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Journal of Magnetism and Magnetic Materials 264 (2003) 36–43 Study of Raman active phonons in NdMnO 3 S. Jandl a, *, S.N. Barilo b , S.V. Shiryaev b , A. A. Mukhin c , V.Yu. Ivanov c , A.M. Balbashov d a Centre de recherche sur les propri ! et ! es ! electroniques des mat ! eriaux avanc ! es, D ! epartement de Physique, Universit ! e de Sherbrooke, 2500 Boulevard Universite, Sherbrooke, Canada J1 K 2R1 b Institute of Solid State and Semiconductors Physics, National Academy of Science, 17 P. Brovka St. Minsk 220072, Belarus c General Physics Institute of the Russian Academy of Sciences, 38 Vavilov St., 119991 Moscow, Russia d Moscow Power Engineering Institute, 14 Krasnokazarmennaya St., 105835 Moscow, Russia Received 13 November 2002 Abstract A g and B 2g Raman active modes in NdMnO 3 single crystals, grown by flux and floating zone techniques, have been studied in the temperature range from 4.2 to 300 K. Similar to orthorhombic LaMnO 3 , the NdMnO 3 structure distorted by a static Jahn–Teller effect is consistent with the D 16 2h space group. The observed phonon frequencies, intensities and bandwidths are influenced by the temperature evolution of the Mn 3+ magnetic sublattice and, particularly, the most intense B 2g mode (B601 cm 1 ) softens below T N B75 K following the paramagnetic to canted-antiferromagnetic phase transition. r 2003 Elsevier Science B.V. All rights reserved. PACS: 78.30.HV Keywords: Manganite; Raman active phonons; Phase transition 1. Introduction Intensive effort has been devoted to the under- standing of colossal magnetoresistance in the rare- earth manganite R 1x A x MnO 3 (R=lanthanides and A=Ba, Sr, or Ca). The substitution of the rare earth ion R 3+ by a divalent cation A 2+ generates Mn 4+ leading to double exchange interactions [1] and a simultaneous observation of metallic and ferromagnetic character [2,3]. Even in the absence of substitution, some amount of Mn 4+ could be present due to non-stoichiometry [4]. The strength of the double exchange interactions in the Nd- based systems is eventually weaker than in the La- based systems, due to possible larger lattice distortions provoked by the smaller Nd ions [5]. Consequently, a closer competition would exist between the electron–phonon, electron–electron and the double exchange interactions in a system like Nd 1x Ca x MnO 3 [6], The undoped RMnO 3 parents that, are characterized by an antiferro- magnetic low-temperature ground state [7] have been comparatively less studied particularly for materials other than R=La. These are insulators and develop static Jahn–Teller distortions. The study of the lattice and magnetic properties of the ARTICLE IN PRESS *Corresponding author. Tel.: +1-819-821-8000x2909; fax: +1-819-821-8046. E-mail address: [email protected] (S. Jandl). 0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(03)00133-1

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Page 1: Study of Raman active phonons in NdMnO3

Journal of Magnetism and Magnetic Materials 264 (2003) 36–43

Study of Raman active phonons in NdMnO3

S. Jandla,*, S.N. Barilob, S.V. Shiryaevb, A. A. Mukhinc, V.Yu. Ivanovc,A.M. Balbashovd

aCentre de recherche sur les propri!et!es !electroniques des mat!eriaux avanc!es, D!epartement de Physique, Universit!e de Sherbrooke, 2500

Boulevard Universite, Sherbrooke, Canada J1 K 2R1b Institute of Solid State and Semiconductors Physics, National Academy of Science, 17 P. Brovka St. Minsk 220072, Belarus

cGeneral Physics Institute of the Russian Academy of Sciences, 38 Vavilov St., 119991 Moscow, RussiadMoscow Power Engineering Institute, 14 Krasnokazarmennaya St., 105835 Moscow, Russia

Received 13 November 2002

Abstract

Ag and B2g Raman active modes in NdMnO3 single crystals, grown by flux and floating zone techniques, have been

studied in the temperature range from 4.2 to 300K. Similar to orthorhombic LaMnO3, the NdMnO3 structure distorted

by a static Jahn–Teller effect is consistent with the D162h space group. The observed phonon frequencies, intensities and

bandwidths are influenced by the temperature evolution of the Mn3+ magnetic sublattice and, particularly, the most

intense B2g mode (B601 cm�1) softens below TNB75K following the paramagnetic to canted-antiferromagnetic phase

transition.

r 2003 Elsevier Science B.V. All rights reserved.

