Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites
We report on the magnetic and transport thermal measurements of nanosize (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganite. The nanoparticles were synthesized with use of co-precipitation method at different (800, 900 and 950 °C) temperatures. Their crystal structure was determined to be perovskite-like with a rhomb...
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| Опубліковано в: : | Физика низких температур |
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| Дата: | 2009 |
| Автори: | , , , , , , , , , , , , , , , , |
| Формат: | Стаття |
| Мова: | Англійська |
| Опубліковано: |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2009
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| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites / V. Dyakonov, A. Slawska-Waniewska, J. Kazmierczak, E. Zubov, S. Myronova, V. Pashchenko, A. Pashchenko, A. Shemjakov, V. Varyukhin, S. Prilipko, V. Mikhaylov, K. Piotrowski, Z. Kravchenko, O. Iesenchuk, A. Szytula, W. Bazela, H. Szymczak // Физика низких температур. — 2009. — Т. 35, № 7. — С. 725-734. — Бібліогр.: 29 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859997990744227840 |
|---|---|
| author | Dyakonov, V. Slawska-Waniewska, A. Kazmierczak, J. Zubov, E. Myronova, S. Pashchenko, V. Pashchenko, A. Shemjakov, A. Varyukhin, V. Prilipko, S. Mikhaylov, V. Piotrowski, K. Kravchenko, Z. Iesenchuk, O. Szytula, A. Bazela, W. Szymczak, H. |
| author_facet | Dyakonov, V. Slawska-Waniewska, A. Kazmierczak, J. Zubov, E. Myronova, S. Pashchenko, V. Pashchenko, A. Shemjakov, A. Varyukhin, V. Prilipko, S. Mikhaylov, V. Piotrowski, K. Kravchenko, Z. Iesenchuk, O. Szytula, A. Bazela, W. Szymczak, H. |
| citation_txt | Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites / V. Dyakonov, A. Slawska-Waniewska, J. Kazmierczak, E. Zubov, S. Myronova, V. Pashchenko, A. Pashchenko, A. Shemjakov, V. Varyukhin, S. Prilipko, V. Mikhaylov, K. Piotrowski, Z. Kravchenko, O. Iesenchuk, A. Szytula, W. Bazela, H. Szymczak // Физика низких температур. — 2009. — Т. 35, № 7. — С. 725-734. — Бібліогр.: 29 назв. — англ. |
| collection | DSpace DC |
| container_title | Физика низких температур |
| description | We report on the magnetic and transport thermal measurements of nanosize (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganite. The nanoparticles were synthesized with use of co-precipitation method at different (800, 900 and 950 °C) temperatures. Their crystal structure was determined to be perovskite-like with a rhombohedral distortion (the space group R3̅c). The phase composition and specific surface nanopowders were determined. The average size of synthesized nanoparticles (from 40 to 100 nm) was estimated by both the method of low-temperature adsorption of argon and x-ray diffraction measurements. All the nanosize samples show ferromagnetic-like ordering with close phase transition temperatures. Their magnetization decreases with reducing the particle size. Comparison of experimental and calculated temperature dependences of the spontaneous magnetic moment shows that the spontaneous magnetization both in magnetic field and without field is well described in the frame of the double exchange model. The decrease of magnetization with decreasing particle size is due to increasing the surface contribution to magnetization. The magnetic entropy was shown to increase with increasing applied magnetic field and to be smaller for the small particles. The resistivity was established to become higher with reducing the particles size at any temperatures.
|
| first_indexed | 2025-12-07T16:35:01Z |
| format | Article |
| fulltext |
Fizika Nizkikh Temperatur, 2009, v. 35, No. 7, p. 725–734
Nanoparticles effect on magnetic and transport
properties of (La0.7Sr0.3)0.9Mn1.1O3 manganites
V. Dyakonov1,2, A. �lawska-Waniewska1, J. Kazmierczak1, E. Zubov2,
S. Myronova2, V. Pashchenko2, A. Pashchenko2, A. Shemjakov2,
V. Varyukhin2, S. Prilipko2, V. Mikhaylov2, K. Piotrowski1, Z. Kravchenko2,
O. Iesenchuk1, A. Szytu�a3, W. Bazela4, and H. Szymczak1
1
Institute of Physics PAS, 32/46 Al. Lotników, 02-668 Warsaw, Poland
2
A.A. Galkin Donetsk Physico-Technical Institute NÀNU, 72 R. Luxembourg Str., Donetsk 83114, Ukraine
E-mail: dyakon@ifpan.edu.pl
3
M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Kraków, Poland
4
Institute of Physics, Technical University, Podchor¹¿ych 1, 30-084 Kraków, Poland
Received March 24, 2009
We report on the magnetic and transport thermal measurements of nanosize (La0.7Sr0.3)0.9Mn1.1O3
manganite. The nanoparticles were synthesized with use of co-precipitation method at different (800, 900
and 950 °C) temperatures. Their crystal structure was determined to be perovskite-like with a rhombohedral
distortion (the space group R c3 ). The phase composition and specific surface nanopowders were deter-
mined. The average size of synthesized nanoparticles (from 40 to 100 nm) was estimated by both the method
of low-temperature adsorption of argon and x-ray diffraction measurements. All the nanosize samples show
ferromagnetic-like ordering with close phase transition temperatures. Their magnetization decreases with
reducing the particle size. Comparison of experimental and calculated temperature dependences of the spon-
taneous magnetic moment shows that the spontaneous magnetization both in magnetic field and without
field is well described in the frame of the double exchange model. The decrease of magnetization with de-
creasing particle size is due to increasing the surface contribution to magnetization. The magnetic entropy
was shown to increase with increasing applied magnetic field and to be smaller for the small particles. The
resistivity was established to become higher with reducing the particles size at any temperatures.
