Microwave Oscillators and Detectors Based …
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of the nanopillar structure and also the in-homogeneous magnetization that arises
from the serrated edges of the nanopillars, which can lead to incoherent precession
of magnetization. To combine the low linewidths of nanocontact structures and high
output power of MTJs, a hybrid nanocontact structure has been proposed [77, 78].
A schematic of this type of hybrid device is shown in Fig. 7a, b. In these type of
devices, two modifications were implemented in order to force the current to go
through the insulating barrier. First, the MTJ cap layer is thinned down to minimize
the lateral current shunting as can be seen in Fig. 7b. Second, a MgO layer with a
low-resistance area (RA) product of 1.5 μm
2 was used to promote the tunneling
through the insulating barrier. As will be seen in the following, these devices show
a rich magneto-static and magneto-dynamic behaviour.
5.1 Magnetostatic Properties of Nanocontact MTJs
The complete layer sequence of the MTJ stack used for the study is: Ta(3)/CuN(30)/
Ta(5)/PtMn(20)/CoFe 30 (2)/Ru(0.85)/CoFe 40 B 20 (2)/CoFe 30 (0.5)/MgO/CoFe 30 (0.5)/
CoFe 40 B 20 (1.5)/Ta(3)/Ru(7), with thicknesses in nm. This stack has an in-plane easy
axis. Figure 7c shows the hysteresis loop of the above-mentioned stack with the magnetic field applied along the in-plane easy axis while the corresponding resistance of
an MTJ-STNO with a nominal diameter of d NC = 150 nm, is shown in Fig. 7d. The
MTJ-STNO device displays a 36% magnetoresistance and the field dependence of
the MTJ-STNO device and the unpatterned film stack shows a very good agreement.
This indicates that during the processing of the MTJ stack there was minimal damage
to the film and more importantly, the free layer (FL) has remained intact. In Fig. 7c
the magnetic state of some of the key points in the hysteresis loop is demonstrated.
Decreasing the magnetic field from the fully saturated state (1), causes the reference
layer (RL) to gradually become antiparallel to FL. This is manifested with a substantial increase in the resistance (R) of the MTJ-STNO, shown in Fig. 7d. Decreasing
the field even more from state 2 → 3, the FL and RL become parallel again and as
a result, R decreases as well. Further decreasing the field from 3 → 4, the pinned
layer (PL) slowly switches to be parallel to the RL, working against a strong antiferromagnetic coupling (AFC). Going from 5 → 6 → 1, The FL switches at low field
and RL switches at high fields to align with the applied field.
5.2 Magnetodynamic Properties of Nanocontact MTJs
To study the magnetodynamical behaviour of MTJ-STNOs, their power spectral
density was extracted as a function of magnetic field strength applied at θ ex = 85
◦
for different drive currents. The results of such measurement are shown in Fig. 8.
At low currents (I dc = −5 & −6 mA), the strongest mode that can be detected is the
spin wave bullet [16, 76, 80], with a much lower frequency than the ferromagnetic
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of the nanopillar structure and also the in-homogeneous magnetization that arises
from the serrated edges of the nanopillars, which can lead to incoherent precession
of magnetization. To combine the low linewidths of nanocontact structures and high
output power of MTJs, a hybrid nanocontact structure has been proposed [77, 78].
A schematic of this type of hybrid device is shown in Fig. 7a, b. In these type of
devices, two modifications were implemented in order to force the current to go
through the insulating barrier. First, the MTJ cap layer is thinned down to minimize
the lateral current shunting as can be seen in Fig. 7b. Second, a MgO layer with a
low-resistance area (RA) product of 1.5 μm
2 was used to promote the tunneling
through the insulating barrier. As will be seen in the following, these devices show
a rich magneto-static and magneto-dynamic behaviour.
5.1 Magnetostatic Properties of Nanocontact MTJs
The complete layer sequence of the MTJ stack used for the study is: Ta(3)/CuN(30)/
Ta(5)/PtMn(20)/CoFe 30 (2)/Ru(0.85)/CoFe 40 B 20 (2)/CoFe 30 (0.5)/MgO/CoFe 30 (0.5)/
CoFe 40 B 20 (1.5)/Ta(3)/Ru(7), with thicknesses in nm. This stack has an in-plane easy
axis. Figure 7c shows the hysteresis loop of the above-mentioned stack with the magnetic field applied along the in-plane easy axis while the corresponding resistance of
an MTJ-STNO with a nominal diameter of d NC = 150 nm, is shown in Fig. 7d. The
MTJ-STNO device displays a 36% magnetoresistance and the field dependence of
the MTJ-STNO device and the unpatterned film stack shows a very good agreement.
This indicates that during the processing of the MTJ stack there was minimal damage
to the film and more importantly, the free layer (FL) has remained intact. In Fig. 7c
the magnetic state of some of the key points in the hysteresis loop is demonstrated.
Decreasing the magnetic field from the fully saturated state (1), causes the reference
layer (RL) to gradually become antiparallel to FL. This is manifested with a substantial increase in the resistance (R) of the MTJ-STNO, shown in Fig. 7d. Decreasing
the field even more from state 2 → 3, the FL and RL become parallel again and as
a result, R decreases as well. Further decreasing the field from 3 → 4, the pinned
layer (PL) slowly switches to be parallel to the RL, working against a strong antiferromagnetic coupling (AFC). Going from 5 → 6 → 1, The FL switches at low field
and RL switches at high fields to align with the applied field.
5.2 Magnetodynamic Properties of Nanocontact MTJs
To study the magnetodynamical behaviour of MTJ-STNOs, their power spectral
density was extracted as a function of magnetic field strength applied at θ ex = 85
◦
for different drive currents. The results of such measurement are shown in Fig. 8.
At low currents (I dc = −5 & −6 mA), the strongest mode that can be detected is the
spin wave bullet [16, 76, 80], with a much lower frequency than the ferromagnetic