-
[show abstract]
[hide abstract]
ABSTRACT: The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant. Comment: 10 pages, 11 figures
07/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: A detailed analysis of the crystal structure in RETiO3 with RE = La, Nd, Sm, Gd, and Y reveals an intrinsic coupling between orbital degrees of freedom and the lattice which cannot be fully attributed to the structural deformation arising from bond-length mismatch. The TiO6 octahedra in this series are all irregular with the shape of the distortion depending on the RE ionic radius. These octahedron distortions vary more strongly with temperature than the tilt and rotation angles. Around the Ti magnetic ordering all compounds exhibit strong anomalies in the thermal-expansion coefficients, these anomalies exhibit opposite signs for the antiferromagnetic and ferromagnetic compounds. Furthermore the strongest effects are observed in the materials close to the magnetic cross-over from antiferromagnetic to ferromagnetic order.
01/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: The crystal and magnetic structure of La_{1-x}Sr_{1+x}MnO_4 (0<x<0.7) has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation. Comment: 8 pages, 6 figures, submitted to PRB
08/2004;
-
C. Yaicle,
C Martin,
Z. Jirak,
F. Fauth,
G. Andre,
E. Suard,
A. Maignan,
V. Hardy,
R. Retoux,
M. Hervieu,
S. Hebert,
R. Raveau Ch. Simon,
D. Saurel,
A. Brulet, F. Bouree
[show abstract]
[hide abstract]
ABSTRACT: Substitutions at the Mn-site of the charge-ordered Pr0.5Ca0.5MnO3 manganite is an effective way to induce abrupt jumps on the magnetic field driven magnetization curve. In order to get new insights into the origin of this remarkable feature, the Pr0.5Ca0.5Mn0.97Ga0.03O3 perovskite manganite has been studied by neutron diffraction, versus temperature and at 2.5K in an applied magnetic field up to 6 Tesla. A weak and complex antiferromagnetic order is found for the low temperature ground-state. Magnetic transitions, associated with structural ones, are evidenced for certain strengths of magnetic field, which gives rise to the step-like behavior corresponding to the magnetization curve. Small angle neutron scattering provides evidence for a nucleation process of micron size ferromagnetic domains which follows the magnetization behavior.
10/2003;
-
[show abstract]
[hide abstract]
ABSTRACT: The crystal and magnetic structure of LaTiO3 ~ has been studied by x-ray and neutron diffraction techniques using nearly stoichiometric samples. We find a strong structural anomaly near the antiferromagnetic ordering, T$_N$=146 K. In addition, the octahedra in LaTiO3 exhibit an intrinsic distortion which implies a splitting of the t2g-levels. Our results indicate that LaTiO3 should be considered as a Jahn-Teller system where the structural distortion and the resulting level splitting are enhanced by the magnetic ordering. Comment: 4 pages 5 figures
02/2003;
-
[show abstract]
[hide abstract]
ABSTRACT: The mixed-valence terbium (III/IV) fluoride KTb <sub>3</sub> F <sub>12</sub> crystallizes with the tetragonal symmetry, space group I4/m. Below T<sub>N</sub>=3.6±0.1 K this compound exhibits an antiferromagnetic collinear structure with k=0 characterized by ferromagnetic chains of edge-sharing Tb <sup>4+</sup> polyhedra and an antiferromagnetic order between the nearest chains. The magnetic moments of the Tb <sup>4+</sup> ions deduced from neutron diffraction are equal to 6.95(4) μ<sub>B</sub> at 1.4 K and aligned along the c axis. The Tb <sup>3+</sup> ions lie in a special position with zero magnetic moments due to a symmetry cancellation effect of the internal molecular field. In other words, Tb <sup>3+</sup> ions present a zero time-averaged magnetic moments in the ordered state, whereas T<sub>N</sub> corresponds to the long-range antiferromagnetic ordering of the moments of the Tb <sup>4+</sup> ions. From the thermal variation of the reciprocal magnetic susceptibility a molar Curie constant of 27.97(1) emu K mol<sup>-1</sup> has been obtained, in good agreement with the theoretical value of 27.64 expected for two Tb <sup>4+</sup> and one Tb <sup>3+</sup> free ions. The Tb <sup>4+</sup> contribution to the magnetization is approximated by the Langevin function and then the Tb <sup>3+</sup> contribution is obtained by subtracting the calculated Tb <sup>4+</sup> contribution to the total magnetization. © 2002 American Institute of Physics.
Journal of Applied Physics 06/2002; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Cr doping in Pr <sub> 0.5 </sub> Ca <sub> 0.5 </sub> MnO <sub> 3 </sub> is known to destroy the low-temperature antiferromagnetic charge-ordered state and to induce a ferromagnetic component as well as colossal magnetoresistance properties. Effects of Cr doping on the magnetic and nuclear structures are studied here by neutron diffraction for Pr <sub> 0.5 </sub> Ca <sub> 0.5 </sub> Mn <sub> 0.95 </sub> Cr <sub> 0.05 </sub> O <sub> 3 </sub>. It is shown that, at low temperature, the strongly distorted charge ordered structure usually observed in Pr <sub> 0.5 </sub> Ca <sub> 0.5 </sub> MnO <sub> 3 </sub> is replaced by a ferromagnetic phase in which the basal-plane distortion of the MnO <sub> 6 </sub> octahedra has disappeared, in agreement with the restoration of the double–exchange interactions. © 1998 American Institute of Physics.
Applied Physics Letters 01/1999; · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have studied the crystal structure of the uncommon phase with k=0 in ZrV2Dx, 2.2<x<2.5, which is an intermediate between the hydrogen-disordered phase and two hydrogen superstructures, ZrV2D<2 with k=(1/2 1/2 1/2) and ZrV2D>2.7 with k=(001). This phase is a primary superstructure combining the features of the disordered phase and, depending on the hydrogen concentration, one or another superstructure with k≠0. Its lattice (translational symmetry) is the same as in the disordered phase, which is k=0. Simultaneously, the lattice sites (the hydrogen arrangement in them) are prototypes of the sites of the subsequent superstructure with k≠0. Specifically, each site of the primary superstructure with k=0 is a mix of the sites with different spatial orientation of the superstructure with k≠0. In this sense the primary superstructure can be considered as a ‘lattice liquid crystal’ whereas usual superstructure with k≠0 is a ‘lattice crystal’. In addition, we have determined the crystal structure of the ‘ordered’ phase with k=(001) in ZrV2D2.73. It is a transitional state between the primary superstructure and the regular superstructure with the same k.
Journal of Alloys and Compounds.
