Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd)
X-ray diffraction and magnetic using dc and ac methods measurements of the polycrystalline and nanosize REMnO₃ (RE = Pr, Nd) powdered samples have been performed. The nanosize manganites were synthesized with a sol-gel method at different (800, 850 and 900 °C) temperatures. The average size of syn...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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irk-123456789-1182732017-05-30T03:06:00Z Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd) Dyakonov, V. Bażela, W. Duraj, R. Dul, M. Kravchenko, Z. Zubov, E. Dyakonov, K. Baran, S. Szytuła, A. Szymczak, H. Низкотемпеpатуpный магнетизм X-ray diffraction and magnetic using dc and ac methods measurements of the polycrystalline and nanosize REMnO₃ (RE = Pr, Nd) powdered samples have been performed. The nanosize manganites were synthesized with a sol-gel method at different (800, 850 and 900 °C) temperatures. The average size of synthesized nanoparticles (from 56 to 89 nm) and polycrystalline powders (above 200 nm) was estimated using the x-ray diffraction data. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room temperature with smaller values of the lattice parameters in the nanosamples. The temperature-dependent ac magnetic susceptibilities show a sharp high-temperature peak connected with Mn magnetic moments ordering. The low-temperature maximum of magnetic susceptibility is proposed to be due to the polarization of the rareearth sublattice by an effective exchange field of the Mn ordered sublattice. The antiferromagnetic ordering of Mn sublattice and paramagnetic Curie temperatures as well as the magnetic moment values for the nanosize samples were found to be smaller than those for polycrystalline sample. 2013 Article Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd) / V. Dyakonov, W. Bażelа, R. Duraj, M. Dul, Z. Kravchenko, E. Zubov, K. Dyakonov, S. Baran, A. Szytuła, H. Szymczak // Физика низких температур. — 2013. — Т. 39, № 4. — С. 452–458. — Бібліогр.: 11 назв. — англ. 0132-6414 PACS: 75.30.–m, 75.60.–d, 75.75.–c http://dspace.nbuv.gov.ua/handle/123456789/118273 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
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English |
| topic |
Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм |
| spellingShingle |
Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм Dyakonov, V. Bażela, W. Duraj, R. Dul, M. Kravchenko, Z. Zubov, E. Dyakonov, K. Baran, S. Szytuła, A. Szymczak, H. Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd) Физика низких температур |
| description |
X-ray diffraction and magnetic using dc and ac methods measurements of the polycrystalline and nanosize
REMnO₃ (RE = Pr, Nd) powdered samples have been performed. The nanosize manganites were synthesized
with a sol-gel method at different (800, 850 and 900 °C) temperatures. The average size of synthesized nanoparticles
(from 56 to 89 nm) and polycrystalline powders (above 200 nm) was estimated using the x-ray diffraction
data. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room
temperature with smaller values of the lattice parameters in the nanosamples. The temperature-dependent ac
magnetic susceptibilities show a sharp high-temperature peak connected with Mn magnetic moments ordering.
