u hartman

P1: SAT/tah/spd P2: KKK/vks QC: KKK/arun T1: KKKMay 18, 1999 15:45 Annual Reviews AR086-03

Annu. Rev. Mater. Sci. 1999. 29:5387Copyright c 1999 by Annual Reviews. All rights reserved

MAGNETIC FORCE MICROSCOPYU. HartmannInstitute of Experimental Physics, University of Saarbrucken, P.O. Box 151150,D-66041 Saarbrucken, Germany; e-mail: [email protected]

KEY WORDS: magnetic domains, magnetic domain walls, magnetic materials, magneticrecording components

ABSTRACTThis review on magnetic force microscopy does not provide an exhaustive over-view of the past accomplishments of the method but rather discusses the presentstate of the art. Magnetic force microscopy is a special mode of noncontact op-eration of the scanning force microscope. This mode is realized by employingsuitable probes and utilizing their specific dynamic properties. The particularmaterial composition of the probes and the dynamic mode of their operationare discussed in detail. The interpretation of images acquired by magnetic forcemicroscopy requires some basic knowledge about the specific near-field mag-netostatic interaction between probe and sample. The general magnetostatics aswell as convenient simplifications of the general theory, which often can be usedin practice, are summarized. Applications of magnetic force microscopy in themagnetic recording industry and in the fundamental research on magnetic mate-rials are discussed in terms of representative examples. An important aspect forany kind of microscopy is the ultimately achievable spatial resolution and inher-ent restrictions in the application of the method. Both aspects are considered, andresulting prospects for future methodical improvements are given.

INTRODUCTIONMagnetic force microscopy (MFM) is a straightforward special mode of opera-tion of the noncontact scanning force microscope. Shortly after the invention ofthe atomic force microscope it was recognized that detection of magnetostaticinteractions at a local scale was possible by equipping the force microscopewith a ferromagnetic probe, which then could be raster-scanned across anyferromagnetic sample. The near-field magnetostatic interaction for a typicalprobe-sample configuration turns out to be fairly strong and largely independent

530084-6600/99/0801-0053$08.00

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


54 HARTMANN

of surface contamination. Consequently, MFM is quite easy to perform andapplicable under various environmental conditions, in most cases even withoutany special sample preparation.

MFM is an important analytical tool whenever the near-surface stray-fieldvariation of a magnetic sample is of interest. This is certainly the case forthe development and application of magnetic recording components. It is thusnot surprising that MFM was first demonstrated on a microfabricated magneticrecording head (1). A big breakthrough occurred when it was demonstratedthat even individual interdomain boundaries (2) and in some cases part of theirinternal fine structure (3) could be analyzed at high-spatial resolution. Thisresolution is comparable to that so far only obtained by electron microscope-based instrumentation.

At the beginning of the 1990s MFM started to become a widely used methodin magnetic materials research and in the development of magnetic devices. Therecording industry became an important field of industrial application (4), andthe number of reported MFM results in basic research, especially on magneticthin film arrangements, soon became vast (5). The era of method developmentin the field of MFM changed more and more toward an era of various dedi-cated applications of a standard scanning probe method. However, this doesnot imply that there have been no method breakthroughs during the past fewyears. One recent breakthrough is certainly the demonstration that MFM can beused to image flux lines in low- and high-Tc superconductors (6). MFMs haveeven extended local detection of magnetic interactions to eddy currents (7) andmagnetic dissipation phenomena (8).

Reviews on past accomplishments can be found in References 5 and 9 and inthe various proceedings of conferences dedicated to scanning probe microscopyor to magnetic materials research. It is not the main goal of this work to addanother review on past achievements by presenting an exhaustive list of ref-erences to the vast original literature. Rather, the purpose of the present workis to analyze the field of MFM by emphasizing the state of the art, the mainapplications in basic research and in the magnetic recording industry, and bylooking into the future from a general viewpoint, which is based on more thanten years of experience with a powerful magnetic imaging method.

FUNDAMENTALS OF NONCONTACT FORCEMICROSCOPYThe simplest mode of operation of a noncontact scanning force microscopeconsists in lifting the cantilever probe up to a certain distance from the samplesurface to measure a long-range interaction in terms of a static force exerted on

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


MAGNETIC FORCE MICROSCOPY 55

the probe. This approach is, however, not preferred in reality, because a far moresensitive detection can be realized by utilizing the dynamic properties of theprobe. An obvious characteristic describing part of these dynamic propertiesis the resonant frequency of the cantilever given by

!0 Dr

c

m; 1.

with its spring constant c and its effective mass m. In order to vibrate the probe,the cantilever may be attached to a bimorph piezoelectric plate. Alternatively,a piezoelectric actuator can be used to excite the sample. In some applicationsit is possible to externally modulate the long-range probe-sample interaction,which also results in cantilever oscillation. The latter possibility is particu-larly relevant if magnetic interactions are caused by electrical driving currents.The noncontact mode of operation involving sinusoidal excitation is frequentlycalled the dynamic or ac mode.

In contrast to the detection of quasistatic forces, the response of the can-tilever in the dynamic mode is more complex and deserves some discussion.If the cantilever is excited sinusoidally at its clamped end with a frequency! and an amplitude 0, the probe tip likewise oscillates sinusoidally with acertain amplitude , exhibiting a phase shift fi with respect to the drive signalapplied to the piezoelectric actuator. The deflection sensor of the force micro-scope monitors the motion of the probe tip provided that its bandwidth is largeenough. The latter requirement clearly favors optical deflection sensors. Theequation of motion describing the output from the cantilever sensor is givenby

@2d@t2C !0Q

@d@tC !20.d d0/ D 0!0 cos.!t/; 2.

where d0 is the probe-sample distance at zero oscillation amplitude and d.t/ theinstantaneous probe-sample separation. Q, apart from the intrinsic properties ofthe cantilever, which are the lumped effective mass and the resonant frequency,is determined by the damping factor :

Q D m!02

; 3.

with !0 from Equation 1. introduces the influence of the environmentalmedium, which could be ambient air, a liquid, or ultrahigh vacuum (UHV).Q thus ranges from values below 100 for liquids, air, or other gases at anappropriate pressure, to more than 100,000 which is sometimes obtained in

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


56 HARTMANN

UHV. After the usual building-up, Equation 2 leads to the steady-state solution

d.t/ D d0 C cos.!t C fi/ 4.for the forced oscillator. The amplitude of the probes oscillation is given by

D 0!20q

!2 !202 C 4 2!2 : 5.

