X-ray absorption spectroscopy and magnetic studies of Sr1−xCexMn1−yCoyO3−δ solid solutions

Accepted Manuscript X-ray absorption spectroscopy and magnetic studies of Sr1−x CexMn1−y CoyO3−δ solid solutions S.N. Shamin, V.V. Mesilov, M.S. Udintseva, A.V. Korolev, T.I. Chupakhina, G.V. Bazuev, V.R. Galakhov PII:

S1567-1739(16)30240-1

DOI:

10.1016/j.cap.2016.09.003

Reference:

CAP 4319

To appear in:

Current Applied Physics

Received Date: 8 June 2016 Revised Date:

4 August 2016

Accepted Date: 5 September 2016

Please cite this article as: S.N. Shamin, V.V. Mesilov, M.S. Udintseva, A.V. Korolev, T.I. Chupakhina, G.V. Bazuev, V.R. Galakhov, X-ray absorption spectroscopy and magnetic studies of Sr1−x CexMn1−y CoyO3−δ solid solutions, Current Applied Physics (2016), doi: 10.1016/j.cap.2016.09.003. 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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X-ray absorption spectroscopy and magnetic studies of Sr1−xCexMn1−y Coy O3−δ solid solutions S. N. Shamina , V. V. Mesilova , M. S. Udintsevab , A. V. Koroleva , T. I. Chupakhinac , G. V. Bazuevc , V. R. Galakhova

M. N. Miheev Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, 620137 Yekaterinburg, Russia b

Ural State University of Railway Transport, 620134 Yekaterinburg, Russia

Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620137 Yekaterinburg, Russia

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Abstract

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We present results of measurements of temperature dependence of the magnetic susceptibility and soft X-absorption spectra of double-substitution solid solutions Sr1−x Cex Mn1−y Coy O3−δ . It was found that in the solid solution Sr0.8 Ce0.2 Co0.2 Mn0.8 O2.96 , cobalt and cerium ions are in the Co2+ and Ce4+ valence states, respectively. About 90 % of manganese ions in Sr0.8 Ce0.2 Co0.2 Mn0.8 O2.96 are in the 4+ state, and the rest of them are in the 3+ state. In the solid solution Sr0.9 Ce0.1 Co0.4 Mn0.6 O2.85 , manganese and cerium ions are in the 4+ states. About 75% of cobalt ions are in the highspin Co3+ state and 25% of cobalt ions are in the 2+ state. Doping Mn position with Co ions reduces the antiferromagnetic interaction in the initial compounds, which leads to a decrease of the N´eel temperature and to the magnetic transition of the material into the spin-glass state. Keywords: X-ray absorption spectra, manganites, cobaltites, magnetic moments, spin state, magnetic susceptibility, N´eel temperature, crystal-field multiplet calculations