The low-temperature maximum of magnetic susceptibility is proposed to be due to the polarization of the rareearth
sublattice by an effective exchange field of the Mn ordered sublattice. The antiferromagnetic ordering of
Mn sublattice and paramagnetic Curie temperatures as well as the magnetic moment values for the nanosize
samples were found to be smaller than those for polycrystalline sample. |
| format |
Article |
| author |
Dyakonov, V. Bażela, W. Duraj, R. Dul, M. Kravchenko, Z. Zubov, E. Dyakonov, K. Baran, S. Szytuła, A. Szymczak, H. |
| author_facet |
Dyakonov, V. Bażela, W. Duraj, R. Dul, M. Kravchenko, Z. Zubov, E. Dyakonov, K. Baran, S. Szytuła, A. Szymczak, H. |
| author_sort |
Dyakonov, V. |
| title |
Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd) |
| title_short |
Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd) |
| title_full |
Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd) |
| title_fullStr |
Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd) |
| title_full_unstemmed |
Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd) |
| title_sort |
grain size effect on magnetic properties of remno₃ (re = pr, nd) |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2013 |
| topic_facet |
Низкотемпеpатуpный магнетизм |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118273 |
| citation_txt |
Grain size effect on magnetic properties of REMnO₃ (RE = Pr, Nd) / V. Dyakonov, W. Bażelа, R. Duraj, M. Dul, Z. Kravchenko, E. Zubov,
K. Dyakonov, S. Baran, A. Szytuła, H. Szymczak // Физика низких температур. — 2013. — Т. 39, № 4. — С. 452–458. — Бібліогр.: 11 назв. — англ. |
| series |
Физика низких температур |
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2025-07-08T13:48:47Z |
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2025-07-08T13:48:47Z |
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| fulltext |
© V. Dyakonov, W. Bażela, R. Duraj, M. Dul, Z. Kravchenko, E. Zubov, K. Dyakonov, S. Baran, A. Szytuła, and H. Szymczak, 2013
Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 4, pp. 452–458
Grain size effect on magnetic properties of REMnO3
(RE = Pr, Nd)
V. Dyakonov1,2, W. Bażela3, R. Duraj3, M. Dul3, Z. Kravchenko2, E. Zubov2,
K. Dyakonov4, S. Baran5, A. Szytuła5, and H. Szymczak1
1Institute of Physics, PAS, 32/46 Al. Lotników, Warszawa 02-68, Poland
2A.A. Galkin Donetsk Physico-Technical Institute, National Academy of Sciences of Ukraine
72 R. Luxembourg Str., Donetsk 83114, Ukraine
E-mail: zubov@fti.dn.ua
3Institute of Physics, Technical University, 1 Podchorążych, Kraków 30-084, Poland
4A.F. Ioffe Physico-Technical Institute RAN, 26 Politekhnicheskaja, St.-Petersburg 194021, Russia
5M. Smoluchowski Institute of Physics, Jagiellonian University, 4 Reymonta, Kraków 30-059, Poland
Received September 25, 2012
X-ray diffraction and magnetic using dc and ac methods measurements of the polycrystalline and nanosize
REMnO3 (RE = Pr, Nd) powdered samples have been performed. The nanosize manganites were synthesized
with a sol-gel method at different (800, 850 and 900 °C) temperatures. The average size of synthesized nanopar-
ticles (from 56 to 89 nm) and polycrystalline powders (above 200 nm) was estimated using the x-ray diffraction
data. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room
temperature with smaller values of the lattice parameters in the nanosamples. The temperature-dependent ac
magnetic susceptibilities show a sharp high-temperature peak connected with Mn magnetic moments ordering.
The low-temperature maximum of magnetic susceptibility is proposed to be due to the polarization of the rare-
earth sublattice by an effective exchange field of the Mn ordered sublattice. The antiferromagnetic ordering of
Mn sublattice and paramagnetic Curie temperatures as well as the magnetic moment values for the nanosize
samples were found to be smaller than those for polycrystalline sample.
PACS: 75.30.–m Intrinsic properties of magnetically ordered materials;
75.60.–d Domain effects, magnetization curves, and hysteresis;
75.75.–c Magnetic properties of nanostructures.
Keywords: x-ray diffraction, magnetic ordering, exchange interactions, nanoparticle, sol-gel method.
1. Introduction
REMnO3 (RE are the rare-earth ions) manganites have
been the subject of intensive investigations which are con-
centrated on their magnetic and electronic properties. In
the REMnO3 series, an increase of the Néel temperature
with increasing number of Z for the rare-earth elements is
observed. The fundamental interest in these compounds is
the explanation of the complex magnetic interactions and
correlation of the magnetic, structural and electronic pro-
perties in these compounds [1].
