In their comment on our publications on pervasive remagnetization of Jurassic-Paleogene carbonate rocks of the Tibetan Himalaya (Huang et al., 2017, Journal of Geophysical Research: Solid Earth, 122, doi: 10.1002/2016JB013662 and 122, doi: 10.1002/2017JB013987), Yi et al. (2017) questioned our fold tests applied to their published paleomagenetic results from the Paleogene Zongpu and latest Cretaceous Zongshan carbonate rocks (Patzelt et al., 1996, Tectonophysics, 259(4), 259-284; Yi et al., 2011, Earth and Planetary Science Letters, 309(1), 153-165). They argued that authigenic magnetite pseudomorphic after pyrite, which is the dominant magnetic carrier within these carbonate rocks as indicated by our thorough rock magnetic and petrographic investigations, was formed during early diagenesis and that the primary natural remanent magnetization (NRM) is retained by these carbonate rocks. However, their statement for the invalidity of our fold tests is based on unrealistic assumptions that these carbonate rocks carry primary NRM and that the upper Zongpu Formation was deposited on a 10 degrees primary dip. Their argument for immediate oxidization of pyrite to authigenic magnetite after carbonate deposition onto the continental passive margin ignores that sulfate-reducing conditions were prevailing during early diagenesis, it is also inconsistent with the timing of the secondary remanence acquisition in remagnetized carbonate rocks elsewhere. As previously demonstrated, and agreed upon by Yi et al. (2017), the Zongpu and Zongshan carbonate rocks in Gamba are remagnetized; here we argue that the timing of remagnetization cannot be demonstrated to shortly postdate sedimentation. These data should therefore not be used for tectonic reconstructions.

The latitudinal motion of the Tibetan Himalayathe northernmost continental unit of the Indian plateis a key component in testing paleogeographic reconstructions of the Indian plate before the India-Asia collision. Paleomagnetic studies of sedimentary rocks (mostly carbonate rocks) from the Tibetan Himalaya are complicated by potentially pervasive yet cryptic remagnetization. Although traditional paleomagnetic field tests reveal some of this remagnetization, secondary remanence acquired prior to folding or tilting easily escapes detection. Here we describe comprehensive rock magnetic and petrographic investigations of Jurassic to Paleocene carbonate and volcaniclastic rocks from Tibetan Himalayan strata (Tingri and Gamba areas). These units have been the focus of several key paleomagnetic studies for Greater Indian paleogeography. Our results reveal that while the dominant magnetic carrier in both carbonate and volcaniclastic rocks is magnetite, their magnetic and petrographic characteristics are distinctly different. Carbonate rocks have wasp-waisted hysteresis loops, suppressed Verwey transitions, extremely fine grain sizes (superparamagnetic), and strong frequency-dependent magnetic susceptibility. Volcaniclastic rocks exhibit pot-bellied hysteresis loops and distinct Verwey transitions. Electron microscopy reveals that magnetite grains in carbonate rocks are pseudomorphs of early diagenetic pyrite, whereas detrital magnetite is abundant and pyrite is rarely oxidized in the volcaniclastic rocks. We suggest that the volcaniclastic rocks retain a primary remanence, but oxidation of early diagenetic iron sulfide to fine-grained magnetite has likely caused widespread chemical remagnetization of the carbonate units. We recommend that thorough rock magnetic and petrographic investigations are prerequisites for paleomagnetic studies throughout southern Tibet and everywhere in general.

In recent years, there has been an intense interest in understanding the microscopic mechanism of laser induced ultrafast magnetization dynamics in picosecond time scales. Magnetization switching on such a time scale has potential to be a significant boost for the data storage industry. It is expected that the writing process will become ~1000 times faster by this technology, compared to existing techniques. Understanding the microscopic mechanisms and controlling the magnetization in such a time scale is of paramount importance at present. In this thesis, laser induced ultrafast magnetization dynamics has been studied for Fe, Co, GdFe, CoMn and Heusler alloys. A multiscale approach has been used, i.e., first-principles density functional theory combined with atomistic spin dynamics utilizing the Landau –Lifshitz-Gilbert equation, along with a three-temperature phenomenological model to obtain the spin temperature. Special attention has been paid to the calculations of exchange interaction and Gilbert damping parameters. These parameters play a crucial role in determining the ultrafast magnetization dynamics under laser fluence of the considered materials. The role of longitudinal and transversal excitations was studied for elemental ferromagnets, such as Fe and Co. A variety of complex temporal behavior of the magnetic properties was observed, which can be understood from the interplay between electron, spin, and lattice subsystems. The very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale was studied. We have shown that the ultrafast thermal switching process can happen above the compensation temperature in GdFe alloys. It is demonstrated that the exchange frustration via Dzyaloshinskii-Moriya interaction between the atomic Gd moments, in Gd rich area of these alloys, leads to Gd demagnetization faster than the Fe sublattice. In addition, we show that Co is a perfect Heisenberg system. Both Co and CoMn alloys have been investigated with respect to ultrafast magnetization dynamics. Also, it is predicted that ultrafast switching process can happen in the Heulser alloys when they are doped with heavy elements. Finally, we studied multiferroic CoCr2O4 and Ca3CoMnO4 systems by using the multiscale approach to study magnetization dynamics. In summary, our approach is able to capture crucial details of ultrafast magnetization dynamics in technologically important materials.

