Electron paramagnetic resonance studies on multiferroic Dy Mn O 3 and Dy 0.5 Sr 0.5Mn O 3S. Harikrishnan, C. M. Naveen Kumar, S. S. Rao, H. L. Bhat, S. V. Bhat, and Suja Elizabeth

Citation: Journal of Applied Physics 104, 023902 (2008); doi: 10.1063/1.2955774 View online: http://dx.doi.org/10.1063/1.2955774 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/104/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Pressure-induced phase transition in Ho 0.8 Dy 0.2 Mn O 3 multiferroic compound J. Appl. Phys. 103, 026102 (2008); 10.1063/1.2829778 Magnetically frustrated behavior in multiferroics R Mn 2 O 5 ( R = Bi , Eu, and Dy): A Raman scattering study J. Appl. Phys. 101, 09M106 (2007); 10.1063/1.2712955 Multiferroic properties of epitaxially stabilized hexagonal Dy Mn O 3 thin films Appl. Phys. Lett. 90, 012903 (2007); 10.1063/1.2429021 Features of the magnetoelectric behavior of the family of multiferroics R Mn 2 O 5 at high magnetic fields(Review) Low Temp. Phys. 32, 709 (2006); 10.1063/1.2219494 Evidence for strong spin-lattice coupling in multiferroic R Mn 2 O 5 ( R = Tb , Dy , Ho ) via thermal expansionanomalies J. Appl. Phys. 99, 08R103 (2006); 10.1063/1.2165586

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Electron paramagnetic resonance studies on multiferroic DyMnO3 andDy0.5Sr0.5MnO3

S. Harikrishnan, C. M. Naveen Kumar, S. S. Rao, H. L. Bhat, S. V. Bhat, andSuja Elizabetha�

Department of Physics, Indian Institute of Science, C. V. Raman Avenue, Bangalore 560012, India

�Received 11 January 2008; accepted 8 May 2008; published online 18 July 2008�

Electron paramagnetic resonance �EPR� studies and magnetic measurements were carried out onsingle crystals of multiferroic DyMnO3 in hexagonal as well as orthorhombic structures. Theinteresting effect of strontium dilution on the frustrated antiferromagnetism of DyMnO3 is alsoprobed using EPR. The line shapes are fitted to broad Lorentzian in the case of pure DyMnO3 andto modified Dysonian in the case of Dy0.5Sr0.5MnO3. The linewidth, integrated intensity, and geff

derived from the signals are analyzed as a function of temperature. The results of magnetizationmeasurements corroborate with EPR results. Our study clearly reveals the signature of frustratedmagnetism in pure DyMnO3 systems. It is found that antiferromagnetic correlations in these systemspersist even above the transition. Moreover, a spin-glass-like behavior in Dy0.5Sr0.5MnO3 isindicated by a steplike feature in the EPR signals at low fields. © 2008 American Institute ofPhysics. �DOI: 10.1063/1.2955774�

I. INTRODUCTION

Multiferroics hold the key to potential four-state memorydevices in the future and have initiated renewed researchactivity among material scientists.1,2 Multiferroics are simul-taneously ferroelectric and ferromagnetic �or antiferromag-netic�, and several compounds have been identified as pos-sessing multiferroic properties but the ferroelectricity indifferent multiferroic systems have different origin.3,4

Among them, RMnO3 �R=Dy,Tb,Gd� show complex mag-netic phase transitions such as commensurate-incommensurate transitions and thus the ferroelectricity inthese systems is believed to originate in the noncollinearmagnetic orderings that break the inversion symmetry. InTbMnO3, noncollinear spin order is observed at 42 K, andthe polarization develops at 27 K with the onset of incom-mensurate order that breaks the inversion symmetry.5 It isamply clear that they offer the possibility of controlling mag-netization by applying electric field and vice versa. Mostmultiferroic RMnO3, for example, TbMnO3, GdMnO3, andDyMnO3 crystallize in orthorhombic structure.6 As ionic ra-dius of R atom decreases across the lanthanide series, hex-agonal structure becomes stable compared to orthorhombicstructure. However, RMnO3 �R=Dy,Ho, . . . � can be crystal-lized in both hexagonal and orthorhombic crystal structuresby careful choice of ambience for crystal growth.7 The lit-erature on multiferroic RMnO3 suggests a strong couplingbetween the lattice and the spins. Thus, DyMnO3 offers thepossibility of studying the spin-lattice interactions for thesame rare earth and metal ions but possessing different crys-tal symmetries. Electron paramagnetic resonance �ESR� ex-periments on multiferroic TbMnO3 reported the presence ofshort-range magnetic interactions well above the antiferro-magnetic ordering temperature TN.8 The fact that the ferro-

