J. A. Hodges

Magnetic density of states at low energy in geometrically frustrated systems

Using muon-spin-relaxation measurements we show that the pyrochlore compound Gd(2)Ti(2)O(7), in its magnetically ordered phase below approximately 1 K, displays persistent spin dynamics down to temperatures as low as 20 mK. The characteristics of the induced muon relaxation can be accounted for by a scattering process involving two magnetic excitations, with a density of states characterized by an upturn at low energy and a small gap depending linearly on the temperature. We propose that such a density of states is a generic feature of geometrically frustrated magnetic materials.

Spin dynamics in geometrically frustrated compounds

Abstract We have probed the low temperature spin dynamics in two pyrochlore lattice compounds R2Ti2O7, where the rare earth (R3+) form a sublattice of tetrahedra linked by their corners such that geometrically derived magnetic frustration is possible. In Yb2Ti2O7 (Yb3+: S′=1/2, XY anisotropy), we show that below 0.24 K , the temperature of the known specific heat λ transition, there is no long range order. The transition, in fact, corresponds to a first-order change in the fluctuation rate of the Yb3+ spins. Above the transition temperature, the rate is in the GHz range and it follows a thermal excitation law, whereas below the transition, the rate is in the MHz range and it is temperature independent. In Gd2Ti2O7, where the S-state Gd3+ ions are antiferromagnetically coupled, there is a magnetic phase transition near 1 K . As the temperature is lowered through the critical region, λZ, the spin-lattice relaxation increases. On lowering the temperature to 21 mK , λZ tends to a temperature independent value. Concomitant with the persisting electronic spin fluctuations, we observe oscillating components evidencing long-range spin correlations. Therefore, temperature independent spin dynamics is observed even after passing through a magnetic phase transition and even when the magnetic correlations are long range.

Transitions and Spin Dynamics at Very Low Temperature in the Pyrochlores Yb2Ti2O7 and Gd2Sn2O7

The very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O7, were investigated using an ensemble of microscopic and bulk techniques. In both compounds, a first order transition is evidenced, as well as spin dynamics persisting down to the 20 mK range. The transition however has a quite different character in the two materials: whereas that in Gd2Sn2O7 (at 1 K) is a magnetic transition towards long range order, that in Yb2Ti2O7 (at 0.24 K) is reminiscent of the liquid–gas transition, in the sense that it involves a 4 orders of magnitude drop of the spin fluctuation frequency, with no long range order. We attribute these unusual features to the frustration of the antiferromagnetic exchange interaction in the pyrochlore lattice.

Spin dynamics in RENi5 ferromagnets by microSR measurements

We report a zero fieldμSR study of single crystals of theRENi5 ferromagnets where RE=Gd, Er, Dy or Tm. Our work points out the strong influence of the crystal electric field on the rare earth total momentum dynamics: whereas the spin-lattice relaxation process belowTc is controlled by a two magnon mechanism for a weakly anisotropic magnet (GdNi5), the phonons drive the relaxation for a strongly anisotropic magnet such asErNi5. ForTmNi5 a comparison is made between the fluctuation time measured byμSR and169Tm Mössbauer spectroscopy.

Magnetic frustration in the disordered pyrochlore Yb2GaSbO7

In the pyrochlore Yb(2)GaSbO(7), the Yb(3+) sublattice forms a network of corner sharing tetrahedra and the second sublattice is made up of disordered, non-magnetic Ga(3+) and Sb(5+) ions. We have examined this compound using magnetic susceptibility, (170)Yb Mössbauer spectroscopy (down to 0.03 K) and muon spin relaxation (μSR) (down to 0.02 K) measurements. We establish the size of the Yb(3+) magnetic moments and that of the Yb(3+)-Yb(3+) coupling. At low temperatures, the correlated moments fluctuate between directions that are well tilted relative to the local [111] axis. The lattice disorder does not quench the frustration induced low temperature spin fluctuations but it does remove the first order dynamic transition that is present in the crystallographically ordered counterpart Yb(2)Ti(2)O(7). Below 1.0 K, the fluctuation rate of the correlated moments decreases progressively as the temperature is reduced and the moments remain dynamic down to 0.02 K where the rate is 7 × 10(7) s(-1). Magnetic frustration is operative in Yb(2)GaSbO(7) where the Yb(3+)-Yb(3+) interaction is antiferromagnetic as it is in Yb(2)Ti(2)O(7) where the interaction is ferromagnetic.

