Antiferromagnet–ferromagnet phase transition in lightly doped manganites
Magnetic and structural phase diagrams of the La₀.₈₈MnOx, La₁₋xSrx(Mn₁₋x/₂Nbx/₂)O₃, Nd₁₋xCaxMnO₃, and Bi₁₋xCaxMnO₃ series constructed on the basis of x-ray, neutron powder diffraction, Young’s modulus, magnetization and resistivity measurements are presented. It is shown that the main factor cont...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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nasplib_isofts_kiev_ua-123456789-1217022025-06-03T16:29:06Z Antiferromagnet–ferromagnet phase transition in lightly doped manganites Troyanchuk, I.O. Khomchenko, V.A. Eremenko, V.V. Sirenko, V.A. Szymczak, H. К семидесятилетию антиферромагнетизма Magnetic and structural phase diagrams of the La₀.₈₈MnOx, La₁₋xSrx(Mn₁₋x/₂Nbx/₂)O₃, Nd₁₋xCaxMnO₃, and Bi₁₋xCaxMnO₃ series constructed on the basis of x-ray, neutron powder diffraction, Young’s modulus, magnetization and resistivity measurements are presented. It is shown that the main factor controlling the antiferromagnet–ferromagnet phase transition in the manganites is a type of an orbital state. The results are discussed in the framework of structurally driven magnetic phase separation model. This work was partly supported by Belarus and Ukrainian Fund for Basic Research (grant F04MS-004 and grant 10.01/001 Ô05K–012) Polish – Ukrainian Academies research project (No.4-2003) and by MES of Ukraine (grant Ì/257-2004). 2005 Article Antiferromagnet–ferromagnet phase transition in lightly doped manganites / I.O. Troyanchuk, V.A. Khomchenko, V.V. Eremenko, V.A. Sirenko, H. Szymczak // Физика низких температур. — 2005. — Т. 31, № 8-9. — С. 1073-1080. — Бібліогр.: 33 назв. — англ. 0132-6414 PACS: 75.30.Vn, 75.30.Cr https://nasplib.isofts.kiev.ua/handle/123456789/121702 en Физика низких температур application/pdf Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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К семидесятилетию антиферромагнетизма К семидесятилетию антиферромагнетизма |
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К семидесятилетию антиферромагнетизма К семидесятилетию антиферромагнетизма Troyanchuk, I.O. Khomchenko, V.A. Eremenko, V.V. Sirenko, V.A. Szymczak, H. Antiferromagnet–ferromagnet phase transition in lightly doped manganites Физика низких температур |
| description |
Magnetic and structural phase diagrams of the La₀.₈₈MnOx, La₁₋xSrx(Mn₁₋x/₂Nbx/₂)O₃,
Nd₁₋xCaxMnO₃, and Bi₁₋xCaxMnO₃ series constructed on the basis of x-ray, neutron powder diffraction,
Young’s modulus, magnetization and resistivity measurements are presented. It is shown
that the main factor controlling the antiferromagnet–ferromagnet phase transition in the manganites
is a type of an orbital state. The results are discussed in the framework of structurally driven
magnetic phase separation model. |
| format |
Article |
| author |
Troyanchuk, I.O. Khomchenko, V.A. Eremenko, V.V. Sirenko, V.A. Szymczak, H. |
| author_facet |
Troyanchuk, I.O. Khomchenko, V.A. Eremenko, V.V. Sirenko, V.A. Szymczak, H. |
| author_sort |
Troyanchuk, I.O. |
| title |
Antiferromagnet–ferromagnet phase transition in lightly doped manganites |
| title_short |
Antiferromagnet–ferromagnet phase transition in lightly doped manganites |
| title_full |
Antiferromagnet–ferromagnet phase transition in lightly doped manganites |
| title_fullStr |
Antiferromagnet–ferromagnet phase transition in lightly doped manganites |
| title_full_unstemmed |
Antiferromagnet–ferromagnet phase transition in lightly doped manganites |
| title_sort |
antiferromagnet–ferromagnet phase transition in lightly doped manganites |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2005 |
| topic_facet |
К семидесятилетию антиферромагнетизма |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/121702 |
| citation_txt |