PACS: 65.05.cp X-ray diffraction;
61.46.–w Structure of nanoscale materials;
61.50.Ks Crystallographic aspects of phase transformations; pressure effects;
62.50.–p High-pressure effects in solids and liquids;
75.47.Gk Colossal magnetoresistance;
75.75.+a Magnetic properties of nanostructures.
Keywords: nanostructures, manganite, magnetic measurement, resistance.
Introduction
At present, the most interesting metal-oxide materials
are rare-earth manganites with the chemical composition
of (RE1–xMx)yMn1+yO3, where RE = La, Pr, Nd, and M =
= Sr, Ca, Ba, manifesting the colossal magnetoresistive
(ÑÌR) effect [1–3]. Their studies were performed
mainly on bulk materials (ceramic, single crystals) and
thin films. Physical properties of mixed valent perov-
skite manganites were shown to depend crucially on
both doping level and the nature of doping element as
well as on various intrinsic inhomogeneities among
which the nanostructure deserves the special attention.
Due to small size, these nanocrystallites exhibit novel
properties which are significantly different from those
of the bulk material. One important factor is increasing
surface to volume of the grains ratio as the particle sizes
reduce to the nanoscale. As a result, the small-size effect
© V. Dyakonov, A. �lawska-Waniewska, J. Kazmierczak, E. Zubov, S. Myronova, V. Pashchenko, A. Pashchenko, A. Shemjakov, V. Varyukhin,
S. Prilipko, V. Mikhaylov, K. Piotrowski, Z. Kravchenko, O. Iesenchuk, A. Szytu�a, W. Bazela, and H. Szymczak, 2009
and surface effect perturb the structure and properties of
nanoparticles. The intensive discussions devoted to syn-
thesis, characterization of nanosized manganites, study
the influence of nanodimension grains on the magnetism
and transport of manganites as well as their application
are present in the Refs. 4–16. dc magnetization measure-
ments of nanosize (the average particles size of 35 nm)
La0.3Ca0.7MnO3 manganite did not show significant
change of the magnetic phase transition temperature in
respect to its bulk counterpart [4]. The magnetization
measurements of La5/8–0.3Pr0.3Ca3/8MnO3 (LPCMO)
manganite under pressure up to 9.5 kbar have shown that
pressure effect on ferromagnetic ordering in nanopow-
der (the grain sizes from 40 to 1000 nm) LPCMO is simi-
lar to that obtained for bulk samples [5]. An applied
pressure enhances the phase transition temperature of
La1–xMnO3+d particles (20–30 nm) with a pressure coef-
ficient of dT /dPC � 1.4–1.9 kbar–1 [6]. The 30 nm nano-
particles of La0.7Sr0.3MnO3 (LSMO) also exhibit the Cu-
rie temperature close to stoichiometric phase-pure LSMO
[7]. A reduction of grain size in the La0.7Sr0.3MnO3 [8]
and La2/3Ca1/3MnO3 [9] compounds was found to de-
crease magnetization due to increasing ratio of surface to
volume of the grains. The grain size (from 53 to 8 nm) in-
duced variations in structural and magnetic properties ob-
served in La0.8Sr0.2MnO3 nano-ferromagnet indicate a
very important role of the particle surface effects [10].
The temperature dependences of magnetization of
La0.8Sr0.2MnO3 show that the nanoparticles with the
grain size below 25 nm exhibit superparamagnetic behav-
ior. Magnetic resonance spectra have been studied to ob-
tain microscopic information on the magnetic structure of
nanosize La0.7Sr0.3MnO3 [11,12].
The particle size effect on the electron transport of
manganites have been also investigated [8,13–16]. A
magnetoresistance above 10 % was obtained in a field of
1 kOe for all the particles sizes (from 20 to 110 nm) of
La0.67Ca0.33MnO3 [8]. Both the resistivity and high-field
magnetoresistance of La2/3Sr1/3MnO3 manganite having
the grain diameters ranging from 10 �m to 20 nm increase
substantially as the particle size decreases, while the max-
imum magnetoresistance exhibited by the nanosize pow-
ders near the Curie temperature is found to be not sensi-
tive to the particle size [13,14]. Unlike the bulk
manganites, in nanostructured manganites a low field
magnetoresistance was observed. The ultrafine (18 nm)
La2/3Ca1/3MnO3 powder is insulating from 5 to 300 K
and is superparamagnetic above the blocking temperature
corresponding to the peak in the ZFC magnetization [15].
When the temperature is below blocking temperature, the
magnetoresistance is shown to be associated with the
spin-polarized tunneling between grains. In Ref. 16, a
model for the nanoparticles based on the existence of a
noncollinear, amorphous surface layer at the grains was
supposed to explain both the higher resistivity and the re-
duction of saturation magnetization in the smaller particle
samples. It should be noted that a clear understanding of
the magnetic and transport properties caused by nano-
structuration of manganite is still lacking.
In this paper, we report on the synthesis and character-
ization of nanopowder (La0.7Sr0.3)0.9Mn1.1O3 mangani-
tes as well as on the study of influence of the nano-
structure on their magnetic and transport properties.
The manganese–lanthanum–strontium manganite
(La0.7Sr0.3)0.9Mn1.1O3 with high phase transition tem-
perature (about of 360 K) has been studied. Since the ex-
cess manganese increases MRE [17], this compound is
perspective for application in the high sensitive sensor of
magnetic field [18,19]. A large magnetic moment at room
temperature allows to use the nanoparticles of this man-
ganite coated with appropriate macromolecules for appli-
cation in biomedical diagnostic [20].
The format of the paper is as a follows. In Introduc-
tion, the analysis of papers devoted to the influence of
grain size down to nanometric dimentions on the mag-
netic and transport properties manganites is presented.
