Estudo da dupla Perovskita La2MnFeO6 preparada por reação de estado sólido

Abstract

In this work, three samples of LFMO were prepared by solid state reaction, calcined at 1200 ◦C, with treatment times equivalent to 24, 48 and 96 hrs. The obtained structures are double perovskite, monoclinic with space group P21/n and network parameters given by a = 5.52 ˚A, b = 5.51 ˚A and c = 7.81 ˚A. The sample crystallite sizes are equivalent to 361.0 nm (LFMO24), 348.9 nm (LFMO48) and 370.8 nm (LFMO96). According to the specific heat measurements, the LFMO’s have a ferrimagnetic phase transition at approximately 10 K, as data fit using the medium field theory. Peaks in the specific heat of the samples at higher temperatures were observed, occurring at different temperatures, depending on the degree of ordering of each LFMO produced. An excess in the specific heat is identified, where the LFMO24 and LFMO48 presented smaller and greater amount of defects, respectively. The contribution of the crystalline lattice was stipulated using the Thirring Model, with Debye temperatures of the order of the expected for this type of material. The measurements of magnetization DC, AC and magnetic hysteresis confirm the existence of weak ferromagnetic behavior at room temperature in all LFMO’s, in samples LFMO48 and LFMO96 it is assigned directly to Mn atoms, but in LFMO24, sample that presented higher magnetization value at this temperature, there is an increase provided to the Fe ions, in spite of the M¨ossbauer spectroscopy for the 57Fe, show a doublet. The real part of the AC susceptibility, χ0AC, complemented by different theoretical models, confirms a cluster-glass type phase, generated by frozen ion clusters in a certain temperature range in the LFMO’s, around 30 K, where χ0AC show peaks that vary with frequency. The strong presence of the Griffiths phase in the curves (χ0AC)−1 of LFMO48 and LFMO96 suggest that this phase is initiated at 65 K, occurring differently at LFMO24.