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Scanning probe microscopy based on magnetoresistive sensing

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2011 Nanotechnology 22 145501(http://iopscience.iop.org/0957-4484/22/14/145501)
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IOP PUBLISHINGNANOTECHNOLOGY

Nanotechnology 22 (2011) 145501 (8pp)

doi:10.1088/0957-4484/22/14/145501

Scanning probe microscopy based onmagnetoresistive sensing
Deepak R Sahoo1 , Abu Sebastian, Walter Häberle,
Haralampos Pozidis and Evangelos Eleftheriou
IBM Research—Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland
E-mail: hap@zurich.ibm.com

Received 30 December 2010, in final form 5 January 2011
Published 24 February 2011
Online at stacks.iop.org/Nano/22/145501
Abstract
Integrated sensors are essential for scanning probe microscopy (SPM) based systems that
employ arrays of microcantilevers for high throughput. Common integrated sensors, such as
piezoresistive, piezoelectric, capacitive and thermoelectric sensors, suffer from low bandwidth
and/or low resolution. In this paper, a novel magnetoresistive-sensor-based scanning probe
microscopy (MR-SPM) technique is presented. The principle of MR-SPM is first demonstratedusing experiments with magnetic cantilevers and commercial MR sensors. A new cantilever
design tailored to MR-SPM is then presented and micromagnetic simulations are employed toevaluate the achievable resolution. A remarkable resolution of 0.84 Å over a bandwidth of
1 MHz is estimated, which would significantly outperform state-of-the-art optical deflection
sensors. Due to its combination of high resolution at high bandwidth, and its amenability tointegration in probe arrays, MR-SPM holds great promise for low-cost, high-throughput SPM.
(Some figures in this article are in colour only in the electronic version)

There are several ways to extend optical beam deflection
sensing to an array of probes. One such method employs
sequential detection using a scanning laser and an arrayof photodiodes [16, 17]. In a similar approach, the laser
beam is focused on the entire area of the array of probes
and the deflected beams are captured through a charge
coupled device (CCD) camera and tracked in real time by
an image processing (centroid-based) algorithm to detect theprobes simultaneously [18]. In another approach, the opticalinterferometric detection technique is extended to an array ofprobes by fabricating the probes with interdigitated fingers,
and by using a cylindrical laser (to illuminate the entire arrayof probes) and an array of photodiodes [19]. The fiber-opticbased interferometric technique can be integrated on probes byfabricating a Bragg grating as a photo-elastic strain sensor [20].
Optical sensing can also be integrated on probes that are
fabricated as (total internal reflection) waveguides and buttcoupled at the free end to another waveguide to sense thedeflection as a change in the optical intensity of the transmitted
light [21–26]. However, such in-plane photonic transduction
techniques cannot easily be extended to a large array of probes
because they use a complex network of waveguides and/or anarray of photodiodes. In general, optical deflection sensing

1. Introduction
Scanning probe microscopy (SPM) based devices havenumerous potential practical applications, such as bio-sensing,maskless lithography, semiconductor metrology and ultrahighdensity data storage [1–6]. However, despite their truly
great potential, stemming from their ability to interrogate andmanipulate matter at the nanometer-scale, SPM devices still
mainly remain niche laboratory instruments, mostly becauseof their limited speed of operation and their small ‘field-ofview’, typically on the order of tens of micrometers. To achieve
high throughput and large field-of-view, these devices typically
employ multiple probes in parallel operation [7–11]. Current
moderate-speed SPM setups typically use an optical deflection
sensing system with a position-sensitive photodetector (PSD)
for moderate bandwidth and high resolution imaging [12–15].
However, these conventional optical detection systems are
too bulky and expensive for parallel probe high-throughput
devices. To develop such devices at reasonable footprint andcost, the deflection sensor needs to be integrated onto themicrocantilever during the fabrication process.
1 Present address: H H Wills Physics Laboratory, University of Bristol,
Tyndall Avenue, Bristol BS8 1TL, UK.

