J. P. Renard

Université Paris-Sud 11, Paris, Ile-de-France, France

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Publications (199)367.06 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: High-quality single crystals of La1.2Sr1.8 −y Cay Mn2O7 (0 ≤ y ≤ 0.2) were studied by means of electrical resistivity and magnetoresistance measurements. They exhibit a phase transition to a metallic ferromagnetic state at low temperature and strongly anisotropic electronic transport properties, which result from their lamellar structure. The temperature range of the resistivity measurements is extended up to about 1000 K, which allows us to discriminate between different theoretical models. In addition, an accurate comparison of parallel and perpendicular resistivity is carried out in order to reveal the effect of the two-dimensional magnetic correlations. The effect of disorder achieved by substitution of Ca2+, of smaller ionic radius, for Sr2+ is also investigated in detail.
    Journal of Physics Condensed Matter 06/2002; 14(26):6667. · 2.22 Impact Factor
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    J.P. Renard, L.P. Regnault, M. Verdaguer
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    ABSTRACT: One-dimensional (1D) magnetic systems are very attractive because they exhibit magnetic properties which are rather different from those encountered at higher dimensionality, D = 2 or 3 [1,2]. In 1D antiferromagnets (AF), in particular, the role of quantum magnetic fluctuations is expected to be very important. Quite generally, in ferromagnets (F), the quantum effects are strongly reduced by the increase of the correlation length as the temperature decreases, since a block of N ferromagnetically coupled spins is likely to behave as a single large spin N S. In AF, with isotropic short-range interactions, quantum effects are sizable even for D = 3. Indeed in Heisenberg AF with nearest neighbor interactions, the spontaneous magnetization has a zero point relative reduction of about 1/z S , where z is the number of nearest neighbors and S the spin value. At D = 1, the spin fluctuations have more drastic effects which were first pointed out by F.D.M. Haldane [3]. Haldane conjectured that the 1D Heisenberg antiferromagnet (1D-HAF) with integer spin, in contrast to the case of half-integer spin, has a singlet ground state separated from the excited states by an energy gap, 􏰲. The spatial correlation function of the 1D-HAF with half- integer spin has a power law decay, while that for integer spin shows an exponential decay. Although fairly controversial at first, the Haldane conjecture was rapidly supported by numerical simulations on finite AF chains and by experiments on quasi- 1D antiferromagnets [4,5]. Such findings renewed the interest in 1D magnetism and induced a large amount of theoretical and experimental work, which is reviewed here.
    01/2002: pages 49-93; , ISBN: 3-527-29772-3
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    ABSTRACT: We have performed electron-spin-resonance measurements on single crystals of the doped spin-Peierls compounds CuGe1- ySiyO3 and Cu1-xMxGeO3 with M=Zn, Mg, Ni (x,yless than or equal to0.1). The first part of our experiments was performed in the paramagnetic and spin-Peierls phases at 9.5, 95, and 190 GHz. All nonmagnetic impurities (Si, Zn and Mg) were found to hardly affect the position and linewidth of the single line resonance, in spite of the moment formation due to the broken chains. In contrast to Si, Zn, and Mg dopings, the presence of Ni (S = 1) at low concentration induces a spectacular shift toward high fields of the ESR line (up to 40% for x = 0.002), together with a large broadening. This shift is strictly proportional to the ratio of Ni to Cu susceptibilities: Hence it is strongly enhanced below the spin-Peierls transition. We interpret this shift and the broadening as due to the exchange field induced by the Ni ions onto strongly exchange coupled Cu spins. Second, the antiferromagnetic resonance was investigated in Ni-doped samples. The frequency vs magnetic-field relation of the resonance is well explained by the classical theory with orthorhombic anisotropy, with g values remarkably reduced, in accordance with the study of the spin-Peierls and paramagnetic phases. The easy, second-easy, and hard axes are found to be a, c, and b axes, respectively. These results, which are dominated by the single ion anisotropy of Ni2+ are discussed in comparison with those in the Zn- and Si-doped CuGeO3.
