Antiferromagnetic optical magnons are observed by means of their far-infrared reflectivity for the first time. The structure of these "magnetic reststrshlenbands" is calculated approximately for normal incidence of the infrared radiation. Spin wave damping can be taken into account by introducing complex eigenfrequencies. The tensor elements of the frequency dependent complex magnetic permeability fulfill a generalized Kurosawa relation.Es werden erstmals antifenomagnetische optische Magnonen aufgrund ihres Reflexionsvermogens im fernen Infraroten beobachtet. Die Struktur dieser ,,magnetischen Reststrahlenbanden" wird fiir senkrechten Lichteinfall niiherungsweise berechnet. Die Diimpfung der Spinwellen liiBt sich dabei iiber den Ansatz komplexer Eigenfrequenzen berlicksichtigen. Die Tensorelemente der frequenzabhangigen, komplexen magnetischen Permeabilitiit geniigen einer verallgemeinerten Kurosaws-Relation.
Far-infrared reflectivity measurements on uniaxial antiferromagnetic CoF, display a characteristic dispersion of the magnetic reststrahlenbands if the wave vector components of the radiation are varied by choosing different angles of incidence. Thus the expected dispersion behaviour of the socalled magnetic polaritons is demonstrated experimentally in a direct manner for the first time. The sirnulabion of t.he infrared reflectivity spectra based on a generalized formula for anisotropic magnetically ordered insulators shows good agreement with the experimental results and can be used to predict even weak polariton dispersion effects.Ref lexionsmessungen am einachsigen antiferromagnetischen CoF, im fernen Infraroten zeigen eine cliarakteristische Dispersion der magnetischen Reststrahlenbanden, sobald durch Veranderung des Einfallswinkels mit unterschiedlichen Wellenvektorkomponenten eingestrahlt wird. Auf diese Weise ist es erstmals gelungen, das Dispersionsverhalten der sogenannten magnetischen Polaritonen direkt nachzuweisen. Die Simulation der Infrarotreflexionsspektren bitsiert auf einer verallgemeinerten Reflexionsfomel fur anisotrope magnetisch geordnete Isolatoren nnd zeigt eine gute ubereinstimmung mit den experimentellen Ergebnissen, sodaB auch sehr kleine Dispersionseffekte der Polaritonen vorhergesagt werden konnen.
The reststrahlen spectrum of yttrium iron garnet is investigated by infrared reflection measurements using an ATR‐hemicylinder or an externally applied magnetic field, respectively. Data on infrared inactive phonons and the magnetic influence on the reststrahlen bands are reported.
By combining the dispersion relations of the refractive index and the boundary conditions in the surfact: the formulae of reflectivity of the antiferromagnetic crystals FeF,, CoF,, and MnF, can be derived for oblique incidence. This is performed for two important orientations of the surface, namely for the surface perpendicular to t'he optical axis and for a surface which contains the optical axis. The results are six different formulae of reflectivity for oblique incidence.Mit Hilfe der Dispersionsgleichungen fur den Brechungsindex und der jeweiligen Randbedingungen in der Oberflache lassen sich die Formeln fur das Reflexionsvermogen bei nicht senkrechtem Einfall fur die antiferromagnetischen Kristalle FeF,, CoF, und MnF, herleiten. Dies wird fiir zwei wichtige Orientierungen der Oberflachen durchgefuhrt, namlich fur eine Oberflache senkrecht zur optischen Achse und fur diejenigen Oberflachen, die die optische Achse enthalten. Dies fuhrt auf sechs verschiedene Formeln fiir das Reflexionsvermogen.
With regard to the optical spectra the magnetic properties of the iron-group difluorides MnF2, CoFZ, and FeF2 are treated'theoretically in /1, 2/. Concerning CoF2 experimental studies include Raman scattering /3, 4/, neutron scattering /5/, and infrared transmission /6 to 8/. As a result of these experiments the frequencies of the magnetic excitations in CoFZ below 300 cm" are sufficiently known. However, optical magnons in the far-infrared region have not been observed by means of their reflectivity until now. Moreover no infrared spectra of magnetic excitations have been published for those frequency ranges, in which the dielectric function E( u) becomes negative. In this case the observation of magnetic resonances is prevented by the much stronger absorption of phonons.That is why the magnetic dipole infrared active exciton (ri) of CoF2 at 204 cm" /9/ could not be measured by transmission spectroscopy so far. In this short note its detection by infrared reflection is reported. Furthermore farinfrared spectra of magnetic excitations which may The experiments were performed using oriented *' t be obtained by modern equipment are presented.
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