Magnetization dynamics in all-optical pump-probe experiments: spin-wave modes and spin-current damping

Abstract

In the field of spintronics, future development demands a new insight into the magnetization relaxation processes in the sub nanosecond regime. With the time resolution inherent, femtosecond laser pulses in all-optical pump-probe experiments can be used to study the basic time constants of ultrafast demagnetization, the magnetic precessional modes and the energy dissipation processes. This thesis investigates magnetization dynamics of thin nickel samples regarding coherent and incoherent relaxation processes. The intense 80fs laser pulse of an amplified Ti:Sapphire oscillator is used both to excite the sample and to monitor the magnetic relaxation using magneto-optical Kerr effect.Ultrafast demagnetization shortly after excitation by the pump pulse is investigated by varying the pump laser fluencies and the external magnetic field. Delayed remagnetization for highly excited samples on the timescales of few picoseconds is observed. This relaxation time describes the evolution of high energy spin waves to spin waves with lower k-vectors.Various magnetic modes are observed upon pump laser excitation depending on the thickness of the ferromagnet. The Kittel mode is present in all films. Standing spin wave modes up to the 3rd order are observed for samples thicker than the optical penetration depth of the pump laser. The dipole modes dominate the relaxation spectra for samples much thicker than the optical penetration depth. Frequency dispersion relations of these modes reveal the material constants (anisotropy constant, exchange constant). Gilbert damping parameter which describes magnetic energy dissipation is assigned to each mode.The energy dissipation processes in ferromagnetic/nonmagnetic double layers are investigated by means of extrinsic and spin-current induced damping. Two-magnon scattering increases the damping parameter for double layers with rough interface. The precession of the magnetization in the ferromagnetic layer pumps spins into the nonmagnetic layer and opens up a new energy dissipation channel. This additional Gilbert damping by spin currents is inversely proportional to the thickness of the ferromagnetic layer and observed only for the samples thinner than 10nm. The efficiency of the spin pumping is determined by the spin accumulation in the spin sink. Spin sink of 5nm dysprosium, palladium or chromium studied within the thesis induce a strong increase of the Gilbert damping parameter.