electricity in these systems is derived from magnetic interac-tions has motivated us to employ EPR for investigating thedynamics of spins and the interactions between the spins andthe lattice in pure and Sr substituted DyMnO3.

II. EXPERIMENTAL

Single crystals of DyMnO3 and Dy0.5Sr0.5MnO3 weregrown in an infrared furnace FZ-T-10000-H-VI-VP �CrystalSystems Inc.� employing optical floating zone method. Dif-ferent atmospheres such as argon, nitrogen, and air wereused for growing the crystals. Chemical composition of thegrown crystals was determined by performing energy disper-sive x-ray analysis as well as inductively coupled plasmaatomic emission spectroscopy using Perkin Elmer Spectrom-eter Optima 2000 for greater accuracy. Powder x-ray diffrac-tograms of pulverized crystal samples were recorded usingPhilips X’Pert diffractometer with Cu K���=1.54 Å� radia-tion. Crystal structures were refined using Rietveld method.9

with FULLPROF code.10 Magnetic measurements were con-ducted in a commercial �Quantum Design� superconductingquantum interference device magnetometer in the tempera-ture range 10–300 K. EPR measurements were carried outin a Bruker spectrometer in X band with microwave fre-quency of 9.43 GHz while the temperature was varied from4 to 300 K.

III. RESULTS AND DISCUSSION

A. Crystal structure

The chemical compositions determined showed onlymarginal deviation from perfect stoichiometry. The crystalstructure of DyMnO3 grown in argon/nitrogen was refined inhexagonal P63cm while that grown in air had Pnma struc-ture. However, Dy0.5Sr0.5MnO3 could only be refined in apseudocubic structure, and no structural modifications in-duced by ambience were observed. The relevant structural

a�Author to whom correspondence should be addressed. Electronic mail:liz@physics.iisc.ernet.in. Tel: 91-80-2293 2721. FAX: 91-80-2360 2602.

JOURNAL OF APPLIED PHYSICS 104, 023902 �2008�

0021-8979/2008/104�2�/023902/6/$23.00 © 2008 American Institute of Physics104, 023902-1

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parameters are collected in Table I. The c /a ratio of hexago-nal DyMnO3 is close to that reported for other hexagonalsystems such as LuMnO3�1.88�.7 The structure of hexagonalRMnO3 consists of MnO5 bipyramids separated by a layer ofR3+ ions. A much-studied hexagonal prototype is YMnO3

which shows ferroelectricity and magnetism. Ferrolectricityin this system has been explained as arising due to the buck-ling of MnO5 polyhedra and asymmetric Y coordination.11

Recent analysis12 confirms this and shows that the R ion andoxygen change their positions along the c axis. For the ortho-rhombic structure, the lattice parameters obey the relation�c /�2��a�b, which is characteristic of the O� structureand implies strong cooperative Jahn-Teller effect and distor-tion of MnO6 octahedra. The tilting of the oxygen octahedraquantified by �, where �=180− ��Mn–O–Mn��, has a valueof 36.005° in contrast to that of LaMnO3 that has a value of24.8°.6 This clearly indicates that as the size of the rare earthcation decreases, the tilting of MnO6 octahedra increases.Dy0.5Sr0.5MnO3 crystal has pseudocubic structure irrespec-tive of the ambience of growth. All the observed peaks couldbe indexed in the cubic unit cell Pm3m. Simple cubic struc-ture Pm3m is observed in R0.5Ba0.5MnO3 �R=La, Pr, andNd� systems also.13 The value of 0.924 calculated for toler-ance factor of Dy0.5Sr0.5MnO3 is slightly lower than that ofthe R0.5Ba0.5MnO3 system. The lattice parameter of 3.824 Åobtained for Dy0.5Sr0.5MnO3 is close to that reported forpolycrystalline Dy0.5Sr0.5MnO3.14

