Magnetic Polaritons, Impurity Modes and Their Interactions.

A high resolution far-infrared laser spectrometer was used to experimentally study magnetic polaritons, impurity modes, and their interactions in antiferromagnetic FeF(,2). The magnetic polariton model not only reproduces complex structures observed in transmission and absorption, but is essential in determining the intrinsic antiferromagnetic resonance (AFMR) linewidth ((DELTA)H(,H)). For the purest FeF(,2) crystal studied (0.0014 (+OR-) 0.0007 at. % Mn) (DELTA)H(,H) is found to be 20 (+OR-) 10 Oe, most of which is attributable to radiative broadening. Fits of polariton model predictions to higher Mn concentration spectra require the inclusion of a frequency-dependent AFMR linewidth as is predicted by two-magnon scattering theory. The field dependence of the magnetic polariton dispersion in FeF(,2) is determined for the region near its AFMR by an analysis of the interference structure observed in the transmission spectra of thick plane-parallel slabs. When Mn impurities are introduced substitutionally for Fe in FeF(,2), a relatively broad and intense, single impurity s(,o) mode appears, even at concentrations as low as 10('-3) at. %. Frequency-pulling on both host and impurity resonances was measured as a function of concentration in FeF(,2):Mn and successfully compared with the predictions of an effective-field, four-coupled-equations-of-motion model for the impurity and host magnetizations. This model also correctly yields the magnitude of the observed impurity mode enhancement. The linewidth of the s(,o) mode has been measured as a function of both concentration and thickness; the thickness-dependent part is attributed to radiative broadening, whereas the zero-thickness, concentration-dependent part is described by impurity banding effects. Mn impurity pairs are observed and identified in FeF(,2) in the concentration range 0.05 < c < 0.8 at. %. To interpret the observed resonances, the spin dynamics of magnon excitations, localized in the vicinity of pairs of substitutional impurity spins in a two-sublattice antiferromagnet, was investigated theoretically. Green function techniques were applied to a spin Hamiltonian which included both inter- (J(,2)") and intra- (J(,1)") sublattice impurity -host Heisenberg exchange terms (as well as an anisotropy term of the form D(S(,z)('2))). . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.