Rare-Earth-Iron Exchange Interactions in Europium Iron Garnet

Recoilless absorption measurements of the 14.4-keV γ rays of Fe⁵⁷ and the 21.6-keV γ rays of Eu¹⁵¹ in europium iron-gallium garnets (Eu₃GaₓFe_5-xO₁₂) were carried out at 4.2^∘{}K for x=0,0.66,1.28,1.60,2.37,and 3.03. The iron concentrations in the octahedral and tetrahedral sites were determined from the Fe⁵⁷ spectra. The results show that about 80% of the Ga ions occupy tetrahedral sites ({~}85% for $x=0.66$ and {~}75% for $x=3.03$). The analysis of the Eu¹⁵¹ spectra shows that (88±{}4)% of the exchange field acting on an Eu³⁺ ion in europium iron garnet is produced by the two nearest iron neighbors in the tetrahedral site, although the Eu-O-Fe angle for these ions is 92^∘{}, an angle often considered unfavorable for superexchange interactions. Excellent agreement between theoretical and experimental spectra is obtained assuming that the remaining 12% of the exchange field is produced by the 4 third nearest neighbors in the tetrahedral site.