New annealing method and CaGe garnet film coercivity origin

A new annealing method which drastically reduces the coercivity of GaGe-garnet films grown by LPE has been developed. The as-grown films of (YSmLuCa)₃(FeGe)₅O₁₂ and (YEuTmCa)₃(FeGe)₅ O₁₂ are sealed with Y₂O₃ and metal Ca in an evacuated tube having three chambers. When heated, the tube is filled with Ca and O₂ gases which are supplied from the metal Ca and Y₂O₃, respectively. Ca and oxygen are believed to be incorporated into the film occupying the dodecahedral cation vacancies and oxygen vacancies, respectively. By this annealing, coercivities higher than 5.0 Oe are drastically reduced to less than 0.5 Oe. At the same time, the 4πMs and lattice constant af are decreased, while the Curie temperature T_c is increased. A point defect structure model containing c-site vacancies, oxygen vacancies and a-site Ge²⁺ ions is proposed. The origin of the coercivity is believed to be the oxygen vacancies.