Multiferroic properties of ErMnO3

Materials Research Bulletin 44 (2009) 2123–2126

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Multiferroic properties of ErMnO3 Jyoti Ranjan Sahu a, Anirban Ghosh a,b, A. Sundaresan a, C.N.R. Rao a,b,* a

Chemistry and Physics of Materials Unit, and International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India b Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 1 July 2009 Accepted 6 July 2009 Available online 15 July 2009

Measurements of magnetic and dielectric properties of single crystalline ErMnO3 establish the Ne´el and ferroelectric transition temperatures to be 77 K and 588 K respectively. The dielectric constant of ErMnO3 shows an anomalous jump at TN. At higher temperatures, the dielectric constant undergoes a significant decrease on application of magnetic fields. The study clearly exhibits multiferroic and magnetoelectric nature of ErMnO3. ß 2009 Elsevier Ltd. All rights reserved.

Keywords: A. Oxides B. Crystal growth D. Magnetic properties D. Dielectric properties

1. Introduction Hexagonal rare earth manganites of the type LnMnO3 (Ln = Ho, Er, Yb, Lu, Y) represent a fascinating family of multiferroic compounds due to the interplay between their charge and spin degrees of freedom [1,2]. Possible applications of these materials in random access memory devices based on the ferromagnetic and ferroelectric properties of epitaxial thin films have been suggested [3]. One of the most intensively studied multiferroic compounds, YMnO3, shows a ferroelectric transition at 910 K and a canted antiferromagnetic transition at 70 K [4]. Canted antiferromagnetism in YMnO3 is explained on the basis of magnetic frustration arising out of the three Mn3+ ions in a triangular lattice (trimer) located in the z = 0 and z = 1/2 planes of the unit cell as evidenced by neutron scattering [5–7]. Other studies based on measurements like thermal conductivity [8] and Raman scattering [9] have also yielded results on the nature of magnetic ordering in YMnO3. Ferroelectricity in YMnO3 arises from the combined effect of the tilting of cornersharing MnO5 polyhedra and buckling of Ln (Ln = Ho, Er, Yb, Lu, Y) planes leading to the absence of centrosymmetry. Ferroelectric polarization occurs perpendicular to the layers along the c-axis [10,11]. The ferroelectric transition involves a change from P63cm symmetry to P63/mmc symmetry [12]. There is also evidence for

* Corresponding author at: Chemistry and Physics of Materials Unit, and International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India. Fax: +91 80 22082760. E-mail address: [email protected] (C.N.R. Rao). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.07.005

coupling between magnons and phonons to support the magnetoelectric nature of YMnO3 [13]. Hexagonal HoMnO3 is also reported to be multiferroic exhibiting a similar spin-lattice coupling [14]. ErMnO3 which belongs to the hexagonal rare earth manganite family is expected to show multiferroic properties similar to YMnO3. While there are several reports on the magnetic properties of ErMnO3, the ferroelectric and multiferroic properties are not clearly documented [15–18]. We have, therefore, investigated the multiferroic properties of single crystals of ErMnO3. 2. Experimental Polycrystalline ErMnO3 was prepared by the conventional high temperature ceramic method. A mixture containing stoichiometric quantities of Er2O3 and MnO2 was ground with propanol and the mixture dried at 800 8C, followed by intermediate grinding and sintering at 1008 intervals for 30 h up to 1200 8C. After final grinding, the resulting monophasic polycrystalline powder was hydrostatically pressed into rods and sintered at 1250 8C to obtain rods of around 4 mm in diameter and 90–100 mm in length. Single crystals were grown by the floating-zone method (SC-M35HD, Nichiden Machinery Ltd., Japan) fitted with two ellipsoidal halogen lamps. The rotation speed of the feed and seed was maintained at 15 rpm and single crystals grown from the melt at a pulling rate of 3 mm/h. The single crystals were annealed at 900 8C for 12 h and at 800 8C for 24 h to improve the oxygen stoichiometry in the sample. X-ray diffraction (XRD) patterns of the single crystal and powder were recorded by a Seifert 3000 TT diffractometer, employing Cu Ka (l = 1.54056 A˚) radiation. The powder of ErMnO3

