Nanoscopic lamellae of centrosymmetric ferromagnetic alloys have recently been reported to host the biskyrmion spin texture; however, this has been disputed as the misidentication of topologically ...trivial type-II magnetic bubbles. Here we demonstrate resonant soft X-ray holographic imaging of topological magnetic states in lamellae of the centrosymmetric alloy (Mn1–x Ni x )0.65Ga0.35 (x = 0.5), showing the presence of magnetic stripes evolving into single core magnetic bubbles. We observe rotation of the stripe phase via the nucleation and destruction of disclination defects. This indicates the system behaves as a conventional uniaxial ferromagnet. By utilizing the holography with extended reference by autocorrelation linear differential operator (HERALDO) method, we show tilted holographic images at 30° incidence confirming the presence of type-II magnetic bubbles in this system. This study demonstrates the utility of X-ray imaging techniques in identifying the topology of localized structures in nanoscale magnetism.
The intense research effort investigating magnetic skyrmions and their applications for spintronics has yielded reports of more exotic objects including the biskyrmion, which consists of a bound pair ...of counter‐rotating vortices of magnetization. Biskyrmions have been identified only from transmission electron microscopy images and have not been observed by other techniques, nor seen in simulations carried out under realistic conditions. Here, quantitative Lorentz transmission electron microscopy, X‐ray holography, and micromagnetic simulations are combined to search for biskyrmions in MnNiGa, a material in which they have been reported. Only type‐I and type‐II magnetic bubbles are found and images purported to show biskyrmions can be explained as type‐II bubbles viewed at an angle to their axes. It is not the magnetization but the magnetic flux density resulting from this object that forms the counter‐rotating vortices.
Research on skyrmions has yielded reports of biskyrmions: bound pairs of counter‐rotating vortices of magnetization. This study combines electron microscopy, X‐ray holography, and simulations to investigate biskyrmions in MnNiGa. It demonstrates that images of biskyrmions can be explained as type‐II magnetic bubbles. It is not the magnetization but the magnetic flux density from this object that forms the counter‐rotating vortices.
Abstract
The creation and annihilation of magnetic skyrmions are mediated by three-dimensional topological defects known as Bloch points. Investigation of such dynamical processes is important both ...for understanding the emergence of exotic topological spin textures, and for future engineering of skyrmions in technological applications. However, while the annihilation of skyrmions has been extensively investigated in two dimensions, in three dimensions the phase transitions are considerably more complex. We report field-dependent experimental measurements of metastable skyrmion lifetimes in an archetypal chiral magnet, revealing two distinct regimes. Comparison to supporting three-dimensional geodesic nudged elastic band simulations indicates that these correspond to skyrmion annihilation into either the helical and conical states, each exhibiting a different transition mechanism. The results highlight that the lowest energy magnetic configuration of the system plays a crucial role when considering the emergence and stability of topological spin structures via defect-mediated dynamics.
Abstract
The GaV
4
S
8−
y
Se
y
(
y
= 0 to 8) family of materials have been synthesized in both polycrystalline and single crystal form, and their structural and magnetic properties thoroughly ...investigated. Each of these materials crystallizes in the
F
$$\bar{4}$$
4
ˉ
3
m
space group at ambient temperature. However, in contrast to the end members GaV
4
S
8
and GaV
4
Se
8
, that undergo a structural transition to the
R
3
m
space group at 42 and 41 K respectively, the solid solutions (
y
= 1 to 7) retain cubic symmetry down to 1.5 K. In zero applied field the end members of the family order ferromagnetically at 13 K (GaV
4
S
8
) and 18 K (GaV
4
Se
8
), while the intermediate compounds exhibit a spin-glass-like ground state. We demonstrate that the magnetic structure of GaV
4
S
8
shows localization of spins on the V cations, indicating that a charge ordering mechanism drives the structural phase transition. We conclude that the observation of both structural and ferromagnetic transitions in the end members of the series in zero field is a prerequisite for the stabilization of a skyrmion phase, and discuss how the absence of these transitions in the
y
= 1 to 7 materials can be explained by their structural properties.
The reduction of polycyclic aromatic hydrocarbons with alkali metals results in solids having intriguing magnetic properties the understanding of which has been hitherto severely hampered by the lack ...of single-phase samples. Here, we report on the successful reduction of triphenylene with stoichiometric amount of potassium in 1,2-dimethoxyethane (DME) solution. Comprehensive diffraction measurements of the obtained K2(C18H12)2(DME) solid demonstrate the importance of cation-π interactions as responsible for the characteristic stacking of the triphenylide molecular ions. Electron paramagnetic resonance and magnetization measurements reveal K2(C18H12)2(DME) is a Mott insulator with strikingly strong nearest neighbor antiferromagnetic interactions between S = 1/2 spins of (C18H12)•– radical anions. Low dimensionality hinders long-range magnetic ordering and establishes a spin state that resembles gapped quantum spin liquid state.
