MLZ is a cooperation between:> Technische Universität München> Helmholtz-Zentrum Geesthacht> Forschungszentrum Jülich
Magnetization dynamics of thin films and laterally patterned stripe arrays subjected to AC magnetic fields is presently of high interest in the field of magnetism. The evolution of sample magnetization in the region of low, quasi-static AC-fields is routinely studied by various magnetometry techniques, such as magneto-optical Kerr effect or SQUID-based susceptometry, while the range of high frequencies is commonly accessible by e.g. ferromagnetic resonance spectroscopy. However comprehensive information about sample magnetization in the mesofrequency region ranging from several dozens of kilohertz up to a few megahertz, which matches typical domain wall propagation and switching times in thin films, micro- and nanostructures, is still missing. This gap could be filled by the use of two AC-polarized neutron reflectivity (AC-PNR) techniques which will be presented in this talk.
The first one is the frequency resolved, but time-integrated AC-PNR technique dedicated to probe
the magnetization dynamics in continuous thin films and in laterally patterned samples subjected to AC field. The response of the samples to AC magnetic fields is quantified via specular reflectivities recorded at several frequencies ranging from 50 up to 800 kHz. Fits of reflectivity curves deliver sets of magnetic parameters which determine the magnetization state at any given AC field frequency and amplitude. Subsequent analysis of these parameters reveals different behavior for continuous and pattern samples. For continuous films a crossover from almost instant adiabatic to retarded non-adiabatic coherent magnetization rotation was found, whereas a complex domain wall scenario was assigned as the dominating mechanism of magnetization response for the laterally patterned sample.
The second technique is the time-resolved AC-PNR method based on neutron heterodyning technique. The use of neutron heterodyning technique establishes a unique possibility to reconstruct the time-structure of the reflected signal in the sub-megahertz range of frequencies, common for AC-PNR measurements, but inaccessible for direct recording due to so called time-frame overlap problem. The latter appears due to uncertainties in neutron fly paths and neutron velocity distribution of the beam. Theory predicts that the time-frame overlap problem can be successfully overcome using a double reflection geometry with two identical samples oriented parallel to each other and subjected to AC fields with close frequencies. For verification the principles and practical implementation of the neutron heterodyning technique a prototype of the heterodyning add-on device was built. Results of pilot experiments performed at three different neutron scattering instruments characterized by significantly different beam characteristics will be presented.
|Time||14:30 - 15:30 h|
|Room||HS 3, Physics Department|
|Speaker||Dr. Dmitry Gorkov (MLZ)|