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crushed from the single crystals gave an XRD pattern consistent with the P63cm space group (a = b = 6.1110 A˚, c = 11.4029 A˚) [19]. Energy dispersive X-ray analysis (EDAX) was used to check the overall chemical homogeneity and composition of the sample. Magnetic properties were measured using the VSM option in a Physical Properties Measurement System (PPMS, Quantum Design, USA) instrument. Specific heat was measured by using the heat capacity option in the PPMS. In order to study the ferroelectric properties of the sample, the single crystals were polished and coated with gold in an argon atmosphere. They were, then, heated for 2 h at 300 8C at a heating rate of 1 8C/min to ensure proper electrical contacts. Dielectric properties of the resulting capacitors were carried out by using a Precision Impedance Analyser (Agilent 4294 A). Magnetic fields were applied on the sample with a superconducting magnet (Cryo Industries, USA). Ferroelectric hysteresis loops were obtained by employing Radiant Technologies Inc. Precision 4 kV HVI instrument. 3. Results and discussion In order to determine the growth axis of the single crystals of ErMnO3, crystals were cut perpendicularly on both sides. The XRD patterns showed the (3 1 6) plane to be perpendicular to the growth direction. The growth axis is [3 2 1] which subtends an angle of 52.328 with the c-axis. The parallel cut corresponds to the plane perpendicular to (3 1 6) plane, the crystallographic direction being [2 0 1]. DC magnetic susceptibility measurements showed only a weak anomaly at the Ne´el temperature. The magnetic susceptibility data indicating AFM transition at 77 K agrees well with the neutron diffraction, thermal expansion and other measurements reported by Park et al. [15]. The derivative of the reciprocal of susceptibility clearly shows a peak at 77 K as can be seen from the inset in Fig. 1(a). As we do not see a distinct magnetic

Fig. 1. Temperature variation of (a) reciprocal dc–magnetic susceptibility and (b) specific heat of ErMnO3 showing the antiferromagnetic transition temperature (TN). Inset in (a) shows a plot of the derivative of reciprocal of magnetic susceptibility against temperature.

transition at the Ne´el temperature in the susceptibility data, we measured the specific heat capacity from 2 K to 160 K. The data shown in Fig. 1(b) show a clear peak at 77 K indicating the onset of magnetic ordering. Below 4 K, we observe a rise in heat capacity, probably due to the Er3+ moment ordering as reported earlier [20]. The magnetic susceptibility follows the Curie–Weiss law with a Curie–Weiss temperature of 20 K and an effective magnetic moment, meff, of 9.9mB. This value of meff is consistent with the 1=2 theoretical value of 10.7mB calculated as meff ¼ ½m2Er þ m2Mn  , where mEr (9.6mB) and mMn (4.9mB) are the effective magnetic moments of Er3+ and Mn3+ respectively. The ratio f = ucw/TN which is a measure of geometric frustration [21] is around 10 for YMnO3. It is significantly reduced in ErMnO3. We have measured the high temperature dielectric properties of ErMnO3 along both the [3 2 1] and [2 0 1] directions of the crystal. The dielectric constant was recorded at various temperatures over a frequency of 100 Hz to 1 MHz range. In Fig. 2, the dielectric constant is plotted as a function of temperature at different frequencies. The curves are characterized by a maximum for each frequency, the maximum being broader for crystals along the [2 0 1] direction compared to the [3 2 1] direction. The maximum values of the dielectric constant observed at 100 kHz along the [2 0 1] and [3 2 1] directions are 157 and 312 respectively. The temperature corresponding to the maximum of the dielectric constant, Tmax, shifts towards higher temperature with increase in frequency. In order to obtain the frequencyindependent ferroelectric transition temperature (TFE) below which frequency dispersion is appreciable [22], we have used the Curie–Weiss plots wherein the reciprocal of dielectric constant is plotted against temperature as shown in the insets of Fig. 2(a) and (b). From these plots, we derive a TFE of 590 K and 586 K for ErMnO3 along the [2 0 1] and [3 2 1] directions respectively. In order to confirm the ferroelectric nature of ErMnO3, we carried out P–E hysteresis measurements for both the crystal orientations at different voltages. We obtain hysteresis loops (Fig. 3) which do not exhibit saturation of polarization, indicating