0957-4484/11/145501+08$33.00

1© 2011 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 22 (2011) 145501

D R Sahoo et alwith a laser and an array of photodiodes may achieve highbandwidth and high resolution, but remains an expensive andbulky way to sense an array of probes. Moreover, none of theaforementioned methods is scalable to large probe arrays, thus
their usage is limited to some niche applications.
In contrast to multi-probe optical detection, which relies
on elements external to the probe for sensing, a variety of othersensors exist that can be integrated on the probe during the
fabrication process. Piezoresistive [27] and piezoelectric [28]elements can be deposited on the probe as strain sensors
to detect the deflection of the micro-structure. However,deposition of such materials results in high stiffness andgenerates unwanted stress in the probe structure. Moreover,the inherent noise in such strain sensors limits the resolution
for nano-scale imaging [29]. MEMS comb structures can be
fabricated on the probes as capacitors, and the probes can bedetected in parallel in an integrated manner [10, 30]. Sucha design requires highly sensitive electronics to detect verysmall currents (in the picoampere range). Moreover, parasitic
capacitances inevitably limit the resolution and bandwidth ofsensing. Thermoelectric sensors can easily be integrated onprobes by fabricating micro-heater regions with differential
doping. Such sensors can provide high resolution when usedappropriately, i.e., with an efficient heat conduction path to thesample. However, the bandwidth of sensing is limited to a few
tens of kHz, owing to the limited bandwidth of the thermal
system [31].
In this paper, a novel cantilever displacement sensing
technique using magnetoresistive (MR) sensing is presented.
MR spin-valve sensors have already been used in scanningmagnetoresistance microscopy by fabricating a MR structureon the back side of the cantilever [32, 33]. These cantilevers
were used to image the magnetic field variation on the surface
of a sample. In contrast, in our work, the MR sensors are used
to sense the cantilever displacement signal for generic SPM
applications. The essential idea is to translate the cantileverdisplacement into a change in the magnetic field as sensed
by the MR sensor. This mode of scanning probe microscopy
is henceforth referred to as MR-SPM. It promises nano-scale
resolution and high bandwidths in excess of 1 MHz for highspeed SPM. Moreover, it is an integrable technique and thus
scalable to large probe arrays for very high throughput. MRSPM possesses all the necessary ingredients for building a new
breed of multi-probe SPM devices that could find widespreaduse. As such, it has the potential to revolutionize scanningprobe microscopy. An earlier version of this work, describing
only the basic concept of MR-SPM and some preliminary
experiments was presented in [34].
The remainder of the paper is arranged as follows.
In section 2, the motivation behind using magnetoresistive
sensors for SPM is presented. In section 3, a proof-of-concept
experiment is presented to illustrate the concept of MRSPM. Magnetic cantilevers and commercial MR sensors are
employed. In section 4, a cantilever design custom-made for
MR-SPM is presented, and micromagnetic simulations serveas the basis to estimate the achievable resolution. Section 5
concludes the paper.

