.13\ \mboxand\ 0.25 samples, longitudinal muon spin relaxation was measured to study non‐magnetic to magnetic transition and the change from itinerant to localized electron state with increasing La impurities. We discuss the dynamics of muon spins in the paramagnetic and antiferromagnetic state. This revised version was published online in July 2006 with corrections to the Cover Date.     

New aspects on URu2Si2 and CeTIn5 (T=Rh, Ir, Co) observed by high pressure NMR and NQR

NMR and NQR studies on two interesting systems (URu₂Si₂, CeTIn₅) were performed under high pressure. (1) URu₂Si₂: In the pressure range 3.0 to 8.3 kbar, we have observed new ²⁹Si NMR signals arising from the antiferromagnetic (AF) region besides the previously observed ²⁹Si NMR signals which come from the paramagnetic (PM) region in the sample. This gives definite evidence for spatially-inhomogeneous development of AF ordering below T ₀ of 17.5 K. The volume fraction is enhanced by applied pressure, whereas the value of internal field (∼91 mT) remains constant up to 8.3 kbar. In the AF region, the ordered moment is about one order of magnitude larger than 0.03 μB. (2) CeTIn₅: The pressure and temperature (T) dependences of nuclear spin-lattice relaxation rate 1/T ₁ of ¹¹⁵In in CeTIn₅ have shown that the superconductivity (SC) occurs close to an AF instability. From the T dependences of 1/T ₁ and Knight shift below T _c. CeTIn₅ has been found to exhibit non-s wave (probable d wave) SC with even parity and line nodes in the SC energy gap.     

Neutron diffraction study of crystal structure and antiferromagnetic ordering in the heavy fermion compound CePd2Al3

Nuclear and magnetic structures of an annealed polycrystalline sample of the heavy-fermion compound CePd₂Al₃ prepared by arc-melting were investigated by neutron powder diffraction. The chemical structure corresponds well to the ordered hexagonal PrNi₂Al₃-type structure. The antiferromagnetic structure of CePd₂Al₃ with an ordered magnetic moment _Ce=0.47(2) _B at saturation is remarkably similar to that in the heavy fermion superconductor UPd₂Al₃. The additional incommensurate magnetic structures reported previously both for UPd₂Al₃ and CePd₂Al₃ are not observed in the present sample of CePd₂Al₃. At 1.4 K the magnetoresistivity of CePd₂Al₃ measured up to 14 T indicates only one field-induced phase transition at 3.0 T.     

Crystals and magnetic structure of RMn2Si2 (R = Pr, Nd, Y) and YMn2Ge2

Crystallographic and magnetic properties of PrMn₂Si₂, NdMn₂Si₂, YMn₂Si₂ and YMn₂Ge₂ intermetallics were studied by X-ray, neutron diffraction and magnetometric measurements. The crystal structure of all four compounds was confirmed to be body-centered tetragonal (space group I4/mmm). All were found to be antiferromagnetic with Néel points at 368, 380, 460 and 395 K respectively. Neutron diffraction results indicate that their magnetic structure consists of ferromagnetic layers composed of Mn ions piled up along the c-axis. Each layer is antiferromagnetically coupled to adjacent layer. The magnetic space group is I_p4/m′m′m′. No magnetic ordering of the R sublattice was observed at 1.8 K in the case of R = Pr and Nd.     

Neutron diffraction study of RECo2Si2 intermetallics

A neutron diffraction study of polycrystalline RECo₂Si₂ intermetallics (RE = Pr, Nd, Tb, Ho, Er) carried out at liquid helium temperature shows the presence of a collinear antiferromagnetic ordering of +?+? type. Magnetic moment is localized on RE ions only and amounts to the RE³⁺ free ion value. In ErCo₂Si₂ the magnetic moment is normal to the tetragonal unique axis, whereas in the remaining compounds the magnetic moment is aligned along it. Néel points were determined from the temperature dependence of magnetic peak heights.