Structural and magnetic properties of La2Ni1−xCoxMnO6 compounds

Accepted Manuscript Title: Structural and magnetic properties of La2 Ni1−x Cox MnO6 compounds Author: Debabrata Pramanik S. Mukherjee Shovan Dan A. Nandy S.K. Pradhan Papri Dasgupta Asok Poddar Manabendra Mukherjee B. Manjunath P.A. Joy PII: DOI: Reference:

S0025-5408(17)33318-4 https://doi.org/doi:10.1016/j.materresbull.2018.01.040 MRB 9811

To appear in:

MRB

Received date: Revised date: Accepted date:

26-8-2017 19-1-2018 25-1-2018

Please cite this article as: Debabrata Pramanik, S. Mukherjee, Shovan Dan, A. Nandy, S.K. Pradhan, Papri Dasgupta, Asok Poddar, Manabendra Mukherjee, B. Manjunath, P.A. Joy, Structural and magnetic properties of La2 Ni1minusx Cox MnO6 compounds, (2018), https://doi.org/10.1016/j.materresbull.2018.01.040 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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Highlights La2Ni1−xCoxMnO6 (x = 0.2, 0.4, 0.8) have been prepared using sol-gel

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technique, in single phase with monoclinic structure (P21/n). 2.

The atomic models generated using Reitveld refined structural

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parameters show that the structure is a double perovskite one, with octahedral distortion.

The study focuses on the dependence of TC and the nature of the

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ferromagnetic interactions on the type of B-site (Ni/Co) ions in A2BBO6 type

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double perovskites.

The study conclusively shows that the origin of short range

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induced by antisite disorders.

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ferromagnetic ordering above the Curie temperature observed in such systems is

Our study involving the measurement of ac susceptibility and IRM (t)

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completely discards the existence of any spin glass like phase in such system at low temperatures.

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Structural and magnetic properties of La2Ni1−xCoxMnO6 compounds

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Debabrata Pramanika , S. Mukherjeea,∗, Shovan Dana , A. Nandya , S. K. Pradhana , Papri Dasguptab , Asok Poddarb , Manabendra Mukherjeeb , B. Manjunathc , P.A. Joyc a

Department of Physics, The University of Burdwan, Burdwan - 713104, India Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata - 700064, India c Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory (CSIR-NCL), Pune 411008, India

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Abstract

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Single phase monoclinic La2 Ni1−x Cox MnO6 (x = 0.2, 0.4, 0.8) compounds, henceforth, referred as LNCx MO (x = 0.2, 0.4, 0.8) have been prepared by Sol-gel technique. The structural study and redox titration suggest that the samples are both La and O deficient. The generated atomic models using Rietveld refined structural parameters show octahedral distortion in the double perovskite structure. X-ray photoelectron spectroscopy (XPS) study has been used to determine the predominant valence states of Ni, Mn and Co ions. LNCx MO (x = 0.2, 0.4) are found to be Heisenberg ferromagnets whereas LNC0.8 MO is a phase segregated system. In LNC0.4 MO and LNC0.8 MO, at temperatures above TC , χ−1 (T) curve shows the characteristics of Griffiths phase. The study of ac susceptibility and isothermal remanent magnetization (IRM) discard the possibility of any glassy state at low temperatures. The antiphase boundaries (APBs), antisite disorders (ASDs) and oxygen vacancies play important roles in this system. Keywords: A. oxides, A. magnetic materials, B. sol-gel chemistry, B. magnetic properties D. crystal structure

∗

Corresponding author Email address: [email protected] (S. Mukherjee)

Preprint submitted to Materials Research Bulletin

January 19, 2018

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1. Introduction

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Rare-earth based double perovskites show various interesting properties such as ferromagnetism, semiconducting behavior, magneto dielectric effect, magneto resistance, luminescences, etc. [1, 2, 3, 4, 5, 6]. The double perovskite, ferromagnetic (FM) semiconductors: La2 NiMnO6 (LNMO) and La2 CoMnO6 (LCMO), having comparatively higher FM ordering temperatures (TC ∼ 280 K for LNMO [1] and TC ∼ 226 K for LCMO [2]) are considered to be promising materials for spintronic devices. LNMO and LCMO also show considerable magnetodielectric effect [1, 3] and magnetoresistance [4, 5]. Such materials whose electric, magnetic and dielectric properties can be tuned by applying suitable electric and/or magnetic fields are advantageous from the point of view of commercial applications. Both LNMO and LCMO have been studied extensively in the past. In LNMO, it is generally accepted that the oxidation state of the metal ions are predominantly Ni2+ and Mn4+ with a small fraction of coexisting Ni3+ and Mn3+ ions, and the metal ions are usually ordered on the B/B0 -site sublattice in A2 BB0 O6 type double perovskites [7, 8, 9, 10]. The perfect ordering gives a value of the saturation magnetization (MS ) at low temperatures close to 5µB /f.u. [1]. The ferromagnetism arises due to the superexchange interaction between the ordered Ni2+ and Mn4+ ions [11]. On the other hand, the compound LCMO with completely ordered Mn4+ and Co2+ ions is a FM one with TC ∼ 226 K and high MS value at low temperatures [2]. The value of TC depends upon the type of B-site cations, and the ordering of the B and B0 -site sublattice in A2 BB0 O6 type double perovskites. For bulk LNMO and LCMO, the value of M at high magnetic field(∼ 5 - 7 T) and low temperature (5 K) has been found to be lower than the theoretical value of MS [2, 11]. Different factors like presence of anti phase bounderies (APBs) [12], anti site disorders (ASDs) [11] and oxygen vacancies [2] have been pointed out by different groups as responsible for such lowering. However, no definite conclusion has been reached yet. A downward deviation in χ−1 (T) for low H, and an upward deviation in the same for high H have been reported for polycrystalline LNMO [12]. Such a behavior is usually considered as a signature of the existence of short-range correlated clusters at temperatures above TC . However, the origin of such short-range FM ordering is not clear yet. Through the study of structural and magnetic properties of La2 Ni1−x Cox MnO6 (x = 0.2, 0.4, 0.8), this article addresses several important points. The article 2

