Chiral Magnetic Domain Wall and Skyrmion Memory Devices
197
the current. Analogous to the Hall effect in electrons. To actually use the skyrmions
for applications there are still a number of open questions that need to be answered.
5 Conclusion
In the chapter, we have reviewed the fundamental physics for designing magnetic
domain wall memories, especially domain wall racetrack memories. An overview of
how the racetrack has been functionally improved and how the fundamental physics
behind the operating mechanism has developed is shown. Material wise, the design
of the racetrack has changed from using in-plane magnetic materials to out-of-plane
magnetic materials. In the process of changing the material design resulted in new
physics such as the SOTs and the DMI which resulted in domain wall motion with
higher efficiency, and stability. The SOTs are the main mechanism in moving the
domain walls efficiently by utilizing the SHE and the ISGE which have shown to
be more efficient than the STT in current induced domain wall motion. The exact
physics behind this SOT is still not well known, but it was well demonstrated that
the SOT shows higher efficiency for DW motion. However, this SOT requires the
DMI in order to act on the DWs. The DMI generates a certain chirality to the domain
walls, especially forcing a Néel type DW. The Néel DW is required for the SOT to
act as a driving force of the DWs. The different sections of the chapter have reviewed
different physics and evidence of the SOT and DMI with the different experimental
methods to quantify the SOT and DMI shown. Furthermore, as an outlook for the
racetrack memory, we have reviewed the new exciting skyrmion racetrack memory
which will be a possible future implementation of the racetrack memory.
References
1. S.S.P. Parkin, M. Hayashi, L. Thomas, Magnetic domain-wall racetrack memory. Science 320,
190 (2008)
2. J.-S. Kim et al., Synchronous precessional motion of multiple domain walls in a ferromagnetic
nanowire by perpendicular field pulses. Nat. Commun. 5, 3429 (2014)
3. T.A. Moore et al., Magnetic-field-induced domain-wall motion in permalloy nanowires with
modified Gilbert damping. Phys. Rev. B 82, 094445 (2010)
4. J.C. Slonczewski, Current-driven excitation of magnetic multilayers. J. Magn. Magn. Mater.
159, L1–L7 (1996)
5. L. Berger, Emission of spin waves by a magnetic multilayer traversed by a current. Phys. Rev.
B 54, 9353–9358 (1996)
6. A. Brataas, A.D. Kent, H. Ohno, Current-induced torques in magnetic materials. Nat. Mater.
11, 372–381 (2012)
7. M. Hayashi et al., Current driven domain wall velocities exceeding the spin angular momentum
transfer rate in permalloy nanowires. Phys. Rev. Lett. 98, 037204 (2007)
8. A. Thiaville, Y. Nakatani, J. Miltat, Y. Suzuki, Micromagnetic understanding of current-driven
domain wall motion in patterned nanowires. EPL 69, 990 (2005)
197
the current. Analogous to the Hall effect in electrons. To actually use the skyrmions
for applications there are still a number of open questions that need to be answered.
5 Conclusion
In the chapter, we have reviewed the fundamental physics for designing magnetic
domain wall memories, especially domain wall racetrack memories. An overview of
how the racetrack has been functionally improved and how the fundamental physics
behind the operating mechanism has developed is shown. Material wise, the design
of the racetrack has changed from using in-plane magnetic materials to out-of-plane
magnetic materials. In the process of changing the material design resulted in new
physics such as the SOTs and the DMI which resulted in domain wall motion with
higher efficiency, and stability. The SOTs are the main mechanism in moving the
domain walls efficiently by utilizing the SHE and the ISGE which have shown to
be more efficient than the STT in current induced domain wall motion. The exact
physics behind this SOT is still not well known, but it was well demonstrated that
the SOT shows higher efficiency for DW motion. However, this SOT requires the
DMI in order to act on the DWs. The DMI generates a certain chirality to the domain
walls, especially forcing a Néel type DW. The Néel DW is required for the SOT to
act as a driving force of the DWs. The different sections of the chapter have reviewed
different physics and evidence of the SOT and DMI with the different experimental
methods to quantify the SOT and DMI shown. Furthermore, as an outlook for the
racetrack memory, we have reviewed the new exciting skyrmion racetrack memory
which will be a possible future implementation of the racetrack memory.
References
1. S.S.P. Parkin, M. Hayashi, L. Thomas, Magnetic domain-wall racetrack memory. Science 320,
190 (2008)
2. J.-S. Kim et al., Synchronous precessional motion of multiple domain walls in a ferromagnetic
nanowire by perpendicular field pulses. Nat. Commun. 5, 3429 (2014)
3. T.A. Moore et al., Magnetic-field-induced domain-wall motion in permalloy nanowires with
modified Gilbert damping. Phys. Rev. B 82, 094445 (2010)
4. J.C. Slonczewski, Current-driven excitation of magnetic multilayers. J. Magn. Magn. Mater.
159, L1–L7 (1996)
5. L. Berger, Emission of spin waves by a magnetic multilayer traversed by a current. Phys. Rev.
B 54, 9353–9358 (1996)
6. A. Brataas, A.D. Kent, H. Ohno, Current-induced torques in magnetic materials. Nat. Mater.
11, 372–381 (2012)
7. M. Hayashi et al., Current driven domain wall velocities exceeding the spin angular momentum
transfer rate in permalloy nanowires. Phys. Rev. Lett. 98, 037204 (2007)
8. A. Thiaville, Y. Nakatani, J. Miltat, Y. Suzuki, Micromagnetic understanding of current-driven
domain wall motion in patterned nanowires. EPL 69, 990 (2005)