    Physical Review B 01/2002; · 3.66 Impact Factor
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    ABSTRACT: Both Pr0.7Sr0.2□0.1MnO3 and Pr0.6□0.1Sr0.3MnO3 ceramic samples have been studied by several techniques including 55Mn nuclear magnetic resonance, magnetic, resistivity and magnetoresistance measurements. Powder X-ray diffraction patterns do not exhibit secondary phases in significant amount. Both samples crystallize in the rhombohedral perovskite structure with R3c space group. They exhibit a paramagnetic (PM)–ferromagnetic (FM) transition at 160 and 290K for Pr0.7Sr0.2□0.1MnO3 (sample I) and Pr0.6□0.1Sr0.3MnO3 (sample II), respectively, while the PM–FM transition in the stoichiometric sample Pr0.7Sr0.3MnO3 occurs at 265K. The PM–FM transition in sample I is followed by a magnetization decrease with decreasing temperature indicating a spin canted state at low temperature. Resistivity measurements as a function of temperature show a semiconducting–metallic transition at Tρ=250K for sample II and a semiconducting–metallic transition at 160K followed by a resistivity increase with decreasing temperature for sample I. Both samples exhibit a magnetoresistance at 2T in the vicinity of the semiconducting–metallic transition temperature Tρ of about 65% and 20% for samples I and II, respectively. Furthermore, we observe for sample I an inversion of the CMR from negative at high temperature, to positive CMR at very low temperatures (T
    Physica B Condensed Matter 01/2002; 321(1):68-73. · 1.33 Impact Factor
  • P Beauvillain, J -P Renard, P -E Hansen
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    ABSTRACT: The principal magnetic susceptibilities chi /sub /// and chi perpendicular to of LiErF4 have been measured in the temperature range 0.32-4.2K. An antiferromagnetic transition has been observed at 0.38K in agreement with the theoretical prediction for dipolar interactions and axial symmetry with gperpendicular to >g/sub ///.
    Journal of Physics C Solid State Physics 02/2001; 10(24):L709.
  • C Dupas, J -P Renard
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    ABSTRACT: The magnetic susceptibilities parallel and perpendicular to the chain axis have been measured in the three linear ferromagnets TMNC, TMNB and CsNiF3 between 1.2 and 77K. Strong anisotropy is observed at low temperature. The variation of chi //c with temperature is in good agreement with the predictions of the quantum given by de Neef. This allows a determination of the parameters J and D in these salts: in CsNiF3 J=10+or-0.5K, D=8.5+or-0.5K; in TMNC, J=1.7+or-0.3K, D=3.4+or-0.5K; in TMNB, J=4.8+or-0.2K, D=3.4+or-0.5K. In the 3D long-range ordered state, a study of the variation with temperature of the spontaneous sublattice magnetisation has been performed by NMR; the curves reflect the existence of an anisotropy in the ab plane in TMNC and TMNB, but not in CsNiF3.
    Journal of Physics C Solid State Physics 02/2001; 10(24):5057.
  • A Dupas, K Le Dang, J -P Renard, P Veillet
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    ABSTRACT: The magnetisation of the nearly two-dimensional weakly anisotropic ferromagnets (CnH2dn+1NH3)2CuCl4 has been studied by means of Cu and Cl nuclear magnetic resonance. The hyperfine interaction of Cu and Cl nuclei and the Cu2+ spin direction were determined. The field dependence of the magnetisation has been measured in the range 0 to 10 kOe. This allowed the intralayer exchange integral J to be determined using simple spin-wave theory taking into account experimental magnetic anisotropy and interlayer exchange. The temperature dependence of spontaneous magnetisation measured in the range 1.4 to 4.2K is well described by the theory using this J value.
    Journal of Physics C Solid State Physics 02/2001; 10(17):3399.
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    ABSTRACT: Ceramic samples of Pr0.7Sr0.3−x□xMnO3 with x⩽0.2 have been investigated by various techniques including 55Mn nuclear magnetic resonance, SQUID magnetometry, resistivity and magnetoresistance measurements. On increasing x, the samples remain ferromagnetic at a low temperature with a decreasing Curie temperature, but a metal–insulator transition is observed. Besides, a positive magnetoresistance (MR) at very low temperatures with respect to a negative colossal MR at high temperatures is observed in the Pr0.7Sr0.2□0.1MnO3 sample.
    Journal of Magnetism and Magnetic Materials 01/2001; 233(3):147-154. · 1.83 Impact Factor
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    ABSTRACT: The critical behaviour of La1.2Sr1.8Mn2O7 has been revisited by means of static macroscopic magnetic properties measurements. We present a renewed insight on its equilibrium properties near the critical point (TC ∼ 108 K) including the determination of the fundamental characteristics of a magnetic system (anisotropy, critical exponents and crossovers in the vicinity of the Curie temperature). It appears that two-dimensional correlations above TC do not spread very fast, and that thus the critical fluctuations regime occurs in a moderately narrow temperature range, assessing the three-dimensional nature of the ferromagnetic ordering. The higher temperature contributions to the physical properties, due to stacking defects, are interpreted within the context of the extensive studies carried out on thin films.
