Electrical, magnetic and magnetocaloric properties of selected b site substituted manganites

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Electrical, magnetic and magnetocaloric properties of selected b site substituted manganites

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ELECTRICAL, MAGNETIC AND MAGNETOCALORIC PROPERTIES OF SELECTED B-SITE SUBSTITUTED MANGANITES SURESH KUMAR VANDRANGI NATIONAL UNIVERSITY OF SINGAPORE 2012 ELECTRICAL, MAGNETIC AND MAGNETOCALORIC PROPERTIES OF SELECTED B-SITE SUBSTITUTED MANGANITES SURESH KUMAR VANDRANGI (M. Phil., University of Hyderabad, Hyderabad, India) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN SCIENCE DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2012 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. Vandrangi Suresh Kumar 13 June 2012 ACKNOWLEDGEMENTS I would like to thank my supervisor Assoc. Prof. R. Mahendiran for his constant guidance. I am very grateful to Dr. C. Krishnamoorti for his valuable suggestions, thought provoking ideas, continuous encouragement and support. I would like to express my sincere thanks to Prof. B.V.R Chowdari for the help I got during first few semesters in accessing his laboratory for sample preparation. My special thanks to A/Prof Chen Lang for his encouragement and support during hard times. I am thankful to Dr. M. V. V. Reddy and Christie for their cooperation. I further thank Mdm. Pang for her assistance on operating xrd and Miss Foo for her support during lab demo classes. I thank my colleagues, Sujit Kumar, Alwyn Rebello, Vinayak, Aparnadevi, Maheswar, Pawan, Mark, Dollie, Zhoubin, Hariom, Dr. Rucha Desai, Dr. Raj sankar and Dr. Kavita for countless discussions during these years. I also would like to thank my friends, Naresh, Rajesh Tamang, Shreya, Abhik, Sourabh, Shuvankar, Satti, Bibin, Prashanth Praveen, Ajeesh, Venkatesh, Saran, Nakul, Anil, Amar, Malli who made my stay in Singapore pleasant. I express my deep gratitude to my parents and my sister without whom it would not have been possible for me to complete my Ph. D. And I thank all my friends overseas for their moral support. TABLE OF CONTENTS  ACKNOWLEDGEMENTS  TABLE OF CONTENTS ----------------------------------------------------------------i  SUMMAY ---------------------------------------------------------------------------------iii  LIST OF PUBLICATIONS -------------------------------------------------------------1  LIST OF TABLES ------------------------------------------------------------------------2  LIST OF FIGURES -----------------------------------------------------------------------2 1. Introduction to the physical properties of manganites 1.1 Introduction --------------------------------------------------------------------------------07 1.2 Crystal structure of manganites ---------------------------------------------------------08 1.3 Orbital Ordering --------------------------------------------------------------------------10 1.4 Charge Ordering --------------------------------------------------------------------------17 1.5 Phase separation --------------------------------------------------------------------------19 1.6 Magnetocaloric Effect -------------------------------------------------------------------21 1.7 Effect of Mn-site doping ----------------------------------------------------------------28 1.8 Scope and motivation of the present work --------------------------------------------31 2. Experimental Techniques 2.1 Introduction--------------------------------------------------------------------------------33 2.2 Synthesis of materials--------------------------------------------------------------------33 2.3 X-Ray Diffraction ------------------------------------------------------------------------34 2.4 Dc magnetization measurements--------------------------------------------------------35 2.5 Dc magnetotransport measurements----------------------------------------------------36 2.6 Calorimetric measurements -------------------------------------------------------------36 2.7 Strain measurements ---------------------------------------------------------------------39 i 3. Magnetic, electric, structural and magnetocaloric characterization of Mn site doped Pr based charge ordered compounds 3.1 Introduction--------------------------------------------------------------------------------42 3.2 Experimental