In the first section, the brief information on the main
experimental details, namely, nanosize samples prepara-
tion, crystalline structure, dc magnetization, 55Mn nu-
clear magnetic resonance and resistance measurements,
are reported. The application of a broad spectrum of ex-
perimental methods has allowed to obtain complete infor-
mation on the particle size effect on a physical properties
as well as on peculiarities of behavior of nanopowder
(La0.7Sr0.3)0.9Mn1.1O3 manganite studied.
The grain size effect on magnetic properties is pre-
sented in the second section. Based on the experimental re-
sults, the magnetic moment is shown to depend strongly on
the average particles size, � �d , while both the magnetic
phase transition temperature and the paramagnetic Cu-
rie–Weiss temperature change insignificantly with varying
� �d . There is a tendency towards a decrease of both mag-
netic moment and magnetic entropy with decreasing � �d .
In the third section, we have focused on the transport
properties changes caused by varying � �d , basing on the
results of resistance (�) measurements both in and without
magnetic field. Temperature dependences of resistance
show that the resistivity becomes higher with reducing
the particles size at any temperatures.
The summary suggests that the nanoparticles size
plays an important role in the formation of magnetic and
transport properties.
1. Sample and experimental
The methods of preparation of manganite nanoparticles
are nontrivial. In this work, the co-precipitation technology
of production of nanosize stochiometric manganese–lantha-
num–strontium perovskites has been used. The mixture of
726 Fizika Nizkikh Temperatur, 2009, v. 35, No. 7
V. Dyakonov et al.
stoichiometric amounts of high purity Mn3O4, La2O3 and
SrCO3 powders was dissolved in diluted nitric acid. This so-
lution was evaporated to dryness, and then, it was fired at
500 °Ñ to decompose the nitrates. Dry remainder was thor-
oughly grinded and again was annealed at different (800,
900 and 950 °Ñ) for duration of 20 h in air, followed by a
slow cooling down to Troom. The resulting material was re-
peatedly grinded, and the nanopowders with the average
particle size of 40, 75 and 100 nm were obtained (Table 1).
The grain size of nanopowders obtained was estimated by
the method of low-temperature adsorption of argon known
as BET’s method [21]. This method has allowed to deter-
mine the specific surface (S SS ) and then to calculate the av-
erage particles size � �d knowing the x-ray density (� � ). The
average grains size was also determined by x-ray diffraction
measurement (XRD) using Scherrer’s relation D /B�
cos ,
where D is the magnitude of grains size,
is the Bragg angle,
B is the difference between half-widths of the Bragg reflex
of nanopowder and standard. The standard was Si powder
with the size 10 �m. A good agreement between these meth-
ods was obtained (Table 1).
Table 1. Physical and chemical characteristics of nanopowders pre-
pared at temperatures of 950 (PS1), 900 (PS2) and 800 (PS3) °C
Sample
Lattice parameters
Specific
surface
Density
Particle
size from
SSS
Particle
size from
XRD
a, � �, deg SSS , m
2
/g ��, g/cm
3 � �d , nm � �d , nm
PS1 5.465 60.38 1 7 0 3. .� 5.891 100 20� 90 17�
PS2 5.467 60.37 2 3 0 4. .� 5.893 75 10� 70 8�
PS3 5.470 60.35 5 8 0 7. .� 5.895 40 5� 37 5�
The magnetization measurements were performed
with a vibrating sample magnetometer in the temperature
range of 5–375 K and in magnetic fields from 500 Oe to
10 kOe. Both zero-field-cooled (ZFC) and field-cooled
(FC) magnetization vs. temperature and at selected mag-
netic field were measured.
55Mn nuclear magnetic resonance (NMR) spectra were
recorded using a two-pulse spin echo method at tempe-
rature between 77 and 300 K and zero external magnetic
field. A noncoherent spectrometer with a frequency
sweep and box-car detector signal averaging has used.
The resistance (�) and magnetoresistive effect,
� �/ 0 �
� �( )� � �0 0H / , have been measured by four-probe me-
thod in a temperature range of 4.2–300 K and in magnetic
field of H � 4 kOe. Contacts were made using silver paste.
For magnetic, transport and resonance measurements,
the nanopowders obtained were pressed at room tempera-
ture under pressure of 0.2 GPa into pellets 1.5 mm thick
and 6 mm in diameter.
In this paper, the experimental and theoretical studies
of magnetism and transport in the (La0.7Sr0.3)0.9Mn1.1O3
manganites with the nanoparticle sizes of � �d = 100 (PS1),
75 (PS2) and 40 (PS3) nm are presented.
Òhe crystallographic structure and lattice parameters
of the samples (Table 1) were determined with room
temperature x-ray powder diffractometer using the Cu K �
radiation XRD patterns were recorded on a Philips
PW-3710X’PERT diffractometer. The 2
scan are per-
formed with the steps of 0.01 and a counting time of 5 s
step. The data are analyzed with the Rietveld-type refine-
ment software FullProf program. The obtained results in-
dicate that the samples are homogeneous single phase
compounds and have rhombohedral structure (the space
group R c3 ). The atoms occupy the following positions:
(La,Sr) in the 2a site 1/4,1/4,1/4; Mn in the 2b 0,0,0 and
O in 6 1 2 1 4e x x, / , /� .
The change of particle size from 100 to 40 nm was es-
tablished to produce an insignificant increase of the a lat-
tice parameter and an insignificant deviation of the � edge
from 60°.
2. Magnetic properties
2.1. dc magnetization
The temperature variation of dc magnetization for
three nanopowders under consideration in magnetic field
of 0.05 Ò near the phase transition is depicted in Fig. 1.