The phase shift between this oscillation and the excitation signal amounts to

fi D arctan 2!!2 !20

: 6.

The above simplified formalism is based on the assumption that the oscillationamplitude is sufficiently small in comparison with the length of the cantilever.Obviously, the results derived so far describe only free cantilever oscillations,e.g. oscillations at the absence of any probe-sample interaction. This means d0is still so large that no influence of the sample on the probes oscillation can bedetected. If d0 is now decreased such that a force F affects the motion of thecantilever then a term F=m has to be added to the left-hand side of Equation 2.In order to consider almost all interactions that could be relevant in MFM, onehas to assume

F D F

d;@d@t

; 7.

which, apart from the static interaction, also accounts for dynamic forces. Anexample of the dynamic forces is eddy currents (7). Because F covers probe-sample interactions of various types, in particular spatially nonlinear ones; thed.t/ curves monitored by the deflection sensor and found according to Equation2 may represent anharmonic oscillations. If, however, F.d / can be substitutedby a first-order Taylor series approximation for 0 d0, then the force micro-scope detects the compliance or vertical component of the force gradient @[email protected] the basis of this approximation, the cantilever behaves under the influenceof the probe-sample interaction as if it had a modified spring constant

cF D c @F@z; 8.

where c is the intrinsic spring constant entering Equation 1. An attractiveprobe-sample interaction with @F=@z > 0 will effectively soften the cantileverspring, while a repulsive interaction with @F=@z < 0 will make it effectively

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



stiffer. According to Equation 1, the change of the apparent spring constantwill modify the cantilevers resonant frequency to

! D !0r

1 1c

@F@z: 9.

Provided that @F=@z c, the shift in resonant frequency is given by

1! 12c@F@z: 10.

According to Equations 5 and 6, a shift in the resonant frequency will resultin a change of the probes oscillation amplitude and of the phase shift fi be-tween probe oscillation and driving signal. 1!; , and fi are experimentallymeasurable quantities that can be used to map the lateral variation of @[email protected] and amplitude additionally contain information about the damping coef-ficient . Thus a local variation of this quantity can be separated from the localvariation of the compliance by measuring the frequency shift and the changein amplitude or the phase shift. The simple harmonic solution in Equation 4evidently shows that the dynamic mode of operation can be based on the em-ployment of lock-in signal detection methods. The additional use of suitablefeedback mechanisms opens up different variants of operation.

The most commonly used detection method, generally referred to as slopedetection, involves driving the cantilever at a fixed frequency ! slightly offresonance. According to Equation 9, a change in @F=@z gives rise to a shift inthe resonant frequency of1! and, according to Equation 5, to a correspondingshift 1 in the amplitude of the cantilever vibration. 1 is maximum at thatpoint of the amplitude-versus-frequency curve where the slope is maximum.The sensitivity is ultimately determined by thermal noise. Careful analysis (10)shows that the minimum detectable compliance is given by

@F@z

minD 1rms

s2kTfl!0 Q

; 11.

where rms is the root-mean-square amplitude of the driven cantilever vibrationand fl is the measurement bandwidth. High Q values can be obtained byoperation in vacuum, reducing air damping (


58 HARTMANN

Thus for a high-Q cantilever in vacuum (Q D 50,000) and a typical resonantfrequency of 50 kHz, the maximum available bandwidth would be only 0.5 Hz,which is unusable for most applications. The dynamic range of the systemwould be similarly restricted. Because of these restrictions it is not useful totry to increase sensitivity by raising the Q to such high values. Moreover, if theexperiments have to be performed in vacuum to prevent sample contamination,it may not be possible to obtain low enough Q for an acceptable bandwidthand dynamic range. Therefore, slope detection is unsuitable for most vacuumapplications.

An alternative to slope detection is frequency modulation (FM). In the FMdetection system, a high-Q cantilever vibrating at resonance serves as thefrequency-determining component of an oscillator. Changes in @F=@z causeinstantaneous changes in the oscillator frequency, which are detected by an FMdemodulator. The cantilever is kept oscillating at its resonant frequency byutilizing positive feedback. The vibration amplitude is likewise maintained at aconstant level. A variety of methods, including digital frequency counters andphase-locked loops, can be used to measure the oscillator frequency with veryhigh precision.

In the case of FM detection, a careful analysis (11) shows that the minimumdetectable force gradient is given by that of Equation 11 multiplied by

p2.

However, in contrast to slope detection, Q and fl are absolutely independent inFM detection. Q depends on only the damping of the cantilever and fl is set byonly the characteristics of the FM demodulator. Therefore, the FM detectionmethod shows the sensitivity to be greatly increased by using a very high Qwithout sacrificing bandwidth or dynamic range.

BASICS OF MAGNETIC CONTRAST FORMATIONIf the probe and sample in a scanning force microscope exhibit a magnetostaticcoupling, the major requirement to perform MFM is fulfilled. The manifes-tation of magnetostatic interactions is obvious if a sharp ferromagnetic tipis brought into close proximity with the surface of a ferromagnetic sample.Raster-scanning of the tip across the surface then allows the detection of spatialvariations of the probe-sample magnetic interaction. The long-range magneto-static coupling is not directly determined by the mesoscopic probe geometry,as for other near-field methods, but rather by the internal magnetic structure ofthe ferromagnetic probe. As shown in the following, this greatly complicatesmatters and requires a detailed discussion of contrast formation.

For simplicitys sake, it is easiest to consider the probe as a needle consistingof bulk material. A sharp ferromagnetic needle naturally exhibits considerable

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



magnetic shape anisotropy, which forces the magnetization vector field near theprobes apex to predominantly align with the axis of symmetry of the probe.On the other hand, sufficiently far away from the apex region, where the probescross-sectional area is almost constant, the more or less complex natural domainstructure obtained in a ferromagnetic wire is established. This domain struc-ture depends on the detailed material properties represented by the exchange,magnetocrystalline anisotropy, and magnetostriction energies. Lattice defects,stresses, and the surface topology exhibit an additional influence on the do-main structure. Because of this complicated situation, it is necessary to de-velop reasonably simplified magnetic models to describe the experimentallyobserved features of magnetostatic probe-sample interaction as accurately aspossible.