Antiferromagnet–ferromagnet phase transition in lightly doped manganites / I.O. Troyanchuk, V.A. Khomchenko, V.V. Eremenko, V.A. Sirenko, H. Szymczak // Физика низких температур. — 2005. — Т. 31, № 8-9. — С. 1073-1080. — Бібліогр.: 33 назв. — англ. |
| series |
Физика низких температур |
| work_keys_str_mv |
AT troyanchukio antiferromagnetferromagnetphasetransitioninlightlydopedmanganites AT khomchenkova antiferromagnetferromagnetphasetransitioninlightlydopedmanganites AT eremenkovv antiferromagnetferromagnetphasetransitioninlightlydopedmanganites AT sirenkova antiferromagnetferromagnetphasetransitioninlightlydopedmanganites AT szymczakh antiferromagnetferromagnetphasetransitioninlightlydopedmanganites |
| first_indexed |
2025-11-25T15:47:12Z |
| last_indexed |
2025-11-25T15:47:12Z |
| _version_ |
1849777858428272640 |
| fulltext |
Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9, p. 1073–1080
Antiferromagnet–ferromagnet phase transition
in lightly doped manganites
I.O. Troyanchuk and V.A. Khomchenko
Institute of Solid State and Semiconductor Physics, NAS, 17 P. Brovka Str., Minsk 220072, Belarus
E-mail: troyan@ifttp.bas-net.by
V.V. Eremenko and V.A. Sirenko
B. Verkin Institute for Low Temperature Physics and Engineering
of the National Academy of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: sirenko@ilt.kharkov.ua
H. Szymczak
Institute of Physics, PAS, 32/46 Lotnikow Str., Warsaw 02-668, Poland
Received February 18, 2005
Magnetic and structural phase diagrams of the La0.88MnOx, La1–xSrx(Mn1–x/2Nbx/2)O3,
Nd1–xCaxMnO3, and Bi1–xCaxMnO3 series constructed on the basis of x-ray, neutron powder dif-
fraction, Young’s modulus, magnetization and resistivity measurements are presented. It is shown
that the main factor controlling the antiferromagnet–ferromagnet phase transition in the manga-
nites is a type of an orbital state. The results are discussed in the framework of structurally driven
magnetic phase separation model.
PACS: 75.30.Vn, 75.30.Cr
1. Introduction
Mixed-valence manganites with a perovskite struc-
ture are the model objects for the physics of strongly
correlated electronic systems. The interest in the study
of manganites is due to a variety of phase states and
transitions and intrinsic correlation of the crystal
structure, magnetic, and transport properties. The na-
ture of the interplay between the crystal structure,
magnetic, and transport properties of manganites is
still a matter of discussion in spite of numerous inves-
tigations. Several models were proposed to explain a
magnetic state evolution under hole doping as well as
a metal–insulator transition at the Curie point. In the
double-exchange model of Zener, simultaneous ferro-
magnetic and metallic transitions have been qualita-
tively explained by the fact that electrons tend to
move between Mn3+ and Mn4+ ions having the same
spin orientation, therefore electron delocalization fa-
vors the ferromagnetic order [1]. More recently Millis
et al. pointed out that double exchange alone cannot
account for many of the experimental results [2]. They
showed that a Jahn–Teller-type electron–phonon cou-
pling should play an important role in explanation of
the colossal magnetoresistance effect. Another mecha-
nism of antiferromagnet–ferromagnet phase transi-
tions in manganites was proposed by Nagaev [3]. He
assumed that the intermediate phase can be described
as a inhomogeneous magnetic state driven by an elec-
tronic phase segregation. In this scenario the ferro-
magnetic regions contain an excess of holes and are
metallic. Goodenough et al. argued that the magnetic
properties of manganites were determined by the type
of orbital state [4]. According to the rules for 180�
superexchange, if the electronic configuration corre-
lates with vibrational modes, Mn3+–O2-–Mn3+ inter-
actions are antiferromagnetic in case of the static
Jahn–Teller effect and ferromagnetic when the
Jahn–Teller effect is dynamic. Thus, antiferromag-
net–ferromagnet phase transitions can occur going
through a mixed state of phases with different orbital
dynamics.