    Physics of Condensed Matter 01/2001; 2380:307-317. · 1.28 Impact Factor
  • P Beauvillain, C Chappert, J P Renard
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    ABSTRACT: The authors describe a fully automated SQUID magnetometer using digital noise reduction techniques which is adapted for low-field measurements on magnetic thin films. Solutions for thermometry, field calibration below 1 mT and shape-dependent moment calibration are presented.
    Journal of Physics E Scientific Instruments 11/2000; 18(10):839.
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    ABSTRACT: The parallel and perpendicular susceptibility of a single crystal of LiTb0.5Y0.5F4 has been measured in the temperature range 0.32-4.2K. A ferromagnetic transition occurs at Tc=1.119K instead of 2.890K for pure LiTbF4. The critical behaviour of the parallel susceptibility has been accurately studied in the range 5*10-4<t<10-1, where t=(T-Tc)/Tc. A marked effect of dilution by yttrium has been observed. In contrast with pure LiTbF4 where chi /sub /// is well fitted by the theoretically predicted law for uniaxial dipolar ferromagnets (i.e. the classical law with small logarithmic corrections), the usual power law t- gamma with gamma =1.215 seems to hold in LiTb0.5Y0.5F4. This could be related to a reduction of effective dimensionality with dilution.
    Journal of Physics C Solid State Physics 11/2000; 13(3):395.
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    ABSTRACT: We have performed magnetic susceptibility measurements on CuGe(1-x)SixO3 single crystals and obtained a detailed (T, x) phase diagram. In order to fully characterize the nature of the SP and AF phases in this diagram, we studied both order parameters by neutron di!raction as a function of temperature and Si concentration.
    Physica B Condensed Matter 01/2000; 280:313-314. · 1.33 Impact Factor
  • J.-P Renard, A Anane
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    ABSTRACT: Single crystals of La1−xSrxMnO3 (0≤x≤0.35) and Pr0.67Ca0.33MnO3 were studied by various techniques including SQUID magnetometry, resistivity and 55Mn nuclear magnetic resonance (NMR). In an untwinned LaMnO3 single crystal, the magnetization versus field at low T exhibits a strongly anisotropic behaviour and a weak ferromagnetic moment depending on oxygen stoichiometry is observed along c axis. The NMR spectra of La0.85Sr0.15MnO3 reveal a coexistence between ferromagnetic insulating and metallic phases. The drastic effect of oxygen deficiency on the Curie temperature is discussed. Slow thermal relaxation at the metal-insulator transition of Pr0.67Ca0.33MnO3 is studied by magnetization and resistivity measurements as a function of time. The experiments support the coexistence of metallic and insulating phases and a percolative behaviour is evidenced in the time variation of resistivity. A tentative interpretation by a phenomenological two-level model is given.
    Materials Science and Engineering B 08/1999; · 1.85 Impact Factor
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    ABSTRACT: The magnetic properties of ( La 1-x Sr x ) MnO 3 crystals have been studied. The ac susceptibility has been measured at frequencies ranging from 10 Hz to 1 000 Hz. The x = 0.25 crystal shows a typical ferromagnetic behaviour with a dimensionless SI susceptibility of 6 below T c = 350 K limited by the demagnetizing field. Both x = 0 and x = 0.06 crystals show very sharp peaks in and at the Néel points, 139 K and 135 K respectively. This is quite unlike the usual behaviour of an antiferromagnet. Magnetization measurements on an untwinned single crystal of LaMnO 3 show a weak moment of at 4.2 K which is directed along the c axis. The origin of this canted antiferromagnetic structure is discussed in terms of anisotropy and of Dzyaloshinskii-Moriya interaction. It is concluded that anistropy is insufficient to explain the canted antiferromagnetism.
    Physics of Condensed Matter 01/1999; 11(3):401-406. · 1.28 Impact Factor
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    ABSTRACT: We have performed magnetic susceptibility measurements on CuGe1-xSixO3 and Cu1-yMyGeO3 (with M=Zn, Mg, Ni) single crystals for a large number of concentrations. This study allowed us to determine an universal (¹(x, y)) phase diagram with a scaling factor y≈3x. We have also studied the coexistence of the AF and SP phases in Si-doped CuGeO3 compounds by neutron diffraction: As the silicon concentration increases, the long-range ordered antiferromagnetic phase over takes the spin—Peierls order, which becomes short-range ordered.
