Femtosecond time-resolved magneto-optical Kerr effect spectroscopy using a resistive 25 T magnet

We demonstrate the measurement of the magnetization dynamics at femtosecond time scales using a split helix resistive magnet reaching magnetic field strengths up to 25 T. The high frequency resolution achieved by ultrafast lasers coupled with the high magnetic fields that can achieve magnetization oscillations in the hundreds of gigahertz makes this a very valuable experimental tool. We demonstrate the technique by using time-resolved magneto-optic Kerr effect spectroscopy to measure the spin dynamics of the ferrimagnet (FeCo)1-xGdx at magnetic fields up to 25 T for different magnetic field orientations. Three orientations were used to investigate the magnetization dynamics, perpendicular to the sample plane, parallel to the sample plane, and 20° off the sample plane. The polar time-resolved magneto-optic Kerr effect data show different dynamics depending on the alignment of the external magnetic field with the rare earth or transition metal magnetization. In time-resolved magneto-optic Kerr effect measurements with magnetic fields at 20° off the sample plane, we obtain oscillations due to the precession of the net magnetization. The oscillation frequency increases linearly with the external magnetic field, reaching 500 GHz at 20 T. When the magnetic field is applied parallel to the sample plane, the observed magnetization dynamics undergoes a 180° phase shift when the magnetic field direction is reversed. The different magnetic field orientations with respect to the sample lead to very different magnetization dynamics, which probe different aspects of the magnetic properties of the alloy, emphasizing the importance of the versatility of the present instrumentation.