Current-Driven Domain Wall Dynamics in Magnetic …
107
× 10
–8 Torr. The hard-axis anisotropy (H K ) of the thin film was 5 kOe. The width
of the devices was kept 300 nm. The current pulses were injected through electrode
(Ta(5 nm) /Cu (100 nm) /Au(10 nm)) to generate a local Oersted field. The successful
nucleation of the DW was detected using anomalous Hall effect (AHE) by measuring
Hall resistance (R Hall ) between the two electrodes. The Hall resistance is the proxy of
perpendicular magnetization which is measured using a constant 50 μA bias current.
The separation between electrodes A (B) and the Hall probe was kept 3.8 μm.
Figure 3b shows the normalized R Hall measured by sweeping an external magnetic
field along the out-of-plane direction. As per our convention, the normalized R Hall =
1 (0) corresponds to field saturation along the z-direction (−z-direction). A square
hysteresis loop was observed, indicating a perpendicular easy axis of magnetization
of the nanowire with a coercivity of 1 kOe [22].
In order to inject a DW, the nanowire is first saturated by applying a large global
out-of-plane external field along z-direction, followed by application of a current
pulse (85 mA, 50 ns) to electrode A. The local Oersted field generated by the pulsed
current, nucleates a DW underneath the electrode A. The DW is then driven by an
external magnetic field. Figure 3c shows the plot of normalized R Hall obtained by
sweeping an external magnetic field along +z direction. Two steps are observed in
R Hall as the field was gradually ramped. The first step at 60 Oe corresponds to DW
propagation and pinning at the junction between the Hall probe and the nanowire.
The second step at 100 Oe corresponds to DW depinning at the Hall probe and
propagation through the nanowire and the Hall probe [22]. This method is commonly
used to inject the DW locally.
2.2 In-line Domain Wall Injection
The second method is through in-line injection, which does not require an additional
DW injector line to the device. Here, DW is injected to the nanowire using the same
contact pad used to drive the DW. There are several variations to this method. In one
variation, Sethi et al., have shown the DWs nucleation at a cross-bar structure within
the nanowire [22]. The demagnetization energy at the Hall structure is higher which
increases the susceptibility of the local magnetization and thus makes it possible for
DWs to be nucleated. An SEM image of the devices is shown in Fig. 3a. The device
parameters are discussed in the previous Sect. 2.1.
A pulsed current was applied between electrodes A & B to study the effect of
in-plane current on the domain wall nucleation process. The nanowire was initially
saturated in −z direction and electrical pulses of density varying from 6.8 × 10
11 to
1.52 × 10
12 A/m
2 were injected through electrodes A & B. The pulse width was kept
constant at 50 ns. Subsequently, the external magnetic field was gradually increased
in the +z-direction and change in the R Hall was measured as shown in Fig. 4a. The
change in R Hall was not observed for low current densities, indicating no change in
the magnetization of the nanowire. However, the R Hall was dropped at external field
strength of 150 Oe when a pulse of current density 8.7 × 10
11 A/m
2 , was injected
107
× 10
–8 Torr. The hard-axis anisotropy (H K ) of the thin film was 5 kOe. The width
of the devices was kept 300 nm. The current pulses were injected through electrode
(Ta(5 nm) /Cu (100 nm) /Au(10 nm)) to generate a local Oersted field. The successful
nucleation of the DW was detected using anomalous Hall effect (AHE) by measuring
Hall resistance (R Hall ) between the two electrodes. The Hall resistance is the proxy of
perpendicular magnetization which is measured using a constant 50 μA bias current.
The separation between electrodes A (B) and the Hall probe was kept 3.8 μm.
Figure 3b shows the normalized R Hall measured by sweeping an external magnetic
field along the out-of-plane direction. As per our convention, the normalized R Hall =
1 (0) corresponds to field saturation along the z-direction (−z-direction). A square
hysteresis loop was observed, indicating a perpendicular easy axis of magnetization
of the nanowire with a coercivity of 1 kOe [22].
In order to inject a DW, the nanowire is first saturated by applying a large global
out-of-plane external field along z-direction, followed by application of a current
pulse (85 mA, 50 ns) to electrode A. The local Oersted field generated by the pulsed
current, nucleates a DW underneath the electrode A. The DW is then driven by an
external magnetic field. Figure 3c shows the plot of normalized R Hall obtained by
sweeping an external magnetic field along +z direction. Two steps are observed in
R Hall as the field was gradually ramped. The first step at 60 Oe corresponds to DW
propagation and pinning at the junction between the Hall probe and the nanowire.
The second step at 100 Oe corresponds to DW depinning at the Hall probe and
propagation through the nanowire and the Hall probe [22]. This method is commonly
used to inject the DW locally.
2.2 In-line Domain Wall Injection
The second method is through in-line injection, which does not require an additional
DW injector line to the device. Here, DW is injected to the nanowire using the same
contact pad used to drive the DW. There are several variations to this method. In one
variation, Sethi et al., have shown the DWs nucleation at a cross-bar structure within
the nanowire [22]. The demagnetization energy at the Hall structure is higher which
increases the susceptibility of the local magnetization and thus makes it possible for
DWs to be nucleated. An SEM image of the devices is shown in Fig. 3a. The device
parameters are discussed in the previous Sect. 2.1.
A pulsed current was applied between electrodes A & B to study the effect of
in-plane current on the domain wall nucleation process. The nanowire was initially
saturated in −z direction and electrical pulses of density varying from 6.8 × 10
11 to
1.52 × 10
12 A/m
2 were injected through electrodes A & B. The pulse width was kept
constant at 50 ns. Subsequently, the external magnetic field was gradually increased
in the +z-direction and change in the R Hall was measured as shown in Fig. 4a. The
change in R Hall was not observed for low current densities, indicating no change in
the magnetization of the nanowire. However, the R Hall was dropped at external field
strength of 150 Oe when a pulse of current density 8.7 × 10
11 A/m
2 , was injected