Structural and magnetic properties of the double perovskite Sr2MnWO6

Journal of Magnetism and Magnetic Materials 237 (2001) 124–134

Structural and magnetic properties of the doubleperovskite Sr2MnWO6

A.K. Azada,c,*, S. Ivanovb, S.-G. Erikssona,c, H. Rundl .ofd, J. Eriksena,R. Mathieue, P. Svedlindhe

aStudsvik Neutron Research Laboratory, Uppsala University, SE-611 82 Nyk .oping, SwedenbKarpov Institute of Physical Chemistry, 103064 Moscow, Russia

cDepartment of Inorganic Chemistry, University of Gothenburg, SE-412 96 G .oteborg, Swedend Inorganic Chemistry, (Angstr .om Laboratory, SE-751 21 Uppsala, Sweden

eDepartment of Materials Science, Uppsala University, Box 534, SE 75121 Uppsala, Sweden

Received 23 May 2001; received in revised form 31 July 2001

Abstract

The nuclear and magnetic structure and the magnetic properties of the polycrystalline double perovskite Sr2MnWO6

have been studied. Rietveld analysis of neutron powder diffraction (NPD) data at T ¼ 295K shows that the sample istetragonal (space group P42/n, a ¼ 8:0119ð4Þ (A, c ¼ 8:0141ð8Þ (A). Some additional magnetic diffraction peaks werefound in the NPD pattern at 10K, which can be accounted for by antiferromagnetic ordering of spins at the Mn sites.

The magnetic unit cell is doubled in all three unit axes directions (a ¼ b ¼ 15:9984ð8Þ (A, c ¼ 16:012ð2Þ (A) and themanganese moments are coupled antiferromagnetically along the unit cell axes. The total magnetic moment of Mn2+ isfound to be 2.27(7) mB. The antiferromagnetic behaviour was confirmed from magnetisation measurements. Thetransition from a paramagnetic to an antiferromagnetic state takes place at 13.070.1K. r 2001 Elsevier Science B.V.

All rights reserved.

PACS: 61.12.Ld; 75.25.+z

Keywords: Neutron diffraction; Double perovskites; Crystal structure; Magnetic materials; Magnetic structure

1. Introduction

The modification of structural and magneticproperties of B-site ordered double perovskite

oxides, caused by a change of B-site cations, is ofgreat interest, and of use when trying to under-stand the mechanism of e.g. colossal magneto-resistance [1–5]. Depending on their valencesand relative radii the B-site is often occupiedby transition metals, and can accommodatetwo different metal ions. Strontium based doubleperovskites, with different B-site cations,Sr2B

0B00O6, show very interesting properties, for

*Corresponding author. The Studsvik Neutron Research

Laboratory, SE 61182, Nykoping, Sweden. Tel.: +46-155-

221871; fax: +46-155-263001.

E-mail address: [email protected] (A.K. Azad).

0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 0 1 - 7

example, half-metallic ferromagnetic behaviour inSr2FeMoO6 [6], and insulating antiferromagneticbehaviour in Sr2FeWO6 [7]. B-site ordered per-ovskite-type oxides, where B0 is magnetic and B00 isnonmagnetic, have been extensively studied byBlasse [4]. Some compounds of this type withordered perovskite structure and a Jahn–Tellerion (Mn3+, Ni3+ (low spin) or Cu2+), show aconsiderable tetragonal distortion with c=a > 1:The structure and physical properties of this typeof double perovskites depend considerably on thevalence of the B0 and B00 cations. They can adoptdifferent types of crystal structure such as Sr2Fe-MoO6 [8] that can show cubic or tetragonal latticesymmetry [3,6,9], Sr2NiWO6 cubic symmetry [4],and Sr2CuWO6 [1,4,10] tetragonal symmetry.When these compounds include transition metalions at the B-sites, the ordering of the B-site ionsalso influences the magnetic properties. Forinstance, the double perovskite oxides, Ba2Co-NbO6 [3], Sr2FeRuO6, BaLaNiBiO6 and Ba2Ln-NbO6 [11–14], show spin-glass behaviour at lowtemperature. More recently, Asano et al. [15] haveshown that it is possible to have either positive ornegative magnetoresistance in thin films of Sr2Fe-MoO6 grown by pulsed laser deposition. Thesestructural distortions are of interest not only froma crystallographer’s point of view, but also becausethey can have important effects on the physicalproperties of perovskite compounds, particularlyin electrical and magnetic properties.Sr2MnWO6 (SMW) has been thought of as a

