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Electronic and magnetic phase diagram of polycrystalline Gd1−xCaxMnO3 manganites
Accepted Manuscript Electronic and magnetic phase diagram of polycrystalline Gd1−xCaxMnO3 manganites A. Beiranvand, J. Tikkanen, H. Huhtinen, P. Paturi PII:
S0925-8388(17)31848-0
DOI:
10.1016/j.jallcom.2017.05.231
Reference:
JALCOM 41962
To appear in:
Journal of Alloys and Compounds
Received Date: 9 March 2017 Revised Date:
16 May 2017
Accepted Date: 21 May 2017
Please cite this article as: A. Beiranvand, J. Tikkanen, H. Huhtinen, P. Paturi, Electronic and magnetic phase diagram of polycrystalline Gd1−xCaxMnO3 manganites, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.05.231. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Electronic and magnetic phase diagram of polycrystalline Gd1−x Cax MnO3 manganites A. Beiranvand∗, J. Tikkanen, H. Huhtinen, P. Paturi
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Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
Abstract
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Based on the structural and magnetoresistive properties of Gd1−x Cax MnO3 (GCMO) (0 ≤ x ≤ 1) polycrystalline manganites, the magnetic phase diagram of GCMO is deduced. The results show that all of the compounds are ferrimagnetic in the ground state due to polarization of the large magnetic moments of Gd in the opposite direction of Mn ions. However, even disregarding the ordering of the Gd spins in the applied magnetic field direction, the series exhibit complicated magnetic behavior below magnetic ordering temperature (TC or TN ). In the hole doped region x ≤ 0.5, Mn ions order ferromagnetically. In the middle doping region (0.5 ≤ x ≤ 0.7), charge ordering can be observed above TN and, below TN , Mn ions are in antiferromagnetic state which is more obvious in the case with x = 0.8. In the electron doped region 0.8 ≤ x ≤ 0.9, Mn ions reveal magnetic cluster glass properties. Except for x = 0.9, which exhibits degenerate semiconductive behavior, the temperature dependence of resistance shows insulating behavior for all other GCMO concentrations. This can be associated with the small average ionic radius of rare earth cations and colossal magnetoresistance (CMR) is observed for GCMO with x = 0.8 and x = 0.9 when the applied magnetic field exceeds 9 T. Keywords: Perovskite structure, Manganite, Phase diagram, Magnetoresistive properties
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1. Introduction
Mixed valence perovskite manganites R1−x Ax MnO3 (R = rare-earth cation, A = alkali or alkaline earth cation) have been intensively studied in recent years [1–3]. As the concentration x of divalent A cations changes from zero to unity, manganites show significant variation in physical properties and the system goes through different phase transitions between various type of structural, magnetic, charge and orbital ordering. However, most of the reports are ∗ Corresponding
author Email address: [email protected] (A. Beiranvand)
Preprint submitted to Elsevier
May 22, 2017
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about light rare earth atoms with large ionic radii and weak or non magnetic nature, strong magnetism of the rare earth ions may be an important factor in magnetic peoperties of mangnites. Also the properties of manganites not only depend on the manganese valency but they are also affected by steric factors such as average A-site cationic radius, hrA i = (1 − x)rRE − xrCa , and A-site cationic size mismatch, which is quantified by the variance of the ionic radii, 2 σ 2 = hrA i2 − hrA i = (x − x2 )(rGd − rCa )2 , that depends on the ionic radii of Ca and Gd in our case. These parameters are related to the eg -electron band width, which determines the transfer probability of a conduction electron between neighboring Mn sites. For instance, with the constant hole concentration, a decrease of hrA i tends to reduce the O–M–O angle from 180◦ [4]. Finally, whenever hrA i and manganese valency are constant, the increase of σ 2 tends to depress magnetic interactions and to minimize charge ordering [5]. One can find several experimental [6–8] and theoretical [9, 10] studies summing up the main physical properties of the manganites. Neutron diffraction studies, allowing researchers to determine the magnetic phase diagram by showing fine details of the shapes of the MnO6 octahedral and magnetic ordering of manganese, are of great importance when analyzing the sequences of magnetic transitions in manganites [11–14]. In low band width manganiets, Sm and Pr manganites, which have been investigated widely by neutron diffraction measurements, in the hole doped region (x ≤ 0.5), the double exchange interaction (DE) between Mn3+ and Mn4+ ions and, thus, the ferromagnetic state (FM) is predominant. In contrast, in the electron doped region where the colossal magnetoresistance (CMR) is observed, charge ordering (CO) and the antiferomagnetic state (AFM) or cluster glass (CG) magnetic behavior appear. However, low bandwidth manganites with small hrA i show CMR effect on different regions based on the variance of the hrA i and mismatched factor. For instance, Sm1−x Cax MnO3 (labeled SCMO), which corresponds to low hrA i (1.132 – 1.18 ˚ A) and small σ 2 (5.8 × 10−4 ˚ A2 ) which were calculated by [15], exhibits CMR only on the electron doped side and Pr1−x Cax MnO3 series (PCMO), with the absence of size mismatch, i.e., a constant hrA i value (1.18 ˚ A) due to identical size of Ca and Pr[16], reveals CMR on both sides, in the hole doped and in the electron doped regions [15, 16]. (Gd, Ca)MnO3 is also considered as a low band width manganite with small A-site ionic radii, smaller than Sm and Pr, and with a high net magnetization. There are few comperehensive studies about the family, probably due to the large neutron absorption cross section of Gd and the magnetic phase diagram of (Gd,Ca)MnO3 has not been studied. There are no detailed data on the crystal and magnetic structures of the Gd manganite systems which, on the other hand, play an important role to explain all the physical properties of these compounds. In this paper, we choosed Gd1−x Cax MnO3 (0 ≤ x ≤ 1) based on samll sized strongly magnetic heavy rare earth, Gd, in comparision with Sm and Pr and present the magnetic phase diagram of the perovskite manganites, utilizing the XRD data and the temperature dependence of the magnetization and resistivity of the compounds. The structure and the magnetoresistive properties were investigated within the complete Ca doping range. 2
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Figure 1: The x-ray diffractograms and Rietveld fits of the GCMO samples at room temperature. The major diffraction peaks are indexed according to the space group Pnma. Note that due to the nearly tetragonal symmetry, every (hkl) reflection is implicitly accompanied by an (lkh) reflection. The goodness-of-fit figure, χ2 , of the phase calculation was below 2 for all samples, indicating a satisfactory agreement between the data and the model.
