Evolution of structural and magnetic properties of multifunctional bismuth iron garnets upon Ca and Y doping

A multiscale approach is reported to understand the influence of the Ca and Y co-substitution in Bi-rich iron garnet thin films. Recently, it has been demonstrated that site-selective co-substitution of ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Y}}^{3+}$ in bismuth iron garnet (BIG) leads to either $n$- or $p$-type semiconductor behavior. However, the evolution of the structural and magnetic properties of bismuth iron garnet upon doping is still unknown. In the (Ca,Y):BIG, thin films grown by pulsed laser deposition onto ${\mathrm{Gd}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ substrates, structural investigations confirm the epitaxial growth of doped thin films with high crystallinity and a complete strain relaxation from 20 nm above the film/substrate interface. While x-ray diffraction only evidences a single-phase garnet, the presence of secondary phase nanocrystallites, that are absent in pure BIG grown in similar conditions, is observed by aberration-corrected scanning transmission electron microscopy. These nanocrystallites form in between garnet grains as textured and poorly crystallized hematite. Despite the presence of nanoneedles, Ca and Y co-substitution preserves the giant Faraday rotation of pure BIG and maintains the Curie temperature above 590 K. Only minor energy shifts (80 meV) or small intensity changes (10%) of the Faraday rotation are observed upon doping or annealing and are mainly related to band-gap evolution and cell volume change. Furthermore, the easy magnetization axis rotates towards in the out-of-plane direction upon doping which is also promising for potential applications in spintronics.