Delta-sigma digital magnetometer utilizing bistable spin-dependent-tunneling magneticsensorsJ. Deak, A. Jander, E. Lange, S. Mundon, D. Brownell, and L. Tran Citation: Journal of Applied Physics 99, 08B320 (2006); doi: 10.1063/1.2171942 View online: http://dx.doi.org/10.1063/1.2171942 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/99/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An optically modulated zero-field atomic magnetometer with suppressed spin-exchange broadening Rev. Sci. Instrum. 85, 045124 (2014); 10.1063/1.4872075 Synchronization sampling method based on delta-sigma analog-digital converter for underwater towed arraysystem Rev. Sci. Instrum. 85, 034701 (2014); 10.1063/1.4868440 Low aspect ratio micron size tunnel magnetoresistance sensors with permanent magnet biasing integrated in thetop lead J. Appl. Phys. 109, 07E506 (2011); 10.1063/1.3537926 Low-frequency noise measurements on commercial magnetoresistive magnetic field sensors J. Appl. Phys. 97, 10Q107 (2005); 10.1063/1.1861375 Torsion cantilever as magnetic torque sensor Rev. Sci. Instrum. 69, 3199 (1998); 10.1063/1.1149084

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Delta-sigma digital magnetometer utilizing bistablespin-dependent-tunneling magnetic sensors

J. Deak,a� A. Jander, E. Lange, S. Mundon, D. Brownell, and L. TranNVE Corp., 11409 Valley View Road, Eden Prairie, Minnesota 55344

�Presented on 1 November 2005; published online 3 May 2006�

A digital magnetometer, utilizing a micron-sized spin-dependent-tunneling �SDT� sensor as afield-dependent bistable difference node within an oversampling first-order delta-sigma ��−��analog-to-digital converter, was designed and prototyped. The �−� magnetometer can be fabricatedas a single-chip device. It is intended for integration directly with digital components, facilitatingdevelopment of low-cost magnetic-field sensing systems for commercial and military navigation,security, and linear-magnetic-field sensing applications. The �−� magnetometer operates bypulsing the SDT element along its hard axis �HA� in order to set the magnetization of the free layerinto an unstable equilibrium orientation along the HA. In the absence of an easy-axis �EA� magneticfield, thermal fluctuations result in an equal probability for the magnetization to relax into one of thetwo stable orientations along the EA. Application of an EA magnetic field increases the probabilityfor the magnetization to relax into the EA orientation parallel to the applied field. Thus, repeatedlypulsing the SDT sensor along the HA and monitoring the magnetoresistance results in a digitalbitstream, where the number of 1s within a fixed length of the bitstream provides a measure of theapplied field. The SDT sensor bitstream is thus integrated in order to produce an EA feedback signal.The raw bitstream from the SDT sensor provides the digital output of the magnetometer. The lengthof the bitstream summing window is the oversampling ratio, which determines the minimumpossible digital resolution. A prototype �−� magnetometer was constructed, and several differentbit shapes were tested in order to optimize performance. The digital output of the device was foundto provide a count that is linearly proportional to the applied field over a range determined by theEA feedback driver current limit and loop gain. © 2006 American Institute of Physics.�DOI: 10.1063/1.2171942�

I. INTRODUCTION

At present there are no commercially available single-chip, multibit resolution, digital magnetic-field sensors. In-stead the market is served by bulky and expensive printed-circuit-board-based modules that combine a sensor having ananalog output with an electronic analog-to-digital converter.A single-chip digital magnetic sensor is described here,where the analog-to-digital conversion occurs in the physicalmechanism of the sensor itself. With this approach only in-expensive digital electronic circuits are needed to completethe sensor system, resulting in a small, inexpensive, andhighly manufacturable design. This gives it a considerablecompetitive advantage over current devices, particularly inapplications where size and cost are of importance. Potentialapplications include embedded digital compasses for phonesand watches and small magnetometers for security and sur-veillance.

The operation of NVE’s all-digital magnetometer isbased upon the bistable magnetic response of a single-domain ferromagnetic element with uniaxial anisotropy. Asingle-domain ferromagnetic element with total magneticmoment m in an applied field H= �Hx ,Hy� and uniaxial an-isotropy has a total energy E=−K cos2 �−m ·H, where K isthe anisotropy constant and � is the angle between moment

m and the easy axis �EA� �see Fig. 1�a��. In the absence of anexternal field, E=−K cos2 �, and the moment will be orientedin one of the two stable �lowest energy� orientations coinci-dent with the easy axis at �=0° or �=180° as shown in Fig.1�b�. If a sufficiently large field is applied along the hard axis�HA�, i.e., Hy �2K / �m�=Hk, then the energy surface allowsonly one stable orientation of m at �=90° as shown in Fig.1�c�. If the HA field is temporarily applied and then re-moved, the moment will be left in an unstable equilibrium at

a�Electronic mail: jdeak@nve.com

FIG. 1. �a� Anisotropy axes and magnetic-moment orientation of a magneticelement. Energy as a function of orientation of the magnetic moment of asingle-domain particle with uniaxial anisotropy. �b� Without external fieldsthe magnetization can be in either of the two stable states shown. �c� Withhard-axis field Hy �Hk only one stable state exists at 90°. �d� When thehard-axis field is removed, the particle is left in an unstable equilibrium.