Since it is generally hopelessly complicated to derive the actual magnetiza-tion vector field of the aforementioned type of probe from first principles, itis reasonable to apply the following model (12). The unknown magnetizationvector field near the probes apex, with all its surface and volume charges, ismodeled by a homogeneously magnetized prolate spheroid of suitable dimen-sion, while the magnetic response of the probe outside this fictitious domainis completely neglected. The second assumption is that the dimensions andthe magnitude of the homogeneous magnetization of the ellipsoidal domain areboth completely rigid, i.e. independent of external stray fields produced by thesample. In this way the micromagnetic problem is simplified to a magnetostaticone.

The model allows interpretation of almost all experimental results obtainedby MFM on the basis of bulk probes. Moreover, the concept of assuming asingle prolate spheroidal domain that is magnetically effective for bulk fer-romagnetic probes approaches reality surprisingly well (12). Using this pseu-dodomain model, the problem is now to determine the probes magnetic prop-erties and the probe-sample magnetostatic interaction for a given experimentalsituation.

The magnetostatic potential created by any ferromagnetic sample is givenby

`s.r/ D 14Z d2s0 Ms.r0/

jr r0j Z

d3r0r Ms.r0/jr r0j

; 13.

where Ms.r0/ is the sample magnetization vector field and s0 an outward nor-mal vector from the sample surface. The first two-dimensional integral coversall surface charges created by magnetization components perpendicular to thebounding surface, whereas the latter three-dimensional integral contains the

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


60 HARTMANN

volume magnetic charges resulting from interior divergences of the magneti-zation vector field. The stray field is then given by Hs.r/ D r`s.r/. Themagnetostatic free energy of a microprobe exposed to this stray field is

.r/ D 0Z

d2s0 Mp.r0/`s.r0/CZ

d3r0rr0 [`s.r0/Mp.r0/]; 14.

where `s.r0/ is given by Equation 13, and Mp.r0/ is the magnetization vectorfield of the probe. The resulting force is then given by F.r/ D r.r/. Thisansatz is rigorously valid for any probe involving an arbitrary magnetizationfield Mp.r/. The first integral, taken over the complete surface of the probe,covers the interaction of the stray field with free surface charges, whereas thelatter volume integral involves the probes dipole moment, as well as possiblevolume divergences. According to the pseudodomain model, Mp.r) is diver-gence free, and the latter integral in Equation 14 reduces to the dipole responseexhibited by the probe.

In many cases of contrast interpretation, even further simplification of theprobes magnetic behavior yields satisfactory results. The effective monopoleand dipole moments of the probe, resulting from a multipole expansion ofEquation 14, are projected into a fictitious probe of infinitesimal size that islocated an appropriate distance away from the sample surface. The a prioriunknown magnetic moments as well as the effective probe-sample separationare treated as free parameters to be fitted to the experimental data. This isknown as the point-probe approximation. The force acting on the probe, whichis immersed into the near-surface sample microfield, is given by

F D 0.q Cm r/H; 15.which implicitly involves the condition r H D 0. q and m are the probeseffective monopole and dipole moments. However, this force is generally notdirectly detected by MFM. Usually the instrument detects the vertical compo-nent of the cantilever deflection. The detected force component is thus givenby Fd D n F, where n is the outward unit normal from the cantilever surface.Well-defined different orientations of the probe with respect to the sample thenallow the successive detection of lateral as well as vertical field components.When Equation 15 is put into component form, one gets the more illustrativeresult

Fd.r/ D 03X

jD1n j

q Hj C

3XkD1

mk@Hk@x j

!; 16.

which is the basis for contrast modeling if the MFM is operated in the staticmode. However, the instruments are usually operated in the dynamic mode,

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



where the probe-sample separation is periodically modulated with an oscillationamplitude, which is small compared with the average probe-sample distance.In this case, the compliance component F 0d.r/ D .n r/.n F.r//, with F.r/according to Equation 15, is detected. Contrast modeling is then based on

F 0d.r/ D 03X

iD1

3XjD1

ni n j

"@q@xiC q @

@xi

Hj .r/

C3X

kD1

@mk

@xi

@

@x jC mk @

2

@xi@x j

Hk.r/

#; 17:

which involves, apart from monopole and dipole moment components, pseu-dopotentials ` pj D @q=@x j and pseudocharges q pki D @mk=@xi . rq D I could,of course, also be associated with a pseudocurrent and r m D Vr M witha pseudodivergence of the probe magnetization within the volume V. How-ever, in the context of Equation 17, the component form is emphasized, andthe denotations pseudopotential and pseudocharge are thus preferred. Thesepseudocontributions result from the fact that the actual magnetic response ofa real probe of finite size clearly depends on its position with respect to thesample surface (12). This aspect has often been completely neglected in theinterpretation of MFM results. In the present context the most important con-sequence is that in dynamic mode MFM, it is not only the second derivativesof the field components that contribute to the ultimately observed contrast but,according to Equation 17, also the first derivatives, as well as the field compo-nents themselves. The number of field derivatives entering Equations 16 and17 is reduced by r H.r/ D 0, leading to

@Hj@xiD @Hi@x j

;@2 Hj@x2i

D @2 Hi

@xi@x j: 18.

The most serious limitation of the point-probe approximation is, of course,that low-pass filtering of the samples stray-field configuration due to the finiteprobe size is completely neglected (13). This latter effect can be accounted forby applying a low-pass filter of type

Hx;y;z.; d/! 412

Z 20

dZ 1=2

0d 0 0Hx;y;z. C 0; d/; 19.

where r D .; d/ determines that geometric center of the probe, which is at aheight d above the sample surface. 0 is a cross-sectional radial vector whoserange is determined by a certain effective probe diameter 1 (12).

