Exchange interaction at the interface of Fe-NiO nanocomposites

Article abstract of DOI:10.1016/j.jssc.2010.09.037

Fe-NiO nanocomposites with Fe to NiO ratio 20:80, 30:70 and 50:50 were prepared by a chemical route. X-ray diffraction and TEM measurements showed the presence of α-Fe and NiO phases in the prepared nanocomposites having an average crystallite size 17 nm. HRTEM image showed a structurally disordered phase at the FeNiO interface. Mssbauer spectra of the nanocomposites consist of a doublet along with a sextet corresponding to relaxed and blocked α-Fe phases, respectively. The dc magnetization measurements in field-cooled condition show a shift of hysteresis loop and an enhancement of coercive field at low temperature confirming the presence of exchange bias at FeNiO interfaces. The irreversibility observed in the FC and ZFC magnetization measurements also points to exchange bias effect.

Full text of DOI:10.1016/j.jssc.2010.09.037

Journal of Solid State Chemistry 183 (2010) 2903–2909
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Exchange interaction at the interface of Fe–NiO nanocomposites
S.P. Pati, B. Bhushan, D. Das n
UGC-DAE Consortium for Scientific Research, Kolkata Centre III/LB-8, Bidhannagar, Kolkata 700 098, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Fe–NiO nanocomposites with Fe to NiO ratio 20:80, 30:70 and 50:50 were prepared by a chemical route.
Received 18 June 2010
Received in revised form
14 September 2010
X-ray diffraction and TEM measurements showed the presence of a-Fe and NiO phases in the prepared
nanocomposites having an average crystallite size 17 nm. HRTEM image showed a structurally
disordered phase at the Fe–NiO interface. Mo¨ssbauer spectra of the nanocomposites consist of a
Accepted 26 September 2010
Available online 1 October 2010
doublet along with a sextet corresponding to relaxed and blocked
magnetization measurements in field-cooled condition show
a
-Fe phases, respectively. The dc
a
shift of hysteresis loop and an
Keywords:
enhancement of coercive field at low temperature confirming the presence of exchange bias at Fe–NiO
interfaces. The irreversibility observed in the FC and ZFC magnetization measurements also points to
exchange bias effect.
Nanocomposites
Mo¨ssbauer spectroscopy
Exchange bias
& 2010 Elsevier Inc. All rights reserved.
Magnetic properties
1. Introduction
In the present paper, structural, hyperfine and magnetic
properties of Fe–NiO nanocomposites prepared by a chemical
route have been reported. A different approach has been adapted
to get better control on the compositional features of the FM–AFM
The study of exchange bias (EB) effect is of current interest
because it plays crucial role in applications involving spin valves
as well as tunnelling and storage devices [1]. It has been shown
interfaces.
a-Fe was chosen as the ferromagnetic phase as it is
Mo¨ssbauer sensitive and has relatively small anisotropy
that exchange interaction at the interfaces of
a composite
(KE0.5 ꢀ 105 J/m3) whereas NiO was chosen as the AFM phase
consisting a ferromagnetic (FM) and an antiferromagnetic (AFM)
component can be useful to overcome the superparamagnetic
limit in FM nanoparticles [2,3]. Exchange bias effect was first
observed by Meiklejohn and Bean in Co–CoO particulate system
[4]. Although exchange bias effect has been widely studied in
multi-layer systems [2,5,6], the polycrystalline powder systems
also show considerable exchange bias effect [4,7–19]. Skumryev
et al. have used the exchange bias effect to overcome the
superparamagnetic limit in Co–CoO multilayer [2]. Most of the
nanocomposite systems used to study the EB effect were prepared
through chemical routes and are composed of a transition metal
and its oxide such as Co–CoO [7,8], Ni–NiO [9,20], Mn–MnxOy [10]
and Fe–FexOy [11] or oxides with different oxidation states such as
in CrO2–Cr2O3 [12,13], Fe3O4–FeO [14,15] and Mn3O4–MnO [16].
Apart from FM–AFM interfaces, the EB effect has also been
observed in samples involving a ferrimagnet (FI) and a spin glass
(SG) e.g. FI–FM, FI–AFM AFM–SG, FM–SG, FI–SG, etc. [17–19].
Traditionally, nanogranular systems showing EB are obtained by
subjecting FM particles to reactive treatment (partial oxidation or
reduction, surface modification by chemical treatment, etc.) to
obtain AFM phases. However, these methods usually give poor
control of the compositional features of the final samples.