cubic double perovskite [4] with a ¼ 8:01 atT ¼ 295K. To our knowledge no work has yetbeen done to characterise this sample in detail. Wehave synthesised the compound in single-phaseform and undertaken X-ray and neutron diffrac-tion studies. Rietveld refinement [16] of NPD datashows that the nuclear and magnetic structure wastetragonal. The space groups P42/n and P1 wereused to determine the nuclear and magneticstructures, respectively. Magnetic refinement at10K shows that the magnetic structure is based ona unit cell related to that of the nuclear structureby a propagation vector ð12

1212Þ: Antiferromagnetic

behaviour was confirmed during magnetisationmeasurement and the antiferromagnetic to para-magnetic transition was found at 13.070.1K.

2. Experimental

Stoichiometric amounts of high purity powderof SrCO3, MnO and WO3, were mixed in an agatemorter using ethanol. The mixed powder wasplaced in an alumina crucible and calcined at1223K for 15 h. The sample was pressed into apellet and reacted at 1373K for 48 h, 1473K for72 h, 1523K for 48 h and 1613K for 48 h in an N2

environment. The sample was reground in eachstep, and grinding and pelleting cycles were carriedout to insure the homogeneity of the sample.X-ray diffraction patterns were obtained from

Guinier film data (CuKa1=1.540598 (A). Thesedata were used to index the pattern. Indexing andrefinement of the lattice parameters were madewith the programs TREOR90 [17] and PCPIRUM[18]. Neutron powder diffraction (NPD) data werecollected at the 50MW R2 Research Reactor atStudsvik, Sweden. The double monochromatorsystem consisting of two parallel copper crystals in(2 2 0) orientation was aligned to give a wavelengthof 1.470(1) (A. The neutron flux at the sampleposition was approximately 106 neutrons cm�2 s�1.A vanadium can was used as the sample holder.The step scan covered the 2y-range 41–139.921with step size 0.081.NPD data sets were refined by the Rietveld

method using the FullProf software [19]. Amagnetic phase was included in the refinement asa second phase for which only the Mn cations weredefined. Diffraction peak shapes were consideredas pseudo-Voigt. Background intensities weredescribed by a Chebyshev polynomial with sixcoefficients. Peak asymmetry corrections weremade during refinements.DC magnetisation measurements were per-

formed in a Quantum Design SuperconductingQUantum Interference Device (SQUID) magnet-ometer. Magnetisation vs. temperature curveswere measured between 5 and 200K in field-cooled (FC) and zero-field-cooled (ZFC) modes.Fields up to m0H ¼ 3T were used. Electronscanning microscopy (ESM) micrographs weretaken in a Philips XL30 at an acceleration voltageof 20 kV. Energy dispersive X-ray (EDX) analysiswas also performed using the same microscope.EDX analyses confirm that the actual cationic

A.K. Azad et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 124–134 125

composition of the perovskite-related crystallites isclose to the nominal composition ‘‘Sr2MnWO6’’and that the sample is homogeneous. Secondharmonic generation (SHG) was measured usinga powder technique with a pulsed Nd :YAG laser(l ¼ 1:064 mm). The measured signal was normal-ised to a-quarts.