2. Magnetoresistive sensors
Magnetoresistance is the property of a material to change thevalue of its electrical resistance when an external magnetic
field is applied to it. Various physical phenomena can leadto magnetoresistance effects in a material. For example, giant
magnetoresistance (GMR) structures use interface scattering ofpolarized conduction electrons as the mechanism for magnetoresistance. Two types of GMR structures are known, namely
multilayer and spin-valve structures. GMR multilayers consistof alternate layers of ferromagnetic and nonferromagnetic conductors [35]. With a proper thickness of the nonmagnetic layer,
there is an indirect exchange coupling between the magnetic
layers, which magnetize adjacent magnetic layers in opposite
directions in the absence of an applied magnetic field. This
state is the higher resistance state. If a sufficiently large magnetic field is applied, the magnetic layers can be magnetizedin the same direction, which is the lower resistance state. In aspin-valve structure, a thin conductor layer (normally copper)
is sandwiched between two magnetic layers. One of them isa ‘soft’ ferromagnetic layer, which responds to the external
magnetic field, and the other is a ‘pinned’ ferromagnetic layer,
which does not. When the magnetizations in the two ferromagnetic layers are parallel, conduction electrons pass more freely
between the magnetic layers than when the magnetizations are
anti-parallel. Thus the resistance in the parallel magnetizationcase is lower than in the anti-parallel case. Pinning is
usually accomplished by coupling of the pinned layer to an
antiferromagnetic material through anti-parallel coupling [36].
MR sensors are typically used as magnetometers(gradiometers), read heads in hard disk drives, and random
access memory (RAM) elements [35–38]. MR (spin-valve)sensors are also proposed for micro-electromechanical-system(MEMS) based sensor applications [39]. MR sensors canoperate at very high bandwidth, as evidenced by their use asread heads and transducers in hard disk drives, whereas theiruse as RAM elements demonstrates their scaling potential.
Moreover, they exhibit high sensitivity and low noise [40, 41].
MR sensors are characterized by their MR ratio, which isdefined as the ratio of the maximum change in resistancewhen an external magnetic field is applied and the nominal
resistance when no magnetic field is applied. GMR sensors
have a particularly high MR ratio (for example, 110% in
sputter-deposited Co/Cu multilayers [35]) and a wide range of
operation.
From the discussion above, it is clear that MR sensors
have very desirable characteristics, such as high bandwidth,
small form factor and excellent scalability. Hence it would be
advantageous to use MR sensors as integrated sensors for highthroughput SPM devices. The key challenge is the ability totranslate the cantilever displacement into a substantial change
in magnetic field as seen by a MR sensor. This aspect isaddressed in this paper.

3. MR-SPM: principle of operation and experimental
results
In this section, the concept of MR-SPM is demonstratedusing experimental results. The essential idea of MR-SPM
2

Nanotechnology 22 (2011) 145501

is shown in figure 1. The four GMR elements are denotedby 1, 2, 3 and 4. They are connected in a Wheatstonebridge configuration to reduce noise. The GMR elements
are approx. 95 μm long and 35 μm wide. The GMR
elements 1 and 2 are exposed to the external magnetic field
during measurement. The GMR elements saturate when themagnitude of the external magnetic field strength exceeds
100 Oe, whereas the linear range is between 10 and 70 Oe.
The typical nominal resistance and the MR ratio of the GMR
elements are 5 k and 20%, respectively. The bandwidth of thecommercial GMR sensor is specified to be larger than 1 MHz.
The cantilever with the micromagnet attached to it was
fixed above GMR element 1 at a central position. Becauseof the small size of the micromagnet, it can be assumed
that GMR element 2 was not affected by the stray magnetic
field of the micromagnet. First an approach–retract curve
was obtained using the cantilever and the sensor. The initialtip–sample separation was approx. 2.5 μm. The GMR chip
was slowly raised by using the Z-scanner to approach thecantilever. This changed the position of the GMR sensing
element relative to that of the fixed micromagnet, which in turnresulted in a change in the magnetic field sensed by the GMR
element and hence its electrical resistance. The Z-scanner was
moved up by approx. 2.75 μm and then retracted to its initial
position. During the experiment, the resistance change of the
GMR element was monitored using an electronic circuit. The
resistance change as a function of the Z-scanner position isshown in figure 2(a). It can be seen that there is an almost
linear change in the resistance as a function of the scanner
position until the tip comes into contact with the sample. The
sensitivity is given by the slope of this curve prior to the tip
coming into contact with the sample and is 9.3  μm−1 . Notethat the topography of the sample will modulate the separation
between the MR sensor and the micromagnet during imaging,which is captured by the off-contact signal. Ideally there would
be no change in the resistance beyond the point of contact inthe design shown in section 4. However, in this setup, further
bending of the cantilever will cause a minor change in theresistance, which has little impact while performing contactmode imaging at low loading force or during tapping-mode
imaging, as will be demonstrated later.

