Strontium ferrite nanoparticles synthesized in presence of polyvinylalcohol: Phase composition, microstructural and magnetic properties

Article abstract of DOI:10.1016/j.jmmm.2006.06.019

Systematic study was devoted to the synthesis of hexagonal strontium ferrite nanoparticles employing polyvinylalcohol as stabilizing agent. Preliminary experiments allowed to select an optimal sol having molar ratio Sr^{2 +} / Fe^{3 +} = 12, weight ratio PVA / [Sr^{2 +} + Fe^{3 +}] = 1.4 and pH = 2.1. The obtained sol were transformed to gels by an evaporation of water at 100 °C and drying at 112 °C under vacuum. The subsequent calcination was carried out for 3 h at 400 °C, achieved by heating rate of 17 K/min. The obtained precursor was used for a detail study of influence of annealing conditions (temperature range 600-700 °C, annealing time 10-190 min) on the resulting properties. Semiquantitative X-ray phase analysis approved a gradual increase of the M-phase content and a gradual growth of M-phase crystallites with temperature and time. Magnetic measurements showed a distinct influence of the phase composition, namely ratio of the contents of M-phase and maghemite on the shape of the magnetic loops, while the crystallite sizes have only a slight effect.

Full text of DOI:10.1016/j.jmmm.2006.06.019

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 309 (2007) 106–112
www.elsevier.com/locate/jmmm
Strontium ferrite nanoparticles synthesized in presence of
polyvinylalcohol: Phase composition, microstructural
and magnetic properties
a
P. Veverka , K. Knızˇ ek , E. Pollert , J. Bohacek , S. Vasseur , E. Duguet , J. Portier
´ ´ ˇ
a
a,
Ã
b
c
c
c
a
Institute of Physics, ASCR, 162 53 Praha 6, Czech Republic
b
ˇ
Institute of Inorganic Chemistry, ASCR, 250 68 Re zˇ u Prahy, Czech Republic
Institut de Chimie de la Mati e` re Condens e´ e de Bordeaux, CNRS/Universit e´ Bordeaux 1, 87 avenue du Dr Schweitzer, 33608 Pessac, France
c
Received 6 February 2006; received in revised form 14 June 2006
Available online 17 July 2006
Abstract
Systematic study was devoted to the synthesis of hexagonal strontium ferrite nanoparticles employing polyvinylalcohol as stabilizing
2
þ
3þ
2þ
3þ
agent. Preliminary experiments allowed to select an optimal sol having molar ratio Sr =Fe ¼ 12, weight ratio PVA=½Sr þ Fe ꢀ ¼
1
:4 and pH ¼ 2:1. The obtained sol were transformed to gels by an evaporation of water at 100 1C and drying at 112 1C under vacuum.
The subsequent calcination was carried out for 3 h at 400 1C, achieved by heating rate of 17 K/min. The obtained precursor was used for
a detail study of influence of annealing conditions (temperature range 600–700 1C, annealing time 10–190 min) on the resulting
properties. Semiquantitative X-ray phase analysis approved a gradual increase of the M-phase content and a gradual growth of M-phase
crystallites with temperature and time. Magnetic measurements showed a distinct influence of the phase composition, namely ratio of the
contents of M-phase and maghemite on the shape of the magnetic loops, while the crystallite sizes have only a slight effect.
r 2006 Elsevier B.V. All rights reserved.
Keywords: Strontium ferrite; Nanoparticles; Magnetic properties
1
. Introduction
A suitable approach to solve the outlined task is the use
of complex magnetic oxides because their magnetic proper-
ties can be suitably tailored by modification of their
intrinsic properties depending on their composition and
structure. Therefore a search of such materials is advisable.
As an example can serve our recent study of lanthanum
manganese perovskite La0.75Sr0.25MnO3 nanoparticles,
where heating power (SAR value) promising for hyperther-
mia applications was found (SAR higher than 350 W/gMn
at 108 kHz) much higher than maghemite SAR values at
Contemporary studies of magnetic nanoparticles are
significantly motivated by their current and potential
applications in biology and medicine like cells and
biomolecules magnetic separation, drug delivery, contrast
agents for magnetic resonance imaging and colloidal
mediators for cancer magnetic hyperthermia [1].
The oxide nanoparticles available at the present consist
almost exclusively from magnetic cores of magnetite Fe O4
3
and maghemite g-Fe O covered by a biocompatible
the same frequency (SAR ¼ 13 W=g ) [2]. Furthermore,
2
3
Fe
polymer corona, such as dextran. These cores are relatively
easy to prepare by alkaline coprecipitation but their use is
associated with inconveniency, namely it is difficult to
adjust their magnetic properties for a specific application.
the Curie temperature in this system can be suitably
adjusted with respect to a surrounding medium so that the
overheating risk is ruled out [3].
Another promising possibility is the exploitation of a
large family of the hexagonal ferrites, so-called hard
ferrites, possessing relatively high saturated and remanent
magnetizations and exhibiting high magnetocrystalline
anisotropy. A high heating effect can thus be expected.
Ã
Corresponding author.
E-mail address: pollert@fzu.cz (E. Pollert).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.06.019

