Open Access Nano Express

Grain size-dependent magnetic and electric properties in nanosized YMnO3 multiferroic ceramics

Tai-Chun Han*, Wei-Lun Hsu and Wei-Da Lee

Author Affiliations

Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan

For all author emails, please log on.

Nanoscale Research Letters 2011, 6:201  doi:10.1186/1556-276X-6-201


The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/6/1/201


Received:26 July 2010
Accepted:8 March 2011
Published:8 March 2011

© 2011 Han et al; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Magnetic and electric properties are investigated for the nanosized YMnO3 samples with different grain sizes (25 nm to 200 nm) synthesized by a modified Pechini method. It shows that magnetic and electric properties are strongly dependent on the grain size. The magnetic characterization indicates that with increasing grain size, the antiferromagnetic (AFM) transition temperature increases from 52 to 74 K. A corresponding shift of the dielectric anomaly is observed, indicating a strong correlation between the electric polarization and the magnetic ordering. Further analysis suggests that the rising of AFM transition temperature with increasing grain size should be from the structural origin, in which the strength of AFM interaction as well as the electrical polarization is dependent on the in-plane lattice parameters. Furthermore, among all samples, the sample with grain size of 95 nm is found to have the smallest leakage current density (< 1 μA/cm2).

PACS: 75.50.Tt, 75.50.Ee, 75.85.+t, 77.84.-s

Introduction

The hexagonal RMnO3 (R = rare earth element or Y) compounds present opportunities for the industrial applications due to their unique nature of multiferroism [1]. Namely, the ferromagnetism, ferroelectricity and ferroelasticity occur simultaneously in the same material. The characteristics of multiferroism include a spontaneous magnetization which can be switched by an applied electric field, a spontaneous electrical polarization which can be reoriented by an applied magnetic field, and a strong coupling between these two properties [2]. Owing to the coupling between ferroelectric and magnetic domains, multiferroism is likely to offer a whole range of new applications and phenomena. Specific device applications that have been proposed for these multiferroic materials include the multiple-state memory elements, the transducer with magnetically modulated piezoelectricity, and the electric-field-controlled ferromagnetic resonance devices [2].

Most of hexagonal RMnO3 exhibit ferroelectric (FE) transitions at high temperatures (TC ≈ 600 to 1,000 K) and antiferromagnetic (AFM) transitions at low temperatures (TN ≈ 70 to 130 K) with a frustrated triangular arrangement of Mn spins in the hexagonal c-plane [1-4]. Additional phase transitions at the temperature below 10 K were observed in the hexagonal RMnO3 with the R3+ ion of high magnetic moment, which is related to the R-R exchange correlations [5]. Several attempts have been directed towards the syntheses of new RMnO3 compounds and the studies of their related properties [6,7]. In particular, the recent work on the hexagonal RMnO3 compounds was focused on the following subjects: (1) the magnetic phases and the magnetic symmetry at low temperatures [8,9], (2) the coupling between the magnetic and FE orderings [10,11], and (3) the strong spin-lattice interaction of the geometrically frustrated Mn-spin system [12]. The studies on YMnO3, HoMnO3 and LuMnO3 indicated that the values of ordering temperatures are associated with the size of R3+ ion. In addition, the size effects in yttrium-based manganites were also reported [13,14]. However, the size effects on the multiferroism remain unclear, and its understanding requires more experimental evidences. In this paper, we prepare a series of YMnO3 samples with different grain sizes by a modified Pechini method to study systematically the effect of grain size on their magnetic and electric properties.

Experimental procedure

The nanosized samples of YMnO3 were synthesized by a modified Pechini method using nitrates as metal precursors. First, yttrium nitrate [Y(NO3)3·6H2O] and manganese nitrate [Mn(NO3)2·4H2O] in stoichiometric proportions (1:1 molar ratio) were dissolved in distilled water. Citric acid (C6H8O7) in 1:1 molar ratio with respect to the metal nitrates was added to the solution as a complexant, and the solution was adjusted to a PH value of 6.5 to 7 by adding ammonia. The mixture was dried at 120°C to form a gel, and then the obtained gel was burned until the combustion process was completed. After that, the precursory powders were reground and pressed into the pellets. Finally, the pellets were sintered at different temperatures ranging from 800°C to 1,050°C for 2 h, respectively. Electrodes were applied to both surfaces to measure electrical properties with silver paste.