© I.O. Troyanchuk, V.A. Khomchenko, V.V. Eremenko, V.A. Sirenko, and H. Szymczak, 2005
The recent magnetic phase diagrams of the
La1–xSrxMnO3 and La1–xCaxMnO3 systems were con-
structed assuming a homogeneous canted magnetic
state in a low doping range [5,6]. On the other hand,
there are numerous experimental data which indicate
the existence of phase separation in manganites. The
results of nuclear magnetic resonance [7,11], neutron
diffraction [11,12], muon spin relaxation [13], x-ray
absorption [14], scanning tunneling spectroscopy
[15], and electron microscopy [16] experiments give
evidence of magnetic and structural inhomogeneities,
but the driving force of magnetic phase separation in
manganites is still not fully clear. In order to contrib-
ute to the solution of this problem we have investi-
gated the features of the antiferromagnet–ferromagnet
phase transition in low-doped La0.88MnOx,
La1–xSrx(Mn1–x/2Nbx/2)O3, Nd1–xCaxMnO3, and
Bi1–xCaxMnO3 manganites.
2. Results and discussion
2.1. La0.88MnOx system
Tentative magnetic phase diagram of the
La0.88MnOx (2.82 � x � 2.96) manganites is shown in
Fig. 1. The most strongly reduced sample
La0.88MnO2.82 is antiferromagnet with a N�el temper-
ature of 140 K. Its properties are found to be similar to
the properties of stoichiometric LaMnO3. Both com-
pounds have very close unit cell parameters, the same
magnetization value, and close temperatures of both
magnetic (TN � 140 K) and orbital orderings (TOO �
� 750 K). The existence of orbital ordering in the
A-type antiferromagnetic structure of La0.88MnO2.82
is corroborated by neutron diffraction measurements
[17]. With increasing oxygen content up to the x =
= 2.85 sample, the magnetic and orbital ordering tem-
peratures lower while the magnetization increases
slightly. Results of the neutron diffraction measure-
ments carried out for the x = 2.84 sample confirm the
appearance of a ferromagnetic component. A further
increase of the oxygen concentration leads to a signifi-
cant enhancement of the ferromagnetic contribution.
The transition temperature to the paramagnetic state
begins to increase and the transition becomes broader.
Neutron diffraction data obtained for the x = 2.87
sample indicate that ferromagnetic coupling becomes
predominant. No long-range antiferromagnetic order
has been observed for this compound. At the same
time, the refined magnetic moment is lower than that
expected for the full spin arrangement. Besides, the
relatively large magnetic anisotropy at low tempera-
ture assumes the presence of an anisotropic magnetic
coupling which differs from the isotropic ferromag-
netic one. This can be attributed to existence of either
short-range antiferromagnetic clusters or a spin-glass
phase. No pronounced thermomagnetic irreversibility
indicating the anisotropic magnetic interactions is ob-
served starting from the x = 2.92 sample. The values of
magnetization estimated for the monoclinic com-
pounds are close to those expected for full spin align-
ment. The ground state of all the orthorhombic com-
pounds 2.82 � x � 2.90 is insulating. It should be
noted that the appearance of metallic conductivity
does not coincide with the transition to monoclinic
phase. Simultaneous first-order magnetic transition
and metal–insulator transition at TC are observed for
x � 2.92 compounds.
A strong correlation between the magnetic and
structural properties of La0.88MnOx (2.82 � x � 2.96)
manganites is observed. The hypothetical structural
phase diagram of La0.88MnOx (2.82 � x � 2.96) con-
structed using x-ray, neuron diffraction, Young’s
modulus, resistivity, and DTA data is shown in Fig. 2.
For La0.88MnO2.82, the sharp anomalies of the
Young’s modulus and resistivity are associated with
the removal of cooperative orbital ordering; it is ob-
served at approximately 650 K. The DTA measure-
ments revealed the release of latent heat in the range
650–730 K. Neutron diffraction data indicate the co-
existence of orbitally ordered OI and orbitally disor-
dered O phases at T = 700 K. Another thermal anom-
1074 Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9
I.O. Troyanchuk, V.A. Khomchenko, V.V. Eremenko, V.A. Sirenko, and H. Szymczak
Oxygen, x
2.82 2.85 2.88 2.91 2.94
T
,K
N
,C
300
200
100
0
Average manganese valency
3.00 3.06 3.12 3.18 3.24
Orth M
FM
AFI
AFI + FI
PI
1 2
Fig. 1. Magnetic phase diagram of the La0.88MnOx
(2.82 � x � 2.96) system. Orth-orthorthorhombic crystal
structure, M is monoclinic crystal structure; PI, AFI, FI,
and FM are paramagnetic insulating, antiferromagnetic in-
sulating, ferromagnetic insulating, and ferromagnetic me-
tallic states, respectively. Areas 1 and 2 correspond to the
concentration regions where an antiferromagnetic or ferro-
magnetic phase predominates, respectively.
