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NTN TECHNICAL REVIEW No.80（2012）

[ Technical Paper ]

TMR: A New Frontier for Magnetic Sensing

1. NTN-SNR magnetic sensingtechnology

1.1 Bearings with active sensor ASB®

NTN-SNR has developed the active sensorbearings, ASB®, the technology for detecting wheelspeed used for Antilock Brake System (ABS) andElectronic Stability Program (ESP) for vehicles, asshown in Fig. 1. The ASB®,, which helps in reducingthe weight and space of vehicles and improves therobustness and zero velocity detection in the severeconditions, is a defacto global standard for detectingwheel velocity in the automobile industry. Thefollowing shows the configuration of an ASB®,:• Target magnet

Multi-pole ring magnet integrated with the bearingseal. It generates an alternating magnetic field withfrequencies proportional to the rotational speed of thewheel. • Detection device

This uses the magnetoresistance effect or Halleffect. It measures the alternating magnetic field thatthe target magnet generates with no contact andcalculates the wheel rotation speed to be used by theABS and ESP.

*NTN-SNR ROULEMENTS Research & Innovation Mechatronics

We present here a new linear magnetic sensing technology, based onTunnel Magneto Resistance (TMR). The TMR principle derives fromSpintronics, a new branch of Physics which received the 2007 NobelPrize in Physics. Contrary to “traditional” electronics, a Magnetic TunnelJunction (MTJ) uses not only the electric charge of the electron, but alsoits quantum spin value.

During the CAMEL project, we have designed MTJs for sensor applications, i.e. for having,compared to other existing technologies, greater performances, extended working ranges and designcapabilities, the robustness needed for harsh mounting and working conditions and of course a costcompatible with a market introduction. The fascinating properties of the TMR sensing elements wehave developed open the door to many new possibilities of magnetic sensing.

Christophe DURET*Shintarou UENO*

Fig. 1 Example of ASB integration in a roller bearing unit

1.2 Development and applications of high-resolution rotation sensor MPS40S

After the development of the ASB®, continuing withthe research of a low-cost sensor for accuratelydetecting angles of rotation and for controlling rudderangle of the steering wheel and electric motor, wedeveloped the special-purpose IC MPS40S 1) with abuilt-in Hall effect device and signal processing circuit.As shown in Fig. 2. MPS40S is applicable for a wide

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TMR: A New Frontier for Magnetic Sensing

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range of pole widths, has the capability of outputtinghigh-resolution pulse signals and origin positionsignals, and satisfies the quality standard AEC-Q100as the electronic device for automobile use.

MPS40S is also targeted at industry machines suchas the rotational control of servo motors. Fig. 3 showsthe high-resolution sensor bearing with origin signaloutput 3), which integrates MPS40S and bearings forimproving ease of assembly with target equipment.

As an application of MPS40S, Fig. 4 shows a high-precision magnetic angle sensor 4), which detects theabsolute angle using the Vernier principle. This sensoris applicable to tubular shafts, where the existingsensors shaft end placements was not applicable. Byintegrating the multi-pole ring magnet and the bearingrotating ring and placing the detection device with abuilt-in signal processing circuit to the fixed ring, theaxial space adjustment is not required whenassembled. The absolute angle can be accuratelymeasured even in high-speed rotation. This is done bysetting the dedicated multi-pole ring magnet, whichhas two rows of concentric magnetic patterns with adifferent number of magnetic poles inside and outside,and calculating the relative displacement of thepatterns from the magnetic sensor signals.

Fig. 4 High precision magnetic angle sensor

Fig. 2 Appearance of MPS40S

2. Characteristics of various magneticsensors

In order to apply magnetic sensing technology to awide range of applications, it is necessary to considerbasic performance of magnetic detection sensors(such as output accuracy, frequency bandwidth, andpower consumption), as well as operation rangeagainst air gap and temperature, size of the sensor,installation allowance, robustness, and cost. Sensorsin severe operating environments such as automobileand industrial machine applications typically use theHall device, anisotropic magnetoresistance (AMR)device, and giant magnetoresistance (GMR) devicefor magnetic field detection 5). The following showstheir respective characteristics:• Hall device

It is of very good linearity and is inexpensive. On theother hand, it has limited sensitivity and large powerconsumption and offset drift.

• AMR elementIt has better sensitivity than the Hall device and thehysteresis is small. Linearity, offset-free operation,and high magnetic field detection can be achievedby a barber pole sensor structure, flip drivingmethod, and arranging a stabilizing magnet.However, in a high-temperature environment, slightdeterioration of sensitivity and linearity is observed.

Fig. 3 High resolution sensor bearing

Type C spring

Sensor caseSensor IC

SubstrateConnectorOuter ring

Innerring

Magnetic encoder

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• GMR elementIt has the highest sensitivity and widest permissiblerange of input magnetic field strength. However, theperformance, such as linearity, significantlydeteriorates as soon as the temperature or inputmagnetic field strength exceeds the operatingrange.