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Polycrystalline samples of Gd1−x Cax MnO3 (0 ≤ x ≤ 1) (hereafter GCMO) were prepared by the solid state ceramic method. The used raw materials were gadolinium(III) oxide (Acros Organics, 99.9+%), calcium carbonate (Merck Millipore, 99+%) and manganese(IV) oxide (Alfa Aesar, 99.9+%). The gadolinium(III) oxides and calcium carbonate were dried overnight at 1600 ◦ C and 200 ◦ C, respectively. The dry powders were weighed with an analytical balance (Sartorius CPA225D) according to stoichiometric formulae, mortared by hand, and compacted into pellets (5 min at 30 MPa). The pellets were initially calcined at 700 ◦ C for 60 h. Then they were repeatedly mortared by hand, recompacted and sintered at 1300 ◦ C for 24 h in air until the equilibrium structure was reached. The crystal structure and the phase purity of the samples were screened by x-ray diffractometry (XRD) using Philips X’pert diffractometer with Cu Kα radiation and Rietveld analysis was made by Maud program [17]. In order to investigate the magnetic transitions of the GCMO compounds, the temperature dependence on magnetization, M (H, T ) was measured with a SQUID magnetometer (Quantum Design MPMS XL SQUID) in the temperature range of 5 K ≤ T ≤ 400 K an applied magnetic field of 0–5 T. The resistivity measurements used the constant current of 0.5 µA at temperature range from
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Figure 2: The lattice parameters of GCMO samples vs. Ca dopant concentration measured by XRD at room temperature and obtained from the Rietveld refinement. The standard errors of the parameters are smaller than the sizes of the plotted symbols.
3. Results
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10 to 400 K and magnetoresistance measurement at 10 K in magnetic fields up to 9 T were made with the Physical Property Measurement System (PPMS, Quantum Design). Unfortunately, the oxygen stoichiometry of the samples could not be verified. In calcium manganites like those studied here, an oxygen deficit of up to 4 percents is possible (depending on x and the sintering temperature) when the final sintering is performed in air, with a corresponding effect on the Mn4+ /Mn3+ valence ratio equivalent to decreasing x by up to 0.08.
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The XRD data at room temperature indicate that all GCMO samples are well crystallized and they are in the orthorhombic space group Pbnm (figure 1). However, the mutual dependences between the lattice parameters vary with the Ca concentration. In the range 0.0 ≤ x ≤ 0.7, the Pbnm lattice belongs to O-type structure with b > a > √c2 . For x ≥ 0.8, the crystal structure of the samples is described by quasi-tetragonal symmetry with √c2 ≈ a < b and
orthorhombicity factor ( ab ) decreases while Ca concentration increases. At the upper limit of the Ca doping, x = 1, a ≈ b ≈ √c2 i.e tetragonal symmetry is dominant (figure 2). The lattice parameters of all concentration are given in table 1. The average size of the A cation, the cation disordering parameter and cell volume are presented in table 1. The Gd and Ca ionic radii are taken from [18] for the ninefold coordination of A-cations, which corresponds to a distorted perovskite lattice. It can be seen that the hrA i values of GCMO are smaller than those of SCMO and PCMO. Thus GCMO is highly distorted in comparison 4
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Table 1: Lattice parameters, an average size and disordering of the cation A, tolerance factor (t) and a cell volume with error limits calculated from the room temperature XRD data. The cation radii are taken from Shannon tables for ninefold coordination [18].
