Unexpected atmosphere effect on the iron oxide growth by the Laser Floating Zone method

Abstract The control of the oxidation state represents an important challenge for materials engineering. It is fairly known that iron oxide can be present in several oxidation states each one with unique physical and chemical properties. In the present work the effects of the different oxygen partial pressure on the growth of Fe3O4 fibres, using hematite (Fe2O3) as a precursor material was studied. The fibres were grown by the Laser Floating Zone (LFZ) method at a constant pulling rate (100 mm/h) under different atmospheres (air, vacuum, argon and nitrogen). The structure and morphology of the as-grown fibres were studied by X-ray diffraction, SEM/EDS/EBSD and optical microscopy. The fibres show a core/shell structure, where Fe3O4 is the core and Fe2O3 the shell, putting in evidence that the partial pressure of oxygen is a crucial parameter to control the thickness and grains morphology of the shell. Low oxygen partial pressure increases the width of the Fe2O3 insulating phase shell. The electrical conductivity, of the samples grown under an atmosphere with low oxygen partial pressure, is improved due to the morphology of the grains in the shell. However, no significant changes in the magnetization measurements were detected. From the results, we can conclude that applying an atmosphere during the LFZ growth allows tunning the transport properties envisaging electrical applications of the as-grown material.