Details----------------------------------------------------------------------44 3.3 DC magnetic properties of Pr0.6Ca0.4Mn1-xCrxO3----- --------------------------------47 3.4 Magnetocaloric properties of Pr0.6Ca0.4Mn1-xCrxO3 ----------------------------------51 3.5 DC magnetic properties of Pr0.6Ca0.4Mn0.96B0.04O3-----------------------------------53 3.6 DC electrical and structural properties of Pr0.6Ca0.4Mn0.96B0.04O3-----------------56 3.7 Magnetocaloric properties of Pr0.6Ca0.4Mn0.96B0.04O3--------------------------------60 3.8 DC magnetic properties of Pr0.6(Ca, Sr)0.4Mn1-xCrxO3-------------------------------71 3.9 DC electrical properties of Pr0.6(Ca, Sr)0.4Mn1-xCrxO3-------------------------------75 3.10 Magnetocaloric properties of Pr0.6(Ca, Sr)0.4Mn1-xCrxO3----------------------------77 3.11 DC magnetic properties of Pr0.5Ca0.5Mn1-xRuxO3------------------------------------82 3.12 Magnetocaloric properties of Pr0.5Ca0.5Mn1-xRuxO3---------------------------------83 3.13 Conclusions-------------------------------------------------------------------------------95 4. Structural, magnetic, electrical and magnetothermal properties of Bi1-yCayMn1-xRuxO3 manganites 4.1 Introduction--------------------------------------------------------------------------------97 4.2 Experimental Details---------------------------------------------------------------------99 4.3 Structural Characterization ------------------------------------------------------------100 4.4 DC magnetic properties and phase diagram-----------------------------------------114 4.5 DC electrical properties ----------------------------------------------------------------131 4.6 Magnetocaloric properties -------------------------------------------------------------139 4.7 Conclusions ------------------------------------------------------------------------------144 Summary and Future work----------------------------------------------------------145 Bibliography ----------------------------------------------------------------------------------149 ii Summary Manganites with a general formula, R1-xAxMnO3 (R is a rare earth element and A is a divalent dopent) have gained considerable interest recently because of the intriguing fundamental physics and wide range of potential applications. In this family of materials, manganites exhibiting charge/orbital ordering (COO) are of particular interest as the destabilization/destruction of charge ordering (CO) simultaneously leads to very interesting structural, electrical, thermal and magnetic properties. The coupling between magnetic and structural order parameters and its response to the external magnetic fields finds potential applications in magnetic refrigeration, which is rigorously investigated in different class of materials. The objective of the present work is to study the influence of impurity doping at Mn-site in charge ordered (CO) manganites, on the destruction of CO state and other physical properties, i.e. structural, electrical, magnetic and magnetothermal properties, with an emphasis on magnetocaloric effect (MCE). A systematic study has been carried out for the CO insulating system, Pr0.6Ca0.4MnO3 by substituting different amounts of Cr at Mn site. Cr-substitution at the Mn-site destablizes the charge-obital ordering and converts the low temperature antiferromgnetic phase into ferromagnetic phase. However, the para-to-ferromagnetic transition temperature (TC) was not considerably changed with Cr substitution from to %. Field induced magnetic phase transitions have been observed as a consequence of coexistence of ferromagnetic nanoclusters and short-range Charge-Orbital ordered clusters just above TC in the compounds less than 4% Cr content and it has been found that 4% Cr content is the optimum value for the magnetocaloric effect (MCE) investigated in these series of compounds. After having studied the effect of Cr substitution, iii impurities of different d-orbital occupancy are doped at Mn-site to get a better insight into how the magnetic and non-magnetic dopants have varying effects. This study essentially revealed that long range ferromagnetism and metallic behavior are induced only for the impurities Cr, Co, Ni and Ru but Fe and Al substitutions made the compound remain antiferromagnetic insulator. This can be because of the fact that the variation of the eg-electron density in these compounds alone is not sufficient to explain the origin of