Contact
Cantilever
Flux concentrator
MagnetVs +OUT-

D R Sahoo et al

3
15 µ
m

95 µ
m

1

Magnetization direction
Sensing direction
Magnetic shield

2

35 µ
m
XYZ
Sca
nne
r

4

OUT+

Vs -

Figure 1. Schematic of the experimental setup to demonstrate
MR-SPM.

is to translate the cantilever displacement into a change inthe magnetic field as sensed by a MR sensor. One way torealize this is to translate the cantilever displacement into achange in the position of a micromagnet relative to a MR
sensor. The micromagnet is attached to the distal end of themoving cantilever whereas the position of the MR sensor isfixed relative to the cantilever. An experiment was designed
using magnetic cantilevers and commercial GMR sensors todemonstrate this scheme.
A schematic of the experimental setup is shown in figure 1.
A FeNdBLa permanent magnetic particle was glued to thebackside of a commercial cantilever above the tip using a heattreatment adhesive [42]. The diameter of the micromagnet wasapprox. 10 μm and it was magnetized in the lateral direction,
as shown in figure 1. The spring constant and the resonant
frequency of the cantilever prior to gluing the magnetic particle
were 0.1 N m−1 and 65 kHz, respectively.
A commercial multilayered GMR sensor chip in die form
(NVE; AA005-01) was mounted on a nanopositioning stage
with XY Z motion capability. One of the four sensing elementson this chip would serve as the sample, and patterns created on
this element could be used for imaging purposes. Note that
this configuration was employed only for the proof-of-concept
experiment. For a cantilever custom-made for MR-SPM, it isbetter to place the MR sensor as described in section 4. The
schematic of the GMR chip, drawn approximately to scale,Figure 2. (a) An approach–retract curve obtained using the experimental setup. (b) Noise characteristic of the experimental setup.

3

X position (nm)Nanotechnology 22 (2011) 145501

D R Sahoo et al

structure (70–100 nm) is in accordance with the actual heightof the fabricated structures.
Tapping-mode imaging is demonstrated next.
Thecantilever chip was mounted on a dither piezo. First, thefrequency response of the cantilever was measured using an
Agilent 4395A network analyzer. A 50 mV peak-to-peakchirp signal was generated by the analyzer and input to thedither piezo for acoustic actuation. The oscillation of thecantilever was measured by the GMR sensor chip. The
deflection signal was fed back to the network analyzer, andthe averaged frequency response data was collected. The
frequency response is shown in figure 4(a). The first dominantresonant frequency and the quality factor of the cantileverare 11.84 kHz and 271.56, respectively. The rather low
resonant frequency is due to the added mass of the gluedon micromagnet. The deflection signal from the GMR
sensor was input to an analog lock-in amplifier (SR530 from
Stanford Research Systems) and the demodulated amplitude
and phase signals from the outputs were used for tapping-mode
imaging. The sensitivity of the amplitude signal for sensing
the topographical changes on the sample surface was measured
in an approach-and-retract experiment. The amplitude of thefreely oscillating cantilever was set to a constant value by
applying a sinusoidal voltage signal of 11.8 kHz frequency
to the dither piezo. An approach–retract curve was obtainedin the same way as in the case of contact-mode operation.
The Z-scanner was slowly moved towards the cantilever andthen retracted to its initial position. The amplitude signal
was calibrated by using the measured Z-scanner position.
Subsequently, the amplitude of the input signal to the dither
piezo was changed appropriately such that the amplitude of
the freely oscillating cantilever was set to approx. 400 nm.
The approach-and-retract experiment was repeated, and thecorresponding experimental data is shown in figure 4(b). Notethat the amplitude signal varies linearly with the position of theZ-scanner. Tapping-mode images of the sample surface wereobtained in constant-height mode. The resulting image of thesample surface is shown in figure 5 (top). The amplitude profilefor one scan line is shown in figure 5 (bottom). The amplitude
signal is modulated by the sample profile as the tip traverses the
sample surface. These results closely resemble those obtainedin the contact-mode operation.

3000
2500

Sensor/sample

2000
1500

X/Y/Z Piezo
1000 2000 3000 4000 5000 6000 7000
scan line Y position (nm)Height (nm)

250
200
150

Sensor/sample

100
50

X/Y/Z Piezo
1000

1500

2000
2500
X position (nm)3000

Figure 3. Top: constant-height, contact-mode image obtained using
MR-SPM. Bottom: scan line across one amplitude profile.

