Visualizing hydrogen diffusion in magnetic film through magneto-optical Kerr effect

The kinematics of hydrogen diffusion in nontransparent metallic materials is crucial to the hydrogen-sensing and -storage technology and remains a challenge. Alongside the conventional optical investigations, the hydrogen absorption-induced reversible changes of magnetic properties in ferromagnetic thin films provides a new method for visualization of hydrogen in solids. Here we monitor real-time hydrogen diffusion in a cobalt-palladium alloy (Co₂₅Pd₇₅) film using a magneto-optical Kerr microscope. The spatially resolved magneto-optical contrasted images provide a noninvasive method of monitoring hydrogen movement. Hydrogen diffusion follows Fick’s diffusion law, and a diffusion coefficient of 3 ± 2 × 10⁻¹² m²/s is obtained. The diffusion velocity of the 2–4% hydrogen concentration fronts reaches 30 ± 15 nm/s in the uniform film area and increases to 50 ± 20 nm/s near a defect site. These results can be applied in detecting hydrogen diffusion in other spintronic materials, such as magnetic palladium-alloy thin films.