Future Directions for 

Magnetic Sensors: HYBRID 

MATERIALS

Our goal is to develop the scientific expertise needed to allow 

modeling and simulation to become the driving force in improving 

magnetic sensors and to implement the resulting advantages. 

Our approach permits industry to target its resources with greater 

effectiveness in developing new and improved magnetic sensors. 

By quantifying the improvement in sensor performance that will 

result from design changes or from improved materials properties, 

our approach reduces guesswork.

Objective

Impact and Customers

• Magnetic sensors are a $1.2 billion per year market, worldwide, 

according to Frost & Sullivan.

• The economic impact of magnetic sensors is far greater than their sales 

volume because they increase economic productivity by playing key roles 

in data storage in computers, in manufacturing technology, and in health 

care.

• NIST is providing the scientific research needed to develop and 

implement a design-software package based on the physics of magnetic 

sensor operation, to predict real-world performance.

• NIST is leading a team of collaborators that include small companies, 

universities and national labs to leverage the best facilities, technologies, 

and personnel for this task.

• NIST interactions with industry leaders such as Seagate, Western Digital, 

Nonvolatile Electronics, MicroMagnetics, et al ensure that the approach is 

closely coupled with industrial needs.

Approach

Materials Science and Engineering Laboratory

Bringing new or improved magnetic sensors to market requires modeling for rapid assessment of the effect of new designs 

or improved materials properties. To this end, we are investigating the fundamental properties of a new class of magnetic 

sensors that is rapidly displacing those based on older technology. 

The new sensors are based on Magnetic Tunnel Junctions (MTJs), 

which use the Tunneling Magnetoresistance Effect (TMR). To optimize 

MTJs, NIST has purchased an instrument that diagnoses the TMR 

properties early in the fabrication process to maximize efficient use 

of time. These instruments are not widely available, and NIST is 

beginning to serve as a clearing house for MTJ testing by offering 

diagnosis of MTJs from outside groups. The resulting collaborations 

with small companies, universities, and national labs allow NIST to 

lead the world in non-proprietary MTJ research. The leverage made 

possible by these collaborations creates an impact beyond what 

would be possible if NIST were to operate independently in magnetic 

sensor research.

Achieving Better Sensitivity, Smaller 

Size, Lower Cost, & Lower Power Use

855-11-12b
Materials Science and Engineering Laboratory

Magnetic tunnel junction (MTJ) sensors are 

rapidly becoming the technology of choice for 

many magnetic sensor applications. They have 

the advantages of high sensitivity, small size, low 

cost, and low power consumption. Often these 

sensors are used in conjunction with magnetic 

flux concentrators. These are magnetic thin 

films designed to concentrate a magnetic field 

on a magnetic sensor to improve its sensitivity to 

small fields, as illustrated below.

However, two critical requirements of flux 

concentrators are that they be both magnetically 

soft and thick enough to absorb a significant 

fraction of the ambient magnetic field. We have 

found that when the softest materials are made 

as thick as they need to be, they are no longer 

soft. The two figures below illustrate the point.

In a), a continuous film of the softest material 

(the alloy Ni77Fe14Cu5Mo4), 400 nm thick, is 

not fully magnetized until a field of 20 mT is 

applied. The inset shows the perpendicular 

stripe domains responsible. In b) the magnetic 

film is split up into four segments of 100 nm 

each and separated by a 5 nm Ag film. The 

result is a 400-fold reduction in the field needed 

to fully magnetize the film. It is rare to find any 

magnetic film as soft as the one in b), much less 

one that is »400 nm thick. This result represents 

an important step forward for flux concentrator 

technology.

We investigated the films to find the source of the 

400-fold improvement. In stress measurements 

we found the film in a) had a biaxial stress 200 

times larger than the film in b). In fact, the stress 

was about equal to the tensile strength of the 

material, meaning the stress was as large as 

it was physically possible for the material to 

sustain. Plastic flow was setting an upper limit 

on the stress. It is common in magnetic films that 

stress induces strongly preferred magnetization 

directions, and prevents them from being 

magnetically soft (responding magnetically to 

small fields). The layering of the film with Ag 

was relieving the stress.

It was of interest to understand from a materials 

point of view to see what was happening inside 

the films to cause such a large effect. The 

grain structure in the two transmission electron 

micrographs below tells the story. 

In a) the continuous 400 nm film shows many 

small grains. Small grains are commonly 

associated with high stress because grain 

boundary tension acts like a dense network of 

springs. In b) the Ag interlayers are nucleating 

significantly larger grains (several are highlighted 

in color). We now understand that a markedly 

different type of grain growth is responsible for 

the effect. This work has just been published in 

the Journal of Applied Physics, and has been 

highlighted in a number of publications and 

newsletters.

Another shortcoming of flux concentrators is that 

they tend to break up into magnetic domains. 

The closure domains make flux concentrators 

hysteretic, which impairs their ability to measure 

small magnetic fields. They also tend to 

introduce noise in the magnetic measurements, 

limiting sensitivity. It does not appear that 

anyone has previously found a way to deal with 

these problems. The illustration below shows 

our solution.

Typical closure domains are illustrated in a 

single-film flux concentrator in a). Energy 

considerations drive the formation of these 

domains. By having the magnetization (red 

arrows) curve around the edges of the flux 

concentrator, the magnetic flux that is forced to 

exit the film into free space is reduced, lowering 

the magnetostatic energy cost. What is needed 

is an innovative design to avoid this cost. Our 

solution is illustrated in b). We have a two layer 

flux concentrator, with the two layers magnetized 

in opposite directions. The two closure domains 

are needed to reduce the magnetostatic energy, 

which is drastically reduced because flux closure 

occurs on a length scale of 100 nm. As in the 

illustration, no closure domains are observed 

in b), and the flux concentrator operates by 

magnetization scissoring. Initial tests indicate 

superior performance for our novel flux 

concentrator.

Our short-term goals are to transfer these 

technological advances to the magnetic sensor 

industry.

Accomplishments

Bob McMichael (CNST)

Philip Pong

Mark Stiles (Physics Lab)

Volker Hoenik

Bill Egelhoff

(Metallurgy Division)

(301) 975-2542

egelhoff@nist.gov

www.nist.gov/metallurgy

Learn More Publications

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Edelstein AS, Burnette J, Fischer GA, Cheng SF, Egelhoff, Jr. WF, McMichael RD and Nowak ER 

Advances in Magnetometry Through Minaturization J. Vac. Sci. Technol., (in press, 2008) 

O SY, Lee CG, Shapiro AJ, Egelhoff, Jr. WF, Vaudin MD, Ruglovsky JL, Mallett J and Pong PWT X-Ray 

Diffraction Study of the Optimization of MgO Growth Conditions for Magnetic Tunnel Junctions 

J. Appl. Phys., (in press, 2008)

Pong PWT, Dennis CL, Castillo A, Chen A and Egelhoff, Jr. WF Detection of Pinholes in Magnetic 

Tunnel Junctions by Magnetic Coupling J. Appl. Phys., 103: 07A902 (2008)

Sudbrack CK, Chiaramonti AN, Hiller JM, Egelhoff WF and Petford-Long AK Comparison of the 

Chemical Heterogeneities and Microstructure Between CoFeB/MgO/CoFeB and CoFeB/Al-O/

CoFeB Magnetic Tunnel Junctions Microsc. and Microanal., 13 Suppl. 2: 620 (2007)

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