Preprint submitted to Current Applied Physics

September 6, 2016

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The type of magnetic exchange interaction in the multicomponent oxide systems containing some transition metals depends on metal valence states and the cation ordering. Influence of atomic ordering and metal oxidation states on magnetic properties of oxides can be seen in double perovskites containing Mn and Co, for example, La2 MnCoO6 [1, 2] and La2−x Srx MnCoO6 [3]. Magnetism of these compounds is determined by the competition between ferromagnetic Mn3+ –O–Mn4+ , Mn3+ –O–Mn3+ , Co2+ –O–Mn4+ and antiferromagnetic Mn4+ –O–Mn4+ , Co2+ –O–Co2+ , Mn3+ –O–Co2+ superexchange. Depending on temperature as well on a method of preparation, two mixedvalence combinations are possible in these systems: Mn2+ –Co4+ and Mn3+ – Co3+ . As a result, the Curie temperature TC of La2 MnCoO6 samples with the ordered cation combination Mn2+ –Co4+ reaches 225 K, but samples with ordered and disordered regions are characterized by two magnetic transitions with TC = 225 and 150 K [2]. Manganite SrMn4+ O3 has a four-layered hexagonal structure. It is an antiferromagnetic with a N´eel temperature TN = 278 K [4]. The substitution of part of Sr ions by Ce ions in SrMnO3 leads to the formation of electrondoped manganites Sr1−x Ce4+ x MnO3 (0.1 ≤ x ≤ 0.3) [5, 6] and to the reducing valence state of some Mn ions from 4+ to 3+. The appearance of Mn3+ (3d4 electron configuration) Jahn–Teller ions in Sr1−x Cex MnO3 leads to crystal structure transitions: at x = 0.1 to the cubic structure and at x ≥ 0.1 to the tetragonal one. Magnetic properties of Sr1−x Cex MnO3 depend strongly on x. Sr0.9 Ce0.1 MnO3 is an antiferromagnetic material of the C-type with a N´eel temperature TN = 290 K [7]. The long-range antiferromagnetic order is degenerated at x ≥ 0.1 and the magnetic susceptibility increases with x [5]. The magnetic behaviour of the tetragonal solid solutions Sr1−x Cex MnO3 in 0.1 ≤ x ≤ 0.3 is determined by strong competition between double exchange and superexchange interactions [8]. Temperature dependence of the magnetic susceptibility χ(T ) of Sr0.8 Ce0.2 MnO3 shows a maximum at 150–300 K, whose appearance is associated with diluted antiferromagnetism with TN = 210 K [9]. This solid solution exhibits spin-glass state at low temperatures. In the paramagnetic region, the Curie–Weiss law is observed only above 300 K. A positive constant Θ indicates the formation of ferromagnetic clusters [8]. The crystal structure and magnetic properties of double substitution solid solutions Sr0.8 Ce0.2 Mn1−y Coy O3−δ (y = 0.3 and 0.4) were studied in Ref. [10]. Solid solutions Sr1−x Cex Mn1−y Coy O3−δ have a tetragonal structure with the

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1. Introduction

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space group I4/mcm similar to that of Sr0.8 Ce0.2 MnO3 [5]. It was found that substitution of a part of manganese with cobalt reduces the oxygen content and leads to a change of the temperature dependence of the magnetic susceptibility χ(T ). The samples are characterized by antiferromagnetic transformations at 176 K (y = 0.3) and 196 K (y = 0.4) and by spin-glass state at temperatures below 40 K (y = 0.3) and 27 K (y = 0.4) [10]. It was concluded on the basis of magnetic measurements in the paramagnetic region that manganese and cobalt ions in the solid solution Sr0.8 Ce0.2 Mn0.7 Co0.3 O2.94 are in the form of Mn3+ , Mn4+ , and high-spin (HS) Co3+ states [10]. In the Sr0.8 Ce0.2 Mn0.6 Co0.4 O2.88 solution, manganese ions are in the Mn4+ state; 2/3 of cobalt ions are in the Co2+ state, and the rest cobalt ions are in the Co3+ state [10]. It was assumed that in these solid solutions Sr0.8 Ce0.2 Mn1−y Coy O3−δ , cerium ions are in the Ce4+ state [10]. However, authors of the works [7, 11] do not exclude mixed oxidation state of cerium, Ce3+ and Ce4+ , in Sr1−x Cex MnO3 . The presence of Ce3+ cations (f 1 electronic configuration) increases the paramagnetic susceptibility of the samples and does not allow to properly estimate effective magnetic moments of Mn and Co. Cobalt may also affect the Ce oxidation state. In connection with the above mentioned, it is obvious that for interpretation of magnetic properties of such systems and especially for understanding the nature of magnetic exchange interactions, detailed studies of the electronic structure and oxidation states of cations are necessary. In this work, we present results of our measurements of temperature dependence of the magnetic susceptibility and Ce M4,5 , Mn L2,3 , and Co L2,3 soft X-absorption spectra of two samples, Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 . These solid solutions having a close chemical composition and related crystal structure, differ in the level of a substitution of Mn and Sr sublattices and in oxygen content. The establishment of valence states of cations and magnetic characteristics of these samples will give an opportunity to understand the interplay of structural, magnetic and electronic properties of the given double substitution solid solutions, and also the nature of magnetic exchange interactions between cations.

