Optical Studies of Exchange in Substituted Garnets

Abstract

The purpose of this study is to see if spectroscopic techniques can be extended to yield information about the strength of coupling between the rare earths and specific iron neighbors in the iron garnets. Trivalent cerium is used as the probing ion because of the simplicity of its spectrum and because of the unusually large exchange splittings which it exhibits in the garnets. Cerium is incorporated in low concentrations in single crystals of the garnets under investigation. Octahedral or tetrahedral iron ions are replaced selectively by other trivalent ions, the influence of these substitutions on the exchange‐split 2.7‐μ line of cerium being observed. For tetrahedral iron replacements in excess of 20 at.%, discrete new spectra are superimposed on the spectrum of unsubstituted YIG:Ce. At low substitution levels (replacements of less than 1% of the iron), the new spectra consist of a small number of additional weak but sharp lines. Such spectra obtained using low substitutions of aluminum and gallium can be interpreted in terms of reduced exchange splittings produced by the replacement of single tetrahedral iron neighbors only. From these preliminary experiments, the magnitudes of the contributions of the nearest (axial) and next‐nearest (nonaxial) tetrahedral iron neighbors to the exchange field at the rare earth can be separately deduced. Both contributions are significant with the axial contribution dominating. No contribution from the octahedral iron neighbors is observed.