PACS: 78.30.HV

Keywords: Manganite; Raman active phonons; Phase transition

1. Introduction

Intensive effort has been devoted to the under-standing of colossal magnetoresistance in the rare-earth manganite R1�xAxMnO3 (R=lanthanidesand A=Ba, Sr, or Ca). The substitution of the rareearth ion R3+ by a divalent cation A2+ generatesMn4+ leading to double exchange interactions [1]and a simultaneous observation of metallic andferromagnetic character [2,3]. Even in the absenceof substitution, some amount of Mn4+ could be

present due to non-stoichiometry [4]. The strengthof the double exchange interactions in the Nd-based systems is eventually weaker than in the La-based systems, due to possible larger latticedistortions provoked by the smaller Nd ions [5].Consequently, a closer competition would existbetween the electron–phonon, electron–electronand the double exchange interactions in a systemlike Nd1�xCaxMnO3 [6], The undoped RMnO3parents that, are characterized by an antiferro-magnetic low-temperature ground state [7] havebeen comparatively less studied particularly formaterials other than R=La. These are insulatorsand develop static Jahn–Teller distortions. Thestudy of the lattice and magnetic properties of the

ARTICLE IN PRESS

*Corresponding author. Tel.: +1-819-821-8000x2909; fax:

+1-819-821-8046.

E-mail address: [email protected] (S. Jandl).

0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0304-8853(03)00133-1

Page 2: Study of Raman active phonons in NdMnO3

parent compounds represents an interesting start-ing point to the understanding of the morecomplex interactions in the doped systems.Recent neutron diffraction measurements per-

formed on the orthorhombic NdMnO3 perovskite(space group D162h-Pnma , with four formula unitsin the cell) suggest the coexistence of ferromagneticand antiferromagnetic interactions [8,9]. The Mnspins order at TNB75K and their momentssaturate at B20K [8]. While no evidence for theNd ordering, down to 1.8K, is reported in Ref. [8],the Nd sub-lattice orders in a ferromagneticarrangement with the moments parallel to they-direction below TB13K according to Ref. [9].Raman spectra, at room temperature, of poly-

crystalline NdMnO3 have been reported [10].Three broad excitations have been detectedaround 340, 500 and 610 cm�1. Similar to ortho-YMnO3 and LaMnO3 [11], they have beenassociated with the corner sharing MnO6 octahe-dra tilt (B340 cm�1) and with symmetric(B610 cm�1) and anti-symmetric (B500 cm�1)stretchings of the same octahedra basal oxygenatoms. The B600 cm�1 orthorhombic LaMnO3stretching mode softens below TN as the materialundergoes a paramagnetic to canted-antiferromag-netic transition [12]. Such a behavior has beeninterpreted in terms of spin–phonon couplingcaused by the phonon modulation of the super-exchange integral [13].In this article we report a study of the NdMnO3

Raman active phonons as a function of tempera-ture. Single crystals were grown by two differentmethods, namely flux and floating zone techni-ques. Objectives of the present work are: (i) todetermine the influence of the crystal growingprocess on the overall phonon characteristics, (ii)to compare the NdMnO3 phonon frequencies andtheir temperature evolutions with those of La-MnO3 and (iii) to confirm the presence of a spin–phonon coupling following the antiferromagnetictransition.

2. Experiment

NdMnO3 Single crystals were grown either bythe flux-based electrochemical technique (sample 1)

or by the floating zone method with radiationheating (sample 2) as described in Refs. [14,15],respectively. 0.5 cm�1 resolution Raman spectrawere measured in the back-scattering configurationusing a Labram-800 Raman microscope spectro-meter equipped with either an attached X-10(B0.014mW/(mm)2) or X-50 (B0.35mW/(mm)2)magnification microscope, and with a nitrogencooled CCD detector. He:Ne (l ¼ 6328 (A) andAr+ ion (l ¼ 4880 (A) lasers and appropriatenotch filters were used with the samples mountedon the cold finger of a micro-Helium Janiscryostat.