ARTICLE IN PRESS
P. Veverka et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 106–112
107
Their structures, of a polytypic nature, can be convention-
Table 1
Properties of selected sols
ally described in terms of an intergrowth of the blocks S, R
and T. The S block is a (1 1 1) slice of the common spinel
structure, while R and T are blocks of hexagonal symmetry
which represent weak links in the magnetic exchange
interactions. Among them let us mention as examples the
Molar ratio, Fe/
Sr
Mass ratio, PVA/
3+
Adjusted pH
2+
(Sr + Fe
)
A
B
12.00
12.00
1.4
2.8
2.1
2.3
*
*
phase M arising from the stacking of the type ySRS R y
and the phase Y formed by the stacking of the type
yS T S T S T y . The hexagonal ferrites are complex
1
1 2 2 3 3
magnetic systems and offer several kinds of the non-
equivalent sites, octahedral, bipyramidal and tetrahedral,
that can be occupied by diverse magnetic or non-magnetic
cations [4,5]. Their magnetic properties thus can be readily
modified by variation of the stacking of the individual
blocks and of the composition in a desirable way.
There are a number of proposed and applied methods
for the synthesis of hexagonal ferrites as submicron
particles, mostly via wet chemistry (e.g. sol–gel methods,
hydroxide coprecipitation) completed by a subsequent
thermal treatment [6–13].
The obtained sols were then transformed to gels by an
evaporation of water at 100 1C and drying at 112 1C under
vacuum. The subsequent calcination of the precursors was
carried out for 3 h at 400 1C, achieved by heating rate of
17 K/min. Finally the precursors were inserted into a
furnace preheated at a given temperature, annealed during
a given time and then rapidly quenched down to liquid
nitrogen. This approach was used in order to minimize
time and temperature uncertainties arising during cooling
from the annealing temperature.
Properties of two sols used in the systematic study aimed
to an optimization of the conditions of the synthesis are
given in Table 1.
The hexaferrite particles of the BaFe O and analogous
1
2
19
SrFe O of the mean size bigger than 100 nm are
19
1
2
synthesized by annealing of precursors above 700 1C while
smaller particles grow up at lower temperatures. The
reaction in the latter case is, however, usually incomplete
and the produced powders are multiphase, containing
g-Fe O and a-Fe O . Evidently it leads to a coercivity
3. Characterization
X-ray powder diffraction using Bruker D8 diffract-
ometer (CuKa, SOL-X energy dispersive detector) was
employed to determine the phase compositions and
crystallite sizes. X-ray diffraction patterns were analyzed
with the Rietveld method using the FULLPROF program
[15]. The structures of detected phases were collected with
the help of ICSD [16]. Thompson–Cox–Hastings pseudo-
Voigt profile was used to resolve instrumental, strain and
size contributions to peak broadening. Instrumental
resolution was determined by measuring strain-free tung-
sten powder with grain size 9.4 mm.
2
3
2
3
decrease undesirable for some applications like magnetic
recording. The problem is, however, less critical for the
potential use of this type of materials in medical and
biological applications, like magnetic fluid hyperthermia.
Therefore the present study, considered as an initial
point, is devoted to the optimization of SrFe O
19
1
2
nanoparticles synthesis with respect to the phase composi-
tions of the products, size of the nanoparticles and
resulting magnetic properties.
Particles size and shape were directly observed by TEM
Philips EM 201). The magnetic hysteresis loops were
(
measured by magnetosusceptometer DSM 10 (Manics) at
2
. Synthesis
room temperature in the fields up to 72 T.
Synthesis of nanoparticles of hexagonal strontium
ferrite was based on a method employing polyvinyl-
alcohol as stabilizing agent, reported previously [14]. The
starting chemicals were Sr(NO ) p.a. (Aldrich Germany),
4. Results and discussion
Comparison of the M-phase growth carried out by
annealing of the calcinated precursors A and B at 600 1C,
see Fig. 1, shows a considerable influence of the sol
properties. It seems that a major role can be attributed to
the influence of composition while the influence of slight
pH variations is probably less important. An origin of this
effect is comprehensible in a context of the starting ratio
3
2
Fe(NO ) ꢁ 9H O p.a. (Fluka Switzerland), polyvinylalco-
3
3
2
hol (M ¼ 30:000270:000 g=mol, Sigma USA) and diethy-
lamine 99.5+ (Aldrich USA). The compounds Sr(NO )
3
2
and Fe(NO ) ꢁ 9H O were chemically analyzed in order to
3
3
2
determine the actual contents of the cationic components.
Precise quantities of cationic compounds were dissolved
3
separately and mixed together in molar ratios of Fe /Sr
+
2+
3+
2+
PVA/(Fe +Sr ), see Table 1. Due to a higher content
3
+
2+
in the range of 11.86–12.02. The mixture of dissolved
cations was heated up to 80 1C and mixed together with a
solution of polyvinylalcohol in the weight ratio of
of (Fe +Sr ), nucleation during the germinal stage of
the sol–gel conversion in the sol A is favored compared to
growth of these nuclei insomuch that a number of arising
nuclei is much higher than in the latter case where one can
suppose a tendency to the growth of ‘‘isolated’’ grains to a
larger size. With respect to a fundamental requirement for
2
þ
3þ
PVA=½Sr þ Fe ꢀ ¼ 0:75525:606. After cooling to room
temperature, pH of the solution was adjusted by diethy-
lamine to values in the range of ꢂ1–10.

