The formation of soap bubbles from thin films is accompanied by topological transitions. Here we show how a magnetic topological structure, a skyrmion bubble, can be generated in a solid-state system in a similar manner. Using an inhomogeneous in-plane current in a system with broken inversion symmetry, we experimentally "blow" magnetic skyrmion bubbles from a geometrical constriction. The presence of a spatially divergent spin-orbit torque gives rise to instabilities of the magnetic domain structures that are reminiscent of Rayleigh-Plateau instabilities in fluid flows. We determine a phase diagram for skyrmion formation and reveal the efficient manipulation of these dynamically created skyrmions, including depinning and motion. The demonstrated current-driven transformation from stripe domains to magnetic skyrmion bubbles could lead to progress in skyrmion-based spintronics.

The well-known Hall effect describes the transverse deflection of charged particles (electrons/holes) in an electric-current carrying conductor under the influence of perpendicular magnetic fields, as a result of the Lorentz force. Similarly, it is intriguing to examine if quasi-particles without an electric charge, but with a topological charge 1-4 , show related transverse motion. Chiral magnetic skyrmions with a well-defined spin topology resulting in a unit topological charge serve as good candidates to test this hypothesis 1-3,5-11 . In spite of the recent progress made on investigating magnetic skyrmions 2,4,6-8,12-19 , direct observation of the skyrmion Hall effect in real space has, remained elusive. Here, by using a current-induced spin Hall spin torque 13,20-23 , we experimentally observe the skyrmion Hall effect by driving skyrmions from creep motion into the steady flow motion regime. We observe a Hall angle for the magnetic skyrmion motion as large as 𝟏𝟓 ∘ for current densities smaller than 𝟏𝟎 𝟕 𝐀/𝐜𝐦 𝟐 at room temperature. The experimental observation of transverse transport of skyrmions due to topological charge may potentially create many exciting opportunities for the emerging field of skyrmionics, including novel applications such as topological selection.Because of their topologically non-trivial spin textures, chiral magnetic skyrmions enable many intriguing phenomena based on their topology 2-4 , such as emergent electrodynamics 10 and effective magnetic monopoles 11 . As compared to most (vortex-like) Bloch skyrmions in bulk chiral materials 2,5,9 , utilizing interfacial inversion symmetry breaking 24 in heavy metal/ultrathin ferromagnet/insulator hetero-structures has enabled

No abstract

We experimentally show that exchange magnons can be detected by using a combination of spin pumping and the inverse spin-Hall effect proving its wavelength integrating capability down to the submicrometer scale. The magnons were injected in a ferrite yttrium iron garnet film by parametric pumping and the inverse spin-Hall effect voltage was detected in an attached Pt layer. The role of the density, wavelength, and spatial localization of the magnons for the spin pumping efficiency is revealed.

We investigate the origin of the spin Seebeck effect in yttrium iron garnet (YIG) samples for film thicknesses from 20 nm to 50 μm at room temperature and 50 K. Our results reveal a characteristic increase of the longitudinal spin Seebeck effect amplitude with the thickness of the insulating ferrimagnetic YIG, which levels off at a critical thickness that increases with decreasing temperature. The observed behavior cannot be explained as an interface effect or by variations of the material parameters. Comparison to numerical simulations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite magnon propagation length. This allows us to trace the origin of the observed signals to genuine bulk magnonic spin currents due to the spin Seebeck effect ruling out an interface origin and allowing us to gauge the reach of thermally excited magnons in this system for different temperatures. At low temperature, even quantitative agreement with the simulations is found. The thermal excitation of a spin current by a temperature gradient is commonly called the spin Seebeck effect (SSE) which is detected by the inverse spin Hall effect (ISHE) [1,2], leading to a thermovoltage similar to the charge analogue, the Seebeck effect. Experimental evidence of the SSE, first in ferromagnetic metals [3], and later, both in semiconductors [4] and in insulators [5][6][7][8], has brought up the question about the origin of the SSE. Of particular interest for spin caloritronics is the observation of the SSE in insulators, which allows us to generate pure spin currents in insulating systems.However, the underlying mechanism, properties, and the origin of the observed signals have been highly controversial. Thermally induced magnonic spin currents have been suggested as the origin [9,10], based on the presence of the effect in magnetic insulators, which excludes charge currents as the source. Despite this explanation of the origin of the effect, direct experimental evidence has not been reported. While parasitic interface effects [11] were suggested as an alternative source of the SSE due to a polarization of the paramagnetic detector layer [12], generally, the observed effects are now primarily attributed to magnonic spin currents [13,14].Time resolved experiments trying to address the problem by probing the temporal evolution of the SSE have obtained contradictory results: For film thickness up to 61 nm, no cut-off frequency due to an intrinsic limitation by the SSE was observed [15]. In contrast, for μm thick films, a characteristic rise time was found, and a finite magnon propagation length of the order of several 100 nm was put forward as a possible explanation [16,17]. This clearly calls for study to reveal the origin of this discrepancy as it underlies the fundamental mechanism of the SSE and to determine the intrinsic length scale.To clarify the origin of the measured SSE signals, we present a detailed study of the relevant length scales of the longitudinal SSE (LSSE) coveri...