This thesis presents the results of a combined magnetic fabric and palaeomagnetic analysis of lower crustal rocks exposed in the Oman (Semail) ophiolite. This has long been an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution, but magnetic investigations in the ophiolite have been limited. Analyses presented here involve using: (i) magnetic anisotropies as a proxy for magmatic petrofabrics in lower crustal rocks in order to contribute to outstanding questions regarding the mode of accretion of fast-spread oceanic crust; and (ii) classical palaeomagnetic analyses to determine the nature of magnetization in these rocks and gain further insights into the regional-scale pattern of tectonic rotations that have affected the ophiolite. The extensive layered gabbro sequences exposed in the Semail ophiolite have been sampled at a number of key localities. These are shown to have AMS fabrics that are layer-parallel but also have a regional-scale consistency of the orientation of maximum anisotropy axes. This consistency across sites separated by up to 100 km indicates large-scale controls on fabric development and may be due to consistent magmatic flow associated with the spreading system or the influence of plate-scale motions on deformation of crystal mushes emplaced in the lower crust. Detailed analysis of fabrics in a single layer and across the sampled sections are consistent with either magmatic flow during emplacement of a melt layer into a lower crustal sill complex, or traction/drag of such layers in response to regional-scale stresses (e.g. mantle drag). Together, results support formation of the layered gabbros by injection of melt into sill complexes in the lower crust. New anisotropy data from the overlying foliated gabbros sampled at two key localities also provide insights into the style of melt migration at this crustal level. Fabrics are consistent with either focused or anastomosing magmatic upwards flow through this layer, reflecting melt migration beneath a fossil axial melt lens. Previous palaeomagnetic research in lavas of the northern ophiolitic blocks has demonstrated substantial clockwise intraoceanic tectonic rotations. Palaeomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization. Systematic sampling resolves for the first time a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used previously to infer lack of significant rotation of the southern blocks. In contrast, E/ENE-directed remanences in the uppermost gabbros imply a large, clockwise rotation of the southern blocks, of a sense and magnitude consistent with that inferred from extrusive sections in the northern blocks. Hence, without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.

In Vakuumanwendungen, wie Molekularpumpen oder Schwungradenergiespeichern, treffen oftmals hohe Drehzahlen auf schwierige Kühlbedingungen. Es besteht daher der Bedarf nach aktiven Magnetlagern mit einem besonders geringem Leistungsbedarf sowie geringen Ummagnetisierungsverlusten im Rotor. Die zur Verlustminimierung prädestinierten Pulververbundwerkstoffe (SMC - Soft Magnetic Composites) finden aufgrund ihrer geringen mechanischen Festigkeit bisher keine Anwendung in industriellen Hochgeschwindigkeitsanwendungen. In diesem Artikel wird das DFG-Projekt „Verlustarme magnetische Radial-/Axiallagerung unter Verwendung von Pulververbundwerkstoffen“ zusammengefasst sowie abschließend der experimentelle Nachweis erbracht, dass die neu entwickelte dreipolige Lagerstruktur mit kombiniertem Radial- und Axiallager den Einsatz von SMC auch bei Drehzahlen von bis zu 30 000 U/min erlaubt. Eine Projizierung der Messergebnisse auf verlustoptimierte industrielle SMC-Sorten verspricht zudem ein Reduzierungspotential der Ummagnetisierungsverluste von mindestens 23 – 44%, wobei insbesondere kompakte und hochausgenutzte Geometrien im Vorteil sind.