For all samples the ferromagnetic-like behavior is ob-
served. Near the phase transition temperature, the magne-
tization begins to rise sharply with decreasing tempera-
ture indicating the onset of ferromagnetic ordering and
tends to the saturation. It is seen that the magnetization is
strongly influenced by the particles size and decreases as
the grain size of the particles diminishes. In inset to Fig. 1
the M TFC ( ) dependences for the PS3 sample in magnetic
Nanoparticles effect on magnetic and transport properties of (La0.7Sr0.3)0.9Mn1.1O3 manganites
Fizika Nizkikh Temperatur, 2009, v. 35, No. 7 727
28
24
20
16
12
8
4
0
1
2
3
M
,
em
u
/g
300 320 340 360 380 400 420
T, K
300 320 340 360 380
T, K
M
,
em
u
/g
70
30
20
10
0
60
50
40
0.5 T
0.3 T
0.1 T
0.05 T
Fig. 1. (Color online) Temperature variation of magnetization for
the PS1, PS2 and PS3 (curves 1–3, respectively) nanopowders
under consideration in magnetic field of 0.05 Ò. Inset: M TFC ( )
dependences for the PS3 sample in magnetic fields of 0.05–0.5 Ò.
fields of 0.05–0.5 Ò are shown. The influence of magnetic
field on the M TFC ( ) dependences for the another samples
is similar.
The low-temperature field-cooled (M FC ) and zero-
field-cooled (M ZFC ) magnetization recorded in magnetic
field of 0.1 T for all samples are presented in Fig. 2,a.
Figure 2,b shows the FC and ZFC magnetization for
PS1 and PS2 as a function of temperature near phase tran-
sition. The insignificant difference between FC and ZFC
magnetization, which begins to diverge below 340, 300
and 275 K for PS1, PS2 and PS3, respectively, and
increases with decreasing temperature, indicates the pres-
ence of a small magnetic anisotropy. The difference
between M ZFC and M FC magnetization diminishes in
magnetic field and disappears between 1 and 3 kOe. The
local maximum observed in the magnetization curves at
low temperatures (about 42 K) suggests a possible frus-
tration effect induced by a competition from different
magnetic interactions, that can result in spin canting at
lower temperature due to an excess manganese [17]. This
fact could be consist with an antiferromagnetic ordering
in Mn3+–O– Mn4+ clusters [22].
The analysis of high-temperature dc susceptibilities
( )M/H of samples studied was performed using the
Curve–Weiss (CW) law:
� �
( )T
C
T
� �
�0 , (1)
where � 0 is the background susceptibility, the Curie con-
stant C / k B� �eff
2 3 , � �eff ( ) ( )S g S SB� �1 is the effective
magnetic moment,
is the paramagnetic CW temperature.
The experimental Í/M T( ) dependences for samples
PS2 in fields of H = 0.05, 0.1, 0.3 and 0.5 Ò are presented
in Fig. 3.
Similar Í/M T( ) dependences were obtained for the an-
other samples. The fitting of the experimental M T( )
dependences to the CW law (1) has shown that Í/M T( )
curves as a function of temperature are linear at tempera-
tures above TC and obey the CW law for all samples. A
root-mean square error of fitting is equal to � 0.2 %. The
CW temperatures,
, calculated as a result of a fitting of
CW law to the experimental data were found to have the
positive sign that is indicative of dominant ferromagnetic
interactions. The
temperatures do not depend practi-
cally on changes of � �d . Using the CW constant values the
effective numbers of Bohr magneton, neff , were calcu-
lated. The �
0, , ,C TC , neff values of as a function of � �d
are summarized in Table 2. In magnetic fields from 0.1 to
728 Fizika Nizkikh Temperatur, 2009, v. 35, No. 7
V. Dyakonov et al.
65
60
55
50
45
40
M
,
em
u
/g
1
2
1
2
1
2
0 50 100 150 200 250
T, K
aPS1
PS2
PS3
6
4
2
0
M
,
em
u
/g
bPS1
PS2
T, K
280 320 360
1
2
1, 2
Fig. 2. (Color online) Low-temperature field-cooled (M FC)
(curve 1) and zero-field-cooled (M ZFC) (curve 2) magnetiza-
tion recorded in magnetic field of 0.1 T for PS1, PS2, PS3
samples (a). The FC and ZFC magnetization (curves 1 and 2,
respectively) as a function of temperature for PS1 and PS2 in
field of 100 Oe (b).
7
6
5
4
3
2
1
0
350 360 370 380
T, K
Fig. 3. (Color online) Experimental Í/M T( ) dependences for
PS2 sample in fields of H = 0.05, 0.1, 0.3 and 0.5 Ò. (Fitting of
the experimental M T( ) dependences to the CW law is shown
by solid lines.)
0.5 T, the � 0,C and neff parameters calculated decrease
insignificantly.
Table 2. The parameters obtained from fitting to the Curie–Weiss
law
Sample
Parameter
�
0
, emu mol/ C /, emu K mol�
� � neff TC , K
H � 0.05 T
PS1 –0.0233 4.98 354 6.3 354
PS2 –0.0276 4.28 356 5.9 356
PS3 –0.016 3.62 355 5.4 354
H � 0.3 T
PS1 –0.0099 4.29 354 5.86 355
PS2 –0.0333 4.62 357 6.08 357
PS3 –0.0215 3.87 355 5.57 355
H � 0.5 T
PS1 –0.0185 4.43 354 5.95 356
PS2 –0.0086 4.03 356 5.68 358
PS3 – – – – –
The theoretical estimation of the effective magnetic
moment in nano-size manganites is of interest. In systems
with variable valency of ions Mn3+ (concentration x) and
Mn4+ (concentration 1� x), the total magnetic moment
(� tot ) can be written in the form:
� � �tot eff eff
2 2
1
2
21� � �x S x S( ) ( ) ( ) , (2)
where S1 = 2 and S 2 = 3/2 are the spins of Mn3+ and Mn4+
ions, respectively, x � 0.7 is the Mn3+ concentration and g
factor is equal to 2. The high value of the total moment
equal to � tot = 4.62 �B was obtained. The � tot value is be-
tween the spin-only values for Mn3+ (4.9 �B ) and Mn4+
(3.87 �B ) ions. In our case, the effective number of Bohr
magnetons determined experimentally are higher than the
total effective moment value calculated. Similar situation
was observed in Ref. 23, where for the system La0.93MnO3
with Mn4+ concentration 1� �x 0.21 the experimental and
calculated values of the total effective moment are equal to
4.7 and 5.8 �B , respectively. It was concluded that in a
paramagnetic phase the polaron effects are responsible for
formation of the magnetic clusters. As the result, it leads to
increase of Curie constants and, respectively, to increase of
the total effective magnetic moment.
