Anisotropy of the Europium-Iron Exchange Interaction in Rare-Earth Iron Garnets

Mössbauer studies of the 21.6-keV transition of ${\mathrm{Eu}}^{151}$ and the 14.4-keV transition of ${\mathrm{Fe}}^{57}$ in europium iron garnet, samarium iron garnet, and mixed samarium-europium iron garnets were performed at various temperatures. The recoilless absorption spectra of the 21.6-keV and 14.4-keV $\gamma$ rays in SmIG and in $\left\{{\mathrm{Sm}}_x{\mathrm{Eu}}_{1-x}\right\}\mathrm{IG}$ at 4.2°K differ substantially from the spectra in EuIG and other rare-earth iron garnets. The main conclusions drawn from the experimental results are the following. (1) The Eu-Fe exchange interaction in the garnets is anisotropic. The principal values of the anisotropic exchange field acting on the ${\mathrm{Eu}}^{3+}$ ions (produced mainly by the ${\mathrm{Fe}}^{3+}$ ions in the tetrahedral sites) are $\frac{\beta H_x}{\mathrm{hc}}=12.7\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\frac{\beta H_y}{\mathrm{hc}}=11.9\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $\frac{\beta H_z}{\mathrm{hc}}=20.3\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. (2) The anisotropic properties of the ${\mathrm{Eu}}^{3+}$ ions in the garnets are mainly produced by the anisotropy in the Eu-Fe exchange interaction and not by the crystalline-field interactions. (3) The direction of the tetrahedral iron sublattice magnetization (${\mathrm{n}}_d$) in SmIG at 4.2 and 20°K and in mixed $\left\{{\mathrm{Sm}}_x{\mathrm{Eu}}_{1-x}\right\}\mathrm{IG}\text{'}\mathrm{s}$ with $x\ge 0.05$ at 4.2°K is close to the [110] direction of the cubic unit cell. The experimental results tend to indicate that the direction of ${\mathrm{n}}_d$ in SmIG and in mixed Eu-Sm garnet at 4.2°K makes a small angle (∼5°) with the [110] direction of the unit cell, and that the direction of ${\mathrm{n}}_d$ at 85°K is slightly canted relative to the [111] direction.