ferromagnetism as the magnetic properties of the compounds with isovalent dopants were different. From the magnetization isotherms and direct calorimetric measurements it has been observed that Co doped sample exhibits maximum value of MCE, which is -7.37 J/kg K followed by Ni, Cr and Ru. It has been found that the applied magnetic field induces a metamagnetic transition above Curie temperature in Co, Ni and Cr substituted samples but not in that of Ru substituted compound that has the lowest resistivity and highest TC. These differences were argued in the light of existence of CO fluctuations and ferromagnetic polarons in the paramagnetic phase of Co, Ni and Cr samples. It was very clear from these studies that among all the elements doped at Mn site, Ru is the most efficient element in destroying the CO state and inducing the ferromagnetism along with the metallic state. When it comes to materials exhibiting more robust CO and high CO temperatures, Bi based manganites are worth mentioning. Especially, Bi 1-xCaxMnO3 system, besides its CO behavior, has distinct magnetic states those include spin-glass and anti-ferromagnetic states. A significant reduction in the resistivity, by several orders of magnitude, has been observed in Bi1-xCaxMnO3 compounds when Ru is doped at Mn-site. Ru doping at the Mn-site is also capable of inducing ferromagnetism and insulator-metal transition without an external magnetic field. iv Magnetoresistance as high as 98% was observed under the magnetic field of T for 5% Ru doping and it decreased to 20% for a doping concentration of 20%. But the change in magnetic entropy in Bi based compounds is considerably low compared to those of Pr based compounds because of the week magnetic signal. It is important to note here that more amount of Ru has to be doped at Mn site in Bi based compounds compared to that of Pr based compounds to effectively destabilize/destroy the CO. It has also been noticed that in Bi based compounds there develops an impurity phase after a critical amount of Ru substitution at Mn-site. In other words, as we go towards Ca rich region, it is possible to substitute more of Ru and consequently the resistivity decreases drastically. Besides the above mentioned studies, the most exciting results are obtained when Mn is completely replaced by Ru. Remarkably, instead of the usual perovskite structure, these Bi based Ruthenites exhibit pyrochlore structure, which is typical of Pyrochlores with general formula A2B2O7, formed by a wide variety of ions and tolerates a high degree of non-stoichiometry on the oxygen anion and ‘A’ cation sites. This phase is observed to be developed gradually with increasing Ru content. v LIST OF PUBLICATIONS Articles  V. Suresh Kumar, R. Mahendiran and B. Raveau “Effect of Ru-doping on magnetocaloric effect in Pr based charge ordered manganites” IEEE transactions on magnetics, 46 1652 (2010).  V. Suresh Kumar and R. Mahendiran “Effect of Ru doping on magnetoresistance and magnetocaloric effect in Bi0.4Ca0.6Mn1-xRuxO3 (0≤x≤0.2) ” J. Appl. Phys., 107 113914 (2010).  V. Suresh Kumar and R. Mahendiran “A comparison of magnetocaloric effect in Pr0.6A0.4Mn1-xCrxO3 (A = Ca and Sr; x = and 0.04)” Solid State Comm., 150 1445 (2010).  V. Suresh Kumar and R. Mahendiran “Effect of impurity doping at the Mn site on magnetocaloric effect in Pr0.6Ca0.4Mn0.96B0.04O3 (B = Al, Fe, Cr, Ni, Co and Ru)” J. Appl. Phys., 109 023903 (2011).  V. Suresh Kumar and R. Mahendiran “Composition dependence of magnetocaloric effect in Pr0.6Ca0.4Mn1-xCrxO3 (x = 0.02-0.08)” J. Nanosci. Nanotechnol., 12 573 (2012).  C. Krishnamoorthi, Z. Siu, V. Sureh Kumar, and R. Mahendiran “Charge order and its destruction effects on magnetocaloric properties of manganites” Thin Solid Films 518 e65 (2010). 