K.A. McEwen

MAGNETIC AND STRUCTURAL PHASE-TRANSITIONS IN UPD3

Abstract We have observed two phase transitions, at T 1 ≈ 7 K and T 2 ≈ 5 K in the localized magnetic moment compound UPD 3 . Several superlattice peaks of the type ( h + 1 2 , 0, l ) have been investigated using polarised neutrons. The transition at T 1 has a structural character, whereas that at T 2 has both structural and magnetic components.

Neutron diffraction study of the magnetic structure of neodymium in a magnetic field

Neutron diffraction studies of neodymium reveal a complicated sequence of modulated magnetic phases below TN = 19.9 K. We have examined the 12-fold array of magnetic satellite reflections around (100) as a function of magnetic field. The intensities of the various satellites were found to have a striking field dependence. These results are in accord with a double-q model for the magnetic structure of Nd.

High energy magnetic neutron scattering in heavy fermion compounds

Abstract High energy magnetic inelastic neutron scattering measurements of intermultiplet transitions in rare earth and actinide metals have established the importance of intra-atomic correlations even in the presence of strong hybridisation. Well-defined transitions are observed up to an energy of 1766 meV in thulium metal. The 3H4→3F2 transition is observed across the alloy series U(Pd1−xPtx)3, showing that localized 5f2 excitations persist in the heavy fermion compound UPt3. The splitting of the spin-orbit transition in CeAl3 suggests that hybridisation is an important component in the crystal field potential.

Magnetic excitations and phase transitions in UPd3

The magnetic excitations in single crystal UPd3 have been studied by inelastic neutron scattering. At 1.8 K, we find at a general wave vector, four modes below 3 meV which we ascribe to crystal field-like excitations on the quasi-cubic sites. Their temperature dependence has been measured and the results are explained in terms of a quadrupolar transition at T1 = 6.5 K, followed by a magnetic transition at T2 = 4.5 K.

Magnetic structures and bulk magnetic properties of YbCu4M, MAu, Pd

Abstract Elastic neutron scattering studies down to 40 mK on YbCu4Au and YbCu4Pd revealed an antiferromagnetic ground state for the former, but ferromagnetism for the latter compound. Due to the Kondo interaction the ordered magnetic moments are considerably reduced with respect to those expected for the Γ7 ground state doublet.

Quadrupolar and magnetic transitions in UPd3

Abstract UPd 3 exhibits phase transitions at T 1 = 6.8K and T 2 = 4.4K, corresponding to antiferroquadrupolar and magnetic ordering. We have now confirmed a third transition at T 0 = 7.8K, where quadrupolar order first develops. The phase diagram for a magnetic field along the a -direction has been examined by neutron-diffraction studies, using both polarised and unpolarised neutrons. In zero magnetic field, the intensities of the superlattice reflections due to the antiferroquadrupolar order are extremely weak. They increase rapidly in a magnetic field, and the underlying quadrupolar structure has been deduced from the induced magnetic structure. This method may be used to elucidate the quadrupolar structures of other systems.

Crystal-field excitations in polycrystalline U(Pd1−xPtx)3

Abstract Neutron inelastic scattering spectra of UPd 3 and U ( Pd 0.75 Pt 0.25 ) 3 measured at IPNS are reported. The 15 meV excitation in UPd 3 decreases significantly in energy and broadens with Pt substitution. We attribute this effect to increasing hybridization between the 4d and 5f electrons.

Neutron intermultiplet spectroscopy of UPd3 and UPt3

Abstract Neutron intermultiplet spectroscopy has been used to re-examine UPd3 and UPt3 at 17 K. In UPd3, the 3H4 → 3F2 transition is observed at 395 meV, confirming a localised 5f2 state for the uranium ions. In the heavy fermion compound UPt3, only a small trace of this transition remains, indicating that the 5f electrons in this material are of a predominantly itenerant nature. The Q dependence of the transition in UPd3 has also been determined. The transition intensities are found to differ by a factor of ∼ 2 from that predicted by calculations.

