Effect of Annealing Environment on Low-Temperature Magnetic and

Article pubs.acs.org/JPCC

Effect of Annealing Environment on Low-Temperature Magnetic and Dielectric Properties of EuCo0.5Mn0.5O3 Vasundhara Katari,† S. N. Achary,*,† S. K. Deshpande,‡ P. D. Babu,‡ A. K. Sinha,∥ H. G. Salunke,§ N. Gupta,§ and A. K. Tyagi† †

Chemistry Division, ‡UGC-DAE Consortium for Scientific Research, R5 Shed, and §Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India ∥ Indus Synchrotrons Utilization Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India S Supporting Information *

ABSTRACT: We report the effect of annealing atmosphere on structure and magnetic and dielectric properties of double perovskite-type EuCo0.5Mn0.5O3 (ECMO). Polycrystalline samples prepared by high-temperature reaction were subjected to annealing in different oxygen partial pressures and investigated by powder X-ray diffraction, magnetometry, and ac impedance spectroscopy. While the samples annealed in air or oxygen environment were found to have cation-disordered orthorhombic structure (Pnma), the samples annealed in inert atmosphere have Eu3+ deficient perovskite-type structure. The temperature- and field-dependent magnetizations indicate the ferromagnetic transition around 130 K. The differences in field-cooled and zero-fieldcooled magnetization and significant effect of magnetic field on magnetic properties suggest metamagnetic behavior in ECMO. Randomly distributed clusters of ferromagnetically (FM) and antiferromagnetically (AFM) paired magnetic ions and their relative proportions are reflected in their magnetic properties. The weak nature of the antiferromagnetic interactions is reflected in the switching of AFM to FM state at appreciably lower field strength (∼300 Oe). Dielectric measurements indicate that the samples annealed in air or oxygen show large dielectric permittivity (≥2000) compared to that in inert atmosphere (∼400). The analyses of temperature- and frequency-dependent loss tangent and modulus data revealed two different types of relaxations which possibly originate from two polaronic conduction processes.

1. INTRODUCTION Recently, transition-metal-containing perovskite-type materials have attracted significant attention because of their magnetoelectric and multiferroic properties.1−6 In particular, the perovskite-type AMO3 (A = rare-earth, Bi3+; M = transitionmetal ions like Fe, Mn, Cr, etc.) and A2MM′O6 (A = rare-earth or alkaline earth ions; M and M′ = 3d transition-metal ions) compounds exhibit wide varieties of magnetic properties like ferro-, ferri-, and antiferromagnetic as well as dielectric properties like colossal magnetocapacitance, relaxor behavior, and ferroelectricity.5−8 Magnetic properties of perovskite-type AMO3 (Ln = rare-earth ions; M = transition-metal ions) and related compositions with heterovalent cation substitution on either A or M sites have been extensively investigated.9−12 The La2MM′O6 (M and M′ are transition-metal ions) compositions show cation-ordered or -disordered structures depending on the ionic radii and oxidation states of the transition-metal ions, and often they exhibit ferromagnetism with high Curie temperature (TC) because of the 180° superexchange of the M and M′ transition-metal ions.10−12 However, dielectric properties of such materials have been investigated only recently in the interest of possible magnetodielectric effects. © 2014 American Chemical Society

A number of studies have been devoted to the dielectric properties of La2NiMnO6 because of its near ambient temperature ferromagnetic transition and large magnetocapacitance near the TC.7,13−20 The significant role of sample preparation conditions on the magnetic and dielectric properties of La2NiMnO6 and La2CoMnO6 have been inferred in these studies. Strong dependency of TC of La2NiMnO6 and La2CoMnO6 on the methods and conditions of preparation has been reported in the literature.10,11,21−26 The La2NiMnO6 samples prepared at high temperature often exhibit coexistence of monoclinic and rhombohedral phase at ambient temperature.18,21,22 The phase fractions of the coexisting phases vary with the post-annealing conditions, such as temperature and oxygen partial pressure (pO2) of the annealing atmosphere, and they are often reflected in their magnetic and dielectric properties.11,18−24 Relaxor-like dielectric and magnetodielectric properties have been observed in La2MMnO6 (M = Ni, Co, and Mg) Received: February 16, 2014 Revised: July 3, 2014 Published: July 9, 2014 17900

dx.doi.org/10.1021/jp501654c | J. Phys. Chem. C 2014, 118, 17900−17913

The Journal of Physical Chemistry C

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and heated at 900 °C for 42 h. The pellets were rehomogenized and sintered at 1250 °C for 24 h in pellet form. These sintered pellets were annealed at 850 °C for 10 h under static air (ECMO-A), flowing oxygen (ECMO-O), or nitrogen (ECMON) atmosphere. Also, one pellet of ECMO sample was annealed for long time (48 h) in flowing N2 atmosphere (ECMO-N (L)) for comparison. The phase purities of samples were investigated by powder X-ray diffraction (XRD) studies. The powder XRD patterns of the samples were recorded on a powder X-ray diffractometer (Rigaku, Japan) equipped with a rotating anode using monochromatic Cu Kα radiation. For structural studies, powder diffraction data was collected in the 2θ range of 10− 100°, with a step width and time of 0.02 and 3 s, respectively, by using this rotating anode-based powder XRD unit. The powder XRD data of the samples were also recorded by using synchrotron radiation on the ADXRD beamline (on bending magnet port BL-12) of the Indus-2 (2.5 GeV, 100 mA) Synchrotron Radiation (SR) source at Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India.32 The diffraction data were collected on a image plate (mar 345) detector. The diffraction images were integrated by using FIT2D program.33 The observed XRD patterns were analyzed by Rietveld method using the Fullprof-2K software package.34 The Mn and Co edge X-ray absorption near-edge spectroscopy (XANES) spectra for the samples were recorded in transmission mode at the EXAFS beamline (BL-9) at the INDUS-2 Synchrotron Source at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India.32 The absorption spectra for standard samples, such as CoO, CoF3, Mn2O3, and MnO2, were also recorded for energy calibration. The magnetic properties of the samples were investigated (between 5 and 300 K) using a superconducting quantum interference device (SQUID) magnetometer and 9T PPMS based vibrating sample magnetometer (VSM) (both Quantum Design) in both zero-field-cooled (ZFC) and field-cooled (FC) conditions at varying magnetic fields in the range of 100−800 Oe. The hysteresis loops (M versus H) of the samples were also recorded at several temperatures between 5 and 300 K. The dielectric properties of the samples were studied by using a Novocontrol Alpha impedance analyzer (Novocontrol Technologies, Germany) equipped with a Quatro liquid nitrogen gas cryosystem. The parallel faces of sintered cylindrical pellets were painted with silver paste for proper electrical contact. All the measurements were done over a frequency range of 100 Hz to 5 MHz at several temperatures while cooling from 293 to 128 K.

