Nanoscale Dichotomy of Ti 3d Carriers Mediating the Ferromagnetism in Co:TiO2 Anatase Thin Films

TL;DR

This study reveals a surface-bulk electronic dichotomy in Co:TiO2 anatase films, showing Ti 3d carriers in the bulk that mediate ferromagnetism through a specific exchange interaction pathway.

Contribution

It provides direct spectroscopic evidence of Ti$^{3+}$ states in the bulk and their role in mediating ferromagnetism, highlighting a surface-bulk electronic dichotomy in Co:TiO2.

Findings

Metallic Ti$^{3+}$ states exist in the bulk but are suppressed at the surface.

Ti$^{3+}$ electrons are present at the Fermi level, indicating their role in conduction.

The charge neutrality condition explains the formation of Ti$^{3+}$ carriers mediating ferromagnetism.

Abstract

We study the surface and bulk electronic structure of the room-temperature ferromagnet Co:TiO2 anatase films using soft and hard x-ray photoemission spectroscopy with probe sensitivities of ~1 nm and ~10 nm, respectively. We obtain direct evidence of metallic Ti$^{3+}$ states in the bulk, which get suppressed to give a surface semiconductor, thus indicating a surface-bulk dichotomy. X-ray absorption and high-sensitivity resonant photoemission spectroscopy reveal Ti$^{3+}$ electrons at the Fermi level (E$_F$) and high-spin Co$^{2+}$ electrons occurring away from E$_F$. The results show the importance of the charge neutrality condition: Co$^{2+}$ + V$_{O}$$^{2-}$ + 2Ti$^{4+}$ $\leftrightarrow$ Co$^{2+}$ + 2Ti$^{3+}$ (V$_O$ is oxygen vacancy), which gives rise to the elusive Ti 3d carriers mediating ferromagnetism via the Co 3d-O 2p-Ti 3d exchange interaction pathway of the occupied orbitals.