180
K. Lee et al.
of nm [9]. In these DWs, a large width of the DWs limits the maximum information
storage density.
To alleviate the listed issues, a different class of magnetic materials that is with outof-plane magnetization has been investigated. In contrast to the relatively large DW
width in the in-plane magnetized thin films, the DW width in an out-of-plane magnetized film is much narrower (~10 nm) [10]. Therefore, the out-of-plane magnetized
films are considered to be technically more promising than the in-plane magnetized
counterparts. In this respect, DWs motion in the out-of-plane magnetized thin films
have been extensively studied, for example, in ferromagnetic multilayers, i.e., Co/Ni,
Co/Pt, and Co/Pd, where a strong perpendicular magnetic anisotropy is present at
their interfaces through spin–orbit coupling [7, 11].
However, experimental results of the current-driven DW motion in the out-ofplane magnetization have been controversial. In Co/Ni multilayers and TbFeCo
alloys with rather thick magnetic films (~> 1 nm), it has been commonly observed
that adiabatic STT plays an important role in DW motion [12], although the effect
of non-adiabatic STT remains controversial. For example, Koyama et al. [12] have
experimentally demonstrated that the current-induced DW velocity in Co/Ni multilayer is rather independent of DW pinning strength, indicating that the adiabatic STT
plays a more dominant role in driving DW motion rather than the non-adiabatic STT.
On the contrary, in ultrathin ferromagnetic multilayers (<1 nm) like Co/Pt multilayers, largely scattered experimental observations were reported. Ravelosona et al.
[13] show a significant reduction in the required field to depin a DW in a Co/Pt multilayer with increasing current density which suggests that the DW is dominated by
non-adiabatic STT. Furthermore, Boulle et al. [11] have reported a similar behavior
with a high efficiency, presumed to be a consequence of strong non-adiabatic STT.
However, note that other torques can lead to similar behavior as discussed in the
next section. In contrast to these reports, a few experimental studies reported that
the efficiency of STTs are very small in Co/Pt [14], and, even in some cases, the
DW motion was measured with the DW direction opposite the direction of the electron flow, which is controversial to the expected results from the conventional STT
[16, 20, 25]. These broad experimental discrepancies in similar magnetic multilayers
imply that a radically new mechanism to explain the current-induced DW motion in
these multilayers is required.
In this chapter, we will discuss the recent progress of chiral domain wall devices
[19]. In the first Sect. 2, we will give an overview of the new mechanisms that have
been revealed for the new phenomena leading to current-induced DW motion. A basic
theoretical introduction about the mechanism and some experimental verification will
be shown. In Sect. 3, we will discuss the chiral domain walls and the mechanism
behind the induced chirality in the DWs. A basic theoretical explanation and some
experimental results about the chiral DWs will be introduced. Furthermore, in the
last section, we will give a short outlook of memory devices beyond domain wall
memory devices based on skyrmions.
K. Lee et al.
of nm [9]. In these DWs, a large width of the DWs limits the maximum information
storage density.
To alleviate the listed issues, a different class of magnetic materials that is with outof-plane magnetization has been investigated. In contrast to the relatively large DW
width in the in-plane magnetized thin films, the DW width in an out-of-plane magnetized film is much narrower (~10 nm) [10]. Therefore, the out-of-plane magnetized
films are considered to be technically more promising than the in-plane magnetized
counterparts. In this respect, DWs motion in the out-of-plane magnetized thin films
have been extensively studied, for example, in ferromagnetic multilayers, i.e., Co/Ni,
Co/Pt, and Co/Pd, where a strong perpendicular magnetic anisotropy is present at
their interfaces through spin–orbit coupling [7, 11].
However, experimental results of the current-driven DW motion in the out-ofplane magnetization have been controversial. In Co/Ni multilayers and TbFeCo
alloys with rather thick magnetic films (~> 1 nm), it has been commonly observed
that adiabatic STT plays an important role in DW motion [12], although the effect
of non-adiabatic STT remains controversial. For example, Koyama et al. [12] have
experimentally demonstrated that the current-induced DW velocity in Co/Ni multilayer is rather independent of DW pinning strength, indicating that the adiabatic STT
plays a more dominant role in driving DW motion rather than the non-adiabatic STT.
On the contrary, in ultrathin ferromagnetic multilayers (<1 nm) like Co/Pt multilayers, largely scattered experimental observations were reported. Ravelosona et al.
[13] show a significant reduction in the required field to depin a DW in a Co/Pt multilayer with increasing current density which suggests that the DW is dominated by
non-adiabatic STT. Furthermore, Boulle et al. [11] have reported a similar behavior
with a high efficiency, presumed to be a consequence of strong non-adiabatic STT.
However, note that other torques can lead to similar behavior as discussed in the
next section. In contrast to these reports, a few experimental studies reported that
the efficiency of STTs are very small in Co/Pt [14], and, even in some cases, the
DW motion was measured with the DW direction opposite the direction of the electron flow, which is controversial to the expected results from the conventional STT
[16, 20, 25]. These broad experimental discrepancies in similar magnetic multilayers
imply that a radically new mechanism to explain the current-induced DW motion in
these multilayers is required.
In this chapter, we will discuss the recent progress of chiral domain wall devices
[19]. In the first Sect. 2, we will give an overview of the new mechanisms that have
been revealed for the new phenomena leading to current-induced DW motion. A basic
theoretical introduction about the mechanism and some experimental verification will
be shown. In Sect. 3, we will discuss the chiral domain walls and the mechanism
behind the induced chirality in the DWs. A basic theoretical explanation and some
experimental results about the chiral DWs will be introduced. Furthermore, in the
last section, we will give a short outlook of memory devices beyond domain wall
memory devices based on skyrmions.