Thallium substitution effects on Co valence states and room-temperature ferromagnetism in co-doped In2O3: Experimental and machine-learning insights

Abstract

We systematically investigate the effects of thallium (Tl) substitution in Co-doped indium oxide (${In}_{1.9-x}{Tl}_x{Co}_{0.1}{O}_3$), focusing on its influence on Co valence states and magnetic properties. X-ray diffraction (XRD) confirmed that the bixbyite structure was preserved across a wide Tl composition range (x = 0.0–0.6). X-ray photoelectron spectroscopy (XPS) revealed that Tl incorporation increased the ${Co}^{3+}$/${Co}^{2+}$ ratio before annealing. Vacuum annealing induced partial reduction of ${Co}^{3+}$ to ${Co}^{2+}$, but higher Tl substitution led to weaker enhancement of ferromagnetic order. Magnetization measurements showed that the pristine (x = 0.0) composition transformed from a paramagnetic to a room-temperature ferromagnetic state upon annealing, while higher Tl content systematically weakened ferromagnetism. These results suggest that the larger ionic radius of ${Tl}^{3+}$ modifies crystal-field environments and magnetic exchange interactions, promoting ${Co}^{3+}$ formation yet suppressing long-range ferromagnetism. A machine-learning model using ridge regression predicted the ${Co}^{3+}$ ratio and room-temperature ferromagnetism (RTF) based on ionic radius and ionization energy with high accuracy (R² ≥ 0.85). Predictions for hypothetical dopants in ${In}_{1.6}{M}_{0.3}{Co}_{0.1}{O}_3$ suggested rare-earth elements such as Pr, Lu, Ce and Nd could enhance ${Co}^{3+}$ content and ferromagnetic properties. This study highlights how ionic size and ionization energy influence valence states and magnetism in ${{In}_{2}O}_3$-based diluted magnetic semiconductors, demonstrating the synergy of experiments and machine learning in spintronics material design.