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doi:10.1016/j.ph

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Physica B 381 (2006) 233–238

www.elsevier.com/locate/physb

Crystal structure, magnetic and electrical-transport propertiesof rare-earth-doped Sr2FeMoO6

Q. Zhanga, G.H. Raoa,�, Y.G. Xiaoa, H.Z. Dongb, G.Y. Liua, Y. Zhanga, J.K. Lianga,c

aBeijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of ChinabUniversity of Qinghai, Xining 810016, People’s Republic of China

cInternational Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China

Received 18 November 2005; received in revised form 9 January 2006; accepted 15 January 2006

Abstract

Crystal structure, magnetic and electrical-transport properties of the rare-earth-doped compounds (Sr1.85Ln0.15)FeMoO6 (Ln ¼ Sr,

La, Ce, Pr, Nd, Sm and Eu) have been investigated by means of X-ray diffraction, magnetic and electric measurements. All the samples

are single phase and belong to the I4/m space group. Due to the competing contributions of electron doping and steric effects, the unit-

cell volume of the doped compounds changes slightly and does not vary systematically with the ionic radius of the rare-earth ions. The

temperature dependence of the magnetization of (Sr1.85Ln0.15)FeMoO6 indicates that the Curie temperature of the doped compounds has

increased upon doping, except for the Eu-doped compound. The saturation magnetization of the compounds at 5K cannot be explained

simply by considering the ferrimagnetic arrangement of the magnetic sublattices of Fe and Mo because of the magnetic contribution of

rare-earth ions. However, the saturation magnetization at 100K of the doped compounds exhibits a linear decrease with the increase of

the anti-site defect concentration. Comparison of the saturation magnetization of the compounds at 5 and 100K suggests that at 5K the

magnetic moments of Ce, Pr, Nd and Sm align parallel with that of Fe, while the magnetic moment of Eu is anti-parallel with that of Fe.

The normalized resistivities of the parent and the La- and Sm-doped compounds exhibit a semiconductor-metal transition, while the Pr-,

Nd- and Eu-doped compounds exhibit semiconducting behavior.

r 2006 Elsevier B.V. All rights reserved.

PACS: 61.72.Ww; 75.50.�y; 75.30.Cr; 75.50.Gg

Keywords: Double perovskite; Electron doping; Saturation magnetization; Degree of ordering

1. Introduction

Ordered double perovskites of the type A2BB0O6 (A is an

alkaline-earth atom, B and B0 are transition-metal atoms)have attracted considerable scientific and technologicalinterest in recent years owing to the room-temperature low-field magnetoresistance (LFMR) of Sr2FeMoO6 (SFMO)[1–4]. The crystal structure of SFMO can be visualized as aregular arrangement of corner-sharing FeO6 and MoO6

octahedra, alternating along the cubic axes in all threedirections with the voluminous Sr cations occupying thevoids between the octahedra. In the case of simpleferrimagnetic arrangement of the magnetic sublattices of

e front matter r 2006 Elsevier B.V. All rights reserved.

ysb.2006.01.467

ng author. Tel.: +861082648089; fax: +86 1082649533.

ss: ghrao@aphy.iphy.ac.cn (G.H. Rao).

Fe and Mo and assuming a spin-only contribution, so thatthe moments of the Fe3+ and the Mo5+ sublattices will be5 and 1mB/f.u., respectively, a saturation magnetization of4mB/f.u. is expected for perfectly ordered SFMO.Band-structure calculations have predicted that SFMO

is a half-metal [1,4], where the Mo t2g spin-down sub-band(hybridized with the Fe t2g spin-down sub-band) crossesthe Fermi level, while the spin-up sub-band, essentiallycomposed of Fe t2g and Fe eg states, is separated by aninsulating gap. Owing to the highly polarized character ofthe conduction electrons, the magnetoresistance (MR) ofSFMO can be interpreted as a spin-dependent electrontunneling through grain boundaries [1]. The grain-bound-ary resistivity can be tuned by the relative angle betweenthe magnetization of ferromagnetic (FM) domains, whichis influenced by an applied magnetic field. This mechanism

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20 40 60 80 100 120 140

Inte

nsity

(ar

b.un

it)

2θ (degree)

(101)

Fig. 1. XRD patterns of (Sr1.85Ln0.15)FeMoO6. From bottom to top,

Ln ¼ Sr, La, Ce, Pr, Nd, Sm and Eu.

