Amorphous yttrium-iron alloys: III. The influence of hydrogen

Abstract

For paper I see Coey et al., ibid., vol.11. p.2707, (1981). The effect of hydrogen on the magnetic properties of a series of sputtered Y1-xFex alloys with x=0.32, 0.48, 0.68, 0.82 and 0.88 is studied by magnetisation measurements and Mossbauer spectroscopy. Hydrogen desorption from all samples was followed by thermopiezic analysis. Dilation, X-ray and UV photoemission and low temperature specific heat were measured on iron-rich alloys (x=0.82 or 0.88). All alloys except x=0.48 absorb significant quantities of hydrogen (up to 4.5 moles per mole of yttrium). The added volume is 1.7 cm3 mol-1. Photoemission spectra of hydrogenated iron-rich alloys resemble those of the crystallised material, having a large density of states at the Fermi level EF and a specific heat coefficient gamma of 13 mJ mol-1 K-2; the density of states of the unhydrogenated material is much smaller although gamma is larger. Magnetic order in the x=0.88 sample evolves continuously from asperomagnetic to ferromagnetic with increasing hydrogen content; corresponding changes in magnetic ordering temperature, aligned moment in 5 T and total magnetic moment on the iron are from 109 to 525 K, 1.5 to 2.3 mu B/Fe and 2.1 to 2.4 mu B, respectively. The decrease in frozen spin disorder on hydrogenation is a consequence of a shift in the exchange distribution towards more positive values due to an increase in the average Fe-Fe distance and an increase in the density of states at EF. The large linear specific heat and a linear temperature dependence of the average hyperfine field at low temperatures in the asperomagnetic state are linked to magnetic excitations of the frozen spin system. The critical concentration for the appearance of magnetism on the iron is reduced to x approximately=0.2 on hydrogenation.