Submicrometer Ferromagnetic NOT Gate and Shift Register

1
Submicrometer Ferromagnetic NOT Gate and Shift
Register

• D.A. Allwood, Gang Xiong, M.D. Cooke, C.C.
  Faulkner, D. Atkinson, N. Vernier, R.P Cowburn
• Science, Vol. 296 Issue 5575 14 June 2002

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Domain Wall Review

• Due to shape anisotropy magnetization in
  nanowires lies along the wire axis
• Where two opposite magnetizations meet a domain
  wall is formed
• Application of a magnetic field parallel to the
  wire can cause motion of the domain wall (T. Ono,
  et al., Science 284, 468 1999)

H
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Domain Wall Motion
? Can we propagate around corners ?
? We can use a circularly polarized magnetic
field
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Schematic of NOT Gate
? Boolean logic states can be defined by whether
or not magnetization arrows point towards or
away from each other
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NOT Gate Fabrication
? Structures consist of 5nm thick Permalloy
films thermally evaporated on silicon
? Focused ion beam milling was used to
create nanowire junction
? Radius of curvature of bend is 1um
? Wire size is 200nm thick
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Measurement of Device
? Magneto-optical Kerr effect was used to
measure the performance of the device
? Input signal of trace I is opposite that
of the output signal in trace II with a
slight delay
? Therefore device works as a NOT Gate
? 1.5 field cycles corresponds to change
magnetization
? ½ cycle for the NOT junction and 1 cycle
for the feedback loop
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Three NOT Junction Device
? From the same measurement point two
different MOKE traces were obtained
? In trace I the gates were initialized by a
150 Oe magnetic pulse followed by a
sinusoidal field of reducing magnitude to
remove adjacent domain walls
? Trace I behaves very much like that of the
1 junction NOT gate because only one domain
wall is present
? In trace II the second demagnetizing field
was not applied
? Consequently, three domain wall are present
so we see six transitions per magnetization
cycle
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Shift Register
? Fully functional logic architecture must
be capable of processing data in two
direction
? By reflecting the junction along the
horizontal axis the chirality of the gate is
switched allowing data transfer in two
directions
? Structure has 11 NOT Gates, 6 shift data
in one direction and 5 in the other direction
? Structure switches with a period of 13
field cycles corresponding to a 13 bit shift
register (1 bit per NOT gate and 2 bits for
the feedback loop)
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Device Performance

• Operational frequency of devices was only
  demonstrated up to 27 Hz due to iron core magnet
  used to switch fields
• Frequency limitations should be limited only by
  domain wall velocity
• For domain wall mobility of 30m/s an operating
  frequency of 200 MHz should be achievable
• In subsequent work this group has demonstrated
  domain wall velocity of 1500m/s (Nature
  Materials Vol.2 Feb 2003)
• Further improvements can be made by reducing the
  size of the gate
• Energy of the NOT gate is estimated to be
  1400KT per transitions so thermal issues should
  not be a problem

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Characterization of Device
? In subsequent work it was determined that
three modes of operation exist
? Normal Operation One transition every 3/2
field cycles for one wall or two transitions
every cycle for three wall operation
? Nucleation By applying a strong magnetic
field two transitions per field cycle are
observed and no information is processed. New
domain walls are created. Equivalent to
reverse breakdown in an electronic device.
? Dephasing Reducing the field down to lower
values and domain walls can be annihilated
by other domain walls in the system
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Modes of Operation
? Normal Mode
? Nucleation
? Three wall operation
? Dephasing
? Field Cycles
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Nanomagnetic Logic
? Three further logic gates are required to make
a fully universal logic architecture
? A two input logic gate that performs AND, OR,
or XOR function
? A fan out structure that converts one domain
wall into two domain walls
? A structure that allows magnetic tracks to
cross over each other
? It is also necessary to read electronic
signals into and out of the logic system
? Signals can be fed into the device by a
current carrying stripline
? Data can be read out by incorporating spin
valves or spin tunnel junctions into
magnetic tracks or by measuring domain wall
resistance
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Conclusions

• Ferromagnetic NOT Gate and Shift Register were
  demonstrated
• Performance limited by domain wall velocity
• Device was determined to have three modes of
  operation
• Future nanomagnetic logic system will need
  addition of three more logic components as well
  as have the capability of interfacing with
  electronic signals