192
K. Lee et al.
4 Beyond Domain Wall Memories: Skyrmion Race-Track
Memory Concepts
4.1 Skyrmions
Magnetic skyrmions are a particle-like chiral spin structure which is stabilized in
systems with DMI. Skyrmions have gathered significant interest most recently in the
spintronics community due to its exciting physics and possibility for applications
in memory and logic devices [77, 79–83]. The topologically protected skyrmions
are well known for the topological stability and predicted to have efficient current
induced motion. These properties predict that skyrmions are promising for future
applications. A skyrmion shows a unique spin structure where the center of the
skyrmion is up (down) and the edges are down (up) and depending on the continuous
spin structure the skyrmion is classified as a Néel type and a Bloch type skyrmion (see
Fig. 12a, b). Skyrmions have been observed in out-of-plane magnetized materials,
whether the out of plane magnetization was achieved via magnetic field or perpendicular magnetic anisotropy or a combination. Skyrmions are stabilized as a result of a
trade-off between the exchange and the magnetostatic energy, and the bulk or interfacial DMI. The Bloch skyrmions are predominantly found in bulk materials while the
Néel skyrmions are a characteristic of the interfacial DMI systems[20, 79, 84, 85].
The aforementioned bulk DMI is due to crystal inversion asymmetry and high spin–
orbit coupling in ferromagnetic alloys (see Fig. 12c), such as B20 materials [54, 55],
while the interfacial DMI has its origin in multilayers of a ferromagnet and a large
spin–orbit material, such as Pt or Ir. In the multilayer systems, a structural inversion
asymmetry exists, which is required for the DMI. In these skyrmion systems, there
have been many different proposals for device applications. Especially, skyrmion
racetrack memories which is analogous to the domain wall racetrack memory, the
idea is to use a single skyrmion as a bit (see Fig. 12d, e). Depending on the topological number, when the topological winding number Q = 1 the bit could be defined as
“1” and when Q = 0 the bit is “0”. For the skyrmion racetrack to function properly,
controlling the distance between the skyrmions is crucial, thus moving the skyrmions
synchronously is crucial. As an advanced version of the skyrnion racetrack memory,
multilane racetrack memory has been proposed [80] (See Fig. 12f). By micromagnetic simulations, it was shown that the by electrical currents the skyrmions could
change lanes. Furthermore, a skyrmion racetrack memory was proposed in synthetic
antiferromagnets where more efficient motion of skyrmions is predicted (Fig. 12g).
In order to use these skyrmions as a racetrack memory device, similar to the domain
wall racetrack memory the means to write, read, and shift the skyrmions is essential. In the following sections we will briefly give an overview of the studies in
skyrmions. We will specially emphasize on the skyrmions in multilayer systems for
these systems are directly applicable to the skyrmion racetrack memories.
K. Lee et al.
4 Beyond Domain Wall Memories: Skyrmion Race-Track
Memory Concepts
4.1 Skyrmions
Magnetic skyrmions are a particle-like chiral spin structure which is stabilized in
systems with DMI. Skyrmions have gathered significant interest most recently in the
spintronics community due to its exciting physics and possibility for applications
in memory and logic devices [77, 79–83]. The topologically protected skyrmions
are well known for the topological stability and predicted to have efficient current
induced motion. These properties predict that skyrmions are promising for future
applications. A skyrmion shows a unique spin structure where the center of the
skyrmion is up (down) and the edges are down (up) and depending on the continuous
spin structure the skyrmion is classified as a Néel type and a Bloch type skyrmion (see
Fig. 12a, b). Skyrmions have been observed in out-of-plane magnetized materials,
whether the out of plane magnetization was achieved via magnetic field or perpendicular magnetic anisotropy or a combination. Skyrmions are stabilized as a result of a
trade-off between the exchange and the magnetostatic energy, and the bulk or interfacial DMI. The Bloch skyrmions are predominantly found in bulk materials while the
Néel skyrmions are a characteristic of the interfacial DMI systems[20, 79, 84, 85].
The aforementioned bulk DMI is due to crystal inversion asymmetry and high spin–
orbit coupling in ferromagnetic alloys (see Fig. 12c), such as B20 materials [54, 55],
while the interfacial DMI has its origin in multilayers of a ferromagnet and a large
spin–orbit material, such as Pt or Ir. In the multilayer systems, a structural inversion
asymmetry exists, which is required for the DMI. In these skyrmion systems, there
have been many different proposals for device applications. Especially, skyrmion
racetrack memories which is analogous to the domain wall racetrack memory, the
idea is to use a single skyrmion as a bit (see Fig. 12d, e). Depending on the topological number, when the topological winding number Q = 1 the bit could be defined as
“1” and when Q = 0 the bit is “0”. For the skyrmion racetrack to function properly,
controlling the distance between the skyrmions is crucial, thus moving the skyrmions
synchronously is crucial. As an advanced version of the skyrnion racetrack memory,
multilane racetrack memory has been proposed [80] (See Fig. 12f). By micromagnetic simulations, it was shown that the by electrical currents the skyrmions could
change lanes. Furthermore, a skyrmion racetrack memory was proposed in synthetic
antiferromagnets where more efficient motion of skyrmions is predicted (Fig. 12g).
In order to use these skyrmions as a racetrack memory device, similar to the domain
wall racetrack memory the means to write, read, and shift the skyrmions is essential. In the following sections we will briefly give an overview of the studies in
skyrmions. We will specially emphasize on the skyrmions in multilayer systems for
these systems are directly applicable to the skyrmion racetrack memories.