Síntese e caracterização de nanoferritas do tipo Mn1-xZnxFe2O4 (x = 0; 0,5; 1) Preparadas por High-Energy Ball Milling.

Abstract

In this thesis, we studied the formation and structural and magnetic properties of the ferrite phases Mn1-xZnxFe2O4 (x = 0; 0, 5; 1), prepared by high-energy ball milling, via X-ray diffraction (XRD), 57Fe M¨ossbauer spectroscopy and magnetic susceptibility measurements (as a function of temperature and AC field frequency). For the mixed and pure Mn ferrites, there was an accident in the preparation process and we verified a contamination of the materials, which compromised the systematic presentation of the results. Nevertheless, we decided to present the results in this thesis to register the experiment. With the reliable portion of the data, we were able to infer that all ferrites, prepared by ball milling under the conditions estabilished in this work, present a quite similar formation kinetics. However, we were sucessful in the production of zinc ferrite (ZnFe2O4), where we accompanied the formation kinetics of this phase, which started in the first 10 h of milling, but was incomplete even after 200 h [there is still 6% of the precursors (ZnO and a-Fe2O3 powders) in the sample]. Moreover, we observed a high rate of formation of ZnFe2O4 until up to 90 h, with a subsequent reduction of the solid state reaction rate between the precursor powders. By 57Fe M¨ossbauer spectroscopy and X-ray diffraction, we distinguished the formation of two distinct ZnFe2O4 ferrite phases milled for 200 h: one with crystal structure of spinel-type and lattice parameter similar to that of its bulk counterpart and one magnetic phase. According to M¨ossbauer spectrums, the crystalline phase of this sample milled for 200 h, with 12 nm grains (as milled), has magnetic ordering temperature (TC ˜ 90 K) well above that found for its bulk counterpart, which is approximately 10 K. This increase in the value of TC is assigned to the cation inversion between A and B sites of the spinel structure which, in turn, should be occupied by Zn and Fe ions, respectively. The second component of the M¨ossbauer spectrums, with magnetic ordering temperature above 300 K, corresponds to a magnetic hyperfine field distribution. The fractions of these components change as we proceed with heat treatments, that is, when: (i) the average grain size increases, (ii) the microtensions produced by the milling process are reduced and (iii) the cation inversion between A and B sites is also reduced. This heat treatment process results in a still nanostructured sample, with average grain sizes in the 14-17 nm range, which are ordered in temperatures lower than 30 K, according to AC magnetic susceptibility and low-temperature M¨ossbauer measurements. We vi propose two models (A and B) to account for our experimental data. In model A, the grain boundary region is negligible from the magnetic point of view and we would have a large grain-size distribution. In model B, we would have a narrower grain distribution, but with major contributions from the grain boundaries, i.e., the region which will govern the magnetic interactions between grains. We were unable to determine which model is the most suitable, but based on physical arguments, we infer that its model B. Either way, the grains with crystalline order would be interacting magnetically and producing a T0 (from the Fulcher model) of 75 K. This value, along with the magnetic susceptiblity data, indicate that the heat treated in 773 or 973 K ZnFe2O4 ferrite exhibits cluster-glass-like behaviour, with well defined freezing temperatures, since ?T ?(Tf , f) = 0.