Skyrmion lattice structural transition in MnSi

Nakajima, Tsuyoshi; Ôike, Hiroshi; Kikkawa, Akiko; Gilbert, Elliot P.; Booth, Norman; Kakurai, Kazuhisa; Taguchi, Y.; Tokura, Y.; Kagawa, Fumitaka; Arima, T.

doi:10.1126/sciadv.1602562

Cited by 108 publications

(84 citation statements)

References 29 publications

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“…In turn, when starting in the skyrmion lattice phase a decrease of temperature or changes of the applied magnetic field cause a transition of the skyrmion lattice to the conical state. Moreover, under thermal quenches in an applied magnetic field the skyrmion lattice survives as a metastable state down to very low temperatures, where a direct transition into the helical or field-polarized phase may be triggered in field-sweeps [2,3,[45][46][47][48].…”

Section: State Of the Art And Main Resultsmentioning

confidence: 99%

Weak Crystallization of Fluctuating Skyrmion Textures in MnSi

et al. 2019

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We report an experimental study of the emergence of non-trivial topological winding and longrange order across the paramagnetic to skyrmion lattice transition in the transition metal helimagnet MnSi. Combining measurements of the susceptibility with small angle neutron scattering, neutron resonance spin echo spectroscopy and all-electrical microwave spectroscopy, we find evidence of skyrmion textures in the paramagnetic state exceeding 10 3Å with lifetimes above several 10 −9 s. Our experimental findings establish that the paramagnetic to skyrmion lattice transition in MnSi is well-described by the Landau soft-mode mechanism of weak crystallization, originally proposed in the context of the liquid to crystal transition. As a key aspect of this theoretical model, the modulation-vectors of periodic small amplitude components of the magnetization form triangles that add to zero. In excellent agreement with our experimental findings, these triangles of the modulation-vectors entail the presence of the non-trivial topological winding of skyrmions already in the paramagnetic state of MnSi when approaching the skyrmion lattice transition. I. MOTIVATIONA pre-requisite for the definition of topological magnetic textures is the presence of a continuous magnetization field with a finite amplitude in space and time. An example par excellence of such textures are magnetic skyrmions, representing topologically nontrivial whirls of this magnetization field [1]. The notions of topological winding and topological stability of such skyrmions are only meaningful when the magnetization is sufficiently smooth on local scales. This condition may be readily satisfied in systems exhibiting long-range magnetic order for temperatures far below the transition temperature T c . In contrast, changes of the topological properties require that the magnetization is capable of vanishing on short length and time scales. The associated microscopic mechanisms underlying the transition of skyrmions into different types of conventional long-range magnetic order have been explored in a large number of theoretical and experimental studies [2][3][4][5][6][7][8].A major unresolved question concerns, in contrast, the formation of skyrmion lattice order when starting from a state that is essentially paramagnetic and dominated by an abundance of fluctuations such that the local magnetization, on a coarse-grained level, practically vanishes on average [9][10][11][12][13][14][15]. This alludes to the question whether topologically non-trivial characteristics exist already in * a paramagnetic state, and how they may be accounted for in the framework of the present-day classification of phase transitions [16][17][18]. It connects also with the relevance of non-trivial topological properties in the search for novel electronic properties of solids, e.g., at quantum phase transitions [19,20].Magnetic skyrmions are ideally suited to address this question. However, they are typically portrayed from either one of two seemingly contrary points of view. On the one hand...

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Section: State Of the Art And Main Resultsmentioning

confidence: 99%

Weak Crystallization of Fluctuating Skyrmion Textures in MnSi

et al. 2019

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“…In planning how to prepare the metastable SkS-L in the area of interest, we noted that the SkS-L can be created as a metastable state by employing rapid cooling (>100 K s -1 ) by way of the thermo-equilibrium SkS-L phase (see Figs. 2a,b) 34,37 . If such thermal quenching is locally implemented, then the metastable SkS-L domain is also expected to be written locally.…”

Section: Resultsmentioning

confidence: 99%

Current-induced viscoelastic topological unwinding of metastable skyrmion strings

et al. 2017

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In the MnSi bulk chiral magnet, magnetic skyrmion strings of 17 nm in diameter appear in the form of a lattice, penetrating the sample thickness, 10–1000 μm. Although such a bundle of skyrmion strings may exhibit complex soft-matter-like dynamics when starting to move under the influence of a random pinning potential, the details remain highly elusive. Here, we show that a metastable skyrmion-string lattice is subject to topological unwinding under the application of pulsed currents of 3–5 × 106 A m–2 rather than being transported, as evidenced by measurements of the topological Hall effect. The critical current density above which the topological unwinding occurs is larger for a shorter pulse width, reminiscent of the viscoelastic characteristics accompanying the pinning-creep transition observed in domain-wall motion. Numerical simulations reveal that current-induced depinning of already segmented skyrmion strings initiates the topological unwinding. Thus, the skyrmion-string length is an element to consider when studying current-induced motion.

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“…Small angle neutron scattering (SANS) measurements were performed by carefully applying the magnetic field along the three major crystallographic directions, either after zero-field cooling (ZFC) or after fast-field cooling (FFC) through the A-phase. [40][41][42][43][44] In this way, extremely robust low-temperature skyrmionic states can be induced for all field directions and over large areas of the phase diagram, stabilized by competing cubic and exchange anisotropies. In particular, when the magnetic field is applied along the cubic axes, triangular arrays of skyrmions are energetically more favorable than both the tilted spiral and conical states near the borderline between modulated and uniform magnetic phases.…”

Section: Introductionmentioning

confidence: 99%

Multiple low-temperature skyrmionic states in a bulk chiral magnet

et al. 2019

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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.

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Skyrmion lattice structural transition in MnSi

Abstract: A triangular-to-square lattice transition of topological magnetic particles (skyrmions) was observed in a chiral magnet MnSi.

Cited by 108 publications

References 29 publications

Weak Crystallization of Fluctuating Skyrmion Textures in MnSi

Weak Crystallization of Fluctuating Skyrmion Textures in MnSi

Current-induced viscoelastic topological unwinding of metastable skyrmion strings

Multiple low-temperature skyrmionic states in a bulk chiral magnet

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