JOURNAL OF APPLIED PHYSICS 99, 08B320 �2006�

0021-8979/2006/99�8�/08B320/3/$23.00 © 2006 American Institute of Physics99, 08B320-1

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�=90° as shown schematically in Fig. 1�d�. Any perturbationwill cause the moment to quickly relax into one of the stableorientations at �=0° or �=180°. In zero EA field, thermalagitation will cause the moment to move with equal prob-ability into the two possible states. A nonzero EA field, how-ever, will increase the probability of the moment relaxinginto one or the other state. The basic principle of the pro-posed magnetometer, then, is to repeat the application of thehard-axis field at a high rate and count the number of timesthat the sensor element ends up in the 0° or 180° state. In-creasing the number of HA pulses within a fixed time win-dow increases the digital resolution of the sensor.

The bistable sensor is implemented using a spin-dependent-tunneling �SDT� junction. Typically, noise inSDT-based sensors has 1/ f frequency dependence at low fre-quency, resulting from magnetic noise in the pinned and freelayers and electronic noise in the tunnel barrier. The elec-tronic noise component is dominant, and it results fromcharge trapping in the barrier.1 Using NVE’s �−� magneto-meter scheme, the electronic component of the SDT junc-tion’s noise will not contribute to sensor resolution since theseparation of the signal levels resulting from the two possibleorientations of the sense layer is significantly larger than thetunnel barrier noise. Provided nonequilibrium noise sourcessuch as domain-wall motion in the sense and pinned layersor drift in the pinning field do not affect the relaxation of thefree layer moment into the stable EA orientation, the lowerlimit on resolution should be determined by the equilibriummagnetic noise of the free layer during the time it relaxesback into its equilibrium orientation. The white-noise limitdue to equilibrium magnetic fluctuations is given by thefluctuation-dissipation theorem, and has a value of SB

=�2�kBT /�MsV with units of T/rtHz.2 Here, �=0.02 is theGilbert damping parameter, kB is Boltzmann’s constant, T thetemperature, � the gyromagnetic ratio, and Ms the saturationmagnetization of permalloy. For the case of a 2 �m�1 �m�5 nm permalloy ellipse the magnetic white noisein the sense layer is on the order of 0.4 nT/rtHz. This isabout a factor of 10 better than a typical analog SDT sensorbridge, which is limited by barrier noise.

II. MAGNETOMETER DESIGN AND OPERATION

An illustration of the bistable SDT sensor used for the�−� magnetometer is shown in Fig. 2�a�. The sensors weredesigned and constructed by NVE Corporation. The sensorsare essentially magnetic random access memory �MRAM�elements. Like many MRAM designs, four metal layers wereused to electrically isolate the read and write conductors. M0is used to apply the HA pulse in order to set the sense layermagnetization into the unstable equilibrium position. M1 isthe lower read conductor. M2 is the upper read conductor.M3 is the EA feedback strap. The bistable SDT sensor ele-ment is roughly a 2�1 �m2 ellipse, and it is located in thelayer between M1 and M2. It uses a bottom pinned stackwith the following layer sequence: seed/IrMn/NiFe/AlOx/NiFe/Ta. It was patterned into an ellipse with reason-able yields using a MicroChem lift-off process.3 In thepresent design, the HA and EA current to field calibrations

are roughly 1.5 Oe/mA. Thinning the M0 and M3 conduc-tors to increase the field calibration at the expense of tighteralignment tolerances would decrease sensor power.

Forty different sensor shapes and sizes were tested, rang-ing from 1 to 3 �m wide with aspect ratios ranging from 1to 2 and with various end shapes. Testing included resistanceas a function of applied field loops, R�H�, and open-loophard-axis pulsing �OL-HA-P� response, where the bit waspulsed along the HA and the statistics of relaxing into an EAorientation were measured as a function of EA field, asshown in Fig. 2�b�. In the micron size range, and for the bitstested, shapes with sharper ends and larger aspect ratio hadhigher coercivity and thus required larger programming cur-rent. On the positive side, larger aspect ratio bits with sharpends were also less likely to exhibit steps in their R�H� loops.Generally, OL-HA-P measurements showed low hysteresisprovided that the HA pulse field was large enough to closethe R�H� loop. Bits with the most square hysteresis loopsshowed the best OL-HA-P response and fewest intermediateresistance states. It appears then that bits with large aspectratio and sharp ends should make the best sensing elements.

A top-level schematic of the �−� magnetometer systemis shown in Fig. 3. In the present implementation, the mag-netometer produces a �−� modulated bitstream, where thenumber of 1s in a fixed time window is proportional to themeasured field. An external clock controls the timing of thedevice. This in combination with an external capacitor usedfor setting the feedback loop time constant allows the over-sampling ratio to be easily adjusted. The clock input triggers

FIG. 2. �Color online� �a� Four-metal-layer bistable SDT sensor element.M0 is used to pulse the bit along the HA, and M3 is used for EA feedback.The resistance measured between M1 and M2 indicates the orientation ofthe sense layer. �b� R�H� loop for a 1.5�3 �m2 bit. and OL-HA-P data fora 1.5�3 �m2 bit using a 50 mA HA pulse amplitude.