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


62 HARTMANN

STANDARD AND ADVANCED PROBESAND THEIR MAGNETIC PROPERTIESIn the force microscope the force-sensing spring consists of a miniaturizedcantilever beam clamped at one end, with the probe tip mounted at the otherend. Originally, electrochemically etched metal wires were used as cantilevers.The increasing demand for cantilevers with integrated sharp tips, manufacturedreproducibly and available in large numbers led to the development of micro-fabrication techniques based on the machining of Si-related materials. Today,a variety of cantilevers with different geometries, mainly finger- and V-shaped,and with pyramidal or conical tips, are commercially available. In order tosensitively measure forces, it is, according to Hookes law, desirable to have alow spring constant. However, this is contradicted by three aspects: First, thespring constant should be maximum in order to achieve a maximum resonantfrequency, and thus minimum vibrational sensitivity and maximum scan rateof the instrument. Second, the ultimate sensitivity of the force measurement isrestricted by thermal excitation of the cantilever. A minimum rms displacementamplitude due to thermal excitation requires a maximum force constant. Third,if the cantilever is subject to long-range attractive forces, which for MFM ap-plications is frequently the case, its position becomes unstable if the magnitudeof the force gradient equals the cantilevers spring constant. Thus a certainminimum spring constant is needed in order for the cantilever to approach thesample sufficiently closely without a jump to contact. Today, commercial can-tilevers have a typical spring constant in the range of 102 N/m c 102 N/m,typical resonant frequencies in the range of 10 kHz !0 500 kHz, a radiusof curvature of the probe tip as low as 10 nm, and are usually fabricated of Si,SiO2 or Si3N4.

MFM requires ferromagnetic probes that interact with the near-surface strayfield of the sample. In order to equip microfabricated cantilevers with somemagnetic sensitivity, they have to be coated with magnetic thin films. This isusually done by thermal evaporation or sputter deposition of suitable ferromag-netic metals or metal compounds. In practice, the sputter deposition of standardcompounds used on magnetic hard disks as recording media has proven suit-able for many standard applications of MFM. In order to ensure a predominantorientation of the probes magnetic vector field along the major probe axis, thethin film is usually magnetized in an electromagnet after deposition.

Unfortunately, the detailed magnetic configuration of the thin film probes isin most cases unknown. Although the general theory of contrast formation stillholds, it is not possible to perform a real modeling of contrast formation fromfirst principles for an unknown domain configuration of the magnetic probe.

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



In particular, it is not possible to simply attribute some well-defined monopoleand dipole moments to the probe because the complicated distribution of theinner and outer magnetic charges of the probe is not known. As a consequence,it is generally not possible to perform MFM in a quantitative way, e.g. in thesense that a stray field is detected in absolute units. Furthermore, even a qualita-tive interpretation of a magnetic contrast obtained by MFM requires minimumknowledge about the probes magnetization. The most detailed informationabout the probe could of course be obtained from a magnetic analysis of theprobe based on a spatial resolution, which is comparable with or even exceed-ing that of MFM. Such resolution makes electron-beam methods promising.For instance, electron holography offers the possibility to obtain some quan-titative information about the magnetic stray field produced by MFM probes(14). Figure 1 gives two examples of electron holograms acquired near the apexof probes of different geometries and magnetic coatings, respectively. The in-formation is displayed as a set of lines, where two successive black or whiteones enclose a magnetic flux quantum h/e. It is obvious that the total magneticflux generated by the probes can be obtained quantitatively from the holograms.Furthermore, by comparing the experimental holograms with modeled ones it ispossible to obtain some information about the local stray-field variation closeto the probes apex and thus ultimately about local variations in the probesmagnetization (14, 15).

The spatial resolution obtained by MFM is clearly related to both the mag-netized part of the probe, which is actually exposed to the sample stray field,and to the probe-sample distance. Thus in order to improve lateral resolution,it is necessary to decrease the magnetically sensitive part of the probe to thesmallest possible size and to operate the probe at close proximity to the samplesurface. This can be realized if the effective volume of the probe is restrictedto a very small particle of magnetic material located at the probe apex. Thefabrication of such a probe is schematically depicted in Figure 2. A standardcantilever is coated from the front with a 50100-nm thick magnetic film of asuitable material (Figure 2ac). At this point, a conventional thin film MFMprobe is achieved. In the next step the cantilever is transferred into a scanningelectron microscope and the electron beam is focused onto the tip apex for aduration of 1015 min. Due to cracking of residual hydrocarbons by the elec-tron beam, a tiny carbon tip is deposited right at the apex of the cantilevers tip(Figure 2d ). In a further processing step, the carbon tip is used as an etch maskduring ArC iron milling of the cantilevers front side. The etching time andthe ion flux are adjusted such that the exposed magnetic material is completelyremoved from the cantilever while the carbon tip is not completely etched away(Figure 2e). This prevents the magnetic coating underneath the cap from being

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


64 HARTMANN

Figure 1 Electron holograms of thin film MFM probes. The scale of the images is 2:351:5m.(a) Equiphase lines for a conical probe covered by a 30-nm thick Co film; (b) a corresponding resultfor a pyramidal tip covered by a 16-nm thick CoCrPt film (sample preparation, Univ. Saarbrucken;holograms, G Matteucci, Univ. Bologna).

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figure 2 Scheme for the electron-beam-assisted fabrication of magnetic supertips.(a,b) A standard cantilever with integratedpyramidal tip. In the first step tip and can-tilever are completely covered with a mag-netic thin film, as shown in (c). A focusedelectron beam is used to grow a tiny carbonwhisker on top of the probes apex, as shown

(d ). Subsequent ArC ion-etching removesthe magnetic thin film except underneath the

cap,which serves as anetch mask (e).

in

carbon

attacked. The result of the whole procedure is a cantilever that exposes a tinymagnetic particle at the probes apex rather than the complete magnetic coating(16). Scanning electron microscope images of such a cantilever are displayedin Figure 3. The carbon tip visible there has a diameter of approximately 50 nmand a length of 100 nm.

The advanced magnetic probes have the potential to produce a much im-proved lateral resolution in comparison with conventional MFM probes.

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


66 HARTMANN

Figure 3 SEM micrographs showing the characteristic dimensions of electron-beamdepositedsupertips: (a) shows a standard cantilever with integrated pyramidal tip; (b) visualizes the supertipin comparison with the original tip of the cantilever; (c) shows the arrangement prior to ArCion-etching. The bright surface contour corresponds to the magnetic thin film.

Figure 4 shows results acquired in the dynamic mode of MFM operation withconventional CoCrPt thin film probes and with the advanced probes consistingof the same magnetic coating. A standard longitudinal recording medium withtwo tracks of bit patterns (periodicities 1 and 2 m, respectively) was used asa test sample. From both the appearance of the MFM images and the detailedcross-sectional profiles, it is obvious that the advanced probes produce a muchimproved lateral resolution. Not only the bit transition zones but also the grainystructure of the recording medium are visible in great detail.