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explores the dependence of TC and the nature of FM interaction on the type of B-site ions in the mixed double perovskite system. The existence of the clustered state at T ≥ TC for LNMO, and the lowering of the value of MS at low temperatures for both LNMO and LCMO have been reported earlier [2, 11, 12]. However, the present study clearly shows that the short-range FM ordering at T ≥ TC is induced by ASDs, and the lowering of the value of MS is also due to ASDs. It may be mentioned that a study of the magnetic properties of LNCx MO nano particles have been reported [13]. However, their observations regarding short range FM ordering do not match with this study for the bulk sample. Although, the existence of low temperature spin glass (SG) phase has been predicted from the large difference between MF C (magnetization measured under field cooled condition) and MZF C (magnetization measured under zero field cooled condition) for both LNMO [4] and LCMO [14], the present study of ac susceptibility as a function of temperature and the relaxation behavior of isothermal remanent magnetization (IRM)discard the existence of any such glassy state in this system. 2. Experimental

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A number of compounds LNCx MO (x = 0.2, 0.4, 0.8) were prepared by Sol-gel technique. Stoichiometric amount of high purity La2 O3 powder, preheated at 950◦ C for 24 hours, stoichiometric amounts of Co & Ni powders, and same of MnO2 with oxalic acid were dissolved in dilute HNO3 to have their respective nitrates. The rare earth oxides like La2 O3 usually absorb moisture from air. Therefore, after a few days, the material transforms into a mixture of La2 O3 , LaOOH and even La(OH)3 . Annealing removes the water and the stoichiometric oxide is obtained. After mixing all the nitrate solutions, citric acid was added to the mixture in the proportion of 2.6 × (mol.wt.of citric acid) × (wt.of the sample in g). Then the mixture was stirred (mol.wt.of the sample) well for 1 hr, followed by evaporation at 80◦ C that resulted into gel formation. After proper heat treatment at 250◦ C, the obtained dark brown powder was ground, pelletized and sintered at 600◦ C for 5 hours. The gel was heated to 250◦ C to remove the organic matter and to decompose the nitrates. The heat treatment at 600◦ C for 5 hours removed the residual organic matter. Finally, the pellets were sintered at 1250◦ C for 24 hours to obtain stable crystal structure with larger grain size. Structural characterization of the compounds LNCx MO (x = 0.2, 0.4, 0.8) were done by analyzing the room temperature powder x-ray diffraction 3

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(XRD) pattern of the samples using Cu Kα radiation (λ = 1.5418 ˚ A) in TTRAX III diffractometer (M/S Rigaku Corp., Japan), with a rotatinganode x-ray source operated at 9 kW(60 kV, 150 mA). The XRD data were recorded using solid state detector at 0.02◦ interval with a scan speed of 10◦ min−1 . Prior to the recording of XRD data, the diffractometer was calibrated using the standard silicon sample supplied by the manufacturer of the diffractometer. Reitveld structure refinement was employed for analysis [15] by using MAUD software [16]. The structural models were prepared using ATOM software. Oxygen stoichiometry of each of the sample was determined by redox titration, using potassium permanganate (KMnO4 ) and ferrous ammonium sulphate (FAS) solutions. Initially, 0.02 M solutions of oxalic acid, potassium permanganate and FAS were prepared. Potassium permanganate was standardized against standard oxalic acid solution, and then FAS was standardized against the permanganate solution. The compound was accurately weighed (about 20 mg), ground well and dissolved in excess amount of FAS solution taken (20 ml). This solution was titrated against KMnO4 , and the titre value corresponded to the amount of Fe2+ unreacted with the compound. Oxygen stoichiometry was calculated from the difference between the theoretical value of Fe2+ required, and the experimental value of the amount of Fe2+ reacted. XPS core-level spectra were taken with an Omicron Multiprobe (Omicron NanoTechnology GmbH., UK) spectrometer fitted with an EA125 hemispherical analyzer. A monochromated Al Kα X-ray source operating at 150W was used for the experiments. The analyzer pass energy was kept fixed at 40 eV for all the scans. As the samples are insulating in nature, a low energy electron gun (SL1000, Omicron) with a large spot size was used to neutralize the samples. The potential of the electron gun was kept fixed at -3 eV for all the samples with respect to the ground. The binding energy of the peaks was corrected by shifting the peak positions by an equal amount that was required to shift the main peak of the corresponding C1s spectrum to 285.0 eV. Magnetization (M) was measured using SQUID VSM (Quantum Design Inc., USA) from 4K to 380K.

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(2 0 0 )

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Figure 1: [left panel] Rietveld refined XRD patterns of LNCx MO (x = 0.2, 0.4, 0.8) compounds at room temperature. Experimental data (Iobs ), simulated data (Ical ) and the difference between Iobs and Ical are indicated by symbol (o), black solid lines, and blue solid lines respectively. Peak positions of the corresponding phase are marked by green vertical lines. [right panel] Zoomed in portion around the main diffraction peak.

