Indirect impurity and host mode excitation via far-infrared level-crossing spectroscopy in antiferromagnets

Abstract

Through the field dependence of its linewidth, the so local mode at 83 cm ~' associated with a V 2+ impurity in FeF2 is used as a novel probe of impurity shell modes, host magnon density of states, and the phonon mixing of the magnon branches at the magnon-acoustic-phonon level crossings. This is the first observation of individual shell modes. 5 z -nonconserving and direct exchange interactions contribute to the so-local-mode relaxation. The method has general applicability to antiferromagnets.PACS numbers: 75.50. Ee, 75.30.Ds, 75.30.Hx, 75.80.+q We have observed by far-infrared (FIR) absorption the fully point-symmetric (so) localized magnon associated with a V 2 " 1 " impurity in antiferromagnetic (AF) FeF2 which lies just above the top of the AF magnon band. While of some importance in itself because its linewidth is narrower than any other magnetic impurity mode in the FIR, what is of most interest is that, through the field dependence of its linewidth, it has been used as an indirect probe of the shell modes associated with the impurity and of the k^O collective excitations of the host, neither of which is directly observable in FIR spectroscopy. This is the first observation of the individual non-s-symmetric shell modes (although their energies have been inferred 1,2 from pair excitations). The method provides an accurate means of locating the Van Hove singularities in the magnon density of states and the crossing points in the magnon and phonon dispersions. We call this novel technique "indirect FIR levelcrossing spectroscopy." It should have general applicability to other AF systems as a probe of impurity and host excitations which are not FIR active, and as a means of generating single magnons (and possibly phonons) for lifetime studies using a pulsed, high-power, tunable FIR source [such as the University of California, Santa Barbara (UCSB), free-electron laser].FeF2 is an ideal, two-sublattice uniaxial AF. Well below the Neel temperature (78 K), the Fe 2+ spins align parallel and antiparallel to the easy (c) axis. The band of collective excitations (magnons) is confined to the region between E mm (H =0) =52.4 and £' max (//=0) =78.1 cm -1 . An applied field H along the c axis (Hllc) lifts the degeneracy of the two magnon branches, with one (p) increasing in energy, and the other (a) decreasing in energy with increasing //, by an amount AEk = ±gFe When an impurity magnetic ion is randomly substituted for Fe 2+ , both the spin S' and exchange interactions J,' of the impurity with the host will, in general, be different from those of the host. If the parameters are such that the resonant energies of the impurity and its neighbors are above E mdx (H) then modes will occur which are spatially localized in the neighborhood of the impurity. The mode with finite amplitude on the impurity itself is fully symmetric under the impurity point group, and is referred to as the ^o mode. The excitations which are localized primarily on the next-nearest neighbors (nnn) to an impurity transform under various irredu...

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“…due to the presence of boundary surfaces, interfaces, impurities and other defects in the crystal. These localized modes appearing in the above mentioned structures are experimentally observed using the neutron and Raman scattering [16,17], or, more recently, far infrared absorption [18]. The Green's function theories have been extensively employed to describe these modes in semi-infinite transverse Ising models [19] and FE thin films [20].…”

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