Off-resonant magnetization dynamics in Co, Fe and Ni thin films driven by an intense single-cycle THz field
2017, Shalaby, Mostafa, Vicario, Carlo, Giorgianni, Flavio, Donges, Andreas, Carva, Karel, Oppeneer, Peter M., Nowak, Ulrich, Hauri, Christoph P.
Spin-wave localization in disordered magnets
2015, Evers, Martin, Müller, Cord A., Nowak, Ulrich
The effect of disorder on magnonic transport in low-dimensional magnetic materials is studied in the framework of a classical spin model. Numerical investigations give insight into scattering properties of the systems and show the existence of Anderson localization in 1D and weak localization in 2D, potentially affecting the functionality of magnonic devices.
Theoretical study of magnetic domain walls through a cobalt nanocontact
2012, Balogh, László, Palotás, Krisztián, Udvardi, László, Szunyogh, László, Nowak, Ulrich
To calculate the magnetic ground state of nanoparticles we present a self-consistent first principles method in terms of a fully relativistic embedded cluster multiple scattering Green's function technique. Based on the derivatives of the band energy, a Newton-Raphson algorithm is used to find the ground state configuration. The method is applied to a cobalt nanocontact that turned out to show a cycloidal domain wall configuration between oppositely magnetized leads. We found that a wall of cycloidal spin-structure is about 30 meV lower in energy than the one of helical spin-structure. A detailed analysis revealed that the uniaxial on-site anisotropy of the central atom is mainly responsible to this energy difference. This high uniaxial anisotropy energy is accompanied by a huge enhancement and anisotropy of the orbital magnetic moment of the central atom. By varying the magnetic orientation at the central atom, we identified the term related to exchange couplings (Weiss-field term), various on-site anisotropy terms, and also those due to higher order spin-interactions.
Off-resonant magnetization dynamics in ferromagnetic thin films initiated by ultrastrong THz field
2017, Shalaby, Mostafa, Vicario, Carlo, Donges, Andreas, Carva, Karel, Oppeneer, Peter M., Nowak, Ulrich, Hauri, Christoph P.
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 . The recent advent of THz pulses with field strength up to several Teslas  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 .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 . 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.
Switching Dynamics of Two Sub-lattice Magnets
2015, Wienholdt, Sönke, Nowak, Ulrich
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.
Magnetic Domain Structure in Coupled Rectangular Nanostructures
2013, Jelli, J., Lebecki, Krzysztof, Hankemeier, Stefan, Frömter, Robert, Oepen, Hans Peter, Nowak, Ulrich
The coupling of rectangular magnetic 2000×1000×20 nm3 structures with flux-closure domain configurations is studied using micromagnetic simulations with periodic boundary conditions. In order to understand the origin of the interaction, the magnetic structure of a single element is analyzed in detail to calculate its magnetostatic fringe field. Both, our simulations as well as earlier experimental data reveal an interesting phenomenon: instead of four domains forming the well-known Landau state there are six domains. A consistent magnitude of the effect can be obtained, when the highly susceptible paramagnetic “coating layer” used in the experiment is included in the simulation. The coupling behavior of both, horizontally and vertically aligned arrays of rectangles is explained by the magnetostatic field of the single element. We show that for arrays of elements that have a coating layer inter-element coupling depends strongly on properties of this coating layer.