PACS numbers:2 Skyrmions, topologically-protected nanometric spin vortices, are being investigated 1-11 extensively in various magnets.Among them, many of structurally-chiral cubic magnets host the triangular-lattice skyrmion crystal (SkX) as the thermodynamic equilibrium state. However, this state exists only in a narrow temperature and magnetic-field region just below the magnetic transition temperature T c , while a helical or conical magnetic state prevails at lower temperatures. Here we describe that for a room-temperature skyrmion material 12 , β-Mn-type Co 8 Zn 8 Mn 4 , a field-cooling via the equilibrium SkX state can suppress the transition to the helical or conical state, instead realizing robust metastable SkX states that survive over a very wide temperature and magnetic-field region, including down to zero temperature and up to the critical magnetic field of the ferromagnetic transition. Furthermore, the lattice form of the metastable SkX is found to undergo reversible transitions between a conventional triangular lattice and a novel square lattice upon varying the temperature and magnetic field. These findings exemplify the topological robustness of the once-created skyrmions, and establish metastable skyrmion phases as a fertile ground for technological applications. Skyrmions are promising for spintronics applications firstly because they are stable due to their topological nature, and secondly because they can be manipulated by an ultra-low current density [17][18][19][20][21] . The recent discovery of skyrmion formation at and above room temperature in a new group of chiral magnets, β-Mn-type Co-Zn-Mn alloys, has provided a significant step toward applications 12 . These materials possess a chiral cubic crystal structure with space group P 4 1 32 as shown in Fig. 1 A square-lattice SkX state, characterized by double-q vectors orthogonal to each other and perpendicular to the magnetic field, also shows up as a 4 spot pattern in the H beam geometry. In the H ⊥ beam geometry, the helical multi-domain state shows 4 spots, the conical state 2 spots on the horizontal axis, and both the triangular and square-lattice SkX states each 2 spots on the vertical axis.Keeping the above relations in mind, we next consider the results (Fig. 3) of the FC process at 0.04 T, i.e. by way of the thermodynamical equilibrium triangular-lattice SkX region (green region in Fig. 1(b)). The SANS images in Fig. 3(b) show that the pattern obtained from the equilibrium triangular-lattice SkX generated at 295 K persists down to 200K. This is a direct demonstration of the realization of the metastable SkX state that exists outside of the equilibrium state for temperatures below 284 K. The lifetime of this metastable SkX is very long and becomes essentially time-independent below 260 K ( Supplementary Fig. S4). At 120 K, the triangular-lattice SkX pattern has partially transformed into 4 spots.At 40 K, the 4 spots become clearer and their |q|(≡ q) values become larger than they were 6 at higher temperatures. The 4 spot patter...

Topological spin textures have attracted much attention both for fundamental physics and spintronics applications. Among them, antiskyrmions possess a unique spin configuration with Bloch-type and Néel-type domain walls due to anisotropic Dzyaloshinskii-Moriya interaction (DMI) in the noncentrosymmetric crystal structure. However, antiskyrmions have thus far only been observed in a few Heusler compounds with D2d symmetry. Here, we report a new material Fe1.9Ni0.9Pd0.2P in a different symmetry class (S4 symmetry), where antiskyrmions exist over a wide temperature region including room temperature, and transform to skyrmions upon changing magnetic field and lamella thickness. The periodicity of magnetic textures greatly depends on crystal thickness, and domains with anisotropic sawtooth fractals are observed at the surface of thick crystals, which are attributed to the interplay between dipolar interaction and DMI as governed by

Magnetic frustration in a chiral magnet stabilizes a new disordered skyrmion phase over an extended temperature region.

We report that in a β-Mn-type chiral magnet Co 9 Zn 9 Mn 2 , skyrmions are realized as a metastable state over a wide temperature range, including room temperature, via field-cooling through the thermodynamic equilibrium skyrmion phase that exists below a transition temperature T c ∼ 400 K. The once-created metastable skyrmions survive at zero magnetic field both at and above room temperature. Such robust skyrmions in a wide temperature and magnetic field region demonstrate the key role of topology, and provide a significant step toward technological applications of skyrmions in bulk chiral magnets. 75.30.Kz * These authors equally contributed to this work

Magnetic skyrmions are vortexlike topological spin textures often observed in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Among them, Co-Zn-Mn alloys with a β-Mn-type chiral structure host skyrmions above room temperature. In this system, it has recently been found that skyrmions persist over a wide temperature and magnetic field region as a long-lived metastable state, and that the skyrmion lattice transforms from a triangular lattice to a square one. To obtain perspective on chiral magnetism in Co-Zn-Mn alloys and clarify how various properties related to the skyrmion vary with the composition, we performed systematic studies on Co 10 Zn 10 , Co 9 Zn 9 Mn 2 , Co 8 Zn 8 Mn 4 , and Co 7 Zn 7 Mn 6 in terms of magnetic susceptibility and small-angle neutron scattering measurements. Robust metastable skyrmions with extremely long lifetime are commonly observed in all the compounds. On the other hand, the preferred orientation of the helimagnetic propagation vector and its temperature dependence dramatically change upon varying the Mn concentration. The robustness of the metastable skyrmions in these materials is attributed to topological nature of the skyrmions as affected by structural and magnetic disorder. Magnetocrystalline anisotropy as well as magnetic disorder due to frustrated Mn spins play crucial roles in giving rise to the observed change in helical states and corresponding skyrmion lattice form.

