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Croissance cristalline, magnétisme critique et magnétorésistance colossale dans les manganites à structure Ruddlesden-Popper La1.2(Sr, Ca)1.8Mn2O7

Abstract : Ruddlesden-Popper layered compounds, of formula (La,Sr,Ca)_3Mn_2O_7, offer the opportunity to explore bidimensional magnetism in the manganites family. In order to acceed to the intrinsic and anisotropic characters of their physical properties, high-quality centimeter size single crystals of La_1.2(Sr, Ca)_1.8Mn_2O_7 were successfully grown using a floating zone method associated with an image furnace. The samples were characterized by X-ray and neutron diffraction and high-resolution transmission electron microscopy, verifying the chemistry, the crystallography and the absence of a second phase over several centimeters. A thorough investigation of the magnetic behaviour of these compounds was performed in a wide range of temperatures, including the determination of the fundamental characteristics of a magnetic system: magnetic anisotropy, critical exponents and crossovers in the vicinity of the Curie temperature, T_c1̃08K. By accurate measurements of magnetic susceptibility and magnetization on single crystals, specially designed to lessen the demagnetizing field, we could demonstrate that two-dimensional (2D) correlations start introducing deviations from the Curie-Weiss behaviour below 4̃20K. La_1.2Sr_1.8Mn_2O_7 can be viewed as a quasi-two-dimensional Heisenberg ferromagnet with notable deviations from this ideal model: firstly, the XY anisotropy inducing a spin dimensionality crossover at T_s4̃57K, secondly, the three-dimensional couplings between perovskite bilayer blocks, which drive a lattice crossover at T1̃17K. The slow development of the two-dimensional ferromagnetic correlations above T_c, and our quantitative measurements (critical exponents delta=(4.3±1.1) and gamma1̃.4 in the vicinity of T_c), lead us to believe that the ferromagnetic transition in La_1.2Sr_1.8Mn_2O_7 is essentially of three-dimensional nature. Intrically related with ferromagnetism are an insulator-metal transition at T_(i-m)1̃28K, and a negative magnetoresistance maximum effect (dp/p(8T))_[001]1̃1502̃x(dp/p(8T))_[110] for La_1.2Sr_1.8Mn_2O_7, somewhat superior to the performances of the perovskites with similar T_c's. The poor metallicity becomes indeed only established within the double perovskite slabs, consistently with spin correlations stronger in the (a,b)-plane than along the c -direction.
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Submitted on : Monday, September 26, 2011 - 11:25:34 AM
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Matias Velázquez. Croissance cristalline, magnétisme critique et magnétorésistance colossale dans les manganites à structure Ruddlesden-Popper La1.2(Sr, Ca)1.8Mn2O7. Matériaux. Université Paris Sud - Paris XI, 2001. Français. ⟨NNT : 2001PA112301⟩. ⟨tel-00626433⟩

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