aly connected with the transition to the monoclinic
phase is observed in the temperature range 915 � T �
� 960 K. With the increase of the oxygen content to
x = 2.83, the temperatures of both orbital order-disor-
der and orthorhombic–monoclinic phase transitions
significantly decrease. The range of coexistence of OI
and O phases becomes broader, while the width of the
anomaly associated with the temperature-induced
orthorhombic–monoclinic transition remains practi-
cally constant. Starting from the x = 2.84 sample, the
differential thermal analysis does not show any signif-
icant heat effect, which could be interpreted as a tran-
sition to a pure orbitally disordered state; however,
the anomaly related to the transition from an
orthorhombic to a monoclinic phase remains well pro-
nounced. Neutron diffraction data coupled with
Young’s modulus measurements indicate the existence
of predominantly static Jahn–Teller distortions at
room temperature and two-phase character of the crys-
tal structure above T � 470 K. Inhomogeneous struc-
tural states are observed up to 650 K. Above this tem-
perature the monoclinic phase is stabilized. A further
increase of the oxygen concentration leads to the
broadening and gradual disappearance of the anomaly
which relates to the transition to the orbitally ordered
state. The neutron diffraction study performed for the
La0.88MnO2.87 compound indicates that the value of
the MnO6 octahedron distortion increases with de-
creasing temperature to 200 K. However, even in the
case of T = 200 K, where the worst agreement factors
for one-phase structural model have been observed,
the introduction of the second orthorhombic phase
was unsuccessful. Apparently, even at 200 K, the or-
bitally ordered clusters are still too small and separate
to distinguish the OI phase in the diffraction experi-
ment. The temperature of orthorhombic–monoclinic
phase transition gradually decreases as the oxygen
content increases and starting from the x = 2.91 sam-
ple, the monoclinic phase is stabilized (Fig. 2). It is
necessary to mention that the x-ray and neutron dif-
fraction experiments can reveal a two-phase structural
state rather in the case of macroscopic structural phase
separation. In the cases of local structural inhomoge-
neities or nanometer scale structural clusters, these
experiments give only an average picture of a struc-
tural state [18]. Thus, the correlation between the or-
bital state and magnetic properties of the La0.88MnOx
manganites is prominent. The static Jahn–Teller dis-
tortions are responsible for the A-type antiferromag-
netic structure, while dynamic orbital correlations
lead to ferromagnetism.
It is worth noting that there are two alternative
models of orbital state corresponding to ferromagnetic
ordering in manganites: 3D dynamic d z r3 2 2– orbital
correlations and staggered ordering of d z r3 2 2– and
dx y2 2– orbitals. Neutron diffraction studies have
shown that LaMnO3 undergoes a structural transition
from OI-orthorhombic to O-orthorhombic phase at
TJT = 750 K [19]. The MnO6 octahedron in the
O-orthorhombic phase becomes nearly regular, i.e.,
the orbital ordering disappears [19]. However, x-ray
absorption near the edge structure and the extended
x-ray absorption fine structure at the MnK-edge mea-
surements have revealed that the MnO6 octahedrons
in LaMnO3 remain tetragonally distorted at T > TJT
[20]. The empty Mn3+ electronic d-states were shown
to be unaltered through the Jahn–Teller transition.
The lowest energy for the eg electron corresponds to
the three possible distortions giving rise to three de-
generate vibronic states, dx r2 2– , dy r2 2– , and dz r2 2– ,
being the electronic orbitals of the vibronic state. The
thermally excited electron jumps between these states
above TJT and is localized in an ordered state below
TJT. The orbital ordering proposed for LaMnO3 arises
then from the ordering of the local Jahn–Teller distor-
tions. The high temperature (O-orthorhombic) phase
can be described as a dynamical locally distorted
phase with the strong antiferrodistortive first neigh-
bour coupling [20].
