Oxygen-deficient dopant-free Ti3O5 and Ti2O3 ferromagnetic two-dimensional nanostructures for spin-based electronic devices

Abstract

Titanium suboxides (Ti_nO_2n−1, n = 2, 3, 4…) have attracted immense research interest due to their unique crystal and band structures. The presence of both Ti³⁺ and Ti⁴⁺ ions in the suboxides provides several possible cation configurations in the crystal structure, various charge-ordered states, and electronic structures that could lead to versatile spin functionality found in magnetism, magneto-transport, and magneto-optics. The creation of oxygen vacancy defects could further disrupt the crystal composition, and thereby spin functionality of the suboxide system. Here, we demonstrate, for the first time, the controlled growth of oxygen-deficient two-dimensional titanium suboxide nanostructures using catalyst-assisted pulsed laser deposition. Not only is the observed magnetization (7.93 emu g⁻¹) significantly higher than those of all the dilute ferromagnetic semiconducting oxides reported to date, but also the ferromagnetic to superparamagnetic phase transition could be tuned by manipulating the thickness of the suboxide nanowalls. We also show that it is the singly charged oxygen vacancy defects that contribute to this remarkable ferromagnetism. To explain the origin of ferromagnetism, we propose an improved defect-induced bound polaron magnetism model, which is generally applicable to not just the present system but indeed other oxygen-deficient metal oxides with unoccupied d orbitals. These results therefore provide new insights about the origin of ferromagnetism in the oxygen-deficient dilute ferromagnetic semiconducting oxides. More importantly, manipulation of magnetization by controlling the crystal composition and the specific surface area of the nanowalls promises a new approach to engineering the charge and spin ground states in semiconductors for novel spin-based electronic device applications.