3. Results and discussion

3.1. Structural refinement

The neutron diffraction data were collected atthree different temperatures, 295, 40, and 10K.The reflection h k 0; h þ ka2n; 0 0 l; la2n and0 k 0; ka2n were systematically absent, whichallows us to propose the tetragonal space groupP42/n (No. 86). All other possible tetragonal spacegroups were checked and finally P42/n was selectedas the correct one, which is a subgroup of the spacegroup describing simple cubic perovskite, Fm-3m.SHG measurement shows that SMW was centro-symmetric at room temperature (the value of SHGsignal is o0.01, which is less than the value foundfor the SiO2 standard). It also indicates that thechoice of a centrosymmetric space group wascorrect. To illustrate the combined effect of cationordering and octahedral tilting consider the tetra-gonally distorted (a�a�a�) [20] tilt system. Theunit cell parameters are related to that of idealcubic perovskite as aE2ap; bE2ap; aE2ap(apE3:89 (A). The refinements resulted in a fairlygood R-factors (Rp ¼ 3:89%; Rwp ¼ 5:15%; Rexp ¼3:82%; RBragg ¼ 2:97%).The observed and calculated patterns, differ-

ences and the peak positions for the neutrondiffraction pattern at room temperature are shownin Fig. 1. Structure parameters, interatomic dis-tances and R-factors obtained from the analysisare summarised in Table 1. The position para-meters are summarised in Table 2. The Wyckoffpositions were Mn on 4c, W on 4d, Sr on 2a, 2b, 4eand O on 8g (3 positions), which gives an orderedperovskite structure of so-called elpasolite type.Mn and W are found to occupy alternate B-sites.Fig. 2 shows a schematic representation of thedouble perovskite structure of SMW in the space

group P42/n. The MnO6 octahedra are elongatedalong the c-axis. The distances between Mn and Oin the ab plane and parallel to c-axis are about2.159(9), 2.129(4) and 2.180(5) (A, respectively(Table 3). WO6 octahedra are compressed alongthe c-axis and the distances between W and O inthe ab plane and parallel to c-axis are about1.932(3), 1.908(4) and 1.866(5) (A, respectively. TheMnO6 octahedra (volume=13.3610(4)) are signifi-cantly larger than WO6 octahedra (volume=9.1504(7)). The MnO6 and WO6 octahedra areordered and alternate along the three directions inthe crystal structure in such a way that each MnO6

octahedra is linked to six WO6 octahedra and vice-versa (see Fig. 2). Based on the ionic radii of Sr2+

(r ¼ 1:58 (A), Mn2+ (r ¼ 0:97 (A), W6+ (r ¼ 0:74 (A)and O2� (r ¼ 1:26 (A) [21] in appropriate coordina-tion number, the tolerance factor was calculated tobe t ¼ 0:949: Bond valence calculation from ourobserved data shows that the charge distributionof Mn and W cations are Mn2+ and W6+,respectively.

3.2. Magnetic structure

The 10K NPD data reveal some extra magneticreflections, an especially strong one at 2yE9:11(see Fig. 3). In the magnetic phase, space group P1was used, resulting in 32 independent atomicpositions for manganese in the magnetic unitcell. An antiferromagnetic (AF) structure wasmodelled where the magnetic moments wereattributed to the Mn positions. The observed extrareflections could be indexed on the basis of a largerunit cell, having the approximate lattice para-meters am ¼ bm ¼ 2an and cm ¼ 2cn; where anand cn are the lattice parameters of the nuclearunit cell.The best possible model we found for the

magnetic structure of this compound is one inwhich manganese-containing layers are arrangedalong the c-axis and these layers are coupledantiferromagnetically. This arrangement may bedescribed as a G-type ordering of the magneticmoments (see Fig. 4). The suggested model ac-counts for the extra intensity, especially for thereflection that occurred at 2yE9:11: This modelalso give a satisfactory agreement between

A.K. Azad et al. / Journal of Magnetism and Magnetic Materials 237 (2001) 124–134126

observed and calculated intensities (Rp ¼ 4:46%;Rwp ¼ 5:83%; Rexp ¼ 3:86%; RBragg ¼ 3:20% andRmag ¼ 7:24%). A resulting magnetic moment of2.27(7) mB was obtained.

