Nanomagnetics-magnetic Nanoparticles Filled Carbon 

Nanotubes 

by Dereje Seifu, Govind Mallick, and Shashi P. Karna 

ARL-TR-5957 March 2012 

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Army Research Laboratory 

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ARL-TR-5957 March 2012 

Nanomagnetics-magnetic Nanoparticles Filled Carbon 

Nanotubes

Dereje Seifu 

Morgan State University, Baltimore, MD 21251 

Govind Mallick 

Vehicle Technology Directorate, ARL

Shashi P. Karna

Weapons and Materials Research Directorate, ARL 

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4. TITLE AND SUBTITLE

Nanomagnetics–metallic Nanoparticles Filled Carbon Nanotubes

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W911QX-08-C-0102

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6. AUTHOR(S)

Dereje Seifu, Govind Mallick, and Shashi P. Karna

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U.S. Army Research Laboratory

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Aberdeen Proving Ground MD 21005-5069

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14. ABSTRACT

The high demand for nanoelectronics devices, high density magnetic memories, sensors, and energy storage for the future force 

and warfighters have led us to develop unique methods to fill vertically aligned nanofibers with magnetic nanoparticles such as 

cobalt ferrite (CoFe2O4), iron (Fe), and nickel cobalt (NiCo). In this work, pulsed laser deposition (PLD) is used to fill CoFe2O4

and DC magnetron sputtering is used to fill Fe and NiCo in carbon nanotubes (CNTs). The filled CNTs are characterized by 

scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), atomic force microscopy (AFM), magnetic force 

microscopy (MFM), and vibrating sample magnetometer (VSM). Magnetization measurements in-plane and out-of-plane with 

respect to the sample’s surface of CNTs filled with CoFe2O4 indicates a reasonable coercivity of 0.4 T. The magnetic 

anisotropy is, however, found out to be randomly oriented indicating a polycrystalline structure. The unique difference between 

the in-plane and out-of-plane magnetizations is the sharing produced by the demagnetizing field in the perpendicular direction.

15. SUBJECT TERMS

CNTs, EDX, AFM, STM, MFM, SEM, VSM

16. SECURITY CLASSIFICATION OF: 

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 ABSTRACT

UU

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Govind Mallick

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(410) 278-2050

Standard Form 298 (Rev. 8/98) 

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iii

Contents 

List of Figures iv

Acknowledgment v

1. Introduction and Background 1

2. Materials and Methods 3

3. Results and Discussions 4

4. Conclusions 9

5. References 10

List of Symbols, Abbreviations, and Acronyms 13

Distribution List 15
iv

List of Figures 

Figure 1. SEM of vertically grown MWCNTs on SiO2. .................................................................4

Figure 2. SEM of vertically grown MWCNTs on SiO2 filled with CoFe2O4 by PLD at high 

resolution (a) and lower resolution (b). ......................................................................................5

Figure 3. Magnetic moment vs. applied field; the solid line represent in-plane and the scatter 

line represents out-of-plane magnetization. ...............................................................................5

Figure 4. SEM of vertically grown MWCNTs on SiO2 filled with Fe by DC magnetron 

sputtering at 0° from the normal of the substrate in (a) and at 70° from the normal (b). ..........6

Figure 5. Magnetic moment vs. applied field; the solid line represent in plane and scatter 

represent out of plane magnetization of vertically grown MWCNTs on SiO2 filled with 

Fe by DC magnetron sputtering at 0° from the normal of the substrate in (a) and at 70° 

from the normal (b). ...................................................................................................................7

Figure 6. (a) AFM scan of MWCNTs filled with Fe sputtered head-on and (b) the 

corresponding MFM scan. .........................................................................................................8

Figure 7. MFM scan of MWCNTs filled with Fe sputtered head-on. ............................................8

Figure 8. 3-D AFM scan of MWCNTs filled with Fe sputtered head-on. ......................................8
v

Acknowledgment 

This work was supported by the U.S. Army Research Laboratory through the contract number 

W911QX-08-C-0102. We would like to thank Professor P. M. Ajayan at Rice University, 

Department of Mechanical Engineering & Materials Science, Houston, TX, and Dr. M.