Superconductivity and magnetism in the HoNiBC system

Abstract We investigated the annealing temperature, the carbon non-stoichiometry and the Fe doping effects on the physical properties of the superconductor HoNi 2 B 2 C 3 (1 ≤ × ≤ 1.35), using X-ray diffraction, magnetisation or electrical resistance measurements and 57 Fe Mossbauer spectroscopy. We show that the spontaneous re-entrant behaviour of HoNi 2 B 2 C which has been attributed to the occurrence of incommensurate magnetic states destructive to superconductivity, is strongly enhanced by quenching or by decreasing the annealing temperature. The re-entrant behaviour also increases with the carbon content and the 57 Fe (0.5 at.%) doping. We conclude that the chemical defects introduced by quenching, deviation from stoichiometry or Fe doping, depress superconductivity and favour the appearance of the re-entrant behaviour. Our 57 Fe Mossbauer investigation shows that the presence of the incommensurate magnetic orders in the re-entrant region does not lead to the introduction of a hyperfine field at the Fe probe.

Thermal Behaviour of the μSR Relaxation Rate at High Temperature in Insulators

We show that, in rare earth based insulators, measurement of the thermal dependence of the muon spin-lattice relaxation rate at high temperature provides information on the nature of the magnetic correlations and on the crystal-field energy splitting, if any.

Mössbauer emission measurements on highly dilute 57Fe impurities in La2−xSrxCuO4

Abstract Mossbauer emission measurements show that the isolated 57 Fe 3+ probe pins a Cu spin-glass cluster in superconducting La 2− x Sr x CuO 4 for x x t ( x t is roughly at the centre of the superconducting x range in the phase diagram), whereas for x > x t the probe is essentially coupled with an induced localized Cu magnetic moment.

Mössbauer measurements on170Yb3+ substituted into YBa2Cu3O7

We present some results obtained by Mössbauer spectroscopy on Yb3+ when substituted into YBa2Cu3Ox-like compounds. For the fully substituted compound YbBa2Cu3O7, we describe the properties of the rare earth sublattice magnetic ordering which coexists with superconductivity. From measurements at the dilute substitution level, we study the Yb3+ crystal field properties and the thermal dependence of the Yb3+ paramagnetic relaxation rate. By making use of the molecular field produced on the Yb3+ probe by the magnetically correlated Cu(2), we examine the evolution of the Cu(2)-based magnetism as a function of carrier density and study the phase separation which occurs in the superconducting samples with intermediate oxygen levels. We also examine how the properties of the Yb3+ probe are influenced by the substitution of Pr3+ or Zn2+ (these substitutions are known to considerably influence the superconducting properties).

Local fields in YBa2Cu3O7−δ measured on 170Yb3+ Mössbauer probes.: II. Vortex alignment in the field-cooled and zero field-cooled configurations

Abstract We examine the directional properties of the vortex lattice in grain-oriented YBa 2 Cu 3 O 7 as a function of the direction of the applied field relative to the c -axis. The measurements are made by following the direction of the field produced by the vortex lattice on non-perturbing paramagnetic 170 Yb 3+ Mossbauer probes. In the field-cooled (FC) configuration, where the vortex density is uniform, the average vortex direction remains parallel to the applied field whatever the temperature and the direction of the applied field relative to the c -axis may be. We relate this behaviour to the competing influence of the effective mass anisotropy and the pinning to the twin planes. In the zero field-cooled (ZFC) configuration, where the vortex density is non-uniform, the average vortex direction may no longer be parallel to the applied field and its direction depends on the size and direction of the field and on the temperature. The critical currents now also play a role.