Electron-beamproduced magnetic supertips represent the state of the art.The ultimately obtainable lateral resolution depends on the dimensions of the

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figure 4 MFM images taken on standard hard disk recording tracks of 1 and 2 m periodicity.Image (a) was recorded with a standard MFM probe, (b) was obtained with a magnetic supertip,and (c) and (d ) represent experimental cross-sectional profiles along the 2m track. Panels (e) and( f ) represent results of model calculations that account for the varying sharpness of the probes. Itis obvious that the supertips produce a much better lateral resolution, which permits resolution ofthe grainy structure of the recording medium.

residual magnetic particle at the probes apex. A physical lower limit for itsdimensions arises because an ultra-small particle becomes superparamagnetic.Technical limitations result from the resolution of the lithographic process andfrom the signal-to-noise ratio that clearly drops for decreasing magnetic dimen-sions of the probe.

ANALYSIS OF MAGNETIC RECORDINGCOMPONENTSThe magnetic recording industry is the most important for industrial applicationof MFM. Most notably, analytical requirements resulting from the tendency to

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


68 HARTMANN

continuously increase the areal bit density of recording devices have made MFMthe industrys most powerful method. Technical breakthroughs such as the em-ployment of giant magnetoresistance (GMR) reading heads in hard disk drivesor the use of blue lasers in magneto-optical recording dramatically decreasethe size needed to store one bit of information. Chraracteristic dimensions arealready far below 1 m.

The employment of MFM to analyze media used for longitudinal magneticrecording has obvious advantages. MFM detects that quantity, namely themagnetic stray field produced by the magnetized medium, which is of ma-jor importance for the recording process. Figure 5 shows that the detailedshape of the magnetization transitions resulting from the pole-piece geometryof the writing head can be obtained at fairly high spatial resolution. In particu-lar, no sample preparation is necessary, and the nonmagnetic surface-protectioncoating does not affect the magnetic contrast by any means. Figure 6 showsanalogous data for high-density digital audio tapes.

Figure 5 Standard MFM image of recorded tracks on a hard disk. With MFM it is possible toanalyze the sharpness of the transitions and the characteristic track profile, which are the result ofthe pole-piece geometry of the writing head. The image size is 12 12 m.

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figure 6 MFM image of written tracks on a digital audio tape (DAT). The image size is 120 120 m. Even for the relatively high areal bit densities achieved, standard MFM is capable ofvisualizing the most important characteristics of the recorded pattern.

Other investigations in which MFM is a useful tool include disk-failure anal-ysis, in particular, the destructive results of head-on-crashes, which can beinvestigated in great detail. Dedicated investigations also concern the read-ing part of the recording head. In order to detect local variations in the sen-sitivity of anisotropic magnetoresistance (AMR) or GMR heads, the strayfield of the MFM probe itself is used to produce a resistance change in theread head. While the probe is raster-scanned across the sensitive area of thehead, the global resistance change is monitored as a function of the probeposition. The procedure allows precise optimization of the measuring cur-rent through the head and its polarity. In general, systematic shortcomingsoccurring during head production can be detected with minimal quality controlmeasurements.

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


70 HARTMANN

The techniques used to characterize longitudinal recording media MFM canalso be used to analyze magneto-optical media and corresponding read/writeprocesses. At present there is a strong tendency in the recording industry toreplace costly read/write quality-control experiments, usually performed oncompletely assembled disk drive systems, with selected MFM experiments inan early stage of the production process.

MICROMAGNETIC INVESTIGATIONSMacroscopic magnetic phenomena, usually detected by magnetometers, havetheir origin in the actual topology of the involved magnetic domains and itsmodification under the influence of an externally applied magnetic field. Thedomains are subdivided by interdomain boundaries of a certain finite width. Inmost cases this width ranges between 1 and 100 nm. For in-plane magnetizedsamples, the interdomain boundaries are the only sources of the magnetic strayfield that could be externally detected by MFM, provided that the sample doesnot contain inner and surface defects that usually also produce stray-field varia-tions. In the vicinity of the interdomain boundaries, the interplay of the material-dependent energy contributions usually causes interior divergences of the mag-netization vector field. Additionally, surface magnetic charges can be pro-duced at the intersection between interdomain boundaries and sample surfaces.In turn, if the domain magnetization has a considerable component orientedperpendicular to the sample surface, extended surface charges determine theexterior stray field of the sample. Numerous methods have been developed forthe analysis of magnetic microstructures (17). The advantages of MFM are thatmagnetic microstructures can be imaged at fairly high lateral resolution witha minimum amount of preparation and a maximum variety of environmentalconditions.

Samples with perpendicular magnetic anisotropy produce extended surfacecharges that correspond to the upward and downward pointing domain mag-netization. In this case, the near-surface stray field of the sample is directlyrelated to the domain topology, as shown for the example of a 500-nm thickTb30Fe62Co8 film in Figure 7.

Although interdomain boundaries produce a much less extended stray fielddue to their small size compared with that within domains, it is neverthelesspossible to image their topology in a routine way with state-of-the-art instru-ments. Figure 8 shows the example of an intersection of two 90 Bloch-typewalls with a 180 Bloch wall in an Fe bulk single crystal. Apart from the over-all wall topology, even fine structures of the walls become apparent owing todistinct differences in their stray fields. These fine structures have their originin the underlying global flux-closure behavior. Another interesting example

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figure 7 A 55 m MFM image of the natural domain arrangement in a Tb30Fe62Co8 film withperpendicular magnetic anisotropy. The 500-nm thick film was deposited on a glass substrate andcoated with a 100-nm thick Si3N4 film for surface protection. The surface coating does not affectthe MFM measurement but restricts the probe-sample separation.

for an internal Bloch wall structure is the subdivision of 180 walls into tiltedsegments of opposite magnetization rotation. For a wall in an Fe bulk crystal(Figure 9), the opposite chirality of the successive segments can be imaged byMFM through the alternating positive and negative surface charges.

Numerous important applications of magnetic materials depend on deviceconfigurations involving lithographically structured magnetic thin films. Inthis field the main task is to image the domain topology at a particularlysmall scale given by the dimensions of the respective thin film elements. Anexample of such an application is given in Figure 10. Arrays of polycrys-talline Permalloy dots deposited on Si/SiO2 substrates were prepared by X-raylithography. The diameter of the dots is 1 m and the thickness 50 nm. Thenearest-neighbor distance (Figure 10a) is 1 m and is decreased to 100 nm

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


72 HARTMANN

Figure 8 Flux-closure domain pattern observed on an Fe single crystal. The 20 20 m MFMimage shows the intersection of two 90 Bloch walls with one 180 wall. Distinct differences inthe stray field of the three walls are clearly visible.

