Robust metastable skyrmions and their triangular–square lattice structural transition in a high-temperature chiral magnet

Abstract

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...

Skyrmion lattice structural transition in MnSi

“…Our results suggest that the triangular-to-square lattice transition is more general of the skyrmion systems and that the square SkL state has topological windings. Also, note that the length of the q vector in Co 8 Zn 8 Mn 4 strongly depends on temperature ( 26 ), whereas that in MnSi does not. This observation suggests that the temperature variation of q is not essential for the formation of the square SkL.…”

Direct evidence for cycloidal modulations in the thermal-fluctuation-stabilized spin spiral and skyrmion states of GaV4S8

“…Moreover, the observations of metastable skyrmions [4,5] and their current-driven dynamics [6,7] inspires new ideas for using skyrmions in data storage and logic devices [8][9][10].…”

Reorientations, relaxations, metastabilities, and multidomains of skyrmion lattices

“…These skyrmion lattices (SkLs) were first identified in cubic chiral magnets by small-angle neutron scattering (SANS) inside the A phase, which is a pocket in the magneticfield-temperature (B-T ) phase diagram just below the critical temperature T C . Soon after their first observation in MnSi [1], skyrmion lattices were found in other cubic chiral magnets, including Fe 1−x Co x Si [8], FeGe [9], Cu 2 OSeO 3 [10][11][12], and Co-Zn-Mn alloys [13,14]. More recently, skyrmions and their lattices have been observed in polar magnets [15], in thin films [16,17], and at surfaces and interfaces of different atomic layers [18].…”