Licentiate thesis defense: Spin dynamics in the terahertz regime

It is known that the use of conventional optical lasers for the study of ultrafast magnetization dynamics imparts heat to the materials under study, which creates a highly non equilibrium state and masks the fundamental coupling processes leading to ultrafast demagnetization. In order to understand, control and manipulate the dynamics of spins in magnetic systems a new kind of radiation called terahertz (THz) is starting to be used in recent years. Due to low photon energy (4 meV) as compared to near infrared radiation (1 eV), THz radiation can directly couple to spins. In the work shown in this thesis, a table-top experimental setup for generating intense THz radiation using organic crystals has been developed for the purpose of carrying out pump probe studies of metallic ferromagnets. With the set up being capable of delivering intense THz fields as high as 1 MV/cm (300 mT) it is possible to initiate magnetization dynamics by directly coupling to the spins. We probe the magnetization change using the femtosecond magneto-optical Kerr effect (MOKE).

In our quest to explore and understand the fast magnetization dynamics on the picosecond timescale we present in this thesis a direct experimental evidence of inertial spin dynamics in ferromagnetic thin films in the THz frequency regime based on the recent modification of the Landau-Lifshitz Gilbert equation (LLG). According to this equation spin nutations should appear at high frequencies which is orders of magnitude higher than the spin precession as observed from the conventional LLG equation. By using the femtosecond MOKE as a probe for the magnetization dynamics we observe a broad resonance in the MOKE response when the films were excited with intense narrowband THz magnetic field pulses of tunable center frequency. The behavior of the spin motion is similar to a forced Lorentz oscillator in the THz magnetic field. From the analysis of our experimental results we could extract the value of momentum relaxation time which is of the order of 10 ps. This quantifies the duration for spin nutation. Our experimental results were consistent with simulations based on the modified LLG equation (also called the inertial LLG equation). The experimental work presented here may be crucial for our current understanding of ultrafast spin dynamics. We believe that the exploitation of the inertial motion of spins may open up a new way of controlling dynamics in magnetism.