DSpace at VNU: Ti-doped A-site deficient lanthanum manganites: Local structure and properties

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 300 (2006) e175–e178 www.elsevier.com/locate/jmmm Ti-doped A-site deficient lanthanum manganites: Local structure and properties Alexander N Ulyanova,Ã, Dong-Seok Yangb, Kyu-Won Leec, Jean-Marc Greneched, Nguyen Chaue, Seong-Cho Yua a Department of Physics, Chungbuk National University, Cheongju 361-763, Korea Physics Division, School of Science Education, Chungbuk National University, Cheongju 361-763, Korea c Korea Research Institute of Standards and Science, Yusong, Taejon 305-600, Korea d Laboratoire de Physique de L’Etat Condense´, UMR CNRS 6087, Universite´ du Maine, 72085 Le Mans, Cedex 9, France e Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam b Available online 16 November 2005 Abstract A study of La0.6Sr0.4ÀxMnTixO3 (x ¼ 0:0, 0.05, 0.1, 0.15, and 0.2) manganites with the Ti in B( ¼ Mn)-position and vacancies in A( ¼ La, Sr)-site is presented The manganites belonged to the rhombohedral phase and small amount of Mn3O4 oxide was observed with increase of Ti content X-ray adsorption fine structure (XAFS) analysis showed an appearance of Mn2+ ions in perovskite cell and tremendous change of local structure We suppose that the change of local structure was mainly caused by the appearance of Mn ions in the A-positions and partially by the formation of vacancies in the above position with the increase of x-value Curie temperature, T C , decreased drastically with x: T C x ẳ 0ị ẳ 355 K and, T C x ẳ 0:05ị ẳ 185 K Further increase of Ti content changed the low-temperature magnetic state from the ferromagnetic to spin/cluster glass state Effects of destruction of the eg electron pathway and change of local structure on Curie temperature, caused by the Ti doping, is discussed r 2005 Elsevier B.V All rights reserved PACS: 75.30.Àm; 75.30.Kz; 61.10.Ht Keywords: Manganites; A- and B-site substitution and deficiency; Curie temperature; Local structure Doped Ln1ÀxRxMnO3 manganese oxides are under the extensive study due to the colossal magnetoresistivity (CMR) effect observed at temperatures close to ferromagnetic ordering temperature, T C (Ln is a rare earth, Y; R is an alkaline earth) [1] The CMR phenomenon was initially explained by the double exchange (DE) interaction between Mn3+ and Mn4+ ions via oxygen 2p orbitals [2] According to the DE model, transfer of itinerant eg electrons between the neighboring Mn ions (local t2g spins) through the O2À ion results in a ferromagnetic interaction due to the on-site Hund’s coupling The strength of the DE interaction is estimated by the transfer integral, teff The electronic bandwidth, W , is proportional to the teff and depends on Mn–O–Mn bond angles and Mn–O bond ÃCorresponding author Tel.: +82 43 271 8146; fax: +82 43 274 7811 E-mail address: a_n_ulyanov@yahoo.com (A.N Ulyanov) 0304-8853/$ - see front matter r 2005 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2005.10.177 distances in MnO6 octahedron through the overlap integrals between the Mn cation 3d orbitals and the O anion 2p orbitals An empirical formula [3] W ẳ W cosy=2ị=d 3:5 (1) was used to describe the dependence (y ¼ pÀ /Mn–O–MnS and d is an average /Mn–OS bond length) Very rich phase diagram and interesting properties of CMR materials are attributed to the A( ¼ Ln, R)- and B( ¼ Mn, transition metal)-site substitution of manganites Deficiency of atoms in A-position in the so-called La1ÀxMnO3 self-doped manganites also causes the CMR effect because of the appearance of the