The FM phase transition temperatures (TC ) were de-
fined from peak of dM/dT in the M T( ) dependence. De-
spite the particle size difference, the Curie temperatures
were found to coincide practically (± 2 K) for all samples
studied (Table 2). This means that the inner parts (cores)
of grains in all nanopowders are magnetically identical,
i.e., a Mn3+/Mn4+ ratio in the nanoparticles is similar, and
their contribution to the magnetization is nearly the same.
It agrees with a model of the nanoparticles presented in
Ref. 16. The TC temperatures determined are close to
those in a bulk manganite of the same composition.
Additional information on the magnetic properties was
obtained by the measurements of both the M T( ) magneti-
zation in applied magnetic field and M H( ) isotherms. The
magnetization is highly dependent on the applied mag-
netic field (inset to Fig. 1). The paramagnet–ferromagnet
phase transition in magnetic field becomes broader and
the M T( ) dependence shifts towards high temperatures
what indicates a extension of the FM phase. Figure 4
shows a comparison of field dependences of magnetiza-
tion, M H( ), dependences for all samples at 5 and 300 K.
The magnetization isotherms display a ferromagnetic be-
havior and reach the saturation at low temperatures above
0.4 T. The magnetization decreases with reducing � �d .
The magnetic moment values observed in nanoparticles
studied are close to the reported values [8,9]. The field
dependences of magnetization at 1 T and 5 K correspond
to uniform ferromagnet having the magnetic moments of
3.4, 3.2 and 3.1 �B /Mn for the PS1, PS2 and PS3 samples,
respectively, which are noticeably smaller than the value
of moment equal to 3.7 �B /Mn taking into account the
presence of about 30 % Mn4+ ions in the samples studied.
It can be supposed to be attributed to the presence of
noncollinear interface layers between grains for the smal-
ler size particles.
Figure 4 shows that at Ò TC� in small fields we have
usual magnetic reversal of domains with a susceptibility
1 4/ N� , where N is the demagnetizing factor of the sam-
ple. In fields of H � 0.2 T, the basic contribution to in-
creasing magnetization is connected with reorientation of
local spin along magnetic field. In this field range, the
Nanoparticles effect on magnetic and transport properties of (La0.7Sr0.3)0.9Mn1.1O3 manganites
Fizika Nizkikh Temperatur, 2009, v. 35, No. 7 729
M
,
em
u
/g
H, T
90
80
70
60
50
40
30
20
10
0
0.2 0.4 0.6 0.8 1.0
1
2
3 4
5
6
7
8
9
Fig. 4. (Color online) Comparison of M H( ) dependences at 5 K
for PS1, PS2 and PS3 (curves 1–3, respectively), at T � 300 K
for ceramic, PS1, PS2 and PS3 (curves 4, 5, 6 and 7, respec-
tively) and at 351 and 373 K for PS1 (curves 8, 9, respectively).
thermodynamic behavior typical for electronic spin sys-
tem with the double exchange is realized.
The measurements of hysteresis loops in field ±1 T
show that a coercive field (Hcoerc) is small for all
nanopowders. A coercivity at T � 4.2 K increases from 10
to 25 Oe with decreasing � �d from 100 to 40 nm (Fig. 5). A
small value of coercive field indicates a very weak aniso-
tropy energy and testifies on soft ferromagnetic behavior
of the samples studied. As known, a multidomain parti-
cles have such low coercive fields. The remanent magne-
tization (M rem) also changes slightly with changing � �d .
To exclude the influence of magnetic dipole interac-
tions in weak fields (demagnetizing field), the Arrot’s me-
thod [24] was used to describe the temperature behavior of
spontaneous magnetization in strong fields near TC .
In Fig. 6, the experimental Arrot’s curves for sample
PS3 near the phase transition are presented. Similar
curves were obtained for PS1 and PS2. The temperature
dependence of spontaneous magnetization was deter-
mined by linear fitting of the experimental data to Arrot’s
curves:
M H T A B
H
M H T
2( , )
( , )
� � , (3)
where A and B are the fitting parameters. Knowing the A
factor, it is possible to find spontaneous magnetization,
M T0( ), as M T A0( ) � , as well as the field dependence
of spontaneous magnetization, M H0( ), without taking
into account of influence of domain structure.
The saturation magnetization M T0( ) at temperatures
below 200 K was determined by fitting to dependence [25]
M H T M T
a
H
a
H
( , ) ( )� � �0
1 2
2
, (4)
where a1 and a2 are the fitting constants.
The left inset to Fig. 6 shows the experimental field
dependences of magnetization (points) and corresponding
interpolation curves M H( ) (lines). Both Fig. 4 and the
right inset to Fig. 6, where the temperature variation of the
spontaneous magnetic moment for the samples with vari-
ous grain sizes is depicted, confirm that the magnetic mo-
ment decreases with decreasing particles size. It may be
due to the surface effect, since in FM and AFM oxides with
the particle size less than 100 nm the local order of atoms
situated in the surface layer can differ considerably from
that of the interior atoms [9,14,16]. An outer shell of parti-
cles will contain most of the oxygen vacancies and faults in
the crystallographic structure that leads to magnetically
disordered state. The atom disorder affecting the magnetic
interactions can lead to a canting of the surface spins, or to
the state when the outer layer has no net moment.
