Exchange constant of calcium- and germanium-substituted yttrium-iron-garnet thin films

The room-temperature exchange constant of single-crystal calcium- and germanium-substituted yttrium-iron-garnet thin films has been measured using ferromagnetic resonance techniques. The germanium concentrations varied in the range of 0 to 1.4 moles per formula unit. The experimental results are compared with the predictions of a percolation model by Kirkpatrick and Harris and reasonable agreement is found when it is assumed that the germanium substitution is confined to the tetrahedral iron sublattice. The results are also compared with the predictions of a model by Slonczewski et al. which relates the exchange constant in a substituted garnet to the Curie temperature. In this case the agreement is qualitative at best and raises the question of whether the exchange constant can be related to the Curie temperature without considering the details of the substitution on the individual iron sublattices. Finally the effective g value has been measured for all samples and the results are shown to be in agreement with the Wangsness model for a two-sublattice system. There is some indication of the existence of a high-field susceptibility in the heavily substituted samples.