Mode Assignment for Magnetic Excitations Associated with Co2+ Impurities in Antiferromagnetic FeF2
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We have performed far-infrared (FIR) absorption and Raman-scattering measurements on the Co-FeF2 system for Co concentrations 0.05 ≤ = x ≤ = 5%, in magnetic fields O ≤ H ≤ 30 kG applied parallel to the crystal c axis. In addition to a careful study of the H dependence of the previously reported impurity associated mode at 85 cm-1, we have observed an impurity-induced shift in the host two-magnon FIR absorption, and an impurity-pair excitation in Raman scattering at a frequency of 223 cm-1. Taken together, these results are shown to be inconsistent with the previous interpretation of the 85 cm-1 mode as being localized on the impurity spin (i.e., an So mode). Instead, from a mean-field cluster model, we show that the new observations are only consistent with an assignment of the original resonance to be that of a shell mode (s1). All of the data agrees with this assignment within the framework of the impurity Green's-function theory as well. The 85 em-1 resonance is, to our knowledge, the first s1 impurity mode to have been observed, a not surprising result when one considers the rather stringent conditions on localization of an SI mode contained in Tonegawa's Green's-function theory.