The comparison of the experimental and calculated
dependences of spontaneous magnetization for nano-
particles is of interest. To perform such comparison it is
necessary to know two basic parameters in the theory of
double exchange: the eg electron bandwidth, W , and the
730 Fizika Nizkikh Temperatur, 2009, v. 35, No. 7
V. Dyakonov et al.
a
b
M
,
em
u
/g
M
,
em
u
/g
80
40
0
–40
–80
3
2
1
3
2
1
–0.10 –0.05 0 0.05 0.10
H, T
20
15
10
5
0
–5
–10
–15
–20
H, T
–0.02 –0.01 0 0.01 0.02
Fig. 5. (Color online) M H( ) dependences (a) and the hysteresis
loops (b) for the PS1, PS2 and PS3 (the curves 1, 2 and 3, re-
spectively) at T � 4.2 K.
M
,
1
0
(e
m
u
/g
)
2
3
2
14
12
10
8
6
4
2
0
0.004 0.008 0.012
1
2
3
4
M
,
em
u
/g
H, T T, K
Arrot's curves
80
60
40
20
0 0.2 0.4 0.6 0.8 1.0
1
2
3
4 1
2
3
4
3
2
1
0 100 200 300 400
Fig. 6. (Color online) Experimental Arrot’s curves for samples
PS3 at temperatures of 200, 300, 327 and 351 K (curves 1–4,
respectively). Left inset: experimental field dependences of
magnetization (points) and corresponding interpolation curves
M H( ) (lines). Right inset: the variation of the spontaneous
magnetic moment with grain sizes for PS3, PS2 and PS1 sam-
ples (curve 1, 2 and 3, respectively).
site concentration of eg electrons, n. According to the
chemical formula, the Mn3+ concentration is equal to 0.7.
If all the eg electrons participate in a double exchange, it
may be suggested that n = 0.7. In the frame of double ex-
change model [26,27], the concentration dependence of
the Curie temperature (in a unit of bandwidth, T /WC ) has
been calculated in Ref. 28. In particular, T /WC = 0.0211
at n = 0.7 and W = 1.45 eV for the TC = 355 K.
In Ref. 27, the self-consistent equations for an average
site spin of � � �� z zS 0 , including spin of eg electron (�),
spin of Mn4+ ion (S z
0 ) and chemical potential (�), are pre-
sented in the form
1
8
5
1
4
0
0
1
0( )� � � � � � � � � � ��
�
�n S F Fz z� � ,
1
8
5
1
4
0
0
1
0( )� � � � � � � � � � ��
�
�n S F Fz z� � ,
(5)
where � ��F 0 and � ��F 0 are the average probability of
electron–hole state, the chemical potential is included in
� �F �0
1 and
� �i
i
i iF
N
f E f� � � � ��0
0
1
4
( ( ) ( ))q
q
. (6)
Here f x( ) is the Fermi function, E t Fi i
i
q q� � � � �� 0
0( ) ,
� 0i are the mean field energy levels of Mn ions and t( )q
the Fourier representation of the hopping integral. The
difference of the mean field energy of hole and electron
with spin upwards or downwards is of
� �
�
0
2
i
Bi
gH
� �
~
, where
~ ( )
H H
J
g
S
B
z z� � � � �
0
0�
� ,
J ( )0 is the indirect exchange parameter and i � + 1 or –1
for the projection of spin electron upwards and down-
wards, respectively. The average probability of elect-
ron–hole state with spin upwards or downwards is de-
fined by the expression
� � � � � � � �F n
i
Si z z0
0
1
8
5
4
( ) � , (7)
where the sign ��� designates averaging over the full
Hamiltonian. The average expression of F i0 operator in a
molecular field approximation has the simple form:
� � � � � � � � �F n S n Si z z0
0 2 0 3 2 01( ) / . (8)
S z
2 and S z
3 2/ spin operators designate the z projections of
S � 2 and S � 3/2 spins, respectively. The � �F i0
1 value in
the expression (5) has the same form, but in which the
mean field � 01 and � 0 1� levels are shifted on energies of
electronic density fluctuations, ��� / 4 and ��� / 4, respec-
tively, where
�� i i
N
t f E� �1
( ) ( )q q
q
. (9)
The set of Eqs. (5) was solved numerically for n � 0.7
in field of H � 0 at J ( )0 � 0 and in field of H � 0.5 T at
J ( )0 � 0 and J ( )0 � 2 Ê, that corresponds to the relations
of H/W � � �3 98 10 5. and J /W( ) .0 119 10 4� � � . The calcula-
tion results of the magnetic moment presented in Fig. 7
show the influence of magnetic field on the phase transi-
tion and an additional biasing of phase transition due to
an indirect exchange.
Comparison of experimental and calculated tempera-
ture dependences of the spontaneous magnetic moment
for the PS2 sample near the phase transition (Fig. 7)
shows that the spontaneous magnetization both in mag-
netic field and without field is well described in the frame
of the double exchange model.
2.2. Nuclear magnetic resonance spectra
The 55Mn nuclear magnetic resonance (NMR) spectra
were obtained by measuring the integrated intensity of the
spin-echo versus frequency. In Fig. 8,a,b the NMR spectra
for PS1 and PS3 are illustrated. The spectrum for the PS2
sample is the same as for PS3 (it is not given). The 55Mn
NMR studies have been performed at 77 K when the local-
ized Mn3+ and Mn4+ states are frequency not resolved. The
NMR signal detected at f res � 375 MHz is typical of mixed
valence metallic-like ferromagnetic manganites and corre-
sponds to fast hopping of electron–holes among Mn sites
[12]. The NMR spectra lines for PS2 and PS3 (Fig. 8,b) are
practically symmetrical about f res indicating the homoge-
neity of nanoparticles intrinsic ferromagnetic region.