c (˚ A) 7.436(3) 7.459(5) 7.489(3) 7.504(8) 7.518(7) 7.523(6) 7,505(5) 7.552(1) 7.489(2) 7.472(2) 7.457(4)
hrA i (˚ A) 1.078 1.084 1.090 1.096 1.102 1.109 1.115 1.121 1.127 1.133 1.140
σ 2 (10−4 ˚ A2 ) 0 3.45 6.15 8.07 9.22 9.61 9.22 8.07 6.15 3.45 0
t 0.84(1) 0.85(3) 0.85 0.86(4) 0.86 0.87(5) 0.87(1) 0.87 0.88(2) 0.88 0.89(2)
Vcell (˚ A3 ) 230.71(3) 227.92(3) 223.93(4) 222.98(4) 218.88(4) 217.14(3) 215.15(5) 214.64(4) 211.67(3) 209.49(2) 208.0(4)
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b (˚ A) 5.861(3) 5.765(1) 5.642(2) 5.569(1) 5.501(3) 5.396(4) 5.375(3) 5.317(2) 5.336(1) 5.307(3) 5.284(4)
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a (˚ A) 5.317(4) 5.311(1) 5.314(2) 5.335(1) 5.347(2) 5.348(4) 5.333(1) 5.313(2) 5.296(5) 5.282(6) 5.269(1)
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with polycrystalline SCMO and PCMO [13]. The unit cell volume decreases as x increases, due to the variation in the Mn3+ /Mn4+ ratio with x. Figure 3 shows the temperature dependences of magnetization and resistivity between 10 and 300 K at magnetic fields of 10 mT and 9 T respectively for 0.0 ≤ x ≤ 0.4. The magnetization increases with decreasing temperature and the maximum value is 2.0 Am2 /kg for the sample with x = 0.2 around 50 K. Then the magnetization sharply declines and goes to negative values for the sample with x = 0.1 at T < 20 K. It is supposed that a negative exchange interaction between Mn and Gd spins makes the Gd sublattice order in opposite direction of the Mn spins. The total magnetic moment (MM n + MGd ) will reach a maximum and then decrease with decreasing temperature to go negative when MGd > MM n . In a mean-field theory, if the local field at the Gd site is mainly due to the magnetic moment of the Mn sublattice, then
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MGd = χGd (T ).HGd = χGd (T ).MM n
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Mtot ∼ MGd − MM n = −|J|.χGd .MM n + MM n
where J is the negative interaction between the two sublattices. It is immediately seen that, at low temperatures, when the gadolinium susceptibility (χGd vs. 1/T) becomes sufficiently large, the total moment will reverse its sign [19, 20]. The magnetic ordering temperature (TC ) was defined as the minimum of the temperature derivative of M (T ) curves. TC of x = 0.3 is indicated with an arrow in fig 3. The resistivity increases gradually as temperature declines and it reaches about 1011 Ω for x = 0.3 and x = 0.4 (see inset figure 3). In this Ca doping region, all samples are ferrimagnetic and insulating at low temperature and it is in agreement withe previous results [19–21].
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Figure 3: Temperature dependence of the magnetization M measured in 10 mT field at temperatures from 10 to 300 K for the low Ca-doped samples x ≤ 0.4. The arrow shows the determined TC for x = 0.3. The inset shows the resistivity data for x = 0.3 and x = 0.4. The data below 50 K is artificially saturated due to hardware limitations.
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The data obtained from magnetization and resistivity measurements at different temperatures indicate charge ordered regions for samples with 0.5 ≤ x ≤ 0.7 near room temperature (figure 4) and Tco , where charge ordering is maximum, is determined from the M (T ) curves. In this system, in contrast to SCMO and PCMO, the highest TCO was observed at x = 0.5. The appearance of charge ordering, also, can be verified from the M (H) curves of the samples by comparison with those of x ≤ 0.4 (see figure 5). The magnetization of x = 0.5 at 10 K at 5 T is small compared with that of x = 0.4. It is obvious that this applied magnetic field (5 T) is insufficient to weaken the CO state and to increase the magnetization. The M (T ) curves also show a peak below the magnetic ordering transition (around 100 K) which is indicated by an arrow in figure 4. A similar peak has been observed in Sm1−x Cax MnO (SCMO) [15, 22] and it is due to AFM transition. Resistance at these Ca concentrations is large at low temperature and drops for all samples at high temperature (inset of figure 4). This is a transition from high resistivity to a highly conducting state. The transition temperature deduced from R(T ) curves coincides with the temperature of appearance of charge ordering phase in M (T ) curves. The temperature of charge ordering phase appearance for x = 0.7 is shown by a red arrow in figure 4. For 0.8 ≤ x ≤ 0.9, corresponding to the electron doped region (Mn4+ rich region) the temperature dependence of magnetization revealed a maximum around 100 K for x = 0.8 and cluster glass properties for x = 0.9 below it. For sample with x = 0.8, the magnetic moment rises at around 100 K leading to antiferomagnetic state (figure 6), whereas the resistance increases sharply (inset of figure 6). In fact, the R(T ) curves exhibit a transition around 100 K from a semiconducting to an insulating state (see inset of figure 6 of x = 0.8). This transition coincides with the appearance of sharp peak in M (T ) curve. The 6
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Figure 4: Temperature dependence of the magnetization M measured in 10 mT field at temperatures from 10 to 400 K for middle range Ca-doped samples 0.5 ≤ x ≤ 0.7. The arrows show the AFM (black) and charge ordering (red) transitions at roughly 100 K and 200 K respectively. The inset shows the resistivity data for the same concentrations at 9 T. The data below 50 K is cut due to the large noise of the device. The resistance drops to below 1 Ω for all samples at high temperature.
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Figure 6: Temperature dependence of the magnetization measured in 10 mT field for GCMO with concentrations x = 0.8 and 0.9 (main panel). The inset shows the resistance vs. temperature curves measured at 0 T (solid line) and 9 T (dash line) field.
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R(T ) curve for the sample with x = 0.9 shows degenerate semiconductive behavior with small gap (inset of figure 6). Both samples with x = 0.8 and x = 0.9 demonstrate CMR phenomenon at 10 K in applied field 9 T. The magnetoresitivity properties are stronger at x = 0.9. Consequently, the structure of the two components was investigated by XRD from 80 K to 450 K, but the samples did not show any change in crystalline structure at this temperature range. Unfortunately, we could not investigate the sample structure at 10 K where the magnetoresistance properties have been observed. From magnetization and resistance measurements the phase diagram of Gd1−x Cax MnO3 was constructed (figure 7). The transition temperatures from paramagnetic state to ferromagnetic (TC ) and antiferromagnetic (TN ) state are determined from M (T ) curves. The charge ordering transition temperature (TCO ) shows the temperature where the maximum charge ordering was observed.
