C.T. Kaiser

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.

Evidence for weak itinerant long-range magnetic correlations in UGe2

Positive muon spin relaxation measurements performed on the ferromagnet UGe2 reveal, in addition to the well-known localized 5f-electron density responsible for the bulk magnetic properties, the existence of itinerant quasistatic magnetic correlations. Their critical dynamics is well described by the conventional dipolar Heisenberg model. These correlations involve small magnetic moments.

Evidence for a Two Component Magnetic Response in UPt3

The magnetic response of the heavy fermion superconductor UPt3 has been investigated on a microscopic scale by muon Knight shift studies. Two distinct and isotropic Knight shifts have been found for the field in the basal plane. While the volume fractions associated with the two Knight shifts are approximately equal at low and high temperatures, they show a dramatic and opposite temperature dependence around T(N). Our results are independent on the precise muon localization site. We conclude that UPt3 is characterized by a two component magnetic response.

Absence of Magnetic Order in Yb3Ga5O12: Relation between Phase Transition and Entropy in Geometrically Frustrated Materials

From muon spin relaxation spectroscopy experiments, we show that the sharp peak (lambda-type anomaly) detected by specific heat measurements at 54 mK for the ytterbium gallium garnet compound, Yb3Ga5O12, does not correspond to the onset of a magnetic phase transition, but to a pronounced building up of dynamical magnetic pair correlations. Beside the lambda anomaly, a broad hump is observed at higher temperature in the specific heat of this garnet and other geometrically frustrated compounds. Comparing with other frustrated magnetic systems we infer that a ground state with long-range order is reached only when at least 1/4-1/3 of the magnetic entropy is released at the lambda transition.

Positive muon diffusion and localization sites in GdNi5

Abstract GdNi 5 is a simple metallic ferrimagnet with well understood magnetic properties. It has been used in the past as a testing ground for validating muon spin relaxation theories. Here we report on the muon diffusion and site localization in its paramagnetic phase, i.e., above 30.85 K. The muon localizes at the interstitial 3f sites over the whole temperature range. Below ∼70 K the interstitial 6m sites become populated as well. These sites form a ring surrounding the (0,0, 1 2 ) site and the muon can occupy all these sites within its lifetime. Above 40 K the muon residing time in these sites is relatively short and it jumps to the 3f site by a quantum diffusion process.

μSR investigation of a cluster of monodisperse Pd nanoparticles

We report the first μSR measurements done on a cluster of monodisperse Pd nanoparticles. The measurements give access to the dipole field from the unpaired electron magnetic moments. We find for a two shell compound that the field is smaller than expected. The moments at the origin of the field are quasi-static, i.e. fluctuate in the microsecond time range.

Muon spin dynamics and sites in GdNi5

In GdNi5, the muon localizes at the 3f interstitial site and below ∼80K a second muon site becomes populated, the so-called 6ring site. This ring site is metastable and the muon hops to the 3f site. We determine the mean time of stay of the muon in the ring site and show that it is governed by a multi-phonon quantum diffusion process. The coincidence energy is measured to be Ea=272(10)K and the tunneling matrix element J=0.11(2)meV.

Magnetism and superconductivity of UPt3 by muon spin techniques

We summarize our recent μSR results on the magnetic and superconducting properties of the heavy-fermion superconductor UPt3. Whereas zero-field measurements show no signature of the superconducting and magnetic phase transitions, the experiments in high transverse field evidence the presence of two inequivalent magnetic regions for temperatures ranging from 0.2 to 115 K with an anomaly near the magnetic transition. From low transverse field experiments in the low-temperature superconducting phase (B phase), we deduce the temperature dependence of the superfluid densities. We briefly discuss all these results in relation to those obtained for other compounds of this family.

Muon localization along a six-fold ring-shaped site and muon quantum hopping in GdNi5

Abstract In GdNi5, the muon localizes at the 3f interstitial site and below ∼80 K a second muon site becomes populated, either at the 6m or 6k site. These latter sites are located in a ring surrounding the (0, 0, 1 2 ) site and the muon occupies all six sites within the time range of the experiment (10−9 s). This ring site is metastable and the muon hops to the preferred 3f site. We determine the mean time of stay of the muon in the ring site and show that it is governed by a multi-phonon quantum diffusion process. The coincidence energy is measured to be Ea=272 (10) K and the tunneling matrix element J=0.11 (2) meV.