The results of studies of crystal and magnetic properties
of REMnO3 manganites, where RE = Pr and Nd, are re-
ported in the number works [2–8]. These compounds crys-
tallize in the orthorhombic crystal structure described by
the Pnma space group. Neutron powder diffraction (NPD)
data indicate that both compounds are antiferromagnets
with the Néel temperatures equal to 86 K for PrMnO3 [2]
and 78 K for NdMnO3 [3] below which the magnetic mo-
ments of the Mn atoms order. In contradiction, the specific
heat and thermal expansion data for PrMnO3 and NdMnO3
single crystals [4,5] indicate the anomalies connected with
the ordering in Mn sublattice at 99 and 88 K, respectively.
It is also proposed that below 13 K in PrMnO3 and below
18 K in NdMnO3 the magnetic moments of rare-earth ele-
ments order [4,6]. In the PrMnO3 single crystal, the Mn
moments is established to form the canted structure de-
scribed by CxFy arrangement whereas Pr moments form
Grain size effect on magnetic properties of REMnO3 (RE = Pr, Nd)
Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 4 453
the ferromagnetic structure of the Fy-type [2]. Similar
magnetic structures are observed in the NdMnO3 single
crystal [3].
The ground state of the rare-earth ions in the PrMnO3
and NdMnO3 single crystals was studied in Ref. 6. Elec-
tron transitions inside of both the ground Pr3+ quasidoublet
split by a crystal field and Nd3+ Kramers doublet split by
exchange Mn–Nd field were revealed and their crystal
field and exchange splitting components were determined.
Physical properties of manganites are known to depend
on various factors including stoichiometry and grain size.
A main motivation for performed studies was to obtain the
data concerning the crystal structure and magnetic proper-
ties of REMnO3 (RE = Pr, Nd) manganites as a function of
the grain size.
In this work, in order to understand the magnetic phe-
nomena in nanopowdered manganites and to compare ob-
tained data with data for polycrystalline powders the de-
tailed studies of PrMnO3 and NdMnO3 have been carried
out. Magnetic data demonstrate the large difference be-
tween the magnetic ordering of poly- and nanopowders.
The nanosamples are characterized by smaller values of
both the Néel temperature and the paramagnetic Curie
temperature. The magnetic moment values for the nanosize
samples are also smaller than for polycrystalline sample.
2. Experiment
The bulk polycrystalline NdMnO3 and PrMnO3 sam-
ples were prepared using the standard solid-state reaction
technique described in Refs. 7, 8. In brief, the mixture
of high-purity praseodymium (Pr2O3) or neodymium
(Nd2O3) oxides and manganese oxide (Mn3O4) taken in
stoichiometric ratio was dissolved in diluted (1:1) nitric
acid. The obtained solution was evaporated to a complete
removal of water followed by a degrading of nitrite salts at
500–700 °C. The product obtained was grinded and then
was heated to 900–950 °C for 2 h to remove a salt-disinte-
gration products. The homogeneous powder compacted
under pressure of 1 GPa into pellets were sintered in air at
1150–1170 °C for 18–20 h followed by temperature de-
crease to the room temperature at the rate of 70 °C/h. Be-
cause of high temperatures of synthesis the grain sizes of
polycrystalline samples has exceeded nanosizes.
The nanoparticle PrMnO3 and NdMnO3 manganites
were prepared using the sol-gel method. To prepare nano-
powder PrMnO3, a stoichiometric amount of Pr6O11 and
Mn3O4 oxides of high purity was dissolved in acetic acid
with added hydrogen peroxide. An urea as a gel-forming
component was added to the solution obtained. This mix-
ture was slowly evaporated to dryness. The dry remainder
was decomposed with smoothly increasing temperature
from 200 to 450 °C. Then the powder obtained was thor-
oughly grinded. The pressed pellets were annealed at tem-
peratures of Tan = 800 and 900 °С for 20 h in air followed
by a slow cooling down to the room temperature.
