Room temperature ferromagnetism of Fe-doped and (Fe, Cu)-codoped TiO2 powders prepared by mechanical alloying

Fe-doped and (Fe, Cu)-codoped TiO2 powders were prepared by mechanical alloying. The structure, chemical composition and magnetic properties of samples were investigated with X-ray diffraction (XRD), inductively coupled plasma atomic emission spectrometry, Fe-57 Mossbauer spectroscopy, X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM). XRD patterns show that all milled samples present pure rutile structure, and no metal iron, copper or their oxides are found in the powders. XPS spectra further support the results of XRD. Mossbauer spectra indicate that iron atoms have dissolved into the TiO2 lattice in Fe3+ state with low concentration of Cu doped in TiO2 and when Cu concentration in TiO2 reaches 5.538 at.%, signal of a small portion of Fe3+ appears in the spectrum, which may be associated with the increase of both oxygen vacancies in the lattice and dopant concentration. VSM measurements reveal that all milled samples exhibit room temperature ferromagnetism. The ferromagnetism does not come from a second magnetic phase such as iron and its oxides particles/clusters but from the (Fe, Cu)-codoped TiO2 matrices. The observed room temperature ferromagnetism of milled samples is attributed to the strong exchange interactions of the large number of overlapping bound magnetic polarons (BMPs). The saturation magnetization increases with increasing Cu content up to 3.076 at.%, which might be due to doping Cu could further increase concentration of oxygen vacancies to form more BMPs. When Cu concentration is increased to 5.538 at.%, the antiferromagnetic interaction could dominance over ferromagnetic interaction, resulting in reduction of magnetization. The magnetic properties of the samples are primarily attributed to the effect of oxygen vacancies, and oxygen vacancies play an important role in the origin of ferromagnetism in the system.