´
because of its high Neel temperature (TN¼523 K) and larger
anisotropy (KE2.8 ꢀ 105 J/m3). Due to higher anisotropy of NiO,
it was expected that it could stabilize the magnetic moment of
a
-Fe nanoparticles over thermal fluctuation. Another advantage of
´
choosing NiO is that since it has a high Neel temperature it may
raise the blocking temperature of relaxing -Fe nanoparticles
a
above 300 K through the exchange bias interaction at the Fe–NiO
interfaces of the nanocomposites.
2. Experimental details
The Fe–NiO nanocomposites were prepared by a two-step
chemical route. In the first step, Fe and NiO nanoparticles were
synthesized separately and in the second step nanocomposites
were prepared by intense sonication of different weight propor-
tions of Fe and NiO nanoparticles dispersed in propanol. For
preparation of Fe nanoparticles, 5 mmol FeCl2 ꢁ 4H2O and appro-
priate amount of polyvinyl alcohol (PVA) was dissolved in 150 ml
of Milli-Q water in a three-necked closed flask under nitrogen
atmosphere. 0.5 M sodium borohydrate (NaBH4) was added drop
wise to the solution up to pH 9 under vigorous stirring. NaBH4
was used as a reducing agent and poly vinyl alcohol (PVA) was
used as a surfactant to prevent coagulation and oxidation.
Precipitates so obtained were separated out, washed several
n Corresponding author. Fax: +91 33 2335 7008.
E-mail address: ddas@alpha.iuc.res.in (D. Das).
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.09.037

2904
S.P. Pati et al. / Journal of Solid State Chemistry 183 (2010) 2903–2909
times with H2O and propanol. Then the collected sample was
dried at room temperature in vacuum and heat treated at 500 1C
in a vacuum furnace for 2 h. For NiO nanoparticles, 5 mmol of
Ni(NO3)2 ꢁ 6H2O was dissolved in 150 ml of water and 0.5 M NaOH
was added drop wise to the solution under vigorous stirring until
pH reaches 9. The precipitate thus formed was collected, dried
and heat treated at 500 1C for 2 h in open atmosphere. In the next
step, total 1 g powder of Fe and NiO were dispersed in 100 ml
propanol with Fe: NiO weight ratio as 50:50, 30:70 and 20:80. To
break apart agglomerates of particles and to obtain a homo-
geneous composite, the mixture was sonicated for 1 h under
nitrogen atmosphere in a sonicator operating at a frequency
3373 kHz. The particles were then filtered and dried in vacuum
at room temperature. The samples were named according to iron
content in them, i.e. the sample FN50 contains 50% iron.
The structural properties of the prepared nanocomposites
were determined by the X-ray diffraction (XRD) technique using a
Brucker D8 Advance X-ray diffractometer with a copper target
(
l¼0.1541838 nm). The diffraction angles corresponding to
different peaks were matched with those in standard JCPDS data
files to identify the phases formed. From the broadened peaks the
average crystallite sizes were determined using the Scherrer
formula. A JEOL 2100 model high resolution transmission electron
microscope (HRTEM) operating at 200 kV was used for imaging
the nanocomposites. The room temperature Mo¨ssbauer measure-
ments were carried out in a standard pc-based multichannel
analyser with 1024 channels working in the constant acceleration
mode. A 25 mCi 57Co in Rh matrix was used as the radioactive
source. The system was calibrated with a high purity iron foil of
thickness 12 mm. The spectra thus obtained were deconvoluted
using LGFIT2 programme [21]. Magnetic properties of the
nanocomposites were investigated by using a vibrating sample
magnetometer (VSM, Lakeshore, model 7407) and a SQUID
magnetometer (Quantum Design, MPMS XL 7).
Fig. 1. X-ray diffractograms of Fe–NiO nanocomposites.
3. Results and discussions
by elliptic shapes in Fig. 2(c)). Such structurally disordered
regions at the interface of different nanocomposite systems
were also observed before [20,22]. The high velocity
interparticle collision produced by ultrasonic irradiation may be
a cause of this disordered interface regions [23]. In the HRTEM
images long range crystalline order of NiO is not clearly visible
while that of Fe is clearly observed. Fig. 2(d) shows a typical high
resolution image of a section of Fe–NiO nanocomposite where
inter planer spacing of Fe is clearly visible corresponding to
[110] plane.
3.1. Structural and morphological analysis
XRD patterns of all the nanocomposites are shown in Fig. 1.
