Cobalt doped titanium dioxide, Co:TiO 2 , is known as a transparent room-temperature ferromagnet. This material has a possibility of being a dilute magnetic semiconductor(DMS), but it has not obtained an absolute proof yet. DMS is an important material in the field of the spintronics. At present two candidates are being suggested as the origins of the ferromagnetism of Co:TiO 2 . One is DMS with sp-d exchange interaction, and the other is cobalt cluster precipitates. The elucidation of the problem has been pursued aggressively to date. Optical second harmonic generation (SHG) can be applied to the investigation of the surface or interface electronic states. It can also be used for investigating the surface magnetization in centrosymmetric media. SHG induced by magnetization is called MSHG. The purpose of this study is to elucidate the origin of the magnetism in Co:TiO 2 using MSHG.
The sample of Co:rutile TiO 2 (110) was made by the combinatorial laser molecular beam epitaxy technique. The film thickness was 100 nm. The light source of the fundamental frequency was an OPG/OPA driven by the mode-locked Nd:YAG laser. The incident and observed angles were both 4 degree with respect to the surface normal. The polarization configurations were S-polarization input and S-polarization output with little intentional angular deviation. The magnetic field was changed within the range of -2.5 to 2.5 kG.
We got the SH intensity from Co:TiO 2 as a function of the strength of the external magnetic field at the SH photon energy of 4.20 eV. The SH intensity took different values between the magnetic fields +2.5 kG and -2.5 kG. Due to the bulk absorption of TiO 2 SH light at this photon energy comes only from the top surface of the film. Since Co cluster cannot exists at the top surface due to the oxidation of Co by air, the observed MSHG comes from the ionic Co at the surface of the Co:TiO 2 film. This fact strongly suggests that ferromagnetism of Co:TiO 2 can be invoked by the surface Co ion.