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Synthesis, structure and magnetic properties of La3Co2SbO9: A double perovskite with competing antiferromagnetic and ferromagnetic interactions
(Elsevier, 2012-10-00) Franco, D.G.; Fuertes, V.C.; Blanco, M.C.; Fernández-Díaz, M.T.; Sánchez, R.D.; Carbonio, R.E.; Gerencia Física. Departamento Materia Condensada. División Bajas Temperaturas
The synthesis, structural characterization, and magnetic properties of La3Co2SbO9 double perovskite are reported. The crystal structure has been refined by X-ray and neutron powder diffraction data in the monoclinic space group P21/n. Co2+ and Sb5+ have the maximum order allowed for the La3Co2SbO9 stoichiometry. Rietveld refinements of powder neutron diffraction data show that at room temperature the cell parameters are a=5.6274(2) Å, b=5.6842(2) Å, c=7.9748(2) Å and β=89.999(3)°. Magnetization measurements indicate the presence of ferromagnetic correlations with TC=55 K attributed to the exchange interactions for non-linear Co2+–O–Sb5+–O–Co2+ paths. The effective magnetic moment obtained experimentally is μexp=4.38 μB (per mol Co2+), between the theoretical one for spin only (3.87 μB) and spin-orbit value (6.63 μB), indicating partially unquenched contribution. The low magnetization value at high magnetic field and low temperature (1 μB/f.u., 5 T and 5 K) and the difference between ZFC and FC magnetization curves (at 5 kOe) indicate that the ferromagnetism do not reach a long range order and that the material has an important magnetic frustration.
Characterization of the Nb-B Superlattice System
(Elsevier, 2016-12-15) Franco, D.G.; Sarmiento-Chavez, A.; Schenone, N.; Allcca, A.E.L.; Berisso, M.G.; Fasano, Y.; Guimpel, J.; Gerencia Física. Departamento Materia Condensada. División Bajas Temperaturas
We study the growth, stacking and superconducting properties of Nb and B thin films and superlattices. The interest in these resides in their possible use in transition edge neutron sensors. The samples were grown by magnetron sputtering over Si (1 0 0) substrates. The X-ray diffraction patterns for all Nb containing samples show a Nb (1 1 0) preferential orientation. From the low-angle X-ray reflectivity we obtain information on the superlattice structure. The superconducting transition temperatures of the superlattices, obtained from the temperature dependence of the magnetization, are higher than those of single Nb films of similar thickness. The temperature dependence of the perpendicular and parallel upper critical fields indicate that the superlattices behave as an array of decoupled superconducting Nb layers.
Magnetic Properties of the Double Perovskites LaPbMSbO6 (M = Mn, Co, and Ni)
(IEEE (Institute of Electrical and Electronics Engineers), 2013-07-23) Franco, D.G.; Carbonio, R.E.; Nieva, G.; Gerencia Física. Departamento Materia Condensada. División Bajas Temperaturas
New double perovskites LaPbMSbO 6 , where M 2+ = Mn 2+ , Co 2+ , and Ni 2+ , were synthesized as polycrystals by an aqueous synthetic route at temperatures below 1000 ° C. All samples are monoclinic, space group P2 1 /n, as it is observed from Rietveld analysis of X-ray powder diffraction patterns. The distribution of M 2+ and Sb 5+ among the two octahedral sites have 3% of disorder for M 2+ = Ni 2+ , whereas for M 2+ = Mn 2+ and Co 2+ less disorder is found. The three samples have an antiferromagnetic transition, due to the antiferromagnetic coupling between M 2 + through super-superexchange paths M 2+ -O 2- -Sb 5+ -O 2- -M 2+ . Transition temperatures are low: 8, 10 and 17 K for Mn 2+ , Co 2+ , and Ni 2 + respectively, as a consequence of the relatively long distances between the magnetic ions M 2+ . Besides, for LaPbMnSbO 6 a small transition at 45 K was found, with ferrimagnetic characteristics, possibly as a consequence of a small disorder between Mn 2+ and Sb 5+ . This disorder would give additional and shorter interaction paths: superexchange Mn 2+ -O 2- -Mn 2+ .
Synthesis and structural and magnetic characterization of the frustrated magnetic system La2Ni4/3-xCoxSb2/3O6
(Elsevier, 2013-11-00) Franco, D.G.; Carbonio, R.E.; Nieva, G.; Gerencia Física. Departamento Materia Condensada. División Bajas Temperaturas
We report the synthesis of double perovskites La2Ni4/3−xCoxSb2/3O6 with x=0, 1/3, 2/3 and 1 by a solid state method. Rietveld refinements of X-ray and neutron powder diffraction data show that all samples crystallize in space group P21/n, with almost perfect occupation of the 2d octahedral site with the transition metals, while all Sb5+ are randomly distributed in a 2c octahedral site. The saturation magnetization in hysteresis loops indicates that the samples are ferrimagnetic throughout all the series. Virgin magnetization curves lie outside hysteresis loops at low temperatures and thermal evolution of Hm – defined as the inflection point of these curves – follows the de Almeida–Thouless dependence for . This spin glass like behavior below 30 K is also supported by thermal evolution of the coercivity, which follows an exponential law typical of magnetic clusters, not found in the pure Ni2+ perovskite, x=0 extreme.
Change in the Magnetic Domain Alignment Process at the Onset of a Frustrated Magnetic State in Ferrimagnetic La2Ni(Ni1/3Sb2/3)O6 Double Perovskite
(IEEE (Institute of Electrical and Electronics Engineers), 2013-07-23) Franco, D.G.; Carbonio, R.E.; Nieva, G.; Gerencia Física. Departamento Materia Condensada. División Bajas Temperaturas
We have performed a combined study of magnetization hysteresis loops and time dependence of the magnetization in a broad temperature range for the ferrimagnetic La 2 Ni(Ni 1/3 Sb 2/3 )O 6 double perovskite. This material has a ferrimagnetic order transition at ~100 K and at lower temperatures (~20 K) shows the signature of a frustrated state due to the presence of two competing magnetic exchange interactions. The temperature dependence of the coercive field shows an important upturn below the point where the frustrated state sets in. The use of hysteresis data, magnetization versus applied magnetic field, together with the magnetization versus time data provides a unique opportunity to distinguish between different scenarios for the low temperature regime. From our analysis, a strong domain wall pinning results in the best scenario for the low temperature regime. For temperatures larger than 20 K, the adequate scenario seems to correspond to a weak domain wall pinning.