An oxide semiconductor doped with a transition metal and exhibiting room-temperature ferromagnetism is disclosed. The transition metal-doped oxide semiconductor is preferably manufactured in powder form, and the transition metal is preferably evenly distributed throughout the oxide semiconductor. The preferred embodiments are iron-doped tin dioxide and cobalt-doped tin dioxide. Gases may be detected by passing them across a material and measuring the change in magnetic properties of the material; the preferred material is iron-doped tin dioxide.