Effect of spacer layer thickness on magnetic interactions in
self-assembledsingle domain iron nanoparticles
Nichole B. Herndon,1 Sang Ho Oh,1,2 Jeremiah T. Abiade,3 Devdas
Pai,1 Jag Sankar,1Stephen J. Pennycook,2 and Dhananjay
Kumar1,2,a1Center for Advanced Materials and Smart Structures,
Department of Mechanical and ChemicalEngineering, North Carolina
Agricultural and Technical State University, North Carolina 27411,
USA2Materials Science and Technology Division, Oak Ridge National
Laboratory, Oak Ridge, Tennesse 37831,USA3Department of Materials
Science and Engineering and Department of Mechanical Engineering,
VirginiaPolytechnic Institute and State University, Blacksburg,
Virginia 24061, USA
Presented on 9 November 2007; received 13 September 2007;
accepted 25 October 2007;published online 29 January 2008
The magnetic characteristics of iron nanoparticles embedded in
an alumina thin film matrix havebeen studied as a function of
spacer layer thickness. Alumina as well as iron nanoparticles
weredeposited in a multilayered geometry using sequential pulsed
laser deposition. The role of spacerlayer thickness was
investigated by making layered thin film composites with three
different spacerlayer thicknesses 6, 12, and 18 nm with fixed iron
particle size of 13 nm. Intralayer magneticinteractions being the
same in each sample, the variation in coercivity and saturation
magnetizationis attributed to thickness dependent interlayer
magnetic interactions of three types: exchange, strongdipolar, and
weak dipolar. A thin film composite multilayer structure offers a
continuously tunablestrength of interparticle dipole-dipole
interaction and is thus well suited for studies of the influenceof
interaction on the magnetic properties of small magnetic particle
systems. 2008 AmericanInstitute of Physics. DOI:
10.1063/1.2833309
Materials consisting of self-assembled nanosized mag-netic
particles embedded in a host matrix have received con-siderable
attention since they offer the potential to satisfy thecritical
needs of data storage devices.13 Magnetic nanopar-ticles assembled
in an insulating matrix can also providegreat insight into the
physics of magnetic interactions.4 As-semblies of small magnetic
particles have been synthesizedpredominantly using chemical
processes where interparticleseparation is manipulated by
controlling the volume fractionof particulate systems in host
matrices.58 Using an exclusivechemical process, however, it is a
challenge to fabricatenanoparticle thin film composite in a
multilayered geometrywhich is important for the device applications
ofnanocomposites.912 In this paper, we report our investiga-tion on
the role of spacer layer thickness on magnetic inter-action and
resulting magnetic properties of nanoparticles as-sembled in a
three dimensional structure using a pulsed laserdeposition PLD
method.
A multitarget PLD system was used where iron and alu-mina
targets were alternately ablated. The details of PLDprocess
parameters are reported in our previouspublications.13,14 The
thickness of first Al2O3 layer, whichserves as barrier layer was 75
nm, is significantly larger thanthe thicknesses of Al2O3 in
subsequent layers, where it actsas spacer layer. While the size of
Fe nanocrystals was keptfixed in all the samples, the thickness of
Al2O3 layer wasvaried from sample to sample by varying the number
of laser
pulses impinging on the A2O3 target. The numbers of alter-nating
layers Fe and Al2O3 were five.
The size of Fe particles, the exact thickness of the
spacerlayer, and interparticle separation were investigated by
cross-sectional scanning transmission electron microscopySTEM.
Shown in Fig. 1a is the STEM image of fivelayered FeAl2O3 sample
prepared using 1500 pulses of la-ser on Fe target and 166 pulses on
Al2O3 target. The averageFe particle size is 135 nm, the average
intralayer par-ticle separation is 3 nm, and the interparticle
separation is6 nm. Though the majority of particles in one layer
arephysically separated from the particles in layers above orbelow
it, in some regions, the interparticles are in direct con-tacts
with each other. A similar STEM image of anothersample prepared
using 500 laser pulses on Al2O3 target isshown in Fig. 1b. The
number of laser pulses on Fe targetwas fixed as in the previous
sample, i.e., 1500. Clearly, thespacer layer thickness has
increased 18 nm and the par-ticles in one layer are well separated
from the particles in theother layers. The particle size as well as
intralayer particleseparation are almost the same as the last
sample. As seen inthe two insets in Fig. 1, the interface between
Fe nanopar-ticle and the surrounding Al2O3 matrix in both samples
isclean and sharp, indicating the presence of pure metallic Feat
the surface. Looking at the absolute value of the heat offormation
Hf for the most stable iron oxide 39.2 kcal /gatom, one might
speculate the formation of an oxide layeron the Fe particle surface
more when the matrix surroundingthe nanoparticles is an oxide.15,16
However, the Hf for ironoxide is almost twice the Hf of aluminum
oxide80.2 kcal /g atom.15,16 The more negative the value of the
aAuthor to whom correspondence should be addressed. Electronic
mail:[email protected].
