Magnonic Band Research Articles

A localized magnon mode in the extended Wolff-Clogston model

The Wolff-Clogston model for a dilute alloy with interactions between electrons in the host metal is considered. A localized magnon mode is suggested to appear above the magnon band for a fcc structure, provided the impurity is “less magnetic” (δ U<0) than a strongly ferromagnetic host.

Inelastic neutron scattering study of the quasi-1d heisenberg antiferromagnet CsMnBr 3

Inelastic neutron scattering on the quasi 1d-antiferromagnet CsMnBr 3 is reported. The magnon band parallel to the hexagonal crystal axis reaches from 0 to 71.4 cm -1. Magnons in this direction are observable up to temperatures far above the 3d-ordering temperature ( T N = 8.3 K). Perpendicular to the hexagonal axis an acoustic and an optical magnon branch can be observed. The first one reaches from 0 to 6.7 cm -1, the second one with a frequency of 6.7 cm -1 shows no measurable dispersion. From a fit of the measured to the calculated dispersion curve the exchange parameters are obtained.

Application of Submillimeter Spectroscopy to Magnetic Excitations

Magnetic excitations are the collective excitations of the magnetic moments in ferro- and antiferromagnets. The frequencies are mostly in the far-infrared spectral range. Their study is of current interest with respect to the properties of phase transitions, since in ordinary 3-dimensional crystals the dominating exchange interactions can be 3-dimensional (MnF/sub 2/, NiO) or may be restricted to 2 dimensions (K/sub 2/MnF/sub 4/CoCl/sub 2/ or even to 1 dimension (CsNiF/sub 3/, CoCI/sub 2/ 2H/sub 2/O). In this paper, an introduction and a review is given of the results on q = 0 magnon modes (ferro- or antiferromagnetic resonance) which can be studied rather directly by submillimeter-wave spectroscopy. Some results about 2 magnon bands are also mentioned. Experimentally, grating monochromators, Fourier-transform interferometers, FIR laser, and microwave techniques have been employed. In the past, not only pure materials have been studied but also doped crystals where localized magnon modes can occur (MnF/sub 2/ :Co/sup 2+/, CoF/sub 2/ :MnF/sub 2/, NiO :Co/sup 2+/).

Effect of Biquadratic Exchange on Two Magnon States of Heisenberg Ferromagnets: Other Neighbor Exchange

We have investigated the two magnon localized states of a one dimensional Heisenberg ferromagnet the Hamiltonian of which is made up of nearest neighbor and next nearest neighbor isotropic bilinear and biquadratic exchange terms, and a single ion anisotropy term. We have restricted our choice of parameters so that the ground state at T = 0 is the fully aligned ferromagnetic state and we have used the thermodynamic Green functions where the averages have been evaluated in the ground state so that our results are good for [Formula: see text]. We have evaluated the probabilities of finding two spin deviations a distance n apart when the system is in a localized state described by total wave vector q. We have (a) compared the effects of ferromagnetic and antiferromagnetic next nearest neighbor exchange, (b) found that localized modes can lie below or above the two free magnon band depending upon the sign and magnitude of the biquadratic exchange, (c) found that in certain cases two spin deviations appear to behave like objects interacting only via a soft core, and (d) found that modes can have a large single ion component when the single ion anisotropy is zero.

Two magnon light scattering in K 2MnF 4

The light scattering spectra of the two dimensional Heisenberg antiferromagnet K 2MnF 4 are reported. At low temperatures a two magnon band is observed and four bands due to phonons as well.

Influence of a narrow conduction band on a Heisenberg spin system

We consider an idealized model, represented by a Heisenberg spin system, which is influenced by a narrow conduction band via ans-d-exchange interaction. By calculating and decoupling the equation of motion of double-time Green's functions by RPA, we derive the magnon dispersion law. The result is a separation of the spectrum into two magnon bands of different shape, similar to the band structure found by Hubbard in his system. We discuss our result by variation of the system parameters, as there are interactions, polarization, temperature, and external field.

Calculations on magnon impurity modes of a pair defect in a face-centred cubic ferromagnet

The energy levels of the magnetic impurity modes are determined for a nearest-neighbour pair defect in a face-centred cubic ferromagnet. The coupling between the host atoms and the impurity atoms is assumed to be ferromagnetic. The calculations are performed by means of a combination of Green functions and group-theoretical methods. According to their transformation properties, eight different types of impurity mode may be distinguished. Only the energy of the modes with B2u symmetry depends on the exchange integral between the two impurity atoms. The numerical computations are restricted to the resonant modes. Particular attention is given to the resonant Ag and B2u modes, which are the only modes with energies in the lower part of the magnon band.