P. Bonville

Absence of zero field muon spin relaxation induced by superconductivity in the B phase of UPt3

Abstract We present muon spin relaxation measurements performed on crystals of the heavy fermion superconductor UPt 3 . In zero applied field, contrary to a previous report, we do not observe an increase of the internal magnetic field in the lower superconducting phase (the B phase). Our result gives an experimental upper bound of the magnetic field that could be associated with the superconducting state.

Magnetic ordering in Gd2Sn2O7 the archetypal Heisenberg pyrochlore antiferromagnet

Low-temperature powder neutron diffraction measurements are performed in the ordered magnetic state of the pyrochlore antiferromagnet Gd2Sn2O7. Symmetry analysis of the diffraction data indicates that this compound has the ground state predicted theoretically for a Heisenberg pyrochlore antiferromagnet with dipolar interactions. The difference in the magnetic structure of Gd2Sn2O7 andof nominally analogous Gd2Ti2O7 is found to be determined by a specific type of third-neighbour superexchange interaction on the pyrochlore lattice between spins across empty hexagons.

Effective hyperfine temperature in frustrated Gd 2Sn 2O 7: two level model and 155Gd Mössbauer measurements

We report on very low temperature (27 mK) Mossbauer absorption spectra on Gd 155 in the Heisenberg frustrated antiferromagnets Gd2 Sn2 O7 (GdSn) and Gd2 Ti2 O7. We show that electronic fluctuations of correlated Gd3+ moments are present near T=0 in GdSn by observing that the hyperfine levels are out of equilibrium. Using a model spin 1/2 system, submitted to a magnetic (hyperfine) field reversing randomly in time, we find an analytical expression for the stationary probability distribution of the level populations (or magnetization). This quantity is a simple function of the ratio of the nuclear relaxation time to the average electronic flip time. These two time scales are found to be of the same magnitude in GdSn, and we discuss a relevant mechanism for nuclear relaxation and the influence of electronic fluctuations on the nuclear specific heat.

Search for incoherent tunnel fluctuations of the magnetisation in nanoparticles of artificial ferritin

Abstract The magnetic behaviour of nanoparticles of antiferromagnetic artificial ferritin, with a mean Fe loading of 410 atoms per core, has been investigated by 57 Fe Mossbauer absorption spectroscopy down to very low temperature (34 mK ) and by magnetometry down to 2.5 K . In previous experiments of frequency-dependent magnetic susceptibility χ ( ω ) and magnetic noise S ( ω ) performed at 25 mK in similar artificial ferritin particles, a resonance observed at a frequency of about 10 8 Hz was claimed to be due to a macroscopic quantum coherent state. In this work, we measured on the one hand the parameters which enter the theoretical expression for the tunnel splitting and for the incoherent tunnel fluctuation rate, namely the anisotropy energy density in ferritin, the transverse antiferromagnetic susceptibility and the excess magnetic moment due to uncompensated spins. We show that, using these measured values, the tunnel splitting is expected to be orders of magnitude lower than claimed in the above mentioned χ ( ω ) experiment. On the other hand, assuming that the tunnel splitting has the value claimed in the above mentioned χ ( ω ) experiments, we performed a search for incoherent tunnel fluctuations in these artificial ferritin particles by performing a 57 Fe Mossbauer spectrum at 34 mK . Mossbauer spectroscopy on 57 Fe has a “window” of measurement of fluctuation frequencies centered around 10 8 Hz , and we show that the expected signature of incoherent tunnel fluctuations is absent from the 34 mK Mossbauer spectrum. These experiments cast a doubt about the previous observation of a macroscopic quantum coherent state in ferritin.

Magnetic properties of Yb2Mo2O7 and Gd2Mo2O7 from rare earth Mössbauer measurements

Abstract:Using 170Yb and 155Gd Mössbauer measurements down to ∼ 0.03 K, we have examined the semiconducting pyrochlore Yb2Mo2O7 where the Mo intra-sublattice interaction is anti-ferromagnetic and the metallic pyrochlore Gd2Mo2O7 where this interaction is ferromagnetic. Additional information was obtained from susceptibility, magnetisation and 172Yb perturbed angular correlation measurements. The microscopic measurements evidence lattice disorder which is important in Yb2Mo2O7 and modest in Gd2Mo2O7. Magnetic irreversibilities occur at 17 K in Yb2Mo2O7 and at 75 K in Gd2Mo2O7 and below these temperatures the rare earths carry magnetic moments which are induced through couplings with the Mo sublattice. In Gd2Mo2O7, we observe the steady state Gd hyperfine populations at 0.027 K are out of thermal equilibrium, indicating that Gd and Mo spin fluctuations persist at very low temperatures. Frustration is thus operative in this essentially isotropic pyrochlore where the dominant Mo intra-sublattice interaction is ferromagnetic.

Magnetic behavior of Yb3Cu4Ge4 and Gd3Cu4Ge4

Abstract Magnetic, specific heat and 170 Yb Mossbauer spectroscopy measurements were carried out in orthorhombic Yb3Cu4Ge4, where the rare earth occupies two inequivalent crystallographic sites. We show that this compound has an unusually high magnetic ordering temperature of 7.5 K , close to that of isostructural Gd3Cu4Ge4 ( 8.6 K ), and that it is a ferromagnet. The breakdown of the de Gennes scaling indicates a strong exchange interaction between Yb ions, presumably due to 4f-conduction band hybridization.

μSR investigation of the quasi-elastic magnetic excitations in strongly correlated compounds

Abstract Although, according to macroscopic measurements, the intermetallics CeRu 2 Si 2 , UGe 2 and YbNiSn have very different electronic ground states, muon spin relaxation measurements show that these three compounds have an anomalous quasi-elastic spectral weight of magnetic excitations in the transfer energy range of 10xa0neV.

Strong axial anisotropy of the magnetic penetration length in superconducting

We report muon spin rotation measurements of the temperature dependence and anisotropy of the magnetic field penetration lengths in the heavy fermion superconductor . We observe a strong axial anisotropy. At 0.05 K we obtain for the penetration length parallel and perpendicular to the c axis (150) A and (130) A, respectively. at low temperatures excludes a superconducting order parameter in the B phase with only a line of nodes in the equatorial plane of the Fermi surface. The combined analysis of and measured in the B phase favours an hybrid order parameter with point nodes at the poles and a line of nodes at the equatorial plane. The A phase is characterized by a larger density of nodes than the B phase.

What does the muon spin-relaxation rate measure in 4f paramagnets with strong crystal fields and weak inter-site correlations?

We investigate the physical meaning of the longitudinal muon spin-relaxation rate measured for a paramagnet at temperatures high with respect to the magnetic phase transition temperature. The depolarization rate is shown to be a function of the relaxation rates of the quasi-elastic and inelastic magnetic excitations. Using this analysis, we consider recently published data recorded on the strongly correlated electron systems CeNiSn and .

Incommensurate modulated magnetic structure in the heavy electron compound YbPtAl

Abstract 170Yb Mossbauer spectroscopy measurements down to 0.025 K and neutron diffraction measurements down to 1.7 K have been performed in the heavy electron compound YbPtAl (TN≃ 5.8 K). Below TN the neutron data show that the magnetic structure is incommensurate, with a wave-vector k =(0.30, 0, 0) . At 0.025 K, the Mossbauer data reveal that the magnetic structure is modulated and that it is not an antiphase structure. YbPtAl is a clear example of a Kramers local moment system where the Kondo singlet ground state allows a modulated magnetic structure to persist down to 0 K.