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3. Results and Discussion

3.1. Structure The Rietveld refined XRD patterns of LNCx MO (x = 0.2, 0.4, 0.8) compounds at room temperature are shown in Fig. 1 [left panel]. The XRD patterns have been simulated with ICSD phase number 98239 (monoclinic, space group P21 /n), and the refined lattice parameters of these compounds are tabulated in Table 1. Similar refinement in the monoclinic structure with space group P21 /n has been done for other double perovskite oxides [17, 18]. All the compositions formed in single monoclinic phase, and the distinct reflections are indexed accordingly. The portion around the highest intensity peak (Fig. 1 [right panel]) has been zoomed in to show that the samples really form in single phase. The sample La2 NiMnO6.05 , prepared using Sol-gel technique after firing at 1350◦ C in Ar has been reported [11] to form in monoclinic phase (space 5

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Table 1: Unit cell parameters and La site vacancy (∆)

a b c β v ˚) (A (˚ A) (˚ A) (◦ ) (˚ A3 ) LNC0.2 MO 5.4603(8) 5.5122(3) 7.7518(6) 89.862(4) 233.315 LNC0.4 MO 5.4690(8) 5.5154(6) 7.7553(3) 89.894(6) 233.928 LNC0.8 MO 5.4748(7) 5.5189(5) 7.7601(9) 89.954(2) 234.470

La site vacancy (∆) 0.574 0.576 0.602

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Sample

x = 0.4

x = 0.8

-0.013 0.0142 0.2490

0.0041 0.0167 0.2472

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Atomic Position x = 0.2 La1 : (x,y,z) x 0.0080 y 0.0108 z 0.2447 Co1 : (0,0.5,0) Co2 : (0.5,0,0) Ni1 : (0,0.5,0) Ni2 : (0.5,0,0) Mn1 :(0,0.5,0) Mn2 :(0.5,0,0) O1 : (x, y, z) x 0.2449 y 0.2542 z 0.0024 O2 : (x, y, z) x 0.2938 y 0.2751 z 0.4788 O3 : (x, y, z) x 0.6057 y 0.0944 z 0.2481 Rexp 5.4592 Rw 7.5633 GoF 1.3854

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Table 2: New sets of refined atomic co-ordinates with reliability factors of the refinement method for LNCx MO (x = 0.2, 0.4, 0.8)

0.2533 0.2443 0

0.2492 0.2435 0.0027

0.2766 0.2764 0.4922

0.2800 0.3165 0.4856

0.6708 0.0855 0.2420 5.4061 6.4401 1.1913

0.6504 0.0897 0.2659 5.3647 5.5113 1.0273

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Figure 2: The generated atomic models of LNCx MO (x = 0.2, 0.4, 0.8), based on the refined structural parameters.

group P21 /n) with ordered arrangement of Ni2+ and Mn4+ ions into distinguishable sites. Table 1 shows that the lattice parameters and lattice volume of the minimum Co-substituted sample LNC0.2 MO are close to the reported values in reference 11. Moreover, neutron diffraction study [19] on bulk stoichiometric LCMO suggests that the compound also possesses monoclinic symmetry (P21 /n) for long range ordering of Co and Mn cations over two distinct sites. From Table 1, it is also evident that with increasing x, all lattice parameters as well as the unit cell volume increase gradually. As the ionic radius of Co3+ (0.61 ˚ A) is less than that of Ni2+ (0.69 ˚ A), and 2+ 2+ ˚ ˚ that of Co (0.74 A) is greater than that of Ni (0.69 A), one may conclude that the continuous increment in lattice parameters and cell volume with increasing Co-substitution suggests substitution of Ni2+ by Co2+ ions in the double perovskite monoclinic lattice. Therefore, the system LNCx MO 7

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Figure 3: Different Mn-O-Mn bond angles of the sample LNC0.2 MO. Bond angle Mn1O2-Mn2 shows octahedral distortion.

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contains ordered distribution of Ni2+ /Co2+ and Mn4+ ions.

Figure 4: Different Mn-O bond lengths for the sample LNC0.2 MO.

Rietveld refinement (Table 1) reveals that all of the Co substituted samples, sintered at 1250◦ C in air are significantly La-deficient. The vacancies on the La-sites have also been observed earlier [11], and it has been suggested that the extruded La forms amorphous La2 O3 in presence of excess O2 , and hence cannot be detected by XRD technique. As Ni2+ and Co2+ ions are of unequal ionic radii, in the monoclinic double perovskite lattices of these compounds different cations and anions are 8

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displaced from their primary atomic positions, and achieve their new equilibrium positions. This causes distortion of the primary monoclinic lattice, and the associated modification of the relative intensity distributions of individual reflections in the XRD pattern. The Rietveld analysis can also account for such change in intensities by refining the respective atomic coordinates of particularly La and O atoms (only variable coordinates) in the lattice. All these new sets of refined atomic coordinates for respective compounds with reliability factors of the refinement method are tabulated in Table 2. Table 3: Different bond lengths, average bond length and bond angles between two octahedra of the compounds LNCx MO (x = 0.2, 0.4, 0.8)

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LNC0.4 MO 1.98 1.91 2.12 1.84 2.05 2.28 2.03

1.91 1.98 1.90 2.04 2.10 2.08 2.00

167.4

166.0

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Compounds LNC0.2 MO Bond length (˚ A) Mn-O1 (×2) 1.90 Mn-O1 (×2) 1.98 Mn-O2 (×2) 1.90 Mn-O2 (×2) 2.03 Mn-O3 (×2) 2.10 Mn-O3 (×2) 2.08 1.99 Bond Angle (o ) Mn-O2-Mn 161.6

Table 4: Oxygen stoichiometry of LNCx MO (x = 0.2, 0.4, 0.8) compounds estimated by redox titration

Sample La1.4 Ni0.8 Co0.2 MnO6−δ La1.4 Ni0.6 Co0.4 MnO6−δ La1.4 Ni0.2 Co0.8 MnO6−δ