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4. Discussion
GCMO is characterized by low hrA i values ranging from 1.078 ˚ A to 1.132 ˚ A within the concentration range from x = 0 to x = 1. The values are smaller than in SCMO and PCMO [6, 15, 23]. This means that the tolerance factor [24] of the samples is always below 1. The tolerance factors are also shown in table 1. GCMO samples show highly distorted perovskite structure, being favorable for charge localization and thus destructive for double exchang (DE) interaction [25]. Consequently, in all samples in the low hole doped region, no ferromagnetic metallic state can be observed. All samples in the GCMO series are ferrimagnetic in the ground state due to Gd polarization in the opposite direction of Mn spins and the ferromagnetic insulating (FMI) is a dominant 8
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Figure 7: The magnetic phase diagram of GCMO. TC and TN TCO are deduced from M (T, H) measurements. The dash area indicates the region of existence of magnetoresistivity properties (CMR). FMI stands for ferromagnetic insulator, AFMI for antiferromagnetic insulator, CGI for cluster glass degenerate semiconductor, CO for charge ordering and PM for paramagnetic.
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state for x ≤ 0.4 like in SCMO (figure 3). Also, it seems that the magnetization that can be read off the M (T ) curves for these concentrations can not delocalize the carriers even under 9 T, so the materials cannot exhibit a metallic state of resistivity and, therefore, no magnetoresistivity properties are observed in this region. In the mid-doped region (0.5 ≤ x ≤ 0.7), where the charge ordering phenomena were observed, the samples show small resistivity at high temperature which was also earlier reported for other manganites [26–28]. This could be related to the dielectric breakdown of the charge ordered state, which leads to the metallic state accompanied by the large number of charge carriers [26]. The observed peak in M (H) curves around 100 K is evidence of an antiferromagnetic (AFM) matrix in the samples. Therefore, the AFM insulating phase dominates below magnetic ordering temperature where the charge localization is characterized. As for SCMO and PCMO [6, 23], the combination of antiferromagnetic insulating (AFMI) state and CO state with TN ≤ TCO shows that the magnetic structure of GCMO is CE or C type in this region. In sample x = 0.8 with pseudotetragonal symmetry and AFMI state below TN = 140 K, the C type AFM can be assumed for its magnetic structure. Transport measurements at Mn4+ rich side (x = 0.9) exhibit degenerate semiconductive behavior. This sample with pseudocubic symmetry (a ≈ b ≈ √c2 ) shows G type AFM structure with FM component similar to other low bandwidth manganites [15, 29, 30]. Nonetheless, the TC values obtained from M (T ) curves for GCMO are smaller than those of SCMO and PCMO in these regions,
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being in agreement with their smaller hrA i values [31]. The ferromagnetic metallic (FMM) state depends on rare earth cation size, following a large bandwidth [4]. For this reason, GCMO, SCMO and PCMO, which have relatively small hrA i and, therefore, too narrow eg electron bandwidth, do not contain FMM domains in the ground state. Although the hrA i value is not sufficient to obtain a FMM behavior anywhere in the doping range 0 ≤ x ≤ 1, CMR is supported for Mn4+ rich compositions of GCMO, x = 0.8 and 0.9, as already seen earlier in Smx Ca1−x MnO3 [15]. With some of the smallest hrA i values among perovskite structured manganites, GCMO provides an excellent testing ground for the effects of extreme structural distortions on the electronic transport properties of manganites. 5. Conclusion
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As a conclusion, we have qualitatively compared in this work developed magnetic phase diagram of GCMO with that of SCMO and PCMO both of which have a larger hrA i, i.e. a larger tolerance factor for a given value of x. The magnetic phase diagram of GCMO is similar to that of SCMO. The FMI state extends over hole concentration region (0 ≤ x ≤ 0.4) and is followed by the CO-AFMI state for the range of 0.5 ≤ x < 0.8. Ferromagnetism with a cluster glass properties appears in electron doped region (x > 0.8) and it could be the evidence of ferromagnetic components in a G type AFM state. In addition, the magnetic phase diagram of GCMO shows that, the CMR phenomenon is obtained in the electron doped region close to cluster glass area, 0.8 < x ≤ 0.9, where FM and AFM are competing.
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The authors wish to thank the Jenny and Antti Wihuri Foundation, Finland. A. Beiranvand also thanks CIMO for financial support of this research. References
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[24] Z. Li, M. Yang, J. S. Park, S. H. Wei, J. J. Berry, K. Zhu, Stabilizing perovskite structures by tuning tolerance factor: Formation of formamidinium and cesium lead iodide solid-state alloys, J. Chem. Mater. 28 (2016) 284– 292. [25] J. P. Zhou, J. T. McDevitt, J. S. Zhou, H. Q. Yin, J. B. Goodenough, Y. Gim, Q. X. Jia, Effect of tolerance factor and local distortion on magnetic properties of the perovskite manganites, Appl. Phys. Lett. 75 (1999) 1146. [26] A. Asamitsu, Y. Tomioka, H. Kuwahara, Y. Tokura, Current switching of resistive states in magnetoresistive manganites, Nature 388 (1997) 50.
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[27] M. Uehara, S. Mori, C. H. Chen, S.-W. Cheong, Structural changes, clustering, and photoinduced phase segregation in Pr0.7 Ca0.3 MnO3 , Nature 399 (1999) 560–563. [28] M.-H. Jo, N. D. Mathur, N. K. Todd, M. G. Blamire, Very large magnetoresistance and coherent switching in half-metallic manganite tunnel junctions, Phys. Rev. B 61 (2000) R14905.