Q. Zhang et al. / Physica B 381 (2006) 233–238234

helps to understand the LFMR observed in polycrystallineSFMO.

The two important parameters for possible applicationsof SFMO are its Curie temperature (TC) and the MR. Onthe one hand, the LFMR decreases as the temperatureapproaches TC, so that a higher TC can extend theworking-temperature range of the spin-electronics devices.As reported by Tovar et al. [5], the strength of magneticcoupling is directly related to the density of electrons at theFermi level. Therefore, in order to enhance the TC ofSFMO, injection of electrons into the conduction band byappropriate doping is a natural strategy. This has beenevidenced by the substitution of trivalent La3+ or Nd3+

for the divalent Sr2+ in SFMO [6–10]. On the other hand,as indicated in Ref. [1], the normalized magnetoresistanceMR(T, H0)/MR(0, H0) of SFMO is approximately inaccordance with (M/MS)

2, where H0 ¼ 0.2 T and MS issaturation magnetization of the compound. This can beexplained by the following scenario: provided the spin-dependent scattering at the grain boundary is responsiblefor MR, a large moment makes it easy for an applied fieldto align the magnetization of the grains, so that tunnelingof the spin-polarized carriers across the grain boundarybecomes easy, resulting in a large LFMR. Computersimulation and extensive experiments manifest that MS ofSFMO decreases with the anti-site (AS) defect content(defined as the fraction of Mo atoms on an Fe site, which isequal to the fraction of Fe atoms on a Mo site) [6,8,11–13].The LFMR of SFMO also shows a linear decrease with theAS content [14]. Band-structure calculations indicate thatinvestigation of the reduction of the magnetization ofSFMO is useful for understanding the mechanism ofmagnetic interactions in the compound [15,16]. For rare-earth-doped SFMO, it would be interesting to examine theinfluence of the rare-earth moment on the magnetization ofthe compound, in addition to the effect of enhancing TC. Inthis work, we synthesized a series of the rare-earth-doped(Sr1.85Ln0.15)FeMoO6 compounds (Ln ¼ Sr, La, Ce, Pr,Nd, Sm and Eu) and investigated the crystal structure,magnetic and electrical-transport properties of the com-pounds at different temperatures.

2. Experiment

Polycrystalline (Sr1.85Ln0.15)FeMoO6 (Ln ¼ Sr, La, Ce,Pr, Nd, Sm and Eu) samples were prepared by standardsolid-state reaction. Stoichiometric powders of SrCO3,La2O3, CeO2, Pr6O11, Nd2O3, Sm2O3, Eu2O3, MoO3 andFe2O3 were mixed, ground and heated at 900 1C for 5 h inair. The pre-reacted mixtures were then finely ground,pressed into pellets and sintered at 1280 1C in a flow of 5%H2/Ar gas for 12 h with several intermediate grindings. Thesamples were heated and cooled at a rate of 5 1C/min underthe same atmosphere.

The crystal structure and phase purity of the sampleswere examined by X-ray diffraction (XRD) using a RigakuD/max 2500 diffractometer with Cu Ka radiation

(50 kV� 250mA) and a graphite monochromator. TheXRD data were collected in the 2y range of 15–1401 by astep-scan mode with a step width of 2y ¼ 0.021 and asampling time of 1 s. The XRD patterns showed that allsamples were single phase without detectable impurity. TheXRD data were analyzed by means of the Rietveldrefinement program FULLPROF [17,18].The field dependence of the magnetization was measured

at 5 and 100K by means of a superconducting quantuminterference device (SQUID). The temperature dependenceof the magnetization was recorded by a vibrating-samplemagnetometer in a field of 500Oe. The transport propertieswere measured by the standard four-probe method.

