U. Steigenberger

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.

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.

Kinetics of phase transitions in modulated ferroelectrics: Time-resolved neutron diffraction from Rb2ZnCl4

Abstract The kinetics of the ferroelectric phase transition between modulated phases in Rb2ZnCl4 at about 190K has been investigated by time-resolved neutron scattering. Observing the time dependence of satellite spectra, it is found that the structural changes associated with the field-induced transformation proceed on a millisecond time scale. Under the influence of fast cycling strong electric fields unusual non-equilibrium states are obtained which are modulated in space as well as in time and characterised by the presence of internal mechanical strains.

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.

Orientational order-disorder phase transition in calcite

Abstract We have performed inelastic neutron scattering experiments on calcite in the ordered phase, focusing on a zone boundary F-point that shows X-ray diffuse scattering. At this position we have observed an incipient soft mode along with a column of inelastic scattering ranging in energy between the soft mode energy and zero energy. The intensity of this scattering varies dramatically with temperature, and is described by an Arrhenius relation with an activation energy of 1035 K.

Time-resolved studies on a millisecond time scale by elastic neutron scattering: transient properties of the ferroelectric phase transition in Rb2ZnCl4

Abstract Using time-resolved neutron scattering, the evolution of the phase transition between the paraelectric incommensurate (IC) phase and the ferroelectric commensurate (C) phase in Rb 2 ZnCl 4 has been observed on a microscopic scale. The transformation was driven by a strong rapidly cycling electric field of 8 kV/cm applied along the ferroelectric a -axis of the crystal. The response of the sample was monitored by measuring satellite reflections corresponding to the IC- and the C-phases. Synchronization of the electric field to the time structure of the ISIS pulsed neutron source allowed us to observe field-induced structural changes on a time scale of milliseconds . Characteristic relaxation times were found to depend strongly on temperature.

Pressure-induced valence and crystal field shifts in YbCu2Si2 and TmTe by neutron scattering.

We present the quasielastic and inelastic neutron scattering of YbCu2Si2 and TmTe at high pressure and compare the results to those at ambient pressure performed at the same samples. In both cases we found variations of the quasielastic line width due to a pressure-induced 4f valence shift. The observations are shown to be in a gross quantitative agreement with theory as based on the Anderson model and therefore comply with the common picture of a dynamic 4f valence fluctuation. A measured variation of the crystal field parameters in YbCu2Si2 indicates a dominant contribution of the conduction electrons to the crystal field rather than the applicability of the Point Charge Model. As a by-product, very general theoretical expressions are provided for the pressure variation of crystal field parameters.

The Crystal Field Scheme of PrCu6 as Obtained by Inelastic Neutron Scattering

The inelastic neutron scattering was studied in both PrCu 6 and the reference system LaCu 6 in order to isolate the magnetic CF-transitions in orthorombic PrCu 6 . Ten CF-transitions could easily be resolved, leading to a CF-scheme with two singlets forming a nearly doubly degenerate groundstate (Δ≈0.02 men) without any coupling through a magnetic dipole matrix element (this doublet shows no quasielastic scattering). The first excited state is found to be 1.48 meV above that quasi-doublet ground state and the overall splitting comes out to be 8.95 meV. The static susceptibility calculated from this CF-scheme is in good agreement with the measured one, which is also presented in this paper.

Neutron scattering study of spin waves in TbFeO3

Abstract Iron sublattice spin waves have been studied in TbFeO 3 by inelastic neutron scattering on the PRISMA spectrometer at ISIS. Magnon dispersion curves were measured along [100] and [001] directions up to energies 80 meV and the results fitted to a model taking into account near-neigbour ( J 1 ) and next-nearest-neigbour ( J 2 ) isotropic Heisenberg interactions. The exchange parameters J 1 = − 28.7(0.3) K and J 2 = − 1.4(0.5) K obtained for TbFeO 3 agree with those found earlier for another orthoferrite TmFeO 3 .

MAGNETIC EXCITATIONS IN SINGLE-CRYSTAL U(PD1-XPTX)3

Abstract The evolution of the magnetic excitations in U(Pd 1 −x Pt x ) 3 has been studied for x = 0, 0.25 and 0.50. Significant changes in the energies, dispersion and temperature dependence were found, together with a considerably increased line width, attributed to damping of the crystal field excitations by increasing 5f-4d hybridization.