δ1 1.08 1.11 1.09

δ2 1.14 1.15 1.15

δ3 Average 1.15 1.13 1.13 1.13 1.12 1.12

The generated atomic models of LNCx MO (x = 0.2, 0.4, 0.8), based on the refined structural parameters have been shown in Fig. 2. The structure is a double perovskite one, in which La, M (Mn/Co/Ni) and O atoms occupy respectively, the corner, the body centre and the face centre positions of the distorted cubic (monoclinic) lattice respectively. M atoms occupy the metal centre of the MO6 octahedra. In an ideal double perovskite structure with cubic symmetry, M -O- M bond angle is 180◦ (linear), and M - O bond 9

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(b)

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Counts ( × 10 )

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lengths are equal in all directions. But in a similar structure with monoclinic symmetry, M -O- M bond angles are well deviated from 180◦ , and thereby introduce an octahedral distortion in the compound. With changing Co:Ni ratios, the resultant M -O- M bond angle changes with a subsequent change in octahedral distortion. The bond angles in LNC0.2 MO are shown in Fig. 3 and the various bond lengths are depicted in Fig. 4. Different bond lengths, average bond length and bond angles are shown in Table 3. Increase in MnO-Mn bond angle towards 180◦ indicates better symmetry in the compound. Comparatively higher Co percentage corresponds to a better symmetry in the compound. The monoclinic angle β increases and moves towards 90◦ with increase in Co percentage as given in Table 1. The oxygen stoichiometry of individual sample, estimated by redox titration has been shown in Table 4.

638

642

646

650

Binding energy (eV)

775

780

785

Binding energy (eV)

Figure 5: XPS spectra of (a) Ni 2p3/2 , (b) Mn 2p3/2 , and (c) Co 2p3/2 for the sample LC0.4 MO.

The XPS spectra of Ni 2p3/2 from LNC0.4 MO compound is shown in Fig. 5(a). The experimental pattern is shown by the black hollow circles and the fitted pattern is shown by the black line through the experimental data points. The fitted pattern is deconvulated with four Gaussian peaks. Ni2+ peak is observed at 856 eV, and this is marked by blue curve. The Gaussian peak at 850 eV (marked with red colour) is from La 3d3/2 contribution. The two deconvulated peaks at 860 and 866 eV are satellite peaks of Ni2+ . No signature of the presence of Ni3+ is detected from the XPS spectra.

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The XPS spectrum of Mn 2p3/2 is shown in Fig. 5(b). The experimental asymmetric peak is deconvulated with two Gaussian peaks. The peak marked by red colour is from Mn4+ reflection at 642 eV. No peak at energy lower than 642 eV can be seen which rules out the presence of Mn+2 or Mn+3 . The blue coloured peak at 643 eV is the asymmetric tail of Mn+4 peak. The XPS spectrum of Co 2p3/2 from LNC0.4 MO compound is shown in Fig. 5(c). The fitted pattern to the experimental data points is shown by the black line. The pattern is deconvulated with two Gaussian peaks. The peak marked by red colour is from Co2+ reflection at 780.5 eV. The blue coloured peak at 782.5 eV is the asymmetric tail of Co2+ peak. No other reflection from other Co valencies is observed which confirms the sole presence of Co2+ . Thus the XPS patterns presented in Fig. 5 confirms that Ni2+ , Mn4+ and Co2+ valencies are predominantly present in the system.

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3.2. Magnetism The ordered matrix of Ni2+ and Mn4+ ions in monoclininc (P21 /n) LNMO becomes ferromagnetic due to superexchange interactions between Ni2+ and Mn4+ ions in accordance with the Goodenough-Kanamori rule [20]. TC lies in between 270 K and 280 K. La-vacancy does not affect TC . Both the stoichiometric (La2 NiMnO6.05 ) and slightly La-deficient (La1.9 NiMnO5.97 and La1.92 NiMnO6.03 ) LNMO samples, all being in the monoclinic phase (P21 /n), have the same TC (∼ 275 K) [11]. In comparison with LNMO, the atomically ordered matrix of Co2+ and Mn4+ ions in monoclinic (P21 /n) lattice, LCMO has a lower TC ∼ 226 K [2]. TC is a measure of the strength of FM coupling. Therefore, the strength of the FM coupling arising out of Ni2+ -O- Mn4+ superexchange is higher than that of Co2+ -O- Mn4+ superexchange. Fig. 6 shows the behavior of the magnetization (M) of three samples LNCx MO (x = 0.2, 0.4, 0.8) measured as a function of temperature (T) at a magnetic field H = 100 Oe, under zero field cooled (ZFC) and field cooled (FC) conditions. For LNC0.2 MO, the enhancement of M both under ZFC and FC conditions with the lowering of T is due to the onset of FM ordering. TC has been estimated as 256 K, the temperature corresponding to the peak of the −dM/dT versus T curve (inset of Fig. 6). The substitution of 20% of Ni2+ ions by Co2+ ions in bulk LNMO reduces TC by almost 20 K. LNCx MO has a double perovskite structure with two different kinds of B ions (Ni/Co). In case of uniform distribution of Ni2+ /Co2+ ions, a fraction of the Ni2+ -OMn4+ block will be replaced by Co2+ -O- Mn4+ block. As the FM coupling 11

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Figure 6: Magnetization (M) as a function of temperature (T) for LNCx MO (x = 0.2, 0.4, 0.8). Open symbols show M measured under ZFC condition, and filled symbols show M measured under FC condition.The solid (red) lines show the fitted curves M = M0 (TC T)β . The inset in each frame shows the derivative (-dM/dT) as a function of T.

