Abstract
Magnetic skyrmions are topologically stable, vortex-like objects surrounded by chiral boundaries that separate a region of reversed magnetization from the surrounding magnetized material1,2,3. They are closely related to nanoscopic chiral magnetic domain walls, which could be used as memory and logic elements for conventional and neuromorphic computing applications that go beyond Moore’s law. Of particular interest is ‘racetrack memory’, which is composed of vertical magnetic nanowires, each accommodating of the order of 100 domain walls, and that shows promise as a solid state, non-volatile memory with exceptional capacity and performance4,5. Its performance is derived from the very high speeds (up to one kilometre per second) at which chiral domain walls can be moved with nanosecond current pulses in synthetic antiferromagnet racetracks. Because skyrmions are essentially composed of a pair of chiral domain walls closed in on themselves, but are, in principle, more stable to perturbations than the component domain walls themselves, they are attractive for use in spintronic applications, notably racetrack memory. Stabilization of skyrmions has generally been achieved in systems with broken inversion symmetry, in which the asymmetric Dzyaloshinskii–Moriya interaction modifies the uniform magnetic state to a swirling state6,7. Depending on the crystal symmetry, two distinct types of skyrmions have been observed experimentally, namely, Bloch7,8 and Néel skyrmions9. Here we present the experimental manifestation of another type of skyrmion—the magnetic antiskyrmion—in acentric tetragonal Heusler compounds with D2d crystal symmetry. Antiskyrmions are characterized by boundary walls that have alternating Bloch and Néel type as one traces around the boundary. A spiral magnetic ground-state, which propagates in the tetragonal basal plane, is transformed into an antiskyrmion lattice state under magnetic fields applied along the tetragonal axis over a wide range of temperatures. Direct imaging by Lorentz transmission electron microscopy shows field-stabilized antiskyrmion lattices and isolated antiskyrmions from 100 kelvin to well beyond room temperature, and zero-field metastable antiskyrmions at low temperatures. These results enlarge the family of magnetic skyrmions and pave the way to the engineering of complex bespoke designed skyrmionic structures.
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Acknowledgements
We thank H. Blumtritt and N. Schammelt for their help in preparing TEM lamellae for this study. This work was financially supported by the ERC Advanced Grant No. 670166 “SORBET” and the ERC Advanced Grant No. 291472 “Idea Heusler”.
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A.K.N., C.F. and S.S.P.P. conceived the original idea for the project. A.K.N. performed the LTEM investigations with the help of E.P. and P.W. The bulk materials were synthesized by V.K. and A.K.N. F.D. and R.S. carried out the neutron diffraction study. A.K.N. and R.S. performed the magnetic measurements. T.M. performed the micromagnetic and LTEM image simulations. A.K.N. and S.S.P.P. wrote the manuscript with substantial contributions from all authors.
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Extended data figures and tables
Extended Data Figure 1 Structural characterization at room temperature.
a, b, Room-temperature XRD pattern with Rietveld refinement for Mn1.4PtSn (a) and Mn1.4Pt0.9Pd0.1Sn (b). The circles represent the experimental data (IExpt), the red line corresponds to a simulation (Ical) and the blue line shows the difference (‘Diff’) between the two.
Extended Data Figure 2 Schematic representations of crystal and magnetic structures.
a, Crystal structure of Mn1.4Pt0.9Pd0.1Sn with different atoms shown in different colours. Pd randomly occupies the Pt position. b, Ferrimagnetic arrangement of Mn atoms from different magnetic sublattices. c, Effective ferromagnetic moment in the unit cell. d, Helical modification of the spin arrangement due to Dzyaloshinskii–Moriya interaction along [100] (upper panel) and spin cycloid along [110] (lower panel).
Extended Data Figure 3 Magnetization measurements versus temperature.
a, b, Field dependence of magnetization, M(H), for Mn1.4PtSn (a) and Mn1.4Pt0.9Pd0.1Sn (b) at various temperatures.
Extended Data Figure 4 LTEM measurements at various temperatures and magnetic fields.
a–f, Under-focused LTEM images of Mn1.4Pt0.9Pd0.1Sn taken at T = 200 K for different magnetic fields H parallel to [001]. g–i, Under-focused LTEM images of Mn1.4Pt0.9Pd0.1Sn taken at T = 368 K for different magnetic fields H parallel to [001].
Extended Data Figure 5 Zero-field metastable antiskyrmions.
Under-focused LTEM images of Mn1.4Pt0.9Pd0.1Sn taken at 100 K at zero magnetic field.
Extended Data Figure 6 Simulated magnetic configuration of skyrmions.
a–c, Magnetization configurations of Bloch skyrmions (a), antiskyrmions (b) and Néel skyrmions (c). The size of the image corresponds to 280 nm × 280 nm in each case. The arrows correspond to the local in-plane component of the magnetization and the colour represents the out-of-plane component of the magnetization.
Extended Data Figure 7 Simulated LTEM images versus defocus distance.
Simulated LTEM images of Bloch skyrmions, antiskyrmions and Néel skyrmions at various defocus distances Δz.
Extended Data Figure 8 Simulated antiskyrmion phase versus perpendicular magnetic field.
a–f, OOMMF simulation of the evolution of the antiskyrmion phase as a function of perpendicular field strength, with: a, Hz = 0.09 T, mixed helix and antiskyrmion phase; b, Hz = 0.15 T, mixed helix and antiskyrmion phase; c, Hz = 0.21 T, antskyrmion phase; d, Hz = 0.39 T, antikyrmion phase; e, Hz = 0.47 T, mixed antiskyrmion and spin-polarized phase; f, Hz = 0.50 T, spin-polarized phase.
Extended Data Figure 9 Simulated antiskyrmion phase versus tilt angle.
a–e, OOMMF simulations as a function of the tilting angle of the magnetic field (0.24 T) with respect to [110], of: a, 20°; b, 10°; c, 0°; d, −10°; e, −20°.
Extended Data Figure 10 Analysis of the antiskyrmion lattice.
Analysis of an LTEM image of the antiskyrmion lattice at 200 K under a perpendicular field of 0.23 T. The red circles show the position and size of the antiskyrmions. The green lines that connect nearby antiskyrmions indicate a hexagonal lattice. The blue numbers are the lengths of the green lines (in nanometres). The red numbers indicate the angles between the green lines (in degrees).
Extended Data Figure 11 Antiskyrmions size and lattice angle analysis.
a, Field dependence of the antiskyrmion size at various temperatures. The error bars represent the standard deviation of the size distribution. b, Field dependence of the mean angle of the antiskyrmion lattice at different temperatures. The inset shows the corresponding standard deviation of the lattice angles.
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Nayak, A., Kumar, V., Ma, T. et al. Magnetic antiskyrmions above room temperature in tetragonal Heusler materials. Nature 548, 561–566 (2017). https://doi.org/10.1038/nature23466
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DOI: https://doi.org/10.1038/nature23466
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