非線形磁気光学効果による人工格子界面の評価

Nonlinear magnetooptical effect is a phenonenon in which second harmonic light shows magnetization-dependence of intensity or polarization. In most of bulk magnetic materials second harmonic generation (SHG) is not allowed due to centrosymmetric nature. On the other hand at the surface or the interface of thin films and superlattices SHG becomes allowed since inversion symmetry is broken there.

In this study we have made survey studies on nonlinear magnetooptical effect, by which we have gained knowledge on macroscopic and microscopic origins of magnetic SHG.Based on the survey works we planned experiments.

For specimens for nonlenear magnetooptical studies we have prepared Fe/Au superlattices and ferromagnet/semiconductor heterostructures. As the fundamental beam for nonlinear magnetooptical effect we adopted a pulsed Ti-sapphire laser and the SH beam was detected by a photon-counting device.

Nonlinear magnetooptical effect was found to be very sensitive to surface conditions of samples. We could not obtain reliable signal from Fe (IML) /Au (IML) superlattice probably due to surface deterioration.

We have succeeded in obtaining strong and reliable SH signals from Fe (15ML) /Au (15ML) as well as Fe (6ML) /Au (6ML) superlattice . Well-defined four-fold symmetry patterns were found in azymuthal anle-dependence of SH signal for Pin-Pout, Pin-Sout, Sin-Pout and Sin-Sout configuration. The azymuthal pattern showed rotaion (or mirror reflection) with the magnetizationrevesal.

This result suggests that the superlattice has a good epitaxial relationship with the (100) MgO subsrate. Since the obsered 4-fold symmetry reflects the symmetry of the substrate. However, if mirror symmetry remains in the plane of the surface we can never expec 4-fold symmetry as far as third rank succeptibility tensor due to the electric dipoleis considered . Succeptibility tensor of the fourth rank is necessary to explane the azymuthal dependence observed. For crystallographic contribution the electric quadruploe should be introduced to get rank four tensor, whereas for magnetic contribution magnetization-expansion of electric dipole term is necessary.

All of these contribution was found to provide an azymuthal dependence of | A+Bcos4f+Csin4f | ^2.

The longitudinal nonlinear magnetooptical Kerr rotation attained as large as 3 degrees.

It is thus elucidated that nonlinear magnetooptical effect presents plenty of information for studies of surafces and interfaces of magnetic superlattices and heterostructured materials.