strength of the Co2+ -O- Mn4+ block is lower than that of the Ni2+ -OMn4+ block, substitution of Ni2+ ions by Co2+ ions will reduce the overall TC of the system. On the other hand, if Co2+ ions congregate in a particular structural domain, an inhomogeneous magnetic system consisting of two different FM phases having different ordering temperatures (TC ) would have been obtained. M (T) behavior of LNC0.2 MO with reduced TC (256 K) suggests a uniform distribution of Ni2+ /Co2+ ions in the compound. For bulk monoclinic LNMO or LCMO, usually B and B0 ions are ordered. However, there exists structural domains with identical boundary atoms (either Ni/Co or Mn). The interface between different such domains is known as APB. APBs contain either Ni2+ -O- Ni2+ or Mn4+ -O- Mn4+ bonds, and ac12

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Figure 7: [Left Panel] shows M of LNC0.2 MO as a function of T for different measuring fields (H). Open symbols show M measured under ZFC condition, and filled symbols show M measured under FC condition. M(T) measured at H = 100 Oe has been plotted using right axis. [Right Panel] shows the inverse susceptibility χ−1 of LNCx MO (x = 0.2, 0.4, 0.8) as a function of T for H = 100 Oe. Both ZFC data (open symbols) and FC data (filled symbol) have been plotted.

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cording to Goodenough-Kanamori rule [20], the superexchange interactions in both these cases are antiferromagnetic (AFM). Two adjacent FM blocks across APBs couple antiferromagnetically in absence of any magnetic field. On the other hand, ASDs in an otherwise ordered matrix couple neighboring spins antiferromagnetically. APBs and ASDs play significant roles in determining the magnetic behavior of bulk LNMO/LCMO samples. In the bulk sample, due to the AFM interactions across APBs, adjoining FM blocks get pinned in presence of zero or sufficiently low magnetic field. MZF C (T) behavior of LNC0.2 MO, below TC , can be explained by the thermally assisted de-pinning of such FM blocks in presence of a low magnetic field H = 100 Oe. In the FC state, below TC , FM blocks remain aligned. As the measuring field is increased more and more, ZFC curve approaches the FC curve and finally merges with it (Fig. 7 [left panel]). For LNC0.2 MO with TC = 256 K, the FM region of the FC data (Fig. 6) can be well described by the power law M = M0 (TC - T)β with β = 0.26. The value of the exponent β is close to the Heisenberg exponent β = 0.365 than the mean field value β = 0.5. The values of the critical exponents give an idea of the range of the exchange interaction J(r) [21]. According to the renormalization group analysis [22], for systems with J(r) = 1/r(d+σ) (d is the

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Θ (K) 263 247 209

TC (K) 256 239 184

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µtheo (µB /f.u.) 4.94 5.08 5.35

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µef f = 8C (µB /f.u.) LNC0.2 MO 5.38 LNC0.4 MO 5.25 LNC0.8 MO 5.82 Sample

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Table 5: The values of the effective magnetic moment per formula unit (µef f ), theocratical value (µtheo ), the paramagnetic Curie temperature (Θ) and TC of LNCx MO (x = 0.2, 0.4, 0.8) compounds √

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M

an

dimension of the system, σ is the range of the interaction), the Heisenberg exponents (β = 0.365, γ = 1.386, and δ = 4.8) are valid for σ > 2. So LNC0.2 MO is more Heisenberg-like rather than the mean field one. It may also be noted that similar fitting of MF C (T) data of bulk LNMO [23] gives β = 0.31, and the study of the critical behavior of LNMO [24] also suggests the system to be a 3D Heisenberg ferromagnet with short range interactions. On increasing the Co substitution to 40% in LNMO (LNC0.4 MO), TC further decreases to 239 K which is lower than that of LNC0.2 MO by 17 K. The value of the exponent β(0.32) obtained by fitting MF C (T) curve for LNC0.4 MO is again close to the Heisenberg value. So LNC0.4 MO is identical to LNC0.2 MO, but with more Ni2+ -O- Mn4+ units replaced by Co2+ -OMn4+ units, and such replacements are random. In both, LNC0.2 MO and LNC0.4 MO, MF C (T) becomes separated from MZF C (T) below the temperature corresponding to the peak of the MZF C (T) curve whereas for LNC0.8 MO, the temperature (Tir ) at which the same irreversibility starts is higher than the peak temperature of the MZF C (T) curve. It may also be noted that Tir (230 K) for LNC0.8 MO is close to the TC (226 K) of bulk LCMO. This suggests the existence of short range correlated clusters containing only Co2+ and Mn4+ ions in addition to FM blocks containing Ni2+ , Co2+ and Mn4+ ions. The FC curve for such phase segregated system could not be fitted by the power law M = M0 (TC - T)β . Fig. 7 [Right Panel] shows the plot of χ−1 (T) for all three samples, obtained from M (M = χH) measured at H = 100 Oe under both the ZFC and FC conditions. The high temperature (T > 300 K) paramagnetic region of each χ−1 (T) curve has been fitted with Curie Weiss (CW) law χ−1 = (T - Θ)/C. The values of the effective magnetic moment per formula unit (µef f ) and the paramagnetic Curie temperature (Θ) estimated from each fit are shown in Table 5. The values of µtheo in Table 5 for 14

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-6

0

3

6 4

2 1

3

0

2

1

T=4K

1

-2 -1

0 H(T)

1

0

0

LNC0.2MO

us

M (µΒ/f.u. )

-1

cr

2

ip t

3

-3

M (µΒ/f.u.)

4

4

LNC0.4MO

-1

-1

M (µΒ/f.u.)

