Role of Entropy in Domain Wall Motion in Thermal Gradients
2014, Schlickeiser, Frank, Ritzmann, Ulrike, Hinzke, Denise, Nowak, Ulrich
Thermally driven domain wall (DW) motion caused solely by magnonic spin currents was forecast theoretically and has been measured recently in a magnetic insulator using magneto-optical Kerr effect microscopy. We present an analytical calculation of the DW velocity as well as the Walker breakdown within the framework of the Landau Lifshitz Bloch equation of motion. The temperature gradient leads to a torque term acting on the magnetization where the DW is mainly driven by the temperature dependence of the exchange stiffness, or—in a more general picture—by the maximization of entropy. The existence of this entropic torque term does not rest on the angular momentum transfer from the magnonic spin current. Hence, even DWs in antiferromagnets or compensated ferrimagnets should move accordingly. We further argue that the entropic torque exceeds that of the magnonic spin current.
Temperature dependence of the frequencies and effective damping parameters of ferrimagnetic resonance
2012, Schlickeiser, Frank, Atxitia, Unai, Wienholdt, Sönke, Hinzke, Denise, Chubykalo-Fesenko, Oksana, Nowak, Ulrich
Recent experiments on all-optical switching in GdFeCo and CoGd have raised the question about the importance of the angular momentum or the magnetization compensation point for ultrafast magnetization dynamics. We investigate the dynamics of ferrimagnets by means of computer simulations as well as analytically. The results from atomistic modeling are explained by a theory based on the two-sublattice Landau-Lifshitz-Bloch equation. Similarly to the experimental results and unlike predictions based on the macroscopic Landau-Lifshitz equation, we find an increase in the effective damping at temperatures approaching the Curie temperature. Further results for the temperature dependence of the frequencies and effective damping parameters of the normal modes represent an improvement of former approximated solutions, building a better basis for comparison to recent experiments.
Controlling the magnetic structure of Co/Pd thin films by direct laser interference patterning
2015, Stärk, Martin, Schlickeiser, Frank, Nissen, Dennis, Hebler, Birgit, Graus, Philipp, Hinzke, Denise, Scheer, Elke, Leiderer, Paul, Fonin, Mikhail, Albrecht, Manfred, Nowak, Ulrich, Boneberg, Johannes
Nanosecond pulsed two-beam laser interference is used to generate two-dimensional temperature patterns on a magnetic thin film sample. We show that the original domain structure of a [Co/Pd] multilayer thin film changes drastically upon exceeding the Curie temperature by thermal demagnetization. At even higher temperatures the multilayer system is irreversibly changed. In this area no out-of-plane magnetization can be found before and after a subsequent ac-demagnetization. These findings are supported by numerical simulations using the Landau–Lifshitz–Bloch formalism which shows the importance of defect sites and anisotropy changes to model the experiments. Thus, a one-dimensional temperature pattern can be transferred into a magnetic stripe pattern. In this way one can produce magnetic nanowire arrays with lateral dimensions of the order of 100 nm. Typical patterned areas are in the range of several square millimeters. Hence, the parallel direct laser interference patterning method of magnetic thin films is an attractive alternative to the conventional serial electron beam writing of magnetic nanostructures.