Chiral Paramagnetic Skyrmion-like Phase in MnSi

Abstract

We present a comprehensive study of chiral fluctuations in the reference helimagnet MnSi by polarized neutron scattering and Neutron Spin Echo spectroscopy, which reveals the existence of a completely left-handed and dynamically disordered phase. This phase may be identified as a spontaneous skyrmion phase: it appears in a limited temperature range just above the helical transition TC and coexists with the helical phase at TC .Chirality is ubiquitous in nature and of fundamental importance both on the microscopic level and in our everyday life. The break of symmetry between right and left manifests itself in parity violation, governs biological structures such as DNA and can also be experienced in the organisation of our own body. In magnetism, chirality is evident in solitons [1], systems with geometric frustration [2] and metallic systems with noncentro-symmetric lattice structures, where the resulting anti-symmetric Dzyaloshinski-Moriya (DM) interactions [3,4] introduce a parity breaking term in the Hamiltonian [5]. The DM term has the form M × ( ∇ × M ) and is more than a perturbation giving rise to the peculiar canted magnetic arrangements found in high temperature superconductors [6] or the cycloid spin structures in multiferroics [7,8]. In the non-centrosymmetric weak itinerant-electron ferromagnet MnSi, DM induced chirality comes in close interplay with Fermi liquid behavior and quantum fluctuations [9]. The Hamiltonian of MnSi comprises three hierarchically ordered magnetic interaction terms with well separated energy scales [10], which allow to distinguish between different contributions. The strongest ferromagnetic exchange interaction aligns the spins, the weaker chiral Dzyaloshinski-Moriya (DM) term twists them into a helix and the weakest Anisotropic Exchange (AE) or crystal field term pins the helix propagation vector τ along the 111 crystallographic directions.The helical order appears below T C ≈ 29 K. It is a lefthanded helix with a period of ℓ ∼ 175Å (τ ≈ 0.036Å −1 ) and all magnetic moments perpendicular to the helix vector [11].In this letter we concentrate on the chiral correlated paramagnetic or spin liquid phase of MnSi just above T C , where intense diffuse neutron scattering spreads homogeneously over the surface of a sphere with radius τ . This unusual feature emerges as a ring on the two-dimensional small angle neutron scattering patterns and the rings reduce to half-moons if the beam is polarized. This is illustrated by figure 1, which reproduces spectra from [12]. Numerous theoretical studies were devoted to explain this phase invoking possibilities such as unpinned helical order [12,13] or condensation of chiral order parameters [14]. Recent local mean-field calculations assuming the hierarchical hamiltonian of MnSi show that the helical phase is preceded by a disordered phase with skyrmionlike short range order similar to the partial order in liquid crystals [15], which sets in at T C ′ ≈ T C + 1K (see supplementary information of [15]). Skyrmions are solutions of the non-linear f...

Confinement of chiral magnetic modulations in the precursor region of FeGe

“…In combination with experimental results on MnSi [37,41], Mn 1−y Co y Si [9], and Mn 1−z Fe z Si [42] obtained by different neutron scattering techniques this provides profound evidence for complex magnetic states in the precursor region of cubic helimagnets. Theoretically these phenomena can be understood by the nucleation of modulated magnetic states composed of confined chiral solitons as described in section 2.…”

“…The sense of the magnetization rotation is fixed due to the presence of the chiral DzyaloshinskiiMoriya (DM) interaction [20,21]. However, in their magnetic field−temperature, (H, T ), phase diagrams, as sketched in figure 1(a), numerous puzzling physical anomalies have been observed in a narrow temperature interval in the vicinity of T C [8,10,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42]. The origin of these "precursor anomalies" (hatched area in figure 1(a)) and notably the magnetic structure of the so-called A phase is a long-standing and intriguing problem in chiral magnetism.…”

“…The prediction of a possible formation of such magnetic vortices in non-centrosymmetric ferromagnets in general [43,44] and in cubic helimagnets in particular [45,46,47] have triggered an intensive quest for these exotic states [7,10,32,33,34,37,38,39,40,41,42,48,49]. Of particular interest in the experimental investigations is the A phase, a small area below T C reported for MnSi many years ago [24,25].…”

A Centrosymmetric Hexagonal Magnet with Superstable Biskyrmion Magnetic Nanodomains in a Wide Temperature Range of 100–340 K

“…including Refs. [1][2][3][4][5][6][7][8][9][10][11][12], [22], [29][30][31], [33]. The red solid squares indicate the experimental data based on the in-situ Lorentz TEM observations.…”

“…Such unique mangetic naondomains have been recently discovered in bulk chiral magnetic materials, such as MnSi [1][2][3][4] , FeGe [5,6] , FeCoSi [7] , Cu 2 OSeO 3 [8][9][10] , -Mn-type Co-Zn-Mn [11] , and also GaV 4 S 8[12] a polar magnet. The crystal structure of these materials is cubic and lack of centrosymmetry, leading to the existence of Dzyaloshinskii-Moriya (DM) interactions.…”