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[29] A. Maignan, C. Martin, F. Damay, B. Raveau, Factors governing the magnetoresistance properties of the electron-doped manganites Ca1−x Ax MnO3 (A = Ln, Th), J. Chem. Mater. 10 (1998) 950–954.
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[30] H. Kawano, R. Kajimoto, H. Yoshizawa, Y. Tomioka, H. Kuwahara, Y. Tokura, Magnetic ordering and relation to the metal-insulator transition in Pr1−x Srx MnO3 and Nd1−x Srx MnO3 with x = 2, Phys. Rev. Lett. 78 (1997) 4253–4256.
[31] O. Chmaissem, B. Dabrowski, S. Kolesnik, J. Mais, D. E. Brown, R. Kruk, P. Prior, B. Pyles, J. D. Jorgensen, Relationship between structural parameters and the Neel temperature in Sr1−x Cax MnO3 and Sr1−y Bay MnO3 , Phys. Rev. B 64.
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Electronic and magnetic phase diagram of polycrystalline Gd 1−x Ca x MnO 3 manganites By A. Beiranvand et al.
Highlights
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Magnetic and resistance properties studied across composition range Charge ordering phenomena and high conductivity found near room temperature in the mid doped concentrations. Gluster glass and degenerate semiconductiv behavior are characterized in the high doped regions. Phase diagram of the series has been determined based on magnetic and resistivity properties.
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S0925-8388(17)31848-0
DOI:
10.1016/j.jallcom.2017.05.231
Reference:
JALCOM 41962
To appear in:
Journal of Alloys and Compounds
Received Date: 9 March 2017 Revised Date:
16 May 2017
Accepted Date: 21 May 2017
Please cite this article as: A. Beiranvand, J. Tikkanen, H. Huhtinen, P. Paturi, Electronic and magnetic phase diagram of polycrystalline Gd1−xCaxMnO3 manganites, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.05.231. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Electronic and magnetic phase diagram of polycrystalline Gd1−x Cax MnO3 manganites A. Beiranvand∗, J. Tikkanen, H. Huhtinen, P. Paturi
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Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
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Based on the structural and magnetoresistive properties of Gd1−x Cax MnO3 (GCMO) (0 ≤ x ≤ 1) polycrystalline manganites, the magnetic phase diagram of GCMO is deduced. The results show that all of the compounds are ferrimagnetic in the ground state due to polarization of the large magnetic moments of Gd in the opposite direction of Mn ions. However, even disregarding the ordering of the Gd spins in the applied magnetic field direction, the series exhibit complicated magnetic behavior below magnetic ordering temperature (TC or TN ). In the hole doped region x ≤ 0.5, Mn ions order ferromagnetically. In the middle doping region (0.5 ≤ x ≤ 0.7), charge ordering can be observed above TN and, below TN , Mn ions are in antiferromagnetic state which is more obvious in the case with x = 0.8. In the electron doped region 0.8 ≤ x ≤ 0.9, Mn ions reveal magnetic cluster glass properties. Except for x = 0.9, which exhibits degenerate semiconductive behavior, the temperature dependence of resistance shows insulating behavior for all other GCMO concentrations. This can be associated with the small average ionic radius of rare earth cations and colossal magnetoresistance (CMR) is observed for GCMO with x = 0.8 and x = 0.9 when the applied magnetic field exceeds 9 T. Keywords: Perovskite structure, Manganite, Phase diagram, Magnetoresistive properties
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1. Introduction
Mixed valence perovskite manganites R1−x Ax MnO3 (R = rare-earth cation, A = alkali or alkaline earth cation) have been intensively studied in recent years [1–3]. As the concentration x of divalent A cations changes from zero to unity, manganites show significant variation in physical properties and the system goes through different phase transitions between various type of structural, magnetic, charge and orbital ordering. However, most of the reports are ∗ Corresponding
author Email address: [email protected] (A. Beiranvand)
Preprint submitted to Elsevier
May 22, 2017
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about light rare earth atoms with large ionic radii and weak or non magnetic nature, strong magnetism of the rare earth ions may be an important factor in magnetic peoperties of mangnites. Also the properties of manganites not only depend on the manganese valency but they are also affected by steric factors such as average A-site cationic radius, hrA i = (1 − x)rRE − xrCa , and A-site cationic size mismatch, which is quantified by the variance of the ionic radii, 2 σ 2 = hrA i2 − hrA i = (x − x2 )(rGd − rCa )2 , that depends on the ionic radii of Ca and Gd in our case. These parameters are related to the eg -electron band width, which determines the transfer probability of a conduction electron between neighboring Mn sites. For instance, with the constant hole concentration, a decrease of hrA i tends to reduce the O–M–O angle from 180◦ [4]. Finally, whenever hrA i and manganese valency are constant, the increase of σ 2 tends to depress magnetic interactions and to minimize charge ordering [5]. One can find several experimental [6–8] and theoretical [9, 10] studies summing up the main physical properties of the manganites. Neutron diffraction studies, allowing researchers to determine the magnetic phase diagram by showing fine details of the shapes of the MnO6 octahedral and magnetic ordering of manganese, are of great importance when analyzing the sequences of magnetic transitions in manganites [11–14]. In low band width manganiets, Sm and Pr manganites, which have been investigated widely by neutron diffraction measurements, in the hole doped region (x ≤ 0.5), the double exchange