B. dc magnetization

Magnetization profiles in the field cooled �FC� and zero-field cooled �ZFC� cycles measured for hexagonal and ortho-rhombic DyMnO3 and Dy0.5Sr0.5MnO3 are presented in Figs.1�a�–1�c�, respectively. It is observed that the magnetizationcurves of hexagonal and orthorhombic DyMnO3 crystals at atypical field of 100 Oe resemble each other. However, atvery low field �20 Oe�, splitting of the FC and ZFC is ob-served for both the crystals. The hexagonal RMnO3 systems,which are geometrically frustrated antiferromagnets, are re-ported to undergo a Neel transition below 90 K. The ortho-rhombic RMnO3 systems, on the other hand, show multiplemagnetic transitions below 40 K when they undergocommensurate-incommensurate magnetic transitions andthus develop spontaneous polarization leading to multiferro-icity. Our specific heat measurements reveal that magnetictransition occurs at 56 K for hexagonal DyMnO3 and at

32 K for orthorhombic DyMnO3.15 These transitions aresimilar in nature to the magnetic anomalies observed in spe-cific heat of noncollinear or frustrated magnets.16 However,no apparent anomaly signifying the onset of antiferromag-netism was seen in the respective magnetization profiles. It is

TABLE I. The structural details of DyMnO3 and Dy0.5Sr0.5MnO3 crystals.

DyMnO3 hexagonal DyMnO3 orthorhombic Dy0.5Sr0.5MnO3

Variance �Å2� ¯ ¯ 0.012Average radius �Å� ¯ ¯ 1.19

Tolerance factor 0.857 0.857 0.924Space group P63cm Pnma Pm3m

Lattice parameters �� a=6.1892�1� a=5.8327�1� a=3.8248�2�b=6.1892�1� b=7.3816�2�c=11.4619�4� c=5.2806�1�

Z 6 4 1Goodness of fit ��2� 2.709 2.351 2.325

FIG. 1. �Color online� dc magnetisation of �a� hexagonal DyMnO3 and �b�orthorhombic DyMnO3 at applied field of 100 Oe. Inset shows the Curie-Weiss fit. �c� Magnetization of Dy0.5Sr0.5MnO3 at applied field of 100 Oe.Inset shows clear bifurcation in FC and ZFC cycles at 20 Oe.

023902-2 Harikrishnan et al. J. Appl. Phys. 104, 023902 �2008�

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possible that the paramagnetic susceptibility of Dy3+ masksthe antiferromagnetic transition of the Mn sublattice. Inset ofFig. 1�a� shows the Curie-Weiss fit for hexagonal DyMnO3.The effective paramagnetic moment calculated from the fit is11�B and the Weiss temperature �CW is −23 K. These valuescompare well with those obtained for HoMnO3.17 Also, theobserved magnetic moment values are close to those calcu-lated from the free-ion Mn3+ �4.9�B� and Dy3+ �10.6�B�moments18 using the equation

�total = ��eff2 �Mn� + �eff

2 �Dy��1/2. �1�

The observed �CW is considerably lower than the TN re-ported for similar RMnO3 systems.17 The deviation fromCurie-Weiss curves at low temperature is suggestive of spin-canting originating from Dzyaloshinsky-Moriya �DM�interaction.19 The relevence of DM interaction in the multi-ferroic parlance is being actively discussed in recent years.20