Nanoparticles effect on magnetic and transport properties of (La0.7Sr0.3)0.9Mn1.1O3 manganites
Fizika Nizkikh Temperatur, 2009, v. 35, No. 7 731
4
3
2
1
0
1
2 3
100 200 300 400
T, K
M
,
�
B
Fig. 7. (Color online) Magnetic moment as a function of tem-
perature calculated for n � 0.7 at Í � 0 and J ( )0 0� (curve 1)
and in field of H � 0.5 T at H/W � � �3 98 10 5. (curve 2) and at
J /W( ) .0 1 19 10 4� � � (curve 3). Experimental temperature depen-
dences of the spontaneous magnetic moment for the sample PS2
near the phase transition at Í � 0 (open circles) and in field of
H � 0.5 T (full circles).
Unlike the PS3 and PS2 samples, the NMR spectrum for
PS1 is broadened (Fig. 8,a). Asymmetrical NMR spectrum
for PS1 is due to nonhomogeneity of manganese ions envi-
ronment by both La3+ and Sr2+ ions and defects. It is ne-
cessary to note the closeness of spin–spin relaxation time
(� �2 10� s) for all systems. Computer decomposition of
NMR spectrum has shown that it resolves on three compo-
nents with maximum frequencies of 363.7, 374.0 and
384.9 MHz for PS1 and on two components with maximum
frequencies of 361.0 and 376.2 MHz for two other
nanopowders, that is indicative of nonequivalence of mag-
netic state of manganese ions in samples with various par-
ticle sizes. In the case of PS1, it is caused by larger defi-
ciency of perovskite structures involving the anion and
cation vacancies as well as nanostructural clusters. In the
case of PS2 and PS3, it is due to smaller concentration of
vacancies. The increase of basic component of resonant
frequency ( . .
F � � �376 2 374 0 2.2 MHz) also indicates
smaller deficiency perovskite structures for PS3 and PS2
nanopowders. Note that in NMR spectra a component with
frequency F � 387 MHz is close to frequency of localized
Mn3+ states, and a component with frequency F �
= 368 MHz is close to frequency of Mn4+ states.
2.3. Magnetocaloric effect
In this paper, the magnetocaloric effect (MCE)
characterazing the magnetic entropy (
S M ) change pro-
duced by changes of the magnetic field applied to the sys-
tem was calculated from initial magnetization curves.
Using the Arrot curves (see the lines in Fig. 6) the
M H( ) dependences and spontaneous magnetization M 0
at H � 0 were obtained. For above temperatures we have
used the spline interpolation to find the M H T( , ) depend-
ence. The temperature dependence of
S M was calcu-
lated using the relation [25]
S T H
M T H
T
dHM
H
H
( , )
( , )
�
�
�
!
"
#
$
%
&
'
0
. (10)
Knowing the M H T( , ) dependence, we have found the
derivative � �M T H / T( , ) , and substituting it in Eq. (10)
S T HM ( , ) was calculated.
In Fig. 9, a change of the absolute
S M value in the
magnetoordered phase as a function of temperature at dif-
ferent magnetic fields for the PS3 sample (d � 40 nm) is
shown. Similar
S TM ( ) dependences were obtained for
other nanosize samples. The
S TM ( ) dependences in the
paramagnetic phase (above TC ) were not calculated, be-
cause the M H( ) were not measured. Therefore, a
S TM ( )
peak around the magnetic phase transition was not ob-
served. As it is seen in Fig. 9, the absolute value of mag-
netic entropy increases with increasing applied magnetic
field. The absolute
S TM ( ) value was found to decrease
for the small particles. This behavior can be explained
with the model of the nanoparticles composed of both an
inner core with physical properties similar to the bulk and
an outer shell. An outer shell has disordered magnetic
structure. Its role increases with reducing particle size as
1/D [15], where D is the particle size. According to the re-
lation (10) the magnetic entropy
S TM ( ) is depended on
the magnetization value. Therefore, the decrease of abso-
lute values of
S TM ( ) can be explained by decreasing
magnetic moment of nanopowders caused by the surface
effect as the grain size decreases. The relative surface
contribution, or the surface/volume of the grains ratio, in-
creases due to the larger surface area of the small parti-
cles. Thus, the total magnetocaloric effect reduces as the
particle size diminishes, since the contribution of the
outer shell having the disordered magnetic state to MCE
will increase.
3. Transport properties
The magnetic properties was shown to be markedly af-
fected by the grain size. Since the magnetic properties of
manganites are closely connected to their transport prop-
erties, it is of interest to study the particles size effect on
the resistivity and magnetoresistance. The temperature
dependences of resistance, �( )T , presented in Fig. 10,
732 Fizika Nizkikh Temperatur, 2009, v. 35, No. 7
V. Dyakonov et al.
100
50
0
100
50
0A
m
p
li
tu
d
e,
ar
b
.
u
n
it
s
340 360 380 400 340 360 380 400
F, MHz F, MHz
a b
A
m
p
li
tu
d
e,
ar
b
.
u
n
it
s
Fig. 8.
55
Mn nuclear magnetic resonance spectra for PS1 (a)
and PS3 (b) at 77 K.
0
–0.2
–0.4
–0.6
–0.8
–1.0
–1.2
280 300 320 340 360
T, K
H = 0.1 T
H = 0.5 T
H = 1.0 T
Fig. 9. Temperature dependences of
SM at various magnetic
fields for the PS3 sample.
show that the resistivity becomes higher with reducing
the particles size at any temperatures.
The higher resistivity observed in the smaller particles
can be caused by the potential barrier between the
nanoparticles as a result of the presence of an amorphous
(or disordered) interface layers between grains in outer-
most shell [9,10,16]. The one of factor affecting the elec-
tron mobility is the larger surface area associated with the
small grain size. The energy level of the Mn ions at the
surface is different from that inside the grain. Thus, eg
electrons near the surface are likely to be localized and
the interfaces between grains are insulating [15]. An insu-
lating-like behavior becomes apparent below � 70 K in
the �( )T dependence for the sample having the smallest
grain size.
