Toward ultrafast magnetic depth profiling using time resolved x ray resonant magnetic reflectivity

During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time resolved x ray resonant magnetic reflectivity, can distinguish local and non local effects. Here, we demonstrate the feasibility of this technique in a pump probe geometry using a custom built reflectometer at the FLASH2 free electron laser FEL . Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 amp; 8201;nm amp; 8771;310 amp; 8201;eV , we were able to probe close to the Fe L3 edge 706.8 amp; 8201;eV of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non homogeneous loss of magnetization and a significant dilation of 2 amp; 8201; of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetizatio

Seismogenic Fault Reactivation in Western Central Africa: Insights From Regional Stress Analysis

Abstract The onshore continental margins of western Central Africa have been hosting potentially damaging earthquake events for decades; yet, the links between the seismicity, the contemporary stress field, and pre‐existing faults are not well understood. Here, we analyze the regional stress fields offshore and onshore along the coastal margin, and in the interior continental areas using earthquake focal mechanisms, map and characterize the detailed structure of preexisting fault systems in outcrops, and assess their reactivation potential. Our results show a switch from NNE‐SSW transpressive stress regime offshore and near the coastal margins, to NE‐SW compressive and transtensive stress regimes in the cratonic interior (Congo Basin and Kasai Block). We show that regional stresses acting on offshore oceanic fracture zones are compatible with those acting along the onshore areas of the continental margin. Field observations reveal the presence of large fault systems that deform both the Precambrian basement and Phanerozoic sedimentary sequences, with widespread calcite veining, quartz veining, and palygorskite mineralization (with evidence of post‐veining shear reactivation) along the fault zones. Along the margin, the preexisting NNE‐, NNW‐, and N‐S ‐trending strike‐slip faults and normal faults show a high slip tendency (>80%–100%), whereas in the continental interior, the NW‐ and N‐S ‐trending thrust faults are the most likely to reactivate. We argue that favorable orientation of the preexisting faults define the susceptibility of the faults to seismic reactivation. We propose that zones of higher stress magnitudes along distal offshore oceanic fracture zones extend further into the continent and may be driving stress loading on pre‐stressed, favorably oriented fault systems onshore, along the continental margin

X ray holography of skyrmionic cocoons in aperiodic magnetic multilayers

The development and characterization of three dimensional 3D topological magnetic textures has become an important topic in modern magnetism both for fundamental and technological perspectives. Among the novel 3D spin textures, skyrmionic cocoons have been successfully stabilized in magnetic multilayers having a variable thickness of the ferromagnet in the vertical direction of the stack. These ellipsoidal 3D magnetic textures remain vertically confined in a fraction of the total thickness, whereas, coexisting with fully columnar skyrmions. Here, we use xray holography with about 15 nm lateral resolution to investigate how their properties depend on the field and temperature. We observe circular objects with different amplitude of contrast, which evidences the presence of different 3D objects located in various vertical parts of the multilayer. Moreover, whereas, sweeping an out of plane magnetic field, we witness an attractive interaction between cocoons located at various heights, mainly due to the stray field, which impacts their horizontal positionings. The x ray holography measurements also allow to determine the size of the cocoons at remanence, which at room temperature, possess diameters close to 100 nm on average. Combining this transmission technique with magnetic force microscopy and micromagnetic simulations gives a precise insight into the 3D distribution of the magnetization, which demonstrates the 3D nature of skyrmionic cocoon