Interestingly, for hexagonal DyMnO3, Ivanov et al.21 report alikely ferrimagnetic ordering of Dy magnetic moments attwo different crystallographic positions. This is suggestive ofhigh anisotropy in microscopic magnetic nature of hexagonalDyMnO3, which is different from other hexagonal RMnO3

systems.In the case of orthorhombic DyMnO3, the effective mag-

netic moment calculated from the fit �as shown in the inset ofFig. 1�b�� is 17�B and the �CW is −18 K. The magnetic mo-ment is higher than the theoretical value of �total=11.67�B

calculated using the spin-only values of Mn3+ and Dy3+ mo-ments. The negative �CW is an indication of the antiferro-magnetic exchange present in the system. However, at verylow temperatures, the rare earth ions order. For DyMnO3, theTDy is nearly 5 K.22 Feyerherm et al.22 find evidence ofstrong interference of Dy and Mn induced structural distor-tions besides magnetic coupling between them.

Magnetization curves of Dy0.5Sr0.5MnO3 show a clearbifurcation in the ZFC and FC cycles at low applied mag-netic field �20 Oe� as seen in inset of Fig. 1�c�. This splittingat Tirr�35 K is a characteristic feature of spin-glass state. Athigher applied fields of 100 Oe, the difference between theZFC and FC cycles is less pronounced and no sign of satu-ration is observed. The inverse susceptibility above 100 K asderived from the magnetization data with an applied field of20 Oe was fitted to the Curie-Weiss law. The effective para-magnetic moment calculated from the fit is �eff�10.90�B.This value is close to the theoretical value of 9.8�B calcu-lated assuming contributions from Dy3+ and Mn3+ /Mn4+

magnetic moments. The Weiss temperature �CW from the fitis −61.5 K. Furthermore, a cusp is observed in the ac sus-ceptibility plots �not shown�, which shifts to higher tempera-ture with increasing frequency. Systems such asY0.5Sr0.5MnO3 �Ref. 23� show a gradual increase in magne-tization as the temperature is lowered. Below 50 K, a glassymagnetic state is established in them as evidenced by theirreversibility of ZFC and FC magnetization curves. Similarfindings have been reported for Eu0.58Sr0.42MnO3 andY0.7Ca0.3MnO3.24,25 In Dy0.5Sr0.5MnO3 single crystals, thelarge paramagnetic moment of Dy apparently dominates themagnetization and the cusplike glass transition is disguised.

C. EPR studies

EPR as a microscopic probe can explore the spin dynam-ics, spin relaxations, and local internal fields.26,27 These in-formation can be very useful for the understanding of themagnetism of frustrated magnets and spin glasses.28,29 Thishas motivated us to undertake a careful EPR experiment tocomplement the magnetization measurements. The EPRspectra obtained on hexagonal and orthorhombic DyMnO3

are presented in Figs. 2�a� and 2�b�, respectively. A broadLorentzian function was used to fit the experimentally ob-served line shapes,

dP/dH = Ad/dH��H/�4�H − H0�2 + �H2�

+ �H/�4�H + H0�2 + �H2�� , �2�

where H0 is the central field, �H is the full width at halfmaximum which when divided by �3 gives the peak-to-peaklinewidth �Hpp, and A is proportional to the area under thecurve. The signals obtained for pure DyMnO3 crystals showparamagnetic behavior in the high temperature region �above100 K�. As the temperature is lowered, antiferromagneticcorrelations set in. The EPR signals of Dy0.5Sr0.5MnO3 atvarious temperatures from 300 K down to 4 K are presentedin Figs. 3�a�–3�c�. The following modified30 Dysonian func-tion was used to fit the line shapes:

FIG. 2. �Color online� EPR spectra of �a� hexagonal DyMnO3 and �b� ortho-rhombic DyMnO3 in the temperature range 150–300 K.

023902-3 Harikrishnan et al. J. Appl. Phys. 104, 023902 �2008�

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dP/dH = d/dH���H + ��H − H0��/�4�H − H0�2 + �H2�

+ ��H − ��H + H0��/�4�H + H0�2 + �H2�� , �3�

where H is the applied field, H0 is the central field, �H is thefull width at half maximum, and � is the asymmetry param-eter which is defined as the ratio of positive and negativeamplitudes.