samples.7,16−26 The relaxor-like dielectric properties of La2NiMnO6 and La2CoMnO6 samples are significantly affected on annealing in oxygen or inert atmosphere.18,22,26 However, not much is reported on the dielectric properties of the double perovskite-type materials with other rare-earth ions. Booth et al.27 have investigated a series of double perovskite-type Ln2NiMnO6 (Ln = La to Y) compounds and reported cation-ordered structure in all of them. From the roomtemperature frequency-dependent dielectric measurement, normal dielectric behaviors with dielectric constants between 15 and 25 have been reported for these materials.27 However, the frequency- or temperature-dependent dielectric studies on such compounds are rather sparse in the literature. The frequency- and temperature-dependent dielectric properties of La2MMnO6 (M = Ni2+, Co2+, and Mg2+) revealed relaxor-like behaviors which depend significantly on the annealing atmosphere.17−19,22−24 Thus, it is likely that the relaxor-like dielectric properties might be observed in other perovskite-type materials containing transition-metal ions. From the earlier literature it has been observed that one end member phase, EuMnO3, exhibits incommensurate antiferromagnetic order below 53 K and A-type antiferromagnetic phase below 48 K.28 However, the other end member, EuCoO3, has a low-spin Co3+ ion and thus is a nonmagnetic material.29 Ferromagnetism is introduced in EuMnO3 by substitution of Co at the Mn site, and the ferromagnetic ordering temperature (TC) increases gradually to a maximum value (TC ∼ 120 K) at 50% Co substitution.30,31 As Eu2CoMnO6 has a cation-disordered perovskite structure, the tunable magnetic and dielectric properties can be expected because of variable valence states of transition-metal ions. To understand the role of rare-earth ions on magnetic and relaxor-like dielectric behavior of perovskite as well as to study the effect of annealing conditions on them, we have investigated details of magnetic and dielectric properties of cationdisordered Eu2CoMnO6 (termed EuCo0.5Mn0.5O3 or ECMO), and the details are presented in this paper. Single-phase EuCo0.5Mn0.5O3 samples were prepared by a combustion method followed by a high-temperature treatment and then annealed under different oxygen partial pressures. The magnetic and dielectric properties of the annealed samples have been investigated below ambient temperature, revealing a significant effect of the annealing conditions on these properties. The differences in the magnetic properties are governed by the coexisting ferromagnetic and antiferromagnetic clusters or domains. The frequency- and temperature-dependent permittivity revealed relaxor-like behavior in EuCo0.5Mn0.5O3. The details of these properties are explained in this paper.

3. RESULTS AND DISCUSSION The phase characterizations of the ECMO samples have been carried out by recording the XRD pattern after each heat treatment. The sample obtained after calcination at 700 °C revealed the formation of a perovskite-type phase but with poor crystallinity, while the samples obtained after sintering at 1250 °C and after subsequent annealing showed crystalline perovskite-type phase. The cation ordering in such systems strongly depends on the preparation conditions, such as annealing temperature and atmosphere as well as heating and cooling protocols. Thus, the Rietveld refinement of the observed powder XRD data considers both cation-ordered (P21/n) and -disordered (Pnma) structural models for EuCo0.5Mn0.5O3. All the observed reflections in the XRD pattern could be accounted by the cation-disordered (Pnma) model. Thus, a cation-

2. EXPERIMENTAL SECTION The EuCo0.5Mn0.5O3 samples used in the experiments were prepared by gel combustion method by using Eu2O3, CoCO3, and MnCO3 as initial reactants. Stoichiometric amounts of reactants were dissolved in a 1:1 nitric acid and water solution. A calculated amount of glycine (for oxidant-to-fuel ratio of 3.5:1) was added to the solution. Highly viscous gels were obtained by slow dehydration of the solution at ∼80 °C on a hot plate. When the temperature was raised further, the gel undergoes autoignition with evolution of a large volume of gases and is then converted to voluminous black colored powders. The product obtained after combustion was calcined at 700 °C for 3h. The calcined powder was pressed into pellets 17901



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disordered orthorhombic structure with composition as EuCo0.5Mn0.5O3 (here onward abbreviated as ECMO) is assigned for the studied phase. Furthermore, we tried to refine the occupation number of Co and Mn with constraint as occCo + occMn = 1. Both the laboratory and SR XRD data did not show significant deviation in occupancy from the expected 1:1 stoichiometry. Thus, the stoichiometries of single-phase samples were assumed as the initial stoichiometry of reactants used for preparation. The refined structural parameters and final Rietveld refinement plot for the sample sintered at 1250 °C are given in section S-I of the Supporting Information. In a similar procedure, the observed XRD patterns of the annealed ECMO samples were analyzed by Rietveld method using refined structural models of the as-prepared sample. The final refined structural parameters of the samples are given in Table 1. The typical refined powder XRD patterns are shown in Table 1. Refined Structural Parameters of EuCo0.5Mn0.5O3 Samples Annealed in Different Atmospheres ECMO-O annealing atmosphere source wavelength (Å) 2θ range (deg) space group unit cell parameters (Å)

volume (Å3) Eu (4c: x, 1/4, z) B (Å2) Co/Mn (4b: 0, 0, 1/2) B (Å2) O1 (4c: x, 1/4, z) B (Å2) O2 (8d: x, y, z) B (Å2) Rwp, Rp χ2, RF

ECMO-A

ECMO-Na

O2 ADXRD, BL-12 0.730 57 5−40 Pnma 5.5567(1)

air ADXRD, BL-12

N2 RA, Cu Kα

0.730 57 5−40 Pnma 5.5627(1)

1.5406, 1.5444 10−100 Pnma 5.5770(1)

7.5680(1) 5.3240(1) 223.893(5) 0.0614(1) 0.25 −0.0144(1) 0.65(1) 0 0 0.5 0.47(3) 0.4658(9) 0.25 0.0999(9) 2.7(2) 0.2943(7) 0.0491(5) 0.7028(7) 1.6(1) 0.0302, 0.0400 1.96, 0.0133

7.5723(1) 5.3276(1) 224.409(0.006) 0.0614(1) 0.25 −0.0142(1) 0.76(1) 0 0 0.5 0.56(3) 0.4660(10) 0.25 0.0983(11) 2.1(2) 0.2943(9) 0.0506(6) 0.7044(9) 1.7(1) 0.0378, 0.0488 2.97, 0.0151

7.5802(1) 5.3295(1) 225.306(5) 0.0616(2)b 0.25 −0.0145(2) 1.18(3) 0 0 0.5 1.2(1) 0.471(1) 0.25 0.094(2) 2.1(4) 0.302(1) 0.049(1) 0.698(1) 2.3(3) 0.1090, 0.0869 3.267, 0.0613

Figure 1. Rietveld refinement plots of powder XRD data of Eu2CoMnO6 sample annealed in different atmospheres.

out on annealing in the atmosphere of low oxygen partial pressure. However, the defects are not likely to be large enough to be detected by XRD, in particular by powder XRD studies. Earlier literature indicated that the La2CoMnO6 samples prepared under extreme conditions (like high-temperature annealing and quenching in different atmospheres exhibit only a small deviation in oxygen stoichiometry, like La2CoMnO6‑δ with 0.00 ≤ δ ≤ 0.05).10,11 To understand the defects in the perovskite phase, the ECMO sample annealed in N 2 atmosphere for longer time (ECMO-N (L)) was investigated by XRD using synchrotron radiation. The analyses of the XRD data of this sample revealed a significant phase separation. The presence of Eu2O3, a solid solution of MnO−CoO and defect perovskite phases is observed in the XRD study. The details of refined structural parameters and refined XRD pattern are given

a

Seconary phase Eu2O3 (cubic: Ia3̅). a = 10.866(1) Å; V = 1283.1(4) Å3; wt. fraction of Eu2O3 phase, 3.1(7) %. b0.96(1) (occ).

