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P. Dalmas de Reotier

Publications and source records attributed to P. Dalmas de Reotier.

At least 37 records · Page 2Linked to original sources
arXiv2014

Core-shell nanostructure in a Ge_0.9Mn_0.1 film from structural and magnetic measurements

We have characterized a film of Ge_0.9Mn_0.1 forming self-organized nanocolumns perpendicular to the Ge substrate with high resolution scanning transmission electron microscopy combined with electron energy loss spectroscopy, and bulk magnetization and positive muon spin rotation and relaxation (muSR) measurements. The Mn-rich nanocolumns form a triangular lattice with no detectable Mn atoms in the matrix. They consist of cores surrounded by shells. The combined analysis of bulk magnetization and muSR data enables us to characterize the electronic and magnetic properties of both the cores and shells. The discovered phase separation of the columns between a core and a shell is probably relevant for other transition-metal doped semiconductors.

cond-mat.mes-hall↗
arXiv2012

Magnetic order, magnetic correlations and spin dynamics in the pyrochlore antiferromagnet Er2Ti2O7

Er2Ti2O7 is believed to be a realization of an XY antiferromagnet on a frustrated lattice of corner-sharing regular tetrahedra. It is presented as an example of the order-by-disorder mechanism in which fluctuations lift the degeneracy of the ground state, leading to an ordered state. Here we report detailed measurements of the low temperature magnetic properties of Er2Ti2O7, which displays a second-order phase transition at T_N \simeq 1.2 K with coexisting short- and long-range orders. Magnetic-susceptibility studies show that there is no spin-glass-like irreversible effect. Heat-capacity measurements reveal that the paramagnetic critical exponent is typical of a 3-dimensional XY magnet while the low-temperature specific heat sets an upper limit on the possible spin-gap value and provides an estimate for the spin-wave velocity. Muon spin relaxation measurements show the presence of spin dynamics in the nanosecond time scale down to 21 mK. This time range is intermediate between the shorter time characterizing the spin dynamics in Tb2Sn2O7, which also displays long- and short-range magnetic order, and the time scale typical of conventional magnets. Hence the ground state is characterized by exotic spin dynamics. We determine the parameters of a symmetry-dictated Hamiltonian restricted to the spins in a tetrahedron, by fitting the paramagnetic diffuse neutron scattering intensity for two reciprocal lattice planes. These data are recorded in a temperature region where the assumption that the correlations are limited to nearest neighbors is fair.

cond-mat.str-el↗
arXiv2012

Evidence for an exotic magnetic transition in the triangular spin system FeGa2S4

We report positive muon spin relaxation measurements on the triangular lattice magnetic system FeGa2S4. A magnetic transition not previously detected by specific heat and magnetic susceptibility measurements is found in zero field at T^* \simeq 30 K. It is observed through the temperature dependencies of the signal amplitude and the spin-lattice relaxation rate. This transition is therefore not a conventional magnetic phase transition. Since persistent spin dynamics is observed down to 0.1 K, the ground state cannot be of the canonical spin-glass type, which could be suggested from hysteresis effects in the bulk susceptibility below T_f \simeq 16 K. These results are compared to those found for the isomorph NiGa2S4. It is argued that the fate of the transition, which has been interpreted in terms of the Z_2 topological transition in this latter system, is probably different in FeGa2S4.

cond-mat.str-el↗
arXiv2010

Absence of magnetic phase separation in MnSi under pressure

We report muon spin spectroscopy data (muSR) obtained under hydrostatic pressure on a large single crystal of the itinerant helimagnet MnSi, and recorded down to 0.235 K and up to 15.1 kbar. Up to the critical pressure p_c = 14.9 (2) kbar, where the magnetic order is suppressed, the muSR data unambiguously demonstrate that the ground state of the system is magnetic with no indication of any phase separation.

cond-mat.str-el↗
arXiv2009

Dynamic correlated Cu(2) magnetic moments in superconducting YBa_2(Cu_0.96Ni_0.04)_3O_y (y ~ 7)

We have examined the magnetic properties of polycrystalline, superconducting YBa_2(Cu_0.96Ni_0.04)_3O_y (y ~ 7, T_sc ~ 75 K) using two local probe techniques: 170Yb Moessbauer down to 0.1 K and muon spin relaxation (muSR) down to 1.5 K. At 0.1 K, the 170Yb measurements show the Cu(2) over essentially all the sample volume carry magnetically correlated moments which are static on the time-scale of 10^{-9} s. The moments show a distribution in size. The correlations are probably short range. As the temperature increases, the correlated moments are observed to fluctuate with measurable rates (in the GHz range) which increase as the temperature increases and which show a wide distribution. The muSR measurements also evidence that the fluctuation rates increase with increasing temperature and there is a distribution. The evidenced fluctuating, correlated Cu(2) moments coexist at an atomic level with superconductivity.

cond-mat.str-el↗
arXiv2009

Dynamic correlated Cu(2) magnetic moments in superconducting YBa_2(Cu_0.96Co_0.04)_3O_y (y ~ 7)

We have examined the magnetic properties of superconducting YBa_2(Cu_0.96Co_0.04)_3O_y (y ~ 7, T_sc = 65 K) using elastic neutron scattering and muon spin relaxation (muSR) on single crystal samples. The elastic neutron scattering measurements evidence magnetic reflections which correspond to a commensurate antiferromagnetic Cu(2) magnetic structure with an associated Neel temperature T_N ~ 400 K. This magnetically correlated state is not evidenced by the muSR measurements. We suggest this apparent anomaly arises because the magnetically correlated state is dynamic in nature. It fluctuates with rates that are low enough for it to appear static on the time scale of the elastic neutron scattering measurements, whereas on the time scale of the muSR measurements, at least down to ~ 50 K, it fluctuates too fast to be detected. The different results confirm the conclusions reached from work on equivalent polycrystalline compounds: the evidenced fluctuating, correlated Cu(2) moments coexist at an atomic level with superconductivity.

cond-mat.str-el↗
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↗