Current Applied Physics 16 (2016) 1597e1602
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X-ray absorption spectroscopy and magnetic studies of Sr1xCexMn1yCoyO3d solid solutions S.N. Shamin a, V.V. Mesilov a, M.S. Udintseva b, A.V. Korolev a, T.I. Chupakhina c, G.V. Bazuev c, V.R. Galakhov a, * a b c
M. N. Miheev Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, 620137 Yekaterinburg, Russia 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
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2016 Received in revised form 4 August 2016 Accepted 5 September 2016 Available online 7 September 2016
We present results of measurements of temperature dependence of the magnetic susceptibility and soft X-absorption spectra of double-substitution solid solutions Sr1xCexMn1yCoyO3d. It was found that in the solid solution Sr0.8Ce0.2Co0.2Mn0.8O2.96, cobalt and cerium ions are in the Co2þ and Ce4þ valence states, respectively. About 90% of manganese ions in Sr0.8Ce0.2Co0.2Mn0.8O2.96 are in the 4þ state, and the rest of them are in the 3þ state. In the solid solution Sr0.9Ce0.1Co0.4Mn0.6O2.85, manganese and cerium ions are in the 4þ states. About 75% of cobalt ions are in the high-spin Co3þ state and 25% of cobalt ions are in the 2þ state. Doping Mn position with Co ions reduces the antiferromagnetic interaction in the el temperature and to the magnetic transition of initial compounds, which leads to a decrease of the Ne the material into the spin-glass state. © 2016 Elsevier B.V. All rights reserved.
Keywords: X-ray absorption spectra Manganites Cobaltites Magnetic moments Spin state Magnetic susceptibility el temperature Ne Crystal-field multiplet calculations
1. Introduction 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, La2MnCoO6 [1,2] and La2xSrxMnCoO6 [3]. Magnetism of these compounds is determined by the competition between ferromagnetic Mn3þeOeMn4þ, Mn3þeOeMn3þ, Co2þeOeMn4þ and antiferromagnetic Mn4þeOeMn4þ, Co2þeOeCo2þ, 3þ 2þ Mn eOeCo superexchange. Depending on temperature as well on a method of preparation, two mixed-valence combinations are possible in these systems: Mn2þeCo4þ and Mn3þeCo3þ. As a result, the Curie temperature TC of La2MnCoO6 samples with the ordered cation combination Mn2þeCo4þ reaches 225 K, but samples with ordered and disordered regions are characterized by two magnetic
* Corresponding author. E-mail address:
[email protected] (V.R. Galakhov). http://dx.doi.org/10.1016/j.cap.2016.09.003 1567-1739/© 2016 Elsevier B.V. All rights reserved.
transitions with TC¼225 and 150 K [2]. Manganite SrMn4þO3 has a four-layered hexagonal structure. It el temperature TN¼278 K [4]. The is an antiferromagnetic with a Ne substitution of part of Sr ions by Ce ions in SrMnO3 leads to the formation of electron-doped manganites Sr1xCe4þxMnO3 (0.1x0.3) [5,6] and to the reducing valence state of some Mn ions from 4þ to 3þ. The appearance of Mn3þ (3d4 electron configuration) JahneTeller ions in Sr1xCexMnO3 leads to crystal structure transitions: at x¼0.1 to the cubic structure and at x0.1 to the tetragonal one. Magnetic properties of Sr1xCexMnO3 depend strongly on x. Sr0.9Ce0.1MnO3 is an antiferromagnetic material of el temperature TN¼290 K [7]. The long-range the C-type with a Ne antiferromagnetic order is degenerated at x0.1 and the magnetic susceptibility increases with x [5]. The magnetic behaviour of the tetragonal solid solutions Sr1xCexMnO3 in 0.1x0.3 is determined by strong competition between double exchange and superexchange interactions [8]. Temperature dependence of the magnetic susceptibility c(T) of Sr0.8Ce0.2MnO3 shows a maximum at 150e300 K, whose appearance is associated with diluted antiferromagnetism with TN¼210 K [9]. This solid solution exhibits spinglass state at low temperatures. In the paramagnetic region, the
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CurieeWeiss law is observed only above 300 K. A positive constant Q indicates the formation of ferromagnetic clusters [8]. The crystal structure and magnetic properties of double substitution solid solutions Sr0.8Ce0.2Mn1yCoyO3d (y¼0.3 and 0.4) were studied in Ref. [10]. Solid solutions Sr1xCexMn1yCoyO3d have a tetragonal structure with the space group I4/mcm similar to that of Sr0.8Ce0.2MnO3 [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 c(T). The samples are characterized by antiferromagnetic transformations at 176 K (y¼0.3) and 196 K (y¼0.4) and by spinglass state at temperatures below 40 K (y¼0.3) and 27 K (y¼0.4) [10]. It was concluded on the basis of magnetic measurements in the paramagnetic region that manganese and cobalt ions in the solid solution Sr0.8Ce0.2Mn0.7Co0.3O2.94 are in the form of Mn3þ, Mn4þ, and high-spin (HS) Co3þ states [10]. In the Sr0.8Ce0.2Mn0.6Co0.4O2.88 solution, manganese ions are in the Mn4þ state; 2/3 of cobalt ions are in the Co2þ state, and the rest cobalt ions are in the Co3þ state [10]. It was assumed that in these solid solutions Sr0.8Ce0.2Mn1yCoyO3d, cerium ions are in the Ce4þ state [10]. However, authors of the works [7,11] do not exclude mixed oxidation state of cerium, Ce3þ and Ce4þ, in Sr1xCexMnO3. The presence of Ce3þ cations (f1 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, Sr0.8Ce0.2Mn0.8Co0.2O2.96 and Sr0.9Ce0.1Mn0.6Co0.4O2.85. 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.