(Figure 10b). The respective MFM images show that the dots obviously exhibita heterogeneous magnetic structure. However, that structure is totally disor-dered for the larger interdot separation (Figure 10a), whereas for the smallerlattice constant (Figure 10b), a magnetic superlattice is clearly visible (18). TheMFM analysis clarifies the manifestation of the interdot magnetostatic couplingand the resulting domain closure configuration.

In general, the inverse problem of deducing a concrete arrangement of innerand surface magnetic charges from the overall stray field they produce is notsolvable. MFM can, however, be used to compare the experimentally detectedstray-field variation of a micromagnetic object with that obtained from certainmodel calculations. Thus it is frequently possible to at least classify the mag-netic object under investigation. One such example is the analysis of different

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figure 9 A 20 20 m MFM image of a subdivided 180 Bloch wall in an Fe single crystal.Adjacent and mutually tilted wall segments exhibit different wall chirality.

types of interdomain boundaries, as shown in Figure 11. Fe thin films withvarying film thicknesses have been produced under ultrahigh vacuum (UHV)conditions. Subsequently, in situ MFM images have been taken from selectedinterdomain boundaries. Figure 11a shows an image of a domain boundary ina 10-nm thick film. The experimentally recorded and the micromagneticallymodeled cross-sectional profiles are displayed in Figure 11b,c. The modelcalculations have been performed on the basis of a 90 Neel wall (19). Theappearance of a 90 Bloch-type wall in an 80-nm thick Fe film is shown inFigure 11df. Both wall types can clearly be distinguished by MFM, and theiridentification in conjunction with micromagnetic modeling is straightforward.

Another phenomenon causing only very tiny stray-field variations is the mag-netic ripple structure, which has its origin in wavy deviations from the majormagnetization orientation in the domains (17). The ripple structure is generally

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


74 HARTMANN

Figure 10 Arrays of 50-nm thick Permalloy dots of 1m diameter. The nearest-neighbor spacingis 1 m in (a) and 100 nm in (b). Although the domain walls within the dots are randomly orientedfor the larger dot spacing (a), all walls are aligned for the smaller one in (b).

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figure 11 MFM results and model calculations for domain boundaries in Fe films for two filmthicknesses. The experimental results have been obtained under UHV conditions. Comparisonbetween experimental and theoretical results clearly confirms that a 90 Neel wall is present forthe 10-nm film, whereas a 90 Bloch wall is present for the 80-nm film.

accessible only to transmission electron Lorentz microscopy (17). Figure 12shows an MFM image of a ripple pattern close to a 90 Neel wall in a 10-nmthick Fe film observed under UHV conditions. The wavy magnetization pat-tern, which can be deduced from the MFM image, is schematically shown inthe lower part of the image (20).

Because MFM is sensitive only to the amount and polarity of near-surfacestray fields produced by ferromagnetic samples rather than to the magnetization

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


76 HARTMANN

Figure 12 MFM image displaying the ripple structure in a 10-nm thick Fe film close to a 90 Neel-type boundary observed under UHV conditions. The deduced wavy alignment of the magnetizationis indicated in the bottom of the image.

itself, it is not always straightforward to deduce the overall domain topologyfrom an MFM image. Such a deduction requires a very detailed interpretationof the experimental data (19). Figure 13 shows the complex domain patternobserved under UHV conditions on a 10 nm-thick Fe film. The experimentalresult shown in Figure 13a is composed of several small-scale MFM imagescarefully matched to each other. The overview thus involves extremely highspatial resolution and allows for zooming in at any location. From a careful

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figure 13 Complex domain arrangement in a 10-nm thick Fe film observed by MFM under UHVconditions: (a) shows the experimental result, which is composed of numerous high-resolutionimages; the deduced magnetization orientations are shown in (b); (c) shows the modeled wallcontrast, which is constructed from the magnetic charge of the individual walls.

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


78 HARTMANN

analysis of locally appearing ripple patterns and stray-field variations causedby morphological defects in the film, as well as from the Neel wall contrast,it is possible to deduce the domain configuration shown in Figure 13b. Giventhis domain configuration, modeling of the magnetic charge of the individualinterdomain boundaries allows one to reconstruct the MFM contrast, as shownin Figure 13c.

OBSERVATION OF MAGNETIZATION REVERSALPROCESSESRaster-scanning a sample in order to obtain an MFM image is a very slowprocess compared with the time scale on which processes occur during mag-netization reversal. Dynamic processes that involve the motion of domainboundaries can be observed only stroboscopically or by quasistatic analysis.Thus a Barkhausen jump, for example, cannot be monitored directly but ratherby observing respective wall positions prior and subsequent to the jump ifthe external field driving the jump is varied sufficiently slowly or maintainedfor a sufficient period at a given level. Actually, the latter strategy turns outto be capable of providing useful information regarding microscopic detailsof magnetization reversal processes. A technical problem in monitoring theseprocesses by MFM is that the microscope head or part of it has to be exposedto an external magnetic field of sufficient strength. Commercial instruments,especially, are frequently not very well suited for this purpose because theycontain a variety of ferromagnetic components. Furthermore, a general prob-lem is that the magnetic probe itself is also subjected to the external field. Thismay cause an unwanted response of the cantilever to the varying external field,which is superimposed on the probe-induced excitation. However, if the wholesetup is carefully optimized (Figure 14), it is possible to observe quasistaticmagnetization reversal processes by MFM at high spatial resolution. The ex-ample shows part of the magnetization cycle of a 4.5-nm thick YSmBiGaFegarnet film with perpendicular magnetic anisotropy. The distinct variation ofthe domain topology with varying external field can be clearly deduced. Trac-ing the whole magnetization cycle, MFM hysteresis loops can be obtained at alocal scale.

Magnetization reversal processes in technical materials are frequently deter-mined by pinning of domain walls at structural defects. The MFM observationof such a pinning event is shown in Figure 15. The circle in the center of theimage marks a pronounced defect in a Co/Pt multilayer film of perpendicularanisotropy. Upon expanding or shrinking domains in the vicinity of this defectunder the influence of a varying external field, the strongly hysteretic behaviorbecomes obvious from the MFM images.