3. Tunnel Magneto Resistance (TMR)effect

The basic theory of the MTJ element *1 is based onspintronics that uses the quantum spin condition inaddition to the electric property of the electron basedon the existing electronics theory. When Pr. A. Fertand Pr. P. Grünberg received the Nobel Prize inPhysics in 2007, Spintronics attracted immediateattention.

As shown in Fig. 5, the MTJ element generally hasthe structure of an insulation layer of aluminum oxide(Al2O3) or magnesium oxide (MgO) in the middle oftwo stacked ferromagnetic layers, similar to a GMRelement. With the MTJ element, the current flowsvertically through these layers while the electrons in aferromagnetic layer pass through the insulator byquantum-mechanical effect and are injected to theother ferromagnetic layer by the tunneling effect. Inthis case, the electric resistance to the currentchanges significantly depends on the relative angulardifference of the magnetized direction of twoferromagnetic layers. When the magnetized directionof two layers is antiparallel, it indicates high

Fig. 5 Representation of a MTJ in an antiparallelmagnetization configuration (left) and in a parallel

magnetization configuration (right). The black arrowrepresents the current going through the junction

resistance, and when it is parallel, it indicates lowresistance. That is, the resistance changes dependingon the relative angle of magnetization direction of twolayers. This phenomenon is called the TunnelMagneto Resistance (TMR) effect *2. In order todetect an external magnetic field using this effect, thedevice is configured keeping the magnetized directionof one ferromagnetic material layer (called fixed or pinlayer) fixed and allowing the magnetized direction ofthe other ferromagnetic material layer (called freelayer) to change depending on the external magneticfield.

The TMR effect under low temperature wasdiscovered in 1975 6); however, it did not attractattention, at that time, as the application was difficult.However, in 1995, when a change of almost 20% ofmagnetic resistance was achieved (the maximumvalue at that time) at room temperature in the lowmagnetic field, the TMR effect was immediately placedunder spotlight 7) 8).

A recent research reports that using crystalline MgO(100) for the insulation layer as an alternative toamorphous Al2O3 yields very high magnetoresistancechange of about 600% 9) in room temperature 10) 11).While the TMR effect has been recently used in thehard disk read head, application development formagnetoresistance memory (MRAM 12)) for read/writeof quantum information of 0 and 1 and for magneticsensor has just begun 13) 14). Also, in order to applythe MTJ element to a magnetic sensor, the resistanceneeds to be changed depending on the externalmagnetic field strength and direction, as opposed tothe MRAM and hard disk head.

4. CAMEL Project

NTN-SNR has been focusing on the research anddevelopment of the TMR effect since 2000 15) 16) 17),and has filed several patents such as laminatedstructure of the MTJ element and sensor structure. Italso accelerated technology development andapplication from 2005 to 2010 in the CAMEL (CApteurMagnétique à effet tunnEL) project. Sponsored by theFrench National Research Agency (ANR), the projectwas conducted as an academic-industry partnershipof NTN-SNR (end-user project leader), Sensitec(making of MTJ element), and two public researchinstitutions.

The project established the MTJ element aimed atsensor applications and linear magnetic sensingtechnology. It evaluated how they would adapt to theproduction and high-volume production using theexisting production line of AMR/GMR elements ownedby Sensitec 18). An example of the linear magnetic

Ferromagnetic layer (fixed layer)

Ferromagnetic layer (free layer)

Insulationlayer

＊1 MTJ element: Magnetic Tunnel Junction element.An element made of a stack of layers with an insulationlayer of a few nm of thickness called a barrier layer ortunnel barrier in the middle, and two ferromagneticlayers made of ferromagnetic metal on both sides.

＊2 TMR Effect: Tunnel Magnetoresistance Effect.A phenomenon that resistance changes by MTJelement.

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sensing using the TMR effect is shown in Fig. 6. Asshown in the graph to the right, it was determined thatthe measured result mostly matched with the linearapproximation and the hysteresis was sufficientlysmall.

Fig. 6 Example of transfer curve for a MTJ with the following structure

In addition, this project received the Yves Rocardprize in 2010 from the French Physics Society for itsachievement in advancing the basic technology topractical utilization.

Fig. 7 Power and Cost comparison of an AMR and aTMR sensor after 6 months of continuous use

AMR SensorChip

Button Cell TMR SensorChip

5. State-of-the-art magnetic sensingusing the TMR effect

The following discusses important properties of thesensor using the TMR effect (hereafter, “TMRSensor”), which was developed applying thistechnology. The details of the structure and testresults are omitted in this paper. For more information,refer to the references 17) 18) 19).