Fig. 2. Temperature dependence of the dielectric constant of ErMnO3 over the 100 Hz to 1 MHz range along (a) [2 0 1] and (b) [3 2 1] directions of the crystal showing the ferroelectric transition. The insets in (a) and (b) show the Curie–Weiss plots to determine the ferroelectric transition temperature (TFE).

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Fig. 3. Ferroelectric hysteresis loops (P–E plots) along the [2 0 1] direction at 5 kHz at various voltages.

ErMnO3 to be a leaky ferroelectric material [23]. Along the [2 0 1] direction, the remnant polarization and coercive field are around 0.38 mC/cm2 and 12 kV/cm respectively at a frequency of 5 kHz at room temperature, corresponding to an applied voltage of 1600 V. We have investigated the dependence of the dielectric constant of ErMnO3 at low temperatures with and without the application of magnetic fields. Fig. 4(a) and (b) shows the plots of dielectric constant at 200 kHz for crystals along the [2 0 1] and [3 2 1] directions. In both the cases, there is a notable increase in the dielectric constant with the increase in temperature. Beyond 250 K, the increase is significantly more rapid in the [2 0 1] direction. The value of the dielectric constant at 310 K is 25 and 34 respectively for crystals along [2 0 1] and [3 2 1] directions. The most remarkable feature of the measurements in Fig. 4 is the anomaly at the Ne´el temperature (77 K) which establishes the presence of coupling between magnetic and ferroelectric order parameters. In order to illustrate the occurrence of the dielectric

Fig. 4. Temperature variation of the dielectric constant at 200 kHz (T < 300 K) along the (a) [2 0 1] and (b) [3 2 1] directions of the ErMnO3 crystal with and without the application of magnetic fields (1 T, 3 T). The inset in (a) shows the differential plot of the dielectric constant along the [2 0 1] direction showing an anomaly at the Ne´el temperature.