Meyyappan, Director and Senior Scientist at Ames’ Center for Nanotechnology in Moffett Field, 

CA, for supplying vertically grown carbon nanotubes (CNTs). This work was also supported in 

part by NSF-MRI-R2 grant (Award No. 0958950). 
vi 

INTENTIONALLY LEFT BLANK. 
1

1. Introduction and Background 

Since the first comprehensive and detailed characterization of carbon nanotubes (CNTs) by 

Iijima in 1991 (1), the saga to fill the low-dimensional space inside CNTs with different types of 

applications-related materials has continued. Pristine CNTs possess incredible mechanical and 

electrical properties (2, 3) besides the hallowed enclosed volume of space. This low-dimensional 

space provides an excellent opportunity to enhance the property of CNTs by filling it with 

applications-related materials (4). A few examples include improved photovoltaic performance 

demonstrated in platinum (Pt)-filled CNTs (5) and the possibility of lightweight wide-band 

microwave absorbers using ferromagnetic-filled CNTs (6). CNTs filled with magnetic 

nanoparticles are attractive candidates for active elements in changeable diffraction gratings, 

filters, and polarizer (7). 

Filling CNTs with both ferromagnetic and ferroelectric materials could be another way of 

coupling the two properties, hence fabricating multi-ferroic materials (8, 9). Iodine-filled CNTs 

have shown high electrical conductivity and excellent mechanical property (10). Molecular 

dynamic simulations (11–13) indicate improved mechanical behavior of filled CNTs compared 

to unfilled CNTs. Increased compressibility of iron (Fe)-filled CNTs has been observed with 

high-pressure x-ray diffraction (14). The mechanical strength of filled CNTs and their buckling 

behavior make them ideal for use as tips for scanning probe microscopes (15). CNTs filled with 

indium (In) can be used as nanoscale mass conveyers in a nano assembly tool (16). 

When filling CNTs with conductive fluids, a voltage is generated between the tube ends 

indicating potential application of CNTs in flow sensing and converting mechanical energy to an 

electrical signal (17). Silver-filled CNTs are promising to be used as spectroscopic enhancers and 

chemical sensors in the visible range (18). Filling CNTs with hydrogen has application for 

mobile energy storage at room temperature (19). 

The list of applications of nanotubes filled with magnetic material also includes materials for 

wearable electronics (20), cantilever tips in magnetic force microscope (21), magnetic stirrers in 

microfluidic devices, and magnetic valves in nanofluidic devices. Biomedical applications 

include capsules or nanosubmarines for magnetically guided drug delivery to desired locations in 

the body, and non-pervasive diagnosis and treatment that can bypass surgery (22). 

Nanomagnets are also important from a fundamental point of view in understanding the physics 

of one-dimensional magnets. The low-dimensional enclosed space of CNTs offer an environment 

in which previously unknown physical phenomena at nanoscale level can be observed. For 

example, confinement of matter on the nanoscale can induce phase transitions not seen in bulk 

systems (23–27). It can also be used as a template to synthesize nanowires and nanomagnets. 
2

The major hurdle for applications-related filling is the assembly of ordered nanoscale structures

and controlled fillings. There has been success in preparing aligned nanotubes vertically on 

silicon dioxide (SiO2) substrate by chemical vapor deposition (CVD) technique (28). In this work

we report a single step procedure used to fill vertically aligned multi-walled (MW) CNTs with 

cobalt ferrite (CoFe2O4) using pulsed laser deposition (PLD) and with Fe using sputtering. This 

is the first attempt ever to fill CNTs using PLD (29) and magnetron sputtering (30, 31), which 

are the two commonly used techniques to prepare various applications related thin and thick 

films.

PLD is a type of physical vapor deposition (PVD) technique where a high power pulse laser

beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. 