From the diffractograms it is clear that all the composites have
both cubic
a-Fe and NiO phases. The intensity of Fe [110] peak
was found to decrease with decrease in iron concentration in the
composites. The average crystallite size of the constituents of the
nanocomposites i.e.
broadening of the corresponding diffraction peaks using the
Debye–Scherrer formula. The average crystallite size of -Fe and
NiO are found to 17 and 18 nm, respectively. None of the
a-Fe and NiO are calculated from the line
a
3.2. Hyperfine analysis
diffractograms showed the presence of
phase of iron oxide.
a-Fe2O3 or any other
The magnetic anisotropy energy of a particle is responsible for
holding the magnetic moment along a certain direction. In the
case of nanoparticles, the magnetic anisotropy energy decreases
with decreasing particle size and below a critical size becomes
comparable with the thermal energy. At this stage the anisotropy
energy is insufficient to hold the magnetic moment along a
certain direction, thus the magnetic moment flips randomly due
to thermal energy. Therefore, a nanoparticle loses its stable
magnetic order and becomes superparamagnetic. To stabilize
the magnetic order of a relaxing ferromagnet, it should be
exchange coupled with an antiferromagnet having higher aniso-
tropy energy. Such exchange-coupled system can be useful
to overcome the superparamagnetic limit of ferromagnetic
nanoparticles [2,3]. In order to study the effect of exchange
bias on superparamagnetic nanoparticles, Mo¨ssbauer study at
room temperature was carried out on the nanocomposites.
In order to get more information on the material microstruc-
tures, the nanocomposites were examined by transmission
electron microscopy. Fig. 2(a) shows a typical particle size
distribution obtained for the sample FN20. It is evident from the
figure that Fe nanoparticles (having darker contrast) are randomly
distributed among the NiO nanoparticles. It is also clear from the
micrograph that all the Fe and NiO nanograins form a well faceted
polygonal shape, having size in the range 16–50 nm. Fig. 2(b)
shows the selected area electron diffraction (SAED) pattern of the
sample FN20 confirming the polycrystalline nature of the sample.
The d-values calculated from the ring diameters confirmed the
presence of both
a-Fe and NiO and the corresponding crystal
planes are labelled. The high resolution transmission electron
micrograph (HRTEM) of the sample FN20 revealed some
structurally disordered region at the Fe–NiO interface (marked

S.P. Pati et al. / Journal of Solid State Chemistry 183 (2010) 2903–2909
2905
Fig. 2. (a) TEM image showing the particle distribution of FN20. (b) SAED pattern of FN20. (c) The HRTEM image showing the disordered regions (marked in elliptic shape).
(d) The HRTEM image showing Fe [110] planes.
Table 1
Mo¨ssbauer parameters of all Fe–NiO nanocomposites.
c
Sample ID
FN20
I.Sa (mm/s)
QSb (mm/s)
Hint (T)
Area (%)
(70.01 mm/s)
(70.01 mm/s)
(70.3 T)
Sd
De
0.09
0.46
0.02
0.71
32.7
–
41
59
FN30
FN50
S
0.06
0.50
0.01
0.79
32.6
–
63
37
D
S
0.10
0.52
0.03
0.72
32.6
–
65
35
D
a
IS: isomer shift.
b
c
QS: quadrupole splitting.
Hint: internal magnetic field.
S: sextet.
d
e
D: doublet.
The Fe nanoparticles, which are located at the Fe–NiO interfaces,
experience strong exchange coupling with the ordered magnetic
moment of NiO nanoparticles. As NiO have comparatively large
magnetic anisotropy, it can hold the magnetic moment of Fe
nanoparticles in a specific direction. So these particles can
overcome the thermal fluctuation and become magnetically
stable. Secondly, the fraction of Fe nanoparticles having size
greater than the critical size (ꢂ25 nm) of becoming super-
paramagnetic at room temperature can also give a sextet pattern.
The central doublets appeared in the spectra are assigned to Fe
Fig. 3. Mo¨ssbauer spectra of Fe–NiO nanocomposites.
Fig. 3 shows room temperature Mo¨ssbauer spectra of all the
Fe–NiO nanocomposites. All the spectra consist of a sextet and a
doublet. The hyperfine parameters obtained after fitting the
spectra are tabulated in Table 1. The sextets having very low
isomer shifts and quadrupole splittings with internal magnetic
field ꢂ33 T are assigned to
a-Fe nanoparticles in the blocked
state [24]. The sextet may have resulted due to two reasons.