For synthesis of the nanoparticle NdMnO3 manganites,
a stoichiometric amount of the Nd2O3 and Mn3O4 oxides
of high purity were used. Neodymium oxide was dissolved
(1:1) in a dilute nitric acid, and Mn3O4 oxide was dis-
solved in acetic acid with added hydrogen peroxide. Both
solutions were mixed together with added solution of urea.
This mixture was slowly evaporated to dryness. The dry
remainder was calcinated at temperature of 200–450 °C.
Then the dry powder obtained was grinded and was
pressed into pellets which were annealed at Tan = 800, 850
and 900 °С for 20 h in air followed by a slow cooling
down to the room temperature.
In order to determinate the crystallographic structure
all the samples studied were characterized at room temper-
ature with x-ray powder diffraction using a Philips PW-
3710 X’PERT diffractometer with CuKα radiation. The 2θ
scans are performed with the steps of 0.01 and counting
time of 5 s step. The data were analyzed with the Rietveld-
type refinement software FullProf program [9]. Magnetiza-
tion and magnetic susceptibility of both nanoparticles and
polycrystalline samples have been measured using a vibrat-
ing sample magnetometer option of the Quantum Design
PPMS over a temperature range 1.9–300 K in magnetic
field up to 90 kOe. The following measurements have been
performed:
— temperature dependence of dc magnetic ZFC (zero
field cooling) and FC (field cooling) susceptibilities (χdc)
in magnetic field of 50 Oe (to determine the phase transi-
tion temperature);
— ( )dc Tχ dependence in temperature range of 1.9–
300 K in magnetic field of 1 kOe (to determine the para-
magnetic Curie temperature, θp, and the effective magnetic
moment value, μeff);
— field dependence of magnetization in magnetic field
up to 90 kOe (to determine the magnetic moment values).
The ac magnetic susceptibility, acχ χ χ= ′ + ″, where
χ′ and χ″ are the real and imaginary components, re-
spectively, was also measured versus temperature and fre-
quency between 10 Hz and 10 kHz in magnetic field with
Hac amplitude equal to 5 Oe.
3. Results
Typical x-ray pattern for nano-PrMnO3 (Tan = 800 °C)
is presented in Fig. 1. The x-ray diffraction data indicate
that all the samples studied are homogeneous, single phase
and have an orthorhombic crystal structure described by
the Pnma space group. In this structure the Pr or Nd and
O1 atoms occupying the 4(c) site: x, ¼, z; Mn atoms in
4(b) site: 0, 0, ½; and O2 atoms in the 8(d) site: x, y, z. The
fitted structural parameters: lattice a, b and c constants and
unit cell volume, V, and positional xi, yi and zi parameters
are listed in Table 1. The decrease of the a, c constants and
V. Dyakonov et al.
454 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 4
unit cell volume, V, and increase of the b constant were
found with decreasing the grain size.
The grain sizes, d, in nanopowder were determined us-
ing the Scherrer relation of d = λ/(B cos θ) [10]. Here λ =
= 1.54178 Å is the x-ray wavelength, θ the corresponding
angle of Bragg diffraction and B = β – β0 the difference
between the Bragg reflex half-widths of the nanosize sam-
ples (β) and standard Si powder sample (β0) with the grain
size of 10 μm [11] which was used to calibrate the intrinsic
width associated to the equipment. The singular (111) re-
flection of the diffraction pattern was used to calculate the
particle size. The d values calculated are listed in Table 2.
Presented data indicate that the grain sizes increase with
increasing annealing temperature.
