154
6 Magnetic Domain Wall Motion
notches. Initially, the resistance has been found to increase at lower applied magnetic
fields, which corresponds to the magnetization reversal of the thin NiFe (5 nm) layer.
Successively, the decrease in resistance observed at higher value of H ext is correlated
with the magnetization reversal of the thick NiFe (20 nm) layer. It should be noted
that our experimental signature does not evidence the pinning of magnetic domain
wall at any of the asymmetric notches during the magnetization reversals of NiFe
(20 nm) layer because the associated nucleation field is quite large.
In an attempt to nucleate a domain wall in the NiFe (20 nm) layer at smaller value
of applied H ext and further to pin the domain wall at the notch, one can adopt the
technique of producing a pulsed local magnetic field at the terminal of this magnetic
wire. Figure 6.9ii presents the consequence of the injection of domain wall into
the NiFe (20 nm) layer of the magnetic wire by H L . This means that in addition to
the normal measurements, a pulsed magnetic field H L is applied at the left end of the
magnetic wire. The magnitude and the duration of the pulsed H L were 200 Oe and
100 ns, respectively. As a result of the application of this pulsed magnetic field H L ,
the resistance has been found to abruptly decrease and remained at an intermediate
value between the largest and the smallest values, as shown in Fig. 6.9ii. This, in turn,
implies that a magnetic domain wall, injected from the left end of the magnetic wire
by the application of H L , gets pinned at the first notch as shown in Fig. 6.9ii. Followed
by the injection of the magnetic domain wall, further increase in H ext results in an
abrupt decrease in resistance to the smallest value. This suggests that the propagation
of domain wall takes place to the right end of the wire overcoming the asymmetric
notches. From this experimental feature, the depinning field for the magnetic domain
wall motion in case of its rightward propagation can be determined.
Figure 6.9iii exhibits magnetoresistance measurement when the magnetic domain
wall is injected from the right end of the wire by the application of H R . Similar
to Fig. 6.9ii, in this case also the depinning field corresponding to the leftward
propagation of the magnetic domain wall can be determined from the experimental
result as shown in Fig. 6.9iii. As is evident from Fig. 6.9ii, iii, the depinning field
corresponding to the leftward propagation is found to be much lower than the rightward propagation. In this scenario, an AC magnetic field, having amplitude lying
between those two depinning fields for both propagation directions, is applied. Quite
expectedly, this induces a unidirectional domain wall motion, which is referred to as
‘magnetic ratchet effect’. More detailed description about the ratchet effect has been
given in Himeno et al. (2005b).
6.4 Domain Wall Motion Velocity Measurements
Another important parameter that is essential to know for the implementation of
magnetic domain wall motion in spintronics is the velocity of domain wall motion. In
fact, experimental research on magnetic domain wall motion and its possible implementation for technological applications has been going on intensively (Sixtus and
Tonks 1931; Ono et al. 1999; Yamaguchi et al. 2004; Allwood et al. 2002; Versluijs
6 Magnetic Domain Wall Motion
notches. Initially, the resistance has been found to increase at lower applied magnetic
fields, which corresponds to the magnetization reversal of the thin NiFe (5 nm) layer.
Successively, the decrease in resistance observed at higher value of H ext is correlated
with the magnetization reversal of the thick NiFe (20 nm) layer. It should be noted
that our experimental signature does not evidence the pinning of magnetic domain
wall at any of the asymmetric notches during the magnetization reversals of NiFe
(20 nm) layer because the associated nucleation field is quite large.
In an attempt to nucleate a domain wall in the NiFe (20 nm) layer at smaller value
of applied H ext and further to pin the domain wall at the notch, one can adopt the
technique of producing a pulsed local magnetic field at the terminal of this magnetic
wire. Figure 6.9ii presents the consequence of the injection of domain wall into
the NiFe (20 nm) layer of the magnetic wire by H L . This means that in addition to
the normal measurements, a pulsed magnetic field H L is applied at the left end of the
magnetic wire. The magnitude and the duration of the pulsed H L were 200 Oe and
100 ns, respectively. As a result of the application of this pulsed magnetic field H L ,
the resistance has been found to abruptly decrease and remained at an intermediate
value between the largest and the smallest values, as shown in Fig. 6.9ii. This, in turn,
implies that a magnetic domain wall, injected from the left end of the magnetic wire
by the application of H L , gets pinned at the first notch as shown in Fig. 6.9ii. Followed
by the injection of the magnetic domain wall, further increase in H ext results in an
abrupt decrease in resistance to the smallest value. This suggests that the propagation
of domain wall takes place to the right end of the wire overcoming the asymmetric
notches. From this experimental feature, the depinning field for the magnetic domain
wall motion in case of its rightward propagation can be determined.
Figure 6.9iii exhibits magnetoresistance measurement when the magnetic domain
wall is injected from the right end of the wire by the application of H R . Similar
to Fig. 6.9ii, in this case also the depinning field corresponding to the leftward
propagation of the magnetic domain wall can be determined from the experimental
result as shown in Fig. 6.9iii. As is evident from Fig. 6.9ii, iii, the depinning field
corresponding to the leftward propagation is found to be much lower than the rightward propagation. In this scenario, an AC magnetic field, having amplitude lying
between those two depinning fields for both propagation directions, is applied. Quite
expectedly, this induces a unidirectional domain wall motion, which is referred to as
‘magnetic ratchet effect’. More detailed description about the ratchet effect has been
given in Himeno et al. (2005b).
6.4 Domain Wall Motion Velocity Measurements
Another important parameter that is essential to know for the implementation of
magnetic domain wall motion in spintronics is the velocity of domain wall motion. In
fact, experimental research on magnetic domain wall motion and its possible implementation for technological applications has been going on intensively (Sixtus and
Tonks 1931; Ono et al. 1999; Yamaguchi et al. 2004; Allwood et al. 2002; Versluijs