XMCD study of magnetism and valence state in iron-substituted strontium titanate

Abstract

Room temperature ferromagnetism was characterized for thin films of SrTi0.6Fe0.4O3-δ grown by pulsed laser deposition on SrTiO3 and Si substrates under different oxygen pressures and after annealing under oxygen and vacuum conditions. X-ray magnetic circular dichroism demonstrated that the magnetization originated from Fe 2+ cations, whereas Fe 3+ and Ti 4+ did not contribute. Films with the highest magnetic moment (0.8 μB per Fe) had the highest measured Fe 2+ :Fe 3+ ratio of 0.1 corresponding to the largest concentration of oxygen vacancies (δ = 0.19).

The Use of Ultra-Small Fe3O4 Magnetic Nanoparticles for Hydrothermal Synthesis of Fe3+-Doped Titanate Nanotubes

“…Since the discovery of room temperature ferromagnetism in Co-doped TiO 2 thin films [ 63 ] understanding the causes of ferromagnetism in TiO 2 materials is a subject of lively discussion [ 64 , 65 , 66 , 67 , 68 , 69 ]. The hysteresis loop diagram for S1 resembles the results obtained in paper [ 70 ] for TiO 2 single crystals annealed in a high vacuum where the investigated samples were diamagnetic at room temperature with a characteristic low-field ferromagnetic behavior.…”

Unravelling the Origin of Ultra‐Low Conductivity in SrTiO3 Thin Films: Sr Vacancies and Ti on A‐Sites Cause Fermi Level Pinning

“…So far, oxygen vacancies as donors were neglected, even though it is known that their concentration can be substantial in acceptor doped SrTiO 3 . [ 97,98 ] However, SrTiO 3 bulk defect chemistry models [ 1 ] suggest that for electronically intrinsic thin films (i.e., low [h • ]), the oxygen vacancy concentration $\[[{\rm{V}}_{\rm{O}}^{ \bullet \bullet }]\]$ is also very low due to the mass action law $$\[{K_\delta } = {[{{\rm{h}}^ \bullet }]^2}{[{\rm{V}}_{\rm{O}}^{ \bullet \bullet }]^{ - 1}}p_{{{\rm{O}}_2}}^{ - 0.5}\]$$of the oxygen incorporation reaction $$\[1{\rm{/}}2{{\rm{O}}_2} + {\rm{V}}_{\rm{O}}^{ \bullet \bullet } \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftharpoonup\over{\smash{\rightharpoondown}}$}} {\rm{O}}_{\rm{O}}^{\rm{x}} + 2{{\rm{h}}^ \bullet }\]$$…”

“…So far, oxygen vacancies as donors were neglected, even though it is known that their concentration can be substantial in acceptor doped SrTiO 3 . [97,98] However, SrTiO 3 bulk defect chemistry models [1] suggest that for electronically intrinsic thin films (i.e., low [h • ]), the oxygen vacancy concentration…”

Dilute iron‐doped polycrystalline strontium titanate: Tracking iron valence and local interactions

“…It is generally accepted that low concentrations of Fe dopants in STO substitute for Ti 4+ and can exist in 4+, 3+, or 2+ oxidation states depending on 𝑃 𝑂 2 and temperature. [6][7][8] When treated or prepared in oxidizing conditions, Fe 3+ and Fe 4+ are expected to be preferred if substituted for Ti 4+ . The valence state of Fe is known to play a role in both the properties mentioned above.…”

“…Lightly Fe‐doped STO is commonly used as a model representative for acceptor‐doped large band gap electroceramics. It is generally accepted that low concentrations of Fe dopants in STO substitute for Ti 4+ and can exist in 4+, 3+, or 2+ oxidation states depending on $${P_{{O_2}}}$$ and temperature 6–8 . When treated or prepared in oxidizing conditions, Fe 3+ and Fe 4+ are expected to be preferred if substituted for Ti 4+ .…”