Photoinduced optical second harmonic generation in Fe-Co metallic spin glasses

For the first time, second-order nonlinear optical effects have been observed in metallic spin glasses. Photoinduced second harmonic generation (PISHG) in reflected light was used as a particular nonlinear optical method. We have varied degree of long-range spin ordering. PISHG is measured immediately after the thermo-treatment using thermoannealing treatment (up to 823 K) of Fe-Co glasses. As a photoinducing beam, Q-switched nitrogen laser (lambda = 337 nm) was used. YAG-Nd pulse laser (W=30 MW, tau = 30 ps, frequency repetition about 12 Hz) was applied as a probe. Only the presence of the photoexcitation stimulates an appearance of the PISHG with values up to 0.128 pm/V. Influence of surface, phase synchronism conditions, fluorescence, etc. is discussed. We have revealed almost a linear correlation between degree of spin ordering and the output PISHG signal. Molecular dynamics geometry optimization and self-consistent quantum chemical simulations have been done. The observed phenomenon can be explained by contributions of long-range ordered spins to electronic dipole moments determining second-order nonlinear optical susceptibilities. Specialty performed X-ray diffractometry structural and magnetically induced nonlinear optical effects show that the observed effects are caused by contribution of collective spin interactions. (C) 2002 Elsevier Science B.V, All rights reserved.