The similar situation seems to be observed for
Mn4+-doped manganites. The atomic pair-density
Antiferromagnet–ferromagnet phase transition in lightly doped manganites
Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9 1075
Average manganese valency
3.00 3.06 3.12 3.18 3.24
Oxygen, x
2.82 2.85 2.88 2.91 2.94
T
,K
800
400
0
1 2
O
O + M
M
O + O
I
Fig. 2. Crystal structure phase diagram of the La0.88MnOx
(2.82 � x � 2.96) system. OI, O, and M are orbitally or-
dered orthorhombic, orbitally disordered orthorhombic,
and monoclinic phases, respectively. Areas 1 and 2 corre-
spond to the concentration regions where the static
Jahn–Teller distortions or dynamic orbital correlations
predominate, respectively.
function of La1–xSrxMnO3 manganites (0 � x � 0.4),
obtained by pulsed neutron diffraction, indicates the
existence of tetragonally distorted MnO6 octahedrons
even in the rhombohedral metallic phase, when the
crystallographic structure shows no JT distortions
[21]. This is possible only in the case of the dynamic
orbital correlations described above. One can assume
that when one puts non-Jahn–Teller Mn4+ ions in the
background of the Mn3+ ions, the eg-orbitals of all the
Mn3+ ions surrounding the localized hole (Mn4+) tend
to be directed towards it, forming an orbital polaron
[22]. Due to the strong antiferrodistortive Mn3+ first
neighbour coupling [20], dynamic correlations of the
d z r3 2 2– orbitals should arise.
According to the rules for 180� superexchange the
dynamic orbital correlations lead to ferromagnetic in-
teraction between the Mn3+ ions [4]. Hence, one can
expect that ferromagnetism in manganites can arise
even in the absence of Mn4+ ions, if only the JT effect
is dynamic. For instance, Mn substitution with
non-Jahn–Teller diamagnetic Nb5+, Al3+, Sc3+, etc,
ions should result in the appearance of ferromagnetic
order. Below we show that this assumption is correct.
Second possibility lies in description of orbital
state as a hybridization of the d z r3 2 2– and dx y2 2–
orbitals as cos (�/2)|3z2-r2� ± sin (�/2)|x2–y2
�. Such
an orbital ordering is recently proposed experimen-
tally and theoretically in the ferromagnetic insulating
phase of La0.88Sr0.12MnO3 and Pr0.75Ca0.25MnO3
[23–25]. The difficulties in determination of priority
of the present models are conditioned by the fact that
staggered ordering of d z r3 2 2– and dx y2 2– orbitals can
exhibit itself in experiments in the same way as 3D
dynamic d z r3 2 2– orbital correlations.
2.2. La1–xSrx(Mn1–x/2Nbx/2)O3 system
Hypothetical magnetic phase diagram of
La1–xSrx(Mn1–x/2Nbx/2)O3 is shown in Fig. 3. The
parent LaMnO3 compound shows the spontaneous
magnetization value at 5 K corresponding to magnetic
moment of 0.07 µB per Mn3+ ion. The Neel point
where spontaneous magnetization develops is 143 K.
According to [26] the spontaneous magnetization has
a relativistic nature. Substitution of Mn with Nb leads
to an enhancement of the spontaneous magnetization
whereas the temperature of transition into paramag-
netic state slightly decreases. In accordance with the
magnetization data the La0.8Sr0.2(Mn0.9Nb0.1)O3 and
La0.7Sr0.3(Mn0.85Nb0.15)O3 samples are ferromagnets
with the magnetic moment per chemical formula
around 2.3 µB and 2.6 µB, respectively. Neutron dif-
fraction study has revealed the magnetic moment of
Mn3+ in the parent LaMnO3 antiferromagnetic com-
pound to be close to 3.5 µB [27] whereas Nb5+ is dia-
magnetic ion, hence the expected moment should be
close to 3µB per formula unit being in a rather good
agreement with the observed one. The Nb doped sam-
ple (x = 0.3) has a well defined Curie point — 123 K.
Both Curie point and spontaneous magnetization start
gradually to decrease when Nb content exceeds 15%
from total sites number in the manganese sublattice.
The magnetic state cardinally changes as the concen-
tration of niobium reaches 25%. We have observed the
magnetic susceptibility of the x = 0.5 sample dramati-
cally decreases. ZFC magnetization shows a peak at
30 K. Below this temperature FC magnetization prac-
tically does not change. Taking into account the char-
acter of M(H) dependence we have concluded that
the sample x = 0.5 can be considered as spin glass with
Tf = 30 K. We can explain the collapse of long range
ferromagnetic ordering by a diamagnetic dilution of
Mn- sublattice. According to resistivity versus tem-
perature measurements La1–xSrx(Mn1–x/2Nbx/2)O3
samples are semiconductors. Below Curie point a large
value of magnetoresistance is observed.