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P. Veverka et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 106–112
108
in vivo applications where the size of magnetic nanopar-
ticles should be as small as possible, the sol A was selected
as the most promising candidate. Therefore, a detail study
of the influence of annealing conditions of precursor
prepared from the sol A on the resulting properties was
carried out.
several remarks which should be taken into consideration
for a successful synthesis of SrFe O nanoparticles of
required properties.
1
2
19
ꢃ There is a gradual increase of the M-phase content with
annealing time at 600 and 650 1C, while for 700 1C the
precursor is nearly immediately converted into M-phase.
ꢃ Maghemite spinel phase, g-Fe O , arising during the
4
.1. Phase composition
2
3
calcination at 400 1C becomes unstable approximately at
600 1C. Its structural affinity provides an easy insertion
of the spinel blocks into the hexagonal ferrite stacked
structure. A slight portion of g-Fe O is simultaneously
Examples of X-ray diagrams illustrating the evolution of
the phase composition with annealing time and tempera-
ture are given in Fig. 2 and the results of semiquantitative
phase analysis of a typical set of the samples are
summarized in Table 2 and Fig. 3. Their inspection offers
2
3
converted to hematite a-Fe O . Because of a structural
2
3
dissimilarity of a-Fe O (corundum structure) compared
2
3
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
60
5
4
3
0
0
0
20
1
0
0
2
0
40
60
80
100
120
140
160
180
200
annealing time [min]
Fig. 1. Comparison of the sol properties (see Table 1) on the growth of SrFe
x
O19ꢄd/(1+x) M-phase at 600 1C:
,
—precursor A;
,
—precursor B.
5
4
3
2
1
000
000
000
000
000
0
1
3
5
c
b
a
3
1
a
a
2
5
30
35
40
45
50
55
60
2
θ (Cu, Kα)
Fig. 2. X-ray phase analysis of the samples 1a, 3a, 5a, 3b, 1c (see also Table 2): —SrFe12
O19; n—g-Fe
O
2 3
; r—a-Fe
O
2 3
;
3
—SrCO .