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2. Experiment

The samples of Sr1−x Cex Mn1−y Coy O3−δ were synthesized by the solidphase reaction method from simple oxides CeO2 , Co3 O4 , and MnO2 and carbonate SrCO3 , which contained not less than 99.95 % of the main sub3

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stance. Stoichiometric mixtures of the oxides and Sr carbonate were thoroughly ground, pressed under the pressure of 3000 kg/cm2 , and sintered during stepwise temperature elevation with a step size of 100 ◦ C and intermediate grinding after each 10 h. Upon calcination, the reaction products were cooled with furnace to room temperature. The initial annealing temperature was 950 ◦ C and the final annealing temperature was 1350 ◦ C. The purity of the synthesized product was verified using X-ray powder diffraction (XRD) on a Shimadzu XRD-7000 S diffractometer. Possible impurity phases were checked by comparing their XRD patterns with those in the PDF2 database (ICDD, USA, Release 2009). The crystal structure refinement was carried out by the full-profile Rietveld analysis using the FULLPROF-2010 software. The oxygen content of the samples was determined by thermogravimetric analysis: measuring loss in mass of the samples as a result of complete reduction in hydrogen flow at 950 ◦ C. It was considered in the calculations of oxygen content that cobalt in the reduction product was in the form of metal, manganese — in the form of MnO, strontium and cerium — in the form of SrO and Ce2 O3 oxides, respectively. Presence of Co and indicated oxides in products of decomposition was confirmed by the X-ray powder diffraction method. Magnetic measurements were performed in the Collaborative Access Center “Testing Center of Nanotechnology and Advanced Materials” of IMP on a MPMS-XL-5 SQUID magnetometer (QUANTUM DESIGN) in the magnetic fields of 0.5 and 5.0 kOe. The measurements were carried while cooling the samples both in magnetic field (FC) and without magnetic field (in zero magnetic field — ZFC). X-ray absorption (XAS) spectra of were obtained at Russian-German beamline at BESSY (Berlin) in the total photoelectron yield mode. All XAS spectra were normalized to the beam flux measured by a clean gold mesh. The XAS spectra were measured in the surface-sensitivity total electronyield mode, therefore, the contribution of surface contamination cannot be excluded. The information depth from which detectable signal intensities can be obtained is about 5–10 nm. The atomic layers of the here studied samples may somewhat differ in atomic composition which therefore can influence on the cation valence state and magnetic moments of samples under studies. To remove surface contamination and to minimize the difference between the surface and the bulk, the samples were mechanically cleaned in the chamber of the spectrometer just before the measurements.

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3.1. Crystal structure According to X-ray diffraction data, the solid solution Sr0.8 Ce0.2 Mn0.8 Co0.2 O3−δ , like Sr0.8 Ce0.2 MnO3 [5] is related to the space group I4/mcm. The tetragonal cell parameters of this sample are a = 5.3962(1) ˚ A, c = 7.6674(1) ˚ A, and V = 223.22(1) ˚ A3 . In comparison with Sr0.8 Ce0.2 MnO3 (a = 5.4013 ˚ A, c = 7.7448 ˚ A, and V = 225.95 ˚ A3 [9]), the parameters of the doping sample were reduced and oxygen content was decreased to δ = 0.04. Therefore, the composition of this sample can be written as Sr0.8 Ce0.2 Co0.2 Mn0.8 O2.96 . The solid solution Sr0.9 Ce0.1 Mn0.6 Co0.4 O3−δ can be indexed at 290 K in the space group P m3m like Sr0.9 Ce0.1 MnO3 at 350 K [7]. Its cubic cell has the parameter a = 3.8328(2) ˚ A. This material is characterized by more high oxygen defectiveness (δ = 0.15).