5.1 Basic performance1) Low power consumption

The power consumption of the TMR Sensor is1/100th of the AMR sensors and 1/1000th of the Hallsensors 19), since the internal resistance of the MTJelement is high. Even when the peripheral signalprocessing circuits are considered, the powerconsumption is still sufficiently low which enables longoperation over the system product life with smallbatteries or energy harvesting devices. Therefore, it isexpected to be used in self-supported wirelesssensors. As an example, a comparison image of thenumber of button cells required for six-monthcontinuous operation is shown in Fig. 7.

TMR: A New Frontier for Magnetic Sensing

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NTN TECHNICAL REVIEW No.80（2012）

The sensitivity of the TMR sensor is 10 times that ofthe AMR sensor and 3 times that of the GMR sensor.Its internal noise is so small that it can also be usedfor magnetic field measurement of geomagnetism inµT scale. Therefore, as shown in Fig 8, detection withmaximum of 5 mm of air gap is possible, whichsignificantly expands the freedom of design. Forexample, a wall of non-magnetic material such asstainless steel, aluminum, or plastic can be installedbetween the sensor and the ring magnet for improvingdust-, oil-, or heat-resistant properties.

2) High detection accuracyThe sensor detection accuracy is determined by the

combination of linearity, hysteresis property,sensitivity, and S/N ratio. The newly developed TMRsensor has the following characteristics compared tothe sensors using the existing AMR/GMR/Halldevices.

The linearity of a Hall sensor for detecting highmagnetic field and the hysteresis property of anAMR/GMR sensor for detecting low magnetic field areboth at a very high level. However, they are notalways good across all the properties because theyexhibit both advantages and disadvantages. On theother hand, the TMR sensor exhibits a high level ofperformance across all the properties compared toother sensors, and thus, it can be used for detecting awide range of magnetic fields. Also, it is possible toverify the position sensing accuracy by the evaluationkit discussed in the next Chapter.

When used for detection of absolute angles, theAMR sensor can only detect a range of 180˚ ofexternal magnetic field, while the TMR sensor candetect a range of 360˚ (one full rotation). The TMRsensor can be applied to angle detection placed onthe shaft end by using its superior properties of wide-range of input magnetic field strength, good linearity,and low hysteresis.

For magnetic detection sensitivity, although theperformance of SQUID *3 is superior, it is proven thatthe 1/f noise and thermal noise of MTJ element can bereduced to the lnT/Hz1/2 level. This implies realizationof high space resolution and low cost for medical andnon-destructive test applications 20).

5.2 Operating range1) Wide operating temperature range

In high-temperature life test, it is confirmed thatthere is no atom diffusion within the MTJ elementeven in high temperature of over 150˚C. Also, differentfrom other sensors, the TMR sensor can be designedto increase sensitivity as the temperature rises tocompensate the reduction of magnetic field in hightemperature. Therefore, it can detect a magnetic fieldeven in the high temperature range with the same airgap as in room temperature. 2) Wide air gap range *4

Air gap setting for the AMR sensor for detectingwheel speed in ABS, etc., is usually set at 2 mm oreven at 1 mm or less when higher resolution oraccuracy is required.

Fig. 8 Magnetic field generated by an elastoferrite polering as a function of the air gap

00

10

20

30

40

50

60

70

80

1 2 3Air Gap mm

Mag

netic

Fiel

d k

A/m

4 5

＊3 SQUID: Superconducting Quantum Interference Device＊4 Air gap: Distance between sensor and magnetic target

5.3 Freedom of design1) Small size

Different from the AMR element and GMR element,the MTJ element uses electric current that flowsvertically through the layers that compose the device.Therefore, it is possible to significantly reduce thearea of the sensor by improving the manufacturingprocess such as the etching machining performanceand side-etching performance. MRAM, where therecording density is the most critical parameter, canbe configured with 1 µm2 or less of area per MTJelement. On the other hand, for sensor application, itis necessary to ensure the operation by securing theminimum dimension in the bonding pad. That is alsopossible with about 30% less space than the AMRsensor. 2) Larger installation tolerance

Magnetic sensors for speed and position detectionare susceptible to inaccuracy of sensor and magnetinstallation. When they are not properly installed,signals may not be detected or the detection accuracymay be deteriorated.

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The TMR sensor operates from a low magnetic fieldof -0.5 to 0.5 kA/m to a high magnetic field of -15 to 15kA/m, as shown in Fig.6. It is possible to apply themost appropriate design for the application and theinstallation tolerance can be larger.

