Optical study of hydrogen diffusion in yttrium iron garnet

Hydrogen diffusion in magnetic garnets is of interest in connection with both H+ ion implantation in bubble films and the proposed application to high-temperature chemical sensors. We have investigated hydrogen diffusion in several pure and doped YIG samples, using optical absorption and magnetic circular dichroism as probes. Differently from the previously reported cases of F− ions and oxygen vacancies, H+ ions are found to diffuse in YIG not obeying the classical diffusion equation. At constant temperature, simple exponential time behaviors are observed, and the time constants are much shorter for inward than for outward diffusion. These facts indicate that a major role is played by surface effects. H+ diffusion in YIG is accompanied by neither additional absorption nor dichroism at wavelengths λ>1.1 μm, so that charge compensation must involve centers of very different character, and more strongly bound, than Fe2+ induced by tetravalent ion dopings. In Ca2+- and Pb2+-doped YIG, besides the absorption peak due to the OH− stretching, a broad peak at λ=3.6 μm is found, similar to that previously reported for YIG(Na): the interaction of the OH− group with dodecahedral low valence ions is thus apparent. For T≤100 K, additional absorption structures arise at λ=3.15 μm indicating the formation of new equilibrium states for the H+ ion.