The magnetoresistance (MR) for all the nanoparticles
studied is of order of 13–18 % at low temperatures in
magnetic field of H = 4 kOå, that agrees with the MR
value for La–Sr–Mn system [29]. This observation im-
plies that a substantial part of MR at low temperatures
arises from the grain boundaries.
Conclusions
In this paper magnetic, transport, thermal and resonance
measurements of nanoparticle (La0.7Sr0.3)0.9Mn1.1O3
manganite having an averaged grain size ranging from 40 to
100 nm have been carried out. The experimental data have
shown that the nanostructuration of the grains plays an im-
portant role in the magnetism and transport of manganite
studied. Its properties are strongly depended on the surface
effects related to the grain sizes. The magnetization of
nanopowders decreases with decreasing particle size unlike
that the phase transition temperature does not show signifi-
cant difference in respect to bulk counterpart. The simplest
explanation of magnetic moment decrease is the existence
of a noncollinear spin structure, or magnetically «dead»
layer at the surface of nanoparticles as a result of the atom
disorder in the surface layer. The relative surface contribu-
tion, or the surface/volume of the grains ratio, increases due
to the larger surface area of the small particles, and therefore
their spontaneous magnetization is diminished. This expla-
nation agrees with a model of the nanoparticles composed of
an inner core with physical properties similar to the bulk and
an outer shell with oxygen vacancies, defects and etc. [16].
However, the spin disorder mechanism at the nanoparticle
surface needs to be ascertained. The magnetic entropy
S TM ( ) was shown to increase with increasing applied
magnetic field and its absolute value is smaller for the small
particles. The total magnetocaloric effect reduces as the par-
ticle size diminishes, since the contribution of the outer shell
having the disordered magnetic state to MCE will increase.
The a presence of an amorphous (or disordered) interface
layers between grains also affects the transport properties.
The resistivity becomes higher with reducing the particles
size due to increasing surface contribution. The MR value of
the nanoparticle samples increases to 13–18 % with reduc-
ing particle size in field of 4 kOe at 4.2 K. This observation
shows that a substantial part of the MR at low temperatures
arises from the grain boundaries.
Acknowledgments
This work was in part of the research program of the
Polish National Scientific Network «Materials with
strongly correlated electrons».
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T, K
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10
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<<
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| id | nasplib_isofts_kiev_ua-123456789-117253 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0132-6414 |
| language | English |
| last_indexed | 2025-12-07T16:35:01Z |
| publishDate | 2009 |
| publisher | Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| record_format | dspace |
| spelling | Dyakonov, V. Slawska-Waniewska, A. Kazmierczak, J. Zubov, E. Myronova, S. Pashchenko, V. Pashchenko, A. Shemjakov, A. Varyukhin, V. Prilipko, S. Mikhaylov, V. Piotrowski, K. Kravchenko, Z. Iesenchuk, O. Szytula, A. Bazela, W. Szymczak, H. 2017-05-21T16:39:48Z 2017-05-21T16:39:48Z 2009 Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites / V. Dyakonov, A. Slawska-Waniewska, J. Kazmierczak, E. Zubov, S. Myronova, V. Pashchenko, A. Pashchenko, A. Shemjakov, V. Varyukhin, S. Prilipko, V. Mikhaylov, K. Piotrowski, Z. Kravchenko, O. Iesenchuk, A. Szytula, W. Bazela, H. Szymczak // Физика низких температур. — 2009. — Т. 35, № 7. — С. 725-734. — Бібліогр.: 29 назв. — англ. 0132-6414 PACS: 65.05.cp, 61.46.–w, 61.50.Ks, 62.50.–p, 75.47.Gk, 75.75.+a https://nasplib.isofts.kiev.ua/handle/123456789/117253 We report on the magnetic and transport thermal measurements of nanosize (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganite. The nanoparticles were synthesized with use of co-precipitation method at different (800, 900 and 950 °C) temperatures. Their crystal structure was determined to be perovskite-like with a rhombohedral distortion (the space group R3̅c). The phase composition and specific surface nanopowders were determined. The average size of synthesized nanoparticles (from 40 to 100 nm) was estimated by both the method of low-temperature adsorption of argon and x-ray diffraction measurements. All the nanosize samples show ferromagnetic-like ordering with close phase transition temperatures. Their magnetization decreases with reducing the particle size. Comparison of experimental and calculated temperature dependences of the spontaneous magnetic moment shows that the spontaneous magnetization both in magnetic field and without field is well described in the frame of the double exchange model. The decrease of magnetization with decreasing particle size is due to increasing the surface contribution to magnetization. The magnetic entropy was shown to increase with increasing applied magnetic field and to be smaller for the small particles. The resistivity was established to become higher with reducing the particles size at any temperatures. This work was in part of the research program of the
 Polish National Scientific Network «Materials with
 strongly correlated electrons». en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Наноструктуры при низких температурах Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites Article published earlier |
| spellingShingle | Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites Dyakonov, V. Slawska-Waniewska, A. Kazmierczak, J. Zubov, E. Myronova, S. Pashchenko, V. Pashchenko, A. Shemjakov, A. Varyukhin, V. Prilipko, S. Mikhaylov, V. Piotrowski, K. Kravchenko, Z. Iesenchuk, O. Szytula, A. Bazela, W. Szymczak, H. Наноструктуры при низких температурах |
| title | Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites |
| title_full | Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites |
| title_fullStr | Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites |
| title_full_unstemmed | Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites |
| title_short | Nanoparticles effect on magnetic and transport properties of (La₀,₇Sr₀,₃)₀,₉Mn₁.₁O₃ manganites |
| title_sort | nanoparticles effect on magnetic and transport properties of (la₀,₇sr₀,₃)₀,₉mn₁.₁o₃ manganites |
| topic | Наноструктуры при низких температурах |
| topic_facet | Наноструктуры при низких температурах |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/117253 |
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