2906
S.P. Pati et al. / Journal of Solid State Chemistry 183 (2010) 2903–2909
nanoparticles undergoing superparamagnetic relaxation. The Fe
nanoparticles that are away from the Fe–NiO interfaces experi-
ence very weak exchange force thus become unstable and
undergo spontaneous relaxation. The Fe nanoparticles that are
in contact with the disordered NiO interfaces (as seen in HRTEM
micrograph) also experience very little exchange force and thus
remain superparamagnetic giving a doublet Mo¨ssbauer pattern.
Finer Fe particles having size less than 25 nm will undergo
spontaneous relaxation and give a doublet pattern. A close
inspection of the spectra reveals that the intensity of the doublet
increases as the amount of Fe in the composites decreases. Since
all the composites are having similar particle size distribution one
can not explain this observation by assuming higher fraction of
smaller particles in the composite with lower concentration of Fe.
As the concentration of Fe in the composite decreases, the
strength of dipolar interaction among them should also decrease
and the doublet intensity is expected to increase. Thus weak
overcome the microscopic torque on FM spins. The material
behaves as if there is an extra internal biasing field that manifests
in a shift in the ferromagnetic hysteresis loop in the field axis.
The M–H loops at room temperature of all the samples are
shown in Fig. 4 and the inset shows magnified view of the central
portion of the loops. It is clear from the nature of the loops that
the nanocomposites are magnetically soft. Calculated saturation
magnetization, MS (at 15 kOe), remanent magnetization, MR
(positive value at H¼0) and coercive field, HC (average of +HC
and ꢃHC) are shown in Table 2. MS, MR and HC were found to
decrease with decrease in Fe concentration in the samples. This
can be explained by considering the fact that strength of dipolar
interaction among a-Fe nanoparticles reduces as concentration of
Fe decreases in the nanocomposites. Mo¨ssbauer results as
discussed in the previous section also support this fact as the
relative area of the doublet increases with decrease in Fe
concentration. The central part of the magnetic hysteresis loops
for all samples measured at 80 K for ZFC and FC conditions in a
14 kOe applied field, cooled from 300 K, are shown in Fig. 5(a)–(c).
It is evident that FC hysteresis loops do not keep good central
symmetry. The shift in the hysteresis loop may be quantified
through the exchange field parameter HE¼ ꢃ(Hright+Hleft)/2,
whereas the coercive field is defined as HC¼(HrightꢃHleft)/2,
Hright and Hleft being the points where the loop intersects the field
axis. The values of HC, MS, MR and HE obtained from the MꢃH
loops are given in Table 2. The observed value of exchange bias
field for all samples is low. This could be due to the reason that
the magnetic anisotropy of NiO is not large enough to prevent the
dipolar interaction among ferromagnetic a-Fe nanoparticles may
be another cause of the observed doubles at room temperature.
3.3. Magnetization analysis
DC magnetization measurements were performed on all Fe–
NiO nanocomposites to investigate their magnetic properties. In
order to observe exchange bias (EB), the system needs to be
´
cooled below Neel temperature (TN) of the AFM component in the
presence of a magnetic field and the Curie temperature of the FM
component should be higher than TN. As a result, the AFM
anisotropy exerts a microscopic torque on FM spins located near
the AFM–FM interface. Thus the field needed to reverse
completely a FM layer magnetization will be larger if it is in
contact with an AFM material, because an extra field is needed to
magnetic moment reversal of
a-Fe nanoparticles on application
of a magnetic field in reverse direction. The enhancement of
coercivity as seen in all nanocomposites in field cooling condition
is also an evidence of the presence of exchange bias in the system
[25]. It is to be noted that highest exchange bias field has been
observed in the sample FN30 (2073 Oe) which has 30% Fe
concentration in contrast to 20% and 50% in the samples FN20 and
FN50, respectively. This indicates that the magnitude of EB field
does not depend on Fe concentration in the nanocomposites,
rather it depends on the Fe–NiO interface area which is more in
case of FN30 due to random distribution Fe and NiO particles in
the nanocomposites.
The temperature dependent magnetization curves of the
sample FN50 in zero field cooled (ZFC) and field cooled (FC)
conditions with an applied field 100 Oe are shown in Fig. 6. It can
be seen that the ZFC and FC magnetization curves show a distinct
irreversible behaviour, and this irreversibility persists beyond
300 K. The observed irreversibility in the ZFC and FC curves
strongly points to the presence of a relaxed magnetic phase in the
sample. The structurally disordered regions at the interfaces as
confirmed by HRTEM studies may be the reason for such
irreversible magnetic behaviour [20,22]. At room temperature,
the local anisotropy of disordered NiO component is small but it
increases as the temperature is lowered. This results in freezing
of AFM spins in the anisotropy direction. As a result of exchange
bias the FM spins at the interface also gradually go to a blocked
state. Thus, the irreversibility as observed in the present case
Fig. 4. Room temperature M–H curves of all Fe–NiO nanocomposites; the inset
shows enlarged central portions.