3.3. Magnetisation measurements

Fig. 5(a–d) shows the FC (MFC=H) and ZFC(MZFC=H) magnetisation vs. temperature curves

Fig. 1. Observed (circles) and calculated (continuous line) NPD intensity profiles for Sr2MnWO6 at room temperature (295K). The

short vertical lines indicate the angular position of the allowed Bragg reflections. At the bottom of the figure the difference plot,

Iobs � Icalc; is shown.

Table 1

Refined structural parameters of Sr2MnWO6 at RT (295K). Chemical cell: s.g. P42/n, a ¼ b ¼ 8:0119ð4Þ (A, c ¼ 8:0141ð8Þ (A.

Rp ¼ 3:89%; Rwp ¼ 5:15%; RBragg ¼ 2:97%; w2 ¼ 1:81

Atom Positions x y z B ( (A)2 Occupancy

Sr(1) 2a 0.25 0.25 0.25 0.29(7) 2.000

Sr(2) 2b 0.25 0.25 0.75 0.66(8) 1.69(22)

Sr(3) 4e 0.75 0.25 0.2671(7) 0.19(6) 4.13(18)

Mn 4c 0.00 0.00 0.00 0.11(8) 3.89(21)

W 4d 0.00 0.00 0.50 0.07(6) 4.01(20)

O(1) 8g 0.2624(9) �0.0399(7) �0.0123(7) 0.34(9) 7.97(34)

O(2) 8g 0.2341(8) 0.0375(8) 0.5234(7) 0.24(9) 8.31(31)

O(3) 8g 0.0148(9) 0.0321(7) 0.2698(8) 0.65(8) 7.12(37)


for SMW measured with the applied fields m0H ¼0:002; 0.05, 0.2, and 3T, respectively. Fig. 5(a)shows that a magnetic transition occurs atTcB40K. The magnetic interactions are predomi-nantly AF, but below 40K a small spontaneousmoment develops, like in a canted-AF state [22],resulting in a magnetic state that can be describedas a weak ferromagnet. The cusp around 20–40Kin the ZFC curves is another property that isfound compounds exhibiting a canted-AF state[22]. The presence of a spontaneous moment isfurther emphasised by the magnetic irreversibilityexhibited by the FC and ZFC curves; below thetransition temperature, the ZFC and FC curvesshow large deviations. With increasing field, thetransition is masked by the larger and linear infield AF-like response of the material (cf Fig. 5(b)).Magnetic hysteresis curves obtained at 5 and 40Kare shown in Fig. 6, confirming the existence of a

Table 2

Fractional atomic coordinates, cell constants, bond lengths and Rietveld refinement reliability factors of Sr2MnWO6 at 295, 40 and

10K. Nuclear structure was refined using tetragonal space group P42/n and magnetic structure at 10K was refined in space group P1

Parameters Sr2MnWO6

295K 40K 10K

an ¼ bn (nucl) 8.0119(4) 7.9995(3) 7.9992(4)

cn (nucl) 8.0141(8) 8.0061(7) 8.0058(8)

am ¼ bm (mag) F F 15.9984(8)

cm (mag) F F 16.012(2)

Vn 514.44(6) 512.23(5) 512.27(6)

Vm F F 4098.1959(1)

mMn F F 2.27(8)

z of Sr(3) 0.2671(7) 0.2699(6) 0.2692(6)

x of O(1) 0.2624(9) 0.2661(8) 0.2661(8)

y of O(1) �0.0399(7) �0.0430(5) �0.0434(5)z of O(1) �0.0123(7) �0.0187(5) �0.0195(6)x of O(2) 0.2341(8) 0.2325(8) 0.2334(9)

y of O(2) 0.0375(8) 0.0325(5) 0.0320(6)

z of O(2) 0.5234(7) 0.5300(6) 0.5297(6)

x of O(3) 0.0148(9) 0.0168(9) 0.0174(9)

y of O(3) 0.0321(7) 0.0271(9) 0.0259(9)

z of O(3) 0.2698(8) 0.2560(9) 0.2576(8)

