Influence of the Host Lattice Electronic Structure on Dilute Magnetic Interactions in Polymorphic Cr(III)-Doped In₂O₃ Nanocrystals

The effect of the host lattice structure on the spectroscopic and magnetic properties of Cr 3+ -doped In 2 O 3 nanocrystals is reported. The influence of the dopant ions on the nanocrystal growth allows for the solution-phase stabilization and separation of doped colloidal In 2 O 3 nanocrystals having different crystal structures – stable cubic phase (bcc-In 2 O 3 ) and metastable rhombohedral (rh-In 2 O 3 ) phase – and comparative study of the electronic structure and magnetic properties of Cr 3+ in both polymorphs. Investigations by a range of complementary spectroscopic techniques, including Raman, X-ray absorption and magnetic circular dichroism spectroscopies, revealed that the change in the In 2 O 3 phase leads to distinctly different electronic structure of Cr 3+ dopants, associated with a different nature of the substitutional doping sites and different electronic structure of the nanocrystal host lattice. Nanocrystalline films prepared from colloidal nanocrystals exhibit ferromagnetism at room temperature, although the average magnetic moment of Cr 3+ in rh-In 2 O 3 is an order of magnitude smaller than that in bcc-In 2 O 3 samples. This difference in magnetization is associated with wider band gap of rh-In 2 O 3 nanocrystals, which prevents effective hybridization of the defect donor band, as a mediator of the Cr 3+ magnetic exchange interactions, and the Cr 3+ 3d states at the Fermi level. The results of this work demonstrate that a change in the defect and electronic structures of the same semiconductor host lattice by nanocrystal phase control in solution allows for tuning of the magnetic properties of diluted magnetic semiconducting oxides.