Table 2
Magnetization data of all samples taken at room temperature and at 80 K in FC condition.
Sample ID
At room temperature
Field cooled to 80 K at 14 kOe field
Hc (Oe)
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
Ms (emu/g)
Mr (emu/g)
HE (Oe)
FN20
FN30
FN50
6575
8375
9775
9.8
22.2
43.2
0.3
0.9
1.9
9975
14475
15075
10.1
23.1
40.5
0.6
1.8
2.7
1573
2073
1273

S.P. Pati et al. / Journal of Solid State Chemistry 183 (2010) 2903–2909
2907
Fig. 5. Magnified central portion of M–H loops under FC (applied cooling field of 14 kOe) and ZFC conditions of (a) FN20, (b) FN30 and (c) FN50 nanocomposite recorded at 80 K.
is
a
consequence of strong exchange coupling across the
there is a change of slope in the temperature range 70–130 K. This
can be due to transition of the disordered Fe spins at the
interfaces to a spin glass (SG) like frozen state as the temperature
interface boundaries of the FM and AFM components in the
nanocomposites. A close inspection of the ZFC curve reveals that

2908
S.P. Pati et al. / Journal of Solid State Chemistry 183 (2010) 2903–2909
reflected in the marked diverging behaviour. The out of phase
component,
//, exhibits a pronounced peak at around 154 K for
w
110 Hz, below which it falls rapidly. With increase of frequency to
710 Hz, this peak shifts towards higher temperature region i.e. to
169 K. This behaviour is attributed to the spin glass like phase,
formed at the AFM–FM interface [26]. This fact is also supported
by temperature dependent dc magnetization measurement that
shows a change of slope around 130 K (Fig. 6).
4. Conclusion
To conclude, Fe–NiO nanocomposites with Fe concentration
varying from 20 to 50 wt% were prepared successfully by a soft
chemical route. XRD and SAED results confirmed the formation of
pure phase nanocomposites. The HRTEM image confirmed the
presence of structurally disordered regions in the FM–AFM
interfaces. Mo¨ssbauer spectroscopy confirmed the presence of
a-Fe nanoparticles in a blocked state giving typical sextet pattern.
Weak dipolar interaction among Fe nanoparticles, Fe nanoparti-
cles at the disordered NiO interfaces and Fe nanoparticles away
from the ordered NiO interface were argued to be possible causes
of appearance of doublets in the Mo¨ssbauer spectra. The observed
hysteresis loop shift along the field axis and enhancement in
coercivity in field-cooled condition confirmed the presence of
exchange bias in the nanocomposites. Irreversibility in the FC
and ZFC curves confirmed strong FM–AFM exchange coupling in
the Fe–NiO nanocomposite system. A change of slope in the ZFC
curve around 130 K has been assigned to transition to a spin
glass like state which is confirmed by ac susceptibility
measurements.
Fig. 6. MꢃT curves of FN50 nanocomposite at an applied field of 100 Oe.
Acknowledgments
The authors are grateful to Dr. S. Das, Jadavpur University,
Kolkata for XRD measurements. Thanks are also due to Dr. K.
Mondal, S N Bose National Centre for Basis Sciences, Kolkata for
VSM measurements. One of the authors (B. Bhushan) thanks CSIR,
Govt. of India for the award of senior research fellowship.
Fig. 7. Temperature dependent out of phase component of ac susceptibility of
FN50. The inset shows the variation of the in-phase component with temperature.
References
[1] G.A. Printz, Science 282 (1998) 1660.
[2] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogues,
Nature 423 (2003) 850.
of the nanocomposite is reduced. The spins of the disordered NiO
component freeze along the random anisotropy direction at low
temperature and consequently the exchange coupled Fe spins
freeze, giving rise to the observed SG behaviour. Another point to
be noted is that the ZFC and FC curves tend to merge at a
temperature beyond 300 K. Apparently the system has a blocking
temperature (TB) above the room temperature. This can be
[3] J. Eisenmenger, I.K. Schuller, Nat. Mater. 2 (2003) 437.
[4] W.P. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413.