Thus, the material is vaporized from the target (in a plasma plume), which deposits itself as a

thin film on a substrate (such as a silicon wafer facing the target). The removal of atoms from the 

bulk material is done by vaporization of the bulk at the surface region in a state of nonequilibrium. In this, the incident laser pulse penetrates into the surface of the material within the 

penetration depth. This dimension is dependent on the laser wavelength and the index of 

refraction of the target material at the applied laser wavelength (248 nm). The strong electrical 

field generated by the laser light is sufficiently strong to remove the electrons from the bulk 

material of the penetrated volume. The free electrons oscillate within the electromagnetic field of 

the laser light and can collide with the atoms of the bulk material, thus transferring some of their 

energy to the lattice of the target material within the surface region. The surface of the target is 

then heated up and the material is vaporized. In the second stage, the material expands in plasma

parallel to the normal vector of the target surface towards the substrate due to Coulomb repulsion 

and recoils from the target surface. The spatial distribution of the plume is dependent on the 

background pressure inside the PLD chamber. PLD is only one of many thin-film deposition 

techniques. Other methods include sputter deposition (radio frequency [RF], magnetron, and ion 

beam).

Sputter deposition is also based on PVD in which materials are sputtered on a substrate from a 

target. Sputtered atoms ejected from the target have a wide energy distribution, typically up to

tens of eV. The sputtered ions are ballistic from the target in straight lines and impact 

energetically on the substrates or vacuum chamber causing re-sputtering. Alternatively, at higher 

gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, 

reaching the substrates or vacuum chamber wall and condensing after undergoing a random 

walk. The entire range from high-energy ballistic impact to low-energy thermal motion is 

accessible by changing the background gas pressure. The sputtering gas is often an inert gas such 

as argon. For efficient momentum transfer, the atomic weight of the sputtering gas should be 

close to the atomic weight of the target, so for sputtering light elements neon is preferable, while 

for heavy elements krypton or xenon are used. Reactive gases can also be used to sputter 

compounds. The compound can be formed on the target surface, in-flight or on the substrate 

depending on the process parameters. The availability of many parameters that control sputter 
3

deposition makes it a complex process, but also allows experts a large degree of control over the 

growth and microstructure of the film. An important advantage of sputter deposition is that even 

materials with very high melting points are easily sputtered. Sputtering sources are usually 

magnetrons that use strong electric and magnetic fields to trap electrons close to the surface of 

the magnetron, which is known as the target. The electrons follow helical paths around the 

magnetic field lines undergoing more ionizing collisions with gaseous neutrals near the target 

surface than would otherwise occur. The extra ions created as a result of these collisions lead to a 

higher deposition rate. It also means that the plasma can be sustained at a lower pressure. The 

sputtered atoms are neutrally charged, and hence, unaffected by the magnetic trap. Though, there 

have been previous attempts (32) to fill multi-walled carbon nantubes (MWCNTs) in aqueous 

suspension with CoFe2O4, the methods presented in this work are more functionally oriented as 

the tubes are aligned and possess high symmetry.

2. Materials and Methods

The vertically aligned MWCNTs used in this filling experiment are grown using a thermal CVD 

method (28) on a SiO2 substrate. This method involves exposing silica structures to a mixture of 

ferrocene and xylene at 770 °C for 10 min. The furnace is pumped down to ~200 mTorr in argon 

bleed and then heated to the temperature of 770 °C. The solution of ferrocene dissolved in xylene 

(~0.01g/ml) is pre-heated in a bubbler to 175 °C and then passed through the tube furnace. The 

furnace is then cooled down to room temperature. The open ended MWCNTs tips are filled with 

CoFe2O4 using PLD and with Fe and nickel cobalt (NiCo) using DC magnetron sputtering 

methods. 