The main sources of measurement noise in the GMR
sensor are the electronic noise in the amplifiers, theintrinsic electromagnetic noise in the GMR elements, and theenvironmental electromagnetic noise in the setup. Following
the approach–retract experiment, the magnetic cantilever wasplaced with its tip approx. 50 nm above the surface, and thenoise signal from the GMR chip was captured. The noise
spectrum of the SPM setup is shown in figure 2(b), where
the previously calculated (out-of-contact) sensitivity value has
been used to translate the measurement into nanometers. Thereis some 1/ f noise, which predominates until approx. 10 kHz.
However, beyond this frequency, the noise can be assumed
to be white with a resolution of 5 pm Hz−1/2 . Note thatthese commercial sensors can sense beyond a bandwidth of
1 MHz. The roll-off in the spectrum is due to the filters in
the measurement circuit.
Contact-mode and tapping-mode imaging were performed
with the MR-SPM setup to further illustrate the efficacy of the
new sensing scheme. A platinum structure spelling ‘IBM’ wasdeposited on the GMR sensing element 1 using the focusedion beam (FIB) technique, so that it could be used for theimaging experiments. A contact-mode (constant-height) imageof the sample surface obtained using the MR-SPM technique isshown in figure 3. As the tip traverses the sample topography,
the relative position between the micromagnet and the GMR
sensor is modulated. The measured height of the platinum

Figure 4. (a) Mechanical frequency response of the cantilever measured using the MR-sensing technique. (b) Approach–retract curve intapping-mode.

4

X position (nm)Nanotechnology 22 (2011) 145501

D R Sahoo et al

3000

MagnetMagnetMRsensorMRsensor

2500
2000

Y
Y

1500

X/Y/Z Piezo

Cantilever
Cantilever400

SampleSample350

XYZscanner
scanner
XYZ

300
250
1000

XX

Vx

Sensor/sample
1000 2000 3000 4000 5000 6000 7000
scan line Y position (nm)Height (nm)Z
Z

Sensor/sample
1500

2000
2500
X position (nm)3000

X/Y/Z Piezo

Figure 5. Top: constant-height, tapping-mode image obtained using
MR-SPM. Bottom: scan line across one amplitude profile.

Vxx
V
Cantilever
Cantilever((deflected)
deflected)
SampleSampleThese experimental results demonstrate the concept of
MR-SPM and the potential of this sensing scheme. It istruly remarkable that with a home-made magnetic cantileverand commercial GMR sensors one can perform regular AFM
operations like contact-mode and tapping-mode imaging withvery high resolution. Clearly, in these experiments, therelative orientation of the micromagnet and the MR sensor
are not optimized to obtain maximum sensitivity. Also, themicromagnet is approximately ten times smaller than theavailable sensing area, which results in reduced sensitivity. In
a custom-designed cantilever for MR-SPM these aspects canbe addressed. In section 4, we describe one such cantilever
design and evaluate the achievable resolution by means of
micromagnetic simulations.

XYZ
XYZscanner
scanner

Figure 6. Schematic of a cantilever design custom-made for
MR-SPM.

this may correspond to the maximal electrical resistance R of
the sensor. When the cantilever experiences a tip–sample force
and as a result deflects, the magnetic field passing through the
MR sensor changes. If the probe deflects by z nm, then themagnetic field strength VX at the sensor changes by Gz Oe.
As a result, the resistance R of the MR sensor changes by
R = Gz S , where S is the MR sensitivity of the sensor in Oe−1 . R can be measured in constant-current or constantvoltage mode as a corresponding change in the voltage or thecurrent, respectively, or it can also be measured by using an
AC bias voltage. R is the measure of the deflection of thecantilever. To sense the normal deflection of the cantileverwith higher resolution, the two MR sensors may be used insumming mode. Alternatively, the two sensors may be used indifferential mode to measure the torsion of the cantilever.
The sensitivity of MR-SPM can be assessed frommicromagnetic simulations of the proposed micromagnet and
MR sensor configuration shown in figure 6. For highsensitivity, a high magnetic field gradient, ∂ VX /∂z , is required.
Because the MR sensors usually have a small range of
operation, also small magnetic field strength VX is required.
The shape, size, orientation and the relative positions of the
micromagnet and the MR sensors dictate VX and ∂ VX /∂z at thesensor. These parameters are calculated from micromagnetic
simulations by using the ‘object oriented micromagnetic
framework’ (OOMMF) [43] for the MR cantilever shown infigure 6.
For the MR cantilever, rectangular, cuboid-shaped
micromagnets are considered because they are the simplest
structure to fabricate by electroplating. The sensing directionof the MR sensor was fixed along the X -axis, and themagnetization direction of the micromagnet was fixed along
the Z -axis. The cantilever deflection was also fixed along
the Z -axis. The center of the micromagnet was fixed at theorigin (X, Y, Z ) = (0, 0, 0). The saturation magnetization