mixed Mn3+-Mn4+ valence state [4,5] The unusual result was obtained in Ref [5]: the occurrence of Mn atoms in A-position was concluded by neutron diffraction when studying the A-site ARTICLE IN PRESS A.N Ulyanov et al / Journal of Magnetism and Magnetic Materials 300 (2006) e175–e178 deficient La1ÀxMnO3 manganites To elucidate this question, we present a study of lanthanum manganites with the simultaneous B-site substitution and creation of vacancies in A-position La0.6Sr0.4ÀxMnTixO3+d (LSMTO) compositions (x ¼ 0:0, 0.05, 0.1, 0.15, and 0.2) were synthesized by solid-state reaction method X-ray absorption fine structure (XAFS) experiments were performed at the 7C1 beam line of the Pohang Light Source (PLS) in Korea PLS operated with electron energy of 2.5 GeV and the maximum current of 230 mA X-rays were monochromatized by the Si(1 1) double-crystal monochromator with the energy resolution, DE/E ¼ Â 10À4 Higher harmonics were removed by a 15 percent detuning of the crystal XAFS spectra were measured near the Mn K-edge (6540 eV) in a fluorescence mode at room temperature Magnetization measurements were carried out with the SQUID (Quantum Design MPMSXL) magnetometer According to X-ray CuKa (XRD) analysis the samples belonged to rhombohedral (R3¯ c) phase and contained a small amount of Mn3O4 oxide, which increased with x (see Fig 1) Temperature dependencies of magnetization in the field of 50 Oe (field cooled, warming rate) are presented in Fig The x ¼ and 0.05 compositions were ferromagnetic, and the compounds with the higher x content were in Mn3O4 La0.6Sr0.4-xMnTixO3+δ Intensity (arb.units) x=0.2 x=0.15 x=0.1 x=0.05 x=0 20 40 60 80 2Θ (degree) Fig XRD patterns of La0.6Sr0.4ÀxMnTixO3+d manganites 18 16 14 50 Oe, warming rate 12 Magnetization (emu/g) e176 10 -2 100 200 300 Temperature (K) 400 Fig Magnetization vs temperature dependencies for the La0.6Sr0.4ÀxMnTixO3+d manganites (x ¼ 0, 0.05, 0.1, 0.15, and 0.2 from the right to the left) spin(cluster)-glass-like state at low temperature The spinglass-like behavior of x ¼ 0:1 composition was reported in Ref [6] The careful analysis of the character of low magnetic state for (xX0:10) samples will be published elsewhere Curie temperature decreased dramatically with small increase of x-value (see Fig 2): T C ðx ¼ 0Þ ¼ 355 K and, T C ðx ¼ 0:05ị ẳ 185 K The T C change was stronger than that observed in La0.7Ca0.3Mn1ÀxTixO3 [7] and La0.7Sr0.3Mn1ÀxTixO3 [8] manganites It was probably due to (i) a non-uniform(multisite) distribution of Mn ions, (ii) appearance of Mn2+ ions, and deficiencies of ions in the A-position of perovskite cell in addition to the removing of pathway for the itinerant eg electrons and change of local structure caused by the Ti occupation in the B-site To carefully elucidate this question, the XAFS analysis was carried out XAFS represents extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) analysis, which give information about the local structure around a central atom and the electronic configuration (valence) of the core Mn cations, respectively XANES spectra were obtained directly by the normalization of absorption spectra, and the Fourier transformations of the EXAFS spectra, which give the rough picture of radial distribution of atoms around the Mn ion in perovskite cell, were obtained by regular way described in Ref [9] XANES (Fig 3), EXAFS (are not shown) and Fourier transform of EXAFS spectra (Fig 4) showed a continuous change with x XANES spectra shifted to lower energy and essentially broadened with x It is important to emphasize ARTICLE IN PRESS A.N Ulyanov