interaction (DE) between Mn3+ and Mn4+ ions and, thus, the ferromagnetic state (FM) is predominant. In contrast, in the electron doped region where the colossal magnetoresistance (CMR) is observed, charge ordering (CO) and the antiferomagnetic state (AFM) or cluster glass (CG) magnetic behavior appear. However, low bandwidth manganites with small hrA i show CMR effect on different regions based on the variance of the hrA i and mismatched factor. For instance, Sm1−x Cax MnO3 (labeled SCMO), which corresponds to low hrA i (1.132 – 1.18 ˚ A) and small σ 2 (5.8 × 10−4 ˚ A2 ) which were calculated by [15], exhibits CMR only on the electron doped side and Pr1−x Cax MnO3 series (PCMO), with the absence of size mismatch, i.e., a constant hrA i value (1.18 ˚ A) due to identical size of Ca and Pr[16], reveals CMR on both sides, in the hole doped and in the electron doped regions [15, 16]. (Gd, Ca)MnO3 is also considered as a low band width manganite with small A-site ionic radii, smaller than Sm and Pr, and with a high net magnetization. There are few comperehensive studies about the family, probably due to the large neutron absorption cross section of Gd and the magnetic phase diagram of (Gd,Ca)MnO3 has not been studied. There are no detailed data on the crystal and magnetic structures of the Gd manganite systems which, on the other hand, play an important role to explain all the physical properties of these compounds. In this paper, we choosed Gd1−x Cax MnO3 (0 ≤ x ≤ 1) based on samll sized strongly magnetic heavy rare earth, Gd, in comparision with Sm and Pr and present the magnetic phase diagram of the perovskite manganites, utilizing the XRD data and the temperature dependence of the magnetization and resistivity of the compounds. The structure and the magnetoresistive properties were investigated within the complete Ca doping range. 2
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Figure 1: The x-ray diffractograms and Rietveld fits of the GCMO samples at room temperature. The major diffraction peaks are indexed according to the space group Pnma. Note that due to the nearly tetragonal symmetry, every (hkl) reflection is implicitly accompanied by an (lkh) reflection. The goodness-of-fit figure, χ2 , of the phase calculation was below 2 for all samples, indicating a satisfactory agreement between the data and the model.
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Polycrystalline samples of Gd1−x Cax MnO3 (0 ≤ x ≤ 1) (hereafter GCMO) were prepared by the solid state ceramic method. The used raw materials were gadolinium(III) oxide (Acros Organics, 99.9+%), calcium carbonate (Merck Millipore, 99+%) and manganese(IV) oxide (Alfa Aesar, 99.9+%). The gadolinium(III) oxides and calcium carbonate were dried overnight at 1600 ◦ C and 200 ◦ C, respectively. The dry powders were weighed with an analytical balance (Sartorius CPA225D) according to stoichiometric formulae, mortared by hand, and compacted into pellets (5 min at 30 MPa). The pellets were initially calcined at 700 ◦ C for 60 h. Then they were repeatedly mortared by hand, recompacted and sintered at 1300 ◦ C for 24 h in air until the equilibrium structure was reached. The crystal structure and the phase purity of the samples were screened by x-ray diffractometry (XRD) using Philips X’pert diffractometer with Cu Kα radiation and Rietveld analysis was made by Maud program [17]. In order to investigate the magnetic transitions of the GCMO compounds, the temperature dependence on magnetization, M (H, T ) was measured with a SQUID magnetometer (Quantum Design MPMS XL SQUID) in the temperature range of 5 K ≤ T ≤ 400 K an applied magnetic field of 0–5 T. The resistivity measurements used the constant current of 0.5 µA at temperature range from
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3. Results
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10 to 400 K and magnetoresistance measurement at 10 K in magnetic fields up to 9 T were made with the Physical Property Measurement System (PPMS, Quantum Design). Unfortunately, the oxygen stoichiometry of the samples could not be verified. In calcium manganites like those studied here, an oxygen deficit of up to 4 percents is possible (depending on x and the sintering temperature) when the final sintering is performed in air, with a corresponding effect on the Mn4+ /Mn3+ valence ratio equivalent to decreasing x by up to 0.08.
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The XRD data at room temperature indicate that all GCMO samples are well crystallized and they are in the orthorhombic space group Pbnm (figure 1). However, the mutual dependences between the lattice parameters vary with the Ca concentration. In the range 0.0 ≤ x ≤ 0.7, the Pbnm lattice belongs to O-type structure with b > a > √c2 . For x ≥ 0.8, the crystal structure of the samples is described by quasi-tetragonal symmetry with √c2 ≈ a < b and
orthorhombicity factor ( ab ) decreases while Ca concentration increases. At the upper limit of the Ca doping, x = 1, a ≈ b ≈ √c2 i.e tetragonal symmetry is dominant (figure 2). The lattice parameters of all concentration are given in table 1. The average size of the A cation, the cation disordering parameter and cell volume are presented in table 1. The Gd and Ca ionic radii are taken from [18] for the ninefold coordination of A-cations, which corresponds to a distorted perovskite lattice. It can be seen that the hrA i values of GCMO are smaller than those of SCMO and PCMO. Thus GCMO is highly distorted in comparison 4
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Table 1: Lattice parameters, an average size and disordering of the cation A, tolerance factor (t) and a cell volume with error limits calculated from the room temperature XRD data. The cation radii are taken from Shannon tables for ninefold coordination [18].
