Estudo das propriedades estruturais e magnéticas em compostos multiferroicos do tipo Yb1-xTbxMnO3 (x = 0; 0,25; 0,50; 0,75 e 1)

Abstract

Multiferroic perovskites of the RMnO3 type present a strong coupling between structural, magnetic and electrical properties. As a consequence, this type of materials has been focus of intensive research works stimulated by the possible technological applications. TbMnO3 and YbMnO3 are two of the most important examples of these multiferroic manganites. TbMnO3 presents electric polarization bellow 27 K that is induced by magnetic reordering that is in turn coupled to structural changes. In contrast to TbMnO3 magnetic and electrical ordering occur independently in YbMnO3 and, in this compound antiferromagnetic ordering appears at around 90 K while ferroelectricity is observed only at high temperatures, up to 1000 K. The main goal in this work is to investigate the correlations between magnetic ordering and crystallographic structure in order a better understanding of the role of structure in the observed magnetic properties. Polycrystalline manganites of Yb1-xTbxMnO3 (x = 0, 0.25, 0.50, 0.75, and 1) were synthesized by using the solid state reaction process. The room temperature structure of these compounds was studied by X-ray diffraction employing a Cu-K? source. The Rietveld refinement method was employed in the structural analysis process. The obtained results indicated a crystallization in an orthorhombic phase with space group Pnma for TbMnO3, whereas, for the YbMnO3 manganite presented a hexagonal phase P63cm; both are in agreement with previously published works. A single hexagonal structure phase P63cm, was observed for Yb0,75Tb0,25MnO3. In the case of the x = 0.50 and 0.75 compounds the coexistence of both hexagonal and orthorhombic phases was evidenced. Rietveld refinement made it possible to model the rare-earth sites occupancy (Tb3+ and Yb3+) in the hexagonal phase. The determination of bond distances and angles enabled to quantify the distortions and tilting of the MnO6 and MnO5 coordination polyhedra. The DC magnetization measurements in ZFC and FC modes, as well hysteresis loops were performed using SQUID magnetometer from 4 K up to room temperature. The results obtained for concentrations x = 0 and 1 are in good agreement with previously reported literature works. TbMnO3 presented an antiferromagnetic phase transition, associated with the Tb3+ ions, as well as metamagnetism and weak ferromagnetism effects. For YbMnO3 it was seen indications of magnetic frustration associated with triangular lattice of Mn3+ ions as well as weak ferromagnetism, caused the Dyzaloshinski-Moriya interaction that produces spin canting on the triangular lattice. The x = 0.25 compound behaves like YbMnO3. Differently, in concentrations x = 0.50 and 0.75 some characteristics that suggest a possible spin glass behavior were noted. Particularly, in these last two compounds it was seen a competition between magnetic orderings. In all compounds, low and room temperature X-ray diffraction measurements have been performed using synchrotron radiation. Nevertheless, some experimental issues and limitations in data collection did not allow to obtain good quality data to be refined, except for the manganite with x=0. In such case it was possible to propose some qualitative relations between structural distortions and magnetic properties based on the observed lattice and atomic position variations. In order to obtain quantitative conclusions more detailed, structural studies at low temperature are necessary and, consequently, better experiments must be performed.