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P. Veverka et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 106–112
109
Table 2
Phase composition of the annealed samples
Annealing temperature
1C)
Annealing time Phase composition
(min)
(
M-phase SrFe
1+x) (mol%)
x
O
19ꢄd
/
Maghemite g-Fe
(mol%)
2
O
3
/2
Hematite a-Fe
(mol%)
2
3
O /2
SrCO
3
(mol%)
(
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
a
a
a
a
a
b
b
b
b
b
c
600
600
600
600
600
650
650
650
650
650
700
700
700
700
700
32
67
20
31
49
54
71
50
80
82
91
90
96
95
94
94
94
72
62
45
41
25
46
16
13
4
2
3
4
4
4
3
4
4
5
5
4
5
6
6
6
6
5
2
1
1
0
0
0
0
0
0
0
0
0
0
120
140
190
15
30
59
121
180
10
4
0
c
28
63
0
c
0
c
121
179
0
c
0
1
00
5
4
3
2
1
0
0
0
0
0
8
6
4
2
0
0
0
0
0
20
40
60
80
100
120
140
160
180
200
annealing time [min]
x
Fig. 3. Influence of annealing time and temperature on the growth of SrFe O19ꢄd/(1+x) M-phase from the precursor A:
n—700 1C.
,
—600 1C;
,
—650 1C; m,
to M-phase, complete dissolution of hematite in
M-phase is difficult and would require annealing at
temperatures well above 700 1C.
composition, examples of the R, S, T blocks ratios for
few samples, where the results seem to be reliable are given
in Table 3. It may be supposed that a large structural
flexibility of the hexagonal ferrites allows to compensate
compositional deviations by stacking defects, in our case
by an insertion of T blocks into the magnetoplumbite
structure, previously reported in a number of studies, see
e.g. Refs. [4,17,18].
Gradual growth of M-phase crystallites with tempera-
ture is shown in Fig. 4. Small crystallites of M-phase
together with large agglomerates of maghemite appear
after annealing at 600 1C, hexagonally shaped crystallites
start to develop at 650 1C and typical platelets are
observable at 700 1C.
The temperature of 700 1C appears to be, at least in our
synthesis conditions, the upper limit for the growth of
crystallites smaller than 50 nm.
Regarding the simultaneous presence of several phases in
the samples the actual composition of the hexaferrite
1
1
M-phase should deviate from the ideal ratio of Sr=Fe ¼ 2.
The semiquantitative character of the X-ray phase analysis,
loaded by inaccuracies, particularly for strontium carbo-
nate, does not allow, however, to calculate the exact Sr/Fe
ratio. Nevertheless, for a better insight of M-phase

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P. Veverka et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 106–112
110
Table 3
Compensation of compositional deviations by stacking defects
Samples
M-phase composition
Ratios of blocks
2+
S
½Me 36 þ
2ꢄ 2þ
2ꢄ
½M2þMe
3þ 2ꢄ 2ꢄ
0
2þ
3þ 2ꢄ 0
O
8
14
O
ꢀ
R
O
ꢀ
T ½M Me
ꢀ
8
6
11
2
3
b
c
SrFe10
SrFe11.5
SrFe12
O
19ꢄd
19ꢄd
19
0.8
1
0.8
1
0.18
0.03
–
1
O
Ideal M-phase
O
1
1
Fig. 4. M—phase crystallites morphology in samples annealed for 3 h: —100 nm; (a) 600 1C; (b) 700 1C; (c) 800 1C. Mean size of the M-phase crystallites
measured by X-ray diffraction: (a) 32 nm; (b) 49 nm; (c) 111 nm.
6
4
2
0
0
0
0
-
-
-
20
40
60
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
H [T]
c
Fig. 5. Magnetic hysteresis loops of the selected multiphase samples: ’—1a; &—3a; n—5a; m—4. Inset—dependences of the coercivity H
remanency M on the relative content of the hexaferrite phase: mol% (SrFe 19/x)/(mol% (SrFe 19/x)+mol%(g-Fe /2)).
c
and
r
x
O
x
O
2 3
O
4
.2. Magnetic properties
There are two main influences which should be
lites while the effect of hexagonal ferrite phase composition
leading to stacking defects is less distinct.
Influence of the phase composition on magnetic proper-
ties can be seen from Fig. 5. The resulting shape of the
hysteresis loops is a combined contribution of both magnetic
considered in the relation between magnetic properties,
phase composition of the samples and size of the crystal-