3. Results and discussion

The XRD patterns of (Sr1.85Ln0.15)FeMoO6 withLn ¼ Sr, La, Ce, Pr, Nd, Sm and Eu are shown in Fig. 1.All reflections can be indexed based on the space group I4/m [19–21]. The space groups I4/mmm [22,23] and P42/m[24] have ever been used to describe the crystal structure ofSFMO. As reported in Ref. [19], oxygen has three non-equivalent positions in the space group P42/m, i.e., O1 at(x y 0), O2 at (x y 0) and O3 at (0 0 z). If the parameters forO1 are coupled as xO1 ¼ yO1, O2 as xO2 ¼ �xO1 andyO2 ¼ xO1+

12, the descriptions of SFMO by I4/m and P42/

m would be equivalent. The difference between I4/m andI4/mmm is that I4/m permits both deformations androtations of octahedral, while I4/mmm allows only thedeformation of octahedra. Recently, several neutron-powder-diffraction results have indicated that the mostprobable space group is I4/m [20,21]. Therefore, the crystalstructure of the compounds investigated in this work hasbeen indexed and refined using space group I4/m. Asshown in Fig. 1, the XRD pattern exhibits the (1 0 1)superstructure reflection resulting from the ordered ar-

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Table 1

Lattice and atomic parameters, degree of Fe/Mo ordering (Z ¼ 1�AS), thermal factor (B), reliability factors (Rwp, Re) and Curie temperature (TC) of

(Sr1.85Ln0.15)FeMoO6

Doping element Sr La Ce Pr Nd Sm Eu

a (A) 5.5704 (1) 5.5698 (1) 5.5704 (1) 5.5676 (1) 5.5679 (1) 5.5676 (1) 5.5695 (1)

c (A) 7.8983 (1) 7.9017 (1) 7.9075 (1) 7.9040 (2) 7.9016 (1) 7.9014 (2) 7.8970 (0)

c=a 1.418 1.420 1.419 1.420 1.419 1.418 1.418

V (A3) 245.07 (1) 245.41 (1) 245.36 (1) 244.99 (1) 244.96 (1) 244.95 (1) 244.96 (1)

Z (%) 86.85 85.72 77.59 77.73 81.89 79.11 80.86

O1 8 h (x y 0)

x 0.2774 (4) 0.2751 (2) 0.2782 (1) 0.2745 (2) 0.2783 (2) 0.2764 (2) 0.2727 (2)

y 0.2226 (4) 0.2249 (2) 0.2218 (1) 0.2254 (2) 0.2217 (2) 0.2235 (2) 0.2272 (2)

O2 4e (0 0 z)

z 0.2510 (4) 0.2541 (3) 0.259 (1) 0.2540 (4) 0.2609 (4) 0.2607 (5) 0.2566 (4)

BSr/Ln (A2) 0.45 (1) 0.44 (1) 0.31 (2) 0.56 (1) 0.57 (1) 0.39 (1) 0.51 (1)

BFe (A2) 0.22 (1) 0.13 (1) 0.12 (1) 0.20 (1) 0.26 (1) 0.10 (1) 0.13 (1)

BMo (A2) 0.02 (1) 0 0 0 0.03 (1) 0 0

Rwp (%) 12.4 12.5 11.1 12.7 12.5 12.2 13.2

Re (%) 6.73 7.16 6.14 7.12 7.00 6.01 7.31

TC (K) 410 427 422 426 428 425 409

The space group is I4/m and the chemical formula per unit cell is 2. The Sr ion is located at 4d (120 14), Fe at 2a (0 0 0) and Mo at 2b (0 0 1

2).