CoFe2O4 is used in this work due to its high anisotropy (4x106ergs/cm2) and magnetostriction 

(800x10–6) at room temperature (33). The target CoFe2O4 for laser oblation is prepared using the 

combination of ferric and cobalt oxides in a 2:1 ratio, respectively (34). The polycrystalline 

CoFe2O4 filling in MWCNT is carried out in a high vacuum (2×10–7Torr) with a 100-W pulsed 

excimer laser (krypton fluoride [KrF]) at 1.5 J/cm2and 3 Hz as the energy density and repetition 

rate at a 248-nm wavelength, respectively. During deposition, the SiO2 template is heated at 

300 °C and the target is rotated in order to ensure its uniform wear. A total of 12,000 shots are 

fired to fill the nanotubes at 3 Hz in an oxygen pressure of 30 mTorr with output energy of 

800 mJ.

DC magnetron sputtering is used to fill vertically aligned MWCNTs with NiCo and Fe. A gun 

power of 50 W and an argon pressure of 0.07 Torr are maintained during the 30-min deposition. 

Then, 51 nm of the sputtering material is deposited over the CNTs at the deposition rate of 

1.7 Å/s. Deposition is carried out at an angle of 0° from the normal to the substrate surface. A 

second sample at an angle of 70° from the normal is also prepared. The latter deposition at 70° is 

an oblique deposition known for constructing nanocolumns. When deposition angle with the 
4

substrate normal is high, a columnar structure is observed (35). The unique structure consisting 

of separated columns inclined toward the deposition direction is due to the “shadowing effect,”

which is caused by the height of the initial nuclei. A shadow is formed behind the initial nuclei 

where the rest of the vapor flux cannot reach. Since the surface mobility of the deposited 

adatoms is low at room temperature the growth rate in the normal to the surface is higher than 

that in the plane.

3. Results and Discussions

Scanning electron microscopy (SEM) of vertically aligned MWCNTs grown on a SiO2 substrate 

before deposition is depicted in figure 1. As shown in the figure, most of the tubes are aligned 

vertically and only very few are misaligned. Filling by PLD method is mediated by the highly 

energetic particles in the plasma formed when an excimer laser of KrF of average power (100 W)

and a wavelength of 248 nm strike the target material, i.e., CoFe2O4. Due to the high energy, in 

the order of several hundreds of eV’s, the particles penetrate deep into the lumen of the CNTs. 

However, these highly energetic particles also increase the probability to damage the CNT 

openings by colliding with the edges. At present, the only mechanism to control this from 

occurring results in slowing all the particles by increasing background pressure. This has a 

negative consequence, however. The particles do not have enough momentum to travel deep into 

the lumen of the CNT. In this work, 30 mTorr of background oxygen pressure is applied to 

prevent CoFe2O4 particles from colliding with the edges of CNTs. Figure 2 depicts SEM of 

vertical tubes after being filled by PLD with CoFe2O4. From SEM measurements, it was not 

feasible to determine the filling depth; however, in single crystals, the depth can be determined 

using in- and out-of-plane magnetization measurements (36).

Figure 1. SEM of vertically grown MWCNTs on SiO2.
5

Figure 2. SEM of vertically grown MWCNTs on SiO2 filled with CoFe2O4

by PLD at high resolution (a) and lower resolution (b).

Magnetization measurements are performed using vibrating sample magnetometer (VSM) both 

in-plane and out-of-plane loops with respect to the sample’s surface. Figure 3 shows the 

magnetic moments with respect to the applied fields for the two loops, indicating the high 

coercivity of 0.4 T. The high coercivity is due to the coupling of the spins of the cobalt (Co) ions 

to the Fe ions in the oxide structure.

Figure 3. Magnetic moment vs. applied field; the solid line represent inplane and the scatter line represents out-of-plane magnetization.

Magnetic properties of a material usually are very sensitive to its shape due to the dominating 

role of anisotropy in magnetism. In particular, the shape of the nanocrystals is a dominating 

factor for the coercivity. Such magnetic nanocrystals with distinct shapes possess tremendous 

potentials in technological applications of magnetic nanocrystals for high-density information 

a b
6

storage and also in the fundamental understanding of magnetism. The coercivity of nanoparticles 

from Stoner-Wohlfarth theory is determined by anisotropy constant and saturation magnetization

(37):

HC = 2K/(μoMS), (1)

where μo is a universal constant of permeability in free space, K is the blocking temperature, and 

MS is saturation magnetization.

