Hyperfine Interact (2009) 191:75–80 DOI 10.1007/s10751-009-9955-2

Substitution effect of Ba at the Sr sites in Sr(Fe,Re)O3 T. Yamakoshi · K. Nomura · T. Kitamori · J. Shimoyama · Z. Nemeth · Z. Homonnay

Published online: 26 March 2009 © Springer Science + Business Media B.V. 2009

Abstract Substitution effect in double perovskites, Sr(FeRe)O3 (SFRO), was studied by Mössbauer spectroscopy and magnetization measurements. The samples were prepared by a sol–gel method. The fraction of alternately ordered Fe1 for SFRO  is 72%, and the magnetization measured is consistent with 3 × 0.72 = 2.16μ B f.u., but the saturation magnetization drops to about one third of its initial value by substituting 5% Ba for Sr. It was considered to be due to the volume expansion of the lattice and the fine grains formed, as a result of the substitution of Ba ions. Keywords Double Perovskite · Sr(FeRe)O3 · Substitution by Ba and Ca · Mössbauer spectroscopy · Magnetization 1 Introduction Double perovskites, AFe0.5 M0.5 O3 (A = Ca, Sr, Ba; M = Mo, Re) have ordered structures where the B sites are occupied alternately by Fe and M ions. It is well established that the magnetoresistance ratio increases with increasing Curie temperature TC . The double perovskite SrFe0.5 Mo0.5 O3 (SFMO) with TC = 415 K shows colossal magnetoresistance (CMR) [1]. SFMO with tetragonal structure has a resistivity which increases with the decrease of TC . The CMR effect reaches maximum by substituting the Sr sites in SFMO by 5% Ca [2]. On the other hand, for SrFe0.5 Re0.5 O3 (SFRO) with TC = 405 K, the resistivity increases with Ca substitution, due to change in the hybridization of states Fe3d(t2g ) and Re5d(t2g ) [3]. The TC of CaFe0.5 Re0.5 O3

T. Yamakoshi · K. Nomura (B) · T. Kitamori · J. Shimoyama · Z. Nemeth Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan e-mail: [email protected] Z. Nemeth · Z. Homonnay Laboratory of Nuclear Chemistry, Eötvös University, Pázmány P. s. 1/A, 1117 Budapest, Hungary

76 Table 1 Lattice parameters of substituted SrFe0.5 Re0.5 O3 perovskites

T. Yamakoshi et al. Composition at the Sr site Ba0.05 Sr0.95 Sr Ca0.05 Sr0.95

Crystal

a (Å)

c (Å)

Tetragonal

5.5675 (3) 5.5589 (3) 5.5589 (3)

7.8657 (5) 7.8496 (5) 7.8537 (5)

is 539 K. SFRO transforms from tetragonal to monoclinic structure if more than 0.75 stoichiometric fraction of Ca is substituted for Sr, whereas SFMO also acquires monoclinic structure by Ca substitution more than 0.1 [1]. Thus the substitution effect for SFRO is not always consistent with that of SFMO. Nakamura et al. reported a Mössbauer study of SFRO [4]. Greneche et al. studied the antiphase domains of SFMO [5], while Klencsár et al. have shown that cation disorder (presence of antisite defects) has essential role in the transport properties of SFMO [6]. Prellier et al. studied the ferromagnetic properties of BaFe0.5 Mo0.5 O3 and CaFe0.5 Mo0.5 O3 [7] and Gopalakrishnan et al. studied the nonmetallic and metallic character of A2 FeReO6 (A = Ca, Ba) [8]. The Ba phase is cubic and metallic, while the Ca phase is monoclinic and nonmetallic. We have studied the chemical pressure effect of SFMO with 5% Ba and Ca [9]. Here we report whether or not the microstructure and magnetic properties of SFRO are affected by substitution with a small amount of Ba and Ca ions so as to keep the whole long range structure.

2 Experimental SrFe0.5 Re0.5 O3 (SFRO), Ba0.05 Sr0.95 Fe0.5 Re0.5 O3 (BSFRO) and Ca0.05 Sr0.95 Fe0.5 Re0.5 O3 (CSFRO) were prepared by using a sol–gel method, and preheated at 500◦ C for 5 h in air and finally heated at 1,000◦ C for 15 h in Ar atmosphere. The CSFRO powder was again heated at 500◦ C for 5 h after annealing at 1,000◦ C because an impurity phase was detected by XRD. Lattice constant a is not changed but c becomes larger, as an effect of substitution, as shown in Table 1. By substitution of 10% Ca for Sr, both a and c decrease [3]. CSFRO is not discussed here. The lattice parameters increased by substitution with 5% Ba. These changes are caused by the different ionic radii of Ba, Sr and Ca ions. These compounds were characterized by X ray diffraction measurements (XRD), Superconducting quantum interference device (SQUID) and 57 Fe Mössbauer spectrometry.

