Magnetic sensors and
logic gates
Ling Zhou
EE698A
EE698A Advanced Electron Devices
Outline
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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
Magnetization M
R=R┴+ΔRAMRcos2 θ
θ
Current I
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
Due to their outstanding
sensitivity, Wheatstone Bridge
Circuits are very advantageous
for the measurement of
resistance, inductance, and
capacitance.
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.
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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: 4~6 nm
• Conductor layer 3~5 nm in sandwich structure
– This thickness is critical in antiferromagnetic
multilayer GMR sensors, typically 1.5~2 nm
• Switching field 3~4 KA/m (35~50 Oe) for
sandwich structure and 250 for multilayer
structures
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Magnetic tunnel junction (MTJ)
Soft Ferromagnetic Layer
Hard Ferromagnetic Layer
Insulation Layer
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
Hc1<Hc2 , 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
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