08B320-2 Deak et al. J. Appl. Phys. 99, 08B320 �2006�

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a nonoverlapping clock generator, which produces properlytimed pulses for the HA reset pulse and output data latch.The first clock pulse sets the HA reset pulse and produces acurrent on the HA field strap to set the SDT sensor into theunstable equilibrium orientation. The next clock pulse setsthe HA field strap current to zero, causing the SDT sensor torelax into an orientation along the EA. The comparator de-termines if the SDT sensor resistance is high or low andproduces a corresponding high or low signal at its output.The third clock pulse triggers the latch to lock onto thepresent state of the comparator. This is necessary to keep theoutput bitstream and feedback loop circuit from tracking theSDT sensor during the time it is being reset. If desired, theoutput bitstream may be used to generate a feedback signal.The advantage in doing this is that it produces a very linearresponse and simplifies sensor fabrication since offsets arecanceled out; the disadvantage is increased power consump-tion in the EA drive circuit.

A �−� ASIC test chip was designed with a die size of1.0�0.7 mm2, and it was fabricated using the MOSIS ser-vice. The ASIC test chip was designed to source 80 mA intothe EA and HA straps of the sensor in order to guarantee thatthe prototype SDT sensor would be fully saturated by the HApulse and to cancel out EA offsets resulting from Néel cou-pling. An optimized sensor would require less than 10 mA tosaturate and would have small offset. The dimensions givenabove thus represent the upper limit on the die size. The SDTsensor could easily be deposited on top of the �−� ASIC,but for proof of concept testing a separate sensor die wasused. Testing of the ASIC revealed a defect related to a tem-perature compensation circuit in the HA drive amplifier. For-tunately, the separate sensor die allowed proof of concepttesting to be accomplished using a printed circuit �PC� boardimplementation of the ASIC. Figure 4�a� shows a plot of theEA feedback current measured as a function of applied fieldusing the PC board �−� modulator circuit. The feedbackloop filter time constant in the test is 1 s, and the drive fre-quency is 4.6 kHz. The output was found to be linearly pro-portional to the applied field over almost 100 Oe. The datawere measured with four-digit precision, and no noise wasevident in the least significant bit, indicating that this tinysensor will have better than microtesla resolution. The hys-teresis observed in the test was small and dominated by theiron-core electromagnet used to apply the external field. Thedigital output of the sensor is shown in Fig. 4�b�. As ex-pected, traces acquired at positive �negative� fields are morelikely to end in a high �low� state.

III. CONCLUSIONS

An all-digital magnetometer suitable for a single-chipdevice was designed and constructed. Performance was veri-fied using a PC board implementation of the �−� modulatorcoupled with 2�1 �m2 SDT sensors. Die size is expected tobe less than 1�0.7 mm2, and resolution will be better than1 �T. The thermodynamic noise limit could be less than1 nT/rtHz in an optimized device. The small size, easy inte-gration with silicon processing, and moderate noise perfor-mance make this magnetometer very attractive for inexpen-sive single-package, multiaxis magnetometers and digitalcompasses.

ACKNOWLEDGMENTS

This work was funded by the National Science Founda-tion under SBIR Grant No. DMI-0321647. Thanks to Dr.Manish Sharma and Dr. Thomas Anthony at Hewlett-PackardLabs, as well as folks at Silicon Magnetic Systems for pro-viding MRAM samples used in the early stages of this work.

1L. Jiang, E. R. Nowak, P. E. Scott, J. Johnson, J. M. Slaughter, J. J. Sun,and R. W. Dave, “Low frequency magnetic and resistance noise in mag-netic tunnel junctions,” Phys. Rev. B 69, 054407 �2004�; N. A. Stutzke, S.E. Russek, D. P. Pappas, and M. Tondra “Low-frequency noise measure-ments on commercial magnetoresistive magnetic field sensors,” J. Appl.Phys. 97, 10Q107 �2005�

2J. L. Garcia-Palacios and F. J. Lazaro, “Langevin-dynamics study of thedynamical properties of small magnetic particles,” Phys. Rev. B 58,14937 �1998�.

3http://www.microchem.com/

FIG. 3. Top-level schematic of the all-digital �−� magnetometer. Thebuffer and filter may be removed to produce a low-power open-loop device.The gain of the feedback loop buffer amplifier controls the field range.

FIG. 4. �a� Closed loop �−� test result. �b� Digital responses of the SDTsensor as a function of applied field measured using an oscilloscope. Theywere captured during the “second pass” measurement shown in Fig. 4�a� andoffset for clarity. The trace labeled “HA” is the HA reset current into thesensor. Traces acquired at positive �negative� fields are more likely to end ina high �low� state.

08B320-3 Deak et al. J. Appl. Phys. 99, 08B320 �2006�

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