3 Results Figure 1 shows Mössbauer spectra of SFRO and BSFRO. The refined values of Mössbauer parameters obtained at 10 K are listed in Table 2. Figure 2 shows a horizontal projection from c axial according to the model proposed by Greneche et al. [5] and Nakamura et al. [4]. By alternating occurrence of Fe and Re through oxygen atoms (oxygen atoms are not shown in Fig. 2), the double exchange (Fe2+ – O2− − Re6+ ←→ Fe3+ − O2− − Re5+ ) occurs quickly. In our discussion the following models will be used. Alternating Fe (ordered; marked as Fe1) atoms are surrounded by six Re atoms. When Fe ions occupy a Re site, this anti-site defect (Fe2) has six Fe atoms as next neighbours. Fe3 are surrounded by five Re atoms and one Fe (Fe2) as next neighbours. Comparing Fe1–3 sites, local spin density of Fe1 is lowest because

Substitution effect of Ba at the Sr sites in Sr(Fe,Re)O3

77 Ba0.05Sr0.95(Re0.5 Fe0.5) O3

Sr(Re0.5Fe0.5) O3 1.00

1.00 0.98

a) 375K

0.96 a) 400K

0.96 -6

-4

-2

0

2

4

0.92

6

1.00

1.00

b) 350K

0.96 b) 367K 0.92

Relative Intensity

Relative Intensity

0.98 1.00

c) RT 0.96

1.00 0.96 0.92 0.88

-4

-2

0

-4 -2

0

2

4

6

8

10 12

c) RT 0.96 1.00 0.96 d) 80K 0.92

d) 80K

0.88 1.00

1.00 0.96

-6

1.00

0.96

e) 10K

0.92

0.92 0.88 -12 -10 -8

e) 10K

0.88 -6

-4 -2

0

2

4

6

8

10 12

-12 -10 -8 -6

Velocity (mm/s)

2

4

6

Velocity (mm/s)

Fig. 1 Mössbauer spectra of SrFe0.5 Re0.5 O3 (SFRO) and Ba0.05 Sr0.95 Fe0.5 Re0.5 O3 (BSFRO)

Table 2 Mössbauer parameters of SRFO and SBRFO at 10 K

SRFO 10 K

SBRFO 10 K

Isomer shift (mm/s)

Quadrupole shift 2ε (mm/s)

Hyperfine field (T)

Line width (mm/s)

Area intensity (%)

Site

0.67 (1) 0.43 (2) 0.62 (2) 0.68 (1) 0.47 (1) 0.58 (1)

−0.04 (1) 0.16 (3) −0.31 (3) −0.03 (1) 0.05 (1) −0.30 (2)

47.1 (5) 51.4 (5) 55.6 (5) 47.0 (5) 51.4 (5) 56.1 (5)

0.71 (1) 0.71 (1) 0.71 (1) 0.57 (1) 0.57 (1) 0.57 (1)

72 (1)% 14 (1)% 14 (1)% 77 (1)% 13 (1)% 10 (1)%

Fe1 Fe2 Fe3 Fe1 Fe2 Fe3

(Isomer shift is given relative to α-Fe at 300 K)

of the electron transfer in six directions to Re sites, while the Bhf is small. The Bhf of Fe3 is intermediate, and the Bhf of Fe2 (S = 5/2) is large, considering the magnetic interactions between Fe3 and Fe2. But, the hyperfine field values of Fe2 and Fe3 were reversed. The antiphase boundary [5] may have been formed, considering the intensity of Fe2 and Fe3. Isomer shift (IS) values between 0.58 mm/s and 0.68 mm/s at 10 K observed for Fe3 and Fe1 show the intermediate valence states of Fe2.9+ and Fe2.8+ , respectively, suggesting thus the double exchange. The low value of IS at antisite Fe2 is due to absence of electron exchange resulting in ionic Fe3+ . Although the deviation of lattice parameters of SFRO due to Ba substitution was found to be large (a = 0.0086 Å, c = 0.0161 Å) as compared with the deviation of lattice parameters of SFMO due to Ba substitution (a = 0.0074 Å, c = 0.0125 Å), neither the Bhf nor IS values show significant deviation with 5% Ba doping. The saturation magnetization for SFRO measured at 10 K (Fig. 3) is consistent  with 3 × 0.72 = 2.16μB Fe because the relative intensity of Fe1 for SFRO was

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Fig. 2 Atomic configuration

Re(Mo) Fe1 Fe2 Fe3

Fig. 3 Magnetization curves of SFRO and SBFRO

2

M (μB / f.u.)

