16 Antiferromagnetic Skyrmions and Bimerons
443
Magnus force is always nonzero and leads to the skyrmion Hall effect [29, 30]. Such
a skyrmion Hall effect may cause the skyrmion to annihilate at the sample edge,
which is detrimental for practical applications. For an antiferromagnetic skyrmion,
it consists of two ferromagnetic skyrmions (one on each sublattice) with opposite
topological charges for magnetization, so that the net Magnus force is zero and the
fast-moving skyrmion will not be destroyed by touching the sample edge.
In order to understand physics of antiferromagnetic textures and explore their
applications, it is necessary to discuss their magnetization dynamics. In Sect. 16.2,
we discuss the current-induced creation, motion and chaos of antiferromagnetic
skyrmions and bimerons. In Sect. 16.3, we review and discuss of the spin torque
nano-oscillators based on antiferromagnetic skyrmions. In Sect. 16.4, we focus on
the study of synthetic antiferromagnetic skyrmions driven by the spin current. The
following two parts, i.e., Sects. 16.5 and 16.6, mainly present and discuss the magnetization dynamics of antiferromagnetic skyrmions in the presence of spatially nonuniform magnetic anisotropy. In Sect. 16.5, we present the calculation results of
using a magnetic anisotropy gradient to drive antiferromagnetic skyrmions, where
the magnetic anisotropy gradient is applicable in both metals and insulators. Then,
in Sect. 16.6, the pinning and depinning processes of antiferromagnetic skyrmions
in a racetrack with a defect caused by the local variation of magnetic anisotropy are
studied. Finally, in Sect. 16.7, we make a summary of this chapter.
16.2 Current-Driven Creation, Motion, and Chaos
of Antiferromagnetic Skyrmions and Bimerons
In this section, we discuss the current-induced creation, motion and chaos of antiferromagnetic skyrmions and bimerons. Magnetic skyrmions are swirling spin textures
carrying an integer topological charge, and they can be stabilized in chiral magnets
with perpendicular magnetic anisotropy [11, 12]. Magnetic bimeron composed of
two merons can be regarded as a counterpart of magnetic skyrmion in systems
with in-plane magnetic anisotropy, which shares the merits of skyrmions, such as
nanoscale size, nontrivial spin structure and low depinning current [18, 19]. These
excellent characteristics make that both skyrmion and bimeron are promising candidates as information carriers in future spintronic devices. In practical applications,
the controllable creation and manipulation of skyrmion and bimeron are crucial. In
2016, Zhang et al. [31] reported the creation of an antiferromagnetic skyrmion by
applying a vertical spin current. In addition, their calculations show that using a
junction geometry, the antiferromagnetic skyrmion is generated from a domain wall
pair, as shown in Fig. 16.2. Similar to the skyrmion, an isolated bimeron is created
in antiferromagnetic films by applying a vertical spin current to reverse the local
magnetic moments, as reported by Shen et al. [19]. It is worth mentioning that in
addition to the spin current, the time-dependent magnetic field can also induce the
generation of an isolated antiferromagnetic skyrmion [32].
443
Magnus force is always nonzero and leads to the skyrmion Hall effect [29, 30]. Such
a skyrmion Hall effect may cause the skyrmion to annihilate at the sample edge,
which is detrimental for practical applications. For an antiferromagnetic skyrmion,
it consists of two ferromagnetic skyrmions (one on each sublattice) with opposite
topological charges for magnetization, so that the net Magnus force is zero and the
fast-moving skyrmion will not be destroyed by touching the sample edge.
In order to understand physics of antiferromagnetic textures and explore their
applications, it is necessary to discuss their magnetization dynamics. In Sect. 16.2,
we discuss the current-induced creation, motion and chaos of antiferromagnetic
skyrmions and bimerons. In Sect. 16.3, we review and discuss of the spin torque
nano-oscillators based on antiferromagnetic skyrmions. In Sect. 16.4, we focus on
the study of synthetic antiferromagnetic skyrmions driven by the spin current. The
following two parts, i.e., Sects. 16.5 and 16.6, mainly present and discuss the magnetization dynamics of antiferromagnetic skyrmions in the presence of spatially nonuniform magnetic anisotropy. In Sect. 16.5, we present the calculation results of
using a magnetic anisotropy gradient to drive antiferromagnetic skyrmions, where
the magnetic anisotropy gradient is applicable in both metals and insulators. Then,
in Sect. 16.6, the pinning and depinning processes of antiferromagnetic skyrmions
in a racetrack with a defect caused by the local variation of magnetic anisotropy are
studied. Finally, in Sect. 16.7, we make a summary of this chapter.
16.2 Current-Driven Creation, Motion, and Chaos
of Antiferromagnetic Skyrmions and Bimerons
In this section, we discuss the current-induced creation, motion and chaos of antiferromagnetic skyrmions and bimerons. Magnetic skyrmions are swirling spin textures
carrying an integer topological charge, and they can be stabilized in chiral magnets
with perpendicular magnetic anisotropy [11, 12]. Magnetic bimeron composed of
two merons can be regarded as a counterpart of magnetic skyrmion in systems
with in-plane magnetic anisotropy, which shares the merits of skyrmions, such as
nanoscale size, nontrivial spin structure and low depinning current [18, 19]. These
excellent characteristics make that both skyrmion and bimeron are promising candidates as information carriers in future spintronic devices. In practical applications,
the controllable creation and manipulation of skyrmion and bimeron are crucial. In
2016, Zhang et al. [31] reported the creation of an antiferromagnetic skyrmion by
applying a vertical spin current. In addition, their calculations show that using a
junction geometry, the antiferromagnetic skyrmion is generated from a domain wall
pair, as shown in Fig. 16.2. Similar to the skyrmion, an isolated bimeron is created
in antiferromagnetic films by applying a vertical spin current to reverse the local
magnetic moments, as reported by Shen et al. [19]. It is worth mentioning that in
addition to the spin current, the time-dependent magnetic field can also induce the
generation of an isolated antiferromagnetic skyrmion [32].