Interestingly, the EPR spectra at low fields develop asmall steplike feature clearly seen till 40 K. Above this tem-perature, this feature vanishes. This can be correlated to thespin-glass state resulting from competing ferromagnetic andantiferromagnetic exchange interactions. Such signatures oftypical spin-glass magnetic state have been observed inLa0.5Sr0.5Co1−xFexO3 �x=0,0.1�.31,32 Near the spin-glasstransition temperature, unmistakably clear humps were ob-served in the EPR spectra of these systems. Similar features

arising due to competing ferromagnetic and paramagneticinteractions were observed in La0.7Ba0.3Mn0.7Ti0.3O3 also.33

The linewidths for hexagonal and orthorhombic DyMnO3

systems as seen in Figs. 4�a� and 4�b� are observed to in-crease as the temperature is lowered, suggesting the exis-tence of short-range antiferromagnetic correlations that per-sist well above TN. As the temperature is increased, thelinewidth decreases. This is characteristic of the paramag-netic phase of systems that exhibit spin-freezing or spin-glass behavior. The temperature dependence of linewidthwas fitted to the equation8

��T� = ��� + A exp�− �T − TN�/T0� , �4�

where ��� is the high temperature linewidth and A and T0

are empirical parameters determined from the fit. In theabove equation, the transition temperature TN was fixed atthe values obtained from specific heat measurements �56 Kfor hexagonal DyMnO3 and 32 K for orthorhombicDyMnO3�. The behavior of linewidth resembles that of other

FIG. 3. �Color online� EPR spectra of Dy0.5Sr0.5MnO3 �a� in the temperaturerange 90–300 K and �b� in the temperature range 50–90 K. �c� The lowfield cusp in the EPR spectra of Dy0.5Sr0.5MnO3. This feature vanishesabove 40 K where the spin-glass transition occurs.

FIG. 4. �Color online� Plot of linewidth of �a� hexagonal DyMnO3, �b�orthorhombic DyMnO3, and �c� Dy0.5Sr0.5MnO3. A small humplike featureis observed at near 200 K. The solid line is the fit to Eq. �4�.

023902-4 Harikrishnan et al. J. Appl. Phys. 104, 023902 �2008�

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antiferromagnets such as TbMnO3 and CaMnO3.8,34 OurEPR spectra show that short-range correlations are promi-nant till 100 K. Among the systems mentioned above,TbMnO3 gives indication of antiferromagnetic correlationpersisting for TTN. Similar observation was made by Kat-sufuji et al.35 in hexagonal LuMnO3.

The linewidth as a function of temperature forDy0.5Sr0.5MnO3 is plotted in Fig. 4�c�. It shows characteristicfeature of the paramagnetic state in spin-glass systems withlinewidth decreasing as the temperature increases. The line-width of canonical spin-glass systems is roughly temperatureindependent at high temperatures. However, for T�2Tg �Tg

is the spin-glass transition temperature�, the linewidth is�exp�−T /Tg�. Unlike in other phase transitions, one does notsee a divergence in linewidth at the spin-glass transition.Thus, the linewidth was fitted to the expression givenbelow,36

��T� = ��� + A exp�− �T/Tg�� , �5�

where ��� is the high temperature term that is independentof temperature. Tg was allowed to vary as a free parameter inthe fit to obtain a value �30 K, which is close to the spin-glass transition temperature. A complete theory for thebroadening of linewidth with temperature has to wait till animproved understanding of spin-glass phenomena is gar-nered. However, the behavior of linewidth can be attributedto slowing down of the spin-relaxation rates on approachingTg or to a broadening from distribution of local fields. Spinglasses such as EuxSr1−xS and dilute magnetic semiconduc-tors such as Cd1−xMnxTe show similar linewidthcharacteristics.37,38 At 220 K, a cusplike feature is observedin the linewidth. The origin of such a feature is not clear tous at present.

