Ultrafast Magnetization Dynamics in High Magnetic Fields

Time: Tuesday, October 23rd, 12:00
Speaker: Anna POGREBNA, Nijmegen

The idea to change properties of media with the help of light has been intriguing people for a long time. Recently it was demonstrated that the laser induced ultrafast spin dynamics in ferrimagnetic GdFeCo can be dramatically influenced by the application of high magnetic fields. It was observed that across a critical field called the spin-flop field, the material shows distinct spin dynamics.  The underlying mechanism was attributed to the spin dynamics associated with the two-different magnetic sublattices. However, these measurements were sensitive only to the iron magnetic sublattice. It has already been shown that, one can element specifically probe in the visible spectral range, the laser triggered spin dynamics in the ferrimagnetic TbFeCo. In this work we therefore consider TbFeCo to study the ultrafast spin dynamics associated with the individual magnetic sublattices across the spin-flop transition.
In our experiment, TbFeCo with out-of-plane magnetic anisotropy was studied in an out-of-plane magnetic field up to 28 T. Our study reveals anomalous hysteresis loops and strong field-dependence of the laser-induced dynamics in the vicinity of the compensation point in this ferrimagnet. In particular, the experiment shows that applying fields weaker than the exchange interaction between Tb and Fe, both sublattices show fast spin dynamics (≈ 200 fs), associated with laser-induced demagnetization. If the field is strong enough to trigger a spin-flop transition (≈ 13 T), both the sublattices show a dramatic slowing down of the spin dynamics. Here we explain these observations as manifestations of the first and second order phase transitions in this ferrimagnet.