M. Costes

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France

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Publications (9)20.83 Total impact

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    ABSTRACT: In this work, we report results of high-field magnetization and electron spin resonance (ESR) measurements performed on η-Na1.286V2O5, a compound that shows a spin gap. This system may be regarded as an assembly of weakly interacting magnetic entities, each of which containing, at low temperature, 18 antiferromagnetically (AF)-coupled S=1/2 spins. The purpose of this work is to determine the gap value and to probe the low-lying energy levels in this compound. On the one hand, the high-field magnetization measurements, performed at temperatures down to 1.7 K on powder samples, suggest a spin gap, Δ, of approximately 39 K, a value which is quite close to the earlier estimate of 35 K, which some of us deduced from the susceptibility measurements at low field [ Duc et al. Phys. Rev. B 69 094102 (2004)]. On the other hand, these measurements show a magnetization step at one ninth of the saturation magnetization, giving strong support to the assumption that this system may be regarded as a finite-size system composed of 18 AF-coupled S=1/2 spins. A theoretical fit of the magnetization indicates the necessity to include couplings to second or more distant neighbors. The ESR data at various frequencies (from 9.6–980 GHz) and at temperatures ranging from 4.2–150 K, obtained on powder samples as well as on single crystals, are in accordance with the gap value mentioned above. In addition, these ESR data indicate that the closure of the gap is not accompanied by any detectable mixing between the singlet level and the lowest-lying level of the first excited triplet. This implies the absence of any appreciable Dzyaloshinskii-Moriya interaction in this compound.
    Physical review. B, Condensed matter 01/2008; 77(22). · 3.77 Impact Factor
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    ABSTRACT: High coercivity and magnetic viscosity in nanocrystalline YCo 5 alloy were observed. The alloy was obtained by arc-melting the raw materials, mechanical milling for 4 h with subsequent heat treatment for short times at 1103 K and a final quenching in water. The x-ray diffraction pattern showed a single hexagonal 1:5 phase. The average crystallite size determined by transmission electron microscopy was 13 nm. Magnetic measurements were carried out at 85 K in a pulsed field magnetometer varying the field in the range 50– 250 kOe and keeping the pulse duration equal to 0.3 s. A maximum coercivity of 50 kOe was measured when applying a maximum magnetic field of 250 kOe. The viscosity parameter was measured varying the dH /dt rate in the range 511– 1113 kOe/ s. The calculated activation volumes from viscosity data were smaller than the crystallite volume from transmission electron microscopy, consistent with their different physical meanings. © 2006 American Institute of Physics.
    Journal of Applied Physics 04/2006; 99(99):08B512. · 2.21 Impact Factor
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    ABSTRACT: The electronic conductivity study of the quasi-one-dimensional Ca3Co2O6 single crystal evidences a Variable Range Hopping conductivity with temperature-induced crossover between 1D and 3D transport and the opening of a Coulomb gap in the d bands along with the ferromagnetic intra-chain ordering. A large negative magneto-resistance is observed at low temperatures. Both spin-dependent hopping and field-induced suppression of the Coulomb gap are discussed. The electronic hopping parameters we infer agree remarkably with the accessible Co sites. Finally, we present the first electronic noise study in a one-dimensional frustrated magnetic single crystal and we discuss the interplay between the low temperature 3D magnetic ordering and the spin-dependent hopping conductivity on the Co sites.
    Microelectronics Journal 10/2005; 36(10):900–906. · 0.91 Impact Factor
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    ABSTRACT: Low-dimensional transition metal oxides provide excellent model systems to study magnetic properties induced by the interplay between low-dimensionality character and frustration. In this work, we study polycrystalline Sr3NiIrO6 and Ca3CoIrO6 compounds which exhibit at low temperatures a frozen state with notable dynamic effects that can be attributed to the low-dimensionality character. These dynamic effects are probed by AC susceptibility measurements.
    Journal of Magnetism and Magnetic Materials 07/2005; 294(2):e123. · 1.83 Impact Factor
