ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 322 (2010) 332–336

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials

0304-88

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/jmmm

Demonstration of ultra-high-resolution MFM images using Co90Fe10-coatedCNT probes

Sang-Jun Choi a, Ki-Hong Kim b,�, Young-Jin Cho c, Hu-san Lee c, Soo-haeng Cho d, Soon-Ju Kwon e,Jung-hwan Moon f, Kyung-Jin Lee f

a Device Architecture Laboratory, Semiconductor R&D center, Samsung Electronics Co. Ltd., Yongin 446-712, Republic of Koreab AE group, Corporate Technology Operations SAIT, Samsung Electronics Co. Ltd., Yongin 446-712, Republic of Koreac Magnetic Device Group, Corporate Technology Operations SAIT, Samsung Electronics Co. Ltd., Yongin 446-712, Republic of Koread Department of Physics, College of Science and Technology, Yonsei University, Wonju 220-710, Republic of Koreae Department of Material Science and Engineering, POSTECH, Pohang 790-784, Republic of Koreaf Department of Materials Science and Engineering, Korea University, Seoul 136-701, Republic of Korea

a r t i c l e i n f o

Article history:

Received 31 October 2008

Received in revised form

4 September 2009Available online 18 September 2009

Keywords:

CNT probe

MFM

Magnetization value

Co90Fe10

FCI

53/$ - see front matter & 2009 Elsevier B.V. A

016/j.jmmm.2009.09.052

esponding author. Tel.: +82 31 280 9193; fax:

ail address: kihong21.kim@samsung.com (K.-H

a b s t r a c t

We demonstrate ultra-high-resolution magnetic force microscopy images of perpendicular magnetic

storage media using carbon nanotube probes coated by ferromagnetic Co90Fe10 films (20, 30, 40, and

50 nm). By optimizing ferromagnetic film thickness (effective tip diameter), we obtained best magnetic

domain image with an 40 nm-Co90Fe10-coated tip (50 nm tip diameter) about a lateral detect density of

1200 k flux per inch on perpendicular magnetic storage medium, one of the highest resolutions in MFM

imaging reported for this material system and structure. The observed dependence of tip dimension on

signal contrast and image resolution was successfully explained by a theoretical analysis indicating that

the signal contrast, along with the physical probe-tip dimension, should be taken into account to design

magnetic probes tips for high-resolution magnetic force microscopy.

& 2009 Elsevier B.V. All rights reserved.

Magnetic force microscopy (MFM), widely used to investigatemagnetic domains, has attracted much attention for its applica-tion as a tool for magnetic research [1,2]. A magnetic-media (ormagnetic random access memory) fabrication technology havingbeen developed, 330 G bits per square inch based on the CoCrPt-oxide material system has recently been deployed commercially[3]. The behavior of magnetic domain with a size below 20 nm,however, has yet to be fully or effectively investigated, partlybecause of the limitation in reducing MFM probe-tip diameter.Among many reports, Hug’s group reported clear MFM image ofwhich resolution is 1015 kFCI [4]. Recently, Kuramochi et al.obtained a magnetic storage media domain structure of 1100 kflux change per inch (kFCI) density [5]. More recently, Hana et al.obtained high-resolution magnetic domain images using acomplex IrMn/CoFe/Ru/CoFe/Ru/CoFe/IrMn (10 nm)-structureddual synthetic tip (1100 kFCI) [6]. Their results provided theprospect to obtain very high MFM resolution (below 20 nm).

In this report, we demonstrate the achievement of very high-resolution MFM images. We employed a carbon nanotube (CNT)probe over a conventional silicon probe, as the CNT probe was

ll rights reserved.

+82 31280 9157.

. Kim).

found to be fabricated in extremely small feature and also to bereliable. On the CNT probe, ferromagnetic Co90Fe10 films withvarious thicknesses (20, 30, 40 and 50 nm) were deposited and weevaluated the imaging capabilities of our sample probes in ultra-high-density perpendicular magnetic storage media with densi-ties ranging from 700 to 1400 kFCI. By optimizing tip size and itseffect on detected signal contrast, we successfully obtained a clearmagnetic domain image with a 1200 kFCI density, one of thehighest resolutions in MFM imaging. Furthermore, we report atheoretical analysis of the behavior of the magnetic fieldinfluencing the surface of the sample as a function of the diameterand CoFe film thickness of the MFM probe-tip, which is inexcellent agreement with the experimental values.