Figure 1. In earlier studies, it has been observed that the cation ordering does not change by annealing at lower temperature.18,22,35 The analyses of the refined structural parameters also indicate similar values for ECMO samples. A comparison of the XRD results of the samples annealed in air (pO2 ∼ 0.2 atm), oxygen (pO2 ∼ 1 atm), and N2 revealed that the structure of the original orthorhombic phase is retained in all. However, the sample annealed in N2 atmosphere shows a partial separation of Eu3+ ion from the lattice, leading to the formation of a cation deficient perovskite phase like Eu1−xCo0.5Mn0.5O3. It needs to be mentioned here that oxygen defect cannot be ruled 17902



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Thus, we conclude that the excess oxygen defect is not tolerated by the structure and either balanced by separation of A site cation or phase separation. From the XRD and TG analyses of the ECMO-N (L) sample, it can be suggested that the perovskite phase is stabilized with only marginal cation and anion defect and the decrease in oxygen concentration is suppressed by separation of Eu from the lattice. A partial decomposition of structure and oxygen vacancies has been observed in the case of La2MgMnO6, La2NiMnO6, and Y2NiMnO6 on annealing in inert atmosphere.18,22 Similar studies on La2NiMnO6 revealed a partial transformation from monoclinic to rhombohedral structure.18 It is worth noting that in such condition only La2CoMnO6 was stable and retains the composition. Despite Eu3+ deficiency, the ECMO-N sample does not show significant deviation in oxygen stoichiometry, which suggests that the oxygen stoichiometry is maintained and charge is balanced by partial oxidation of Co2+ or Mn3+ present in the lattice. Sazonov et al.38 have investigated magnetic properties of partially cation-ordered Nd2−xCoMnO6 perovskite with varying concentration of oxygen stoichiometry and concluded that the Co2+ and Mn4+ are favored with x = 0.0. This observation is similar to that reported by Dass et al.11 in La2CoMnO6. Sazonov et al.38 have also indicated a variation in cationic ordering with oxygen stoichiometry, but this may not be the case in the present study as all the heat treatments have been carried out at a temperature (850 °C) lower than the temperature adopted for the preparation (1250 °C). The exact oxidation state of the Co and Mn in the ECMO samples can be Co2+, Co3+, Mn3+, and Mn4+. Such variable oxidation states have also been observed in earlier studied systems, such as La2CoMnO6 and La2NiMnO6.11,17,18,21 In particular, they are more prominent in the Co−Mn systems because of marginal difference in the redox potentials between Co2+−Mn4+ and Co3+−Mn3+ couples (electrochemical potential differences between Co2+−Co3+ = 1.8 eV and Mn3+−Mn4+ = 1.6 eV).11 The internal strain arising from the differences in the ionic radii of Co2+ and Mn4+ can also facilitate the variable oxidation state in these samples. Thus, the coexistence of variable oxidation states of these transition-metal ions is common in such perovskites. The XPS spectra for the annealed samples shows broad peaks around 640 and 778 eV attributable to Mn 2p and Co 2p photoelectrons.39−41 Typical XPS spectra in the region of Co 2p and Mn 2p are given in section S-IV of the Supporting Information. Because the differences in binding energies of Co2+ and Co3+ as well as Mn3+ and Mn4+ are not significant, the exact quantitative information on the valence states could not be revealed from the observed XPS peaks. However, the presence of Co2+, Co3+, Mn3+, and Mn4+ ions in the samples can be inferred from the broad nature of the peaks. From the variation of shape of Mn 2p peak and from deconvolution of the peaks, it could be inferred that the relative proportion of Mn3+ and Mn4+ varies with the annealing conditions, viz., Mn3+ > Mn4+ (in ECMO-A), Mn3+ ∼ Mn4+ (ECMO-O), and Mn3+ < Mn4+ (ECMO-N). Thus, it is likely that Mn4+ and Co2+ are favored upon annealing in atmospheres with low oxygen partial pressure. The XANES studies of Kyomen et al.35 indicated that the cation ordering and fluctuation of the valence states of transition-metal ions in analogous perovskites, such as La2CoMnO6, have second-order phase transition character. The thermal activation of charge carriers and defects in the structure drive the valence fluctuation in such materials. The samples prepared or annealed at lower

in section S-II of the Supporting Information. The analyses of the structural data for ECMO-N (L) indicated the composition of the perovskite phase is Eu1−xCo0.5Mn0.5O3−y, with x ∼ 0.03 and y ∼ 0.02. From XRD studies of ECMO-N samples, we had observed a deficiency of Eu site (∼0.04). The observed unit cell parameter of segregated cubic Eu2O3 phase is 10.8660(2) Å, which is similar to that observed for pure Eu2O3. However, the unit cell parameter observed for cubic CoO−MnO solid solution phase (a = 4.3562 Å) is between those of CoO (4.261 Å; JCPDS-PDF entry 48-1719) and MnO (4.442 Å; JCPDSPDF entry 78-0424). Thus, we can suggest the solid solution phase has nearly 50% contribution from both CoO and MnO. The weight fraction analyses indicated decomposition of almost half of the original phase. The feeble anion deficient is further supported by the marginal higher value of unit cell volume of the ECMO-N or ECMO-N (L) compared to those of ECMO-A or ECMO-O samples. The decrease in valence state of transition-metal ions may be a reason for the increasing average radius of the octahedral site. The XANES study (explained later) indicated an increasing fraction of Co2+ on annealing in N2 atmosphere, which compensates the unit cell contraction due to the Eu3+ or O2− vacancies. This will be the case only with marginal deficiency of Eu3+ or O2− ions. It can be mentioned here that the unit cell parameters of (ABO3) perovskite structures are more sensitive to the octahedral (B) cation radius than to the interstitial site (A) cations. Also, to compare the oxygen defects, thermogravimetry (TG) studies under reduction (Argon−Hydrogen, Ar−H2) on ECMO-O and ECMO-N (L) samples were performed. The TG traces were recorded while heating the sample from 25 to 800 °C; the sample was then held at 800 °C for 60 min. From the thermogravimetry studies, the total weight loss observed for ECMO-O sample is 6.22%, which is similar to that expected for reduction of Eu2CoMnO6 leading to final products Eu2O3, Co, and MnO (i.e., 6.223%). The XRD analyses of the residue obtained from the TG run confirm these products. Thus, it can be assumed that the oxygen-annealed sample (ECMO-O) has stoichiometry EuCo0.5Mn0.5O3. To study the effect of inert atmosphere, TG study on sample ECMO-N (L) was carried out under a similar procedure. The XRD analyses of the TG residue revealed the presence of Eu2O3 and solid solution of CoO−MnO and Co. The net weight loss (3.32%) observed in TG is nearly half that observed of ECMO-O and is comparable to slightly more than 0.5 O loss from EuCo 0.5Mn0.5O3 composition. This suggests the average valence octahedral site is a little higher than 3.0 despite the Eu deficient perovskite phase. The expected average valence of the octahedral site of the perovskite phase as observed from SR XRD study is 3.047 (for composition Eu0.973Co0.5Mn0.5O2.98). This is also indicated by the XANES data at the Co and Mn edges. Thus, we can suggest the oxygen content values in ECMO-O, ECMO-A, and ECMO-N are between 3.00 and 2.98. The XRD patterns of TG residue do not show any peak attributable to perovskite-type phase. This is consistent with the observed lower weight loss in TG. No indication for the formation of cubic EuCoO1.7 type phase (a = 3.78 Å) as reported earlier by Arakawa et al.36 has been observed in the inert atmosphere-annealed sample or reduced samples. The decomposition behavior of ECMO under reducing conditions is similar to the observation of Yankin et al.37 for EuMnO3. Typical TG traces and powder XRD pattern of the TG residues are given in section S-III of the Supporting Information. 17903



