Growth‐Induced Anisotropy in Bubble Garnet Films Containing Calcium

The growth‐induced anisotropy, K_u measured on (111) LPE films of ${\mathrm{Eu}}_{\text{3−}\mathrm{x}}{\mathrm{Ca}}_{\mathrm{x}}{\mathrm{Si}}_{\mathrm{x}}$ ${\mathrm{Fe}}_{\text{5−}\mathrm{x}}{\mathrm{O}}_{12}$ grown on ${\mathrm{Sm}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ substrates is large and positive (${\mathrm{K}}_{\mathrm{u}}\text{\hspace{0.17em}=\hspace{0.17em}4.0\hspace{0.17em}}\mathrm{to}{\text{\hspace{0.17em}8.1×10}}^4\hspace{0.17em}{\mathrm{ergs/cm}}^3$ for $\mathrm{x}\text{\hspace{0.17em}=\hspace{0.17em}0.5}$ ). Many experiments have shown that K_u in ${\mathrm{Eu}}_{\text{3−}\mathrm{x}}{\mathrm{Y}}_{\mathrm{x}}{\mathrm{Fe}}_{\text{5−}\mathrm{y}}{\mathrm{Ga}}_{\mathrm{y}}{\mathrm{O}}_{12}$ films is also positive. In films of compositions $(\mathrm{YSmCa}{)}_3$ $(\mathrm{GeFe}{)}_5{\mathrm{O}}_{12}$ , the effects of Ca and of Y on the growth‐induced anisotropy are the same in sign and similar in magnitude. Since Ca²⁺ is larger in ionic radius than Eu³⁺ or Sm³⁺, whereas Y³⁺ is smaller, the simple site selectivity model of growth‐induced anisotropy would predict opposite signs of K_u for the Ca and Y additions. This contradiction shows that ionic radius is not the only relevant parameter which determines growth‐induced anisotropy in garnets, although size differences have been shown to account for the observed effects within the rare earth series. The charge difference between the Ca²⁺ ions and the rare earth ions, or the pairing of Ca²⁺ and Ge⁴⁺ or Si⁴⁺ ions during crystal growth may influence the site preference of the Ca ions and the resulting anisotropy. The growth‐induced anisotropy of the ${\mathrm{Eu}}_{\text{2.5}}{\mathrm{Ca}}_{\text{0.5}}{\mathrm{Si}}_{\text{0.5}}{\mathrm{Fe}}_{\text{4.5}}{\mathrm{O}}_{12}$ film, grown at 835°C at a rate of 2 μm/min, annealed out after several hours at 900°C in oxygen. This temperature is much lower than the usual annealing temperatures of 1200°C to 1300°C in O₂ which are required to reduce K_u in other bubble garnets. This is attributed to a high concentration of defects due to an imbalance in the Ca‐Si concentrations in films grown at low temperatures and large growth rates.