Figure 5 shows the temperature dependence of geff forthe pure and doped �inset of Fig. 5� DyMnO3 crystals. It isobserved that geff for hexagonal DyMnO3 decreases initiallywith temperature and reaches a constant value �2. Fororthorhombic DyMnO3, geff shows an almost monotonic de-crease. A comparison with the behavior of geff for ortho-

rhombic TbMnO3 �Ref. 8� suggests a trend typical of man-ganite systems. However, the doped crystal exhibits acharacteristically different behavior than pure DyMnO3. Thehexagonal RMnO3 are geometrically frustrated systems thatbelong to the class of stacked triangular antiferromagnetswith 120° spin configuration, whereas orthorhombic RMnO3

are noncollinear magnets with sinusoidal or helical spin ar-rangement. A highly frustrated magnetism along with crys-talline electric field effects and possible anisotropic exchangemechanisms could explain the variation of linewidth and geff

in multiferroic DyMnO3, whereas evidences of spin-glassstate are seen in line shapes and linewidth ofDy0.5Sr0.5MnO3.

For canonical manganite systems, the inverse of inte-grated intensity is observed to follow the same trend as1 /�.39 In the pure DyMnO3 systems, the inverse intensitydoes not scale with 1 /�. The 1 /� plots of these systems fit toCurie-Weiss equation. It must be noted that since DyMnO3

crystals belong to frustrated/noncollinear magnets, Curie-Weiss equation will not describe its microscopic magnetism.A complete understanding of the properties of hexagonalDyMnO3 would be possible only through a careful evalua-tion of the bond distances and their changes as a function oftemperature. The ferroelectric polarization has its origin inthese displacements. Studies on similar compounds such asYMnO3 �Ref. 11� indicate the necessity of such an investi-gation. On the other hand, for orthorhombic RMnO3, theinteresting multiferroic properties are closely linked to itsmagnetic order. Previous studies40,41 have found that themagnetism of RMnO3 changes from A type to E typethrough incommensurate structures. A close relative,HoMnO3,42 shows coexistence of commensurate antiferro-magnetic phases along with incommensurate phase at lowtemperatures. DyMnO3 also belong to the incommensurategroup. Thus, we think that to explain the properties of thesesystems, a clear understanding of the magnetic structure isrequired. However, in the doped system, inverse susceptibil-ity follows Curie-Weiss curve till 200 K. This temperature iswell above the spin-glass transition temperature and mightsuggest that the magnetic correlations exist even above Tg.

IV. CONCLUSIONS

The negative value of �CW calculated from Curie-Weissfits indicates antiferromagnetic behavior in DyMnO3 sys-tems. The EPR spectra also indicate such a behavior below�100 K. Even though specific heat measurements revealed amagnetic transition at 56 K, we observe from the EPR stud-ies that short-range antiferromagnetic correlations exist evenfor TTN. This illustrates the frustrated magnetic naturecharacteristic of hexagonal RMnO3 systems. Similar featuresobserved in magnetization and EPR curves of orthorhombicDyMnO3 are attributed to the complex noncollinear mag-netic orderings. Interestingly, strontium doping forcesDyMnO3 to adopt a pseudocubic structure, and the antiferro-magnetism in the parent compound is replaced by a spin-glass-like state in Dy0.5Sr0.5MnO3. The EPR signals of thissystem show a clear steplike feature at low fields that van-ishes above 40 K, which is very close to the spin-glass tran-

FIG. 5. �Color online� Plot of geff as a function of temperature for hexagonaland orthorhombic DyMnO3. Inset shows the geff for Dy0.5Sr0.5MnO3.

023902-5 Harikrishnan et al. J. Appl. Phys. 104, 023902 �2008�

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sition temperature as determined from magnetization and acsusceptibility measurements. The fits to observed linewidthsconfirm the frustrated magnetism in pure DyMnO3 and spin-glass state in Dy0.5Sr0.5MnO3.

ACKNOWLEDGMENTS

We acknowledge the financial support for optical float-ing zone furnace from the Department of Science and Tech-nology �DST�, Government of India through the FIST pro-gram. DST is also thanked for NSTI project funding to oneof us �S.V.B.�. One of us �H.L.B.� also acknowledges thefinancial support from CSIR, India.

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