The in-plane (solid line, figure 3) and out-of-plane (scattered line, figure 3) magnetization loops 

exhibit the same hysteresis. The in-plane and out-of-plane hysteresis curves coincide, indicating 

a high coupling between the in-plane and out-of plane anisotropy. The slight difference between 

the in-plane and out-of-plane magnetizations is the sharing produced by the demagnetizing field 

in the perpendicular direction.

Figure 4 shows the SEM and energy dispersive spectroscopy (EDS) of vertically grown 

MWCNTs filled by DC magnetron sputtering with Fe. In figure 4a, the deposition is parallel to 

the normal of the substrate surface and in figure 4b the deposition is oblique at an angle of 70° to 

the normal of the substrate surface. The EDS measurements show Fe peak and silicon (Si) peak 

which is due to the SiO2 substrate on which the CNTs are grown.

Figure 4. SEM of vertically grown MWCNTs on SiO2 filled with Fe by DC 

magnetron sputtering at 0° from the normal of the substrate in (a) 

and at 70° from the normal (b).

The graph of the magnetic moment of the Fe-filled MWCNTs as a function of applied field is 

shown in figure 5. Solid line represents in-plane and scatter represents out-of-plane 

magnetization. In both cases, the in-plane and out-of-plane magnetization hysteresis loops 

coincide, indicating a strong coupling between the in-plane and out-of-plane anisotropy, which is 
7

mainly shape anisotropy due to the constricted spacing of the lumen of CNT. The hysteresis loop 

of Fe-filled MWCNT’s deposited at 0° from the normal exhibits an anomalous narrowing of the 

loop at the zero magnetization axis. This anomalous hysteresis loop indicates somewhat 

indifferent domain behavior and is subject to further investigation.

Figure 5. Magnetic moment vs. applied field; the solid line represent in plane and scatter represent out of plane 

magnetization of vertically grown MWCNTs on SiO2 filled with Fe by DC magnetron sputtering at 0° 

from the normal of the substrate in (a) and at 70° from the normal (b).

Figure 6 shows 5 x 5 μm2scan areas of atomic force microscopy (AFM) and magnetic force 

microscopy (MFM) images of Fe-filled MWCNTs at the same spot of the sample prepared by 

DC magnetron sputtering. The distinct bright and dark regions in the MFM images have a greater 

length scale than the corresponding atomic force morphology images. The average domain size 

D is determined by measuring the distance between successive bright to dark regions using line

scans across the MFM image. The competition between magnetostatic, exchange, 

magnetocrystalline, and any other growth-induced anisotropy energies lead to stripe domain 

configurations in which the domain size depends on the sample thickness and the domain 

configuration depends on the sample magnetic history. The theoretical explanation of the 

formation of magnetic domains is to minimize the magnetic energy of a ferromagnetic crystal.

Figures 7 and 8 illustrate two-dimensional (2-D) and three-dimensional (3-D) versions of the 

1.5 x 1.5 μm2scan area of magnified version of MFM image shown in figure 6b. The stripe 

domains in both figures are clearly visible with typical domain width thickness of 0.25 μm.
8

Figure 6. (a) AFM scan of MWCNTs filled with Fe sputtered 

head-on and (b) the corresponding MFM scan.

Figure 7. MFM scan of MWCNTs filled with 

Fe sputtered head-on.

Figure 8. 3-D AFM scan of MWCNTs filled with Fe sputtered head-on.
9

4. Conclusions 

In summary, for the first time, PLD technique and magnetron sputtering are used to fill vertically 

aligned CNTs with high-yield magnetic nanoparticles. The magnetization measurements suggest 

that the filled magnetic nanoparticles have a polycrystalline nature, as evidenced from the 

randomly oriented magnetic anisotropy. 

Spin reorientation due to competition between shape anisotropy of nanotubes and magnetocrystalline component of anisotropy was observed while filling the MWCNTs with CoFe2O4, and

seemed to be dependent on the lattice of the material.

