The role of iron doping on magnetic properties of hydrothermal anatase TiO2:57Fe (0–1 at. %) nanoparticles is investigated by combining superconducting quantum interference device magnetometry with Mössbauer and electron paramagnetic resonance techniques. The results on both as-prepared and thermally treated samples in reduced air atmosphere reveal complexity of magnetic interactions, in connection to certain iron ion electron configurations and defects (oxygen vacancies, F-center, and Ti3+ ions). The distribution of iron ions is predominantly at nanoparticle surface layers. Formation of weak ferromagnetic domains up to 380 K is mainly related to defects, supporting the bound magnetic polaron model.

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A phenomenological 2D model, simulating the martensitic transformation, is built upon existing experimental observations that the size of the formed plates -in direct transformation-decreases as the temperature is lowered; then they transform back in reversed order. As such, if a reverse transformation is incomplete ("arrested"), the subsequent direct one will show anomalously large number of big size plates-old plus newly formed-but consequentially a depletion of intermediate sizes, due to geometrical constraints, phenomenon that generates thermal memory.

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