Magnetic-Domain Structure Analysis of Nd-Fe-B Sintered Magnets Using XMCD-PEEM Technique

A combination of X-Ray Magnetic Circular Dichroism and PhotoEmission Electron Microscopy (XMCD-PEEM) was applied to the magnetic domain analysis of Nd-Fe-B sintered magnets. The XMCD-PEEM high-resolution images revealed both the magnetic domain structures and the microstructural morphologies. In the thermally demagnetized state, each grain in a polycrystalline sample exhibits a multidomain structure, which is magnetically coupled across grain boundaries. After the DC field-demagnetization, it changed to a single domain structure. The magnetization vector in each surface grain reversed to the negative direction during the field-demagnetization procedure because of the small coercivity in the surface region. In the present study, we observed this surface domain reversal for the first time by means of XMCD-PEEM imaging method, which is important in order to understand the surface phenomena of Nd-Fe-B magnets

Dynamik des Ummagnetisierungsvorgangs in magnetisch gekoppelten Dreischichtsystemen

Title page 1 Introduction 2 Magnetic domain theory in static 3 Magnetic domain theory in dynamics 4 Experiment 5 Samples and Results 6 Discussion 7 Conclusion Appendix Bibliography List of publications Curriculum vitae Acknowledgement ZusammenfassungThroughout this dissertation, the spin dynamics from the nanosecond (ns) range to infinitely long time scales has been studied. A combination of x-ray magnetic circular dichroism and photoelectron emission microscopy was used to investigate the magnetic domain structure with element selectivity. In addition, a micro-coil, which can supply ns-short magnetic field pulses, was employed to study either the magnetic domain evolution induced by single pulses (so called single-pulse technique) or the domain dynamics during pulses (pump-probe technique). The magnetization reversal in the ns range of the magnetically soft layer in spin valve and magnetic tunnel junction like trilayer systems has been investigated. The influence of magnetic anisotropy and interlayer magnetic coupling between the two ferromagnetic layers on the magnetization reversal have been studied. Also the surface/interface roughness and micromagnetic effects related to the domain wall energy and stray fields from a domain wall in the hard magnetic layer influence the reversal. Magnetization reversal in the timescale of seconds range was observed in a region with an out-of-plane easy axis of magnetization of a Co/Ni double layer epitaxially grown on a Cu(001) clean surface. The driving forces for the magnetization reversal were the thermal energy, upon heating the sample, or the change in anisotropy energy with time by depositing Co.In dieser Arbeit wurde die Spindynamik im Bereich von Nanosekunden (ns) bis hin zu einigen Sekunden untersucht. Hierzu wurde Photoelektronenmikroskopie unter Ausnutzung des magnetischen Rontgenzirkulardichroismus angewendet, um die Domanenstruktur elementspezifisch darstellen und analysieren zu konnen. Zusatzlich wurde ein Mikrospulensystem zur Erzeugung von ns-kurzen Magnetfeldpulsen eingesetzt. Dies ermoglichte es, die zeitliche Entwicklung der Domanen nach Anregung mit einem Einzelpuls (single-pulse Technik) oder durch Pulsfolgen im Pump-Probe-Verfahren zu verfolgen. Das Ummagnetisieren der weichmagnetischen Schicht von Spin-valves und magnetischen Dreifachlagen, die als Tunnelkontakt aufgebaut sind, wurde im Nanosekundenbereich untersucht. Das Hauptaugenmerk lag dabei auf den Einflussen, die die magnetische Anisotropie und die Interlagenaustauschkopplung der beiden ferromagnetischen Lagen auf das Umklappen der Magnetisierung haben. Der Ummagnetisierungsprozess wird aber auch durch Oberflachen- und Grenzflachenrauhigkeit sowie mikromagnetische Effekte Im Bereich der Domanenwand-Energie und der Streufelder der Domanenwande der hartmagnetischen Schicht beeinflusst. Umklappprozesse im Sekundenbereich wurden in Regionen von Co/Ni-Zweifachlagen mit senkrechter leichter Magnetisierungsrichtung beobachtet, die epitaktisch auf Cu(001)-Einkristallen gewachsen wurden. Die treibenden Krafte hinter dem Umklappen der Magnetisierung sind zum einen thermische Effekte durch das Heizen der Probe, aber auch zeitliche Anderungen der Anisotropieenergie durch das Aufdampfen von Co

Spatio-temporal observation of photogenerated electron dynamics in twisted graphene

International audienc

Domain wall dynamics and interlayer interactions in magnetic trilayer systems studied by XMCD-PEEM

International audienceWe have used time-resolved x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM) to investigate the layer-resolved microscopic magnetization reversal in FeNi/X/Co (with X = Cu, Al2O3) trilayer systems. These measurements were performed in pump-probe mode, synchronizing magnetic pulses with synchrotron x-ray pulses. The good magnetic contrast observed for most samples reveals that in many cases the magnetization reversal is reproducible. We have used the measurements to obtain domain wall propagation speeds as a function of applied magnetic field, and to investigate the influence of domain wall interactions on the magnetic switching

Spatio-temporal observation of photogenerated electron dynamics in twisted graphene

Graphene is one of the most studied 2D materials. However, the ultrafast carrier dynamics influenced by the crystallographic structures is not well studied, because of the instrumental limitations. Locally different twisting angle between graphene layers was detected by Raman spectro-microscopy. Photogenerated carrier lifetimes in these selected regions were estimated by time-resolved photoemission electron microscopy with 100 nm spatial and 100 fs temporal resolutions. We concluded that the interaction between layers and to the substrate influence the optoelectronic properties