Spintronic materials for hydrogen sensing

[Pd/Fe]2 multilayers were deposited on a flat MgO(001) substrate to study the effect of hydrogen on magnetic interlayer coupling. In an Fe/Pd/Fe interlayer coupled system, complex magnetic hysteresis behavior, including single, double, and triple loops, were measured as a function of the azimuthal angle ( ) in a longitudinal and transverse direction. It was hypothesized that with a combination of a 2fold magnetic anisotropy energy in the bottom-Fe and a 4-fold MAE in the top-Fe, the complex magneto-optical Kerr effect hysteresis behavior could be clearly explained. Two well-split hysteresis loops with Kerr remanence of almost zero were measured by choosing a suitable Pd thickness and applying the magnetic field perpendicular to the easy axis of the bottom-Fe. The split double loops originated from the 90°-rotation of the top-Fe moment. On exposure to a hydrogen gas atmosphere, the separation of the two minor loops increased, indicating that Pd-hydride formation enhanced the ferromagnetic coupling between the two Fe layers. On the basis of these observations, we proposed that, by applying a suitable constant magnetic field, the top-Fe moment could undergo reversible 90°rotation following hydrogen exposure. The results suggest that the Pd space layer used for mediating the magnetic interlayer coupling is sensitive to hydrogen, and therefore, the [Fe/Pd]n multilayer system can function as a giant magnetoresistance-type sensor suitable for hydrogen gas.