4.7 Conclusions In summary, we have investigated the effect of Ru doping at the Mn site in Bi1-yCayMn1xRuxO3 (0.5 ≤ y ≤ 0.8; ≤ x ≤ 0.4) samples. With Ca content, more amount of Ru can be doped at Mn site without any impure phase. Ru is capable of inducing ferromagnetism and insulatormetal transition without an external magnetic field. Remarkably Ru is very efficient in transforming the ferromagnetic Curie temperature (TC) to higher temperatures in these series of compounds. A large magnetoresistance under H = T, a complex magnetic behaviour together with the magnetic phase diagram are reported here. Besides, it is shown that the magnetic entropy for a second order phase transition compound Bi0.4Ca0.6Mn0.8Ru0.2O3 is determined and it shows a maximum value (Sm = -1.81 J/kg K for H = T) close to its Curie temperatures. The greater ability of Ru to induce long range ferromagnetism and insulator-metal transition in the robust charge-ordered materials such as BiCaMnO3 is quite interesting from the point of view of fundamental physics. Further theoretical and experimental studies which can probe the band structure of these materials will help to understand the origin of ferromagnetism in these compounds. Another important highlight of this work is the study of ruthenates. Instead of partial doping of Ru at Mn site, a complete replacement of Mn by Ru results in formation of different structure which is counterintuitive from the structural point of view. We report for the first time, the Bi based ruthernates of the form ABO3 with pyrotchlore structure, metallic and weak ferromagnetism. It will definitely be an interesting to carry out more details of the origin of structure and magnetism in these compounds from the fundamental perspective. 144 Summary and Future Work The main interest of the present work is to understand the direct effect of impurity doped B-site disorder in manganites with CO state, on the destruction of CO state and the other physical properties, i.e. structural, electrical, magnetic and magnothermal properties, with an emphasis on magnetocaloric effect (MCE). We first started with a CO insulating system, Pr0.6Ca0.4MnO3 and carried out a systematic substitution of Cr at Mn site. The magnetic characterization revealed that the para-to-ferromagnetic transition temperature (TC) was not considerably changed with maximum Cr substitution of 8%. The coexistence of ferromagnetic nanoclusters and short-range Charge-Orbital ordered clusters just above TC has been observed in the compounds less than 4% Cr content. Above 4% Cr content, there is an increase in the size of FM clusters destroying the short-range Charge Ordering resulting in metamagntic transition. We have further continued the study for magnetocaloric effect (MCE) and observed that the 4% of Cr doping at the Mn site is an optimum value for MCE in these compounds. After having studied the effect of Cr substation, we extended our work to impurities of different d-orbital occupancy. This was to get a better insight into how the magnetic and non-magnetic dopants have varying effects. During this study, we found that long range ferromagnetism and metallic behavior are induced only for the impurities Cr, Co, Ni and Ru but Fe and Al substitutions made the compound remain antiferromagnetic insulator. We realized that the variation of the eg-electron density in these compounds alone is not sufficient to explain the origin of ferromagnetism as the magnetic properties of the compounds with isovalent dopants were different. From the magnetization isotherms and direct calorimetric measurements it was observed that the maximum MCE value is exhibited by Co doped sample, which is -7.37 J/kg K. 145 Material TC -∆Sm RCP (K) (J/kg K) (J/kg) Ref. La0.7Sr0.3MnO3 365 4.44 128 [160] La0.65Bi0.05Sr0.3MnO3 353 5.02 216 [160] La0.7Ca0.05Sr0.25MnO3 341 6.86 364 [161] La0.67Ca0.33MnO3 252 2.06 175 [100] Gd 294 10.2 410 [39] Gd5Si2Ge2 276 18.4 535 [39] La(Fe0.88Si0.12)13 195 23 - [117] La0.67Sr0.33Mn0.9Cr0.1O3 328 5.00 200 [162] La0.7Sr0.3Mn0.95Fe0.05O3 317 4.4 - [163] La0.7Sr0.3Mn0.95Ti0.05O3 308 4.4 - [108] La0.7Sr0.3Mn0.80Cr0.20O3 286 2.6 240 [164] Pr0.6Ca0.4Mn0.96Cr0.04O3 155 286 Present Work Pr0.6Ca0.4Mn0.96Ni0.04O3 191 6.77 319 Present Work Pr0.6Ca0.4Mn0.96Co0.04O3 190 7.37 317 Present Work TABLE 2: Maximum entropy change (-Sm), and relative cooling power (RCP), values for H = T for the present samples and for materials with different values of TC from the literature. 