The reactivity of antimony trifluoride, SbF3, in non-polar aprotic solvents has been unexplored due to its negligible solubility. The reaction of the SbF3(tmen) complex (2) with the N-heterocyclic ...carbene, 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene, (LDipp), probably leads to the formation of a neutral (LDipp)SbF3 intermediate (3) in non-polar solvents. Mild heat treatment of the reaction mixture unexpectedly activates the SbF3 molecules to undergo the auto-ionization reaction. In this process, one of the (LDipp) ligands undergoes rearrangement into the mesoionic carbene (MIC) and further coordinates to the cation to form (LDipp)2SbF2+SbF4-, product (4). Studies have confirmed that N,N,N',N'-tetramethylethane-1,2-diamine (tmen) present in the solution is not involved in the conversion mechanism and serves solely as an auxiliary ligand to increase the solubility of SbF3. The compound (4) shows a rare pnictogen(iii) halide auto-ionization process with the first mesoionic NHC rearrangement on any metal/metalloid fluoride compound and the first reported reaction system where a N-heterocyclic carbene ligand facilitates the auto-ionization process on a fluorido substrate.
AlF3-based aerogels, a new class of inorganic aerogels, are prepared in a novel direct process that combines fluoride sol-gel synthesis with high temperature supercritical drying. The bulk structure ...of the solid products depends decisively on the applied solvent(s); very voluminous bulk aerogels are obtained only with MeOH that is used either alone or in combination with some other polar solvents. MeOH acts as a methoxylation agent; and formed methoxy (MeO) species are remarkably stable and deactivate the surface acidic sites. Removal of MeO species under moderate conditions results in catalytically active fluorides with a preserved nanostructure. In preparations with MeOH, preferential growth of anisotropic nanoparticles (nanorods) is the key step that leads to the formation of very open aerogel structures. Another process, dehydration of alcohols, results in some hydroxylation and hydration that lead to the formation of distinctive surface and bulk OH/H2O species. The structure of AlF3-based aerogels is consistent with the hexagonal tungsten bronze (HTB) β-AlF3 although their composition corresponds to a formula AlF3-x(OH, OMe)x·yH2O (x = 0.1 ± 0.05). Some other characteristics of the fluoride nanoparticles, like crystallinity, particle size, and uniformity, can be effectively controlled by the temperature of the solvothermal process. The described methodology allows a controllable preparation of catalytically active fluorides in the form of regularly shaped and uniformly sized nanoparticles.
We present the results of a muon-spin spectroscopy investigation of GaV4S8−ySey with y=0,2,4, and 8. Zero-field measurements suggest that GaV4Se8 and GaV4S8 have distinct magnetic ground states, with ...the latter material showing an anomalous temperature dependence of the local magnetic field. It is not possible to evolve the magnetic state continuously between these two systems, with the intermediate y=2 and 4 materials showing glassy magnetic behavior at low temperature. The skyrmion lattice (SkL) phase is evident in the y=0 and 8 materials through an enhanced response of the muon-spin relaxation to the emergent dynamics that accompany the SkL. For our polycrystalline samples of GaV4Se8, this enhanced dynamic response is confined to a smaller region of the magnetic field-temperature phase diagram than the previous reports of the SkL in single crystals.
Imaging of nanoscale magnetic textures within extended material systems is of critical importance to both fundamental research and technological applications. While high-resolution magnetic imaging ...of thin nanoscale samples is well established with electron and soft x-ray microscopy, the extension to micrometer-thick systems currently requires hard x rays, which limits high-resolution imaging to rare-earth magnets. Here, we overcome this limitation by establishing soft x-ray magnetic imaging of micrometer-thick systems using the pre-edge phase x-ray magnetic circular dichroism signal, thus making possible the study of a wide range of magnetic materials. By performing dichroic spectroptychography, we demonstrate high spatial resolution imaging of magnetic samples up to 1.7 μm thick, an order of magnitude higher than conventionally possible with soft x-ray absorption-based techniques. We demonstrate the applicability of the technique by harnessing the pre-edge phase to image thick chiral helimagnets, and naturally occurring magnetite particles, gaining insight into their three-dimensional magnetic configuration. This new regime of magnetic imaging makes possible the study of extended non-rare-earth systems that have until now been inaccessible, including magnetic textures for future spintronic applications, non-rare-earth permanent magnets for energy harvesting, and the magnetic configuration of giant magnetofossils.
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