Hyperfine field distribution in amorphous speromagnets (abstract)

Abstract

The 57Fe Mössbauer Zeeman spectra for amorphous ferric speromagnets a-Y3Fe5O12 (YIG) and a-FeF3 consist of well-resolved broad experimental lines, which indicates the existence of a distribution of electric field gradients, hyperfine fields, and isomer shifts at the iron sites. A quantitative analysis of mean line positions, root-mean-square widths, and line shapes of the Mössbauer lines enables us to obtain information concerning the statistical distribution of iron environments in the glassy matrix. The hyperfine field distribution in amorphous speromagnets a-YIG and a-FeF3 is shown to result dominantly from local spin disorder via the supertransferred components HST. Since this same spin disorder determines local exchange fields Hex, the hyperfine field distribution indirectly contains much information concerning the distribution of Hex in the ‘‘spin glass’’ environment. We find, in particular, the mean exchange field 〈Hex〉 is only some 6% of its equivalent in crystalline YIG and FeF3. The hyperfine-field distribution p(H) possesses a small-amplitude low-field ‘‘tail’’ which at 4.2 K contains ∼2% and 5% of the total sites a-YIG and a-FeF3, respectively. A theoretical explanation of the origin and shape of the tail is given, and it is expected to disappear completely in the limit T→0. The limiting shape of p(H) as T→0 is quantitatively established and is significantly asymmetric about its peak.