1

SBFRO 0 -1

SFRO 10K

-2 -60

-40

-20

0

20

40

60

H ( kOe)

72%. However, the magnetization for BSFRO became about one third of that of SFRO. The question arises why the bulk magnetization decreased largely, while the hyperfine parameters concluded from Mössbauer spectra show only slight changes. This paradox suggests that although the local structure of iron ions is not affected by the Ba doping, long range correlations change drastically due to the introduction of Ba ions with large ionic radius. The cationic inversion may introduce some local topological frustration between Fe moments as observed in Ref. [10]. From the scanning microscope images measured, it is found that the grain sizes of BSFRO were about 120 nm in the narrow range of 80–180 nm in diameters, whereas the grain sizes of SFRO were distributed in the wide range of 80–800 nm in diameters. TC was found to be about 367 K for BSFRO and 375 K for SFRO, which are lower than the previously reported values. This finding may be caused by the fine grain size of submicron because of sol–gel synthesized powders. Investigating the temperature dependence of Mössbauer spectra of the studied compounds, an other interesting feature emerges. Although the 10 K and 80 K spectra of both SFRO and BSFRO resemble each other very well, Mössbauer spectra of the studied compounds differ above the magnetic transition temperature. Mössbauer spectrum of SFRO consists of one doublet with IS = 0.27 mm/s, quadrupole splitting (QS) of 0.96 mm/s, line width (LW) of 0.76 mm/s, and one singlet with IS = 0.29 mm/s,

Substitution effect of Ba at the Sr sites in Sr(Fe,Re)O3

79

LW = 0.76 mm/s at 375 K. On the other hand, Mössbauer spectrum of BSFRO consists of a doublet with IS = 0.26 mm/s, QS = 0.73 mm/s, LW = 0.59 mm/s, and a singlet with IS = 0.40 mm/s, LW = 0.37 mm/s at 400 K. The main difference is that the peak intensity of the singlet (15%) is smaller in the case of SFRO, than it is in the case of BSFRO (34%), while these relative intensities are much lower than those observed by Nakamura et al. [4]. The singlet can be associated with the ordered Fe1 atoms, while the doublets refer to the disordered and disorder-neighbour irons [4]. The surprisingly high intensity of the doublets in our samples should be a consequence of very low crystalline size. In small crystals one would expect higher cation disorder, and atoms at the grain boundary should also contribute substantially to the number of disordered Fe atoms. Difference in Mössbauer–Lamb factor should also be a determining source of the observed intensity ratio, and it could also explain why the intensity ratios of Fe1, Fe2 and Fe3 change significantly with lowering the temperature. On the other hand, the singlet’s isomer shift values are also in contrast with those measured in Ref. [4]. That is, in our samples iron ions are in 3+ state, and do not show the intermediate valence states. Moreover, IS value increases when the intensity of the singlet increases (as in the case of BSRFO). This may be explained as follows. If the number of ordered Fe1 atoms is lower, they must be better separated from each other. The separation suppresses the delocalization of the electrons of Re, that is the reduction of Fe3+ ions into Fe3+ /Fe2+ due to hybridization with Re electrons. This causes low IS for the Fe1 atoms, as well. All of this should be a consequence of small grains due to the sol–gel method.

4 Conclusion Mössbauer spectrometry and magnetization measurement allowed studying the substitution effect in double perovskites of SFRO and BSFRO, prepared by a solgel method. The magnetization changed drastically by the substitution by 5% Ba for Sr site although the hyperfine structures did not change appreciably. The intensity of alternately ordered Fe1 for SFRO  was 72%, and the magnetization measured was consistent with 3 × 0.72 = 2.16μB Fe. The saturation magnetization became about one third of its initial value by substituting 5% Ba for Sr. It is considered to be basically due to the expansion of lattice volume and the fine grains produced by the substitution of Ba ions. Acknowledgements Z. Nemeth thanks to Matsumae International Found for visiting to Tokyo University. Authors thank Dr. J.M. Greneche, Université du Maine for the comments.

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