Magnetic sensors and

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Magnetic sensors and logic gates
Ling Zhou
EE698A
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Outline

• Anisotropic magnetoresistive sensors
• Giant magnetoresistive sensors
• Colossal magnetoresistive sensors
• Using magnetoresistive elements to build up logic
  gates
• Hall sensors and devices

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Conventional Vs. Magnetic sensing
The output of conventional sensors will directly
report desired parameters On the other hand,
magnetic sensor only indirectly detect these
parameters
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Magnetic sensor technology field ranges
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Anisotropic magnetoresistive (AMR) sensor
The theory of the AMR sensor is based on the
complex ferromagnetic process in a thin film
Magnetoresistance variation with angle between M
and I
AMR ratio for typical ferromagnetic materials at
room temperature is around 1-3
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AMR sensor circuit
Wheatstone bridge configuration is used to ensure
high sensitivity and good repeatability
Disadvantage of AMR sensor can only sense the
magnitude, but not the direction non-linear
output.
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AMR effect for small wire
Effect of bar width on magnetoresistance of
nanoscale nickel and cobalt bars J. Appl. Phys.
81(8) 1997
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Giant magnetoresistive (GMR) sensor
Two different ferromagnetic materials sandwiched
by a thin conduction layer
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GMR circuit technique

• GMR resistors can be configured as a Wheatstone
  bridge sensor. Two of which are active. Resistor
  is 2 µm wide, which makes the resistors sensitive
  only to the field along their long dimension.

• Due to their outstanding sensitivity, Wheatstone
  Bridge Circuits are very advantageous for the
  measurement of resistance, inductance, and
  capacitance.

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Obtaining parallel, antiparallel magnetic
alignment

• pinned sandwiches
• Consist of two magnetic layers, soft layer and
  hard layer
• Antiferromagnetic multilayers
• Consist of muliple repetitions of alternating
  magnetic and nonmagnetic layers
• The polarized conduction electrons cause
  antiferromagnetic coupling between magnetic
  layers
• Spin valves
• An additional layer of an antiferromagnetic
  material is provided on the top or bottom

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Antiferromagnetic multilayers
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Use GMR in hard drive read head
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Parameters for GMR sensor

• Magnetic layers 46 nm
• Conductor layer 35 nm in sandwich structure
• This thickness is critical in antiferromagnetic
  multilayer GMR sensors, typically 1.52 nm
• Switching field 34 KA/m (3550 Oe) for sandwich
  structure and 250 for multilayer structures

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Magnetic tunnel junction (MTJ)
Sandwiches of two ferromagnetic layers separated
by a very thin insulation layer as tunneling
barrier
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Use MR element as logic gates
Hc1ltHc2 , layer 1 is easier to be switched
Only IA and IB together can switch layer 1
For rotation of layer 2, an additional input line
IC is required
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AND gate
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OR gate and NAND, NOR gates
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Advantages of MR element

• A single MR element is sufficient to realize and
  store four basic logic functionalities.
  Integration density is increased.
• The output is non-volatile and repeatedly
  readable without refreshing, which reduces the
  heat evolution.
• Fast operation the switching of frequency of
  magnetic films can be pushed to several GHZ.
• Low power consumption.

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Colossal magnetoresistive (CMR) and extraordinary
magnetoresistive (EMR)

• Under certain conditions, mixed oxides undergo a
  semiconductor to matallic transition with the
  application of an external magnetic field.

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Hall sensor
The Hall voltage is generated by the effect of an
external magnetic field acting perpendicularly
to the direction of the current.
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Hybrid hall effect devices
An HHE device is a layered structure composed of
an input wire, ferromagnetic element, insulation
layers, and a conducting output channel.
Can be used as magnetic field sensor, storage
cell and logic gates
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Magnetic p-n junction