The results presented here deal with the facts that
the Nb-doped La1–xSrx(Mn Nb1 2
3
2
5
�
� �
x/ x/ )O3 samples
enriched with Mn3+ ions are ferromagnetic and show a
large magnetoresistance. It is worth noting that the
possibility of the existence of ferromagnetic ordering
in the manganites, despite the absence of Mn3+ ions,
reject the double exchange and the electronic phase
separation concepts. The result obtained indicates an
important role of ferromagnetic superexchange via ox-
ygen scenario of magnetic interactions in manganites.
According to the superexchange mechanism the
Mn3+–O–Mn3+ and Mn3+–O–Mn4+ 180� magnetic in-
teractions are strongly ferromagnetic for the orbitally
1076 Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9
I.O. Troyanchuk, V.A. Khomchenko, V.V. Eremenko, V.A. Sirenko, and H. Szymczak
200
160
120
80
40
0
T
,K
0.1 0.2 0.3 0.4 0.5
A + F F
P(O)
La Sr (Mn Nb )O1–x x 1 – x/2 x/2 3
P(O )
X
SG
I
Fig. 3. Magnetic phase diagram for
La1–xSrx(Mn1–x/2Nbx/2)O3 series (A is antiferromagnet, F
is ferromagnet, P is paramagnet, SG is spin glass; OI and
O is orbitally ordered and orbitally disordered phases, re-
spectively).
disordered state whereas the Mn4+–O–Mn4+ ones are
strongly antiferromagnetic [4]. The Curie point asso-
ciated with Mn3+–O–Mn3+ positive superexchange
may be close to room temperature for manganites with
perovskite structure because our samples contain dia-
magnetic Nb5+ ions which should strongly decrease
the Curie point. Stoichiometric LaMn3+O3 compound
also shows ferromagnetic interactions between Mn3+
ions when cooperative Jahn–Teller distortions are
vanished at T = 750 K. The orbital ordering changes
character of superexchange magnetic interactions
which in the orbitally ordered state become
anisotropic [4,28].
2.3. Nd1–xCaxMnO3 system
The hypothetical magnetic phase diagram of the
Nd1–xCaxMnO3 system at low Ca doping level is pre-
sented in Fig. 4. Neutron diffraction shows that the
samples with x < 0.08 consist mainly of antiferro-
magnetic phase while at x � 0.08 ferromagnetic com-
ponent dominates. Under hole doping the temperature
of the transition into paramagnetic state at first de-
creases and then around x = 0.1 increases. We have ob-
served two magnetic phase transitions in the range
0.06 � x � 0.1 as temperature decreases.
The Nd1–xCaxMnO3 solid solutions contain two
types of magnetically active sublattices: neodymium
and manganese ones. At first we discuss the Nd contri-
bution into magnetic properties. The f–f exchange in-
teraction in rare-earth sublattice is as a rule rather
weak in comparison with d–d interaction between
manganese ions. One can expect that neodymium mag-
netic moments should order as a result of f–d ex-
change interactions between neodymium and manga-
nese sublattices. The study of magnetic properties of
Nd1–xCaxMnO3 samples confirm this viewpoint. Ac-
cording to neutron diffraction data the magnetic mo-
ments of neodymium ions start to be ordered slightly
below TN. Magnetic moment of Nd ion is about 1.2�B
at 2 K and directed opposite to weak ferromagnetic
vector in NdMnO3 while in the sample x = 0.12 the
orientation of Nd and Mn magnetic moments is the
same. In the range 0.06 � x � 0.10 the metamagnetic
behavior was observed in large magnetic fields (triple
hysteresis loops with a negative remanent magnetiza-
tion).