considered in the calculations of oxygen content that cobalt in the reduction product was in the form of metal, manganese d in the form of MnO, strontium and cerium d in the form of SrO and Ce2O3 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 d ZFC). X-ray absorption (XAS) spectra of were obtained at RussianGerman 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 electron-yield mode, therefore, the contribution of surface contamination cannot be excluded. The information depth from which detectable signal intensities can be obtained is about 5e10 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. 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 spineorbit interaction [12,13]. Slater integrals were calculated by the HartreeeFock 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 X-ray 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 chargetransfer effects. 3. Results 3.1. Crystal structure
2. Experiment The samples of Sr1xCexMn1yCoyO3d were synthesized by the solid-phase reaction method from simple oxides CeO2, Co3O4, and MnO2 and carbonate SrCO3, which contained not less than 99.95% of the main substance. 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
According to X-ray diffraction data, the solid solution Sr0.8Ce0.2Mn0.8Co0.2O3d, like Sr0.8Ce0.2 MnO3 [5] is related to the space group I4/mcm. The tetragonal cell parameters of this sample are a¼5.3962(1) Å, c¼7.6674(1) Å, and V¼223.22(1) Å3. In comparison with Sr0.8Ce0.2MnO3 (a¼5.4013 Å, c¼7.7448 Å, and V¼225.95 Å3 [9]), the parameters of the doping sample were reduced and oxygen content was decreased to d¼0.04. Therefore, the composition of this sample can be written as Sr0.8Ce0.2Co0.2Mn0.8O2.96. The solid solution Sr0.9Ce0.1Mn0.6Co0.4O3d can be indexed at 290 K in the space group Pm3m like Sr0.9Ce0.1MnO3 at 350 K [7]. Its cubic cell has the parameter a¼3.8328(2) Å. This material is characterized by more high oxygen defectiveness (d¼0.15). 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 Sr1xCexMnO3, Ce is predominantly 4þ at low doping levels (x0. It is an evidence of the formation of ferromagnetic clusters. 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 Sr0.8Ce0.2Mn0.8Co0.2O2.96, parameter relation c/a¼1.42 for the tetragonal cell decreased in comparison with that for Sr0.8Ce0.2MnO3 (1.44) that is due to a decrease in the number of JahneTeller cations Mn3þ. The solid solution Sr0.9Ce0.1Mn0.6Co0.4O2.85 at 300 K has a cubic cell, unlike Sr0.9Ce0.1MnO3 [7], which can be explained by increase in the number and high-valence small-size Co3þ and Mn4þ cations. According to Ref. [7], cerium ions in Sr0.9Ce0.1MnO3 are in the 3þ 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 Sr0.9Ce0.1Mn0.6Co0.4O2.85, cerium is only in the Ce4þ state. Longrange antiferromagnetism (C-type) in tetragonal Sr0.9Ce0.1MnO3 is stabilized by JahneTeller Mn3þ ions at TN¼295 K. Absence of Mn3þ cations in Sr0.9Ce0.1Mn0.6Co0.4O2.85 (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 Mn4þ and Co3þ (HS) cations. The sample Sr0.8Ce0.2Mn0.8Co0.2O2.96 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 Mn4þeCo2þ 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 5e10 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.