LNC0.8MO

an

2

-2 -3

0

-4 -6

M

0

-3

0

3 H(T)

-2 -3

6

-4 3

6

d

H(T)

te

Figure 8: M as a function of H for LNCx MO (x = 0.2, 0.4, 0.8) at T = 4 K. The lower inset shows the virgin curves, and the upper inset shows the central regions of the M-H curves.

Ac ce p

the samples LNCx MO (x = 0.2, 0.4, 0.8) have been estimated using for1/2 mula µtheo = [µM n 2 + x ∗ (µCo 2 ) + (1 − x) ∗ (µN i 2 )] , using spin only value µM 2 = 4SM (SM + 1) [M = Ni2+ /Co2+ /Mn4+ , SNi2+ = 1, SCo2+ = 3/2, and SMn4+ = 3/2]. For each sample, paramagnetic Weiss constant Θ is greater than TC , and this is a characteristic of a FM system. The values of µtheo for all the samples, assuming Ni/Co ions in 2+ state, and Mn ions in 4+ state are less than the experimental values. The mismatch may indicate that some of the M ions are in 3+ state. M(H) curves of all three samples LNCx MO (x = 0.2, 0.4, 0.8) at T = 4 K have been shown in Fig. 8. The virgin curves have been shown in the lower inset of Fig. 8. Saturation has not been attained for any of the samples even at H = 7 T. M (T = 4 K, H = 7 T), remanent magnetization (Mr ) and coercive field (HC ) for LNCx MO have been given in Table 6. For LNC0.2 MO, if one considers the perfect ordering of B (Ni2+ (t62g e2g , S 15

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LNC0.2 MO 3.74

LNC0.4 MO 3.67

LNC0.8 MO 3.99

Mtheo (µB /f.u.)

5.2

5.4

∆M (µB /f.u.)

1.46

1.73

1.81

Mr (µB /f.u.)

1.40

1.45

1.60

Hc (T )

0.17

1.04

cr

Sample M (4 K, 7 T) (µB /f.u.)

an

ip t

Table 6: M (T = 4 K, H = 7 T), theoretical value Mtheo , ∆M = Mtheo - M (T = 4 K, H = 7 T), remanent magnetization (Mr ) and coercive field (Hc ) for LNCx MO (x = 0.2, 0.4, 0.8)

us

5.8

M

0.40

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te

d

= 1)/Co2+ (t52g e2g , S = 3/2)) and B 0 (Mn4+ (t32g e0g , S = 3/2)) cations, one can expect theoretically MS (Mtheo ) to be 5.2 µB . The spin polarized calculations [25] for LNMO and LCMO estimate the local magnetic moments for Ni, Co and Mn ions as µN i = 1.73 µB , µCo = 2.54 µB , and µM n ≈ 3.20 µB . If the local moments are considered to be the same for LNC0.2 MO, the estimated MS (Mest ) would be 5.1 µB . For LNC0.2 MO, the MS value calculated by extrapolating the virgin curve to 1/H = 0 has been found to be 3.83 µB . This calculated MS value as well as the observed value of M (4 K, 7 T) (3.74 µB /f.u.) is lower than both Mtheo and Mest . For bulk LNMO samples, different MS values have been reported by different groups at low temperatures [1, 11]. For LNMO prepared by solid state synthesis, MS (5 K) has been found as 4.96 µB /f.u. [1]. This value is close to the theoretical value of 5 µB /f.u., suggesting a perfect ordering of Ni2+ and Mn4+ ions. Magnetic saturation has never been attained at low temperatures even at a field of H = 5 T for any LNMO sample prepared by Sol-gel synthesis, and the value of M (5 K, 5T) is ≤ 4.05 µB /f.u. [11] whereas the same sample prepared by the same group using solid-state route shows saturation with MS = 4.31 µB /f.u [11]. The low value of MS has been explained by the presence of ASDs [11]. This suggests that the ASDs are more probable in samples prepared by Sol-gel technique. The study on thin films of LNMO [23] and the study on bulk LCMO [2] suggest that the oxygen vacancies may also lower the value of MS . The oxygen vacancies help transfer of an electron from the e 16

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band of Ni2+ (Co2+ ) ions to neighboring Mn4+ ions. As a result, there will be Ni3+ (Co3+ ) and Mn3+ ions. Thus the long-range ordering of Ni2+ (Co2+ ) and Mn4+ ions is interrupted [2]. This will also reduce the value of MS . Following the discussion for LNMO and LCMO, the lowering of MS may be due to any or both of the two reasons: i) presence of ASDs, ii) oxygen deficiency. All the studied samples have oxygen vacancies to a large extent, and such vacancies will definitely lower the value of MS . Moreover, although the δ remains almost the same for all the studied samples, the difference 4M = Mtheo − M (4 K, 7 T) (Table 6) increases with x. This can only be explained if it is admitted that the ASDs play a role in lowering of MS , and their concentration increases with increasing concentration of Co2+ ions. For LNC0.2 MO, Mr and HC values are respectively 1.40 µB /f.u. and 0.17 T. These values increase with increasing x (Table 6). At low temperatures (≤ 8 K), bulk [11] as well as epitaxial film samples [23] of LNMO show low values of Mr (≤ 1µB /f.u.) and HC (≤ 400 Oe), whereas LCMO shows a higher value of Mr (2.90 µB /f.u.) and HC (5520 Oe)[2]. Higher values of Mr and HC of LCMO compared to LNMO may be associated with the large crystalline anisotropy of the octahedral-site Co2+ ions. This also explains the increasing values of Mr and HC with increasing x. For LNC0.2 MO, χ−1 (T) behavior obeys CW law down to TC (Fig. 7 [Right Panel]). LNC0.4 MO and LNC0.8 MO samples obey CW behavior above T∗ = 280 K (Fig. 7 [Right Panel]). In the temperature range TC < T < T∗ at a low H = 100 Oe, a downward deviation of χ−1 (T) from ideal CW behavior is observed for both the samples. With the increase of measuring field, a decrease in downturn is observed, and finally at sufficiently high field the deviation becomes an upward one (Fig. 9). A downward deviation of χ−1 (T) above TC has been observed in a number of oxide systems [12, 26, 27]. Such an observation is usually associated with the presence short-range correlated clusters at temperatures above TC . Above TC the low field value of χ enhances due to these clusters, hence there is a downward deviation of χ−1 (T). To find the origin of such clusters, different models have been proposed. According to Griffith’s model [28], the long-range ordering temperature TC (x) (x is the dilution) of a randomly diluted ferromagnet will be lower than the TC of the pure ferromagnet (TPC ). The Griffiths phase (GP) within the temperature region TC < T < TPC , contains small FM clusters. TPC is also known as the Griffiths temperature TG . The observation of GP is possible in systems having random quenched disorder that can randomly dilute the 17

