Formation of Fe–Fe Antiferromagnetic Dimers in Doped TiO₂:Fe Nanoparticles

In this work, we report the results of comprehensive experimental and theoretical study of magnetic properties of TiO₂ nanoparticles (20 nm) doped with Fe at various concentrations ranging from 0.1 to 4.6 at. %. Our electron paramagnetic resonance and magnetic measurements data evidence the mixed magnetic state, where paramagnetic Fe³⁺ ions coexist with short-range antiferromagnetic correlations caused by negative exchange interaction between neighboring Fe³⁺ ions. A quantum mechanical model of the Fe-based magnetic cluster represented as a set of dimers with strong ∼(100–300) K and weak (∼1 K) exchange interactions has been proposed. Our model was found to provide a good description of magnetic properties of TiO₂:Fe nanopowders. Density-functional theory (DFT) calculations revealed Fe³⁺ oxidation state of the iron center in the vicinity of an oxygen vacancy in the crystal structure of anatase. DFT calculations confirmed that two types of Fe³⁺ spin-pairs with weak and strong exchange interactions can be formed in the vicinity of an oxygen vacancy. Accumulation of magnetic moment carriers and formation of magnetic clusters in TiO₂ nanoparticles with anatase structure were found to be a general tendency for all studied TiO₂:Fe nanopowders.