According to our hypothesis, samples in the range
0.06 � x � 0.10 consist of antiferromagnetic (weak fer-
romagnetic) and ferromagnetic phases which are ex-
change coupled at the boundary. The neodymium
sublattice in both weak ferromagnetic and ferromag-
netic phases orders nearby the N�el point (Curie
point). However, the orientation of neodymium mag-
netic moments in both these phases is different: f–d
exchange is positive for ferromagnetic phase whereas
it is negative in weak ferromagnetic phase. The ferro-
magnetic phase strongly affects magnetic properties of
weak ferromagnetic phase due to exchange coupling at
the boundary. This interaction may induce a
reorientational transition from antiparallel orienta-
tion of neodymium moments and weak ferromagnetics
vector to parallel one. We believe that nearby certain
temperature the ground state of Nd3+ ions becomes de-
generate because opposite contributions from ex-
change coupled ferromagnetic and weak ferromagnetic
phases at Nd site become equal. According to theoreti-
cal consideration this state should be unstable thus
leading to magnetic structure transformation [29].
Neutron diffraction study carried out for the x = 0.08
Antiferromagnet–ferromagnet phase transition in lightly doped manganites
Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9 1077
0 0.03 0.06 0.09 0.12 0.15
x, Ca
wF + F
Mn
Nd Teff
Mn
Nd
T
,K
120
90
60
30
P
Nd Ca MnO1 – x x 3
Fig. 4. Magnetic phase diagram of the Nd1–xCaxMnO3
low-doped manganites: wF is weak ferromagnet, F is
ferromagnet, P is paramagnet, Teff is effective temperature
of the reorientational phase transition.
5 10 15 20 25
T, K
Mn
Nd
Mn
Nd
Mn
Nd
or
60
50
40
30
20
10
0
H
,k
O
e
Mn
Nd
Nd Ca MnO0.92 0.08 2.98
Fig. 5. The H–T magnetic phase diagram for
Nd0.92Ca0.08MnO2.98 compound.
sample is in agreement with this interpretation of low
temperature phase transition.
On the basis of magnetization data we propose
H–T magnetic phase diagram of Nd0.92Ca0.08MnO2.98
compound (Fig. 5). Depending on the prehistory in
the wide range of magnetic field the phases with par-
allel or antiparallel orientation of neodymium and
manganese sublattices in weak ferromagnetic phase
can be realized. One can see that the value of magnetic
field required for the change of relative orientation of
the Nd and Mn magnetic moments in the weak ferro-
magnetic phase increases as temperature rises. The
width of a field range in which the hysteresis is ob-
served practically does not depend on a temperature.
This type of magnetic phase diagram is in agreement
with crossover of energy sub-levels of Nd ions.
2.4. Bi1–xCaxMnO3 system
Figure 6 presents a magnetic phase diagram of the
Bi1–xCaxMnO3 manganites. As the calcium content in
the Bi1–xCaxMnO3 system increases, the latter passes
through three different magnetic states, namely, ferro-
magnetic (x � 0.1), spin-glass (0.15 � x � 0.25), and
antiferromagnetic (x > 0.25). In the case of anti-
ferromagnetic compositions, the magnetic-ordering
and structural-transformation temperatures vary only
weakly within the concentration interval from x =
= 0.25 to 0.6. The ferromagnetic ordering in BiMnO3
is most likely due to cooperative ordering of the
dx y2 2– orbitals [30,31]. With orbital ordering of this
type, according to the Goodenough—Kanamori rules,
ferromagnetic ordering becomes more energetically fa-
vorable than antiferromagnetic ordering. We may re-
call that rare-earth manganites exhibit orbital order-
ing of the dz2 -type, which stabilizes the A-type
antiferromagnetic structure [4]. Orbital disorder in
BiMnO3 sets in, apparently, at a fairly high tempe-
rature, near 760 K. Replacement of bismuth ions by
calcium results in the formation of quadrivalent man-
ganese ions, which should be accompanied by destruc-
tion of orbital ordering due to the appearance of non-
Jahn–Teller Mn4+ ions in the lattice. However, the or-
bitally disordered phase in manganites should be fer-
romagnetic [4,28], whereas we observed a state of the
spin-glass type. A direct transition from the anti-
ferromagnetic to the spin-glass phase without passing
through the ferromagnetic state was observed to occur
in the rare-earth manganites Sm1–xBaxMnO3 and
Y1–xCaxMnO3 (x � 0.12) [32,33]. It should be point-
ed out that at approximately this concentration of
rare-earth ions, the ferromagnetic–spin glass transi-
tion takes place in Bi1–xCaxMnO3.
There is more than one opinion on the nature of ex-
change interactions in manganites. The antiferromag-
netic state certainly forms through oxygen-mediated
superexchange interactions of the type Mn–O–Mn.
