Hyperfine and exchange field distributions in amorphous magnetic insulators (abstract)

Abstract

The 57Fe Mössbauer Zeeman spectra of amorphous magnetic ferric insulators, a-Y3Fe5O12 (YIG), a-FeF3, and a-NaFeF4, consist of well-resolved broad experimental lines, which indicate the existence of a distribution of electric field gradients, hyperfine fields, and isomer shifts at the iron sites.1,2 The hyperfine-field distribution in amorphous magnetic insulator is shown to result dominantly from local spin disorder via its supertransferred component HST. Since the same spin disorder determines local exchange fields Hex, the hyperfine-field distribution indirectly contains much information concerning the distribution of exchange, in the spin-glass environment. This work probes the nature of the nearest-neighbor spin correlations in such systems, by analysis of hyperfine field distribution, as measured by 4.2 K Mössbauer spectroscopy. Low-field tails are found in the hyperfine field distribution spectra.3 They indicate the presence of low-energy exchange-field states caused by the cancellation of the nearest-neighbor contributions within the frustrated spin network. The distribution of exchange fields for a-YIG, a-FeF3, and a-NaFeF4 has been determined and, by comparing the results to those expected for a set of randomly oriented spins, the nature of nearest-neighbor spin frustration has been monitored in these amorphous magnetic insulators. The implied spin correlation is one of a quasiconstraint of the nearest neighbors of any given spin which restricts their vector sum to small values.