Table 2. The grain sizes, d, of REMnO3 nano- and polycrys-
talline powders calculated using XRD data and the Scherrer re-
lation
Sample <d>, nm
PrMnO3
NdMnO3
800 °C
900 °C
poly
800 °C
850 °C
900 °C
poly
66
85
250
56
70
89
245
Figiure 2(a)–(c) present the results of magnetic studies
of the PrMnO3 samples. The temperature dependences of
dc magnetic susceptibility, ( )dc Tχ , for poly- and nano-
samples are shown in Fig. 2(a). Presented data point to a
distinct splitting between the dc susceptibilities measured
under field-cooled (FC) and zero-field-cooled (ZFC) con-
ditions. These curves are separated below the temperatures
of 90 K for poly-, 65 K for nano-900 °C and 60 K for
nano-800 °C samples. The ZFC curves have the maximum
at 68, 45 and 35 K for poly-, nano-900 °C and nano-800 °C
samples, respectively. Such behavior of ( )dc Tχ seems to
be connected with the freezing of the domain dynamics
within the ordered Mn sublattice. The antiferromagnetic
(AFM) phase transition temperature, TN, defined as maxi-
mum of dχdc/dT dependence was found to decrease with
decreasing the grain sizes (Table 3).
Above 150 K, the reciprocal magnetic susceptibili-
ties (Fig. 2(b)) obey the Curie–Weiss (CW) law: iχ =
0 / ( )i i iC Tχ θ= + − , where Ci is the Curie–Weiss con-
stant, 0iχ is the background susceptibility and θi is the
Curie–Weiss temperature. The values of the paramagnetic
Fig. 1. X-ray diffraction pattern of nano-800 °C PrMnO3 ob-
served (solid squares) and calculated (solid lines). Bragg reflec-
tions are indicated by tick marks and the difference pattern is also
plotted.
Table 1. Structural parameters for NdMnO3 and PrMnO3 obtained in the Rietveld refinement from x-ray patterns at room tem-
perature
Parameter NdMnO3 PrMnO3
poly nano poly nano
900 °C 850 °C 800 °C 800 °C 900 °C
a, Å
b, Å
c, Å
V, Å3
RE x
z
O1 x
z
O2 x
y
z
RBragg, %
Rf, %
5.7119(9)
7.5890(13)
5.4191(8)
234.60(11)
0.0597(7)
0.9909(9)
0.4837(8)
0.0817(8)
0.3106(6)
0.0405(4)
0.7108(6)
6.8
4.4
5.6512(5)
7.6117(7)
5.4140(5)
232.88(7)
0.0563(3)
0.9929(6)
0.467(3)
0.103(4)
0.294(3)
0.042(2)
0.725(3)
8.4
7.4
5.6283(4)
7.6199(6)
5.4129(4)
232.14(6)
0.0552(3)
0.9914(5)
0.467(3)
0.101(4)
0.290(3)
0.040(2)
0.727(3)
13.8
8.8
5.6125(2)
7.6234(2)
5.4120(2)
231.56)2)
0.0536(2)
0.9913(3)
0.481(2)
0.090(2)
0.302(1)
0.041(1)
0.713(2)
9.0
9.8
5.6049(5)
7.6654(7)
5.4616(5)
234.65(6)
0.0464(8)
0.9949(11)
0.4854(7)
0.0760(7)
0.3013(4)
0.0385(3)
0.7169(4)
7.8
6.4
5.5192(2)
7.6995(3)
5.4547(2)
231.80(2)
0.0391(2)
0.9955(4)
0.493(2)
0.073(3)
0.292(2)
0.034(1)
0.716(2)
5.2
6.6
5.5490(2)
7.6908(3)
5.4550(2)
232.88(2)
0.0427(2)
0.9927(4)
0.496(2)
0.074(3)
0.287(2)
0.038(1)
0.721(2)
6.6
7.1
Grain size effect on magnetic properties of REMnO3 (RE = Pr, Nd)
Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 4 455
Curie temperature and effective magnetic moment are listed
in Table 3.
The θp temperature for PrMnO3 decreases from 104 K
for poly- to 88 and 81 K for nano-900 °C and nano-800 °C
samples, respectively. A shift of the CW temperature to-
wards lower temperatures indicates a decrease of magnetic
interactions between Mn moments with decreasing the
grain size. A positive Curie–Weiss temperature despite the
AFM transition can be understood considering the magnet-
ic structure with ferromagnetic coupling in the plane and
AFM coupling between planes.