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3.2. Determination of Ce valence state It is necessary to determine whether Ce ions in these materials are in the tetravalent state or in the trivalent state. It is known that for Ce-doped manganites Sr1−x Cex MnO3 , Ce is predominantly 4+ at low doping levels (x < 0.2). For Ce content x ≥ 0.2, trivalent Ce ions are introduced [11]. To estimate cerium valence state, Ce M4,5 X-ray absorption spectra are necessary to use because they are highly sensitive to the valence state of cation. Fig. 1 shows Ce M4,5 X-ray absorption spectra for theses two solid solutions. For comparison, spectra of CeO2 (Ce4+ ) and CeCl3 (Ce3+ ) taken from [16] are

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Crystal field multiplet calculations of L2,3 XAS spectra for Ce, Co, and Mn ions in an oxygen octahedral (Oh ) environment were carried out using a computer program for calculation of spectra with a multiplet structure determined by the Coulomb and exchange interactions between 2p holes and 3d electrons, the splitting by the crystal field, and spin–orbit interaction [12, 13]. Slater integrals were calculated by the Hartree–Fock method. Crystal field parameters (10Dq) were taken identical for the basic and final states of the system. It is known that charge-transfer effects are important for L2,3 Xray photoelectron spectra whereas X-ray absorption spectra are dominated by multiplet effect [14, 15]. Therefore, in calculations of L2,3 X-ray absorption spectra, we neglect charge-transfer effects.

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3.3. Determination of Mn valence state For the determination of the valence states of the transition metal ions in the Sr1−x Cex Mn1−y Coy O3−δ solid solutions, we used L2,3 X-ray absorption spectra which a well understood and has become a standard tool. Due to strong electronic correlations, the multiplet structure of the transition metal L2,3 spectrum can be well reproduced by crystal-field multiplet calculations. In Fig. 2, we present the Mn L2,3 X-ray absorption spectra of Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 together with those of LaMnO3+δ and La0.5 Ca0.5 MnO3 for comparison. The experimental spectra are compared with results of crystal-field multiplet calculations for Mn3+ , Mn3+ + Mn4+ , and Mn4+ ions shown in Fig. 2 by solid lines. One can see that the spectra of the solid solutions Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 are shifted toward higher energy relative to the spectrum of the La0.5 Ca0.5 MnO3 manganite where manganese ions have a mean valence state of 3.5+ [18]. Taking into account the linear dependence of the Mn L3 XAS energy versus the oxidation state of manganese ions in LaMnO3+δ and La0.5 Ca0.5 MnO3 , we concluded that Mn ions in here studied solid solutions are in the 4+ state. This statement is supported by the shape of the spectra. The Mn L2,3 spectra of here studied solid solutions are similar to those of SrMnO3 [19] and in LaMn0.5 Co0.5 O3 [20] in which a Mn4+ state was found.

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presented. One can see that the spectra of our samples are similar to that of CeO2 . The Ce M4,5 X-ray absorption spectra of the formally tetravalent Cecompound CeO2 consist of main structures at 883.5 eV and 901.5 eV and a satellites at about 5 eV above the main peaks. This satellite structure is explained by the charge-transfer process. The final state of the X-ray absorption process for tetravalent compounds such as CeO2 can be presented as a sum of the bonding and antibonding states, 3d9 4f 1 and 3d9 4f 2 L [17]. Here, L refers to the ligand hole. A higher energy antibonding state appears usually as a weak satellite [17]. The main spectral features of the trivalent Ce compound like CeCl3 exhibits a more complicated multiplet structure. Therefore, Ce ions in the solid solutions Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 are in the tetravalent state.

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Figure 1: (Color online) Ce M4,5 X-ray absorption spectra for solid solutions Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 . For comparison, spectra of CeO2 and CeCl3 taken from [16] are presented.

