Abstract
[Pd/Fe]2 multilayers were deposited on a flat MgO(001) to study the effect of hydrogen on magnetic interlayer coupling. 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. With a combination of a 2-fold magnetic anisotropy energy (MAE) in the bottom-Fe and a 4-fold MAE in the top-Fe, the complex magnetic hysteresis behavior could be clearly explained. Two well-split hysteresis loops with almost zero Kerr remanence 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. Based on 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 multilayer system can function as a giant magnetoresistance-type sensor suitable for hydrogen gas.
Highlights
Pd has long been used as a high-efficient catalyst for the dissociation of hydrogen molecules into individual atoms[1,2,3,4]
Childress studied the magnetic properties of epitaxial Fe(001)/Pd superlattices on MgO(001) and concluded that no evidence for antiferromagnetic coupling between Fe layers through Pd interlayers could be found in Pd with a thickness of 10–50 Å32
When the sample was exposed to H2 pressure, hydrogen molecules dissociated on the top Pd layer and diffused into the underlayers
Summary
The uniaxial MAE of the bottom-Fe on MgO(001) was attributed to the miscut of the substrate. The complex magnetic hysteresis loops (Fig. 7) originated from the coexistence of a uniaxial MAE and a cubic MAE. If a constant biasing magnetic field Ha ≈ 6 Oe is applied to the Fe/Pd/Fe sample, the hydrogen that is charged or discharged will shift the minor loop leftward or rightward, causing a reversible 90° rotation of the top-Fe layer. This effect can be readily applied for magnetoresistance, such as that in a GMR device, for hydrogen sensing
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