et al / Journal of Magnetism and Magnetic Materials 300 (2006) e175–e178 1.4 Absorption spectra (arb.units) 1.2 MnO 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 6.54 6.55 E (keV) 6.56 Fig XANES spectra of La0.6Sr0.4ÀxMnTixO3+d manganites (x ¼ 0:0; 0.05; 0.1; 0.15; and 0.2, from the right to the left) and MnO oxide |Fourier transform| (arb.units) x=0.0 x=0.05 x=0.1 x=0.15 x=0.2 R (Å) Fig Fourier transform of EXAFS spectra for La0.6Sr0.4ÀxMnTixO3+d compositions e177 that in the case of the La1ÀxCaxMnO3 compositions [10,11], the XANES spectra showed almost the same shape and only shifted parallel to each other The shift of the absorption edge from the lower to higher energy with x was caused by the change of average Mn valence from 3+ (in LaMnO3) to 4+ (in CaMnO3) The main absorption for the Mn3+ ion (in LaMnO3) was observed at the interval from 6550 to 6556 eV The absorption for the Mn2+ ion in MnO oxide was observed at lower energies than that for the Mn3+ ion in LaMnO3 The very different picture has been observed in our study (see Fig 3, where the LSMTO and MnO XANES spectra are presented) Really, the spectrum for the La0.6Sr0.4MnO3 composition showed almost the same shape and position as that in La0.7Ca0.3MnO3 one [10] But, a small amount of Ti (x ¼ 0:05) only caused considerable changes in XANES spectra: (i) the spectra became broader and low energy ‘‘tail’’ appeared; (ii) the ‘‘tail’’ became wider (spread to lower energy) and more intensive with x; (iii) visible Xray absorption appeared just at the 6.547 keV for the x ¼ 0:05 sample and increased with x The changes of XANES spectra probably originated from (a) the occurrence of divalent Mn ions, which was manifested by the appearance of X-ray absorption at energies lower than 6.550 keV, and (b) a nonuniform distribution of Mn ions—partial occupation of A-position by the Mn ions—as indicated by the broadening of XANES spectra The nonuniform(multisite) distribution of Mn ions in perovskite cell was also confirmed by the Fourier transform of EXAFS spectra (Fig 4) Namely, it is well established [9], that the regularity in appearance of high-intensity peaks, as for the x ¼ samples, clearly evidences for uniform distribution of Mn atoms in lattice, and a complete disappearance of third and fourth peaks with x, as for the xX0:10 compositions, is an evidence of nonuniform distribution of Mn ions in the perovskite cell We suppose thus that Mn2+ occupy the A-position, and Mn3+,4+ ions, as usually, occupy the B-site Really, in the La0.6Sr0.4ÀxMnTixO3+d compositions one concludes to a deficiency of atoms in Aposition of perovskite cell, and an excess of Mn and Ti atoms, which almost always occupy the B-site The Aposition is occupied by La3+ and Sr2+ ions with ionic radii 1.216 and 1.31 A˚, respectively (all ionic radii are taken according to Shannon [12]) The most preferable ions, which can occupy the A-position among the Mn2+( ¼ 0.83 A˚), Mn3+( ¼ 0.645 A˚), and Mn4+( ¼ 0.53 A˚) ones, are the Mn2+ ion as the largest one We have to note that if the Ti4+( ¼ 0.605 A˚) ions occupy the A-position there will not be so strong change in the Fourier spectra There will be only a weak change in intensity of second peak, which is caused by the backscattering of electrons by the atoms, located in the A-positions (see, for example, the case of La0.7Ca0.3ÀxBaxMnO3 manganites in Ref [13]) So, it is finally possible to describe the compositions as (La0.6Sr0.4ÀxMny)(Mn1ÀyÀzTix)O3+d1 +(z/3)Mn3O4, where y and z depend on x; the atoms in first and second brackets occupy the A- and B-positions, respectively The