Grenoble, France
Grenoble, France

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Lorenzo J.E.,CNRS Neel Institute | Regnault L.P.,SPSMS | Boullier C.,SPSMS | Martin N.,SPSMS | And 4 more authors.
Physical Review Letters | Year: 2010

We report the direct observation by inelastic neutron scattering experiments of a spin triplet of magnetic excitations in the response associated with the ladders in the composite cuprate Sr14Cu24O41. This appears as a peak at qQ1D=π and energy Δ1=32.5meV, and we conjecture that all the triplets making up this conspicuous peak have the same phase and therefore interpret it as the signature of the occurrence of quantum coherence along the ladder direction between entangled spin pairs. From the comparison with previous neutron and x-ray data, we conclude that the temperature evolution of this mode is driven by the crystallization of holes into a charge density wave in the ladder sublattice. © 2010 The American Physical Society.

Bourdarot F.,SPSMS | Martin N.,SPSMS | Raymond S.,SPSMS | Regnault L.-P.,SPSMS | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The aim of this study is to compare the magnetic behavior of URu 2Si2 under uniaxial stress along the a axis with the behavior under hydrostatic pressure. Both are very similar, but uniaxial stress presents a critical stress σxa [0.33(5) GPa] that is smaller than the hydrostatic critical pressure px (0.5 GPa) where the ground state switches from a HO (hidden-order) to AF (antiferromagnetic) ground state. From these critical values and Larmor neutron diffraction, we conclude that the magnetic properties are governed by the shortest U-U distance in the plane (a lattice parameter). Under stress, the orthorhombic unit cell stays centered. A key point shown by this study is the presence of a threshold for the uniaxial stress along the a axis before the appearance of the large AF moment, which indicates no mixture of the order parameter between the HO ground state and the AF one as under hydrostatic pressure. The two most intense longitudinal magnetic excitations at Q0 =(1,0,0) and Q1 =(0.6,0,0) were measured in the HO state: the excitation at Q0 decreases in energy while the excitation at Q1 increases in energy with the uniaxial stress along the a axis. The decrease of the energy of the excitation at Q 0 seems to indicate a critical energy-gap value of 1.2(1) meV at σxa. A similar value was derived from studies under hydrostatic pressure at px. © 2011 American Physical Society.

Klein C.,SPSMS | Klein C.,Karlsruhe Institute of Technology | Petitjean C.,SPSMS | Waintal X.,SPSMS
Physical Review Letters | Year: 2012

We discuss the current induced magnetization dynamics of spin valves F 0|N|SyF where the free layer is a synthetic ferrimagnet SyF made of two ferromagnetic layers F 1 and F 2 coupled by RKKY exchange coupling. When the magnetic moment of the outer layer F 2 dominates the magnetization of the SyF, the sign of the effective spin torque exerted on the layer F 1 is controlled by the coupling's strength: for weak coupling the spin torque tends to antialign F 1's magnetization with respect to the pinned layer F 0. At large coupling the situation is reversed and tends to align F 1 with respect to F 0. At intermediate coupling, numerical simulations reveal that the competition between these two incompatible limits leads generically to spin torque oscillator (STO) behavior. The STO is found at zero magnetic field, with very significant amplitude of oscillations and frequencies up to 50 GHz or higher. © 2012 American Physical Society.

Tabis W.,AGH University of Science and Technology | Tabis W.,University of Minnesota | Lorenzo J.E.,CNRS Neel Institute | Kozlowski A.,AGH University of Science and Technology | And 9 more authors.
Journal of Physics Condensed Matter | Year: 2013

Following the controversy between two previous publications (Lorenzo et al 2008 Phys. Rev. Lett. 101 226401 and Garcia et al 2009 Phys. Rev. Lett. 102 176405), we report on the influence of mechanical polishing, and subsequent sample storage, on the electronic order at the Verwey transition of highly pure magnetite, Fe3O4, by resonant x-ray scattering. Contrary to expectations, mechanically polishing the surface induces an inhomogeneous micron deep dead layer, probably of powdered Fe3O4. In addition, we have found that polishing the sample immediately before the experiment influences and favors the appearance of long range order electronic correlations, whereas samples polished well in advance have their electronic order quenched. Conversely, lattice distortions associated with the Verwey transition appear less affected by the surface state. We conclude that mechanical polishing induces stresses at the surface that may propagate into the core of the single crystal sample. These strains relax with time, which affects the different order parameters, as measured by x-ray resonant diffraction. © 2013 IOP Publishing Ltd.

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