146 The table above, provides a brief over view of the MCE on a few selected manganites, elemental Gd and Gd based alloys that have been previously investigated. La based manganites have been studied extensively for MCE, whereas not many studies of MCE on Pr based manganites and moreover handful of reports of MCE on Mn-site doped manganites in the literature. It has been found that the applied magnetic field induces a metamagnetic transition above the Curie temperature in Co, Ni and Cr substituted samples but not in that of Ru substituted one, that has the lowest resistivity and highest TC. We suggested that these differences are due to existence of CO fluctuations and ferromagnetic polarons in the paramagnetic phase of Co, Ni and Cr samples. It can be concluded from our observations that the reduction in magnetic entropy is more in the earlier case than that of later because of the competition between the coexisting short range charge-orbital ordered clusters and long-range ferromagnetic regions. It was very clear from these studies that among all the elements doped at Mn site, Ru is the most efficient in destroying the CO state and inducing the ferromagnetism along with the metallic state. We then proceeded towards more robust CO state, Pr 0.5Ca0.5MnO3, by selecting the potential element, Ru, as the impurity at the Mn-site. It was found that Ru content as low as 3% was sufficient to destroy the CO, AFM phase of the parent compound Pr0.5Ca0.5MnO3. When compared to other higher doped Ru levels, the 3% doped sample exhibited the largest value of MCE. We further extended the idea of Ru substitution at Mn site to Bi based manganites where in, the CO state exists over a wide composition range. Our investigations revealed the significant reduction in the resistivity of these compounds by several orders of magnitude. Ru doping at the Mn-site is also capable of inducing ferromagnetism and insulator-metal transition without an external magnetic field. Magnetoresistance as high as 98% was observed under the magnetic 147 field of T for 5% Ru doping As we go towards Ca rich region, it is possible to substitute more of Ru and consequently the resistivity decreases drastically. We have found the magnetic signal to be week in these compounds and hence has no considerable contribution to the change in magnetic entropy. Besides the above mentioned studies, the most exciting results are obtained when Mn is completely replaced by Ru. Remarkably, instead of the usual perovskite structure, these Bi based Ruthenites exhibit pyrochlore structure, which is typical of Pyrochlores with general formula A2B2O7, formed by a wide variety of ions and tolerates a high degree of nonstoichiometry on the oxygen anion and ‘A’ cation sites. This phase is observed to be developed gradually with increasing Ru content. So, in the intermediate composition range there exist both the phases and we have not reported any properties of those compounds. The physical properties connected with the series of samples Bi1-xCaxRuO3 have not been reported earlier and our studies in this regard definitely set a direction and interest to resolve the puzzle of compounds of general formula ABO3 exhibiting pyrochlore structure. Besides structural properties, electrical and magnetic transport properties are very interesting and challenging too from the fundamental point of view. As we see from the preliminary reports, these materials are highly metallic in nature and the low temperature transport is worth paying more attention. A detailed investigation can reveal several interesting features connecting the structure and magnetism that is possible by means of neutron diffraction studies at different temperatures. Spectroscopic studies are very much recommended which are helpful to understand the changes in electron band structure brough tout by Ru doping. 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Nabil Kallel, Sami Kallel, Ahmed Hagaza, and Mohamed Oumezzine, Physica B: Condensed Matter 404 (2), 285 (2009). 159 [...]... potential magnetocaloric < /b> candidate materials for magnetic < /b> refrigeration in the sub-room and < /b> room-temperature range are shown in Fig 1.10 and < /b> a brief summary of < /b> comparison of < /b> the MCE in inter-metallics and < /b> manganites can be found in a review by Phan and < /b> Yu46 Besides intermetallic alloys, the CMR manganites also show appreciable MCE over a wide temperature and < /b> hence are considered to be potential candidates... magnetocaloric < /b> effect and < /b> effect of < /b> Mn -site doping and < /b> the objective of < /b> current work 7 1.2 Crystal structure of < /b> manganites Manganites, with the general formula ABO3, belong to perovskite structure, where A site is occupied by bigger size cations such as rare earth or alkaline earth ions and < /b> B site is occupied by smaller size cation, Mn, a transition metal ion In an idealized cubic unit cell, the A