Stability of skyrmion lattices and symmetries of quasi-two-dimensional chiral magnets (original) (raw)
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Spontaneous atomic-scale magnetic skyrmion lattice in two dimensions
Nature Physics, 2011
Skyrmions are topologically protected field configurations with particle-like properties that play an important role in various fields of science. Recently, skyrmions have been observed to be stabilized by an external magnetic field in bulk magnets. Here, we describe a two-dimensional square lattice of skyrmions on the atomic length scale as the magnetic ground state of a hexagonal Fe film of one-atomic-layer thickness on the Ir(111) surface. Using spin-polarized scanning tunnelling microscopy we can directly image this non-collinear spin texture in real space on the atomic scale and demonstrate that it is incommensurate to the underlying atomic lattice. With the aid of first-principles calculations, we develop a spin model on a discrete lattice that identifies the interplay of Heisenberg exchange, the four-spin and the Dzyaloshinskii-Moriya interaction as the microscopic origin of this magnetic state.
Disordered skyrmion phase stabilized by magnetic frustration in a chiral magnet
Science advances, 2018
Magnetic skyrmions are vortex-like topological spin textures often observed to form a triangular-lattice skyrmion crystal in structurally chiral magnets with the Dzyaloshinskii-Moriya interaction. Recently, β-Mn structure-type Co-Zn-Mn alloys were identified as a new class of chiral magnet to host such skyrmion crystal phases, while β-Mn itself is known as hosting an elemental geometrically frustrated spin liquid. We report the intermediate composition system CoZnMn to be a unique host of two disconnected, thermal-equilibrium topological skyrmion phases; one is a conventional skyrmion crystal phase stabilized by thermal fluctuations and restricted to exist just below the magnetic transition temperature , and the other is a novel three-dimensionally disordered skyrmion phase that is stable well below . The stability of this new disordered skyrmion phase is due to a cooperative interplay between the chiral magnetism with the Dzyaloshinskii-Moriya interaction and the frustrated magneti...
Equilibrium Skyrmion Lattice Ground State in a Polar Easy-plane Magnet
Scientific Reports, 2017
The skyrmion lattice state (SkL), a crystal built of mesoscopic spin vortices, gains its stability via thermal fluctuations in all bulk skyrmion host materials known to date. Therefore, its existence is limited to a narrow temperature region below the paramagnetic state. This stability range can drastically increase in systems with restricted geometries, such as thin films, interfaces and nanowires. Thermal quenching can also promote the SkL as a metastable state over extended temperature ranges. Here, we demonstrate more generally that a proper choice of material parameters alone guarantees the thermodynamic stability of the SkL over the full temperature range below the paramagnetic state down to zero kelvin. We found that GaV 4 Se 8 , a polar magnet with easy-plane anisotropy, hosts a robust Néel-type SkL even in its ground state. Our supporting theory confirms that polar magnets with weak uniaxial anisotropy are ideal candidates to realize SkLs with wide stability ranges. Whether skyrmions, as nanometric bits 1, 2 , can boost the information density of magnetic memory devices depends on three key factors: i) their thermal stability range 3-11 , ii) size 12-14 and iii) controllability by external stimuli, preferably via electric fields or weak electric currents 7, 15-19. Some of the currently existing host materials at least partially fulfill these conditions and may reach the level of applications. As a common feature, all of them lack spatial inversion symmetry. When inversion symmetry is broken in a material with predominantly ferromagnetic interactions, the relativistic Dzyaloshinskii-Moriya interaction (DMI) emerges that can destabilize the ferromagnetic state (FM), giving rise to the formation of spin spirals and skyrmions 20-26. Additionally, SkL-like phases may also exist in centrosymmetric magnets as a consequence of frustrated magnetic interactions 27, 28 , though this theoretical prediction has not found experimental realization yet. In multilayer systems, consisting alternating magnetic and heavy metal layers, the inversion symmetry is broken by the interfaces and strong DMIs emerge in the magnetic layers due to their proximity to the heavy element layers. Recently, in such Pt/CoFeB/MgO, Pt/Co/Ta and Pt/Co/MgO stacks the stabilization of mid-sized skyrmions (typically 100 nm in diameter) has been demonstrated at room temperature 5-7. The same interface-induced DMI mechanism leads to the spontaneous formation of SkL in iron mono-, bi-and triple layers with unprecedently small atomic-scale skyrmions at low temperatures 13, 14, 16-18. Room-temperature SkLs with periodicities of ~100 nm were also reported to emerge in ultrathin films of FeGe 4 and in bulk crystals of β-Mn-type Co-Zn-Mn alloys 8, 9. In these cubic magnets, the spatial inversion symmetry is broken by the chirality of the lattice, similarly to the case of MnSi 3, 29, 30 and its isostructural analogues 31-34. In terms of current-driven skyrmion manipulation, a possible drawback of multilayers and Co-Zn-Mn compounds is the strong pinning of skyrmions due to atomic-scale disorder inevitably present in these systems. Of the most desirable functionalities, the manipulation, writing and erasing of skyrmions solely by electric fields, i.e. without electric currents, have been demonstrated only in iron triple layers so far 17. The possibility
Superlattices and Microstructures, 2019
Magnetic skyrmions have attracted great research interest in recent years due to their exotic physical properties, scientific merit and potential applications in modern technology. Here, we apply a quantum computational method to investigate the spin configurations of Fe 0.5 Co 0.5 Si-like quasi-twodimensional ferromagnetic system with coexistence of Dzyaloshinsky-Moriya and Heisenberg exchange interactions. We find that within a weak magnetic field perpendicular to the film plane, skyrmion crystal (SkX) of hexagonalclose-packed pattern can be induced, the spin configurations evolve with applied magnetic field and temperature. This quantum model, if scaled, is able to qualitatively reproduce the experimental results of SkX with long periodicity. Especially, when the skyrmion size is around a few nano-meters in diameter, or more general, when the characteristic length of the material approaches the lattice scale, the quantum model is expected to be more accurate than the classical ones.
