Investigation of V residence in Sr2(Fe1-xVx)MoO6 compounds

Qin Zhang1, a, Zhenfeng Xu1 Zhencui Sun1 and Keyan Wang1 1 Department of Mathematics & Physics, Shandong Jiaotong University,

Jinan, 250023, People’s Republic of China

azhangqin@sdjtu.edu.cn

Keywords: Double perovskite; V doping

Abstract. Crystal structure and magnetic properties of the double perovskite compounds

Sr2(Fe1-xVx)MoO6 (0 ≤ x ≤ 0.1) have been investigated in this work. Rietveld refinement of the crystal

structure shows that the cation ordering decrease monotonously with the V content. Magnetic

measurement shows that saturation magnetization of the compounds decrease with increasing x due

to the reduced degree of ordering. Analysis of saturation magnetization based on the ferrimagnetic

model (FIM) indicates that the V atoms in Sr2(Fe1-xVx)MoO6 selectively occupy the Mo sites instead

of Fe sites.

Introduction

Ordered double perovskite Sr2FeMoO6 (SFMO) and related oxides have been intensively studied due

to its large room-temperature low-field magnetoresistance (LFMR) [1]. The crystal symmetry of

Sr2FeMoO6 is known as cubic [2]or tetragonal [3] with the regular arrangement of the alternating

FeO6 and MoO6 octahedra. In the simple ionic picture, the itinerant Mo 4d1 electrons (S = 1/2) couple

antiferromagnetically with the localized Fe 3d5 electrons (S = 5/2) and thus a saturation magnetization

(MS) of 4µB is expected. However, due to the existence of anti-site defects (the fraction of Mo on the

Fe site or that of Fe on the Mo site), the MS of bulk materials reported so far are systematically smaller

(3.1µB [1], 3.5µB [4]) than the theoretical value (4µB). It is also reported that the LFMR of Sr2FeMoO6

increases with the decrease of anti-site defects [5]. Therefore, the anti-site defects among the Fe and

Mo sites have large influence on the magnetic and transport properties of Sr2FeMoO6. In order to

optimize its magnetic properties, researchers have investigated a lot of compounds doped on Fe site,

such as Sr2(Fe1-xCrx)MoO6 [6,7], Sr2(Fe1-xMnx)MoO6 [8], and Sr2(Fe1-xCux)MoO6 [9]. It is shown that

the variation of the degree of ordering （1-AS, AS is the concentration of anti-site defects） strongly

depends on the dopant. For example, Blasco et al [6] and Feng et al [7] found that the degree of

ordering in Sr2(Fe1-xCrx)MoO6 compounds decreased monotonously with x, in contrast to the case of

Sr2(Fe1-xMnx)MoO6, where the degree of ordering decreased at low doping level (x ≤ 0.05) and then

increased with x （0.05＜x ≤ 0.45）[8]. The degree of ordering in Sr2(Fe1-xCux)MoO6 compounds was

also found to decrease firstly (x ≤ 0.15) and then increase with x (0.15＜ x ≤ 0.3). Thus,

comprehensive studies are necessary to understand the influence of the dopant on the degree of

ordering and magnetic properties of the double perovskites. In this work, we prepare and report the

crystal structure and magnetic properties of Sr2(Fe1-xVx)MoO6 (0 ≤ x ≤ 0.1).

Experiments

Polycrystalline Sr2(Fe1-xVx)MoO6 (SFVMO) samples were prepared by standard solid-state reaction.

Stoichiometric powders of SrCO3, MoO3, Fe2O3 and V2O5 were mixed, ground and fired at 900 ℃ for

5 hours in air. The pre-reacted mixture was then finely ground, pressed into pellets and sintered at

1280 ℃ in a stream of 5% H2/Ar gas for 15 hours with several intermediate grindings. Samples were

heated and cooled at a rate of 5 ℃/min under the same atmosphere.

Advanced Materials Research Vols. 284-286 (2011) pp 2255-2258Online available since 2011/Jul/04 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.284-286.2255

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.118.88.48, Pennsylvania State University, University Park, United States of America-02/06/14,04:23:52)

Crystal structure and phase purity of the samples were examined by X-ray diffraction (XRD).

Field dependence of magnetization of the compounds was measured at 5 K on a superconducting

quantum interference device (SQUID).

Results and discussion

20 40 60 80 100

Intensity (arb .unit)

2θ θ θ θ (degree)

(101)

(200)

(112)

20 40 60 80 100

0.00 0.02 0.04 0.06 0.08 0.10

78

81

84

87

90

93

Intensity (arb .unit)

2Θ Θ Θ Θ (degree)

Obs

Cal

Obs-Cal

1-AS (%)

X

Figure 1: XRD patterns of Sr2(Fe1-xVx)MoO6. From bottom to top, x = 0, 0.03, 0.05, 0.08, 0.1.

Figure 2: The observed and calculated XRD patterns for Sr2(Fe0.92V0.08)MoO6, the vertical bar at

the bottom indicates the Bragg reflection positions, the lowest curve is the difference between the

observed and calculated XRD patterns, inset shows the composition dependence of the degree of

ordering among (Fe, V) and Mo sites in Sr2(Fe1-xVx)MoO6.

