Chiral Magnetic Domain Wall and Skyrmion Memory Devices
191
Lo Conte et al. [51], have reported asymmetric current-induced DW’s velocity under
the in-plane bias field in Ta/CoFeB/MgO nanowires. In the measurement, the DMI
can be estimated by the field where DWs stop moving when a complete Bloch configuration is introduced by the in-plane field [64, 65]. However, the methods using DW
dynamics have been pointed out that it may entail some drawbacks quantifying the
interfacial DMI, as analysis can be hampered by possible additional effects [66, 67]
from spin structure deformations and/or chiral damping [68], in addition to DMI.
(see Fig. 11b).
3.2.3 Asymmetric Spin Wave Dispersion Relation
More recently, the measurements of the strength of the interfacial DMI via nonreciprocal spin waves’ behavior have also been performed by using Brillouin Light
Scattering (BLS) [69–72], time-resolved magneto-optical Kerr effect (TR-MOKE)
[73], spin-polarized electron energy loss spectroscopy (SPEELS) [74], etc [75]. For
the case of spin waves, the chirality of non-collinear spin configurations induced
by spin waves is determined by their propagation direction (or wave vectors). For
a Damon-Eshbach spin wave mode [76], where the wave vector is normal to the
surface of magnetic thin films as well as the magnetization direction (see Fig. 11c),
the total energy of two spin waves propagating into the opposite direction are differed
by the interfacial DMI—the cross products of two neighboring spins vector, S 1 ×
S 2 , for both spin waves are all on the axis of the interfacial DMI vector, D, but
antiparallel to each other. The main advantage of this technique for measuring the
interfacial DMI is that it does not require an exchange stiffness for quantifying the
strength of the interfacial DMI that is difficult to experimentally address in ultrathin
ferromagnetic films. However, the accuracy of this approach is also contested, as
asymmetric anisotropies at the two interfaces of the ferromagnetic materials (FM)
can lead to similar asymmetries of the dispersion relation, making the interpretation
for the DMI determination rather complex.
3.2.4 Other Measurement Techniques
Together with the techniques using spin dynamics, the measurements based on the
statics of magnetic entities have been also developed. For example, Woo et al. [77],
estimated the magnitude of the interfacial DMI by measuring the domain width
of labyrinth stripe domain structures that is determined by the competition among
micromagnetic energies, i.e., the interfacial DMI, magnetic anisotropic energy,
exchange energy, dipolar energy, and Zeeman energy. Very recently, asymmetric
magnetic hysteresis loop in laterally asymmetric structures under the application of
an in-plane field is demonstrated by Han et al. (see Fig. 11d) [78].
191
Lo Conte et al. [51], have reported asymmetric current-induced DW’s velocity under
the in-plane bias field in Ta/CoFeB/MgO nanowires. In the measurement, the DMI
can be estimated by the field where DWs stop moving when a complete Bloch configuration is introduced by the in-plane field [64, 65]. However, the methods using DW
dynamics have been pointed out that it may entail some drawbacks quantifying the
interfacial DMI, as analysis can be hampered by possible additional effects [66, 67]
from spin structure deformations and/or chiral damping [68], in addition to DMI.
(see Fig. 11b).
3.2.3 Asymmetric Spin Wave Dispersion Relation
More recently, the measurements of the strength of the interfacial DMI via nonreciprocal spin waves’ behavior have also been performed by using Brillouin Light
Scattering (BLS) [69–72], time-resolved magneto-optical Kerr effect (TR-MOKE)
[73], spin-polarized electron energy loss spectroscopy (SPEELS) [74], etc [75]. For
the case of spin waves, the chirality of non-collinear spin configurations induced
by spin waves is determined by their propagation direction (or wave vectors). For
a Damon-Eshbach spin wave mode [76], where the wave vector is normal to the
surface of magnetic thin films as well as the magnetization direction (see Fig. 11c),
the total energy of two spin waves propagating into the opposite direction are differed
by the interfacial DMI—the cross products of two neighboring spins vector, S 1 ×
S 2 , for both spin waves are all on the axis of the interfacial DMI vector, D, but
antiparallel to each other. The main advantage of this technique for measuring the
interfacial DMI is that it does not require an exchange stiffness for quantifying the
strength of the interfacial DMI that is difficult to experimentally address in ultrathin
ferromagnetic films. However, the accuracy of this approach is also contested, as
asymmetric anisotropies at the two interfaces of the ferromagnetic materials (FM)
can lead to similar asymmetries of the dispersion relation, making the interpretation
for the DMI determination rather complex.
3.2.4 Other Measurement Techniques
Together with the techniques using spin dynamics, the measurements based on the
statics of magnetic entities have been also developed. For example, Woo et al. [77],
estimated the magnitude of the interfacial DMI by measuring the domain width
of labyrinth stripe domain structures that is determined by the competition among
micromagnetic energies, i.e., the interfacial DMI, magnetic anisotropic energy,
exchange energy, dipolar energy, and Zeeman energy. Very recently, asymmetric
magnetic hysteresis loop in laterally asymmetric structures under the application of
an in-plane field is demonstrated by Han et al. (see Fig. 11d) [78].