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Figure 9: The inverse susceptibility χ−1 of LNCx MO (x = 0.4, 0.8) as a function of T for different H. Both ZFC data (symbols) and FC data (solid lines) have been plotted. The insets show the lower portion of χ−1 (T) curves for two samples separately.

exchange interaction (J). The existence of GP has been proposed in the doped manganite La1−x Srx MnO3 (0.07 ≤ x ≤ 0.16) to explain the downturn of χ−1 (T) from the ideal CW behavior [26]. The same phase has been identified with the help of ESR spectra, showing the presence of FM resonance (FMR) signal in addition to the PM signal due to Mn3+ and Mn4+ spins at a temperature above TC . There, the random disorder has been associated with the random substitution of La3+ ions by Sr2+ ions, and such random disorder enhances the probability of a FM bond. However, the GP is not the only model which can explain the clustered state. A downturn of χ−1 (T) below a characteristic temperature TG , in the Cobaltite La1−x Sr1+x CoO4 [29] and Nd1−x Sr1+x CoO4 18

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[30] has been explained by the presence of the FM clusters, although the existence of GP is not very clear. Like LNC0.4 MO and LNC0.8 MO, a downward deviation for low H and an upward deviation for high H in χ−1 (T) has been reported for polycrystalline LNMO also [12]. However, they excluded the possible existence of GP on the basis of the results of their ESR measurement. They did not get any trace of FM signals in ESR measurement in the temperature range TC < T < T∗ . They considered the relative orientations of the FM blocks across APBs depending upon the magnitude of the applied magnetic field, responsible for the behaviour of χ−1 (T) in the range TC < T < T∗ . According to them, at a lower field, the AFM interactions between FM blocks across APBs result in a residual magnetization for FM blocks of unequal volumes. This causes an enhancement of χ compared to the paramagnetic value, and an associated downward deviation of χ−1 (T) is observed at low field. They also suggest that the susceptibility will decrease at a higher field when the FM blocks get aligned. However, for the same polycrystalline LNMO, another group [27] observed an upward deviation from the CW law in inverse susceptibility above TC for a low as well as a high measuring field. Such an upward deviation has also been explained by the AFM interactions between FM blocks across APBs. Similar upturn in χ−1 (T) has been observed in Cobaltite La1−x Srx CoO3 [31] also, where the AFM interactions between FM clusters are held responsible for such deviation. In reference 27, a downward deviation from the CW behavior in χ−1 (T) has been reported for polycrystalline LNMO when the grain size reduces to nanometer scale. Such a downward deviation has been suggested to occur due to the short-range ordered FM clusters. The presence of short-range FM correlations in a temperature range above TC (280 K) in bulk LNMO and the same in epitaxial thin films of LNMO have been established by ESR study [32] and XMCD (x-ray magnetic circular dichroism) signals [23]. Such short-range FM correlation has been predicted to be induced by antisite defects against long-range ordering of the Ni/Mn sublattice. In the present study, no downward or upward deviation at low or high H in LNC0.2 MO is observed. It is detectable in LNC0.4 MO, and it is more prominent in LNC0.8 MO. For LNC0.4 MO and LNC0.8 MO, the deviation is downward at low H, and becomes upward at high H. Thus the deviation increases with increasing Co-concentration. In the present work it has also been observed that the concentration of ASDs increases with increasing Co-concentration. According to the proposition given in reference 27, the 19

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1 .0 3

M O

T = 1 0 0 K

1 .0 1

0 .4

M O

T = 1 0 0 K

1 .0 0 1 .0 3

an

1 .0 4

M /M 4 0 0 0

T = 2 0 0 K

4 0 0 0

1 .0 3 T = 2 0 0 K

M /M

1 .0 2

1 .0 1 1 .0 0

1 .0 0 0

1 0 0 0

2 0 0 0

3 0 0 0

t (s) 1 .0 1 2

L N C

4 0 0 0

0 .8

0

4 0 0 0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

t (s) 1 k H z

M O

d

(c )

1 .0 0 8

(d )

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1 .0 0 0 1 .0 4

4 0 0 0

3 3 3 H z

c ’ (A r b . U n it)

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T = 4 K

1 .0 0 4

M /M

1 .0 2

M

1 .0 1

M /M

1 .0 2 1 .0 1

1 .0 0

L N C

(b )

us

1 .0 2

0 .2

4 0 0 0

L N C

(a )

M /M

M /M 4 0 0 0

1 .0 3

cr

ip t

FM clustered state in LNMO is induced by ASDs against long-range FM ordering of the Ni/Mn sublattice. Therefore, for higher Co concentration there will be FM clusters. They are responsible for the downward deviation of χ−1 (T) at low H. As the field is increased, FM clusters are aligned, and a decrease in susceptibility is observed.

