Donnelly, David W.2010-06-022012-02-241995-07Donnelly, D. W. (1995). Mode assignment for magnetic excitations associated with Co2+ impurities in antiferromagnetic FeF2. Physical Review B, 52(2), pp. 1042-1049.https://hdl.handle.net/10877/4047We 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 s0 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 cm-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 s1 mode contained in Tonegawa's Green's-function theory.Text8 pages1 file (.pdf)enmode assignmentmagnetic excitationsCo2+ impuritiesantiferromagneticFeF2PhysicsArticle