Showing papers by "Markus Münzenberg published in 2019"

Light-wave dynamic control of magnetism

Abstract: The enigmatic interplay between electronic and magnetic phenomena observed in many early experiments and outlined in Maxwell’s equations propelled the development of modern electromagnetism1. Today, the fully controlled evolution of the electric field of ultrashort laser pulses enables the direct and ultrafast tuning of the electronic properties of matter, which is the cornerstone of light-wave electronics2–7. By contrast, owing to the lack of first-order interaction between light and spin, the magnetic properties of matter can only be affected indirectly and on much longer timescales, through a sequence of optical excitations and subsequent rearrangement of the spin structure8–16. Here we introduce the regime of ultrafast coherent magnetism and show how the magnetic properties of a ferromagnetic layer stack can be manipulated directly by the electric-field oscillations of light, reducing the magnetic response time to an external stimulus by two orders of magnitude. To track the unfolding dynamics in real time, we develop an attosecond time-resolved magnetic circular dichroism detection scheme, revealing optically induced spin and orbital momentum transfer in synchrony with light-field-driven coherent charge relocation17. In tandem with ab initio quantum dynamical modelling, we show how this mechanism enables the simultaneous control of electronic and magnetic properties that are essential for spintronic functionality. Our study unveils light-field coherent control of spin dynamics and macroscopic magnetic moments in the initial non-dissipative temporal regime and establishes optical frequencies as the speed limit of future coherent spintronic applications, spin transistors and data storage media. The magnetic properties of a ferromagnetic layer stack are controlled on attosecond timescales through optically induced spin and orbital momentum transfer, demonstrating a coherent regime of ultrafast magnetism.

Microscaffolds by Direct Laser Writing for Neurite Guidance Leading to Tailor-Made Neuronal Networks

Abstract: While modern day integrated electronic circuits are essentially designed in a 2D fashion, the brain can be regarded as a 3D circuit. The thus enhanced connectivity enables much more complex signal processing as compared to conventional 2D circuits. Recent technological advances in the development of nano/microscale 3D structuring have led to the development of artificial neuron culturing platforms, which surpass the possibilities of classical 2D cultures. In this work, in vitro culturing of neuronal networks is demonstrated by determining predefined pathways through topological and chemical neurite guiding. Tailor-made culturing substrates of microtowers and freestanding microtubes are fabricated using direct laser writing by two-photon polymerization. The first scaffold design that allows for site-specific cell attachment and directed outgrowth of single neurites along defined paths that can be arranged freely in all dimensions, to build neuronal networks with low cell density, is presented. The neurons cultured in the scaffolds show characteristic electrophysiological properties of vital cells after 10 d in vitro. The introduced scaffold design offers a promising concept for future complex neuronal network studies on defined neuronal circuits with tailor-made design specific neurite connections beyond 2D.

Tunnel magneto-Seebeck effect

Abstract: The interplay of charge, spin and heat transport is investigated in the fascinating research field of spin caloritronics, the marriage of spintronics and thermoelectrics. Here, many new spin-dependent thermal transport phenomena in magnetic nanostructures have been explored in the recent years. One of them is the tunnel magneto-Seebeck (TMS) effect in magnetic tunnel junctions (MTJs) that has large potential for future nanoelectronic devices, such as nanostructured sensors for three-dimentional thermal gradients, or scanning tunneling microscopes driven by temperature differences. The TMS describes the dependence of the MTJ's thermopower on its magnetic cofiguration when a thermal gradient is applied. In this review, we highlight the successful way from first observation of the TMS in 2011 to current ongoing developments in this research area. We emphasize on different heating techniques, material designs, applications, and additional physical aspects such as the role of the thermal conductivity of the barrier material. We further demonstrate the efficient interplay between ab initio calculations and experiments within this field, as this has led, e.g., to the detection of large TMS ratios in MTJs with half-metallic Heusler electrodes.