The values of effective magnetic moments are com-
pared with calculated values from the relation of
RE 2 Mn 2
eff eff eff( ) ( )μ μ μ= + , (1)
where Pr
effμ = 3.58 μB and Mn
effμ = 4.90 μB are the theoreti-
cal values for Pr3+ and Mn3+ ions, respectively.
The effective magnetic moments are larger than theoreti-
cal value μeff = 6.07 μB. It can be connected with existence
of ferromagnetic clusters which at high temperatures be-
have as the superparamagnetic particles having, as a rule,
the large value of effective magnetic moment.
Magnetic field dependences of magnetization of
PrMnO3 are shown in Fig. 2(c). As it is seen in Fig. 2(c),
the magnetic moments per formula unit are equal to 3.3
(nano-800 °C), 3.6 (nano-900 °C) and 3.9 μB (poly-) at T =
= 1.9 K in magnetic field H = 90 kOe (Table 3). The mag-
netization increases as magnetic field is increased, without
reaching the saturation. The lack of saturation at T = 1.9 K
and H = 90 kOe is due to Mn antiferromagnetism.
Table 3. Magnetic data for REMnO3 (RE = Pr and Nd) com-
pounds
Compound TN,
K
Tt,
K
θp,
K
μeff,
μB
μ,
μB
μ,
μB, Mn3+
PrMnO3
poly 74 10 104 6.5 3.9 2.1
nano-800 °C 50 81 6.4 3.3 1.4
nano-900 °C 56 15 88 6.4 3.6 1.7
NdMnO3
poly 70 11 68 6.2 4.2 2.0
nano-800 °C 50 7 58 6.1 3.7 1.7
nano-850 °C 55 9 62 6.1 3.8 1.8
nano-900 °C 55 10 67 6.2 4.0 1.9
Comments : TN is the temperature of antiferromagnetic ordering
of Mn sublattice determined as the dχdc /dT maximum; Tt is the
temperature of rare-earth polarization; θp is the paramagnetic
Curie temperature; μeff is the effective magnetic moment de-
termined from the Curie–Weiss law; μ is the magnetic moment
per formula unit determined from magnetization at T = 1.9 K
and H = 90 kOe; μ, Mn3+ is the magnetic moment per formula
unit of Mn3+ subsystem in applied magnetic field without con-
tribution of Mn–Pr and Mn–Nd exchange interactions.
For polycrystalline PrMnO3 samples, the temperature
dependence of acχ ′ susceptibility shows the sharp maxi-
mum at 70 K and weak anomaly at 10 K (Fig. 3(a)), which
correspond to two peaks in temperature dependence of
acχ ′′ susceptibility at the same temperatures. Their intensi-
ty decreases with increasing frequency.
For nano-900 °C, the ( )ac Tχ′ dependence exhibits the
sharp maximum at 48 K and a slightly pronounced anoma-
ly at 15 K, while the ( )ac Tχ′′ dependence has two maxima
at 46 and 13 K (Fig. 3(b)). The position and intensity of
( )ac Tχ′′ maximum change with changing frequency.
For nano-800 °C, alone ( )ac Tχ′ maximum is observed
at 41 K (Fig. 3(c)). Both an intensity and temperature of
the ( )ac Tχ′′ peak increase with increasing frequency.
For NdMnO3 the temperature and magnetic field de-
pendences of susceptibility and magnetization are analo-
gous to those for PrMnO3 shown in Figs. 2 and 3 (there-
fore they are not presented). They show the following:
— the temperature-dependent dc magnetic susceptibili-
ties of NdMnO3 resemble the results obtained for PrMnO3.