Modified cantilevers (nano-tools) of multi-walled CNTs(1000 nm length, 20 nm diameter, and 151 tilted) on a Si micro-fabricated tip were employed over conventional Si probes/tips.Ferromagnetic Co90Fe10 thin films with various thicknesses (20, 30,40, and 50 nm) were deposited on/around the bare CNT probes(TIP0) using a radio-frequency (RF) sputtering system (LA-440) andthe incidence angle for deposition is 901. The RF power, argon flowrate, and deposition pressure were 400 W, 50 sccm, and 5 mTorr,respectively. We did not do any surface functionalization andsummarized the dimension of the prepared probes in Table 1. Inorder to prove the capability of the prepared tips, we investigated

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S.-J. Choi et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 332–336 333

magnetic domains of ultra-high-density perpendicular magneticstorage media provided by Samsung electronics company.

The Co90Fe10-coated CNT probes and TIP0 (for comparison) wereinvestigated by UHR-SEM (S-5500, Hitachi). Magnetic force micro-scopy measurements were carried out by scanning probe microscopy(DI 3100, VEECO) in a double pass with an amplitude modulationdetection scheme at room temperature under atmospheric pressure.The scan rate and lift height were set to be 1 Hz and 20 nm,respectively, and the resonance frequency was about 300 kHz.

First of all, the physical structure of the prepared probes wereinvestigated by UHR-SEM. Fig. 1 displays tip images of the bareand Co90Fe10-coated CNT probes, showing the shape and effectivediameter of each tip. The shape of TIP0 (bare CNT probe) is conical,and the diameter of the tip surface is about 20 nm (Fig. 1(a)). Asthe thickness of the deposited Co90Fe10 film increases, the overalldiameter of the tip surface accordingly increases. The observedeffective diameter of the 30 nm-Co90Fe10-coated tip (TIP30) was�30 nm, that of 40 nm-Co90Fe10-coated tip (TIP40) was about�50 nm, and that of 50 nm coated tip (TIP50) was about �90 nm.

We scanned high-density perpendicular magnetic storagemedia of varying densities (ranging from 700 to 1400 kFCI) usingthe prepared tips to gauge the detecting capability. A low-density(700 kFCI) medium was first scanned to determine reasonable andgeneral measurement conditions which were also used toinvestigate higher density media. Fig. 2 shows MFM images andphase signals of magnetic-media with 1200 kFCI density and onlyMFM images of 1400 kFCI density. Those images were obtainedunder the same conditions determined by evaluating the 700 kFCI

Table 1The thickness and diameter of Co90Fe10 films on CNT probe.

Sample name Co90Fe10 thickness (A) Diameter (A)

Tip0 0 20

Tip20 20 20

Tip30 30 30

Tip40 40 50

Tip50 50 90

Fig. 1. UHR-SEM images of prepared probes. (a) Bare CNT probe, (b) 30 nm-Co90Fe10-c

coated CNT probe.

medium and the total scan length of Fig. 2 was 2mm. It wasobserved that, whereas the resolution of the images for bothmedia improved as the Co90Fe10 film thickness increases (20–40 nm), the resolution slightly degraded when the medium wasinvestigated using TIP50. In the image of 1200 kFCI magnetic-media using the finest tip (TIP20), the small signal-to-noise (S/N)ratio of the image made it difficult to identify magnetic bitsprobably due to the small magnetic moment.

It is noteworthy that, in the images of the 1200 kFCI densityobtained by TIP30 and TIP40, each magnetic bit was clearlydistinguishable and the measured length between bits is 21.7 nm.The line profile of the MFM image in the inset of Fig. 2 confirms thereasonable detecting capability of our probe tips. This observationclearly demonstrates the achievement of a lateral detect density ofas high as 1200 kFCI on perpendicular magnetic storage media usingTIP30 or TIP40. In contrast, when measuring the same specimenusing TIP50, the number of distinguishable bits decreased.

