Substitutions of divalent manganese, iron and nickel in yttrium iron garnet

Abstract

Various amounts of the divalent ions of manganese, iron and nickel have been substituted for trivalent iron in yttrium iron garnet. The effect of substitution of Fe 2+ or Mn 2+ ions in dodecahedral sites has also been studied. Electrical balance was accomplished by the simultaneous substitution of tetravalent silicon or germanium. It appears that under the conditions of the experiments, the divalent ions investigated prefer octahedral sites. The great affinity of silicon for the tetrahedral sites in the garnets results in reduction of trivalent to divalent iron at high temperatures even in air atmosphere when appropriate amounts of finely divided silica are present. In Y 3Fe x 2+Fe 5−2 x Si x O 12, the maximum attainable value of x appears to be 0.45. In the case of Ni 2+, an analogous value of 0.50 was attained for x when Si 4+ was used for electrical balance. Single garnet phases were not obtained when small octahedral site substitutions of Ni 2+ or Mn 2+ or large octahedral site substitutions of Mn 2+ with Ge 4+ ions for electrical balance were attempted. However, it was possible to make Y 3Ni x Fes 5−2 x Ge x O 12 with x = 1.50 and 2.00. Both are ferrimagnetic with 0°K moments extrapolated to H a = 0 of 1.5 and 0.5-0.6 μ B respectively. The Curie temperature of the garnet with x = 2.0 indicates that the Ni 2+—O 2−—Fe 3+ magnetic interaction is less than one-third as strong as the Fe 3+—O 2−—Fe 3+ magnetic interaction in {YCa 2}[Fe 2](FeSi 2)O 12. The 0°K spontaneous moments of garnets with divalent ions (and 0.5 Cr 3+ ion) substituted in octahedral sites are all smaller than those calculated on the basis of the Néel model. This appears to be caused by a weakening of a- d interactions and canting of the d-site Fe 3+ ion moments resulting from the d- d antiferromagnetic interactions. The divalent magnetic ions do not behave like the trivalent magnetic lanthanide ions when substituted in the dodecahedral sites : they appear not to contribute appreciably to the net spontaneous moment. It is possible that the antiferromagnetic interactions among the dodecahedrally substituted divalent ions are much stronger than the antiferromagnetic interactions between these ions and iron ions on either of the other sites.