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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Datum:	2005
Hauptverfasser:	Troyanchuk, I.O., Khomchenko, V.A., Eremenko, V.V., Sirenko, V.A., Szymczak, H.
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Veröffentlicht:	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2005
Schriftenreihe:	Физика низких температур
Schlagworte:	К семидесятилетию антиферромагнетизма
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spelling	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
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
topic	К семидесятилетию антиферромагнетизма К семидесятилетию антиферромагнетизма
spellingShingle	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
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first_indexed	2025-11-25T15:47:12Z
last_indexed	2025-11-25T15:47:12Z
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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. 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B63, 174405 (2001). 1080 Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9 I.O. Troyanchuk, V.A. Khomchenko, V.V. Eremenko, V.A. Sirenko, and H. Szymczak

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