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P. Veverka et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 106–112
111
6
4
2
0
0
0
0
-
-
-
20
40
60
-2
-1
0
1
2
H [T]
c
Fig. 6. Magnetic hysteresis loops of the selected single phase samples: ’—1c; &—5c. Inset—dependence of the coercivity H
phase crystallites.
c
on the mean size of the M-
phases hexaferrite M-phase and maghemite and can be
readily explained by an important difference of magnetic
Table 4
Annealing at 700 1C, samples 1c and 2c of this study, samples from
previous studies [12,9]
5
3
anisotropies, where K1ðSrFe12O19Þ
¼ 3:8 ꢅ 10 J=m
2
98 K
4
3
and K1ðg
ꢄ
Fe2O3Þ
¼ ꢄ2:5 ꢅ 10 J=m . Consequently,
2
98 K
Time of
annealing (min)
D
M
1.6 T
H
c
M
r
an increase of the content of the hexaferrite M-phase leads
to a gradual increase of coercivity and remanency.
(nm)
(emu/g)
(T)
(emu/g)
Ref. [12]
Sample 1c
Ref. [9]
120
10
25
29
34
35
53
56
55
58
0.55
0.45
0.07
0.53
?
29
30
29
A slight effect of the crystallite size is demonstrated in
Fig. 6. An increase of the size leads to an expected increase
of the coercivity while the remanency does not change.
120
28
Sample 2c
4
.3. Comparison with previous studies
The results of the synthesis of SrFe O nanoparticles
2
reported in the previous works employing polyvinylalcohol
14] or polyacrylic acid [11] as a stabilizing sol agent differ
An inspection of the XRD patterns given in Fig. 2 of this
work show unambiguously presence of maghemite phase
solely for To600 1C, accompanied for T ¼ 600 1C only by a
small content of strontium ferrite phase. The authors of the
second work [11] determined an extremely low coercivity of a
sample annealed at T ¼ 700 1C and refer to a superpar-
amagnetic behavior. This result and conclusion seems to be
quite surprising, particularly with respect to a high
magnetocrystalline anisotropy of strontium ferrite. It would
be thus interesting to find a reasonable explanation of the
observed behavior, sharply contrasting e.g. with the coerciv-
ity of our related sample given in Table 4. On the other hand,
the coercivity values amounting to more than 0.6 T for the
particles size of ꢂ70 nm reported in Ref. [19] agree at least
qualitatively with our results obtained on single phase
samples, see inset of Fig. 6.
1
19
[
in some points from the results found in the present study,
see Table 4. One can suppose that it comes from
two sources, the first one is undefined starting ratio
3
+
2+
PVA/(Fe +Sr ), whose importance is now undoubt-
edly evidenced, see Table 1 and Fig. 1, and the second is the
different thermal processing of the gels. The gel in the
former study employing polyvinylalcohol was dried at
1
20 1C and then directly annealed at selected temperatures
in the range of 400–800 1C for 2 h. The gel in the present
study, however, was processed stepwise and the product of
the gel decomposition became substantially more reactive.
A similar situation appears for the precursor prepared in
the medium of polyacrylic acid subsequently treated in one
step. It can be easily evidenced e.g. by a comparison of the
results of annealing at 700 1C. In this work, annealing for
4.4. Preliminary cytotoxicity test for potential biomedical
applications
1
0–28 min was sufficient to obtain nearly the same quality
of the SrFe O samples like for samples annealed for
1
2
19
1
20 min in the previous studies.
In addition, let us give small comments to the previous
Two samples denoted as A (100% of M-phase, mean size
of crystallites ꢂ80 nm) and B (100% of M-phase, mean size
of crystallites ꢂ100 nm) were used for preliminary cyto-
toxicity tests.
works. At first the interpretation of the phase composition of
the samples annealed for Tp600 1C in Ref. [14] is doubtful.

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P. Veverka et al. / Journal of Magnetism and Magnetic Materials 309 (2007) 106–112
112
1
20
The magnetic properties, i.e. shape of the hysteresis
loops, are strongly dependent on the ratio of the present
magnetic phases, M-phase/maghemite. A gradual increase
of coercivity and remanency is enhanced by an increase of
the content of M-phase. The behavior can be understood in
a relation to the magnetic anisotropies of both phases.
A slight influence of the M-phase crystallite size is a
consequence of the extremely high magnetocrystalline
anisotropy of SrFe O . Due to that, an effect of the
surface magnetically dead layer is substantially suppressed.
Negative cytotoxicity tests confirmed an applicability of
the material for biomedical applications, particularly for
magnetic hyperthermia when the shape of the magnetic
loops can be suitably adjusted by the phase composition.
1
00
8
6
4
2
0
0
0
0
0
1
2
19
Acknowledgment
This study was performed under support of the Academy
of Sciences of the Czech Republic (project 1QS100100553:
new hybrid magnetic nanocomposite materials for selected
applications in medicine, magnetic resonance imaging and
magnetic hyperthermia) and support of the Ministry of
Education, Youth and Sports (project 1K05019: hybrid
magnetic nanocomposite materials for diagnosis and
therapy in medicine).
2
4
48
72
96
120
culture time (h)
Fig. 7. Cytotoxicity tests of M-phase SrFe12O19 nanoparticles: black—
mean size of crystallites 80 nm; white—mean size of crystallites 100 nm.
Crystallites were sequentially washed in demineralized
water at 95 1C then in absolute ethanol and chloroform at
room temperature, dried under sterile conditions and
stocked in polypropylene microtube at room temperature.
Cells used in cytotoxicity tests were human K562 cells
line in RPMI-1640 medium supplemented with fetal bovine
serum (10% v/v) without antibiotics. Culture conditions
were as follows: 50 000 cells were cultured per mL and 75%
of medium was renewed every 48 h.
Blank culture was cell culture without magnetic particles.
Test cultures were obtained by adding 2 mg of strontium
ferrite particles per mL. Dead cells counting was done by
Blue Trypan coloration and visualization under optical
microscope.
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[
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