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temperature favor the Co2+ and Mn4+ ions, while Co3+ and Mn3+ ions are favored at higher temperature,35 which has a bearing on their electrical and magnetic properties. A number of possible ion pairs of Co and Mn in cation-disordered structure of ECMO may further influence their properties compared to the cation-ordered lattices in which only two dominating Co2+−Mn4+ and Co3+−Mn3+ interactions control their properties. Furthermore, to support the oxidation states of transitionmetal ions in the ECMO samples, XANES spectra at the Kedges of Co and Mn were recorded. The XANES spectra were normalized by using the Athena program of the IFEFFIT package42, and the energy threshold E0 was obtained by set point at the edge of each μ(E) spectrum. The normalized spectra are shown in Figure 2. The observed edge energies (E0)

Table 2. Threshold Energies of Co and Mn K-Edges of the ECMO Samples and Standards E0 (Co K-Edge) CoF3 MnO2 ECMO-O ECMO-A ECMO-N Mn2O3 CoO

E0 (Mn K-Edge)

7726.03 7723.99 7723.85 7722.85

6556.0 6555.7 6554.2 6554.5 6551.8

7721.21

it can be suggested that in ECMO-N sample has more Co2+ than Co3+. Similar comparison of edge energies of the Mn-K edge XANES spectra indicates that the oxygen-annealed sample has more Mn4+ compared to the others. However, in all cases the edge energies are closer to MnO2; thus, all ECMO samples have predominant Mn4+ ions. It can be mentioned here the white line peak of XANES spectra of each sample appears as a single peak, which might be due to strong interaction of the cations of different oxidation states. As mentioned earlier, stoichiometry of the perovskite phases in a sample obtained after long annealing (ECMO-N (L)) has only a marginal deviation in oxygen content; the increase in average oxidation state is more of Mn4+ contribution in it. Thus, it can be expected the increasing fraction of Co 2+ favors the ferromagnetic interaction of the Co2+ and Mn4+ in ECMO-N sample, as is explained later. Further insights on such variable oxidation state ion pairs and oxygen defects were inferred from the magnetic and dielectric studies as explained below. The temperature-dependent magnetizations plots (M versus T) of the ECMO samples in both field-cooled and zero-fieldcooled conditions are shown in Figure 3. It is apparent from

Figure 2. XANES spectra of ECMO samples at Co and Mn K-edges. Figure 3. Temperature-dependent magnetic susceptibility (FC and ZFC) of ECMO-A, ECMO-O, and ECMO-N measured at applied field of 100 oe.

for different samples are given in Table 2. The observed XANES spectra and edge energies are similar those of LnCo0.5Mn0.5O3 (Ln = La, Tb) reported in the literature.43,44 It is observed that for all the annealed samples the Co K-edges appeared between those of CoO and CoF3 and similarly the Mn K-edges appeared between those of Mn2O3 and MnO2. This suggests that the oxidation state of Co is between 2 and 3 while that of Mn is between 3 and 4. Furthermore, it is noticed that the XANES spectra of ECMO-A and ECMO-O are more similar and close to that of CoF3, while the spectrum of ECMO-N is close to that of CoO. Considering the edge shifts,

Figure 3 that all the samples are paramagnetic above 136 K. All samples show a sharp increase in magnetization below 136 K, which is close to but little higher than the value reported earlier for ECMO (124 K).30,31 The temperature-dependent FC magnetization of all the ECMO samples indicates that magnetization increase with decreasing temperature. However, the magnetization of ECMO-A and -O samples do not tend to saturate, while the magnetization of the ECMO-N tends to 17904



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saturate at lower temperature. Also, the observed transition temperature in ECMO-N (136 K) is higher than those observed at observed for ECMO-A and ECMO-O (132 K). A closer look on the magnetization behaviors both in FC and ZFC shows multiple magnetic transitions in ECMO-O and -A samples, while a single magnetic transition is observed in the ECMO-N sample. In both ECMO-O and -A samples, the second magnetic transition is observed at around 115 K. The multiple magnetic−ferromagnetic transitions with a large difference in TC due to different magnetically coupled ion pairs have been earlier reported in La 2 CoMnO 6 , La2NiMnO6.11,18,19,21 The studies on magnetic properties of partially cation-ordered Nd2−xCoMnO6 perovskite with varying concentration of oxygen stoichiometry indicate that the Co2+ and Mn4+ are the favored ion pair at x = 0.0.38 The authors have also observed that the ferromagnetic TC decreases systematically with increasing oxygen content, which has been attributed to the formation of low-spin Co3+.38 Also, it has been reported that the fraction of the ferromagnetically coupled ion pairs can be altered by annealing in different oxygen partial pressures at a temperature relatively lower than that of the preparation conditions.18,19 Thus, it is possible that the presence of larger fraction of Co3+as observed in XANES and XPS is likely to be a reason for the lower TC for the ECMO-A and ECMO-O sample compared to that of the ECMO-N sample. The ZFC magnetizations of all the samples (Figure 3) were almost similar to FC magnetization above the transition temperature, but they diverge below TC. All the ZFC magnetization curves show a peak at the transition temperature and then gradually decrease with decreasing temperature. The peaklike feature of ZFC shows increasing diffusiveness with the increases in the oxygen containing annealing atmospheres. The peak-like features in ZFC magnetizations at the transition temperatures are quite similar to antiferromagnetic or spin glass systems. In addition, the ZFC traces of ECMO-A and -O samples gradually decrease and even showed a negative trace. However, the same for ECMO-N remains positive down to 5 K. The decreasing magnetization or even negative magnetization in ZFC in such double perovskites and other related spinel-type ferrimagnetic systems have been reported in the literature.18,21,22,30,31,38 The ZFC traces show increasing negative values with the increase in the measurement field, which is observed in the oxygen-annealed sample more predominantly than in the other samples. Such decreasing magnetization of the ZFC and the field-dependent magnetization have been reported by Vasiliev et al.30 for ECMO samples. The authors have attributed it to a metamagnetic nature of ECMO and also to an appreciable contribution from the residual trapped field in the magnetometer. The origin of such decreasing magnetization and negative magnetization has been attributed to several factors, namely the contribution of antiferromagnetically coupled rare-earth magnetic moments, dominating antifferomagnetic clusters of transition-metal ion pairs at low temperature, as well as residual trapped magnetic field superconducting solenoids of the magnetometer.30,45,46 The measurements carried out at various applied fields indicate a significant role of the applied field on their FC (Figure 4), and the net magnetization gradually increases with the increase in the applied field for measurement. This indicates that the antiferromagnetically coupled magnetic ions pairs contribute to the ZFC magnetization behavior of all at lower temperature. However, the differences in the ZFC and FC behavior indicate