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figu

re14

Ase

quen

ceof1

00

100-

m

MFM

imag

esta

ken

on

YSm

BiG

aFe

garn

etfil

mof4

.5

mth

ickn

ess

ina

var

ying

exte

rnal

field

.The

soft

mag

netic

film

hasp

erpe

ndic

ular

aniso

tropy

.A

com

plet

ese

rieso

fim

ages

can

beuse

dto

obt

ain

alo

cal

hyste

resis

loop

oft

hesa

mpl

e.

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


80 HARTMANN

Figure 15 Sequence of MFM images showing the pinning and nucleation of domains at a structuraldefect in a 5 .4 A Co C 15 A Pt) multilayer film with perpendicular anisotropy. The image sizeis 7 7 m. The structural defect is marked and the sequence starts with the upper left image andis terminated with the lower right image.

A subtle phenomenon occurring during magnetization reversal processesis the field-induced modification of the internal structure of interdomainboundaries. Figure 16 gives the example of a subdivided 180 Bloch wallin a bulk Fe crystal. The upper image has been taken at zero external field,whereas in the lower image a downward-oriented external field causes the wallto move to the left. This global wall motion, however, is accompanied bythe local expansion of the upper wall segment downward along the wall di-rection. It is expected that such internal restructurings of domain boundaries

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


Figure 16 7:5 7:5 m MFM images showing the internal reconstruction of a subdivided 180Bloch wall in an Fe single crystal upon the application of an in-plane field. The magnetizationcomponent perpendicular to the sample surface in the individual wall segments is indicated. Theexternal field causes motion of the wall to the left and, at the same time, a downward motion of thetransition between the adjacent wall segments.

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


82 HARTMANN

and the related motion of Bloch lines can be an important contribution to thecoercivity of certain materials (17). However, microscopic processes at thisscale are not well understood.

LIMITS OF THE METHODA most essential question concerning any microscopic method is that of theachievable spatial resolution. The lateral resolution, which can be obtained byMFM, cannot simply be quantified in terms of a precisely defined characteristiclength. It is important to keep in mind that MFM is sensitive to the near-surfacevariation of the stray field produced by certain magnetic objects. In general,the magnetic object with all its stray-field sources cannot be reconstructed bydetecting the resulting stray-field variation. In many experiments, however, oneis interested in the magnetic object rather than its stray field. In particular, thedetailed internal structure and even the lateral extent of interdomain boundariescan a priori not be deduced from MFM images. An example that illustrates thisaspect is shown in Figure 17. The MFM image in Figure 17a shows two domainwalls in a 10-nm thick Fe film observed under UHV conditions. At a certainlocation indicated in the image, cross-sectional profiles have been taken whilecontinuously varying the probe-sample distances between 50 and 250 nm. Theresult is shown in Figure 17b,c, where the latter representation (c) displays linescans at selected probe-sample spacings. The line-scan analysis clearly showsthat upon retracting the probe from the sample surface the stray-field variationrelated to the two walls becomes more and more smeared out. This is a naturalconsequence of the increasing broadening and decreasing strength of the strayfield when moving away from its source. It is thus obvious that, in principle,a high resolution can be obtained in operating the probe as close as possibleto the sample surface. However, even in contact with the sample surface, thelateral extent of the domain wall cannot be determined because the probe hasfinite horizontal and vertical dimensions leading to some averaging over thelocal stray-field variation.

Unfortunately, there is an aspect that limits the minimum possible probe-sample separation in many experiments. MFM relies on the magnetostaticinteraction between probe and sample. Interaction means that the probe some-how acts on the sample and vice versa. The interaction consists in a modifiedmagnetostatic energy balance with respect to an arrangement where probe andsample are completely separated. Whether the perturbed energy balance hasan influence on the magnetization of probe or sample depends only on the sig-nificance of the magnetostatic contributions to the total energy balance. Thelatter is, apart from magnetostatic contributions, determined by the exchangeand magnetocrystalline anisotropy energies. For soft magnetic samples, the

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figure 17 UHV MFM data obtained for two adjacent domain boundaries in a 10-nm thick Fefilm: (a) shows the wall topology; (b) represents the cross-sectional variation of the wall stray fieldat the indicated position when increasing the probe-sample separation from 50 to 250 nm; (c) dis-plays some selected cross-sectional profiles at five different values of probe-sample separation.

magnetostatic contribution can become the most important part in determiningthe overall domain and wall configurations. In such a situation, magnetostaticperturbation caused by the MFM probe can have a very destructive influenceon the samples domain configuration. Figure 18 shows the example of avery soft magnetic garnet film with perpendicular anisotropy. Figure 18acshows a sequence of images taken upon successively decreasing the probe-sample separation. The destructive influence of the probe in terms of domain

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


84 HARTMANN

Figure 18 A sequence of 25 25 m MFM images obtained on a YSmBiGaFe garnet film of4.5 m thickness at probe-sample separations of (a) 910 nm, (b) 520 nm, (c) 390 nm, and (d )910 nm. It is obvious that the domain configuration of the film with perpendicular anisotropy isperturbed if the probe-sample separation becomes too small. If, after such a destructive probe-sample interaction, the probe is retracted to the original working distance, the sample magnetizationhas completely changed to a new remanent state. The images have been taken with a standard MFMprobe.

instabilities caused by the scanning process is clearly obvious. If the probeis again retracted from the sample surface (Figure 18d ), it becomes apparentthat the initial domain configuration has completely changed to a new remanentstate.

Much smaller probe-sample distances are possible with the magnetic su-pertips, as shown in Figure 3. This is shown again in Figure 19, where anMFM image of the soft garnet film has been taken by continuously decreasingthe probe-sample separation, starting at the bottom of the image with somefinite value to zero at the top. The probe now permits MFM imaging without

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



Figure 19 A 25 25 m MFM image obtained with a magnetic supertip on the garnet filmshown in Figure 18. The image has been taken at continuously decreasing probe-sample separation,starting with 200 nm at the bottom of the image. At the top of the image a mechanical contactbetween probe and sample had been obtained. The supertip does not cause any perturbation of thesample magnetization.

any sample perturbation until mechanical contact between probe and sample isachieved.

CONCLUSIONS AND OUTLOOKSince its invention, MFM has become a veritable work horse in research onmagnetic materials, device development, and quality control in the magneticrecording industry. The main strength of the method is to achieve a fairly highresolution in magnetic imaging without special sample preparation and undervarious environmental conditions. The lateral resolution, which can be ob-tained routinely with probes commonly used today, amounts to below 100 nm

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.