R-factors, Rp(%) 3.89 3.94 4.46

Rwp(%) 5.15 5.08 5.83

RBragg(%) 2.97 3.26 3.20

Rmag(%) F F 7.24

w2 1.81 2.94 2.27

Fig. 2. The crystal structure of the tetragonal perovskite

Sr2MnWO6.


weak ferromagnetic response but indicating thatthe predominant character of the response is linearin field. This behaviour is analogous to thatobserved for Ba2MnWO6 [23], where a canted-AF phase was observed belowB45K. In the NPDdata for Ba2MnWO6, diffuse magnetic scatteringwas observed in the temperature range from 10 to50K, which was explained by the existence ofsmall magnetic domains, showing antiferromag-netic order, but being of a size too small to giverise to distinct magnetic Bragg peaks. Since forSr2MnWO6, we do not observe distinct Braggpeaks at 40K, it is our strong belief that also inthis compound the canted-AF phase consists ofsmall AF domains. The closely related compoundCa2MnWO6 shows a similar behaviour [24].

On further lowering of the temperature atransition to a low temperature AF phase isobserved at TN ¼ 13:070:1K, as also evidencedby the appearance of resolution limited magneticBragg peaks in the NPD data at 10K. However,the magnetic state below TN is not that of an idealantiferromagnet, since magnetic irreversibilityremains down to the lowest temperature studied(cf Figs. 5 and 6). Thus, a small canting angleexists even below TN giving rise to a netspontaneous magnetic moment.Above the transition temperature Tc the sample

shows typical Curie–Weiss behaviour. The datawere fitted to a Curie–Weiss law, w ¼ C=ðT � yÞ;where C is the Curie constant and y is the Weisstemperature, yielding C ¼ 0:696 and y ¼ �71:3K.

Table 3

Main bond distances (dp3:5 (A) of Sr2MnWO6 from 295, 40 and 10K from neutron diffraction data refinements

295K

Sr1-O1 ( (A)� 4 3.134(6) Mn-O1 ( (A)� 2 2.129(4)

Sr1-O2 ( (A)� 4 2.777(5) Mn-O2 ( (A)� 2 2.159(9)

Sr1-O3 ( (A)� 4 2.574(1) Mn-O3 ( (A)� 2 2.180(5)

Sr2-O1 ( (A)� 4 3.005(9) Distortion (D)a 0.02369

Sr2-O2 ( (A)� 4 2.492(9) W-O1 ( (A)� 2 1.932(3)

Sr2-O3 ( (A)� 4 3.103(6) W-O2 ( (A)� 2 1.908(4)

Sr3-O1 ( (A)� 2 2.648(3)/2.588(8) W-O3 ( (A)� 2 1.866(5)

Sr3-O2 ( (A)� 2 2.853(1)/3.277(5) Distortion (D) 0.03458

Sr3-O3 ( (A)� 2 2.747(7)/2.957(4)

40K

Sr1-O1 ( (A)� 4 3.184(4) Mn-O1 ( (A)� 2 2.162(6)

Sr1-O2 ( (A)� 4 2.841(6) Mn-O2 ( (A)� 2 2.168(6)

Sr1-O3 ( (A)� 4 2.580(9) Mn-O3 ( (A)� 2 2.064(1)

Sr2-O1 ( (A)� 4 2.990(7) Distortion (D) 0.04902

Sr2-O2 ( (A)� 4 2.479(4) W-O1 ( (A)� 2 1.907(6)

Sr2-O3 ( (A)� 4 3.077(7) W-O2 ( (A)� 2 1.893(6)

Sr3-O1 ( (A)� 2 2.607(5)/2.592(5) W-O3 ( (A)� 2 1.970(7)


Sr3-O3 ( (A)� 2 2.782(9)/2.905(1)

10K

Sr1-O1 ( (A)� 4 3.191(4) Mn-O1 ( (A)� 2 2.162(7)

Sr1-O2 ( (A)� 4 2.871(5) Mn-O2 ( (A)� 2 2.160(6)

Sr1-O3 ( (A)� 4 2.583(7) Mn-O3 ( (A)� 2 2.077(1)

