Study of the relation between oxygen vacancies and ferromagnetism in Fe-doped TiO2 nano-powders

Abstract

In this work, we present an experimental and theoretical study of structural and magnetic properties of Fe doped rutile TiO2 nanopowders. We show that Fe-doping induces the formation of oxygen vacancies in the first-sphere coordination of iron ions, which are in +2 and +3 oxidation states. We found that Fe ions form dimers that share one oxygen vacancy in the case of Fe3+ and two oxygen vacancies in the case of Fe2+. The saturation magnetization is almost independent of iron concentration and slightly increases with the relative fraction of Fe2+. Ab initio calculations show that two Fe ions sharing an oxygen vacancy are coupled ferromagnetically, forming a bound magnetic polaron (BMP), but two neighbor BMPs are aligned antiparallel to each other. Extra electron doping plays a fundamental role mediating the magnetic coupling between the ferromagnetic entities: carriers, possibly concentrated at grain boundaries, mediate between the BMP to produce ferromagnetic alignment.

Structural and Magnetic Properties of 57Fe‐Doped TiO2 and 57Fe/Sn‐Codoped TiO2 Prepared by a Soft‐Chemical Process

“…The lower H c values exclude the possibility of segregation of a metallic Fe phase 16. In addition, M s seems to be independent of the iron concentration, that is, only a small part of the total volume of the sample is related to the ferromagnetic contribution of the system 14. The M s value obtained in this study (ca.…”

“…Figure 2 shows the temperature dependence of the susceptibility for 57 Fe‐TiO 2 and 57 Fe‐Sn‐TiO 2 . The curves for samples with different iron concentrations show a temperature dependence similar to that expected for a pure paramagnet, but they do not tend to zero at high temperatures, and thus indicate the presence of a fraction of ions in a magnetically ordered phase 14. Doubtless, doping of Fe 3+ ions into the TiO 2 lattice leads to the formation of oxygen vacancies to maintain the charge balance, and the higher the degree of Fe doping, the higher the concentration of oxygen vacancies.…”

“…In addition, a linear variation of the magnetic moment of the sample versus the applied field is observed at higher fields, corresponding to the paramagnetic component 19. Thus, the room‐temperature hysteresis loop is composed of two contributions: paramagnetic and ferromagnetic 14. However, the paramagnetic component cannot reach a constant value at high field.…”

“…Similarly, Fe‐doped anatase TiO 2 annealed in an oxygen‐deficient atmosphere8 or hydrogen atmosphere9 also exhibited RTFM. Some groups also reported that Fe‐doped rutile TiO 2 samples exhibited RTFM, not only in the form of thin films, but also as nanoparticles 12,14,21. However, other studies claimed that the ferromagnetism in Fe‐doped rutile TiO 2 was due to the presence of extrinsic impurity phases, such as Fe oxides or Fe clusters 3,4.…”

Bandgap Engineering and Signature of Ferromagnetism in Ti1−xMnxO2­ Diluted Magnetic Semiconductor Nanoparticles: A Valence Band Study

“…They reported sharp ferromagnetic to paramagnetic transition with a high Curie temperature (T c ) ≈793 K for 3% Co‐doped TiO 2 NPs. Navarro et al showed that Fe‐doped rutile TiO 2 samples have ferromagnetic, paramagnetic, and antiferromagnetic contributions . They suggested that the ferromagnetism in Fe‐doped rutile TiO 2 strongly depends on defects.…”