V. Magnetic Excitationsraman Scattering Studies of Spin Order and Fluctuations in the Europium Chalcogenides

Magnetic phase dependent, inelastic light scattering from phonons in the europium chalcogenides is reviewed with an emphasis on the behavior of the spectra near magnetic phase transitions. As a result of the one phonon-one spin Raman scattering mechanism which gives rise to these spectra, both broad (Av % 30 cm) and sharp (Av 5 l cm) first-order Raman lines are observed. The calculated temperature dependence of the broadline Raman spectra is related to spin fluctuations and compared with experiment. The sharp lines, observed in the ordered magnetic phases at non g; = 0 phonon energies, are shown to be a direct measure of the square of the order parameter (exponent 3). The observed Raman spectra are correlated with the magnetic properties of EuSe and EuTe near the metamagnetic (EuSe) and spin flop to paramagnetic (EuTe) phase transitions. Both theory and experiment indicate that the temperature and magnetic field dependence of these Raman spectra is a sensitive probe of spin fluctuations and spin order in these materials.