c (˚ A) 7.436(3) 7.459(5) 7.489(3) 7.504(8) 7.518(7) 7.523(6) 7,505(5) 7.552(1) 7.489(2) 7.472(2) 7.457(4)
hrA i (˚ A) 1.078 1.084 1.090 1.096 1.102 1.109 1.115 1.121 1.127 1.133 1.140
σ 2 (10−4 ˚ A2 ) 0 3.45 6.15 8.07 9.22 9.61 9.22 8.07 6.15 3.45 0
t 0.84(1) 0.85(3) 0.85 0.86(4) 0.86 0.87(5) 0.87(1) 0.87 0.88(2) 0.88 0.89(2)
Vcell (˚ A3 ) 230.71(3) 227.92(3) 223.93(4) 222.98(4) 218.88(4) 217.14(3) 215.15(5) 214.64(4) 211.67(3) 209.49(2) 208.0(4)
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b (˚ A) 5.861(3) 5.765(1) 5.642(2) 5.569(1) 5.501(3) 5.396(4) 5.375(3) 5.317(2) 5.336(1) 5.307(3) 5.284(4)
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a (˚ A) 5.317(4) 5.311(1) 5.314(2) 5.335(1) 5.347(2) 5.348(4) 5.333(1) 5.313(2) 5.296(5) 5.282(6) 5.269(1)
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with polycrystalline SCMO and PCMO [13]. The unit cell volume decreases as x increases, due to the variation in the Mn3+ /Mn4+ ratio with x. Figure 3 shows the temperature dependences of magnetization and resistivity between 10 and 300 K at magnetic fields of 10 mT and 9 T respectively for 0.0 ≤ x ≤ 0.4. The magnetization increases with decreasing temperature and the maximum value is 2.0 Am2 /kg for the sample with x = 0.2 around 50 K. Then the magnetization sharply declines and goes to negative values for the sample with x = 0.1 at T < 20 K. It is supposed that a negative exchange interaction between Mn and Gd spins makes the Gd sublattice order in opposite direction of the Mn spins. The total magnetic moment (MM n + MGd ) will reach a maximum and then decrease with decreasing temperature to go negative when MGd > MM n . In a mean-field theory, if the local field at the Gd site is mainly due to the magnetic moment of the Mn sublattice, then
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Mtot ∼ MGd − MM n = −|J|.χGd .MM n + MM n
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Figure 3: Temperature dependence of the magnetization M measured in 10 mT field at temperatures from 10 to 300 K for the low Ca-doped samples x ≤ 0.4. The arrow shows the determined TC for x = 0.3. The inset shows the resistivity data for x = 0.3 and x = 0.4. The data below 50 K is artificially saturated due to hardware limitations.
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The data obtained from magnetization and resistivity measurements at different temperatures indicate charge ordered regions for samples with 0.5 ≤ x ≤ 0.7 near room temperature (figure 4) and Tco , where charge ordering is maximum, is determined from the M (T ) curves. In this system, in contrast to SCMO and PCMO, the highest TCO was observed at x = 0.5. The appearance of charge ordering, also, can be verified from the M (H) curves of the samples by comparison with those of x ≤ 0.4 (see figure 5). The magnetization of x = 0.5 at 10 K at 5 T is small compared with that of x = 0.4. It is obvious that this applied magnetic field (5 T) is insufficient to weaken the CO state and to increase the magnetization. The M (T ) curves also show a peak below the magnetic ordering transition (around 100 K) which is indicated by an arrow in figure 4. A similar peak has been observed in Sm1−x Cax MnO (SCMO) [15, 22] and it is due to AFM transition. Resistance at these Ca concentrations is large at low temperature and drops for all samples at high temperature (inset of figure 4). This is a transition from high resistivity to a highly conducting state. The transition temperature deduced from R(T ) curves coincides with the temperature of appearance of charge ordering phase in M (T ) curves. The temperature of charge ordering phase appearance for x = 0.7 is shown by a red arrow in figure 4. For 0.8 ≤ x ≤ 0.9, corresponding to the electron doped region (Mn4+ rich region) the temperature dependence of magnetization revealed a maximum around 100 K for x = 0.8 and cluster glass properties for x = 0.9 below it. For sample with x = 0.8, the magnetic moment rises at around 100 K leading to antiferomagnetic state (figure 6), whereas the resistance increases sharply (inset of figure 6). In fact, the R(T ) curves exhibit a transition around 100 K from a semiconducting to an insulating state (see inset of figure 6 of x = 0.8). This transition coincides with the appearance of sharp peak in M (T ) curve. The 6
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Figure 4: Temperature dependence of the magnetization M measured in 10 mT field at temperatures from 10 to 400 K for middle range Ca-doped samples 0.5 ≤ x ≤ 0.7. The arrows show the AFM (black) and charge ordering (red) transitions at roughly 100 K and 200 K respectively. The inset shows the resistivity data for the same concentrations at 9 T. The data below 50 K is cut due to the large noise of the device. The resistance drops to below 1 Ω for all samples at high temperature.
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R(T ) curve for the sample with x = 0.9 shows degenerate semiconductive behavior with small gap (inset of figure 6). Both samples with x = 0.8 and x = 0.9 demonstrate CMR phenomenon at 10 K in applied field 9 T. The magnetoresitivity properties are stronger at x = 0.9. Consequently, the structure of the two components was investigated by XRD from 80 K to 450 K, but the samples did not show any change in crystalline structure at this temperature range. Unfortunately, we could not investigate the sample structure at 10 K where the magnetoresistance properties have been observed. From magnetization and resistance measurements the phase diagram of Gd1−x Cax MnO3 was constructed (figure 7). The transition temperatures from paramagnetic state to ferromagnetic (TC ) and antiferromagnetic (TN ) state are determined from M (T ) curves. The charge ordering transition temperature (TCO ) shows the temperature where the maximum charge ordering was observed.