On the other hand, for the higher density of 1400 kFCI medium,only TIP40 could produce an image of the magnetic domains butthe small S/N ratio made it difficult to distinguish each magneticbit clearly.

Intuitively, we could think that as the ferromagnetic filmthickness on top of the CNT tips increases, the effective diameterof the MFM probe-tip also increases and, thus, the lateral resolutiondegrades. Nevertheless, contrary to the expected behavior, weobserved that TIP40 still yielded a better image than TIP30 inmeasuring 1400 kFCI media as clearly shown in Fig. 2(b). In view ofthis, when comparing two 1200 KFCI images obtained by TIP30 andTIP40 again, it was found that the image obtained by TIP40 hashigher intensity contrast than the image by TIP30 even though thesame resolution was achieved as discussed previously (Fig. 2(a)).

In an attempt to properly understand and explain the observedvariation in the resolution of MFM images as a function of the tipdiameter and Co90Fe10 film thickness, we theoretically calculatedthe resolution by micro-magnetic simulation [7]. In simulations,we divided magnetic volume on CNT probe into 2�2�2 nm3

cubic cells and calculated the force gradient between the recordedmagnetic bit and active ferromagnetic volume as functions of thediameter and thickness of the ferromagnetic film. We assumed

oated CNT probe, (c) 40 nm-Co90Fe10-coated CNT probe, and (d) 50 nm-Co90Fe10-

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S.-J. Choi et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 332–336334

that the magnetic bits were aligned normal to the surface with adensity of 1200 kFCI. Only two up-and-down vertical magneticdomains were considered and we simulated the magnetization ofthe probe in the direction to normal to the surface. In addition, itwas assumed that the ferromagnetic coated tip moves along themedia surface and it contains point magnetic poles as displayed inFig. 3(a). We calculated the force gradient between the tip and thesurface of the media at a given position in every cell. Then, thetotal force gradient profile was obtained by summing up eachvalue and the contrast (or the intensity) and the resolution weredetermined by the algorithm [7] (see Fig. 3(b)). The definitions ofresolution and contrast were indicated in Fig. 3(b). Fig. 3(b)implies that contrast and resolution simultaneously influence onthe image quality of MFM and that the quality of MFM image willbe poorer even using a smaller tip when the contrast is low.

Using these assumptions and conditions, the dependence ofthe resolution and the signal contrast on the ferromagnetic filmthickness and effective diameter of the probe-tip were simulated.First, we calculated the resolution as a function of the diameterwith a fixed 10 nm thick ferromagnetic coating layer (solidsquares in Fig. 3(c)). It was found that the lateral resolutiondegrades when tip diameter increases as predicted. On the otherhand, the dependence of the contrast on film thickness shows aninteresting tendency (open squares in Fig. 3(c)). The drasticincrease of contrast was observed with increasing effective tipdiameter up to 40 nm and the contrast degrades with tip diameterlarger than 40 nm. This behavior can be attributed to two effects:(1) One would be the increase of the number of actual magneticpoles with the increased effective diameter. The more magneticpoles are inside the tip, the stronger interaction occurs betweenthe tip and magnetic domain to enhance the contrast. (2) Theother is the intrinsic property of magnetic field line. Fig. 3(d)shows the behavior of the interaction between the tip andmagnetic domain according to the diameter of the tip. Whenthe diameter is too small, only a few magnetic field lines pass thetip and the number of lines which pass the tip increases as theincrease of the diameter. However, when the size of diameterreaches a certain value, the magnetic field lines which haveopposite direction (downward arrows in blue) also interact withthe tip and complementarily reduce the force gradient.

The behavior implies that the adequate diameter is recom-mended for obtaining MFM images of high-quality at a givenmagnetic domain size. In a separate study which is not shown

500 nm

50 nm Co90Fe10

500 nm40 nm Co90Fe10

500 nm30 nm Co90Fe10

500 nm20 nm Co90Fe10

0

-0

0

-0

0

-0

0

-0

Fig. 2. (a) MFM images of media of 1200 kFCI density and (b) enlarged images in the r

media of 1400 kFCI density. The measured thicknesses of the tip from top to bottom are

signal was 500 nm.

here, we also determined the effect of ferromagnetic filmthickness on the MFM image with the same effective outerdiameter and we found that the thicker ferromagnetic film, asexpected, produced the higher contrast whereas the resolution didnot change noticeably.