JOURNAL OF APPLIED PHYSICS 103, 07D515 2008
0021-8979/2008/1037/07D515/3/$23.00 2008 American Institute of
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Hf, the more strong the element bonds to oxygen. Besides,all the
sample fabrications are carried out in high vacuumcondition 107
torr. Our previous atomic level structuralcharacterization study by
means of STEM and electron en-ergy loss spectroscopy on
nanoparticles of Ni which has asimilar Hf for its stable oxide 28.8
kcal /g atom embed-ded in A2O3 thin film matrix using the PLD and
similardeposition conditions had proved the absence of any
oxidelayer on the Ni nanoparticle surface.14 Thus, based on theheat
of formation data and the structural characterization in-formation
of NiAl2O3,14 it is reasonable to assume that theFe nanoparticles
in Al2O3 matrix have very high likelihoodto be free from any
appreciable oxide dead layer in thepresent study.
The magnetic properties of FeAl2O3 systems weremeasured using a
vibrating sample magnetometer interfacedwith a physical property
measurement system. Figure 2shows the magnetic moment versus field
loops at 10 K forFeAl2O3 samples having three different spacer
layer thick-nesses. The magnetic moment versus field loops were
alsorecorded at several other temperatures between 10 and 300 Kfor
samples with different spacer layer thicknesses. The re-
sults are plotted in Fig. 3 in the form of Hc vs T.
Thecoercivity values of the sample with 12 nm thick Al2O3spacer
layer are higher than that of the other two samples atall the
temperatures. In an assembly of single domain mag-netic particles,
the magnetic field Hc required to reversetheir magnetization is
given by the equation,
Hc = 2K/Ms1 T/TB1/2 , 1
where TB =KV /25kB for uniaxial anisotropy is called theblocking
temperature where the metastable hysteretic re-sponse is lost for a
particular observation time. Here, K is theanisotropy constant of
the particle material, V is the volumeof the particle, and Ms is
the saturation magnetization. KV isthus the anisotropy energy
barrier for the reversal of magne-tization. As shown in Fig. 3, the
iron particles follow thisrelationship closely in all three samples
with different spacerlayer thicknesses. This trend suggests the
presence of assem-blies of noninteracting magnetic particles with
uniaxial an-isotropy on the basis of Nels concept of
superparamag-netism. However, in a dense systems, such as in the
presentstudy volume fraction of nanoparticles=0.43 for 6 nmAl2O3,
0.33 for 12 nm Al2O3, and 0.26 for 18 nm Al2O3spacer layer samples,
the interparticle interaction is knownto modify the magnetic
behavior of the assembly.7,17,18 Thevalues of TB obtained from the
fitting of the Hc T data to
FIG. 1. STEM high angle annular dark-field images of a
FeAl2O3multilayer with 6 nm thick Al2O3 spacer layers and b
FeAl2O3multilayer with 18 nm thick Al2O3 spacer layers. The insets
are STEMbright-field images of single isolated Fe particles
embedded in Al2O3 spacerlayers.
FIG. 2. Color online Magnetic moment vs field applied loops at
10 K forsamples with 6, 12, and 18 nm thick spacer layer Al2O3.
FIG. 3. Color online Coercivity as a function of temperature
with the datafitted to equation Hc=2K /Ms1 T /TB1/2.
07D515-2 Herndon et al. J. Appl. Phys. 103, 07D515 2008
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Eq. 1 are 690, 846, and 336 K, respectively, for sampleswith 6,
12, and 18 nm thick Al2O3 spacer layers. The varia-tion of TB for
samples with the same particle size in a matrixwith different
spacer layer thicknesses can be explained us-ing the concept of
magnetic interactions in dispersed mag-netic nanoparticle
assemblies.1720
Two types of interaction mechanismsdipolar andexchangecan
operate between nanoparticles depending onthe interparticle
distances and agglomeration. Since the ma-trix Al2O3 in the present
study is insulating and nonmag-netic, the interaction among Fe
nanoparticles is expected tobe predominantly dipolar. However, if
the spacer layer isvery thin, it may not be able to separate the
magnetic par-ticles completely in one layer from magnetic particles
in thenext layer and so on over the entire macroscopic sample in
auniform fashion. As seen in Fig. 1a, some of the Fe nano-particles
in one layer are indeed in direct contact with Feparticles in
neighboring layers. When the magnetic particlesare in contact with
each other, the exchange type of interpar-ticle interaction
dominates where atomic spins within a par-ticle are coupled. When
the Al2O3 spacer layer thickness isincreased to 12 nm, the
possibility of interlayer Fe particlecontacts is greatly diminished
which, in turn, promotes thedipolar interaction mechanism. The
strength of dipolar inter-action is determined by two parameters,
namely, the averagedistance between the particles and their
size.20,21 Since theparticle size and interparticle separation in a
given layer arefixed, it is reasonable to assume that magnetic
interaction inthe intralayer is the same in all the three samples.
