Chapter 6
Magnetic Domain Wall Motion
6.1 Introduction on Magnetic Domain Wall Motion
6.1.1 What Is Magnetic Domain Wall?
In nature, often we found ferromagnetic materials, i.e., 3d transition metals, Fe,
Co and Ni, in unmagnetized condition. In this demagnetized state, i.e., M = 0,
ferromagnetic materials are divided into a number of small regions. Each of those
small regions is magnetized to saturation and is called magnetic domains (Weiss
1907). More subtle point is that within a single magnetic domain magnetization is
saturated, but magnetizations of different magnetic domains are oriented in different
directions (Fig. 6.1). This implies that the magnetization orientations of neighbouring
domains are not parallel. Consequently, in this demagnetized or M = 0 state of
ferromagnetic materials, any two neighbouring domains are separated by a region
in which the direction of magnetic moments gradually changes from one direction
to another. This transition region of the orientation of magnetic moments is called
magnetic domain wall.
6.1.2 Why Do Domains Exist?
Figure 6.2 exhibits two different hypothetical configurations of domain. Figure 6.2a
represents the configuration where both the exchange and the anisotropy energy
are minimal. The reason behind minimal exchange energy is that all the magnetic
moments are aligning parallel in this configuration. On the other hand, minimal
anisotropy energy is attained because in this configuration, the magnetization axis is
an easy axis. But, as it is evident from Fig. 6.2a, numerous uncompensated magnetic
‘poles’ appear at the ferromagnetic sample surface that gives rise to magnetostatic
energy deriving from dipole–dipole interaction. The situation arises where at one
surface there is magnetic North Pole, whereas at another there is magnetic South
© Springer Nature Singapore Pte Ltd. 2021
P. Dey and J. N. Roy, Spintronics,
https://doi.org/10.1007/978-981-16-0069-2_6
145
Magnetic Domain Wall Motion
6.1 Introduction on Magnetic Domain Wall Motion
6.1.1 What Is Magnetic Domain Wall?
In nature, often we found ferromagnetic materials, i.e., 3d transition metals, Fe,
Co and Ni, in unmagnetized condition. In this demagnetized state, i.e., M = 0,
ferromagnetic materials are divided into a number of small regions. Each of those
small regions is magnetized to saturation and is called magnetic domains (Weiss
1907). More subtle point is that within a single magnetic domain magnetization is
saturated, but magnetizations of different magnetic domains are oriented in different
directions (Fig. 6.1). This implies that the magnetization orientations of neighbouring
domains are not parallel. Consequently, in this demagnetized or M = 0 state of
ferromagnetic materials, any two neighbouring domains are separated by a region
in which the direction of magnetic moments gradually changes from one direction
to another. This transition region of the orientation of magnetic moments is called
magnetic domain wall.
6.1.2 Why Do Domains Exist?
Figure 6.2 exhibits two different hypothetical configurations of domain. Figure 6.2a
represents the configuration where both the exchange and the anisotropy energy
are minimal. The reason behind minimal exchange energy is that all the magnetic
moments are aligning parallel in this configuration. On the other hand, minimal
anisotropy energy is attained because in this configuration, the magnetization axis is
an easy axis. But, as it is evident from Fig. 6.2a, numerous uncompensated magnetic
‘poles’ appear at the ferromagnetic sample surface that gives rise to magnetostatic
energy deriving from dipole–dipole interaction. The situation arises where at one
surface there is magnetic North Pole, whereas at another there is magnetic South
© Springer Nature Singapore Pte Ltd. 2021
P. Dey and J. N. Roy, Spintronics,
https://doi.org/10.1007/978-981-16-0069-2_6
145