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Figure 2: (Color online) Mn L2,3 X-ray absorption spectra for solid solutions Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 . For comparison, there are presented spectra of manganites where Mn-ion oxidation states are closely to 3+ (LaMnO3+δ ) and 3.5+ (La0.5 Ca0.5 MnO3 ). The solid lines display crystal-field multiplet calculations assuming Mn3+ , Mn4+ , and a sum of Mn3+ and Mn4+ ground states. Position of Mn L3 maxima for the solid solutions Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 is shown by the dashed line.

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3.4. Determination of Co valence state Co L2,3 X-ray absorption spectrum of the solid solutions Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 (S1) and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 (S2), CoO, and LaCoO3 are shown in Fig. 3. For comparison, the spectra of single crystals of CoO, and LaCoO3 and theoretical Co 2p spectra for high-spin and low-spin Co3+ ions and Co2+ ions in oxygen octahedral (Oh ) surrounding are shown. The XAS data show strong splitting structure on both L2 and L3 absorption edges due to the interplay of crystal field 10Dq and electronic interactions. The spectra of the solid solutions Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 show extra peaks at 784.4 eV and 799.2 eV which correspond to Ba M5 and M4 lines due to Ba impurities. The Ba indexes in the chemical formulas are estimated to be 0.03 and 0.001 for solid solutions Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 , respectively. The Co L2,3 spectrum of Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 is very similar to the experimental spectrum of CoO and the theoretical spectrum calculated for Co2+ ions. It means that cobalt ions in this solid solution are in the high-spin 2+ oxidation state. On the other hand, the spectrum of Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 show features both of Co2+ and Co3+ ions. We have subtracted the spectrum of the S1 sample from the spectrum of the S2 sample to show the availability of the Co3+ ions in the Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 sample (S2). This was demonstrated in the insert in Fig. 3. The spectra of the solid solutions were plotted in the following suggestions: (i) the integral intensities of the Co L2,3 spectra are proportional to the number of 3d holes: 3 and 4 for Co2+ and Co3+ ions, respectively; (ii) since the sample S2 contains 25% of Co2+ and 75% of Co3+ ions, the intensity of its spectrum was multiplied by 4. The concentrations of Co2+ and Co3+ ions in these solid solutions were estimated from the results of magnetic measurements (see below). Therefore, the difference spectrum is formed only by Co3+ ions. This difference spectrum coincides well with the spectrum of LaCoO3 and with the theoretical spectrum for high-spin Co3+ ions. The X-ray spectral results demonstrate that Co3+ ions are in the high-spin state.

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3.5. Magnetic measurements The results of measurements of the magnetic characteristics for the solid solution Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 and Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 in the range of 2–400 K are presented in Fig. 4. Inverse magnetic susceptibility χ−1 versus T is given in the inset. The temperature dependences of magnetic susceptibility χ for all the examined samples at H = 5 kOe are characterized 9

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Figure 3: (Color online) Co L2,3 X-ray absorption spectra for the solid solutions Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 , CoO, and LaCoO3 . Experimental spectra are compared with results of crystal field multiplet calculations for Co2+ and Co3+ ions (solid lines). For Co3+ ions, calculations were carried out for the highspin (HS) and low-spin (LS) states. Insert shows proceeding of obtaining the difference spectrum. The “difference” spectrum was obtained by the subtraction of the spectrum of Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 from that of Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 taking in the relation of 1 : 4.

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Figure 4: (Color online)Magnetic susceptibility χ and reciprocal susceptibility 1/χ (inset) as functions of temperature for Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 and Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 . The measurements were carried while cooling the samples both in magnetic field (FC) and in zero magnetic field (ZFC). The arrows indicate temperature of a transition of the solid solution Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 to the spin-glass state (35 K) and N´eel temperatures, 225 K and 138 K for Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 and Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 , respectively.

