126
S. Krishnia and W. S. Lew
4.2 Current-Induced Domain Wall Motion in Synthetic
Antiferromagnetic Wires
As discussed in the previous section, the exchange coupling torque can improve the
DWs velocities significantly in SAF wires which is also observed in experiments. In
this section, we will discuss how the SHE induced perturbation in antiferromagnetic
alignment of the DWs enhances the DW velocity in SAF wires.
Krishnia et al., have demonstrated high speed current-induced
DW motion in Pt/SAF/Ta wires[89]. An out-of-plane hysteresis
loop
of
a
SiO 2 /Ta(3)/Pt(3)/[Co(0.4)/Ni(0.7)/Co(0.4)]/Ru(0.8)/[Co(0.4)/
Ni(0.7)/Co(0.4)]/Ta(3) SAF thin film is shown in Fig. 20a. In the thin film
stack, lower (M L ) and upper (M U ) ferromagnetic layers are formed from Co/Ni/Co
trilayers and are coupled through ultrathin Ru spacer layer. Here, numbers in
parenthesis represent the layer thickness in ‘nm’. The magnetization loop of the
thin film exhibits double switching at 320 Oe and 6500 Oe. The hysteresis loop is
categorized into five regions. At zero magnetic field, the two ferromagnetic layers
are coupled in antiferromagnetic manner as marked by region I. Regions II and IV
show the flipping of a layer along the external magnetic field. In regions III and
V, the exchange coupling is broken and the two ferromagnetic layers are saturated
along the external magnetic fields. The magnetic field at which the exchange
coupling is broken and the two ferromagnetic layers are aligned in same direction, is
termed as exchange field (H ex ). The spin configurations of the two layers in all five
regions are represented by the blue and purple arrows. The double switching in the
hysteresis confirm the presence of RKKY antiferromagnetic coupling between the
Fig.
20 a
Out-of-plane
VSM
hysteresis
loop
measurement
of
the
Ta(3)/Pt(3)/[Co(0.4)/Ni(0.7)/Co(0.4)]/Ru(0.8)/[Co(0.4)/Ni(0.7)/Co(0.4)]/Ta(3) SAF stack. The
double switching behavior in the measurement indicates the antiferromagnetic coupling between
two magnetic trilayer Co(0.4)/Ni(0.7)/Co(0.4) structures. Each region drawn in the hysteresis loop
is illustrated with the relevant magnetic configurations of the SAF structure b In-plane (black) and
out-of-plane (red) hysteresis loops of the SAF stack[89]
S. Krishnia and W. S. Lew
4.2 Current-Induced Domain Wall Motion in Synthetic
Antiferromagnetic Wires
As discussed in the previous section, the exchange coupling torque can improve the
DWs velocities significantly in SAF wires which is also observed in experiments. In
this section, we will discuss how the SHE induced perturbation in antiferromagnetic
alignment of the DWs enhances the DW velocity in SAF wires.
Krishnia et al., have demonstrated high speed current-induced
DW motion in Pt/SAF/Ta wires[89]. An out-of-plane hysteresis
loop
of
a
SiO 2 /Ta(3)/Pt(3)/[Co(0.4)/Ni(0.7)/Co(0.4)]/Ru(0.8)/[Co(0.4)/
Ni(0.7)/Co(0.4)]/Ta(3) SAF thin film is shown in Fig. 20a. In the thin film
stack, lower (M L ) and upper (M U ) ferromagnetic layers are formed from Co/Ni/Co
trilayers and are coupled through ultrathin Ru spacer layer. Here, numbers in
parenthesis represent the layer thickness in ‘nm’. The magnetization loop of the
thin film exhibits double switching at 320 Oe and 6500 Oe. The hysteresis loop is
categorized into five regions. At zero magnetic field, the two ferromagnetic layers
are coupled in antiferromagnetic manner as marked by region I. Regions II and IV
show the flipping of a layer along the external magnetic field. In regions III and
V, the exchange coupling is broken and the two ferromagnetic layers are saturated
along the external magnetic fields. The magnetic field at which the exchange
coupling is broken and the two ferromagnetic layers are aligned in same direction, is
termed as exchange field (H ex ). The spin configurations of the two layers in all five
regions are represented by the blue and purple arrows. The double switching in the
hysteresis confirm the presence of RKKY antiferromagnetic coupling between the
Fig.
20 a
Out-of-plane
VSM
hysteresis
loop
measurement
of
the
Ta(3)/Pt(3)/[Co(0.4)/Ni(0.7)/Co(0.4)]/Ru(0.8)/[Co(0.4)/Ni(0.7)/Co(0.4)]/Ta(3) SAF stack. The
double switching behavior in the measurement indicates the antiferromagnetic coupling between
two magnetic trilayer Co(0.4)/Ni(0.7)/Co(0.4) structures. Each region drawn in the hysteresis loop
is illustrated with the relevant magnetic configurations of the SAF structure b In-plane (black) and
out-of-plane (red) hysteresis loops of the SAF stack[89]