3. Results and discussion

There are 60 phonon modes associated with theG-point of orthorhombic NdMnO3 space groupD162h: Twenty-four of them (7 Ag+5 B1g+7 B2g+5B3g) are Raman active [11]. In Figs. 1 and 2Raman spectra, at T ¼ 4:2K, of samples 1 and 2are shown for the Ag (zz) and B2g (xz) symmetries,respectively. Raman active phonons have slightlydifferent energies in samples 1 and 2 (B2–6 cm�1).Their observed values in sample 2 at 4.2K are:205, 245, 335, 468, and 495 cm�1 for Ag symmetryand 314, 453, 482, 500, and 601 cm�1 for B2gsymmetry. Also, the Raman lines are broader insample 1 as compared to sample 2; typically for the335 cm�1 Ag line, 8 cm

�1 versus 4 cm�1 and for the601 cm�1 B2g line, 12 cm

�1 versus 7 cm�1 atT ¼ 4:2K. Line widths are temperature dependentand in order to insure minimum heating an X-10objective was used. For a given temperature, theincident intensity was reduced till the minimumline width was reached. A Raman line around650 cm�1 is detected, more pronounced, in sample1 and attributed as in the case of LaMnO3 to somedefect [11]. Heating the sample by using an X-50objective did not induce a rhombohedral phasewith its corresponding shift and broadening ofsome phonons as observed in LaMnO3 [11]. Byincreasing the power density (B0.70mW/(mm)2) aband appears around 430 cm�1 due, probably, tosome local induced defect. Such a band has beenreported in LaMnO3 and could not be clearlyassigned [12].

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S. Jandl et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 36–43 37

Page 3: Study of Raman active phonons in NdMnO3

Temperature evolutions of samples 1 and 2strongest B2g phonons (symmetric stretching of theoctahedra basal plane oxygens) are presented inFigs. 3(a) and (b), respectively. For samples 1 and2, the 606.8 cm�1 (604.2 cm�1) B2g phonon hard-ens between T ¼ 300 and 100K from 606.8 cm�1

(604.2 cm�1) up to 609.5 cm�1 (607.3 cm�1) thensoftens between T ¼ 80 and 4.2K from 608.2 cm�1

(606.7 cm�1) down to 604.8 cm�1 (601.5 cm�1).The frequencies softenings of samples 1 and 2A2g phonons B496.9 cm�1 (491.1 cm�1) thatcorrespond to anti-symmetric stretching of theoctahedra basal plane oxygens, and B329.9 cm�1

(330.87 cm�1) phonons that are associated with the

octahedra tilting are weaker than the B2g phonons.Between T ¼ 80 and 4.2K their frequencies varyfrom 501.2 cm�1 (495.9 cm�1) down to 500.5 cm�1

(495.2 cm�1), and from 335.76 cm�1 (335.45 cm�1)down to 335.16 cm�1 (334.8 cm�1). The tempera-ture evolution of the B600 cm�1 phonon fre-quency for samples 1 and 2 are presented in Fig. 4.Fig. 5, and Figs. 6(a) and (b) show, as a functionof temperature, the 601, 495, and 335 cm�1

phonon intensities and their widths at half-max-imum intensity, respectively.Frequencies of the Raman active phonons in

orthorhombic YMnO3 and LaMnO3 have beenmeasured, assigned to definite atomic motions,

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Fig. 2. B2g NdMnO3 Raman active phonons at 4.2K obtained with the exciting line l ¼ 6328 (A for: (a) sample 1 and (b) sample 2.

* mark plasma lines.

Fig. 1. Ag NdMnO3 Raman active phonons at 4.2K obtained with the exciting line l ¼ 6328 (A for: (a) sample 1 and (b) sample 2.

* mark a plasma line.