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 Sr0.8Ce0.2Co0.2Mn0.8O2.96 contains Ce4þ, Mn4þ, and Co2þ cations. Magnetic measurements specify that about 10% of manganese ions present as cation Mn3þ. The solid solutions Sr0.9Ce0.1Co0.4Mn0.6O2.85, according to the spectroscopic studies, consists of Ce4þ, Mn4þ, Co2þ, and Co3þ cations. According to the magnetic susceptibility measurements, all the manganese ions are in the 4þ oxidation state and cobalt ions are in a mixed state (75% of Co3þ and 25% Co2þ ions). Co3þ 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. Acknowledgement 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 “RussianeGerman Laboratory at BESSY”. Magnetic experiments were carried out on equipment of The Collective Services Center of the Mikheev Institute of Metal Physics. References [1] R.I. Dass, J.B. Goodenough, Multiple magnetic phases of La2CoMnO6d (0d0.05), Phys. Rev. B 67 (2003) 014401. n-Gonz [2] A.J. Baro alez, C. Frontera, J.L. G.-Munoz, 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. [3] G.V. Bazuev, A.V. Korolyov, M.A. Melkozyorova, T.I. Chupakhina, Magnetic phases in lanthanumestrontium manganiteecobaltite La1.25Sr0.75MnCoO6, J. Magn. Magn. Mater 322 (2010) 494e499. [4] P.D. Battle, T.C. Gibb, C.W. Jones, The structural and magnetic properties of SrMnO3x: A reinvestigation, J. Solid State Chem. 74 (1988) 60e66. [5] H. Wu, K. Zhu, G. Xu, H. Wang, Magnetic inhomogeneities in electron-doped manganites Sr1x CexMnO3 (0.10x0.30), Phys. B 407 (2012) 770e773. [6] Z. Zhang, B.J. Kennedy, C.J. Howard, M.A. Carpenter, W. Miller, K.S. Knight, M. Matsuda, M. Miyake, Crystal structures, strain analysis, and physical properties of Sr0.7Ce0.3MnO3, Phys. Rev. B 85 (2012) 174110. [7] A. Sundaresan, J.L. Tholence, A. Maignan, C. Martin, M. Hervieu, B. Raveau, E. Suard, Jahn-Teller distortion and magnetoresistance in electron doped Sr1xCexMnO3 (x¼0.1,0.2,0.3 and 0.4), Eur. Phys. J. B 14 (2000) 431e438. [8] P. Mandal, A. Hassen, A. Loidl, Effect of Ce doping on structural, magnetic, and transport properties of SrMnO3 perovskite, Phys. Rev. B 69 (2004) 224418. [9] W.J. Lu, B.C. Zhao, R. Ang, W.H. Song, J.J. Du, Y.P. Sun, Internal friction evidence of uncorrelated magnetic clusters in electron-doped manganite Sr0.8Ce0.2MnO3, Phys. Lett. 346 (2005) 321e326. [10] T.I. Chupakhina, G.V. Bazuev, Synthesis, structure, and magnetic properties of Sr0.8Ce0.2Mn1yCoyO3d (y¼0.3, 0.4), Inorgan. Mater 47 (2011) 1361e1366. [11] Z. Zhang, B.J. Kennedy, C.J. Howard, L.-Y. Jang, K. Kevin S, M. Matsuda, M. Miyake, X-ray absorption and neutron diffraction studies of (Sr1x Cex) MnO3: transition from coherent to incoherent static JahneTeller distortions, J. Phys. Condens. Matter 22 (2010) 445401. [12] E. Stavitski, F. de Groot, CTM4XAS 3.1 d Charge Transfer Multiplet Calculations for X-Ray Absorption Spectroscopy: Simulations of XAS, XPS and XES, Spectra of Transition Metal Systems, Utrecht University, 2010. [13] E. Stavitski, F.M. de Groot, The CTM4XAS program for EELS and XAS spectral shape analysis of transition metal L edges, Micron 41 (2010) 687e694. [14] K. Okada, A. Kotani, Complementary roles of Co 2p X-ray absorption and photoemission spectra in CoO, J. Phys. Soc. Jpn. 61 (1992) 449e453. [15] F.M.F. de Groot, M. Abbate, J. van Elp, G.A. Sawatzky, Y.J. Ma, C.T. Chen, F. Sette, Oxygen 1s and cobalt 2p X-ray absorption of cobalt oxides, J. Phys. Condens. Matter 5 (14) (1993) 2277e2288. [16] S.O. Kucheyev, B.J. Clapsaddle, Y.M. Wang, T. van Buuren, A.V. Hamza, Electronic structure of nanoporous ceria from X-ray absorption spectroscopy and atomic multiplet calculations, Phys. Rev. B 76 (2007) 235420. [17] C. Mitra, Z. Hu, P. Raychaudhuri, S. Wirth, S.I. Csiszar, H.H. Hsieh, H.-J. Lin, C.T. Chen, L.H. Tjeng, Direct observation of electron doping in La0.7Ce0.3MnO3 using x-ray absorption spectroscopy, Phys. Rev. B 67 (2003) 092404.
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