Interfacial effect on Mn-doped TiO2 nanoparticles: from paramagnetism to ferromagnetism

Abstract

Manganese-doped TiO₂ nanoparticles with different crystalline structures, namely, anatase, rutile, mixed-phase of anatase and rutile, have been synthesized by a template method. We found that different structures lead to different defect concentrations and, as a result, strongly influence the magnetic properties. The pure anatase and rutile Mn-doped TiO₂ are paramagnetic while the mixed-phase exhibits robust room temperature ferromagnetism. Extended X-ray absorption fine structure data reveal increasing oxygen vacancies when the mixed-phase formed. The key factor for activating ferromagnetism is found to be the interfacial defects created during phase-transformation, which is proposed to be oxygen vacancies. The defect effective Bohr radii are sufficiently large to overlap dopant ions, causing a net alignment of the dopant spins and then leading to a long-range ferromagnetic order. This discovery demonstrates that controlling the interfaces is an effective route to regulate magnetic properties and establish a prominent example of emergent phenomena at oxide interfaces.