A cobalt film two atoms thick has a magnetization direction perpendicular to the film plane when grown on ruthenium. Spin-polarized low-energy electron microscopy allows to observe its local magnetization, and follow in real time and real space changes in the magnetic domains of the film. When exposed to minute amounts of hydrogen, the out-of-plane magnetic domains in the film first break into smaller domains and eventually the magnetization direction switches on an in-plane orientation. The effect is understood with theoretical calculations that show that the origin is the change in the electronic structure of the topmost cobalt atoms bonded to hydrogen. This effect might be used to make gas sensors based on magnetic detection. The hydrogen pressure required for the effect is just one billionth of the atmospheric pressure, for a few minutes. Given the prevalence of hydrogen in ultra-high-vacuum experimental instruments, this effect also points to the risk hydrogen effects can pose for magnetization studies.
Reference: B. Santos, S. Gallego, A. Mascaraque, K.F. McCarty, A. Quesada, A.T. N’Diaye, A.K. Schmid, and J. de la Figuera. "Hydrogen-induced reversible spin-reorientation transition and magnetic stripe domain phase in bilayer Co on Ru(0001)",  Phys. Rev. B 85 (2012) 134409, DOI: 10.1103/PhysRevB.85.134409 (arxiv 1203.3945)