A. S. Borovik-Romanov

Brillouin-Mandelstam scattering from thermal and excited magnons

Abstract Brillouin-Mandelstam scattering (BMS) is the scattering of light from acoustical quasiparticles (phonons, magnons and others). The frequency shift under BMS is 10–100 GHz. The observation of BMS from magnons became possible only after J. Sandercock had designed a multi-pass Fabry-Perot interferometer with a high contrast (1971). BMS from magnons has, by now, been observed in CrBr 3 (Sandercock), YIG, FeBO 3 (Jantz, Sandercock, Wettling), CoCO 3 (Borovik-Romanov, Jotikov, Kreines), EuO, EuS (Grunberg, Metawe), Ni, Fe (Sandercock, Wettling), metglasses (Chang, Malozemoff, Grimsditch, Senn, Winterling). In this review the main results of the above works are presented. The dispersion laws of magnons were studied by BMS in the energy range (inaccessible for neutron diffraction) where the contributions due to three types of interaction: magnetic, dipole-dipole, and exchange can be separated. Investigation of BMS in EuO and metals has led to the discovery of surface magnons. BMS from standing spin waves has been observed in thin films of metglasses. By observing BMS, it is possible to study quasiparticles pumped by microwave power. It was found that under ferro- or antiferromagnetic resonance an excess of quasiparticles arises, these quasiparticles being magnons with the frequency equal to that of microwave power and phonons with half the microwave frequency. Scattering of light from parametrically excited magnons has also been observed. This opens new possibilities for studying relaxation processes in magnetic materials.

NONCOLLINEAR MAGNETIC STRUCTURES IN ANTIFERROMAGNETIC La2CuO4

Weak-noncollinear antiferromagnetic structure of La2CuO4 (space symmetry D2h18) is determined on the basis of symmetry analysis. The transition of the first order in the magnetic field into weak ferromagnetic phase is possible, the corresponding reconstruction of antiferromagnetic structure being predicted.