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anomaly at TN more prominently, we have plotted the derivative of the dielectric constant against temperature along the [2 0 1] direction in the inset of Fig. 4(a). There are reports in the literature wherein such an anomaly in the dielectric constant is observed at the magnetic transition in other members of the LnMnO3 family [24–27]. The coupling between the ferroelectric and antiferromagnetic domain walls can be interpreted as a local magnetoelectric effect and is explained microscopically as a piezomagnetic interaction between the domain walls [28]. Hexagonal manganites like ErMnO3 undergo an isostructural transition at TN without breaking the P63cm symmetry, simultaneously producing giant displacements of every atom in the unit cell [29]. The large magnetoelastic coupling is the origin of the multiferroic phenomenon in ErMnO3. The magnetoelectric effect in ErMnO3 is also evidenced by the effect of magnetic fields on the dielectric constant at high temperatures. We see from Fig. 4 that the response to the application of magnetic field of 1 T is less pronounced in the [2 0 1] direction as compared to the [3 2 1] direction. When subjected to a magnetic field of 3 T, it brings about a dramatic impact in the reduction of the dielectric constant. 4. Conclusions ErMnO3 exhibits a ferroelectric transition temperature of around 588 K. It is clearly a multiferroic with an antiferromagnetic transition at 77 K (TN). There is coupling between the electrical and magnetic order parameters since we observe a dielectric anomaly at TN and magnetic field-induced decrease in the dielectric constant above 250 K. Acknowledgement One of us (JRS) thanks University Grants Commission (UGC), Government of India for a fellowship. References [1] J.S. Zhou, J.B. Goodenough, J.M. Gallardo-Amores, E. Moran, M.A. Alario-Franco, R. Caudillo, Phys. Rev. B 74 (2006) 014422. [2] C.N.R. Rao, C.R. Serrao, J. Mater. Chem. 17 (2007) 4931–4938. [3] N. Fujimura, H. Sakata, D. Ito, T. Yoshimura, T. Yokota, T. Ito, J. Appl. Phys. 93 (2003) 6990. [4] T. Katsufuji, S. Mori, M. Masaki, Y. Moritomo, N. Yamamoto, H. Takagi, Phys. Rev. B 64 (2001) 104419. [5] A. Munoz, J.A. Alonso, M.J. Martinez-Lope, M.T. Casais, J.L. Martinez, M.T. Fernandez-Diaz, Phys. Rev. B 62 (2000) 9498. [6] J. Park, J.-G. Park, G.S. Jeon, H.-Y. Choi, C. Lee, W. Jo, R. Bewley, K.A. McEwen, T.G. Perring, Phys. Rev. B 68 (2003) 104426. [7] M.C. Sekhar, S. Lee, G. Choi, C. Lee, J.-G. Park, Phys. Rev. B 72 (2005) 014402. [8] P.A. Sharma, J.S. Ahn, N. Hur, S. Park, S.B. Kim, S. Lee, J.-G. Park, S. Guha, S.-W. Cheong, Phys. Rev. Lett. 93 (2004) 177202. [9] H. Fukumura, S. Matsui, H. Harima, K. Kisoda, T. Takahashi, T. Yoshimura, N. Fujimura, J. Phys.: Condens. Matter 19 (2007) 365239. [10] B.B. Van Aken, T.T.M. Palstra, A. Filippetti, N.A. Spaldin, Nat. Mater 3 (2004) 164. [11] T. Katsufuji, M. Masaki, A. Machida, M. Moritomo, K. Kato, E. Nishibori, M. Takata, M. Sakata, K. Ohoyama, K. Kitazawa, H. Takagi, Phys. Rev. B 66 (2002) 134434. [12] I.G. Ismailzade, S.A. Kizhaev, Sov. Phys. Solid State 7 (1965) 236. [13] S. Petit, F. Moussa, M. Hennion, S. Pailhes, L. Pinsard-Gaudart, A. Ivanov, Phys. Rev. Lett. 99 (2007) 266604. [14] C. dela Cruz, F. Yen, B. Lorenz, Y.Q. Wang, Y.Y. Sun, M.M. Gospodinov, C.W. Chu, Phys. Rev. B 71 (2005) 060407(R). [15] J. Park, U. Kong, S.I. Choi, J.-G. Park, C. Lee, W. Jo, Appl. Phys. A 74 (Suppl.) (2002) S802–S804. [16] F. Yen, C. dela Cruz, B. Lorenz, E. Galstyan, Y.Y. Sun, M. Gospodinov, C.W. Chu, J. Mater. Res. 22 (2007) 2163. [17] J. Vermette, S. Jandl, M.M. Gospodinov, J. Phys.: Condens. Matter 20 (2008) 425219. [18] Th. Lonkai, D.G. Tomuta, U. Amann, J. Ihringer, R.W.A. Hendrikx, D.M. Tobbens, J.A. Mydosh, Phys. Rev. B 69 (2004) 134108. [19] B.B. van-Aken, A. Meetsma, T.T.M. Palstra, Acta Crystallogr. E 57 (2001) i38. [20] M. Fiebig, C. Degenhardt, R.V. Pisarev, Phys. Rev. Lett. 88 (2002) 027203. [21] A.P. Ramirez, Geometrical Frustration, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, vol. 13, North-Holland, Amsterdam, 2001. [22] L.E. Cross, Ferroelectrics 151 (1994) 305.

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