The fillings of Fe and NiCo in the CNTs using magnetron sputtering method, showed an 

anomalous hysteresis in the VSM measurement and the stripe magnetic domains in the MFM 

measurement with an average thickness of 0.25 μm. This is indicative of a soft magnetic 

material. 

We believe that our present work further extends the applications of CNT-based materials in 

nanoscale electronics technologies. This study opens up not only a simple and easy way 

preparation method and low-cost synthesis procedures, but also state-of-the-art industrial-scale 

preparation techniques. The fabrication of oxide nanowires using MWCNTs as templates will 

provide new approach to nanofabrication of ferromagnetic materials, high Tc superconductors, or 

colossal magnetoresistance (CMR) materials. 
10

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2004, 42, 1395–1399. 

33. Bozorth, R. M.; Tilden, E. F.; Williams, A. J. Anisotropy and Magnetostriction of Some 

Ferrite. Phys. Rev. 1955, 99, 1788–1798.

34 Benjamin, J. S. Mechanical Alloying. Sci. Am. 1976, 234, 40.

35. Smith, D. L. Thin Film Deposition – Principle and Practice; McGraw-Hill, 1995.

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Alloys. Philos. Trans. R. Soc. London 1948, Ser. A240, 599.
13

List of Symbols, Abbreviations, and Acronyms 

2-D two-dimensional 

3-D three-dimensional 

AFM atomic force microscopy 

CMR colossal magnetoresistance 

CNT carbon nanotube 

Co cobalt 

CoFe2O4 cobalt ferrite 

CVD chemical vapor deposition 

EDS energy dispersive spectroscopy 

Fe iron 

In indium 

KrF krypton fluoride 

MFM magnetic force microscopy 

MW multi-walled 

MWCNT multi-wall carbon nanotube 

Ni nickel 

NiCo nickel cobalt 

PLD pulsed laser deposition 

Pt platinum 

PVD physical vapor deposition 

RF radio frequency 

SEM scanning electron microscopy 

Si silicon 
14

SiO2 silicon dioxide 

VSM vibrating sample magnetometer 
15

NO. OF 

COPIES ORGANIZATION 

 1 ADMNSTR 

 ELEC DEFNS TECHL INFO CTR 

 ATTN DTIC OCP 

 8725 JOHN J KINGMAN RD STE 0944 

 FT BELVOIR VA 22060-6218 

 1 US ARMY RSRCH DEV AND ENGRG 

 CMND 

 ARMAMENT RSRCH DEV & ENGRG 

 CTR 

 ARMAMENT ENGRG & TECHNLGY 

 CTR 

 ATTN AMSRD AAR AEF T J MATTS 

 BLDG 305 

 ABERDEEN PROVING GROUND MD 

 21005-5001

 1 US ARMY INFO SYS ENGRG CMND 

 ATTN AMSEL IE TD A RIVERA 

 FT HUACHUCA AZ 85613-5300

 1 US GOVERNMENT PRINT OFF 

 DEPOSITORY RECEIVING SECTION 

 ATTN MAIL STOP IDAD J TATE 

 732 NORTH CAPITOL ST NW

 WASHINGTON DC 20402 

NO. OF 

COPIES ORGANIZATION 

22 HCS US ARMY RSRCH LAB 

 2 CDS ATTN RDRL VT (ODIR) 

 (1 HCS) 

 ATTN RDRL VTM G MALLICK 

 (10 HCS, 1 CD) 

 BLDG 4603 

 ATTN RDRL WM (A) 

 ATTN RDRL WM S KARNA 

 (10 HCS, 1 CD) 

 BLDG 4600 

 ABERDEEN PROVING GROUND MD 

 21005 

 3 US ARMY RSRCH LAB 

 ATTN IMNE ALC HRR 

 MAIL & RECORDS MGMT 

 ATTN RDRL CIO LL TECHL LIB 

 ATTN RDRL CIO LT TECHL PUB 

 ADELPHI MD 20783-1197 

TOTAL: 31 (1 ELEC, 2 CDS, 28 HCS) 
16

INTENTIONALLY LEFT BLANK.