Magnon squeezing in conical spin spirals
2023-04-13, Wuhrer, Dennis, Rózsa, Levente, Nowak, Ulrich, Belzig, Wolfgang
We investigate squeezing of magnons in a conical spin spiral configuration. We find that while the energy of magnons propagating along the k and the −k directions can be different due to the non-reciprocal dispersion, these two modes are connected by the squeezing, hence can be described by the same squeezing parameter. The squeezing parameter diverges at the center of the Brillouin zone due to the translational Goldstone mode of the system, but the squeezing also vanishes for certain wave vectors. We discuss possible ways of detecting the squeezing.
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.
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.
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.
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.
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.