Skyrmion–skyrmion interaction in a magnetic film
Journal of Physics: Condensed Matter, 2020
Interaction of two skyrmions stabilized by the ferromagnetic exchange, Dzyaloshinskii-Moriya interaction (DMI), and external magnetic field has been studied numerically on a 2D lattice of size large compared to the separation, d, between the skyrmions. We show that two skyrmions of the same chirality (determined by the symmetry of the crystal) repel. In accordance with earlier analytical results, their long-range pair interaction falls out with the separation as exp(−d/δH), where δH is the magnetic screening length, independent of the DMI. The prefactor in this expression depends on the DMI that drives the repulsion. The latter results in the spiral motion of the two skyrmions around each other, with the separation between them growing logarithmically with time. When two skyrmions of the total topological charge Q = 2 are pushed close to each other, the discreteness of the atomic lattice makes them collapse into one skyrmion of charge Q = 1 below a critical separation. Experiment is proposed that would allow one to measure the interaction between two skyrmions by holding them in positions with two magnetic tips. Our findings should be of value for designing topologically protected magnetic memory based upon skyrmions.
Towards an Effective Theory of Skyrmion Crystals
JETP Letters, 2019
We consider multiskyrmion configurations in 2D ferromagnets with Dzyaloshinskii-Moriya (DM) interaction and the magnetic field, using the stereographic projection method. In the absence of DM interaction, D, and the field, B, the skyrmions do not interact and the exact multiskyrmion solution is a sum of individual projections. In certain range of B, D = 0, skyrmions become stable and form a hexagonal lattice. The shape of one skyrmion on the plane is fully determined by D and B. We describe multiskyrmion configurations by simple sums of individual skyrmion projections, of the same shape and adjusted scale. This procedure reveals pairwise and triple interactions between skyrmions, and the energy of proposed hexagonal structure is found in a good agreement with previous studies. It allows an effective theory of skyrmion structures in terms of variables, referring to individual skyrmions, i.e., their position, size and phase, elliptic distortions etc.
Multiple low-temperature skyrmionic states in a bulk chiral magnet
npj Quantum Materials
Magnetic skyrmions are topologically protected nanoscale spin textures with particle-like properties. In bulk cubic helimagnets, they appear under applied magnetic fields and condense spontaneously into a lattice in a narrow region of the phase diagram just below the magnetic ordering temperature, the so-called A-phase. Theory, however, predicts skyrmions to be locally stable in a wide range of magnetic fields and temperatures. Our neutron diffraction measurements reveal the formation of skyrmion states in large areas of the magnetic phase diagram, from the lowest temperatures up to the A-phase. We show that nascent and disappearing spiral states near critical lines catalyze topological charge changing processes, leading to the formation and destruction of skyrmionic states at low temperatures, which are thermodynamically stable or metastable depending on the orientation and strength of the magnetic field. Skyrmions are surprisingly resilient to high magnetic fields: the memory of skyrmion lattice states persists in the field polarized state, even when the skyrmion lattice signal has disappeared. These findings highlight the paramount role of magnetic anisotropies in stabilizing skyrmionic states and open up new routes for manipulating these quasi-particles towards energy-efficient spintronics applications.
A novel chiral spin texture: Antiferromagnetic Skyrmionium
arXiv (Cornell University), 2018
Exotic spin textures viz. chiral domain wall, vortices, skyrmion, skyrmionium, etc. have recently emerged as active field of research because of their potential applications in high density data storage technology and logic gate computing. Magnetic skyrmionium is a skyrmion like soliton, which carries zero topological quantum number. Skyrmioniums are superior to conventional skyrmions in ferromagnets due to their negligible skyrmion hall effect and higher velocity. The physical properties of both skyrmion and skyrmionium have been investigated rigorously in ferromagnetic systems. Recent observations hint that such chiral spin structures in antiferromagnetic (AFM) systems are more promising in comparison to the ferromagnetic ones because of their robustness towards external perturbation, absence of Skyrmion hall effect, etc. However skyrmionium in AFM materials are not reported in literature so far. In this work, we demonstrate that skyrmionium can be created and stabilized in AFM materials by application of spin polarized current in an experimentally feasible geometry. We have further studied the dynamics of AFM skyrmionium by applying spin polarized current.