Q. Zhang et al. / Physica B 381 (2006) 233–238 235

rangement of Fe and Mo atoms on the B and B0 sites in theperovskite structure. The degree of cationic orderingamong B and B0 is defined as Z ¼ 1�AS, then AS ¼ 0.5means a complete disorder on the Fe and the Mosublattices. The intensity of the (1 0 1) reflection of thedoped compounds decreases with respect to the parentcompound SFMO, indicating that the degree of cationicordering is decreased by the doping with trivalent rare-earth ions. The variation of the unit-cell volume with thedoping elements is listed in Table 1. It is interesting to notethat, in comparison to the parent compound SFMO, theunit-cell volume of the doped compounds changes onlyslightly and does not vary regularly with the ionic radius ofrare-earth ions. This can be explained by two factors: one isthe steric effect, the other is the electron-doping effect. Onthe one hand, the cell volume of the compounds will bereduced because of the substitution of smaller rare-earthions for the larger Sr ions. On the other hand, as reportedin Refs. [6,7], electron doping can increase the unit-cellvolume of the compounds because the extra electronprovides delocalized carriers selectively to the spin-downt2g metallic spin channel, which will increase the density ofstates at the Fermi level. These carriers can modify thebond lengths through screening of the ionic potentials thatdetermine the interatomic distances. The influence of theelectronic state on the volume has been reported formanganite perovskite [25]. For our compounds, the pureelectron-doping effect counteracts the steric effect, whichprobably results in an almost unchanged unit-cell volume.A similar phenomenon has been found in the Nd-dopedSFMO [10].

The structure of the compounds has been refined basedon powder XRD data, in which the parameter for O1 on8 h (x y 0) is coupled as x+y ¼ 1

2. In the preliminary

refinement, the thermal factors for atoms at differentpositions were fixed to the values of SFMO derived fromneutron diffraction [21], then the degree of cationicordering among Fe and Mo was refined. Afterwards, werefined the thermal factors under the condition that thedegree of cationic ordering is not changed. However, therefined thermal factors for O atoms are negative or havelarge uncertainty because of the small X-ray-scatteringfactor for O atoms. Thus, in the subsequent refinement, thethermal factors of O atoms have been given based on theresults of powder neutron diffraction [21], and those for Sr(Ln), Fe and Mo atoms were refined. After several cycles ofthe alternately refinement of the thermal factors and thedegree of ordering, all the parameters, including profileparameters, atomic parameters, thermal factors and thedegree of ordering, have been refined simultaneously. Thefinal atomic parameters, degree of the cationic ordering,thermal factors for Sr (Ln), Fe and Mo atoms and thereliability factors of the refinement for (Sr1.85Ln0.15)Fe-MoO6 are listed in Table 1. As an example, Fig. 2 showsthe refinement result of (Sr1.85Nd0.15)FeMoO6. The experi-mental and calculated XRD patterns, their difference andthe expected peak positions are shown in the figure.Compared to the parent compound, the degree of orderingof the doped compounds (Table 1) has decreased. It is wellknown that the degree of cationic ordering is correlated tothe charge difference between the Fe and Mo ions. Asevidenced by powder neutron diffraction [26], nuclearmagnetic resonance [27] and Mossbauer [28] experiments,the doping electrons selectively enter the Mo sites andreduce the valence of the Mo ions, while the valence of theFe ions is almost unchanged. Thus, the charge differencebetween the Fe and the Mo ions has reduced and thedegree of ordering has decreased accordingly.

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-10000

0

10000

20000

30000

40000

50000

Inte

nsity

(co

unts

)

2θ (degree)

(Sr1.85Nd0.15)FeMoO6

Rwp: 12.5%

Fig. 2. Observed (filled curve) and calculated (crosses) XRD patterns for

(Sr1.85Nd0.15)FeMoO6. The vertical bars at the bottom indicate the Bragg

reflection positions, and the lowest curve is the difference between the

observed and the calculated XRD patterns.

1.26 1.32 1.38 1.442.4

2.6

2.8

3.0

3.2

SrNdLa

PrEu

MS (

µ B/f.

u.)

Sm

Ce

Ionic radius (Å)

SrLaCePrEuNdSm

0 1 2 3 4 50

10

20

30

40

14 16 18 20 222.5

2.6

2.7

2.8

2.9

M (

emu/

g)

0

10

20

30

40

M (

emu/

g)

H (104 Oe)

0 1 2 3 4 5

H (104 Oe)

Sr

La Nd

Pr EuSm

Ce

MS (

µ B/f.

u.)