Figure 4. Temperature-dependent FC magnetic magnetization of ECMO-A, ECMO-O, and ECMO-N samples measured at different fields.

the antiferromagnetic interactions are weak; thus, on applied field conditions, the spins rotate along the field direction and show net magnetization. In such coexisting antiferromagnetically and ferromagnetically coupled system, the ZFC magnetization depends on the applied field strength, strength of antiferromagnetic interaction and magnetic anisotropy energy of the domains. In general, the ZFC magnetization is directly proportional to the applied field strength and inversely proportional to the magnetic anisotropy energy. A lower magnetic anisotropy in ECMO samples has been observed as they have metamagnetic behaviors with smaller critical fields (explained later in this paper). Furthermore, as the nature of magnetic interactions is altered by the annealing conditions, the ZFC and FC magnetization show differences. It can be noticed that the ZFC behavior of ECMO-O and -A samples are drastically different below TC2 17905



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compared to TC1. This might be due to the presence of a larger proportion of antiferromagnetically coupled ion pairs in these compared to ECMO-N. However, the variation of magnetic susceptibility with temperature (M/H versus T) of all three samples measured at different applied field (shown section S-V of the Supporting Information) shows a decreasing trend with the increasing field and tends to saturate at higher field at lower temperature. Such behavior indicates magnetic interaction in all these samples which is sensitive to the magnetic field. Furthermore, the magnetization of the ECMO-N sample has a greater tendency to saturate compared to the ECMO-O or -A samples. Considering the observed negative magnetization in ZFC, the crossover temperature for negative-to-positive magnetization is found to decrease with the increase in the applied field. Thus, it can be suggested that the metamagnetic nature of the sample has a significant role in the negative ZFC magnetization. Thus, contributing antiferromagnetic and metamagnetic behavior can be inferred in the ECMO samples. Further evidence for these have been obtained from the magnetic hysteresis loops recorded at different temperatures. The magnetic hysteresis (MH) loops for ECMO samples recorded at several temperatures between 5 and 150 K are shown in Figure 5. All the samples show ferromagnetic-like hysteresis loops below 150 K, which further confirms their ferromagnetic nature. However, the magnetizations of all the ECMO samples do not show any saturation up to an applied field of about 9 T. This can be due to the existence of antiferromagnetic ordering and their contribution being reduced with the increasing applied external field. It can be noticed from the hysteresis loop of the ECMO-N at 5 K (Figure 6) that magnetization increases like a step at around 30 kOe, which is similar to metamagnetic materials. Similar metamagnetic-like hysteresis loops have been observed by Vasiliev et al.30 in EuCo0.5Mn0.5O3. A comparison of the hysteresis loops of ECMO-samples at 5 K indicates that the critical field increases on annealing in the oxygen-containing atmosphere. All the samples show two critical magnetic fields (HC1 and HC2) (inset in Figure 6) in the first cycle magnetization in the positive field, and they are reproduced in the successive cycles. A comparison of the M versus H loops of the three samples indicate that the HC1 remains almost similar while the HC2 shifts to lower field on annealing in ECMO-N. This suggests that the antiferromagnetic interactions are weakened by annealing in lower oxygen partial pressure. The magnetic phase diagram for EuCo0.5Mn0.5O3 reported by Troyanchuk et al. suggests ferrimagnetic structure with oppositely aligned spins of Co2+ and Mn4+ ions, which transforms to ferromagnetic state with increasing field.31 The magnetic behavior of the system indicates a field-dependent switching of the magnetic interactions in the ECMO can be further tuned by simple annealing conditions. The paramagnetic region of the temperature-dependent inverse susceptibility of ECMO samples (Figure 7) were fitted with Curie−Weiss (χ = (C/T − θ)) relation, where C is the Curie constant defined as C = ((Nμeff2)/3k) and θ = Weiss constant. The effective magnetic moment (μeff/F.U.) observed for the ECMO-O and ECMO-A samples are 4.94 and 4.96 μB, respectively, while ECMO-N shows a larger value (6.63 μB). However, the Weiss constant (θ) for all the samples are closely similar, viz., 100.6, 103.4, and 102.5 K for ECMO-O, ECMO-A, and ECMO-N, respectively. The positive values of θ indicate ferromagnetic interaction of the transition-metal ions in all of them. In all cases, the observed θ is lower than the TC because

Figure 5. Magnetic hysteresis loops of ECMO samples measured at some representative temperatures.

of the contributions of antiferromagnetic interactions in the system. The observed paramagnetic moments μeff/F.U. for the ECMO-A and -O samples are similar to those reported in the literature.30 The effective magnetic moments comparable to that expected in such Co containing compounds (as: μeff = 0.5 μB2[gMn2SMn(SMn + 1) + gCo2SCo(SCo + 1)]; with SCo2+ (HS) and SMn4+ are 3/2, gCo2+ = 3.00, gMn4+ = 2.00, the μeff = 4.94 μB). It is known that low spin configuration Co2+ (3d7) (t62ge1g ; S = 1/2) 17906



