Investigation of structural, electronic and magnetica properties of Ca2-xLaxFelrO6, Sr2-xLaxFelrO6 e TbMnO3 oxides / Investigação as propriedades estruturais, eletronicas e magneticas dos oxidos Ca2-xLaxFelrO6, Sr2-xLaxFelrO6 e TbMnO3

AUTOR(ES)
DATA DE PUBLICAÇÃO

2010

RESUMO

For many decades the transition metal oxides are subject of great scientific interest because of the wide variety of interesting physical properties and their potential technological applications. More recently, for example, oxides of transition metals with multiferroic properties have been considered as potential magneto-electronic devices. Many transition metal oxides form in the perovskite crystalline structure, with cubic or distorted symmetry. Many other oxides can crystallize in a variant of the simple perovskite, called the ordered double perovskite (ODP), which has the general formula A2B B O6, where the A ion occupies the vertices of the cube while the cations B and B alternate in the centers of the oxygen octahedra. Sr2FeReO6 and Sr2FeMoO6 are two compounds with the ODP structure which were extensively studied due to their interesting properties such as half-metal behavior, tunneling magnetoresistance at room temperature and ferrimagnetic order (TC above 400 K). The structural, electronic and magnetic properties of these ODPs are highly correlated and are strongly dependent on the strong d orbitals hybridization of the of the B cations. Therefore, studies of new ODP compounds are important in order to investigate the current ideas proposed in the literature and improve the understanding of their physical properties. We report here our results of synthesis and characterization of the unpublished series Ca2-xLaxFeIrO6 and Sr2-xLaxFeIrO6, where the Ir such as Re and Mo are transition metal, with d character that can assume different valence states. The magnetic measurements indicated that those systems tend to evolve from antiferromagnetics at the ends of the series, x = 0 and x = 2, to ferrimagnetic for intermediate regions of the series. Measurements performed in the compound of higher magnetization in the Sr serie, Sr1.2La0.8FeIrO6 indicated that this compound orders ferrimagnetic around 700 K, which is the highest TC ever reported for double perovskites. Resistivity measurements as a function of temperature indicated that these compounds also exhibit insulating behavior and virtually no magneto-resistive effect. In the antiferromagnetic compound Sr2FeIrO6, the effect of pressure on the resistivity was investigated, and although no metal-insulator transition was seen, there is a systematic decrease of the resistance and the slope of the curve as the pressure increases, indicating a Mott insulator-like behavior in this compound. This work also presents results on the TbMnO3 perovskite. We have performed magnetic susceptibility, specific heat, Electron Paramagnetic Resonance (EPR) and microwave absorption measurements at various temperatures. Magnetic susceptibility and specific heat data confirmed the ocurrence of a magnetic (TN = 41 K) and ferroelectric (Tlock) phase transition. The EPR spectra showed, for the entire temperature range measured, a single Lorentzian line shape and T independent g-value = 1.96 (3), consistent with the resonance of Mn3+ in an insulating environment. The width line broadens with the decreasing temperature following a C/T law. This broadening prevented the observation of the resonance spectra near the magnetic and ferroelectric phase transitions. Because of the strong frequency dependence of the dielectric constant, the measurements performed with the electric field cavity also did not allow observation of any anomaly around the ferroelectric transition

ASSUNTO(S)

multiferroic ferrimagnetism ferrimagnetismo perovskite multiferroico perovskita

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