S. Jandl et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 36–4338

Page 4: Study of Raman active phonons in NdMnO3

and compared to the results of lattice dynamicalcalculations [11]. The corner-shared MnO6 octa-hedra (with two O1 and four in-plane O2 oxygensat the summits) are the building units of NdMnO3as well as of the orthorhombic YMnO3 andLaMnO3. In the MnO6 octahedra, the Mn–O1bond is along the y-axis while the Mn–O2 bondsare in the xz plane. The NdMnO3 unit cellcontains two MnO6 octahedra along the y-direc-tion and their oxygen atoms may vibrate in-phaseor out-of-phase. For the Ag and B2g modes, theMn atoms are immobile and the motions of the Ndand O1 atoms are restricted within the xz plane incontrast to the O2 atoms that could vibrate either

parallel or perpendicular to the xz plane. Ag andB2g Raman active phonons of YMnO3, LaMnO3and NdMnO3 are colligated in Table 1. The 7 Agand 7 B2g, group analysis predicted modes, havebeen observed in YMnO3, while 6 Ag and 5 B2gmodes have been reported for LaMnO3. We havedetected 5 Ag and 5 B2g modes in NdMnO3(Figs. 1 and 2) in comparison to the three roomtemperature modes reported and assigned pre-viously [10]. The lattice parameters of LaMnO3(a ¼ 5:724 (A, b ¼ 7:696 (A, and c ¼ 5:534 (A [13]),and NdMnO3 (a ¼ 5:712 (A, b ¼ 7:689 (A, c ¼5:412 (A [9]) are close and differ from YMnO3 para-meters (a ¼ 5:844 (A, b ¼ 7:358 (A, c ¼ 5:262 (A

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Fig. 3. Temperature evolution of the B601 cm�1 B2g phonon and the B495 cm�1 A2g phonon (inset) for: (a) sample 1 (from 300 to

4.2K) and (b) sample 2 (from 100 to 4.2K).

S. Jandl et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 36–43 39

Page 5: Study of Raman active phonons in NdMnO3

[11]). Considering also the atomic masses MLa ¼138:9; MNd ¼ 144:24; and MY ¼ 88:90; the differ-ences between the YMnO3 and NdMnO3 Ag andB2g Raman active phonon frequencies are due tomass and lattice parameter variations. In contrast,

the close phonon frequencies in LaMnO3 andNdMnO3 indicate that the force constants in bothmaterials are of the same order as well as the staticJahn–Teller distortion effects. This is reflected inalmost equal phonon line widths (p.e.B8 cm�1 for

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Fig. 4. Temperature evolution of the B601 cm�1 B2g phonon frequency in sample 1 (square) and sample 2 (circle).

Fig. 5. Sample 2, 601 cm�1 B2g phonon intensity temperature evolution, inset: typical temperature evolution of the A2gB495 cm�1 and

335 cm�1 phonons.

S. Jandl et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 36–4340

Page 6: Study of Raman active phonons in NdMnO3

both the 600 cm�1 mode in NdMnO3 (sample 2)and the 604 cm�1 mode in LaMnO3 at T ¼ 4:2K[16]). The Jahn–Teller distortions that are at theorigin of the relatively large line widths do notmask frequency splittings above B10 cm�1; forinstance Mukhin et al. [17] succeeded to observe inNdMnO3 the Nd3+ crystal-field ground-state

Kramers doublet splitting (B14.5 cm�1) as a resultof the Mn–Nd exchange interaction. For theB600, 500, and 335 cm�1 phonons, Martin-Car-ron et al. [10] have correlated the small frequencydifferences between LaMnO3 and NdMnO3 withthe Mn–O2 distances (2.05 (A in LaMnO3 and2.06 (A in NdMnO3), the Jahn–Teller distortion

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Fig. 6. (a) Sample 2 temperature evolution of the A2gB335 cm�1 phonon width at half-maximum (inset: idem for the A2gB495 cm

�1

phonon ), (b): Temperature evolution of the B2g B601 cm�1 phonon width at half-maximum.

S. Jandl et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 36–43 41

Page 7: Study of Raman active phonons in NdMnO3

(0.27 (A in LaMnO3 and 0.31 (A in NdMnO3), andthe octahedra tilting (16� in LaMnO3 and 18

� inNdMnO3), respectively.Sample 1 phonon line widths are broader than

those of sample 2 and their frequencies are slightlyhigher (e.g. 605 versus 601.5 cm�1 at T ¼ 4:2K).This could reflect a more pronounced oxygen non-stoichiometry in sample 1 as compared to sample2. Such a non-stoichiometry effect has beenobserved in LaMnO3+d for the B600 cm

�1 mode,whose frequency varied from 605 to 616 cm�1 for dchanging from 0.00 to 0.07 [13].The frequency temperature evolutions of the

