Importance of Intrasublattice Magnetic Interactions and of Substitutional Ion Type in the Behavior of Substituted Yttrium Iron Garnets

The results of measurements at moderate to high magnetic fields on a large number of nonmagnetic ion substituted yttrium iron garnets suggest that intrasublattice interactions play an important role in determining their spontaneous magnetizations and Curie temperatures. It is shown that the system {Y3−xCax}[Fe2](Fe3−xSix)O12is continuously related to the system {Y3−xCax}[ZrxFe2−x](Fe3)O12or {Y3}[ScxFe2−x](Fe3)O12. It is concluded that in these systems the tetrahedral-tetrahedral (d-d) antiferromagnetic interactions are stronger than octahedral-octahedral (a-a) antiferromagnetic interactions. The changes in magnetic structure from an ideal ferrimagnet, yttrium iron garnet, to an end-member in which there are at least short-range antiferromagnetic interactions (i.e., in {Ca3}[Fe2](Si3)O12or a hypothetical {YCa2}[Zr2](Fe3)O12) should bear an analogy to the crystal chemical changes. It is therefore proposed that when substitution is made exclusively in one sublattice, the moments of the Fe3+ions in that sublattice remain parallel (as in the Yafet-Kittel theory), while the weakened average a-d interactions and the intrasublattice interactions lead to random canting of the Fe3+ion moments of the other. This tendency occurs as soon as substitution begins. On continued substitution, a point is reached beyond which canting increases much more rapidly with increasing substitution. In this region, the intrasublattice interactions dominate the a-d interactions, but it is probable that the canting continues to be random.