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by the presence of two anomalies and by a reduction of χ with increasing cobalt content. One can see that replacement of 40% Mn ions with Co in Sr0.9 Ce0.1 MnO3 leads to the decrease of the transition temperature from 290 K [8] to 225 K. Above 225 K, the magnetic susceptibility of Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 obeys a modified Curie–Weiss law

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C , T −Θ where χ0 takes into account all temperature-independent contributions (in our case, χ0 = 0), C is a Curie constant, and Θ is a Curie–Weiss constant. For this solid solution, Θ = −125 K. χmol = χ0 +

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Results of the XAS analysis of the valence and spin state of Mn and Co ions and magnetic measurements of χ(T ) in the paramagnetic region of the Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 and Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 are summarized in Table 1. For comparison, the magnetic moment for Sr0.8 Ce0.2 MnO3 is shown, taken from Ref. [9]. In calculations of the valence and spin states of the Mn and Co and their concentrations in the solid solutions, we took into account the oxygen nonstoichiometry and experimentally measured effective magnetic moments. We have used the following electronic configurations of cations in an octahedral environment: Mn4+ (t32g e0g , S = 3/2), Mn3+ (t32g e1g , S = 2), Co2+ (t52g e2g , S = 3/2), Co3+ (t42g e2g , S = 2, high-spin state). It is necessary to take unto account a spin-orbital contribution in the effective magnetic moment of Co2+ ions. The theoretical magnetic moment for Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 corresponds with the experimentally estimated moment in the suggestion that manganese ions are in the Mn4+ state and that Co3+ (HS), and Co2+ (HS) states are in the ratio of 2/1. For other combinations of Co and Mn concentrations and their valence and spin states, the system of linear equations based on theoretical and experimental magnetic moments and charge characteristics has no solution. For the Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 sample, the effective magnetic moment µexp deduced from experiment is about 4.76 µB , which is larger than the calculated value (3.96 µB ). Such effect found early for Sr0.8 Ce0.2 MnO3 was ascribed to the appearance of magnetic clusters in the paramagnetic region formed by combination of Mn3+ –Mn4+ ions [8, 9]. One can see from Table 1 that manganese in this solid solution is also in the Mn4+ and Mn3+ cations and the Curie–Weiss constant Θ > 0. It is an evidence of the formation of ferromagnetic clusters.

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The N´eel temperature for the solid solution Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 was found to be 138 K. For comparison, Sr0.8 Ce0.2 MnO3 is characterized by TN = 210 K [8]. The constant Θ is positive for the both compounds. For Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 , Θ = +11 K. There is a divergence between the FC and ZFC curves of χ(T ) below 35 K which indicates a transition into the spin-glass state. Both solid solutions exhibit dominant antiferromagnetic interactions.

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Table 1: Magnetic moments of Srx Ce1−x Mn1−y Coy O3−δ

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The data on the oxidation states of Ce, Mn, and Co ions in the investigated solid solutions are in good agreement with the change of the crystal structure parameters. Thus, in Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 , parameter relation c/a = 1.42 for the tetragonal cell decreased in comparison with that for Sr0.8 Ce0.2 MnO3 (1.44) that is due to a decrease in the number of Jahn–Teller cations Mn3+ . The solid solution Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 at 300 K has a cubic cell, unlike Sr0.9 Ce0.1 MnO3 [7], which can be explained by increase in the number and high-valence small-size Co3+ and Mn4+ cations. According to Ref. [7], cerium ions in Sr0.9 Ce0.1 MnO3 are in the 3+ state. This conclusion has been made on the basis of magnetic susceptibility measurements contradicts results of the works [6, 11]. Our researches have shown that in the solid solution Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 , cerium is only in the Ce4+ state. Long-range antiferromagnetism (C-type) in tetragonal Sr0.9 Ce0.1 MnO3 is stabilized by Jahn–Teller Mn3+ ions at TN = 295 K. Absence of Mn3+ cations in Sr0.9 Ce0.1 Mn0.6 Co0.4 O2.85 (Table 1) leads to the increase the symmetry to cubic and preserves the antiferromagnetic order at reasonable high temperature. This effect correlates well with the sample composition, in particularity, with the existence of significant amounts of Mn4+ and Co3+ (HS) cations. The sample Sr0.8 Ce0.2 Mn0.8 Co0.2 O2.96 shows a magnetic transition with a wide maximum at 138 K which characterizes this solid solution as a diluted antiferromagnetic. The second maximum at 35 K in the ZFC curve indicates the formation of a spin-glass state originated from competition between antiferromagnetic and ferromagnetic correlation. This competition is probably determined by short-range ferromagnetic Mn4+ –Co2+ interactions which existence was established by these studies. Note that X-ray absorption spectra in total electron yield mode are surface sensitive and allow to analyse only about 5–10 nm of depth. Magnetic measurements are bulk sensitivity. The atomic layers of the here studied samples may somewhat differ in atomic composition which therefore can influence on the cation valence state and magnetic moments of solid solutions.