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transition in the cation-ordered Co−Mn-containing double perovskites originate from the Co2+−O−Mn4+ super exchange interactions.10,12,17−21 Different ferromagnetic interactions due to different magnetically coupled pairs has been observed in La2CoMnO6.11,21−23 As mentioned earlier, because the Coand Mn-containing double perovskite can retain Co2+, Co3+, Mn3+, and Mn4+ ions in the structure, a number of possible interactions of magnetic ions can be expected. In such perovskites, the ferromagnetic interactions mainly arise from the Mn3+−O−Mn4+, Co2+−O−Co3+ (LS), Co2+−O−Mn4+, and vibronically coupled Co3+−O−Mn3+, while all other interactions of magnetic ions are of an antiferromagnetic nature.11,21,29,46,47 Even in the cation-ordered rare-earth double perovskite structures, the antisite defects cause an appreciable contribution of antiferromagnetic interactions.11,17−25,47 Besides, the spin states of Co2+ and Co3+ are also strongly dependent on the temperature and composition of the samples.48−50 Thus, a wider range of effective paramagnetic moments for such materials is expected. Because the coexistence of Co2+, Co3+, Mn3+, and Mn4+ is evident in the presently studied ECMO samples, a number of possible magnetic interactions between these transition-metal ions are likely to exist. The ferromagnetism in the ECMO samples mainly arise from the Co2+−O−Mn4+ and Co3+−O−Mn3+ clusters as all the other interactions have negative superexchange interaction.49−52 Besides, the presence of all the magnetic interacting pairs in a disordered system as in the present case may lead to spin glass type behavior. Investigation of magnetic structure of partially cation-disordered monoclinic lattices (∼35% disordered) systems, such as La2CoMnO6, Tb2CoMnO6, Y2CoMnO6, etc., revealed ferromagnetic ordering in the transition-metal ions but with a significant contribution from the spin glass or paramagnetic phases.21,22,49−53 Thus, such systems always show a significantly lower effective magnetic moment compared to that expected from the spin only values. Metamagnetic transitions with fieldinduced ferromagnetic phase having significantly lower magnetic moment in double perovskite-type compositions are reported with Ln = Gd, Tb, Dy, Nd, Eu, Y, etc.30,31,53−59 However, later studies on Tb2CoMnO6 suggest the reorientation of antiparallel Tb3+ moments is the origin of the metamagnetic transition.51 However, this reason cannot be generalized because similar metamagnetic behavior has also been observed in perovskites with diamagnetic ions as well as ions with low magnetic contribution as the A-site cation.31 Thus, the clusters of different magnetically coupled ion pairs and their reorientation with the magnetic field might be the origin of the metamagnetic transition. To understand the effect of annealing conditions on the electrical characteristics and their defect properties, the dielectric properties of ECMO samples were investigated. The variations of the real part of the relative permittivity (ε′) with frequency as well as with temperature are shown in Figures 8 and 9, respectively. Both the temperature- and frequency-dependent permittivities of all the samples show two step-like relaxations with large dispersion. The ECMO-A sample shows a large dielectric constant of about 800 at 273 K at frequencies around 2 kHz. Because the dielectric measurements could not be measured up to the magnetic transition temperature, the behavior of the samples at the magnetic transition temperature is not delineated. Above the magnetic transition temperature, all the studied samples show two relaxations, one weaker toward the lower frequency and the

Figure 6. Magnetic hysteresis loops of ECMO-A, -O, and -N samples at 5 K.

Figure 7. Variations of inverse magnetic susceptibility of ECMO samples with temperature.

shows only nearly isotropic g (g⊥∼ 2.2 > g∥ = 2.02−2.01) at room temperature because of sufficiently long spin−lattice relaxation time, while the high-spin configuration (t52ge2g ; S = 3/ 2), Co2+ can exhibit large anisotropic g (g⊥= 4.5 > g∥ = 2.02− 2.04) because of shorter spin−lattice relaxation time.22,30 The EPR studies on La2CoMnO6 indicate a larger g value (g ∼ 2.98) for the high-spin Co2+ (S = 3/2).22 From the magnetic measurements of EuCo0.5Mn0.5O3, Vasiliev et al. have reported g ∼ 3.1 for high-spin Co2+ ions.30 It can be mentioned here that the EuCo0.5Mn0.5O3 sample studied by Vasiliev et al.30 has about 8% Co3+ (3d6) in low-spin t62ge0g state. The observed effective moment for the ECMO-N indicates a g value of 4.4 for Co2+ ion. However, the exact spin state and oxidation states of the Co and Mn cannot be concluded in such strongly interacting systems. Thus, it can only be suggested that the effective spin state of Co and distortion around the Co ions varies appreciably on annealing in inert atmosphere. The magnetic behaviors of the Co−Mn containing rare-earth double perovskites show diversified magnetic properties, like ferromagnetic, antiferromagnetic to spin glass, or metamagnetic behavior.11,18,30,31,38 The magnetic phase diagram investigated in such a system revealed the presence of different magnetically coupled transition-metal pairs which govern their magnetic properties. It has been suggested that the ferromagnetic 17907



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Figure 8. Frequency-dependent real part of relative permittivity of ECMO samples measured at different temperatures. Figure 9. Temperature-dependent real part of relative permittivity of ECMO samples measured at different frequencies.

main relaxation at a higher frequency. The permittivity in ECMO-A for the main relaxation varies around 250−650 in the entire temperature range. However, the low-frequency relaxation shows permittivity of about 150 at low temperature but increases with the increase in temperature. Similar behaviors are observed in ECMO-O samples also, but the permittivity increases to about 2000 at 273 K, which remains almost unchanged between 102 and 105 Hz. However, the

ECMO-N sample shows a reduced permittivity in both lowand high-frequency regions. The variation of loss tangent, tan δ (= ε″/ε′), with temperature at different frequencies of different samples is shown in Figure 10. The ECMO-O sample shows two distinct relaxation peaks while others show only the one main relaxation 17908



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with strong frequency dispersion in tan δ compared to that of other samples (Figure 10). In addition, a large increase in the loss tangent in ECMO-O sample is observed at higher temperature. In the ECMO-N sample, the relaxation peaks are hardly distinguishable, especially at higher frequencies, but the permittivities increase sharply at higher temperatures. This suggests that the annealing conditions play a crucial role in tailoring the dielectric response of ECMO samples and that the permittivity can be enhanced by almost an order of magnitude with oxygen annealing. The dielectric relaxation observed in the samples can have two types of origin, viz., conduction or dipolar origin. The large increase in loss tangent in both ECMO-O and -N at higher temperature suggests enhanced electrical conductivities in them. In all the samples, the peak relaxation temperature shifts toward higher temperature with the increase in frequency. The relation between the relaxation frequency and temperature Tm of the tan δ peaks was found to follow the Arrhenius relation of eq 3, as shown in Figure 11. The activation energy values for

Figure 11. Arrhenius fit of the peak relaxation frequency for ECMO samples.