3 3 H z

T = 1 0 0 K 3 .3 H z

1 .0 2

L N C

0 .2

M O

1 .0 0

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

1 5 0

2 1 0

2 7 0

T (K )

t (s)

Figure 10: Relative M measured as a function of time t for (a) LNC0.2 MO, (b) LNC0.4 MO, (c) LNC0.8 MO at different temperatures. (d) shows the in-phase part of the ac susceptibility (χ0 ) as a function of T of LNC0.2 MO measured at an ac field of 1 Oe and different frequencies.

A large difference between MZF C and MF C has been observed in both LNMO [4] and LCMO [14]. Such difference has been pointed out as the typical characteristics of the glassy state [14]. Since the present system shows a 20

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large difference between MF C and MZF C , to observe any existence of SG like phase below TC , ac susceptibility measurement and relaxation experiments have been carried out. In order to investigate the nature of the state below TC , the IRM as a function of time (t) has been measured at two different temperatures (below TC ) for all three samples (LNCx MO). The measurement has been carried out after cooling the sample in zero field (ZFC), followed by cycling an external field 0 -1 kOe- 0 (Fig. 10(a), (b), (c)), and the measurement starts with the switching off of the external field i.e., the waiting time tw is negligibly small. The total time of observation (tobs ) is fixed to 4000 s. IRM relaxes in the same way in all three samples following the functional form M/M4000 = A0 + A1 e−t/τ1 + A2 e−t/τ2 + A3 e−t/τ3 , where the respective values of τ1 , τ2 , and τ3 are 20 s, 200 s and 2000 s respectively. Various functional forms have been proposed to describe the IRM as a function of time (t). One of the most popular form is the stretched exponential [33] M(t) = M0 exp[−(t/τ )β ] [M0 = M(t = 0), τ is a characteristic relaxation time, 0 < β < 1]. The stretched exponential relaxation of various physical quantities have been observed in different systems and areas of research. Such behavior is explained to originate from the exponential relaxation of many independently relaxing components having different relaxation times [33]. The system LNCx MO also shows similar behavior i.e., the system contains independent relaxing components having different relaxation times. As tobs is 4000 s, the relaxing components having relaxation time an order magnitude greater than 103 s could not be identified. Thus, in LNCx MO the magnetic state of the sample below TC is a non-equilibrium one, and it approaches equilibrium via relaxation of different components. The minimum value of τ observed is 20 s. Such multiple relaxing components may be associated with different kinds of pinning centers like APBs, ASDs and oxygen vacancies. One more interesting point is, the relaxation spectra does not change with temperature. Such relaxation mechanism is different from SG systems where the relaxation time increases with the lowering of temperature. For glassy systems obeying stretched exponential relaxation M(t) = M0 exp[−(t/τ )β ], τ depends upon T. This is further supported by the measurement of ac susceptibility. The ac susceptibility measurement (Fig. 10(d)) does not show any peak beyond TC , and the peak corresponding to TC does not have any shift with frequency.

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4. Conclusions

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Acknowledgment

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In the present studies structural and magnetic properties of LNCx MO (x = 0.2, 0.4, 0.8) compounds have been investigated. The structural analysis suggests the presence of an octahedral distortion in these double perovskite compounds. Such octahedral distortion changes with changing Co:Ni ratios. The XPS core-level spectra suggests that the Ni, Mn and Co ions are predominantly in the 2+ , 4+ and 2+ states respectively. However, as the estimated values of µtheo assuming Ni/Co ions in 2+ state are less than the experimental values, a few Ni/Co ions may be in the 3+ state. The magnetic study suggests that the Co2+ substitution replaces Ni2+ ions uniformly, and reduces the TC upto 40% Co2+ substitution. In 80% Co2+ ion substituted samples, phase segregation is observed. The magnetization for LNCx MO (x = 0.2, 0.4, 0.8) does not show saturation even at H = 7 T, and M(4K, 7T) for each sample is lower than the theoretically estimated value Mtheo . The lowering is due to ASDs whose concentration increases with increasing concentration of Co2+ ions. The remanent magnetization and coercivity of LNCx MO increases with increasing x due to large crystalline anisotropy of the octahedral site Co2+ ions. The clustered state in the LNCx MO system is induced by ASDs against long range FM ordering. The ac susceptibility study as a function of temperature and IRM as a function of time discard the possibility of any SG like phase.

We thank Prof. Sangam Banerjee, Surface Physics and Material Science Division, Saha Institute of Nuclear Physics for providing us ac susceptibility data. We also thank Mr. Goutam Sarkar, Surface Physics and Material Science Division, Saha Institute of Nuclear Physics for helping in XPS data acquisition. P. Dasgupta is grateful to the Department of Science and Technology (DST), India for the financial assistance through Women Scientist Project. References [1] N.S. Rogado, J. Li, A.W. Sleight, and M.A. Subramanian, Adv. Mater., 2005, 17, 2225. [2] R.I. Dass, and J.B. Goodenough, Phys. Rev. B, 2003, 67, 014401. 22

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[26] J. Deisenhofer, D. Braak, H.-A. Krug von Nidda, J. Hemberger, R.M. Eremina, V.A. Ivanshin, A.M. Balbashov, G. Jug, A. Loidl, T. Kimura, and Y. Tokura, Phys. Rev. Lett., 2005, 95, 257202.

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[33] D.C. Johnston, Phys. Rev. B, 2006, 74 184430.

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