The ZFC and FC curves are observed to be separated at
74 K (for poly-) and 55 K (for nano-900 °C) and 50 K (for
nano-800 and 850 °C);
Fig. 2. PrMnO3: (a) Temperature dependence of the dc magnetic
susceptibility for poly- and nanosamples. Lower and upper curves
correspond to ZFC and FC susceptibilities, respectively; (b) Tem-
perature dependences of the reciprocal magnetic susceptibility;
(c) Experimental field dependences (points) of magnetization of
PrMnO3 at 1.9 K for nano-800 °C (squares), nano-900 °C (cir-
cles) and poly (triangles). The solid lines 1–3 correspond to mag-
netization of nano-800 °C, nano-900 °C and poly without mag-
netic contribution of Pr3+ ions, respectively.
V. Dyakonov et al.
456 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 4
— the reciprocal magnetic susceptibility for poly- and
nanosamples obeys the Curie–Weiss law at temperatures
above the AFM phase transition. The paramagnetic Curie
temperatures and the effective magnetic moments calculat-
ed are displayed in Table 3. The experimental data are
close to the theoretical value of effective moment μeff =
= 6.09 μB for the superposition of the Nd(4f3) and the
Mn(3d4) sublattices;
— the total magnetic moments calculated are equal to
4.2, 4.03, 3.87 and 3.75 μB/f.u. for poly-, nano-900, nano-
850 and nano-800 °C, respectively, at T = 1.9 K in magne-
tic field H = 90 kOe.
The temperature dependences of acχ ′ and acχ ′′ compo-
nents of ac susceptibility resemble the results obtained
from dc measurements. For example, in Fig. 4 the ( )ac Tχ
dependences are presented for nano-800 °C NdMnO3, which
exhibit two sharply pronounced maxima. The temperature
and intensity of both maximum are slightly dependent on
frequency.
The characteristic peculiarity of the ( )ac Tχ′ susceptibi-
lity is related to an occurrence of a cusp at T ≈ 11 K which
is accompanied by a peak in the ( )ac Tχ′′ susceptibility. It
may be assumed (as in Ref. 6) that this low-temperature
maximum is due to the polarization of the Nd sublattice by
an effective exchange field of the Mn sublattice and obvi-
Fig. 3. Temperature dependences of the real
acχ′ and imaginary
acχ′′ components of the ac magnetic susceptibility of the PrMnO3
samples: (a) poly-, (b) nano-900 °C and (c) nano-800 °C mea-
sured in magnetic field of H = 5 Oe at the frequencies varied
between 10 Hz and 10 kHz.
Grain size effect on magnetic properties of REMnO3 (RE = Pr, Nd)
Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 4 457
ously results in the additional contribution of the Nd ions
to a weak magnetic moment.
4. Discussion
To calculate the magnetic moment of PrMnO3 in ap-
plied magnetic field it is necessary to take into account the
contribution of the Pr3+ sublattice. As is shown in Ref. 4,
for the PrMnO3 single crystal the lowest level of Pr3+ is
split in quasidoublet by crystal field and superexchange
rare-earth ion with Mn3+ ion. The Δ value of this splitting
can be written as
1/22 2 22 ( )cf zΔ Δ Δ⎡ ⎤= + +⎣ ⎦Hμ , (2)
where 2 cfΔ and 2 zΔ are the splitting by the crystal field
and Pr–Mn exchange interactions, respectively, and μ =
= (μa, μb, 0) is the magnetic moment of ground state in ab
plane. The Pr3+ contribution to Δ is caused by both cfΔ =
= 18.7 K [4] and energy of interaction of μ magnetic mo-
ment with applied field. One can write the next expressions
for magnetic moment per formula unit along the x = a, b
and z = c directions [4]:
22
tanh ,
2
tanh ,
2
VVx x
x x x
B
VV
z z z z
B
H
M H
k T
M H
k T
μ Δ
χ
Δ
Δ
μ χ
⎛ ⎞
= +⎜ ⎟⎝ ⎠
⎛ ⎞
= +⎜ ⎟⎝ ⎠
(3)
where ,
VV
x zχ are the Van-Vleck susceptibilities of rare earth
ions. For Pr3+ we have μx = 2.1 μB, μz = 0 and ,
VV
x zχ =
= 0.15·10–4 emu/f.u. [6].