the main relaxation as obtained from the Arrhenius relation are 0.20, 0.20, and 0.22 eV for ECMO-A, ECMO-O, and ECMO-N samples, respectively. In all cases, the activation energies are more or less similar, which suggests an identical conduction mechanism in them. Furthermore, the values of the activation energies suggest conduction of the polarons associated with the defects. Although no detectable anion vacancies are observed in any of the system under study, a feeble concentration of point defects cannot be ruled out in any system containing transitionmetal ions having variable oxidation state. The oxygen vacancies, due to (O2− = V02− + 2e + 1/2O2), can be expected in such systems. Also, the Eu3+ vacancies observed in ECMO-N samples are also likely to form holes (Eu3+ = VEu3+ + 3h+). Thus, these holes may be balanced by equivalent increase in valence states of Co2+ or Mn3+ to Co3+ or Mn4+ in the lattices or can associate with the anions forming a localized bound-state polaron. However, the presence of extra polarons in the ECMO-N sample might be a reason for the loss in it being higher than that in the others. To further differentiate the relaxation due to the conduction or dipolar fluctuations from the interfacial or electrode polarization at low-frequency ends, the temperature- and frequency-dependent dielectric data were analyzed in electric

Figure 10. Variation of loss tangent of ECMO samples with temperature.

process in the tan δ versus T plots. The temperature dependence of dielectric permittivity and tan δ revealed the presence of the dielectric relaxations in all samples, but the features of relaxation peaks are different for different samples. It can also be observed that the ECMO-A sample shows a peak 17909



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modulus representation. In general, the modulus representations of frequency dispersion data suppress the electrode/grain boundary effects and clearly delineate the hopping conduction and dipolar relaxations. The modulus data were extracted from the permittivity data using the following relation: M *(ω) = M′(ω) + iM″(ω) = 1/ε*(ω)

(1)

where M*(ω) = M′(ω) − iM″(ω) and ε*(ω) = ε′(ω) − iε″(ω) are complex modulus and permittivity, respectively, and ω= 2πf is the angular frequency of the applied electric field. The real part of the modulus (M′) shows significantly lower values, which suggests negligible contribution from the electrodes for the dielectric properties of the ECMO samples (see plots of M′ versus T in section S-VI of the Supporting Information). Thus, the dielectric and relaxation phenomena observed in the samples are their intrinsic effects. The modulus data can also provide more information to differentiate the hopping conduction or dipolar relaxation compared to the permittivity data. The variations of M″ with frequency as well as with temperature are shown in Figures 12 and 13, respectively. In general, the conduction-induced relaxation process is observed as a peak in the M″ versus f plots but not in the ε″ versus f plots. However, for the dielectric relaxation due to localized dipolar relaxation and long-range hopping, conductivity is observed as peaks in both M″ and ε″ plots.54−57 In the modulus representation, the relaxation time τM = 1/f m, where f m is the position of relaxation peaks in M″(f), and is related to the dielectric relaxation time τD = 1/f D in the Debye model as54 fm = 1/τM = (εs /ε∞)(fD )

(2)

The variations of imaginary parts of the modulus (M″ versus f) of all three samples show clear peaks which are similar to that observed in tan δ versus f. In the frequency region below the peak maximum (M″max), the carriers are mobile at longer distances, while in the frequency region above the peak the carriers are confined and have short-range mobility. The extended dipole formed in the anion defective lattices may favor a long relaxation process and hence larger dielectric permittivity as well as loss. Such features are also likely to exist in the ECMO-O and -A samples. However, in the ECMO-N sample, the presence of Eu3+ ion vacancies might result in holes at the Eu3+ site and the hole and electron may form a trapped polaron. But the polaron migration is favored at higher temperature for migration. Thus, the dielectric permittivities are significantly reduced with the concurrent increase in loss. A comparison of the loss peaks in M″ versus f of the ECMO samples indicates a systematic shift toward higher frequency with the increase in temperature. Thus, the frequency region for the long-range conductivity increases with the increasing temperature. The temperature dependence of the relaxation frequency has been obtained from the positions of the maxima (f m) in the frequency-dependent M″ plots, and they are shown in Figure 14, which shows typical Arrhenius behavior (eq 3). fm = f0 exp[−E /(kBT )]

Figure 12. Variation of real part of modulus (M′) of ECMO samples with temperature and frequency.

frequency f m and the M″ peak temperature Tm (Figure 15). The observed values of activation energy are 0.185, 0.194, and 0.21 eV, for the ECMO-A, -O, and -N samples, respectively. These values are close to the activation energy in the chargeordered magnetodielectric La2CoMnO6, which shows polaronic conduction.17,22 It is notable that, according to electrochemical data, the energy required to transfer an electron from the Co2+ to Mn4+ in La2CoMnO6 is 0.20 eV.17,18,22 We therefore presume that the observed low-temperature dielectric relaxations in EuCo0.5Mn0.5O3 are polaronic in nature and that they are dependent upon the annealing conditions. From the above observations, it is clear that the dielectric relaxations of the samples, though qualitatively different based on sample treatment conditions, most likely originate from

(3)

where f 0 is a prefactor, kB the Boltzmann constant, T the temperature, and E the activation energy. The activation energy values were 0.19, 0.20, and 0.19 eV for the ECMO-A, -O, and -N samples, respectively. Similar Arrhenius type behaviors are observed for the relaxation 17910



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Figure 14. Variation of imaginary part of modulus (M′′) of ECMO samples with temperature.

Figure 15. Arrhenius fit of the peak relaxation frequency of ECMO samples as observed from M′′ versus f plots.

also evident that the dielectric constant can be tuned by altering the annealing conditions.

4. CONCLUSIONS The magnetic and dielectric studies on three samples of cationdisordered EuCo0.5Mn0.5O3 obtained by different annealing conditions revealed a significant effect of oxygen partial pressure of oxygen annealing atmosphere on these properties. The transition temperature, critical field, and temperaturedependent magnetization indicates that the variation of oxygen partial pressure not only varies the relative concentration of various ions but also affects the spin state transition of the Co2+ ions. Thus, the magnetic properties of the samples are mainly governed by the clusters of ferromagnetically and antifferomagnetically paired magnetic ions and their relative proportions. Similarly, the dielectric properties of the EuCo0.5Mn0.5O3 show larger permittivity (≥2000) on annealing in oxygen-containing atmosphere than that in inert atmosphere (∼400). The analyses of temperature- and frequency-dependent loss tangent and modulus data revealed two different types of relaxations which are related to the polaronic conduction process in them. In addition, the study revealed a switching of magnetic properties by the field as well as by the preparation conditions of EuCo0.5Mn0.5O3; electrical characteristics can be a useful for tuning the properties for desired applications.