cations... (0, 0, 0), B cations occupy the body centered positions (1/2, 1/2, 1/2) and < /b> oxygen anions occupy the face centered positions (1/2, 1/2, 0), this can be better illustrated in Fig 1.1(a) So, the coordination number of < /b> A, B and < /b> O atoms are 12, 6, and < /b> 8 but, the ion size requirements for stability of < /b> cubic structure are quite stringent Hence, the buckling and < /b> distortion of < /b> MnO6 octahedra stabilize in lower... coordination number of < /b> A and < /b> B site ions are reduced For example, tilting of < /b> MnO6 octahedra reduces the coordination number of < /b> A site ions from 12 to as low as 8 The general description of < /b> distorted perovskite structure is shown in Fig 1.1 (b) Here, two non-equivalent positions of < /b> oxygen determine the degree of < /b> distortion of < /b> MnO6 octahedra in terms of < /b> the Mn-O-Mn bond angles and < /b> Mn-O bond lengths Similar... the structure belongs to cubic and < /b> t < 1 for the orthorhombic and < /b> rhombohedral structures that are commonly observed in manganites It has been observed that, 9 the tilting of < /b> MnO6 octahedra has a large influence on transport properties < /b> of < /b> manganites It is clear from the above equation that, the distortion mainly depends on the ionic radii of < /b> the ions present at the A -site Another possible cause for... region of < /b> manganites3 7,38 1.6 Magnetocaloric < /b> Effect The magnetocaloric < /b> effect (MCE) is defined as the heating or cooling of < /b> a magnetic < /b> material under the application of < /b> magnetic < /b> field This has also been referred as adiabatic demagnetization for years, even though this is one practical application of < /b> the MCE in magnetic < /b> materials It was first observed in iron by E Warburg in 1881, and < /b> explained by E Debye... temperatures for (a) B = Fe and < /b> (b) Al Fig 3.10 Magnetization isotherms at selected < /b> temperatures for (a) B = Ni, (b) B = Ru, (c) B = Co, (d) B = Cr Fig 3.11 Magnetic < /b> entropy change (ΔSM) as a function of < /b> temperature for different impurities for a field interval of < /b> (a) ΔH = 1 T and < /b> (b) 5 T Inset shows -ΔSM, RC and < /b> RCP as a function of < /b> doping element Fig 3.12 Universal behavior of < /b> the scaled entropy...LIST OF < /b> TABLES Table1 The list of < /b> lattice parameters obtained from the Reitveld fit (a, b and < /b> c) for all the Bi based compounds with different doping levels of < /b> Ru at Mn site Table2 Maximum entropy change (-∆Sm), and < /b> relative cooling power (RCP) values for ∆H = 5 T for the present samples and < /b> for materials with different values of < /b> TC from the literature LIST OF < /b> FIGURES Fig 1.1 Schematic diagram of < /b> the... coupling of < /b> magnetic < /b> sublattice to the applied magnetic < /b> field results in MCE, which changes the magnetic < /b> contribution to the entropy of < /b> the system The entropy of < /b> a magnetic < /b> material depends on the temperature and < /b> magnetic < /b> field All magnetic < /b> materials intrinsically show MCE, but the intensity of < /b> the effect depends on the properties < /b> of < /b> the individual material 21 1.7 Schematic picture illustrating the two basic... can be ferromagnetic or antiferromagnetic depending on the relative orbital orientation Hence, there is a possibility of < /b> 16 correlation among orbital, spin and < /b> lattice degrees of < /b> freedom in manganites Here is an example of < /b> hole doped manganite, Nd1-xSrxMnO3, shown in Fig 1.5 demonstrating the correlation between orbital and < /b> spin leading to different electronic and < /b> magnetic < /b> states depending the level of . ELECTRICAL, MAGNETIC AND MAGNETOCALORIC PROPERTIES OF SELECTED B-SITE SUBSTITUTED MANGANITES SURESH KUMAR VANDRANGI NATIONAL UNIVERSITY OF SINGAPORE 2012 ELECTRICAL, MAGNETIC AND MAGNETOCALORIC PROPERTIES. MAGNETOCALORIC PROPERTIES OF SELECTED B-SITE SUBSTITUTED MANGANITES SURESH KUMAR VANDRANGI (M. Phil., University of Hyderabad, Hyderabad, India) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY. Pr 0.6 Ca 0.4 Mn 1-x Cr x O 3 47 3.4 Magnetocaloric properties of Pr 0.6 Ca 0.4 Mn 1-x Cr x O 3 51 3.5 DC magnetic properties of Pr 0.6 Ca 0.4 Mn 0.96 B 0.04 O 3 53 3.6 DC electrical and structural properties of Pr 0.6 Ca 0.4 Mn 0.96 B 0.04 O 3

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