No abstract

We show that a femtosecond spin-current pulse can generate terahertz (THz) transients at Rashba interfaces between two nonmagnetic materials. Our results unambiguously demonstrate the importance of the interface in this conversion process that we interpret in terms of the inverse Rashba Edelstein effect, in contrast to the THz emission in the bulk conversion process via the inverse spin-Hall effect. Furthermore, we show that at Rashba interfaces the THz-field amplitude can be controlled by the helicity of the light. The optical generation of electric photocurrents by these interfacial effects in the femtosecond regime will open up new opportunities in ultrafast spintronics.

Magnetization dynamics in an artificial square spin-ice lattice made of Ni80Fe20 with magnetic field applied in the lattice plane is investigated by broadband ferromagnetic resonance spectroscopy. The experimentally observed dispersion shows a rich spectrum of modes corresponding to different magnetization states. These magnetization states are determined by exchange and dipolar interaction between individual islands, as is confirmed by a semianalytical model. In the low field regime below 400 Oe a hysteretic behavior in the mode spectrum is found. Micromagnetic simulations reveal that the origin of the observed spectra is due to the initialization of different magnetization states of individual nanomagnets. Our results indicate that it might be possible to determine the spin-ice state by resonance experiments and are a first step towards the understanding of artificial geometrically frustrated magnetic systems in the high-frequency regime.Frustrated magnetic systems, such as spin ices, have been of scientific interest for a long time due to their highly degenerated ground states, which result in complex magnetic ordering and collective behavior [1][2][3][4][5]. In contrast to the prototypical crystalline materials that started the exploration of spin-ice systems, such as the pyrochlores Dy 2 Ti 2 O 7 , Ho 2 Ti 2 O 7 and Ho 2 Sn 2 O 7 [6,7], artificially structured spin-ice lattices offer the unique opportunity to control and engineer the interactions between the elements by their geometric properties and orientation [1,8,9]. Another outstanding advantage of artificial spin ices is that the magnetization state of each individual spin (i.e., macrospin/single domain magnetic particle) is directly accessible through magnetic microscopy (e.g., scanning probe, electron, optical or X-ray microscopy). The 16 possible magnetization configurations of a square spin ice are shown in Fig. 1(a).Spin dynamics in magnonic crystals, materials with periodic perturbations or variations in one of the magnetic properties of the system, have been extensively investigated [10-13]. One-and two-dimensional magnonic crystals were studied and the research community paid particular attention to nano-structured materials [10], such as chains of dots or arrays of discs [14], antidot lattices with different shapes and alignments [15][16][17], gratings or nanostripes [18], etc.Although artificial spin ices offer a fascinating playground to investigate how specific magnetization states of individual islands or defects can affect the collective spin dynamics, there are only very few works on dynamics in the GHz-regime [19,20] reported. Sklenar et al. show broadband ferromagnetic resonance (FMR) measurements on an artificial bicomponent square spin-ice lattice utilizing a macroscopic meanderline approach and find a field-dependent behavior in remanence where interactions between individual elements presumably play a less important role. Furthermore, the geometrical arrangement of the structures in the artificial lattice leads to frustration by d...