4. Cantilever design for MR-SPM
In this section, we introduce a cantilever custom designed
for MR-SPM. The essential idea remains to translate thecantilever displacement into a change in the position of a
micromagnet relative to a MR sensor. Unlike in the proof-ofconcept experiment presented in section 3, the micromagnet
and the MR sensors are now integrated on the same cantileverstructure. The proposed cantilever design is henceforthreferred to as the ‘MR cantilever’ and is schematically shownin figure 6. In the MR cantilever, a permanent micromagnet isfabricated at the flexible end on the backside of the cantileverjust above the tip. A MEMS-compatible electroplating process
can be used to fabricate such micromagnets. The MR sensors,which can be fabricated by sputtering, are deposited onto thebottom side of two rigid structures fabricated on either side of
the moving part of the cantilever for easy electrical connection.
The sensors are fabricated such that they are sensitive to themagnetic field component of the micromagnet along the widthof the cantilever, i.e. the ‘ X ’-axis in figure 6. The size and theposition of the MR sensors and the permanent micromagnet arechosen such that the stray magnetic field strength, VX , at thesensors remains small and the magnetic field gradient G of VX
along the vertical direction z , i.e., G = ∂ VX /∂z in Oe nm−1
is high. When the cantilever is at its unforced position, theaverage magnetic field strength at the MR sensor along thesensing direction is low. Depending upon the MR material,
5Nanotechnology 22 (2011) 145501

D R Sahoo et al

Figure 7. (a) Stray magnetic field strength and (b) gradient of the 15 μm × 15 μm × 3 μm large micromagnet.

of the magnetic material was set to 1.4 × 106 A m−1 . A
uniform magnetic field of 1 T was applied along the Z -axis
to magnetize the micromagnet.
Rectangular cuboid micromagnets of various sizes wereconsidered in the simulations. The stray magnetic fields V ofthe micromagnets were analyzed over the three-dimensional
space around the magnet to find a region in which themagnitude of the X -component of V , |VX |, is low and themagnitude of the gradient of VX along the Z -axis, |∂ VX /∂z|,is high. The minimum separation between the adjacent sidesof the micromagnet and the MR sensor in the simulations ischosen to be 3 μm to ease the fabrication process.
Based on the simulation results, a rectangular cuboid
micromagnet with length, depth and height of 15 μm, 15 μm
and 3 μm, respectively, was chosen for the MR cantilever
design. For this micromagnet, simulations showed thatfavorable conditions for |VX | and |∂ VX /∂z| can be achievedin the XY -plane near z = 0. The distribution of VX in the
XY -plane at z = 25 nm is shown in figure 7(a). The Z -stepsize in the simulation was set to 50 nm, which generated grid
points at −25 and 25 nm. Note that the stray magnetic field
V of the micromagnet is distributed nonuniformly in the threedimensional space around the micromagnet. In particular, VX
is distributed nonuniformly in the XY -plane at Z = 25 nm,
as shown in figure 7(a). The magnitude of VX is high near the
micromagnet, and decreases in the XY -plane away from themicromagnet.
In this configuration, a MR sensor having a 1 μm ×
15 μm size was chosen, positioned approx. 3 μm fromthe edge of the micromagnet. The average and maximum
magnitude of VX over the MR sensor were approx. 10.5 Oe
and 15 Oe, respectively. It is essential that the magnitude of themagnetic field does not saturate the sensor. Most commercial
MR sensors can comfortably operate in this regime. The
distribution of the magnetic field gradient ∂ VX /∂z in the XY plane at z = 25 nm is shown in figure 7(b). The averagemagnitude of the field gradient |∂ VX /∂z| over the area of the
MR sensor was approx. 22.64 Oe μm−1 . Note that larger MR
sensors may be considered by compromising on |∂ VX /∂z|.