Sr2-O1 ( (A)� 4 2.988(1) Distortion (D) 0.040122

Sr2-O2 ( (A)� 4 2.483(3) W-O1 ( (A)� 2 1.909(1)

Sr2-O3 ( (A)� 4 2.3.074(8) W-O2 ( (A)� 2 1.900(2)

Sr3-O1 ( (A)� 2 2.596(6)/2.600(9) W-O3 ( (A)� 2 1.957(1)


Sr3-O3 ( (A)� 2 2.792(5)/2.894(6)

aDistortion (D) in MO6 octahedra=(Longest M–O bond distanceFshortest M–O bond distance)/average M–O bond distance.


The effective magnetic moment, as calculated fromthese results is p ¼ 5:69 mB: The Weiss temperatureof the sample is negative in sign, confirming thatthe magnetic interaction between Mn ions is AF toits nature.The ordered magnetic moment of 2.27 mB found

from NPD data at 10K is significantly lower thanthe effective moment obtained from high tempera-ture magnetisation data. This discrepancy can beattributed either to the effect of disorder in themagnetic interactions or to spin fluctuations closeto the antiferromagnetic transition temperature.The antiferromagnetic exchange interactions areof Mn–O–W superexchange type, where theMn–O–W bond angle is 1801. Disorder can arisefrom mis-site disorder on the octahedral cationsublattices (Mn,W) and/or the presence of oxygenvacancies [9]. Since we have not found anysignificant evidence of cation mis-site disorder,

Fig. 3. Observed (dots), calculated (lines) and difference (bottom) plots of NPD Rietveld profiles at 10K.

Fig. 4. The orientation of magnetic moments in Sr2MnWO6.

The arrows indicate the direction of the magnetic moments at

the Mn-sites.


cation vacancies or oxygen vacancies during ourrefinement from NPD data, it is our belief thatdisorder in the superexchange interaction cannotbe at the origin of the comparably small momentfound from NPD data. However, the N!eeltemperature of the compound is TN ¼ 13K andthe magnetic moment was estimated at T ¼ 10K,which is rather close to the N!eel temperature(T=TN ¼ 0:77). The estimated moment will bereduced because of spin fluctuations (Mn momentfluctuations), since spin fluctuations imply that the

time average of the moment is smaller than themagnitude of the moment. If the NPD measure-ments would be repeated at 2K, one would obtaina closer match between the two estimates of themanganese magnetic moment.

4. Conclusions

The B-site ordered double perovskite-typeoxide, Sr2MnWO6, has a tetragonal structure.

Fig. 5. (a–d). MFC=H and MZFC=H vs. temperature for Sr2MnWO6 measured with the applied fields m0H (a) 0.002, (b) 0.05, (c) 0.2

and (d) 3 T.


Rietveld analysis of NPD data shows that thisdouble- perovskite crystallises in space groupP42/n. The magnetic unit cell is related to thenuclear one as am ¼ 2� an; bm ¼ 2� bn and cm ¼2� cn: From BVS calculation the charge distribu-tion between Mn and W ions in SMW wascalculated, indicating the presence of Mn2+ andW6+. The magnetisation results indicate a transi-tion from a paramagnetic state to a weakferromagnetic state at B40K. Furthermore, thereis a transition to an AF state at 1370.1K.

Analysis of the 10K NPD data supports theoccurrence of an AF state at low temperature.

Acknowledgements

The authors are grateful to the Royal SwedishAcademy of Sciences, to the Swedish NaturalScience Research Council (NFR), and to theSwedish foundation of Strategic Research (SSF),for financial support. One of the authors,

Fig. 5. (Continued).


A.K. Azad, is grateful for the financial supportfrom ‘‘Research, development and training projectof Bangladesh Atomic Energy Commission’’. Theauthors are also grateful to Dr. S. Stefanovich forhis SHG study of the sample.

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Structural and magnetic properties of the double perovskite Sr2MnWO6

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Transcript of Structural and magnetic properties of the double perovskite Sr2MnWO6