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4. Discussion
GCMO is characterized by low hrA i values ranging from 1.078 ˚ A to 1.132 ˚ A within the concentration range from x = 0 to x = 1. The values are smaller than in SCMO and PCMO [6, 15, 23]. This means that the tolerance factor [24] of the samples is always below 1. The tolerance factors are also shown in table 1. GCMO samples show highly distorted perovskite structure, being favorable for charge localization and thus destructive for double exchang (DE) interaction [25]. Consequently, in all samples in the low hole doped region, no ferromagnetic metallic state can be observed. All samples in the GCMO series are ferrimagnetic in the ground state due to Gd polarization in the opposite direction of Mn spins and the ferromagnetic insulating (FMI) is a dominant 8
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state for x ≤ 0.4 like in SCMO (figure 3). Also, it seems that the magnetization that can be read off the M (T ) curves for these concentrations can not delocalize the carriers even under 9 T, so the materials cannot exhibit a metallic state of resistivity and, therefore, no magnetoresistivity properties are observed in this region. In the mid-doped region (0.5 ≤ x ≤ 0.7), where the charge ordering phenomena were observed, the samples show small resistivity at high temperature which was also earlier reported for other manganites [26–28]. This could be related to the dielectric breakdown of the charge ordered state, which leads to the metallic state accompanied by the large number of charge carriers [26]. The observed peak in M (H) curves around 100 K is evidence of an antiferromagnetic (AFM) matrix in the samples. Therefore, the AFM insulating phase dominates below magnetic ordering temperature where the charge localization is characterized. As for SCMO and PCMO [6, 23], the combination of antiferromagnetic insulating (AFMI) state and CO state with TN ≤ TCO shows that the magnetic structure of GCMO is CE or C type in this region. In sample x = 0.8 with pseudotetragonal symmetry and AFMI state below TN = 140 K, the C type AFM can be assumed for its magnetic structure. Transport measurements at Mn4+ rich side (x = 0.9) exhibit degenerate semiconductive behavior. This sample with pseudocubic symmetry (a ≈ b ≈ √c2 ) shows G type AFM structure with FM component similar to other low bandwidth manganites [15, 29, 30]. Nonetheless, the TC values obtained from M (T ) curves for GCMO are smaller than those of SCMO and PCMO in these regions,
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being in agreement with their smaller hrA i values [31]. The ferromagnetic metallic (FMM) state depends on rare earth cation size, following a large bandwidth [4]. For this reason, GCMO, SCMO and PCMO, which have relatively small hrA i and, therefore, too narrow eg electron bandwidth, do not contain FMM domains in the ground state. Although the hrA i value is not sufficient to obtain a FMM behavior anywhere in the doping range 0 ≤ x ≤ 1, CMR is supported for Mn4+ rich compositions of GCMO, x = 0.8 and 0.9, as already seen earlier in Smx Ca1−x MnO3 [15]. With some of the smallest hrA i values among perovskite structured manganites, GCMO provides an excellent testing ground for the effects of extreme structural distortions on the electronic transport properties of manganites. 5. Conclusion
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As a conclusion, we have qualitatively compared in this work developed magnetic phase diagram of GCMO with that of SCMO and PCMO both of which have a larger hrA i, i.e. a larger tolerance factor for a given value of x. The magnetic phase diagram of GCMO is similar to that of SCMO. The FMI state extends over hole concentration region (0 ≤ x ≤ 0.4) and is followed by the CO-AFMI state for the range of 0.5 ≤ x < 0.8. Ferromagnetism with a cluster glass properties appears in electron doped region (x > 0.8) and it could be the evidence of ferromagnetic components in a G type AFM state. In addition, the magnetic phase diagram of GCMO shows that, the CMR phenomenon is obtained in the electron doped region close to cluster glass area, 0.8 < x ≤ 0.9, where FM and AFM are competing.
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The authors wish to thank the Jenny and Antti Wihuri Foundation, Finland. A. Beiranvand also thanks CIMO for financial support of this research. References
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[8] C. Martin, A. Maignan, M. Hervieu, S. Hbert, The catalytic activity of rare earth manganites, Phys. Rev. B 77. [9] L. Y. Kuai, Y. Y. Wei, L. X. Guang, Colossal magnetoresistance in manganites and related prototype devices, Chin. Phys. B 22 (2013) 087502.
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[10] P. K. Siwach, H. K. Singh, O. N. Srivastava, Low field magnetotransport in manganites, J. Phys. Cond. Mat. 20 (2008) 273207. [11] P. Raychaudhuri, C. Mitra, P. D. A. Mann, S. Wirth, Phase diagram and hall effect of the electron doped manganite La1−x Cex MnO3 , J. Appl. Phys. 93 (2003) 8328–8330. [12] E. O. Wollan, W. C. Koehler, A time-dependent order parameter for ultrafast photoinduced phase transitions, Phys. Rev. 100 (1955) 545–563.
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[13] A. Kurbakov, Electronic, structural and magnetic phase diagram of Sm1−x Srx MnO3 manganites, J. Magn. and Magn. Mater. 322 (2010) 967– 972.
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Electronic and magnetic phase diagram of polycrystalline Gd 1−x Ca x MnO 3 manganites By A. Beiranvand et al.
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Magnetic and resistance properties studied across composition range Charge ordering phenomena and high conductivity found near room temperature in the mid doped concentrations. Gluster glass and degenerate semiconductiv behavior are characterized in the high doped regions. Phase diagram of the series has been determined based on magnetic and resistivity properties.
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