2256 Materials and Design

Fig. 1 presents the variation of XRD patterns vs composition of Sr2(Fe1-xVx)MoO6 for 0 ≤ x ≤ 0.1. All

the samples exhibit a superstructure reflection peak (101) arising from the ordered arrangement of the

Fe and Mo atoms in the double perovskite structure. The intensity of the superstructure reflection

obviously decreases with increasing V content, indicating an increase of the anti-site defects.

Based on the space group I4/m, we refined the crystal structure for all the compounds. As an

example, Fig. 2 shows the experimental and calculated XRD patterns of Sr2(Fe1-xVx)MoO6 for x =

0.08. The degree of ordering is defined as 1-AS, where AS is the concentration of anti-site defects (the

fraction of Mo on the Fe site or that of (Fe,V) on the Mo site). The degree of ordering derived from the

Rietveld refinement decreases slightly for x ≤ 0.03 and then pronouncedly with the doping level as

shown in the inset of Fig. 2, similar to the case in Sr2(Fe1-xCrx)MoO6 [7]. This indicates that V doping

increases the anti-site defects arising from the misplacement of Fe and Mo atoms.

0 1 2 3 4 5

0

10

20

30

40

M (emu/g)

H ((((104Oe))))

x=0

x=0.03 x=0.05

x=0.08

x=0.1

The magnetization curves measured at 5K is shown in Figure 3. The magnetization curves for the

compounds with x = 0 and x = 0.03 is almost identical, indicating a slight influence of the low doping

(x ≤ 0.03) on the magnetic properties of the compounds. For the compounds with x ≥ 0.05, the

magnetization increases more and more gradually at low fields, indicating that there are more and

more pinning centers preventing the domain walls from moving in the compounds.

Figure 4 shows the composition dependence of saturation magnetization (MS, at 5 K) derived by

extrapolating 1/H to zero on the M-1/H curves. The MS changes slightly for x ≤ 0.03 and decreases

rapidly with increasing V concentration, similar to the concentration dependence of the degree of

ordering. The inset shows an almost linear correlation between MS and AS, indicating the strong

influence of the V doping on the magnetization of the compounds essentially via increasing the

anti-site defects. Based on the ferrimagnetic model (FIM) [10] and the degree of ordering derived

from the Rietveld refinements, it is plausible to understand the composition dependence of MS shown

in Fig. 4 by considering different distributions of the trivalent vanadium ions. Firstly, we assume a

random distribution of V on the Fe and Mo sites, then the Fe site will be occupied by

Fe(1-x)(1-б)Vx(1-б)Moб and the Mo site by Mo1-бVxбFe(1-x)б (x is the content of V, б is the anti-site

concentration, AS). The calculated MS is shown as open squares. The difference between the

experimental value and the calculated one is relatively large. Considering the similar variation trend

of the degree of ordering between Sr2(Fe1-xVx)MoO6 and Sr2(Fe1-xCrx)MoO6 [7], we assume a

preferential substitution of V on Mo site,

Figure 3: Magnetization curves of Sr2(Fe1-xVx)MoO6 ( 0 ≤ x ≤ 0.1) at 5K.

Advanced Materials Research Vols. 284-286 2257

0.00 0.02 0.04 0.06 0.08 0.10

2.0

2.4

2.8

3.2

3.6

6 8 10121416182022

2.6

2.8

3.0

3.2

3.4

3.6

MS(( ((

µµ µµB/f.u. )) ))

X

Experimental

FIM

V3+ in Mo

MS (( ((

µµ µµB/f.u. )) ))

AS (%)

then the Fe site is occupied by Fe1-бMoб and the Mo site by Mo1-бVxFeб-x, the calculated MS is shown

as filled squares in Fig. 4. The small difference between the experimental value and the calculated one

as well as their similar concentration dependence indicate that the preferential substitution of V for

Mo may be the case in our samples.

Conclusions

Crystal structure and magnetic properties of the double perovskite compounds Sr2(Fe1-xVx)MoO6 (0 ≤

x ≤ 0.1) have been investigated systematically. Analysis of the XRD pattern indicates that all the

compounds are of single phase and belong to the I4/m space group. Saturation magnetization of the

compounds decrease with increasing x due to the reduced degree of ordering among Fe and Mo

atoms. Analysis of saturation magnetization based on the FIM model indicates that the V atoms in

Sr2(Fe1-xVx)MoO6 selectively occupy the Mo sites instead of Fe sites.

Acknowledgements

This work is supported by the Natural Science Foundation of Shandong Jiaotong University

(Z201029).

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Figure 4: Dependence of saturation

magnetization on doping content. Open

circle is the experimental value, open

square represents the calculated value

assuming a random distribution of V on

Fe and Mo sites, filled square represents

the calculated value for a preferential

substitution of V for Mo.

2258 Materials and Design

Materials and Design 10.4028/www.scientific.net/AMR.284-286 Investigation of V Residence in Sr2(Fe1-XVX)MoO6 Compounds 10.4028/www.scientific.net/AMR.284-286.2255

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http://dx.doi.org/10.1088/0953-8984/14/47/323