Discovery of a Giant Kerr Rotation in a Ferromagnetic Transition Metal by M-edge Resonant Magneto-Optic Kerr Effect
I. Matsuda and Shin Groups
Magneto-optical effects are one of the central topics in condensed matter physics. The effects have been used to study magnetism and spin transport in materials. Many experiments with the magneto-optical Kerr effect (MOKE) have been performed using linearly polarized laser light of a single wavelength typically in the visible region. Recently, there has been remarkable progress in new-generation light sources, such as the X-ray free-electron laser (FEL) and the high-harmonic generation (HHG) laser. These new monochromatic lasers have ultra-short pulse widths and tunable photon energies ranging from the extended violet to the X-ray region. Thus, MOKE measurements with photon energy tuned at the absorption edge (so-called resonant MOKE or RMOKE) are of considerable interest. Measurements with the new-generation light sources are expected to be element-specific and to trace spin dynamics in real time via time-resolved measurements.
In the present research, we have studied the Kerr rotation angle and ellipticity in the RMOKE at photon energies corresponding to the M-shell absorption edge of transition metal. We investigated the RMOKE in the well-known nickel film both theoretically and experimentally. The simulation was carried out with resonant scattering theory based on the Kramers-Heisenberg formula. The predicted Kerr rotation angle indicated a large MOKE (> 10˚). Then, the large Kerr rotation angle was confirmed by rotating analyzer ellipsometry using vacuum ultraviolet (VUV) synchrotron radiation at KEK PF BL-18A. Figure 1 compares experimentally measured and calculated variations of θk with photon energy. The experiment reproduced large θk values (~10 degree) at the absorption edge: the overall features are in good agreement with the calculations, where the model parameter set has been independently determined with other core-level spectroscopies.
Since the RMOKE measurement is inherently element specific and is a photon-in and photon-out experiment, it is able to probe spin states of selected elements in complicated magnetic materials under external electromagnetic fields. Moreover, we observed a large Kerr rotation angle (> 10˚) in the RMOKE, which is much larger than angles (<1˚) typically found for the MOKE with visible light. Conventional MOKE measurement has always required lock-in amplifier techniques to obtain a high enough signal-to-noise ratio. However, the large Kerr rotation angle in the RMOKE we observed does not require such techniques. This advantage allows us to detect the magnetism or spin dynamics of a very small amount of the magnetic element in a material, such as a dilute magnetic semiconductor, or to measure very small structure, such as the magnetic nanostructure on a surface. This feature makes it possible to trace these ultrafast spin dynamics in terms of individual chemical species in real time when combined with FEL or HHG lasers.
References
- [1] Sh. Yamamoto, M. Taguchi, M. Fujisawa, R. Hobara, S. Yamamoto, K. Yaji, T. Nakamura, K. Fujikawa, R. Yukawa, T. Togashi, M. Yabashi, M. Tsunoda, S. Shin, and I. Matsuda, Phys. Rev. B 89, 064423 (2014).