Hyperfine tails and exchange-field distributions in amorphous magnetic spin glasses

Almost nothing is yet known about the nature of the ‘‘frozen disorder’’ of magnetic moments in concentrated magnetic spin glasses (or ‘‘speromagnets’’) such as (amorphous) a-YIG (yttrium iron garnet), a-${\mathrm{FeF}}_3$, and a-${\mathrm{NaFeF}}_4$, beyond the qualitative description of ‘‘randomlike.’’ This paper probes the nature of the nearest-neighbor spin correlations in such systems by analysis of hyperfine-field distributions as measured by 4.2-K Mössbauer spectroscopy. Low-field tails are found to exist on the hyperfine distribution spectra of amorphous speromagnets but not amorphous ferromagnets. They indicate the presence of low-energy exchange-field states in the former caused by the cancellation of nearest-neighbor contributions within the frustrated spin network. Their interpretation has been used to determine the detailed exchange distribution function for a-YIG, a-${\mathrm{FeF}}_3$, and a-${\mathrm{NaFeF}}_4$ and, by comparison of the results to the expectation for a set of randomly oriented spins, to monitor the nature of nearest-neighbor spin frustration in those amorphous speromagnetic systems.