Propriedades microestruturais e magnéticas de policristais de V2O5, CoV2O6 e Co3O4 sintetizadas por método Pechini
Dreifus, Driele Von
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In this work we performed an extensive research about the magnetic properties of three materials, vanadium pentoxide (V2O5), cobalt tetraoxide (Co3O4) and cobalt vanadate II (CoV2O6). All the three oxides were synthesized by a sol-gel technique named Pechini method. These oxides present remarkable distinct characteristics. The first one, V2O5, although being reported as a diamagnetic material in literature, presented a paramagnetic behavior accompanied by an antiferromagnetic transition at 80K. This behavior has its origin in structural defects in the sample such as oxygen vacancies. The presence of these structural defects in the sample was confirmed by photoluminescence measurements as a function of the temperature which showed a characteristic indirect band gap emission and another emission at lower energy, characteristic of oxygen vacancies. The second oxide, Co3O4, presented the usual antiferromagnetic transition at about 32K, and an additional peak characterized as a metastable transition at a lower temperature. This behavior was observed in the AC susceptibility measurements that the position of this peak exhibits a variation as the drive frequency is changed. This metastable transition might has it origin in the presence of particles with different sizes in the sample. This can provide changes to grains surface condition giving rise to canted moments which can contribute to frustrations in the system, in addition to generating an increase in the magnetization for temperatures below this transition. The third oxide studied in this work is CoV2O6. This material has very peculiar magnetic properties like plateaus in its magnetization curve which are induced by the applied magnetic field. At temperatures below 7K, the system changes from an anisotropic antiferromagnetic state to a ferrimagnetic state, with a saturation tendency with the magnetic field increase. The interactions between the quasi one-dimensional chains of cobalt in this, that comprises the material, lead to frustrations in the system structure that allow the emergence of various transitions. We also observed in this system a transition at about 2,8K, with low intensity when compared to that one at 7K. This transition has an intensity dependence with AC drive frequency observed in AC susceptibility measurements as a function of temperature.