Síntese e caracterização de perovskitas quádruplas multiferróicas

Abstract

Materials that display induced electric polarization by magnetic ordering have attracted a lot of attention due to their peculiar physical properties and respective applications in devices as memories, sensors and actuators. However, comprehend the induced electric polarization in these materials has been a challenging. In this investigation we performed the synthesis and characterization (structural, dielectric, magnetic and vibrational) of pure quadruple multiferroic perovskites CaMn7O12 (CMO), Sr2+-doped CMO and Co3+-doped CMO in order to clarify the mechanisms that drive the induced ferroelectricity in CMO. In pure CMO, we investigated the temperature-dependent Raman spectra from room temperature down to 10 K. The temperature dependence of the Raman mode parameters shows remarkable anomalies for both antiferromagnetic and incommensurate transitions that this compound undergoes at low temperatures. The anomalies observed in this material at the magnetic ordering transition indicate a spin-phonon coupling at higher- temperature magnetic transition in this material and magnetostriction effect at the lower-temperature magnetic transition. In Sr2+-doped CMO, we observed that the induced electric polarization is drastically reduced by Sr2+ doping into the A-site. Our preliminary results suggest that the induced electric polarization reduction in CMO is due to the increase of Mn4+ - O - Mn3+ bond angle. Regarding the Co3+-doped CMO sample we showed, by using X-ray diffraction and magnetic measurements, that the doping with trivalent Co3+ cation into the CMO octahedral sites implies in Mn3+ and Mn4+ ions structural disorder, causing the destruction of the magnetic helicity in CMO.