86 HARTMANN

for sufficiently strong field variations. Individual flux quanta have been detectedon superconductors. The spatial resolution is, however, not sufficient to detectdetails of the magnetization rotation within interdomain boundaries. On thebasis of adequate model calculations, it is at least possible to distinguish be-tween different wall types and to deduce other micromagnetic features hardlyaccessible to many analysis methods. The stray field produced at a local scaleby MFM probes can be used to induce magnetoresistance on suitable devices.It can, on the other hand, seriously perturb the magnetization of a sample.

MFM is a well-established method with well-known limitations and short-comings. For the future it cannot be expected that dramatic breakthroughs willhappen, e.g. in improving the resolution or sensitivity by an order of magnitudeor more. However, special areas of application, such as low-temperature MFMand/or MFM under UHV conditions, will become further developed and easierto perform. Even in these exotic fields commercial instruments will be avail-able. For standard applications under ambient conditions, technical improve-ments will allow MFM to be used under fairly high externally applied magneticfields. An increasing availability of easy-to-use dedicated commercial instru-ments will ultimately make MFM a widely distributed standard method acces-sible to anyone involved in the research and technical application of magneticphenomena.

ACKNOWLEDGMENTThe author thanks U Memmert and AN Muller (University of Saarbrucken) forsupplying most of the beautiful images. Part of the work presented here hasbeen supported by the German Research Association (SFB 277).

Visit the Annual Reviews home page athttp://www.AnnualReviews.org

Literature Cited

1. Martin Y, Wickramasinghe HK. 1987.Appl. Phys. Lett. 50:145557

2. Goddenhenrich T, Hartmann U, AndersM, Heiden C. 1988. J. Microsc. 152:52736

3. Goddenhenrich T, Lemke H, Hartmann U,Heiden C. 1990. Appl. Phys. Lett. 56:257880

4. Rugar D, Mamin HJ, Guethner P, LambertSE, Stern JE, et al. 1990. J. Appl. Phys.68:116983

5. Grutter P, Mamin HJ, Rugar D. 1992. InScanning Tunneling Microscopy II, ed. RWiesendanger, HJ Guntherodt, pp. 151207. Heidelberg: Springer

6. Moser A, Hug HJ, Parashikov I, Stiefel B,Fritz O. 1995. Phys. Rev. Lett. 74:184750

7. Hoffmann B, Houbertz R, Hartmann U.1998. Appl. Phys. A 66:S40913

8. Lin Y, Grutter P. 1998. J. Appl. Phys.83:733338

9. Wadas A. 1997. In Handbook of Mi-croscopy. Methods II, ed. S Amelinckx, Dvan Dyck, J van Landhuyt, G van Tendeloo.pp. 84553. Weinheim: VCH

10. Durig U, Gimzewski JK, Pohl DW, Schlit-ter R. 1986. IBM Res. Rep. RZ1513

11. Albrech TR, Grutter P, Horne D, Rugar D.1990. IBM Res. Rep. RJ7681

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.



12. Hartmann U. 1994. Adv. Electr. ElectronPhys. 87:49200

13. Schonenberger C, Alvarado SF. 1990.Phys. B 80:37378

14. Matteucci G, Muccini M, Hartmann U.1993. Appl. Phys. Lett. 62:183941

15. Matteucci G, Maccini M, Hartmann U.1994. Phys. Rev. B 50:682327

16. Leinenbach P, Memmert U, Schelten J,Hartmann U. 1999. Appl. Surf. Sci. In press

17. Hubert A, Schafer R. 1998. Magnetic Do-mains. Heidelberg: Springer

18. Mathieu C, Hartmann C, Bauer M, Buett-ner O, Riedling S, et al. 1997. Appl. Phys.Lett. 70:291214

19. Memmert U, Leinenbach P, Losch J, Hart-mann U. 1998. J. Magn. Magn. Mater. 190:12429

20. Leinenbach P, Losch J, Memmert U, Hart-mann U. 1998. Appl. Phys. A 66:S119194

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.

Annual Review of Materials Science Volume 29, 1999

CONTENTSWhat Next for Departments of Materials Science and Engineering? M. C. Flemings 1

Modern Resonant X-Ray Studies of Alloys: Local Order and Displacements, G. E. Ice, C. J. Sparks 25

Magnetic Force Microscopy, U. Hartmann 53Skutterudites: A Phonon-Glass-Electron-Crystal Approach to Advanced Thermoelectric Materials Research, G. S. Nolas, D. T. Morelli, Terry M. Tritt

89

Scanning SQUID Microscopy, John R. Kirtley, John P. Wikswo Jr. 117COMBINATORIAL MATERIALS SYNTHESIS AND SCREENING: An Integrated Materials Chip Approach to Discovery and Optimization of Functional Materials, X.-D. Xiang

149

Surface Roughening of Heteroepitaxial Thin Films, Huajian Gao, William D. Nix 173

Nanocrystalline Diamond Films, Dieter M. Gruen 211Heat Conduction in Novel Electronic Films, Kenneth E. Goodson, Y. Sungtaek Ju 261

Applications of Ultrasound to Materials Chemistry, Kenneth S. Suslick, Gareth J. Price 295

Electrophoretic Deposition of Materials, Omer O. Van der Biest, Luc J. Vandeperre 327

Kelvin Probe Force Microscopy of Molecular Surfaces, Masamichi Fujihira 353

Spin-Tunneling in Ferromagnetic Junctions, Jagadeesh S. Moodera, Joaquim Nassar, George Mathon 381

Characterization of Organic Thin Film Materials with Near-Field Scanning Optical Microscopy (NSOM), P. F. Barbara, D. M. Adams, D. B. O'Connor

433

Two-Dimensional Dopant Profiling by Scanning Capacitance Microscopy, C. C. Williams 471

Scanning Thermal Microscopy, A. Majumdar 505

Ann

u. R

ev. M

ater

. Sci

. 199

9.29

:53-

87. D

ownl

oade

d fro

m w

ww

.annu

alre

view

s.org

A

cces

s pro

vide

d by

Indi

an In

stitu

te o

f Sci

ence

- Ban

galo

re o

n 07

/13/

15. F

or p

erso

nal u

se o

nly.

u hartman

Documents