Driving magnetization dynamics in an on-demand magnonic crystal by magneto-elastic interaction

Abstract: Using spatial light interference of ultrafast laser pulses, we generate a lateral modulation in the magnetization profile of an otherwise uniformly magnetized film, whose magnetic excitation spectrum is monitored via the coherent and resonant interaction with elastic waves. We find an unusual dependence of the magnetoelastic coupling as the externally applied magnetic field is angle- and field-tuned relative to the wave vector of the magnetization modulation, which can be explained by the emergence of spatially inhomogeneous spin-wave modes. In this regard, the spatial light interference methodology can be seen as a user-configurable, temporally windowed, on-demand magnonic crystal, potentially of arbitrary two-dimensional shape, which allows control and selectivity of the spatial distribution of spin waves. Calculations of spin waves using a variety of methods, demonstrated here using the plane-wave method and micromagnetic simulation, can identify the spatial distribution and associated energy scales of each excitation, which opens the door to a number of excitation methodologies beyond our chosen elastic wave excitation.

Remagnetization in arrays of ferromagnetic nanostripes with periodic and quasiperiodic order

Abstract: We investigate experimentally and theoretically the magnetization reversal process in one-dimensional magnonic structures composed of permalloy nanostripes of the two different widths and finite length arranged in a periodic and quasiperiodic order. We showed that dipolar coupling between rectangular nanostripes is significantly reduced as compared to the analytical and numerical predictions, probably due to formation of the closure domains at the nanostripe ends. Although the main feature of the hysteresis loop is determined by different shape anisotropies of the component elements and the dipolar interactions between them, the quasiperiodic order influences the hysteresis loop by introducing additional tiny switching steps and change of the plateau width. We also showed that the dipolar interactions between nanostripes forming a ribbon can be counterintuitively decreased by reduction of the distance between the neighboring ribbons.

Spin-current manipulation of photoinduced magnetization dynamics in heavy metal / ferromagnet double layer based nanostructures

Abstract: Spin currents offer a way to control static and dynamic magnetic properties, and therefore they are crucial for next-generation MRAM devices or spin-torque oscillators Manipulating the dynamics is especially interesting within the context of photo-magnonics In typical $3d$ transition metal ferromagnets like CoFeB, the lifetime of light-induced magnetization dynamics is restricted to about 1 ns, which eg strongly limits the opportunities to exploit the wave nature in a magnonic crystal filtering device Here, we investigate the potential of spin-currents to increase the spin wave lifetime in a functional bilayer system, consisting of a heavy metal (8 nm of $\beta$-Tantalum (Platinum)) and 5 nm CoFeB Due to the spin Hall effect, the heavy metal layer generates a transverse spin current when a lateral charge current passes through the strip Using time-resolved all-optical pump-probe spectroscopy, we investigate how this spin current affects the magnetization dynamics in the adjacent CoFeB layer We observed a linear spin current manipulation of the effective Gilbert damping parameter for the Kittel mode from which we were able to determine the system's spin Hall angles Furthermore, we measured a strong influence of the spin current on a high-frequency mode We interpret this mode an an exchange dominated higher order spin-wave resonance Thus we infer a strong dependence of the exchange constant on the spin current

Petahertz Magnetization Dynamics

Abstract: In contrast to conventional electronics, where only the charge of electrons is considered, spintronics is based on the utilization of both charge and spin. Due to this additional degree of freedom, spintronic devices can potentially provide higher processing speed or better energy efficiency [1,2], However, while sub-femtosecond control of the electronic properties of solids has previously been demonstrated [3], the lack of direct coupling between light and spin has limited the manipulation speed of magnetic properties to the few-tens-of-femtoseconds timescale. Here we introduce a technique able to follow the magnetic properties of a solid with attosecond resolution and demonstrate the direct sub-femtosecond all-optical manipulation of its spin degrees of freedom.

Emission Properties of Structured Spintronic Terahertz Emitters

Abstract: The inverse Spin Hall effect offers a promising approach for the generation of intense broadband terahertz radiation from optically driven ultrathin magnetic samples. Here, we present our approach for tailoring the spatial and spectral emission properties of such spintronic thin film systems, utilizing terahertz resonator designs.