Superconductivity and magnetic order in CeCo2

We report the study of the magnetisation of CeCo 2 at temperature below 1 K, in which a superconducting transition (T c = 0.82 K) is observed. Field cooling the sample from above T c in a 5 mT field suppressed the superconducting transition for temperatures down to 0.4 K. Magnetisation measurements below T c show behaviour typical of a type II superconductor, except for an unusual feature below Hoe, with Hcl = 7.5 mT and Hc2 = 0.275 T at 0.4 K. Recent progress in both high-T c compounds and heavy fermion compounds has led to a re-examination of superconductivity in CeRu 2 [1-4]. Yagasaki et al. [2] found that CeRu 2 exhibits behaviour that may be described as a re-entrant superconductor. Whilst this is not the first rareearth compound to show such behaviour [5], it is amongst the first binary compounds to be described as such. Band calculations [6] and X-ray photoemission studies [7] for CeRu 2 both suggest that the Ce 4f electrons are to some extent delocalised and form a narrow 4f band near to the Fermi level. Coles [8] has suggested that the development of a gap at the Fermi surface on becoming superconducting would reduce the Kondo screening below Hc2. An early study of CeCo 2 made by Luo et al. [9] considered the effect of changing composition on T c. They found T c to be strongly peaked (T c = 1.5 K) for a lattice parameter of 7.165 A. However a pressure of 20 kbar produced a negligible change in the value of T c. From this they concluded that the maximum in T c is not associated with a critical value of the volume, but that T c reflects the degree of order in the sample. In this study of CeCo 2 we present magnetisation data which will be compared with the earlier results [9] for CeCo 2. We also compare the results with those of CeRu 2. The sample was made by arc melting the constituents under an argon atmosphere and then annealed at 900°C for 1 week. X-ray powder diffraction (XRD) measurements reveal that the sample contains a second phase which might be accounted for by the presence of CeCo 3 or Ce2Co 7. However neither of these patterns provided a close fit to the second phase. The lattice parameter for this sample of CeCo 2 is calculated from XRD to be 7.161 A. * Corresponding author. Fax: +44-171-631 6220; email: ubap754@ccs.bbk.ac.uk. The magnetisation was measured using a vibrating sample magnetometer (VSM) built by Oxford Instruments. The VSM is equipped with a 12 T vertical field magnet and a cryostat which operates between T = 0.3-80 K. The base temperature is achieved through the use of liquid 3He which is pumped using a sorption pump. Fig. 1 shows the magnetisation as a function of temperature. The sample was first cooled to 0.3 K in zero field and then heated in an applied field of 5 mT. The superconducting ground state gradually collapses in a broad transition which is completed at T = 0.82 K. On subsequently reducing the temperature from T = 0.9 K in a 5 mT field, the sample did not undergo a superconducting transition. Instead the moment remained positive down to T = 0.4 K. All magnetisation measurements were made on the sample after zero-field cooling. Measurements made at T = 0.4 K show that CeCo 2 exhibits type II superconducting behaviour with He1 = 7.5 mT and He2 = 0.275 T. Further experiments indicate that He1 decreases rapidly from 7.5 mT at 0.4 K to 1 mT at 0.8 K. Fig. 2a shows the low field detail of the hysteresis loop

Non-Fermi liquid behaviour in the dynamic susceptibility of Ce(Rh0.8Pd0.2)Sb

We have made inelastic neutron scattering studies of Ce(Rh0.8Pd0.2)Sb, which is located at the crossover from non-magnetic to magnetically ordered states (i.e. close to a quantum critical point). In the imaginary part of the dynamic susceptibility, we have found clear evidence of E/T scaling behaviour. This E/T scaling behaviour appears to break down at high temperatures and energies. The transition from non-Fermi liquid to localized behaviour in Ce(Rh0.8Pd0.2)Sb on an energy scale above similar to15 meV is demonstrated by the observation of both crystal-field and spin-orbit excitations of the Ce3+ ions.