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4.31 3.96 4.21

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µcalc (µB ) µexp (µB )

4+ 3+ Sr0.8 Ce4+ 0.2 Mn0.6 Mn0.4 O3 4+ 3+ 2+ Sr0.8 Ce4+ 0.2 Mn0.72 Mn0.08 Co0.2 O2.96 3+(HS) 4+ Sr0.9 Ce4+ Co2+ 0.1 Mn0.6 Co0.3 0.1 O2.85

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Compound

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Acknowledgement

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The research was carried out within the state assignment of FASO of Russia (theme “Electron” No. 01201463326), supported in part by RFBR (project No. 16-02-00577). The measurements at BESSY were supported by the bilateral Program “Russian–German Laboratory at BESSY”. Magnetic experiments were carried out on equipment of The Collective Services Center of the Mikheev Institute of Metal Physics.

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[1] R. I. Dass, J. B. Goodenough, Multiple magnetic phases of La2 CoMnO6−δ (0 ≤ δ ≤ 0.05), Phys. Rev. B 67 (2003) 014401.

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[2] A. J. Bar´on-Gonz´alez, C. Frontera, J. L. G.-M. noz, B. Rivas-Murias, J. Blasco, Effect of cation disorder on structural, magnetic and dielectric properties of La2 MnCoO6 double perovskite, J. Phys.: Condens. Matter 23 (2011) 496003.

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Comparing the oxidation state of the cations derived from the magnetic measurements and absorption spectra, we can conclude that there is qualitative agreement between the methods used. Analysis of the absorption spectra showed that the solid solution Sr0.8 Ce0.2 Co0.2 Mn0.8 O2.96 contains Ce4+ , Mn4+ , and Co2+ cations. Magnetic measurements specify that about 10 % of manganese ions present as cation Mn3+ . The solid solutions Sr0.9 Ce0.1 Co0.4 Mn0.6 O2.85 , according to the spectroscopic studies, consists of Ce4+ , Mn4+ , Co2+ , and Co3+ cations. According to the magnetic susceptibility measurements, all the manganese ions are in the 4+ oxidation state and cobalt ions are in a mixed state (75 % of Co3+ and 25 % Co2+ ions). Co3+ ions are in the high-spin of state. In general, the study found that a combination of these methods makes it possible to estimate reliably the charge state of the transition metals in multicomponent oxide compounds.

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ACCEPTED MANUSCRIPT Solid solutions Sr0.8Ce0.2Mn0.8Co0.2O2.96 and Sr0.9Ce0.1Mn0.6Co0.4O2.85 were produced. Soft X-ray absorption spectroscopy and crystal field atomic multiplet calculations were applied for characterization of these objects. Cerium ions are in the 4+ states.

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About 90 % of Mn ions in Sr0.8Ce0.2Co0.2Mn0.8O2.96 are in the 4+ state, and the rest of them are in the 3+ state. In Sr0.9Ce0.1Co0.4Mn0.6O2.85, Mn ions are in the 4+ states. About 75% of Co ions are in the high-spin Co3 + state and 25% of Co ions are in the 2+ state. Measurements of magnetic susceptibility as function of temperature were carried out.

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Doping Mn position with Co ions leads to a decrease of the Néel temperature and to the magnetic transition of the material into the spin-glass state.