    Physica B Condensed Matter 01/1999; 259-261:954-955. · 1.33 Impact Factor
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    ABSTRACT: We performed magnetisation (M) and transport measurements along the ab plane and the c direction in La2−2xSr1+2xMn2O7 single crystals for x=0.4. In addition to a sharp para-ferromagnetic transition at Tc=125K, a weak maximum in M was found around 265K. A sharp drop of resistivity at Tc associated with a semi-conductor to metal transition was found for both current orientations. The colossal magnetoresistance of the compound CMR=−(ρH−ρ0)/ρ0 was found to be ∼100% in a field of 8T near Tc. In contrast with the well-known La1−xSrxMnO3 type manganites, a CMR effect larger than 10% was present at all temperatures ranging between 5 and 300K.
    Cold Regions Science and Technology - COLD REG SCI TECHNOL. 01/1999; 259:833-834.
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    ABSTRACT: We report here on an experimental study of slow thermal relaxation effects from the metastable metallic-ferromagnetic phase to the insulating one in a charge-ordered manganese oxide, Pr0.67Ca0.33MnO3. The metal-insulator transition is evidenced by an abrupt jump of the resistivity by several orders of magnitude at a well-defined time tau while there is no apparent singularity in the magnetization relaxation. This supports the view of a percolative behavior of current transport. The magnetic relaxation is discussed in terms of a two-level phenomenological model with a distribution of energy barriers.
    Physical Review B 01/1999; 59(1):77-80. · 3.66 Impact Factor
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    ABSTRACT: We report on the resistivity (ρab), magnetisation (M), and X-ray synchrotron measurements on a single crystal of the two-layer manganite LaSr2Mn2O7. ρab showed a maximum at 180 K and increased rapidly for In a field of 8 T, colossal magnetoresistance (CMR) data showed two maxima at 150 and 40 K. In low fields, M (H∥ab) showed also two maxima at these temperatures. X-ray investigations revealed satellite reflections whose intensity showed a strong maximum at 170 K followed by a re-entrant behaviour below about 40 K. The data seem to reflect the structural, charge and orbital degrees of freedom present in this compound.
    Solid State Communications 01/1999; 113(5):267-271. · 1.53 Impact Factor
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    ABSTRACT: We report here on resistance and magnetoresistance (MR) studies of ultrathin Co/Au(111) single sandwiches and bilayers with perpendicular magnetization. Resistance of the films was measured in situ in ultrahigh vacuum, during depositions and as a function of a perpendicular applied magnetic field. A large MR variation with the thickness of Au coverage was observed and compared to calculations. The coercive field of the Co films shows a drastic variation with the Au coverage thickness, which reflects the theoretical anisotropy variation. It was measured as a function of temperature. For the first time, the effect of interlayer interaction on the resistivity of a Co bilayer during the growth of Co top layer, is evidenced and compared to calculations. Finally, hysteresis loops of strongly antiferromagnetically coupled bilayers are investigated. PACS. 75.70.Cn Interfacial magnetic properties (multilayers, magnetic quantum wells, superlattices, magnetic heterostructures - 75.70.Pa Giant magnetoresistance
    Physics of Condensed Matter 01/1999; 10(2):223-231. · 1.28 Impact Factor
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    ABSTRACT: We have studied the effect of doping (up to 3%) with Ni, Zn and Si on the ESR spectra of CuGeO3. Our experiments were performed on single crystals at 9.5, 95 and 190 GHz, above and below the spin–Peierls transition. In contrast to Zn or Si-doping, which hardly affects the position of the resonance, the presence of Ni induces a large shift towards high fields of the ESR line. This shift is shown to be proportional to the ratio of Ni to Cu susceptibility: Hence it is strongly enhanced below the spin–Peierls transition.
    Physica B Condensed Matter 01/1999; 259:961-962. · 1.33 Impact Factor

Publication Stats

2k Citations
367.06 Total Impact Points

Institutions

  • 1976–2008
    • Université Paris-Sud 11
      • • Institut d'Electronique Fondamentale
      • • Laboratoire de Physique des Solides
      Paris, Ile-de-France, France
  • 2006
    • University Joseph Fourier - Grenoble 1
      • Institut Néel
      Grenoble, Rhône-Alpes, France
  • 2004
    • Atomic Energy and Alternative Energies Commission
      • SPEC - Service de Physique de l'Etat Condensé
      Gif-sur-Yvette, Ile-de-France, France
  • 1980–1995
    • Institute of Fundamental Electronics
      Orsay, Île-de-France, France
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 1976–1988
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1981–1983
    • Paul Sabatier University - Toulouse III
      Tolosa de Llenguadoc, Midi-Pyrénées, France