The resolution of the MR cantilever can be estimatedby using the magnetic field gradient and the magnetic field
resolution of the MR sensors. For typical commercial MR
sensors, the resolvable magnetic field typically varies between
0.15 and 1.5 nT Hz−1/2 [40]. For our calculations we chosethe resolution figures associated with GMR sensors from NVE
Corporation, which is ResGMR ≈ 0.27 nT Hz−1/2 = 2.7 ×
10−6 Oe Hz−1/2 .
From the micromagnetic simulations, we had obtained anaverage value of the magnitude of the magnetic field gradientat the sensors, |∂ VX /∂z|ave , of 22.64 Oe μm−1 . Hence,
the resolution of the MR-sensor-based deflection sensing
technique using one sensor is estimated as 1/|∂ VX /∂z|ave ×
ResGMR = 1/22.64 × 2.7 × 10−6 μm Hz−1/2 ≈ 1.19 ×
10−4 nm Hz−1/2 . Because two MR sensors are employed in the
MR cantilever√ in figure 6, the resolution is further improved bya factor of 2 to 8.43 × 10−5 nm Hz−1/2 . Assuming a white
noise level of 8.43 × 10−5 nm Hz−1/2 , a resolution of 84 pm
can be achieved over a bandwidth of 1 MHz. This is a truly
remarkable number given that the achievable resolution using
optical means in state-of-the-art AFMs exceeds 200 pm overthe same bandwidth. Further optimization of the cantilevergeometry and the selection of more advanced MR sensors,
such as tunnel magnetoresistive (TMR) sensors, can lead toeven better resolution. Because the torsion of the cantileveris measured by operating the two MR sensors in differential√
mode, the resolution of torsion measurement is 2 times
worse than the resolution of a single MR sensor.

5. Conclusions
MR-sensor-based cantilever displacement sensing is a newparadigm for SPM that promises high resolution at very
high bandwidth, enabling high-speed and high-throughput
operation. The key idea in MR-SPM is to translate thecantilever displacement into a relative motion between amicromagnet and a MR sensor. Thus the displacement signal
is translated into a magnetic field change as seen by the MR
sensor. The electrical resistance of the MR sensor is a functionof the magnetic field, and hence the deflection signal can
6Nanotechnology 22 (2011) 145501

D R Sahoo et al

be measured as a resistance signal. The concept of MRSPM is demonstrated experimentally using commercial GMR
sensors and magnetic cantilevers. A spherical micromagnet
is glued onto the cantilever, and the MR sensor, which
also serves as the sample, is used to sense its deflection.
Images of a platinum micro-structure with a nominal heightof 70 nm were obtained in both contact-mode and tappingmode operation. A custom-made MR cantilever design withan integrated micromagnet and MR sensors is presented thatcan be used for SPM applications. Micromagnetic simulations
are performed to estimate the resolution of the sensing scheme
using such a cantilever. The resolution is estimated to be aremarkable 84 pm over a 1 MHz bandwidth. MR-SPM has
the potential to address the need for high-bandwidth, highresolution integrated sensing. SPM-based devices employing
several MR cantilevers for high throughput and large field-ofview can be manufactured. At the same time, MR cantilevers
can also be employed to realize compact and low-cost scanningprobe microscopes, as alternatives to the current AFMs, which
employ bulky and costly optical deflection sensing means.

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Acknowledgments
We would like to acknowledge the contributions of several
colleagues at IBM Research—Zurich, in particular Mark
Lantz for providing the magnetic microcantilevers and PeterBächtold for the electronic setup. We would also like to thankour colleagues Gian Salis, Michel Despont, Urs Dürig andArmin W Knoll for stimulating discussions.

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