3D Nanomagnetic Logic
287
Spin-Hall-effect or switching with multiferroic materials) could also be suited for
electrical input, but have not yet been in the scope of our work.
Regarding the electrical output, the Extraordinary-Hall effect (EHE) has been
investigated with a special sensing geometry [74–76]. In the so-called split-current
arrangement, three current contacts are needed to directly contact the output magnet
of a pNML circuit. The side of the magnetic strip is free for incorporating an ANC for
field-coupling to its next neighbor. An advantage is, that small volume magnets can
be sensed and the output signal is proportional to the magnetization, but the EHEeffect is very likely too small for industrial applications with high data-throughput.
Instead, MTJ or GMR sensors (see e.g. [77, 78]), either as remote stray-field sensors,
or integrated with the free layer as pNML output magnet, could be applied. The latter
concept, a path towards a GMR stack for pNML output sensing was envisioned in
[79] and experimentally demonstrated in [68, 80].
5.3 A New Sensing Concept: Split-Stack Magneto-Resistance
(MR) Device
As described above, there are highly optimized and industry-ripe sensors for MRAM
and hard-drive technology. In principle, one could apply those sensors for pNML
technology. However, there are operating conditions, that have to be addressed and
tailored for pNML integration. One could make e.g. use of the stray field emanating
from the pNML output magnet to flip the free-magnet (FM) of a GMR stack for
sensing but fabrication is complicated by putting complex GMR stacks as new devices
above the pNML output magnets. In our point of view, it is most efficient to tailor
a perpendicular GMR sensor where the free-layer of the magneto-resistance (MR)
device is at the same time the pNML output device and the permanent magnet is
stacked or buried below. Hence, summarizing the results of [68, 80], a sensor was
developed, which has a split structure as depicted in the cross-sectional views of
Fig. 19a–c.
First, a pNML stack is deposited—ending with an inert Pt-layer—followed by
hard-mask patterning for later Co/Pt island etching through a lift-off process. After
that, the device is coated with a Co/Cu/Co GMR interface (Fig. 19b), followed by a
Co/Pt stack forming the latter permanent magnet (PM). In a further step the second
hard-mask is deposited, overlapping with the buried first hardmask. As a last step
(Fig. 19c), the split-stack MR sensor is physically etched by Ar ion etching. The
ANC creation can take place in step (a) or after (c). The fabrication steps result in a
split-structure forming a standard pNML output device with ANC and domain-wall
conduit (left) and an MR-stack with Co/Cu/Co GMR interface. The pNML stack
serves as free-magnet (FM) and will be switched by a domain wall moving from the
ANC throughout the lower stack. In Fig. 19d an SEM image shows the fabricated
structure, where the hard-mask overlap and the ANC formation are clearly visible.
287
Spin-Hall-effect or switching with multiferroic materials) could also be suited for
electrical input, but have not yet been in the scope of our work.
Regarding the electrical output, the Extraordinary-Hall effect (EHE) has been
investigated with a special sensing geometry [74–76]. In the so-called split-current
arrangement, three current contacts are needed to directly contact the output magnet
of a pNML circuit. The side of the magnetic strip is free for incorporating an ANC for
field-coupling to its next neighbor. An advantage is, that small volume magnets can
be sensed and the output signal is proportional to the magnetization, but the EHEeffect is very likely too small for industrial applications with high data-throughput.
Instead, MTJ or GMR sensors (see e.g. [77, 78]), either as remote stray-field sensors,
or integrated with the free layer as pNML output magnet, could be applied. The latter
concept, a path towards a GMR stack for pNML output sensing was envisioned in
[79] and experimentally demonstrated in [68, 80].
5.3 A New Sensing Concept: Split-Stack Magneto-Resistance
(MR) Device
As described above, there are highly optimized and industry-ripe sensors for MRAM
and hard-drive technology. In principle, one could apply those sensors for pNML
technology. However, there are operating conditions, that have to be addressed and
tailored for pNML integration. One could make e.g. use of the stray field emanating
from the pNML output magnet to flip the free-magnet (FM) of a GMR stack for
sensing but fabrication is complicated by putting complex GMR stacks as new devices
above the pNML output magnets. In our point of view, it is most efficient to tailor
a perpendicular GMR sensor where the free-layer of the magneto-resistance (MR)
device is at the same time the pNML output device and the permanent magnet is
stacked or buried below. Hence, summarizing the results of [68, 80], a sensor was
developed, which has a split structure as depicted in the cross-sectional views of
Fig. 19a–c.
First, a pNML stack is deposited—ending with an inert Pt-layer—followed by
hard-mask patterning for later Co/Pt island etching through a lift-off process. After
that, the device is coated with a Co/Cu/Co GMR interface (Fig. 19b), followed by a
Co/Pt stack forming the latter permanent magnet (PM). In a further step the second
hard-mask is deposited, overlapping with the buried first hardmask. As a last step
(Fig. 19c), the split-stack MR sensor is physically etched by Ar ion etching. The
ANC creation can take place in step (a) or after (c). The fabrication steps result in a
split-structure forming a standard pNML output device with ANC and domain-wall
conduit (left) and an MR-stack with Co/Cu/Co GMR interface. The pNML stack
serves as free-magnet (FM) and will be switched by a domain wall moving from the
ANC throughout the lower stack. In Fig. 19d an SEM image shows the fabricated
structure, where the hard-mask overlap and the ANC formation are clearly visible.