very similar ARTICLE IN PRESS e178 A.N Ulyanov et al / Journal of Magnetism and Magnetic Materials 300 (2006) e175–e178 LayMnO3+(z/3)Mn3O4 (y % 0:9) segregation in the range 0.9XLa/MnX0.7 was reported [4] when studying the La1ÀxMnO3+d compositions The change in Curie temperature and magnetization of the B-site substituted manganites mainly originates from the weakening of the DE interaction because the breaking of the pathway for the itinerant eg electrons, caused by the difference in electron configurations between the Mn3+, Mn4+ ions and transition metal ions-change of W in (1)(E-factor); and by the structural S-factor: change of /Mn–OS bond distances and /Mn–O–MnS bond angles because the difference in Mn and dopant size ionic radii (see, e.g., [14] and references therein) The observed T C and magnetization decrease in La0.6Sr0.4ÀxMnTixO3+d was stronger than that observed in La0.7Ca0.3Mn1ÀxTixO3 [7] and La0.7Sr0.3Mn1ÀxTixO3 [8] The stronger T C decrease is obviously caused by the occurrence of Mn2+ ions and deficiency of atoms in A-position of perovskite cell in addition to the E- and S-factors In summary, the segregation of La0.6Sr0.4ÀxMny Mn1ÀyÀzTixO3+d1 phase and fallout of Mn3O4 oxide with x increase was observed The x increase caused the Mn2+ ions appearance and deficiency of atoms in Aposition, which together with the substitution of Ti for Mn in B-site caused the strong decrease in magnetization and Curie temperature, and change the character of low temperature magnetic state of high x value samples The Research at Chungbuk National University was supported by the Korean Research Foundation Grant (KRF—2003-005-C00018) A.N Ulyanov was supported by Brain Pool Program of the Korean Ministry of Educations The authors are indebted to H.D Quang for the ac susceptibility measurements References [1] J.M.D Coey, M Viret, S von Molnar, Adv Phys 48 (1999) 167 [2] R.N Zener, Phys Rev 82 (1951) 403 [3] M Medarde, J Mesot, P Lacorre, S Rosenkranz, P Fischer, K Gobrecht, Phys Rev B 52 (1995) 9248 [4] G Dezanneau, M Audier, H Vincent, C Meneghini, E Djurado, Phys Rev B 69 (2004) 014412 [5] M Wo"cyrz, R Horyn´, F Boure´e, E Bukowska, J Alloys Compd 353 (2003) 170 [6] M Phan, S Yu, K Lee, N Chau, N Tho, Abstracts of 49th Annual Conference on Magnetism and Magnetic Material, Jacksonville, Florida, USA, November, 2004 [7] X Liu, X Xu, Y Zhang, Phys Rev B 62 (2000) 15112 [8] N Kallel, G Dezanneau, J Dhahri, M Oumezzine, H Vincent, J Magn Magn Mater 261 (2003) 56 [9] D.C Koningsberger, R Prins (Eds.), X-ray absorption: Principles, Applications, Techniques of EXAFS, and XANES, Wiley Interscience, NewYork, 1988 [10] C.H Booth, F Bridges, G.H Kwei, J.M Lawrence, A.L Cornelius, J.J Neumeier, Phys Rev B 57 (1998) 10440 [11] G Subı´ as, J Garcı´ a, M.G Proietti, J Blasco, Phys Rev B 56 (1997) 8183 [12] R.D Shannon, Acta Crystallogr A 32 (1976) 751 [13] A.N Ulyanov, D.-S Yang, S.-C Yu, J Phys Soc Jpn 72 (2003) 1204 [14] A.N Ulyanov, S.-C Yu, J Appl Phys 97 (2005) 10H702  ... absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) analysis, which give information about the local structure around a central atom and the electronic configuration (valence)... strong decrease in magnetization and Curie temperature, and change the character of low temperature magnetic state of high x value samples The Research at Chungbuk National University was supported... (degree) Fig XRD patterns of La0.6Sr0.4ÀxMnTixO3+d manganites 18 16 14 50 Oe, warming rate 12 Magnetization (emu/g) e176 10 -2 100 200 300 Temperature (K) 400 Fig Magnetization vs temperature dependencies