An XMCD study of magnetism and valence state in iron-substituted strontium titanate

TL;DR

This study uses XMCD to investigate the origin of room temperature ferromagnetism in iron-doped SrTiO3 thin films, revealing that Fe$^{2+}$ ions and oxygen vacancies are key to magnetic properties and can be controlled by annealing.

Contribution

It provides new insights into the microscopic origin of magnetism in Fe-substituted SrTiO3 and shows how oxygen vacancies influence magnetic moments and Fe valence states.

Findings

Fe$^{2+}$ ions are responsible for magnetism

Magnetic moment correlates with Fe$^{2+}$/Fe$^{3+}$ ratio

Annealing affects Fe valence and magnetism

Abstract

Room temperature ferromagnetism was characterized for thin films of SrTi$_{0.6}$Fe$_{0.4}$O$_{3-{\delta}}$ grown by pulsed laser deposition on SrTiO$_{3}$ and Si substrates under different oxygen pressures and after annealing under oxygen and vacuum conditions. X-ray magnetic circular dichroism demonstrated that the magnetization originated from Fe$^{2+}$ cations, whereas Fe$^{3+}$ and Ti$^{4+}$ did not contribute. Films with the highest magnetic moment (0.8 {\mu}B per Fe) had the highest measured Fe$^{2+}$:Fe${^3+}$ ratio of 0.1 corresponding to the largest concentration of oxygen vacancies ({\delta} = 0.19). Post-growth annealing treatments under oxidizing and reducing conditions demonstrated quenching and partial recovery of magnetism respectively, and a change in Fe valence states. The study elucidates the microscopic origin of magnetism in highly Fe-substituted SrTi$_{1-x}$Fe$_x$O$_{3-{\delta}}$ perovskite oxides and demonstrates that the magnetic moment, which correlates with the relative content of Fe$^{2+}$ and Fe$^{3+}$, can be controlled via the oxygen content, either during growth or by post-growth annealing.