Most researchers believe that the ferromagnetic state
in manganites is created through double exchange,
i.e., via direct carrier transfer between various lattice
sites. In order for such an exchange mechanism to op-
erate, manganese ions in different valence states must
be present and the electrical conductivity must be
high. The presence of manganese ions of different va-
lencies is not a sufficient condition for high electrical
conductivity; indeed, the 3d-orbitals of manganese
and the 2p-orbitals of oxygen should also overlap
strongly. It is believed that this parameter is con-
trolled by the Mn–O–Mn bond angle [4,30]. The
larger the lanthanide ion, the larger should be the
Mn–O–Mn angle, the wider the 3d-band, and, ac-
cordingly, the higher the magnetic ordering tempera-
ture and the electrical conductivity. It was observed
that the magnetic state of the manganites also depends
on the difference between the ionic radii of the
rare-earth and the lanthanide ions. A large difference
between the radii lowers, as a rule, the magnetic or-
dering temperature as a result of competition between
various exchange interactions characterized by a large
difference in the Mn–O–Mn angles. This is why the
spin-glass state sets in in the Sm1–xBaxMnO3 system,
wherein the average radius of the Sm and Ba ions is far
larger than that between the Y and Ca cations in the
Y1–xCaxMnO3 system [32,33]. However, in all rare-
earth manganites, the Mn3+–O–Mn4+ exchange cou-
pling in the orbitally disordered phase is apparently
ferromagnetic. The Mn–O–Mn angles in bismuth-
based manganites are fairly large, which is supported
by structural studies [31] and the quite high Curie
temperature of BiMnO3. Hence, in the case of an or-
bitally disordered phase, one can expect the ferromag-
netic part of exchange interactions to be dominant,
which is at odds with experiment. Therefore, we be-
1078 Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9
I.O. Troyanchuk, V.A. Khomchenko, V.V. Eremenko, V.A. Sirenko, and H. Szymczak
0
0.2 0.4 0.6 0.8 1.0
x
300
200
100
T
,K
P
+
C
OP
A
F
CO
SG
SG + A A + F
Fig. 6. Magnetic phase diagram of the Bi1–xCaxMnO3
manganites: A is antiferromagnet, F is ferromagnet, P is
paramagnet, SG is spin glass, CO is charge-ordered state.
lieve that, in contrast to the rare-earth manganites, no
orbitally disordered phase forms in the BiMnO3 sys-
tem in the concentration interval 0.1 � x � 0.3. The
spin-glass state forms in the Bi1–xCaxMnO3 system
most likely as a result of competition between ferro-
magnetic interactions in BiMnO3-type clusters and
antiferromagnetic coupling in clusters in which the
Mn3+ orbitals are frozen in random orientations.
As the Ca2+ concentration increases, a new type of
antiferromagnetic clusters, apparently due to charge
ordering, appears. The existence in Bi0.75Ca0.25MnO3
of large clusters, charge-ordered in a similar way
to those in Bi0.5Ca0.5MnO3, is suggested in studies
of its elastic properties. Despite the presence of the
spin-glass-type ground state, there is a certain fraction
of states characterized by short-range order of the type
of a charge-ordered phase, which is indicated by the
fact that the Young modulus minima for the x = 0.25
and 0.35 compositions are close in temperature. We
believe that the extremely high stability of the orbit-
ally and charge-ordered states in bismuth-based man-
ganites derives from the strongly anisotropic character
of the Bi–O covalent bonding.
Conclusions
The magnetic and structural phase diagrams of
La0.88MnOx, La1–xSrx(Mn1–x/2Nbx/2)O3,
Nd1–xCaxMnO3, and Bi1–xCaxMnO3 manganites have
been proposed. It has been shown that the magnetic
properties of the samples under study are determined
by the type of their orbital state. The dynamic correla-
tions of d z r3 2 2– orbitals favor ferromagnetic ordering
in the manganites, while A-type antiferromagnetic
structure is typical for the static Jahn–Teller distor-
tions. It has been argued that concentrational transi-
tion from an antiferromagnetic to a ferromagnetic
state occurs via the formation of inhomogeneous state
due to structural phase separation mechanism.
This work was partly supported by Belarus and
Ukrainian Fund for Basic Research (grant F04MS-004
and grant 10.01/001 Ô05K–012) Polish – Ukrainian
Academies research project (No.4-2003) and by MES
of Ukraine (grant Ì/257-2004).
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