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GMR sensors

Brinta N. R.Roll No:20S7 EEE, SNGCE

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Contents

Introduction Features comparison GMR Sensors- Technology GMR Materials-Types GMR Circuit Techniques GMR Sensors-Applications Conclusion

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Introduction

GMR - “Giant Magnetoresistance" Discovered in the late 1980s Very large resistance changes in

materials with magnetic field The GMR effect states significant

decrease in electrical resistance in the presence of a magnetic field.

Size of magnetic field is 10-100 Oe

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Features

High sensitivity for low magnetic fields

Measures field parallel to its sensitive axis

Good signal-to-noise ratio High spatial resolution Frequency independent sensitivity Low cost, Less power, Small size

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Comparison

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GMR Sensors- Technology

Stands for Giant magnetoresistance (GMR) sensors and can achieve a change of resistance of 10%–20%

In contrast with conventional magnetoresistance (MR), which exhibit a change of resistance of <3%

GMR films have two or more magnetic layers separated by a nonmagnetic layer. 

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GMR Sensors-Technology, Cont.

Significant decrease in electrical resistance in the presence of a magnetic field.   No magnetic field: The direction of

magnetization of adjacent ferromagnetic layers is anti parallel.

High-resistance, magnetic scattering.  

With magnetic field: The magnetization of the adjacent ferromagnetic layers is parallel.

Lower resistance

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GMR Sensors-Technology, Cont.

The new GMR Technology is SDT An insulating layer separates two

magnetic layers A resistance change of 10-40% with

magnetic field

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GMR Materials -Types

Unpinned sandwiches

Antiferromagnetic multilayers

Spin valves

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Unpinned sandwiches

Consist of two magnetic layers separated by a conductor layer

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Anti ferromagnetic multilayers

Consist of multiple repetitions of alternating magnetic and nonmagnetic layers

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Spin valves

An additional layer of an antiferromagnetic material is provided on the top or bottom

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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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GMR circuit technique, Cont. 

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GMR circuit technique, Cont. 

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GMR circuit technique, Cont. 

Doted linesWith No field

Solid linesWith field of 40mA

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GMR Sensors-Applications

Detection of motor vehicles

Eddy current detection

Detection of magnetic ink

Sensing of body position

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Detection of motor vehicles

The Earth field acts as a biasing magnet resulting in a magnetic signature from various parts of the automobile as it passes a sensor.

The x, y, and z components of the signature can be detected by magnetic sensors buried in the road

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Detection of motor vehicles

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Eddy current detection

A coil applies an ac field to the material under test.

Eddy currents generated by the applied ac field in a continuous conducting sheet below the sensor will create a mirror image of the field from the coil and will also not affect the magnetic sensor.

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Eddy current detection, Cont.

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Detection of magnetic ink

The use of iron oxide as a pigment in black ink is utilized here

This is used for reading MICR The magnetic sensor averages the

signal over the entire height of the characters as they pass the magnetic sensor at a high rate of speed.

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Detection of magnetic ink

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Sensing of body position

Plays a role in various medical evaluation

Small magnet attached to the body part to be monitored

The position of magnet is monitored

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Conclusion

Available for over 25 years Magnetic field produced either by bias

magnet or electric current Magnetic field should be above earth’s

field SDT structures are recent addition to

the GMR materials

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References Robert W. Schneider, Carl H. Smith. “Low

Magnetic Field Sensing with GMR Sensors.” Nonvolatile Electronics, Inc. Sensors, Sep 1, 1999.

Carl H. Smith and Robert W. Schneider. 1997. "Expanding The Horizons Of Magnetic Sensing: GMR," Proc Sensors Expo Boston:139-144.

J. Daughton and Y. Chen. 1993. "GMR Materials for Low Field Applications," IEEE Trans Magn, Vol. 29:2705-2710.

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GMR Sensors