Felix Groß's research while affiliated with Max Planck Institute for Intelligent Systems and other places

Supplimentary data for the manuscript "Ni80Fe20 thickness optimization of magnetoplasmonic crystals for magnetic field sensing"

A promising approach to enhance the transverse Kerr effect with potential applications in the detection of weak magnetic fields is the use of magnetoplasmonic crystals based on ferromagnetic metals. The sensitivity, measuring field range, and limit-of-detection of 1D magnetoplasmonic crystals with 5–20 nm thick Ni80Fe20 layers are analyzed in this study based on magnetic, optical, and magneto-optical characterization. The magnetoplasmonic crystal with 10 nm-thick Ni80Fe20 layer provided a sensitivity of 21.9 µV/mOe, a limit-of-detection of 3.6 mOe, and a measuring field range of 1.134 Oe. This sample was also utilized as a magnetic field probe to reconstruct the magnetic configuration of a multicore cable and a planar induction coil, thereby highlighting its potential for the visualization of DC magnetic fields.

We have investigated the size dependent energy barrier regarding the transition between magnetic vortex and collinear states in dense arrays of magnetic cap structures hosting magnetic vortices. The cap structures were formed by the deposition of soft magnetic thin films on top of large arrays of densely packed polystyrene spheres. The energy barrier associated with the magnetic field assisted switching from a collinear magnetic state to a non-uniform vortex state (or vice versa) was tuned by tailoring the diameter and thickness of the soft magnetic caps. At a sufficient temperature, known as the bifurcation temperature, the thermal energy overcomes this energy barrier and magnetic bistability with a hysteresis-free switching occurs between the two magnetic states. In magnetic caps with a fixed thickness, the bifurcation temperature decreases with increasing cap diameter. On the other hand, for a fixed diameter, the bifurcation temperature increases with an increase in film thickness of the cap structure. This study demonstrates that the bifurcation temperature can be easily tailored by changing the magnetostatic energy contribution which in turn affects the energy barrier and thus the magnetic bistability.

Supplimentary data for the manuscript "Magnetic field sensing elements made of quasi-trapezoidal magnetoplasmonic crystals based on thin permalloy films"

This work is devoted to the study of magnetic, optical, and magneto-optical properties of quasi-trapezoidal magnetoplasmonic crystals based on Ni80Fe20 permalloy films ranging in thickness from 5 to 20 nm. Magnetoplasmonic crystals’ dependencies of measuring range, required modulation magnetic field, sensitivity, and limit-of-detection on the thickness of the permalloy layer are comprehensively studied. It is shown, that an increase in magnetoplasmonic crystals’ differential susceptibility leads to the sensitivity increase of the proposed magnetic field sensing method, but also results in a decrease in the sensing elements’ measuring range. Obtained results can be used for sensing elements fabrication from magnetoplasmonic crystals with sensitivity and measuring range, suitable for particular applications.

Being able to accurately control the interaction of spin waves is a crucial challenge for magnonics in order to offer an alternative wave-based computing scheme for certain technological applications. Especially in neural networks and neuromorphic computing, wave-based approaches can offer significant advantages over traditional CMOS-based binary computing schemes with regard to performance and power consumption. In this work, we demonstrate precise modulation of phase- and amplitude-sensitive interference of coherent spin waves in a yttrium–iron–garnet based magnonic analog adder device, while also showing the feasibility of frequency-division multiplexing. Using time-resolved scanning transmission x-ray microscopy, the interference was directly observed, giving an important proof of concept for this kind of analog computing device and its underlying working principle. This constitutes a step toward wave-based analog computing using magnons as an information carrier.

Frequency multiplication is an essential part of electronics and optics which led to numerous indispensable applications. In this paper, we utilize a combination of scanning transmission x-ray microscopy and micromagnetic simulations to directly image magnonic frequency multiplication by means of dynamic real-space magnetization measurements. We experimentally demonstrate frequency multiplication up to the seventh order, which enables the generation of nanoscale spin waves at 6GHz with excitation frequencies of less than 1GHz. Good agreement between the experiment and micromagnetic simulations allows us to build a micromagnetic model capable of predicting conversion efficiencies and multiplexing capabilities of the system. Furthermore, simulations reveal that more than two rows of antidots do not increase the conversion efficiency substantially. By enabling magnonic multiplexing with low input frequencies while not exceeding the size of a few microns, the device will lead to numerous applications, further advancing the capabilities of magnonic data transmission.

Magnetic droplets are non-topological magnetodynamical solitons displaying a wide range of complex dynamic phenomena with potential for microwave signal generation. Bubbles, on the other hand, are internally static cylindrical magnetic domains, stabilized by external fields and magnetostatic interactions. In its original theory, the droplet was described as an imminently collapsing bubble stabilized by spin transfer torque and, in its zero-frequency limit, as equivalent to a bubble. Without nanoscale lateral confinement, pinning, or an external applied field, such a nanobubble is unstable, and should collapse. Here, we show that we can freeze dynamic droplets into static nanobubbles by decreasing the magnetic field. While the bubble has virtually the same resistance as the droplet, all signs of low-frequency microwave noise disappear. The transition is fully reversible and the bubble can be thawed back into a droplet if the magnetic field is increased under current. Whereas the droplet collapses without a sustaining current, the bubble is highly stable and remains intact for days without external drive. Electrical measurements are complemented by direct observation using scanning transmission x-ray microscopy, which corroborates the analysis and confirms that the bubble is stabilized by pinning. Magnetic droplets are a type of non-topological magnetic soliton, which are stabilised and sustained by spin-transfer torques for instance. Without this, they would collapse. Here Ahlberg et al show that by decreasing the applied magnetic field, droplets can be frozen, forming a static nanobubble

Terahertz (THz) spin dynamics and vanishing stray field make antiferromagnetic (AFM) materials the most promising candidate for the next-generation magnetic memory technology with revolutionary storage density and writing speed. However, owing to the extremely large exchange energy barriers, energy-efficient manipulation has been a fundamental challenge in AFM systems. Here, we report an electrical writing of antiferromagnetic orders through a record-low current density on the order of 106 A cm−2 facilitated by the unique AFM-ferromagnetic (FM) phase transition in FeRh. By introducing a transient FM state via current-induced Joule heating, the spin-orbit torque can switch the AFM order parameter by 90° with a reduced writing current density similar to ordinary FM materials. This mechanism is further verified by measuring the temperature and magnetic bias field dependences, where the X-ray magnetic linear dichroism (XMLD) results confirm the AFM switching besides the electrical transport measurement. Our findings demonstrate the exciting possibility of writing operations in AFM-based devices with a lower current density, opening a new pathway towards pure AFM memory applications. Electrical manipulation of antiferromagnetic order is crucial for future memory devices, but existing switching schemes require a large current density. Here, the authors achieve record low current density switching in FeRh by taking advantage of its antiferromagnetic to ferromagnetic phase transition.