Efeitos da polarização elétrica sobre as propriedades magnéticas, térmicas e magnetoelétricas em compósitos multiferroicos de 0,675Pb(Mg1/3Nb2/3)O3-0,325PbTiO3/CoFe2O4

Abstract

In this doctoral thesis, the magnetization, specific heat and magnetoelectric response (ME) measurements were carried out under remaining electric polarization conditions in the PMN-PT / CFO particulate multiferroic composites, with the objective of studying the effects of the electric condition boundary of the ferroelectric phase on the magnetic, thermal and magnetoelectric properties. The magnetization measurement results in the function of the magnetic field and the temperature showed that under the remaining electric polarization conditions, there were decreases in the saturation and remnant magnetization values, indicating the magnetic moment reductions. However, no change was observed for the coercive magnetic field values under the electric polarization condition, showing that the mechanical stresses of the ferroelectric phase due to the remaining electrical polarization did not contribute to the increase or decrease of these values. The effect of decreasing magnetization was attributed to the partial exchange of electronic valences of the iron ions in the cobalt ferrite. Furthermore, we observed by means of the DC magnetic susceptibility that at temperatures higher than the room temperature, during electrical depolarization process, showed the ferroelectric and ferrimagnetic phase transitions. The results of the specific heat measurement (Cp) in the function of the magnetic field and temperature, under remaining electric polarization effect, showed an increase its thermal capacity, as compared with the electric unpoled condition. This change was ascribed to the structural crystal distortion, probably, by reduction of the electronic valence of the iron. The measurements at low temperature showed the Schottky-like anomaly, being independent of the remaining electric polarization state of the sample. The ME effect measurements were carried out under two electric polarization process conditions: at room temperature and higher than that temperature, 75 °C, below at the ferroelectric-paraelectric transition temperature, 124 °C. The result of the ME coefficient showed an increase in the maximum peak, to which attributed to better performance of the piezoelectric and magnetostrictive effects by at-high temperature electrical polarization process. Furthermore, the ME dynamic effect at several temperatures was attributed to the spin/lattice relaxation and the adiabatic state under different rates variation of the magnetic field. The results showed that the remaining electric polarization state in the multiferroic composite materials go beyond influencing the ferroelectric response, and consequently the magnetoelectric response, but also, changes significantly the behavior of the saturation magnetization of the ferrimagnetic phase, indicating the importance to consider this effect in the interpretation of the physical properties of multiferroic composites.