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A. Yaouanc

Publications and source records attributed to A. Yaouanc.

51 records · Page 3Linked to original sources
arXiv2008

Short-range magnetic ordering process for the triangular lattice compound NiGa2S4: a positive muon spin rotation and relaxation study

We report a study of the triangular lattice Heisenberg magnet NiGa2S4 by the positive muon spin rotation and relaxation technique. We unravel three temperature regimes: (i) below T_c = 9.2(2) K a spontaneous static magnetic field at the muon site is observed and the spin dynamics is appreciable: the time scale of the modes we probe is ~ 7 ns; (ii) an unconventional stretched exponential relaxation function is found for T_c < T < T_{cross} where T_{cross} = 12.6 K, which is a signature of a multichannel relaxation for this temperature range; (iii) above T_{cross}, the relaxation is exponential as expected for a conventional compound. The transition at T_c is of the continuous type. It occurs at a temperature slightly smaller than the temperature at which the specific heat displays a maximum at low temperature. This is reminiscent of the behavior expected for the Berezinskii-Kosterlitz-Thouless transition. We argue that these results reflect the presence of topological defects above T_c.

cond-mat.str-el↗
arXiv2006

Spin dynamics and magnetic order in magnetically frustrated Tb2Sn2O7

We report a study of the geometrically frustrated magnetic material Tb2Sn2O7 by the positive muon spin relaxation technique. No signature of a static magnetically ordered state is detected while neutron magnetic reflections are observed in agreement with a published report. This is explained by the dynamical nature of the ground state of Tb2Sn2O7: the Tb3+ magnetic moment characteristic fluctuation time is ~ 10^{-10} s. The strong effect of the magnetic field on the muon spin-lattice relaxation rate at low fields indicates a large field-induced increase of the magnetic density of states of the collective excitations at low energy.

cond-mat.str-el↗
arXiv2006

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 magnetic structures of Gd2Sn2O7 and of nominally analogous Gd2Ti2O7 is found to be determined by a specific type of third-neighbor superexchange interaction on the pyrochlore lattice between spins across empty hexagons.

cond-mat.mtrl-sci↗
arXiv2005

Studies of R2Ti2O7 (R=Gd and Yb); new results

Specific heat and muon spin rotation and relaxation data are presented for two geometrically frustrated systems: Gd2Ti2O7 which antiferromagnetically orders and Yb2Ti2O7 which presents dynamical short range correlations at low temperature. The muon data help to characterize the spin dynamics of these two compounds.

cond-mat.str-el↗
arXiv2005

Magnetic density of states at low energy in geometrically frustrated systems

Using muon-spin-relaxation measurements we show that the pyrochlore compound Gd2Ti2O7, in its magnetically orderered phase below ~ 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.

cond-mat.str-el↗
arXiv2003

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.

cond-mat↗
arXiv2003

Transition 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 20mK range. The transition however has a quite different character in the two materials: whereas that in Gd2Sn2O7$ (at 1K) is a magnetic transition towards long range order, that in Yb2Ti2O7 (at 0.24K) is reminiscent of the liquid-gas transition, in the sense that it involves a 4 orders of magnitude drop of the spin fluctuation frequency; furthermore, no long range order is observed. These unusual features we attribute to frustration of the antiferromagnetic exchange interaction in the pyrochlore lattice.

cond-mat.mtrl-sci↗
arXiv2002

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 quasi-static magnetic correlations. Their critical dynamics is well described by the conventional dipolar Heisenberg model. These correlations involve small magnetic moments.

cond-mat.str-el↗
arXiv2002

Magnetic character of the empty density of states in uranium compounds from X-ray magnetic circurlar dichroism

We present a discussion of published x-ray magnetic circular dichroism (XMCD) measurements performed at the uranium M_4,5 edges of metallic uranium compounds, focusing on the shape of the dichroic signal at the M_5 edge. A well resolved double lobe structure, comprised of a positive and a negative peak, is sometimes observed. Out of the twelve metallic uranium compounds so far investigated by XMCD, six exhibit an intense double-lobe structure at the M_5 edge. This line shape gives information on the empty 5f magnetic density of states with angular quantum number j=7/2. Conclusions about the difference between these two families of compounds are given regarding the splitting of the j=7/2 band and the occupation among the different m_7/2 sublevels.

cond-mat.str-el↗
arXiv2002

First Order Transition in the Spin Dynamics of Geometrically Frustrated Yb2Ti2O7

Using neutron diffraction, 170Yb Mossbauer and muSR spectroscopies, we have examined the pyrochlore Yb2Ti2O7 where the Yb3+ S'=1/2 ground state has planar anisotropy. Below ~0.24K, the temperature of the known specific heat lambda transition, there is no long range magnetic order. We show that the transition corresponds to a first order change in the fluctuation rate of the Yb3+ spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is essentially temperature independent indicative of a quantum fluctuation regime.

cond-mat.str-el↗
arXiv2000

Evidence for a two component magnetic response in UPt3

The magnetic response of the heavy fermion superconductor UPt_3 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 UPt_3 is characterized by a two component magnetic response.

cond-mat.str-el↗
arXiv1997

Effect of the vortex core on the magnetic field in hard superconductors

Using approximate analytical and new numerical solutions of the conventional Ginzburg-Landau equations we calculate the small angle neutron scattering cross-section and the variance of the field distribution as measured by muon-spin rotation for superconductors with large Ginzburg-Landau parameter $κ$. Our results prove that a proper account of the finite size of the vortex core is important, even at relatively low fields. This finding provides a natural explanation for the recently observed field dependence of the CeRu_2 form factor and of the YBa_2Cu_3O_{6.95} penetration depth.

cond-mat.supr-con↗
arXiv1995

Absence of zero field muon spin relaxation induced by superconductivity in the B phase of UPt$_3$

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.

cond-mat↗