Role of intensive milling in the processing of barium ferrite/magnetite/iron hybrid magnetic nano-composites via partial reduction of barium ferrite

Abstract

In this research a mixture of barium ferrite and graphite was milled for different periods of time and then heat treated at different temperatures. The effects of milling time and heat treatment temperature on the phase composition, thermal behavior, morphology and magnetic properties of the samples have been investigated using X-ray diffraction, differential thermal analysis, high resolution transmission electron microscopy and vibrating sample magnetometer techniques, respectively. X-ray diffraction results revealed that BaFe12O19/Fe3O4 nanocomposites form after a 20h milling due to the partial reduction of BaFe12O19. High resolution transmission electron microscope images of a 40h milled sample showed agglomerated structure consisting of nanoparticles with a mean particle size of 30nm. Thermal analysis of the samples via differential thermal analysis indicated that for un-milled samples, heat treatment up to 900°C did not result in α-Fe formation, while for a 20h milled sample heat treatment at 700°C resulted in reduction process progress to the formation of α-Fe. Wustite was disappeared in an X-ray diffraction pattern of a heat treated sample at 850°C, by increasing the milling time from 20 to 40h. By increasing the milling time, the structure of heat treated samples becomes magnetically softer due to an increase in saturation magnetization and a decrease in coercivity. Saturation magnetization and coercivity of a sample milled for 20h and heat treated at 850°C were 126.3emu/g and 149.5Oe which by increasing the milling time to 40h, alter to 169.1emu/g and 24.3Oe, respectively. High coercivity values of milled and heat treated samples were attributed to the nano-scale formed iron particles.