AS (%)

La

NdEu

PrSm

Ce

(a)

(b)

Fig. 3. Magnetization curves of (Sr1.85Ln0.15)FeMoO6 (a) measured at

5K. The inset shows the ionic-radius dependence of the saturation

magnetization at 5K (filled squares) and 100K (open squares) (b)

measured at 100K. The inset shows the correlation between the saturation

magnetization at 100K and the anti-site defect concentration.

Q. Zhang et al. / Physica B 381 (2006) 233–238236

The field dependence of the magnetization of the(Sr1.85Ln0.15)FeMoO6 compounds (Ln ¼ Sr, La, Ce, Pr,Nd, Sm and Eu) measured at 5 and 100K is shown in Figs.3(a) and (b), respectively. The magnetization increasesrapidly with external field, and then saturates at a value MS

that is derived by extrapolating 1/H to zero in M vs. 1/Hplots, indicating that all the compounds are FM. As shownin the inset of Fig. 3(a), in comparison with the parentcompound SFMO, the value of MS at 5K (filled squares)has decreased markedly for the La-, Ce-, Pr- and Eu-doped compounds, while that of the Sm- and Nd-dopedcompounds has decreased only slightly. Among the dopedcompounds, MS of the Sm- and Nd-doped compounds isobviously larger than that of the La-, Ce-, Pr- and Eu-doped compounds. Based on the ferrimagnetic model(FIM, i.e. the moments on the Fe- and Mo-sublatticescouple ferrimagnetically [11]), if the amount of dopingelectrons is the same and the Ln atoms do not form a long-range magnetic order, the saturation magnetization of the(Sr0.85Ln0.15)FeMoO6 should be the same in case of perfectFe/Mo ordering, then the saturation magnetization of thecompounds should decrease linearly with increasing ASconcentration. Thus, MS of the La-doped compoundshould be larger than those of the Sm- and Nd-dopedcompounds, which is inconsistent with the observation.Therefore, the irregular variation of MS at 5K cannot besimply explained by the FM arrangement between themoment of Fe and Mo. Taking into account the lowordering temperature of the magnetic moments of the rare-earth ions, we measured the magnetization curves of thecompounds at 100K (Fig. 3(b)), expecting to eliminate thecontribution of the moments of the rare-earth ions to themagnetization of the compounds. The derived MS values at100K are shown in the inset of Fig. 3(a) as open squares,where a minimum of MS occurs for the Ce-dopedcompound due to the higher AS concentration. Comparedto MS at 5K (filled squares), MS at 100K of the non-

magnetic La- and Sr-doped compounds has changed only alittle, whereas MS of the Ce-, Pr-, Sm- and Nd-dopedcompounds are reduced pronouncedly, but MS of(Sr1.85Eu0.15)FeMoO6 has increased by about 0.27mB/f.u.These observations suggest that the moments of Ce, Pr, Smand Nd ions may be ordered at low temperature (such as5K) and parallel to that of the Fe ion, while the moment ofEu is anti-parallel to that of the Fe ion (noting that Eu3+ isusually in an excitated state already at low temperature dueto the small energy difference between the first excited leveland the ground state level). At higher temperature (such as100K), the long-range order of the moments of rare-earthions collapses and, consequently, the magnetization ofthe Ce-, Pr-, Sm- and Nd-doped compounds decreases,while that of Eu-doped compound increases. As shown inthe inset of Fig. 3(b), for the doped compounds, MS at100K shows an almost linear correlation with the AS

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300 350 400 450 500

M(T

)/M

(300

K)

(arb

.uni

t)

T (K)

Nd

La

Pr

Eu

Sr

Sm

Ce

Fig. 4. Temperature dependence of the magnetization of (Sr1.85Ln0.15)Fe-

MoO6.

0 50 100 150 200 250 3000.8

1

1.2

1.4

1.6

1.8

T (K)

Eu

Nd

Pr

Sr

La

ρ (T

) / ρ

(273

K)

Sm

Ce

Fig. 5. Temperature dependence of the normalized resistivity of

(Sr1.85Ln0.15)FeMoO6 compounds. The dotted lines in the figure are

drawn to clarify the behavior of the semiconductor–metal transition.

