Broad magnetic transitions in cobalt-substituted yttrium iron garnet

Mössbauer spectra of polycrystalline specimens of yttrium iron garnet containing small amounts of divalent cobalt ions, compensated by ${\mathrm{Ge}}^{4+}$ ions (${\mathrm{Y}}_3{\mathrm{Fe}}_{5-2x}{\mathrm{Co}}_x{\mathrm{Ge}}_x{\mathrm{O}}_{12}$) indicate that in zero applied magnetic field, they have very broad magnetic transition regions. However, there is clear evidence that two magnetic phases exist in these regions, one with $〈111〉$, the other with $〈100〉$ easy directions. The broad transition region is attributed to the low ${\mathrm{Co}}^{2+}$ concentration and statistical fluctuation thereof, small particle size, and inhomogeneous internal stresses in the particles.