Influence of hydrogen on the magnetic properties of iron-rich metallic glasses

Melt spun and sputtered amorphous alloys from the a FexM100−x series with M=Y or Zr can be electrolytically hydrogenated up to a maximum of about 3 hydrogens per M, while retaining their mechanical integrity. Hydrogen uptake has been monitored in situ during hydrogenation by following the increase in length of the melt-spun ribbons. Values of up to 7% have been found, and the corresponding increase in volume per absorbed hydrogen is approximately 0.5 Å3. Hydrogen diffusion constants have been estimated from the mechanical relaxation, and activation energies of order 0.4 eV have been deduced. The binding energy is about 1 eV per hydrogen. Desorption at elevated temperatures preceeds crystallization of the alloys studied, and the hydrogen-loaded materials retain much of their charge at room temperature for periods of order one year. Iron-rich yttrium alloys have a moment close to 2 μB/Fe, but they are asperomagnets, the iron moments freezing in a random noncollinear arrangement which possesses a net moment, below their spin freezing temperature of about 100 K. On hydrogenation they become excellent soft ferromagnets with a Curie point at 400–500 K, although there is little change in the magnitude of the iron moment. The effect is attributed to a shift in the exchange distribution towards more positive values on dilation of the interatomic spacings. Iron-rich zirconium alloys are quite different. They are essentially weak itinerant ferromagnets with a greatly reduced iron moment, and pure amorphous iron, judging from a FexM100−x as x→100, would be nonmagnetic. On hydrogenation however they tend towards the same ferromagnetic state as the hydrogenated yttrium alloys.