Therefore,an increase in TB of 12 nm Al2O3 spacer layer sample
bynearly 20% with respect to that of 10 nm Al2O3 spacer layersample
can be attributed to change in interlayer interactionmechanism to
dipolar type from exchange type. An increasein TB by a factor of up
to 2 has been observed for magneticnanoparticle dispersions with
the same particle size but witha different volume fraction, which,
in turn, affects the inter-actions among magnetic particles.1720
The decrease in TBand Hc with further increase in spacer layer
thickness i.e., at18 nm can be explained by the decrease in the
strength ofinterparticle dipolar strength. The dipolar interaction
can beweak, moderate, or strong, depending on the level of
volumefraction.1719
In summary, we have fabricated multilayered sampleswith fixed Fe
nanoparticle size and variable spacer layer
thickness using a physical vapor deposition based PLD pro-cess.
The variation of coercivity values as a function ofspacer layers
indicates the existence of an optimum thicknessof a spacer layer to
realize strong magnetic dipolar interac-tion and, hence, to realize
high values of coercivity at highertemperatures in a given system.
The ability to change inter-layer particle interaction without
changing intralayer particleinteraction by means of spacer layer
thickness variation canprovide a convenience means to study the
physics of inter-actions among magnetic nanoparticles.
This work was supported by a NSF-NIRT under GrantNo. DMR-0403480
and by the Center for Advanced Materi-als and Smart Structures.
1S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser,
Science 287,1989 2000.
2C. L. Chien, Annu. Rev. Mater. Sci. 25, 129 1995.3O. Santini,
D. H. Mosca, W. H. Schreiner, R. Marangoni, J. L. Guimares,F.
Wypych, and A. J. A. de Oliveira, J. Phys. D 36, 428 2003.
4J. S. Moodera, L. R. Kindler, T. M. Wong, and R. Meservey,
Phys. Rev.Lett. 74, 3273 1995.
5V. Russier, C. Petit, J. Legrand, and M. P. Pileni, Phys. Rev.
B 62, 39102000.
6Y. Lalatonne, L. Motte, V. Russier, A. T. Ngo, P. Bonville, and
M. P.Pileni, J. Phys. Chem. B 108, 1848 2004.
7D. Farrel, Y. Cheng, R. W. Mccallum, M. Achan, and S. A.
Majetich, J.Phys. Chem. B 109, 13409 2005.
8X. M. Lin, C. M. Sorensen, K. J. Klabunde, and G. C.
Hadjipanayis,Langmuir 14, 7140 1998.
9C. P. Collier, J. O. Jeppesen, Y. Luo, J. Perkins, E. W. Wong,
J. R. Heath,and J. F. Stoddart, J. Am. Chem. Soc. 123, 12632
2001.
10T. Cui, F. Hua, and Y. Lvov, IEEE Trans. Electron Devices 51,
503 2004.11C. Joachim, J. K. Gimzewski, and A. Aviram, Nature
London 408, 5412000.
12M. Ollsen, B. Persson, and L. Salford, Magn. Reson. Imaging 4,
4371986.
13D. Kumar, J. Narayan, A. V. Kvit, A. K. Sharma, and J. Sankar,
J. Magn.Magn. Mater. 232, 161 2001.
14D. Kumar, A. Lupini, S. J. Pennycook, J. Narayan, and G.
Duscher, Appl.Phys. Lett. 81, 4204 2002.
15R. K. Singh and D. Kumar, Mater. Sci. Eng., R. 22, 113
1998.16R. S. Williams and S. Chaudhury, in Chemistry of High
Temperature Su-
perconductors II, edited by D. L. Nelson and T. F. George
AmericanChemical Society, Los Angeles, CA, 1988, p. 291.
17D. Kechrakos and K. N. Trohidou, Phys. Rev. B 58, 12169
1998.18J. L. Dorman, L. Bessais, and D. Fiorani, J. Phys. C 21,
2015 1988.19S. Gangopadhyay, G. C. Hadjipanayis, C. M. Sorenson,
and K. J. Kla-
bunde, IEEE Trans. Magn. 29, 2619 1993.20R. W. Chantrell, M.
El-Hilo, and K. OGrady, IEEE Trans. Magn. 27,
3570 1991.21A. Ceylan, C. C. Baker, S. K. Hasanain, and S. Ismat
Shah, Phys. Rev. B
72, 134411 2005.
07D515-3 Herndon et al. J. Appl. Phys. 103, 07D515 2008
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