Nowak, Ulrich
0000-0003-2925-6774 Berufsbeschreibung
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Off-resonant magnetization dynamics in Co, Fe and Ni thin films driven by an intense single-cycle THz field
Inverse Faraday effect as mechanism for ultrafast all-optical magnetic switching
Dynamics of driven interfaces in heterogenous magnetic systems
We analyze interface motion in the random field Ising model with quenched disorder by means of Monte Carlo simulations. In the absence of thermal fluctuations a depinning transition occurs at some critical threshold field. We study the interface motion in the vicinity of the threshold field by varying the temperature as well as the driving field. It turns out that thermal fluctuations yield a rounded transition which can be characterized by a critical exponent.
Off-resonant magnetization dynamics in ferromagnetic thin films initiated by ultrastrong THz field
Summary form only given. The speed of magnetization switching is a key feature in next generation magnetic storage devices. The ongoing pursue towards faster magnetization control has triggered the development of laser sources at terahertz frequencies. In fact pulses in this spectral range are more suited than optical laser for coherent magnetization excitation by Zeeman precession [1]. The recent advent of THz pulses with field strength up to several Teslas [2] opens novel opportunities to drive ultrafast magnetization dynamics in the strong-field regime, which is different from the commonly used optical lasers where the magnetization control is mediated by heat deposition [3].Here we report on time-resolved measurements exploring the sub-cycle THz-induced magnetization dynamics in the ferromagnetic thin film samples Co, Fe and Ni [4]. We present the induced magneto-optical Kerr dynamics as function of the THz field strength up to extreme amplitudes of 7 T and 21 MV/cm, respectively. By increasing the THz pump fluence, we find a continuous transition from the regime of purely coherent Zeeman oscillations, to the incoherent regime, where spin oscillations are superimposed by thermal demagnetization. Our observations indicate that while the coherent response is driven only by the magnetic field, the incoherent dynamics are dominated by the associated THz electric field component. The observed magnetization evolution over sub-picosecond time scale is excellently reproduced by simulations based on ab-initio calculations for the Heisenberg spin Hamiltonian and the stochastic Landau-Lifshitz-Gilbert equation to describe the spin dynamics at finite temperature.
Domain walls in thermal gradients : entropic torque and angular momentum transfer
Thermally activated magnetization reversal in classical spin chains
We investigate the thermally activated magnetization switching in a classical Heisenberg spin chain driven by an external magnetic field. For small system sizes we expect that the magnetic moments rotate coherently, while in the case of larger system sizes the magnetization reversal is proposed to be due to soliton-antisoliton nucleation. We compare Monte Carlo simulations with the direct integration of the Landau-Lifshitz-Gilbert equation of motion with Langevin dynamics as well as with asymptotic solutions for the escape rates following from the Fokker-Planck equation, finding agreement for low temperatures and high damping. We also discuss deviations in the intermediate temperature regime.
Switching Dynamics of Two Sub-lattice Magnets
After thermal excitation ferrimagnets can switch via a transient ferromagnetic-like state. It is shown by spin model simulations that this state follows from a dissipationless dynamics on picosecond time scales, while slower dissipative relaxation leads back to the ferrimagnetic state which might or might not be switched.