1. Magnetic sensors and
logic gates
Ling Zhou
EE698A

2. Outline Anisotropic magnetoresistive sensors
Giant magnetoresistive sensors
Colossal magnetoresistive sensors
Using magnetoresistive elements to build up logic gates
Hall sensors and devices

3. 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

4. Magnetic sensor technology field ranges

5. Anisotropic magnetoresistive (AMR) sensor
Current I θ
Magnetization M
R=R┴+ΔRAMRcos2
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%

6. 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.

7. AMR effect for small wire
“Effect of bar width on magnetoresistance of nanoscale nickel and cobalt bars” J. Appl. Phys. 81(8) 1997

8. Giant magnetoresistive (GMR) sensor
Two different ferromagnetic materials sandwiched by a thin
conduction layer

9. 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.

10. 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

11. Antiferromagnetic multilayers

12. Use GMR in hard drive read head

13. 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

14. Magnetic tunnel junction (MTJ)
Insulation Layer
Soft Ferromagnetic Layer
Hard Ferromagnetic Layer
Sandwiches of two ferromagnetic layers separated by a very thin
insulation layer as tunneling barrier

15. 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

16. AND gate

17. OR gate and NAND, NOR gates

18. 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.

19. 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.

20. Hall sensor
The Hall voltage is generated by the effect of an external magnetic
field acting perpendicularly to the direction of the current.

21. 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

22. Magnetic p-n junction