Vacancy-Driven Noncubic Local Structure and Magnetic Anisotropy Tailoring in FexO-Fe3−δO4 Nanocrystals

In contrast to bulk materials, nanoscale crystal growth is critically influenced by size- and shape21 dependent properties. However, it is challenging to decipher how stoichiometry, in the realm of mixed22 valence elements, can act to control physical properties, especially when complex bonding is implicated by short- and long-range ordering of structural defects. Here, solution-grown iron-oxide nanocrystals (NCs) of the pilot wüstite system are found to convert into iron-deficient rock-salt and ferro spinel subdomains but attain a surprising tetragonally distorted local structure. Cationic vacancies within chemically uniform NCs are portrayed as the parameter to tweak the underlying properties. These lattice imperfections are shown to produce local exchange-anisotropy fields that reinforce the nanoparticles’ magnetization and overcome the influence of finite-size effects. The concept of atomic-scale defect control in subcritical-size NCs aspires to become a pathway to tailor-made properties with improved performance for hyperthermia heating over defect-free NCs.