X-ray Anomalous Scattering of Diluted Magnetic Oxide Semiconductors: Possible Evidence of Lattice Deformation for High Temperature Ferromagnetism

TL;DR

This study used X-ray anomalous scattering to investigate the local structure of Co ions in ferromagnetic TiO2 films, finding evidence of lattice deformation and suggesting Co ions are not simply substituting Ti sites, unlike in ZnO.

Contribution

The paper demonstrates that Co ions in ferromagnetic TiO2 are not located at Ti sites, indicating lattice deformation, and confirms Co substitution at Zn sites in ZnO using anomalous scattering.

Findings

Co ions in TiO2 do not substitute Ti sites as expected.

Lattice deformation around Co ions likely caused by oxygen vacancies.

Co ions are confirmed to substitute Zn sites in ZnO.

Abstract

We have examined whether the Co ions crystallographically substitute on the Ti sites in rutile and anatase Ti_{1-x}$Co$_{x}$O$_{2-delta}$ thin films that exhibit room-temperature ferromagnetism. Intensities of the x-ray Bragg reflection from the films were measured around the $K$-absorption-edge of Co. If the Co ions randomly substitute on the Ti sites, the intensity should exhibit an anomaly due to the anomalous dispersion of the atomic scattering factor of Co. However, none of the anatase and rutile samples did exhibit an anomaly, unambiguously showing that the Co ions in Ti$_{1-x}$Co$_{x}$O$_{2-delta}$ are not exactly located at the Ti sites of TiO$_2$. The absence of the anomaly is probably caused by a significant deformation of the local structure around Co due to the oxygen vacancy. We have applied the same method to paramagnetic Zn$_{1-x}$Co$_{x}$O thin films and obtained direct evidence that the Co ions are indeed substituted on the Zn sites.