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Femtosecond laser-pulse excitation provides an energy efficient and fast way to control magnetization at the nanoscale, providing great potential for ultrafast next-generation data manipulation and nonvolatile storage devices. Ferromagnetic van der Waals materials have garnered much attention over the past few years due to their low dimensionality, excellent magnetic properties, and large response to external stimuli. Nonetheless, their behaviour upon fs laser-pulse excitation remains largely unexplored. Here, we investigate the ultrafast magnetization dynamics of a thin flake of Fe3GeTe2 (FGT) and extract its intrinsic magnetic properties using a microscopic framework. We find that our data is well described by our modelling, with FGT undergoing a slow two-step demagnetization, and we experimentally extract the spin-relaxation timescale as a function of temperature, magnetic field and excitation fluence. Our observations indicate a large spin-flip probability in agreement with a theoretically expected large spin-orbit coupling, as well as a weak interlayer exchange coupling. The spin-flip probability is found to increase when the magnetization is pulled away from its quantization axis, opening doors to an external control over the spins in this material. Our results provide a deeper understanding of the dynamics van der Waals materials upon fs laser-pulse excitation, paving the way towards two-dimensional materials-based ultrafast spintronics.
Magnetic skyrmions are localized topological excitations that behave as particles and can be mobile, with great potential for novel data storage devices. In this work, the current-induced dynamics of large skyrmion bubbles is studied. When skyrmion motion in the direction opposite to the electron flow is observed, this is usually interpreted as a perpendicular spin current generated by the spin Hall effect exerting a torque on the chiral Néel skyrmion. By designing samples in which the direction of the net generated spin current can be carefully controlled, we surprisingly show that skyrmion motion is always against the electron flow, irrespective of the net vertical spin-current direction. We find that a negative bulk spin-transfer torque is the most plausible explanation for the observed results, which is qualitatively justified by a simple model that captures the essential behaviour. These findings demonstrate that claims about the skyrmion chirality based on their current-induced motion should be taken with great caution.
Protecting our natural resources is one of the important thrusts of every organization. In this study, the waters of Iloilo Batiano River, Philippines is explored. This study was conducted because there is a dearth of information on the water physico-chemical characteristics of the river. Since this is an initial assessment, this study will serve as a baseline data and the results will serve as a basis for future reference for conservation measures of the river. The present study aimed to determine the selected water physico-chemical characteristics of the river such as the electrical conductivity (EC), temperature, pH, and calcium content; assess if these parameters exceed the limit set by the DENR Administrative Order No. 20F16-08 of the Republic of the Philippines and other available literatures; and to determine if there was no significant difference in the water physico-chemical characteristics among the six sampling stations. Results revealed that EC and calcium content of the river exceed their limits except for temperature and pH levels. Furthermore, EC and temperature were statistically different among the six sampling stations except for pH values. This study concludes that the river is saline due to the mixing of saltwater and surface water as the river is near to the sea and has high calcium content that can threaten the life of freshwater aquatic organisms.
Large skyrmion bubbles in confined geometries of various sizes and shapes are investigated, typically in the range of several micrometers. Two fundamentally different cases are studied to address the role of dipole-dipole interactions: (I) when there is no magnetic material present outside the small geometries and (II) when the geometries are embedded in films with a uniform magnetization. It is found that the preferential position of the skyrmion bubbles can be controlled by the geometrical shape, which turns out to be a stronger influence than local variations in material parameters. In addition, independent switching of the direction of the magnetization outside the small geometries can be used to further manipulate these preferential positions, in particular with respect to the edges. We show by numerical calculations that the observed interactions between the skyrmion bubbles and structure edge including the overall positioning of the bubbles are fully controlled by dipole-dipole interactions. a) Electronic mail: t.lichtenberg@tue.nl 1 arXiv:1905.10304v1 [cond-mat.mes-hall] 24 May 2019Magnetic skyrmions are whirls in the magnetization in which neighbouring spins are rotated with respect to each other with a specific chirality. They cannot be removed by continuous deformation of the magnetization without creating a singularity, which provides a topological barrier that makes them robust against annihilation. They are less hindered by pinning sites or defects than magnetic domain walls (DWs), and their size can be in the order of nanometers. These properties make them suitable for data storage. For the envisioned skyrmion racetrack memory 1-3 , the skyrmions are required to be present in small geometrically confined structures, instead of infinite sheets of material. Therefore, the interaction between skyrmions and the edge of the magnetic structure is crucial. In fact, this interaction is necessary to prevent skyrmions from being expelled from the track, it can stabilize skyrmions in absence of an external magnetic field 4,5 , assist in their formation 6,7 , and by reducing the width of the track it could be possible to reduce the size of the skyrmion and hence to achieve larger data storage densities 8 .In the research field on skyrmions, usually a distinction is made between a 'compact skyrmion' and a 'skyrmion bubble'. These objects share many properties, but the latter has typically a much larger size and has a constant magnetization at its core 8 . Numerical and experimental work on compact skyrmion confinement show that there is indeed a repulsive interaction between skyrmions and sample edges that is a result of tilting of the magnetic moments at the edge, which is caused by the Dzyaloshinskii-Moriya interaction (DMI) 9,10 . For skyrmion bubbles, dipolar interactions are paramount in their stabilization, and because near the sample edge these stray fields will change, it is intuitively expected that the edges will influence the skyrmion bubbles via this mechanism. Though this has been realized before 1...
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