[5] J. Nogues, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203.
[6] A.E. Berkowitz, K. Takano, J. Magn. Magn. Mater. 200 (1999) 552.
[7] S. Gangopadhyay, G.C. Hadjipanayis, C.M. Sorensen, K.J. Klabunde, J. Appl.
Phys. 73 (1993) 6964.
[8] J.B. Tracy, D.N. Weiss, D.P. Dinega, M.G. Bawendi, Phys. Rev. B 72 (2005)
064404.
´
attributed to high (ꢂ523 K) Neel temperature of NiO that has
[9] I.S. Lee, N. Lee, J. Park, B.H. Kim, Y.W. Yi, T. Kim, T.K. Kim, I.H. Lee, S.R. Paik,
T. Hyeon, J. Am. Chem. Soc. 128 (2006) 10658.
[10] P.Z. Si, D. Li, J.W. Lee, C.J. Choi, Z.D. Zhang, D.Y. Geng, E. Bruck, Appl. Phys. Lett.
87 (2005) 133122.
effectively raised the superparamagnetic blocking temperature of
the nanocomposites above room temperature.
To get more information on the magnetic properties of the
nanocomposites, the real (in-phase) and imaginary (out-of-phase)
components of the ac magnetic susceptibility were measured as a
function of temperature at different frequencies. Data were
collected while cooling the samples from 220 down to 5 K.
Fig. 7 shows the results obtained with a driving field of amplitude
H0¼3.5 Oe, oscillating at frequencies 110 and 710 Hz. The in-
phase susceptibility component, w/ (see inset of Fig. 7) is seen to
increase continuously with increase in temperature. As
temperature is increased above 60 K, the in-phase susceptibility
component is seen to depend strongly on frequency, which is
[11] R.K. Zheng, G.H. Wen, X.X. Zhang, Phys. Rev. B 69 (2004) 214431.
[12] M.H. Yu, P.S. Devi, L.H. Lewis, P. Oouma, J.B. Parise, R.J. Gambino, Mater. Sci.
Eng. B 103 (2003) 262.
[13] R.K. Zheng, H. Liu, X.X. Wang, Appl. Phys. Lett. 84 (2004) 702.
[14] F.X. Redl, C.T. Black, G.C. Papaefthymiou, R.L. Sandstrom, M. Yin, H. Zheng,
C.B. Murria, S.P. O’Brien, J. Am. Chem. Soc. 126 (2004) 14583.
[15] C.J. Bae, Y. Hwang, J. Park, K. An, Y. Lee, J. Lee, T. Hyeon, J.G. Park, J. Magn.
Magn. Mater. 310 (2007) E806.
[16] G. Salazar-Alvarez, J. Sort, S. Surinach, M.D. Baro, J. Nogues, J. Am. Chem. Soc.
129 (2007) 9102.
[17] P.J. Vander Zaag, R.M. Wolf, A.R. Ball, C. Bordel, L.F. Feiner, R. Jungblut,
J. Magn. Magn. Mater. 148 (1995) 346.
[18] R.K. Zheng, G.H. Wen, K.K. Fung, X.X. Zhang, J. Appl. Phys. 95 (2004)
5244.

S.P. Pati et al. / Journal of Solid State Chemistry 183 (2010) 2903–2909
2909
[19] L.D. Bianco, D. Fiorani, A.M. Testa, E. Bonetti, L. Signorini, Phys. Rev. B 70
[23] K.S. Suslick, Science 247 (1990) 1439.
(2004) 052401.
[24] R.S. Preston, S.S. Hanna, J. Heberle, Phys. Rev. 128 (1962) 2207.
[20] L.D. Bianco, F. Boscherini, M. Tamisari, F. Spizzo, M.V. Antisari, E. Piscopiello,
J. Phys. D: Appl. Phys. 41 (2008) 134008.
[25] J. Nogues, J. Sort, V. Langlais, V. Skumryev, S. Surinach, J.S. Munoz, M.D. Baro,
Phys. Rep. 422 (2005) 65.
[21] E.V. Meerwall, Comput. Phys. Commun. 9 (1975) 117.
[22] Z.M. Tian, S.L. Yuan, S.Y. Yin, L. Liu, J.H. He, H.N. Duan, P. Li, C.H. Wang, Appl.
Phys. Lett. 93 (2008) 222505.
[26] E. Bonetti, L.D. Bianco, D. Fiorani, D. Rinaldi, R. Caciuffo, A. Hernando, Phys.
Rev. Lett. 83 (1999) 14.