Exotic topological spin textures such as emergent magnetic monopole/anti-monopoles (hedgehog/anti-hedgehog) in the metastable extended skyrmion-strings attract much attention to the fundamental physics owing to their novel electromagnetic properties. However, the direct imaging of such spin textures is lacking. Here, we report the real-space observation of emergent magnetic monopoles involved in extended skyrmion-strings by Lorentz transmission electron microscopy (TEM) in combination with micromagnetic simulations. The in-plane extended skyrmionstrings are observed directly by Lorentz TEM to accompany the topological hedgehog-like defect, where the skyrmion-string terminates or merges with another skyrmion-string, as well as the surface-related defects where skyrmion-string bends 90°and ends on the surface. We also demonstrate the transformation of a metastabilized lattice of out-of-plane short skyrmion-strings into an in-plane array of extended skyrmion-strings by tuning the magnitude of oblique fields in a room-temperature helimagnet, revealing the stability of such topological spin textures and the possibility to control them.

Propagation character of spin wave was investigated for chiral magnets FeGe and Co-Zn-Mn alloys, which can host magnetic skyrmions near room temperature. On the basis of the frequency shift between counter-propagating spin waves, the magnitude and sign of Dzyaloshinskii-Moriya (DM) interaction were directly evaluated. The obtained magnetic parameters quantitatively account for the size and helicity of skyrmions as well as their materials variation, proving that the DM interaction plays a decisive role in the skyrmion formation in this class of room-temperature chiral magnets. The propagating spin-wave spectroscopy can thus be an efficient tool to study DM interaction in bulk single-phase compounds. Our results also demonstrate a function of spin-wave diode based on chiral crystal structures at room temperature.1 Recently, the concept of magnetic skyrmions, i.e. vortex-like swirling spin texture with topologically stable particle nature, has attracted much attention as potential information carriers for novel magnetic information storage and processing devices [1][2][3][4][5][6][7][8]. The skyrmion and associated helical spin texture can be stabilized by several distinctive mechanisms, such as Dzyaloshinskii-Moriya interaction (DMI) [1,8], frustrated exchange interactions [9,10] or the competition between magnetic dipole-dipole interaction and magnetic anisotropy [11,12].So far, the experimental observation of skyrmions has mainly been reported for a series of noncentrosymmetric ferromagnets, where the sizable contribution of DMI is expected [2,4,[13][14][15]. However, the full understanding for DMI in the metallic system is often more difficult than the case for the insulating system, and recent theories suggest its relevance to quantum Berry phase and band anti-crossing that causes the complicated E F (Fermi energy)-dependence of DMI [16][17][18]. To unambiguously elucidate the microscopic origin of skyrmion formation for each compound, the direct quantitative evaluation of relevant magnetic parameters, in particular the magnitude and sign of DMI, is important.To directly evaluate DMI, one promising approach is the analysis of spin wave dispersion in the ferromagnetic state. It is generally symmetric (i.e. even function) with respect to the wave number k, but can become asymmetric only under the existence of k-linear term originating from DMI that causes the energy shift between ±k [19]. The direct observation of DMI based on this idea has recently been reported for the interface of bilayer films by employing several surface-sensitive methods such as Brillouin light scattering [20,21] and spin-polarized electron energy loss spectroscopy [22]. For bulk single-phase compounds, on the other hand, the direct quantitative evaluation of DMI has rarely been reported. Only recently, the alternative method based on neutron inelastic scattering technique [23] and propagating spin-wave spectroscopy (PSWS) [24][25][26][27][28] has been proposed.Among a series of single-phase compounds hosting magnetic skyrmions, th...

Magnetic skyrmions in CoZnMn thin plates are observed to deform in a metastable state prepared in a magnetic-field-cooling process by way of the thermal-equilibrium skyrmion phase. In cooling, the disk-shape skyrmions change to bar- or L-shaped elongated form, whereas the skyrmion density is nearly conserved. The deformation of the skyrmions in the supercooled metastable phase is observed irrespective of the crystallographic orientation of the thin plate, whereas the elongation direction nearly aligns along the magnetic easy axis. It is proposed that the deformation should be induced by a large increase in magnetic modulation wavenumber when decreasing the temperature, whereas the topological protection of the skyrmions keeps the averaged skyrmion density constant.