Caracterizações estruturais e magnéticas de nanocompósitos do tipo Fe/Fe3O4, Fe/FeyO e Fe/Fe3O4/FeyO obtidos por moagem de alta energia

Abstract

In this work, Fe/Fe3O4, Fe/FeyO and Fe/Fe3O4 /FeyO nanocomposites were synthesized using the high energy milling method. The effect of the milling the formation of nanocomposites was investigated considering the following conditions: milling times of 24 and 40 h and rotation speeds of 300, 400 and 600 rpm, as well as for the milling time of 99 h only the velocity of 300 rpm was evaluated. The ground samples were characterized from the structural and magnetic perspective using X-Ray Diffraction, M¨ossbauer Spectroscopy, Magnetometric techniques and Magnetic Hyperthermia. From X - ray diffraction and Rietveld refinements, it was possible to identify, quantify and determine the average crystallite sizes and lattice parameters of the nanocomposite crystalline phases. The Rietveld refinemet for the samples milled at 24 h indicated a reduction of the crystallite average sizes with the increase of the rotation speed due to the higher fragmentation of particles. For the samples produced at 600 rpm during 40 h, the magnetite crystalline structure changed to the w¨ustite one. Such structural evolution was confirmed by M¨ossbauer spectroscopy and is in agreement with the X-ray diffraction data. The sample containing w¨ustite presented interesting thermal properties with potential for hyperthermia applications in cancer therapy. When the effect of the milling time 24, 40 and 99 h was evaluated for a fixed speed 300 rpm, a significant decrease in the Fe concentration of the sample was verified. The data obtained from the magnetic measurements revealed a reduction of the magnetization saturation as the time and speed of milling increased, a fact due to the reduction of iron concentration in the samples. Magnetic measurements as a function of magnetic field at different temperatures from 5 K to 25 K for the samples produced during 40 and 99 h with speeds of 600 and 300 rpm, respectively, revealed the effect of exchange bias, which is associated with the coupling between the ferromagnetic and antiferromagnetic phases. In the magnetization results as a function of the temperature for the sample milling during 99 h, a displacement of the irreversibility temperature was observed, suggesting a spin-glass behavior according to Almeida-Thouless.