Anisotropy of the europium-iron exchange interaction in rare-earth iron garnets

Mössbauer studies of the 21.6-keV transition of Eu151 and the 14.4-keV transition of Fe57 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 γ rays in SmIG and in {SmxEu1-x}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 Eu3+ ions (produced mainly by the Fe3+ ions in the tetrahedral sites) are βHxhc=12.7 cm-1, βHyhc=11.9 cm-1, and βHzhc=20.3 cm-1. (2) The anisotropic properties of the Eu3+ 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 (nd) in SmIG at 4.2 and 20°K and in mixed {SmxEu1-x}IG's with x0.05 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 nd 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 nd at 85°K is slightly canted relative to the [111] direction.