Finally, the theoretically calculated dependence of resolution andcontrast on the actual physical dimensions of film thickness anddiameter of the probe tips obtained by UHR-SEM was summarizedand compared in Fig. 3(e). The open circles indicate the contrast andsolid squares are the resolution. The solid line is just connecting thevalues to show the tendency. The resolution of MFM imagesbecomes worse as the tip’s dimension gets larger. However, thecalculated contrast increases drastically and the contrast shows asaturation at and over 40 nm of ferromagnetic film thickness.

The simulated resolution obtained by TIP40 is about 20 nm,which is in excellent agreement with the experimental value(21.7 nm) and is the measurable value for obtaining magneticdomain image at 1200 kFCI. The results of simulation match wellwith the experimental results. In the experimental data, either TIP30or TIP40 could produce the comprehensible MFM images at1200 kFCI but TIP40 still generated a better image than TIP30. Inview of these results and discussions, it can be interpreted that theimprovement in resolution would have to be complemented with anenhancement of the probe sensitivity and that the actual physical tipsize and the high contrast have to be considered simultaneously forhigher resolution MFM images. Accordingly, it can be concluded thatthe reduction in tip size may not be the main factor to obtain high-quality MFM images (e.g. 20 nm coated tip) and the magneticdomain size should also be taken into account in practice.

In conclusion, we fabricated ultra-high-resolution CNT MFMprobes by coating them with Co90Fe10 ferromagnetic film. Thoseprobes were used to demonstrate the achievement of the veryclear images of a magnetic domain of as small as unprecedented21 nm length (1200 kFCI). Especially, the 40 nm-Co90Fe10-coatedprobe produced a clearer MFM image than the physically smaller30 nm-Co90Fe10-coated probe, indicating that the resolution ofMFM image is not determined just by the smaller tip size. Wefound out that the high contrast is also one of the importantfactors to obtain highly resolute MFM image. The contrastincreases according to the diameter and thickness of ferromag-netic film on CNT probe. However, as the tip gets broader, thecontrast degrades because broad tip senses both up-and-downfields arising from neighboring bits, resulting in an overall

500 nm

500 nm

500 nm

500 nm20 nm Co90Fe10

30 nm Co90Fe10

40 nm Co90Fe10

50 nm Co90Fe10

.200

.20

.200

.20

.200

.200 0.25 0.50

.200

.200 0.25 0.50

0 0.25 0.50

0 0.25 0.50

ectangle of Fig. 2(a). (c) Phase signals obtained from Fig. 2(b). (d) MFM images of

labeled. The number of bits was obtained from the signals; the total length of each

-1.0

-0.5

0.0

0.5

1.0

F'C

NT

(A. U

.)

Position (nm)

Resolution

Con

trast

0 100 200 300 400

0

20

40

Resolution Contrast

Res

olut

ion

(nm

)

Diameter (nm)

0

2

4

6

8

Con

trast

(A. U

.)

0 10 20 30 40 50 60 70 80 90

0

20

40

Diameter Thickness#1 20nm 20nm#2 30nm 30nm#3 50nm 40nm#4 90nm 50nm

Con

trast

(A.U

.)

Res

olut

ion

(nm

)

Thickness (nm)

Resolution Contrast

4

6

8

20 30 40 50

Fig. 3. (a) Magnetization of the MFM probe-tip by micro-magnetic simulation. The white arrow represents the vector of magnetization in each cell. (b) Determination of

resolution and contrast using force gradient profile. (c) Change of resolution and contrast as a function of probe diameter with fixed thickness (10 nm). (d) Schematic

drawings of the magnetic interactions as the variation of the diameter. (e) Change of resolution and contrast when both thickness and diameter are varied.

S.-J. Choi et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 332–336 335

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decreased value and degraded MFM image. Therefore, thediameter and thickness of ferromagnetic film of the tip must becarefully considered by simulating the contrast and resolution at agiven magnetic domain for high-resolution magnetic forcemicroscopy.

Acknowledgement

This research was supported by a grant from the FundamentalR&D Program for Core Technology of Materials funded by theMinistry of Knowledge Economy, Republic of Korea.

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