We report that Co0.1Ti0.9O2 prepared by the sol-gel technique is a paramagnet following the Curie–Weiss law: x= x0+C/ (T+theta). However, hydrogenation at 673 K in H2/Ar 5% /95% gas converts a part of the paramagnetic sample to room temperature ferromagnet RTFM and reheating the sample at 573 K in air converts it back to a paramagnet completely. This reversible 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.
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