Temperature dependence of the exchange constant in magnetic−bubble materials

A recent theory has related, through their different Curie temperatures, the exchange constant of an epitaxial garnet film to that of pure YIG at the same temperature. We have experimentally determined the exchange constants A of a SmGa:YIG and of a EuGa:YIG epitaxial garnet film from anisotropy and domain wall energy measurements in the range −30 to 120°C. Despite comparable Curie temperatures, the variation of A with temperature in the two compositions showed a markedly different behavior, which would not be expected from the theoretical model. There is reasonable agreement with the behavior of EuGa:YIG, but not for that of SmGa:YIG or of three other compositions whose exchange constants have been obtained from data in the literature.