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    ABSTRACT: We report an electron spin resonance (ESR) study of the s=1/2-Heisenberg pseudo-ladder magnet CaCu2O3 in pulsed magnetic fields up to 40 T. At sub-Terahertz frequencies we observe an ESR signal originating from a small amount of uncompensated spins residing presumably at the imperfections of the strongly antiferromagnetically correlated host spin lattice. The data give evidence that these few percent of ''extra'' spin states are coupled strongly to the bulk spins and are involved in the antiferromagnetic ordering at T_N = 25 K. By mapping the frequency/resonance field diagram we have determined a small gap for magnetic excitations below T_N of the order of 0.3 - 0.8 meV. Such a small value of the gap explains the occurrence of the spin-flop transition in CaCu2O 3 at weak magnetic fields H_{sf} ~ 3 T. Qualitative changes of the ESR response with increasing the field strength give indications that strong magnetic fields reduce the antiferromagnetic correlations and may even suppress the long-range magnetic order in CaCu2O3. ESR data support scenarios with a significant role of the ''extra'' spin states for the properties of low-dimensional quantum magnets. Comment: revised version, accepted for publication in the New Journal of Physics
    New Journal of Physics 01/2005; · 4.06 Impact Factor
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    ABSTRACT: We present a high-field magnetization study of the S=1/2 antiferromagnetic Heisenberg chain [PM Cu(NO$_3$)$_2$(H$_2$O)$_2$]$_n$. For this material, as result of the Dzyaloshinskii-Moriya interaction and a staggered $g$ tensor, the ground state is characterized by an anisotropic field-induced spin excitation gap and a staggered magnetization. Our data reveal the qualitatively different behavior in the directions of maximum and zero spin excitation gap. The data are analyzed via exact diagonalization of a linear spin chain with up to 20 sites and on basis of the Bethe ansatz equations, respectively. For both directions we find very good agreement between experimental data and theoretical calculations. We extract the magnetic coupling strength $J/k_B$ along the chain direction to 36.3(5) K and determine the field dependence of the staggered magnetization component $m_s$.
    Physical Review B 12/2003; 68(22):220406(R). · 3.66 Impact Factor
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    ABSTRACT: The magnetic properties of a polycrystalline Sr 3 NiIrO 6 sample have been investigated by means of susceptibility and magnetization measurements. On the one hand, it is found that this compound, made of magnetic chains on a triangular lattice, behaves very similarly to Ca 3 CoRhO 6 and Ca 3 CoIrO 6 . In particular, at low temperature ( T > T 2 (=21 K)), a frozen state, characterized by a very slow spin dynamic, is evidenced. On the other hand, for T 2 > T > T 1 (=70 K), this phase exhibits magnetization values, smaller than the expected ones, with a 1/3 plateau reminiscent of the ferrimagnetic state of Ising spins on a triangular lattice. Nonetheless, the absence of saturation in 35 T and the low magnetization values are consistent with an antiferromagnetic intrachain coupling between Ir 4+ (S=1/2) and Ni 2+ (S=1). Sr 3 NiIrO 6 can be viewed as made of antiferromagnetically coupled ferrimagnetic chains on a triangular lattice. On the basis of these results, a magnetic (H, T) phase diagram is proposed. Copyright Springer-Verlag Berlin/Heidelberg 2003
    Physics of Condensed Matter 01/2003; 35(3):317-323. · 1.28 Impact Factor
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    ABSTRACT: After a brief review underlining the power of electronic noise studies to probe local magnetic instabilities, we present new data in three different systems in terms of dimensionality and magnetic ordering: the first results we present deal with low-frequency longitudinal and Hall resistance fluctuations in Ni nanostructures, where we address the questions of the inhomogeneous behavior of the noise at a nanometric scale. In order to get some insight into the atomic process, we report on noise generated in atomic contacts of Ni obtained by the break junction technique. And finally, we present the first electronic noise study in a one-dimensional frustrated magnetic Ca3Co2O6 single crystal composed of parallel Co spin chains. From the magnetic noise analysis, we discuss the interplay between the low-temperature three-dimensional magnetic ordering and the spin-dependent hopping conductivity on the Co sites.
    Journal of Magnetism and Magnetic Materials 01/2003; · 1.83 Impact Factor
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    ABSTRACT: We have measured the transport and magnetic properties of the model spin chain single crystal Ca3Co2O6 in high pulsed fields. A crossover between 1D and 3D transport is observed, with the opening of a Coulomb gap below the order temperature of the individual chains. The samples show changes in the variable range hopping transport dimensionality and gap; the magnetic field suppresses the gap, inducing a significant reduction of the resistance, while dimensionality seems governed by temperature. At fields associated with the levels of magnetization of the frustrated triangular lattice formed by the Co lines, and below the critical magnetic temperature, a further increase of the conductivity is observed. This effect is associated with planar magnetic states perpendicular to the lines.
    Physica B Condensed Matter 01/2002; 320(1):337-339. · 1.28 Impact Factor