We suppose that the change of local structure was mainly caused by the appearance of Mn ions in structure on Curie temperature, caused by the Ti doping, is discussed.. PACS: 75.30.m; 75.
Trang 1Journal of Magnetism and Magnetic Materials 300 (2006) e175–e178
Ti-doped A-site deficient lanthanum manganites:
Local structure and properties Alexander N Ulyanova, , 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
b 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
Available online 16 November 2005
Abstract
tremendous change of local structure We suppose that the change of local structure was mainly caused by the appearance of Mn ions in
structure on Curie temperature, caused by the Ti doping, is discussed
PACS: 75.30.m; 75.30.Kz; 61.10.Ht
Keywords: Manganites; A- and B-site substitution and deficiency; Curie temperature; Local structure
Doped Ln1xRxMnO3manganese oxides are under the
extensive study due to the colossal magnetoresistivity
(CMR) effect observed at temperatures close to
ferromag-netic ordering temperature, TC(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 t2gspins)
through the O2 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
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 ¼ W0 cosðy=2Þ=d3:5 (1) was used to describe the dependence (y ¼ p /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
La1xMnO3 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
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doi:10.1016/j.jmmm.2005.10.177
Corresponding author Tel.: +82 43 271 8146; fax: +82 43 274 7811.
E-mail address: a_n_ulyanov@yahoo.com (A.N Ulyanov).
Trang 2deficient La1xMnO3 manganites To elucidate this
ques-tion, we present a study of lanthanum manganites with the
simultaneous B-site substitution and creation of vacancies
in A-position
La0.6Sr0.4xMnTixO3+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
struc-ture (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
monochroma-tized by the Si(1 1 1) double-crystal monochromator with
the energy resolution, DE/E ¼ 2 104 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
(seeFig 1)
Temperature dependencies of magnetization in the field
of 50 Oe (field cooled, warming rate) are presented in
Fig 2 The x ¼ 0 and 0.05 compositions were
ferromag-netic, and the compounds with the higher x content were in
spin(cluster)-glass-like state at low temperature The spin-glass-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): TCðx ¼ 0Þ ¼ 355 K and,
TCðx ¼ 0:05Þ ¼ 185 K The TC change was stronger than that observed in La0.7Ca0.3Mn1xTixO3 [7] and
La0.7Sr0.3Mn1xTixO3 [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 infor-mation 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
x=0.2
x=0.15
x=0.1
x=0.05
x=0
La0.6Sr0.4-xMnTixO3+δ
O4
2 Θ (degree)
Fig 1 XRD patterns of La0.6Sr0.4xMnTixO3+d manganites.
-2 0 2 4 6 8 10 12 14 16 18
50 Oe, warming rate
Temperature (K)
Fig 2 Magnetization vs temperature dependencies for the La0.6Sr0.4xMnTixO3+d manganites (x ¼ 0, 0.05, 0.1, 0.15, and 0.2 from the right to the left).
Trang 3that in the case of the La1xCaxMnO3 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 (seeFig 3, where the LSMTO and MnO XANES spectra are presented) Really, the spectrum for the La0.6Sr0.4
M-nO3 composition showed almost the same shape and position as that in La0.7Ca0.3MnO3one [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 X-ray 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 occupa-tion of A-posioccupa-tion 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 0 samples, clearly evidences for uniform distribu-tion 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 La0.6Sr0.4xMnTixO3+d
compositions one concludes to a deficiency of atoms in A-position of perovskite cell, and an excess of Mn and Ti atoms, which almost always occupy the B-site The A-position 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
La0.7Ca0.3xBaxMnO3 manganites in Ref [13]) So, it is finally possible to describe the compositions as (La0.6Sr0.4xMny)(Mn1yzTix)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
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
MnO
E (keV)
Fig 3 XANES spectra of La0.6Sr0.4xMnTixO3+d manganites (x ¼ 0:0;
0.05; 0.1; 0.15; and 0.2, from the right to the left) and MnO oxide.
x=0.0
x=0.05
x=0.1
x=0.15
x=0.2
R (Å)
Fig 4 Fourier transform of EXAFS spectra for La0.6Sr0.4xMnTixO3+d
compositions.
Trang 4LayMnO3+(z/3)Mn3O4 (y 0:9) segregation in the range
0.9XLa/MnX0.7 was reported [4] when studying the
La1xMnO3+dcompositions
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 egelectrons, caused by the
difference in electron configurations between the Mn3+,
Mn4+ions and transition metal ions-change of W0 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 TCand
magnetization decrease in La0.6Sr0.4xMnTixO3+d was
stronger than that observed in La0.7Ca0.3Mn1xTixO3 [7]
and La0.7Sr0.3Mn1xTixO3[8] The stronger TCdecrease 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 La0.6Sr0.4xMny
Mn1yzTixO3+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
A-position, 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
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