Figure 13. Variation of imaginary part of modulus (M′′) of ECMO samples with frequency.

polaron hopping. For charge-ordered double perovskites like La2MMnO6 (M=Co, Ni), the ordering of M2+ and Mn4+ induces a strong local polarization leading to high dielectric constants and polaronic hopping at low temperatures.17,58,59 It is also worth noting that while the Mott variable range-hopping (VRH) behavior was seen in La2CoMnO6,17 where the behavior deviated from the Arrhenius law (eq 3), we find that our data can be very well fit to eq 3, suggesting a simple thermally activated hopping of the polarons to nearest neighbors in EuCo0.5Mn0.5O6. Such behavior has been observed in La2MgMnO6,59 in which coexistence of Mn3+ and Mn4+ was observed to produce the polaronic conduction. It is therefore likely that both the ordering of Co2+ and Mn4+ and mixed valence of Mn lead to the dielectric relaxations in the present studied samples. The significant differences in the dielectric relaxations in the three samples suggest that the synthesis conditions play a significant role in the polaron dynamics. It is 17911



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(15) Yang, W. Z.; Liu, X. Q.; Zhao, H. J.; Lin, Y. Q.; Chen, X. M. Structure, Magnetic and Dielectric Characteristics of Ln2NiMnO6 (Ln = Nd and Sm) Ceramics. J. Appl. Phys. 2012, 112, 064104 (1−6). (16) Guo, Y.; Shi, L.; Zhou, S.; Zhao, J.; Liu, W. Near RoomTemperature Magnetoresistance Effect in Double Perovskite La2NiMnO6. Appl. Phys. Lett. 2013, 102, 222401 (1−4). (17) Lin, Y. Q.; Chen, X. M. Dielectric, Ferromagnetic Characteristics, and Room-Temperature Magnetodielectric Effects in Double Perovskite La2CoMnO6 ceramics. J. Am. Ceram. Soc. 2011, 94, 782− 787. (18) Sayed, F. N.; Achary, S. N.; Jayakumar, O. D.; Deshpande, S. K.; Krishna, P. S. R.; Chatterjee, S.; Ayyub, P.; Tyagi, A. K. Role of Annealing Conditions on the Ferromagnetic and Dielectric Properties of La2NiMnO6. J. Mater. Res. 2011, 26, 567−577. (19) Singh, M. P.; Grygiel, C.; Sheets, W. C.; Boullay, Ph.; Hervieu, M.; Prellier, W.; Mercey, B.; Simon, Ch.; Raveau, B. Absence of LongRange Ni/Mn Ordering in Ferromagnetic La2NiMnO6 Thin Films. Appl. Phys. Lett. 2007, 91, 012503 (1−3). (20) Barilo, S. N.; Gatalskaya, V. I.; Shiryaev, S. V.; Kurochkin, L. A.; Shimchak, R.; Baran, M. Magnetic Properties of a LaMn0.46Co0.54O3 Single Crystal. Low Temp. Phys. 2002, 28, 853−855. (21) Joy, P. A.; Khollam, Y. B.; Date, S. K. Spin States of Mn and Co in LaMn0.5Co0.5O3. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 8608−8610. (22) Sayed, F. N.; Achary, S. N.; Deshpande, S. K.; Rajeswari, B.; Kadam, R. M.; Dwebedi, S.; Nigam, A. K.; Tyagi, A. K. Role of Annealing Atmosphere on Structure, Dielectric and Magnetic Properties of La2CoMnO6 and La2MgMnO6. Z. Anorg. Allg. Chim. DOI: 10.1002/zaac.201400215. (23) Baŕon-Gonzźalez, A. J.; Frontera, C.; Garćıa-Mũnoz, J. L.; RivasMurias, B.; Blasco, J. Effect of Cation Disorder on Structural, Magnetic and Dielectric Properties of La2MnCoO6 Double Perovskite. J. Phys.: Condens. Matter 2011, 23, 496003 (1−11). (24) Yánez-Vilar, S.; Sánchez-Andújar, M.; Rivas, J.; SénarísRodríguez, M. A. Influence of the Cationic Ordering in the Dielectric Properties of the La2MnCoO6 Perovskite. J. Alloys and Comp. 2009, 485, 82−87. (25) Singh, M. P.; Troung, K. D.; Jand, S.; Fournier, P. Multiferroic Double Perovskites: Opportunities, Issues, and Challenges. J. Appl. Phys. 2010, 107, 09D917 (1−3).. (26) Singh, M. P.; Truong, K. D.; Fournier, P. Magnetodielectric Effect in Double Perovskite La2CoMnO6 Thin Films. Appl. Phys. Lett. 2007, 91, 042504 (1−3). (27) Booth, R. J.; Fillman, R.; Whitaker, H.; Nag, A.; Tiwari, R. M.; Ramanujachary, K. V.; Gopalakrishnan, J.; Lofland, S. E. An Investigation of Structural, Magnetic and Dielectric Properties of R2NiMnO6 (R = rare earth, Y). Mater. Res. Bull. 2009, 44, 1559−1564. (28) Goto, T.; Kimura, T.; Lawes, G.; Ramirez, A. P.; Tokura, Y. Ferroelectricity and Giant Magnetocapacitance in Perovskite RareEarth Manganites. Phys. Rev. Lett. 2004, 92, 257201 (1−4). (29) Baier, J.; Jodlauk, S.; Kriener, M.; Reichl, A.; Zobel, C.; Kierspel, H.; Freimuth, A.; Lorenz, T. Spin-State Transition and Metal-Insulator Transition in La1−xEuxCoO3. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 014443 (1−10). (30) Vasiliev, A. N.; Volkova, O. S.; Lobanovskii, L. S.; Troyanchuk, I. O.; Hu, Z.; Tjeng, L. H.; Khomskii, D. I.; Lin, H.-J.; Chen, C. T.; Tristan, N.; et al. Valence States and Metamagnetic Phase Transition in Partially Β-site-Disordered Perovskite EuMn0.5Co0.5O3. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 104442 (1−5). (31) Troyanchuk, I. O.; Khalyavin, D. D.; Lynn, J. W.; Erwin, R. W.; Huang, Q.; Szymczak, H.; Szymczak, R.; Baran, M. Magnetic Phase Diagrams of the Ln(Mn1−xCox)O3 (Ln = Eu, Nd, Y) Systems. J. Appl. Phys. 2000, 88, 360−367. (32) Sinha, A. K.; Sagdeo, A.; Gupta, P.; Upadhyay, A.; Kumar, A.; Singh, M. N.; Gupta, R. K.; Kane, S. R.; Verma, A.; Deb, S. K. Angle Dispersive X-ray Diffraction Beamline on Indus-2 Synchrotron Radiation Source: Commissioning and First Results. J. Phys.: Conf. Ser. 2013, 425, 072017 (1−4).

ASSOCIATED CONTENT

S Supporting Information *

Details of structural parameters of the as-prepared EuCo0.5Mn0.5O3 sample and EuCo0.5Mn0.5O3 sample annealed for longer time under inert atmosphere, thermogravimetric data of EuCo0.5Mn0.5O3 samples and XRD of TG resides, XPS spectra, field-dependent magnetic susceptibility, and real part of dielectric modulus. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0092-22-25592328. Fax: 0091-22-25505151. E-mail: [email protected]; acharysn@rediffmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Department of Atomic Energy’s Science Research Council (DAE-SRC) is acknowledged for supporting this work (vide sanction 2010/21/9-BRNS/2025, dated 7-12-2010). The authors thank Dr. S. K. Sali and Dr. N. K. Kulkarni, Fuel Chemistry Division, BARC, for their help in thermogravimetric studies.

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