AgB500 cm�1 phonon and the B2g B607 cm�1

phonon, presented in Figs. 3(a) and (b), indicatethat for both samples 1 and 2 the B2g phononsoftens more than the Ag phonon below theantiferromagnetic temperature ordering of theMn sublattice at B75K. The pseudo-parabolicbehavior of the softening, as a function oftemperature, of the 607 cm�1 phonon in NdMnO3(DnðTÞ in Fig. 4) maps the 604 cm�1 phononsoftening in LaMnO3 below TN: DnðTÞ; which isproportional to the square of the Mn3+ sublatticemagnetization reflects, similarly to LaMnO3, a

possible spin phonon coupling caused by thephonon modulation of the nearest neighborsexchange integral [13].Phonon intensities are strongly reinforced at low

temperatures below 80K (Fig. 5). Typically afactor of 3 for the B2g phonon and a factor of 2for the Ag phonons. Such renormalization of thephonon polarizabilities, specially below 40K,could imply additional magnetic interactionsbetween the Nd3+ and the Mn3+ sublattices.While the phonon widths are further reducedbetween 40 and 20K (Figs. 6(a) and (b)), theyremain constant below 20K possibly reflecting theMn3+ moment saturation around 2.21 mB asreported by the neutron study of Wu et al. [8].Phonon frequencies, line widths, and intensitiesexhibit no particular anomaly that would confirmthe Nd3+ sublattice ferromagnetic orderingaround TB13K as inferred by Munoz et al. [9].Similarly to Nd2CuO4 [18], the ordering of the Ndmoments in NdMnO3 would gradually build upover a large temperature range as a result of theMn–Nd exchange interaction. The splitting of theNd3+ 4f electron ground-state doublet induced bythe staggered magnetic field at the Nd3+ site inNdMnO3 [17] as well as in Nd2CuO4 [18] reflectssuch ordering evolution.

4. Conclusion

Study of Raman active Ag and B2g NdMnO3phonons, as a function of temperature in samplesgrown by the flux and floating zone techniques,indicates strong similarities with orthorhombicLaMnO3 phonons and some relative non-stoichio-metry that is manifest in phonon bandwidthbroadening. The high frequency in-phase stretch-ing 601 cm�1 phonon softens more than the Agout-of-phase stretching 495 cm�1 and out of phaseoctahedra tilt 335 cm�1 phonons following theMn3+ sublattice magnetic moment ordering belowTNB75K. The 601 cm

�1 phonon frequency-shiftevolution confirms that the spin–lattice interactionobserved in LaMnO3 below the N!eel temperatureis rather universal in the RMnO3 family. Phononfrequencies, intensities, and bandwidths inNdMnO3 are mainly sensitive to the magnetic

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Table 1

Experimental Raman frequencies, in cm�1, of YMnO3 and

orthorhombic LaMnO3 at 300K and of NdMnO3 at 4.2K

YMnO3Ref. [11]

LaMnO3Ref. [11]

LaMnO3Ref. [12]

NdMnO3this work

Ag Ag Ag Ag151 140 142

188 198 210 205

288 257* 260 245

323 284*(s) 291(s)

396 333 335(s)

497 468

518 493*(s) 493(s) 495(s)

B2g B2g B2g B2g151 109 149

220 170

317 308* 309 314

341 450 453

481 481* 484 482

537 500

616 611*(s) 609(s) 601(s)

*Indicates vibrations involving mainly oxygen atoms and (s)

refers to strong intensities.

S. Jandl et al. / Journal of Magnetism and Magnetic Materials 264 (2003) 36–4342

Page 8: Study of Raman active phonons in NdMnO3

evolution of the Mn3+ sublattice as a function oftemperature with no particular indication forNd3+ sudden moment ordering.

Acknowledgements

S.J. acknowledges support from the NationalScience and Engineering Research Council ofCanada and from le Fonds de Formation desChercheurs et l’Aide "a la Recherche du Gouverne-ment du Qu!ebec.A.M. and V.Yu.I. were partly supported by the

Russian Foundation for Basic Researchers (00-02-16500) and the program ‘‘Quantum Macrophy-sics’’ of the Russian Academy of Sciences. Thework in Minsk was partly supported by theSwitzerland National Scientific Foundation underSCOPES program grant No. 7BYPJ65732.

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