Q. Zhang et al. / Physica B 381 (2006) 233–238 237

concentration, indicating that the influence of AS on MS

dominates and is responsible for the decrease of MS.Fig. 4 shows the temperature dependence of the

magnetization of the compounds. For sake of clarity, allcurves have been normalized to the magnetization at 300Kand have been shifted upward. The transition temperature(TC) is determined as the inflection point of the M–T curve.The FM–paramagnetic (PM) transition becomes broaderfor the doped compounds. The strong magnetic couplingbetween the Fe and Mo moments can be explained in termsof the double-exchange mechanism via Fe–O–Mo–O–Fecharge transfer. Therefore, the strength of magneticinteraction should be closely related to the density ofcharge carriers. As shown in Table 1, similar to the resultsin Refs. [6–9], the derived TC values of the La-, Ce-, Pr-,Sm- and Nd-doped compounds increase by about 15K dueto the reinforcement of magnetic interaction arising fromelectron doping. The rate of increase (dTC/dx) is about 2Kper percent rare earth, which is comparable with 1.6K/(%of Nd) reported in Ref. [29]. As evidenced by Arrott plots[9] and nuclear magnetic resonance [24], the enhancementof the magnetic interaction upon electron doping isessentially due to the band filling effects arising from thepreferred occupancy of the doping electron in the spin-down t2g metallic channel. However, TC of (Sr1.85Eu0.15)-FeMoO6 is almost unchanged compared to that of theparent compound. It is well known that the usual oxidationstate of Eu is +2. Some of the Eu in (Sr1.85Eu0.15)FeMoO6

may exhibit a divalent state which may weaken theelectron-doping effect on TC, thus explaining the almostunchanged TC of (Sr1.85Eu0.15)FeMoO6.

As a consequence of different preparation conditions,SFMO may exhibit insulating, semiconducting or metallicbehavior [30]. In our case, as shown in Fig. 5, the

normalized resistivity of the parent compound and theLa- and Sm-doped compounds exhibits semiconductingbehavior at low temperature and metallic behavior at hightemperature. A similar metal–semiconductor-transitionphenomenon has been reported in the systems (La1�xSrx)-VO3 [31] and Sr2(Fe1�xCrx)MoO6 [32]. The resistivity ofthe Ce-, Nd-, Pr- and Eu-doped compounds exhibitssemiconducting behavior in the investigated temperaturerange from 5 to 300K.

4. Conclusions

We have presented a systematic study on the structural,magnetic and electric properties of rare-earth-dopeddouble-perovskite (Sr1.85Ln0.15)FeMoO6 (Ln ¼ Sr, La,Ce, Pr, Nd, Sm and Eu) compounds. The structural studyindicates that all compounds are single phase and can beindexed according to the I4/m space group. The unit-cellvolume of the doped compounds changes slightly becauseof the competing contribution of the electron doping andthe steric effects. The injection of doping electrons into theMo orbital reduces the charge difference between the Feand the Mo ions, leading to a decrease of the degree ofcationic ordering of the doped compounds. Magneticmeasurements reveal that the saturation magnetizationMS of the compounds at 5K cannot be explained by simplyconsidering the ferrimagnetic arrangement of the moments

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on the Fe and Mo sublattices due to the magneticcontribution of rare-earth ions. Eliminating the influenceof magnetic ordering of rare-earth ions on MS of thecompounds, MS at 100K of the doped compoundsdecreases linearly with increasing AS defect concentration.Contrary to the case of the Ce-, Pr-, Sm- and Nd-dopedcompounds, MS of the Eu-doped compound at 100K islarger than that at 5K, which indicates that the moment ofEu is probably anti-parallel to that of the Fe ions at lowtemperature, while the moments of Ce, Pr, Sm and Nd areparallel to that of the Fe ions. Similar to the results in Refs.[6–10], the Curie temperature of the doped compoundsincreases upon doping except for the Eu-doped case. Thenormalized resistivity of the parent compound and the La-and Sm-doped compounds exhibits a semiconductor–metaltransition, while the Ce-, Pr-, Nd- and Eu-doped com-pounds exhibit semiconducting behavior.

Acknowledgments

This work is supported by the National NaturalFoundation of China and by the State Key Project ofFundamental Research.

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