Magnetic properties of RuSr{sub 2}GdCu{sub 2}O{sub 8} using the temperature dependence of the Mossbauer spectrum.

Moessbauer spectra were measured from 4.2 to 145 K on a {sup 99}Ru enriched sample of RuSr{sub 2}GdCu{sub 2}O{sub 8} which magnetically orders at 138 K and has a full transition to superconductivity at 8.7 K with an onset at {approx}13 K. The superconducting transition has no effect on the spectrum which is determined by the hyperfine magnetic field due to the magnetic order. At low temperatures there is a rapid decrease of this hyperfine magnetic field with increasing temperature indicating a gapless or a very low energy magnon spectrum. We use a local-moment model which includes coupling between nearest-neighbor in-plane Ru moments and between the Ru and Gd moments which are separated by a superconducting CuO layer and a SrO layer to calculate the magnon spectrum and use this to estimate the strength of the exchange interactions based on the hyperfine magnetic-field temperature dependence. The coupling strength is {approx}275 K for Ru-Ru coupling and {approx}30 K for Ru-Gd coupling.