5.4 Robustness1) Electrostatic discharge (ESD) capability

The MTJ element can be regarded as a capacitorwith a very thin insulator of nanometer scalesusceptible to high insulation breakdown risk.However, the risk is reduced as follows: ¡To increase dielectric strength voltage by improving

the quality of the insulation layer¡To distribute ESD voltage per MTJ element by

serially connecting multiple MTJ elements¡To reduce the risk of ESD by controlling the process

of connecting the TMR sensor and signalprocessing circuit for quick handling, as the ESD ismostly found in the sensor packaging process. After introducing the above measures, the testing

based on the machine model after installation onto theprinted wiring board revealed improvement ininsulation breakdown risk. 2) Tolerance against high magnetic field

As shown in Fig.6, the resistance of the TMR hasreversed property against the external magnetic field;therefore, the TMR sensor does not break down evenwhen it is exposed to a strong magnetic field producedby a rare-earth magnet.

5.5 Cost1) Facility investment cost

As previously noted, the MTJ element is made bythe same manufacturing facility as the AMR/GMRelements that Sensitec owns. In other words, the TMRsensors can be manufactured by using similar processand materials as the manufacturing for the GMRelements, and any special facility investment for thevolume production is not required. 2) Cost reduction effect by TMR sensor

The following shows the cost reduction effects:

¡Sensor chips can be made smaller so that morechips can be manufactured from one wafer.

¡Inexpensive ferrite magnets can be deployedinstead of expensive rare-earth magnets since avery low magnetic field can be detected.

¡Low power consumption drive is possible due tosimplified signal processing circuit, as theresistance of the device itself is high and amplifyingcircuitry is not required.

3) Cost reduction of the entire systemIn applications such as electric motors and ABS,

consideration for assembly and installation of theentire system configuration including peripheralcomponents is required, not only the sensingfunctionality.

When the air gap and installation tolerance isbroader, such as the case of the TMR sensor, therequirement for assembly/installation can be reducedand overall reduction of manufacturing cost isachieved from a simplified assembly process.

6. TMR sensor evaluation kit

NTN-SNR and Sensitec created three types ofevaluation kits for detection of rotation and positionand are providing them to potential users for thebroader use of the TMR sensor technology (Fig. 9 and10). The configuration of the evaluation kits is shownas follows:

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Fig. 9 TMR Evaluation PCB

Fig. 10 The three TMR Evaluation Kits

TMR: A New Frontier for Magnetic Sensing

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7. Summary

The TMR sensor has the following significantlysuperior properties and high potential as a next-generation magnetic sensor for automobile andindustrial machine applications:¡Basic performance (low power consumption/highoutput accuracy/high sensitivity)

¡Operating range (high temperature/wide airgap/360˚ angle detection/detection of extremelylow magnetic field)

¡Freedom of design (smaller sensor size/largerinstallation tolerance)

¡Robustness (ESD capability/high magnetic fieldtolerance)

¡Low cost

Taking advantage the TMR sensor and its use inmany broader applications is expected, such aselectronic compass, switches, non-destructive testing,

current sensors, speed sensors and position sensors,image processing equipment, and the medical field.NTN group including NTN-SNR will actively pursuefurther improvement and new market development ofthe TMR sensors.

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Resolution Sensor Element MPS40S and Dual TrackMagnetic Encoder for Rotational Speed and PositionMeasurement, NTN TECHNICAL REVIEW, 75(2007) 36-41.

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NTN TECHNICAL REVIEW No.80（2012）

• Sample board with TMR sensor for 360˚ angledetection to be placed at shaft end

• SPI communication by low-power consumptionmicro-computer

• Solar cell• Magnet to be placed at shaft end

Low power consumption drive demo set

• Sample board with TMR sensor• High-resolution pulse output (phase A/B)• Analog output• Multi-pole magnet (linear and ring magnet)

Active sensing *5 demo set

• Sample board with TMR sensor and biasmagnet

• High-resolution pulse output (phase A/B)• Analog output• Linear and ring gear

Passive sensing *6 demo set

Kit (1)

Kit (2)

Kit (3)

＊5 Active sensing: A detection method with multi-pole magnet as the detection target＊6 Passive sensing: A method, with a magnet placed behind the sensor, to detect change of magnetoresistance as a magnetic

body passes through.

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13) www.nve.com/Downloads/AAT001.pdf14) http://cache.freescale.com/files/sensors/doc/data_

sheet/MAG3110.pdf?pspll=115) G. Malinowski, M. Hehn, S. Robert, O. Lenoble, and

A. Schuhl: Magnetic origin of enhanced topexchange biasing in Py/IrMn/Py multilayers , Phys.Rev. B 68, 184404, 2003

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20) R. Guerrero, M. Pannetier-Lecoeur, C. Fermon, S.Cardoso, R. Ferreira and P.P. Freitas: Low frequencynoise in arrays of magnetic tunnel junctionsconnected in series and parallel, Journal of AppliedPhysics, 105, 113922 (2009)

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Christophe DURET

NTN-SNRResearch & Innovation

Mechatronics

Photo of authors

Shintaro UENONTN-SNR

Research & InnovationMechatronics

TMR: A New Frontier for Magnetic Sensing

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