Nature of the reversible paramagnetism to ferromagnetism state in cobalt-doped titanium dioxide.

We report that Co_0.1Ti_0.9O_2-δ prepared by the sol-gel technique is a paramagnet following the Curie-Weiss law: x =x0^+C/(T+θ) with θ≃5 K. However, hydrogenation at 673 K in H₂/Ar (5%/95%) gas converts a part of the paramagnetic (P) sample to room temperature ferromagnet (RTFM) and reheating the sample at 573 K in air converts it back to a paramagnet completely. This reversible P⇄RTFM transition has been observed for additional cycles by alternately heating in air and H₂/Ar. It is argued that this RTFM is intrinsic and it is due to Co²⁺-Co²⁺ exchange interaction mediated by oxygen holes which are produced by hydrogenation but eliminated by oxidation. [ABSTRACT FROM AUTHOR]