Therefore we can write the spatially averaged total
magnetic moment MPr–Mn of Pr–Mn system:
( )Pr Mn
1 2– 3 x zM M M= + . (4)
The calculation results of magnetic moment of Mn3+ in
PrMnO3 in applied magnetic field without contribution (4)
are presented in Fig. 2(c). From Fig. 2(c) it is easy to find
that the magnetic moments of Mn3+ are 1.4, 1.8 and 2.1 for
nano-800 °C, nano-900 °C and polysamples, respectively.
Using Eq. (2) for the Nd3+ Kramers ion, when the split-
ting of ground doublet is caused by superexchange be-
tween Mn and Nd ions only, one can write the next expres-
sion for splitting Δi:
( )
1/2222i z i iHΔ Δ μ⎡ ⎤= +⎢ ⎥⎣ ⎦
, (5)
where Δz = 10.1 K is the splitting by Mn–Nd exchange,
μz = 1.9 μB, μx = 1.8 μB and μy = 1.2 μB [4]. Substituting
(5) in (3) with account for isotropic magnetic moment
( )Nd Mn
1
– 3 x y zM M M M= + + ,
we obtain the magnetic moment values of Mn ion equal to
1.7, 1.8, 1.9 and 2 μB for nano-800 °C, 850 °C, 900 °C and
polysamples, respectively. A decrease of magnetic moment
of Mn ion with decreasing grain size indicates the weaken-
ing of magnetic interactions in nanosamples.
5. Summary
The detailed magnetic and x-ray diffraction investiga-
tions of the PrMnO3 and NdMnO3 manganites have al-
lowed to study and to clarify interesting aspects related to
the grain size effect on their structural and magnetic prop-
erties.
The REMnO3 (RE = Pr, Nd) samples prepared at tem-
peratures below 900 °C using the sol-gel method had the
grains corresponding to the nanoparticles, while the pow-
der synthesized at temperatures of 1150–1170 °C was the
large-grain (> 240 nm) sample. The grain sizes determined
from the x-ray data decrease with decreasing the annealing
temperature. The samples investigated had the orthorhom-
bic crystal structure, in which the lattice parameters depend
on the grain sizes.
Fig. 4. Temperature and frequency dependences of the real
acχ′
(a) and imaginary
acχ′′ (b) components of the ac magnetic sus-
ceptibility of the nano-800 °C NdMnO3 sample.
V. Dyakonov et al.
458 Low Temperature Physics/Fizika Nizkikh Temperatur, 2013, v. 39, No. 4
Magnetic data obtained demonstrate the difference be-
tween magnetic ordering of poly- and nanopowdered sam-
ples. The nanosamples are characterized by smaller value
of both the Néel temperature and the paramagnetic Curie
temperature that indicates decrease of magnetic interac-
tions in the nanoparticle samples. The values of magnetic
moments obtained from the magnetization measurements
decrease as the grain size is decreased.
The temperature-dependent ( )ac Tχ′ and ( )ac Tχ′′ mag-
netic susceptibilities show a sharp high-temperature peak
connected with Mn magnetic moments ordering. The low-
temperature maximum is proposed to be due to the polari-
zation of the rare-earth sublattice by an effective exchange
field of the Mn ordered sublattice. The ac susceptibility
data for poly- and nanosamples demonstrate the strong
frequency dependence of ( )ac Tχ susceptibility, that indi-
cates the relaxation process near the critical temperatures.
Acknowledgments
The macroscopic magnetic measurements was carried
out with the equipment purchased thanks to the financial
support of the European Regional Development Fund in
the framework of the Polish Innovation Economy Opera-
tional Program (contract No. POIG.02.01.00-12-023/08).
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