The crystalline structure and the phase purity of the samples were examined with a typical X-ray diffraction (XRD), acquired by a Bruker D8 Advance X-ray diffractometer (Bruker UK Ltd., Coventry, Warwickshire, UK) equipped with a monochromatized Cu Kα1 radiation and field emission scanning electron microscopy. The magnetization was measured with a Quantum Design superconducting quantum interference device (Quantum Design, Inc., San Diego, CA, USA) with an applied magnetic field of 500 Oe. For the dielectric measurements, a capacitance bridge (Agilent 4284A; Agilent Technologies, Inc., Palo Alto, CA, USA) hooked to a probe station with a closed-cycle low temperature system was used. The leakage currents of the samples were measured using a commercial FE test system (TF Analyzer, aixACCT Systems GmbH, Aachen, Germany).

Results and discussion

Figure 1 shows the XRD patterns of the YMnO3 samples sintered at different temperatures from 800°C to 1,050°C. Based on the standard reference, all the observed peaks can be indexed on the basis of a hexagonal unit cell of space group P63cm (JCPDS:25-1079), suggesting that all samples are pure phases without any impurity. In addition, with the increase in sintering temperature, there is a gradual intensity increasing and narrowing of the diffraction peaks, indicative of better crystallization and the grain growth. The lattice parameters were determined by Rietveld refinement method and shown in Figure 2. With increasing of sintering temperature, the value of c lattice parameter is slightly expanded, while the value of a lattice parameter decreased. The typical scanning electron microscopy (SEM) images of the YMnO3 samples sintered at different temperatures are shown in Figure 3. From the images, it can be found that the grain size becomes larger as the sintering temperature increases. The estimated average grain size is about 25, 45, 95, and 200 nm for the samples sintered at 800°C, 850°C, 900°C, and 1,050°C, respectively.

thumbnailFigure 1. The standard reference and the XRD patterns of YMnO3 samples sintered at different temperatures. Samples were sintered at temperatures ranging from 800°C to 1,050°C for 2 h, respectively.

thumbnailFigure 2. The evolution of lattice parameters for YMnO3 samples sintered at different temperatures. The uncertainty is contained within the area of the suitable mark.

thumbnailFigure 3. The SEM micrographs for YMnO3 samples. Samples sintered at (a) 800°C, (b) 850°C, (c) 900°C, and (d) 1,050°C, respectively.

The temperature-dependent magnetization curves M(T) were measured in a magnetic field of 500 Oe under the conditions of zero-field-cooled (ZFC) and field-cooled (FC). Figure 4 displays the temperature dependence of magnetization for the powders with different grain sizes. Open symbols are the data with the ZFC mode, while the solid ones with FC mode. As can be seen, typical AFM to paramagnetic (PM) phase transition is observed for the sample with grain size of 200 nm, and the Néel temperature (TN) is about 74 K. As the grain size decreases, the value of TN shifts to the lower temperatures and is equal to 52 K for the sample with grain size of 25 nm. This size-dependent TN is similar to the observation in the BiFeO3 nanoparticles [15], where the increase in TN with increasing size has been discussed both in terms of phenomenological scaling relations and possible correlations with the decreasing electrical polarization. To further explore the magnetic properties of the samples, magnetic hysteresis loops for the YMnO3 samples with different grain sizes have been measured at 5 K, as presented in Figure 5. For the samples with grain size of 25 and 45 nm, weak ferromagnetic (FM) behavior is observed with corresponding coercivity (Hc) about 395 and 260 Oe, respectively. The inset in Figure 5 shows the magnetic hysteresis curve for the sample with grain size of 25 nm has been measure at 55 K. It indicates the PM behavior which confirms that the FM component disappears above TN. Therefore, the weak FM component does not come from FM impurity phase. As the grain size increases, the weak FM behavior transforms into paramagnetism. Similar effect of grain size on magnetism was also reported in nanosized YMn2O5 [16] and BiFeO3 particles [17]. In fact, weak surface FM component is a universal feature for nanosized AFM systems, which is attributed to the deviation of the AFM arrangement to the disordered surface spin due to the lattice strain [17,18]. Based on the above consideration, the magnetic structure of the nanosized YMnO3 can be considered as a core/shell system, where the inner part of the particle is AFM phase and the surface is FM component.

thumbnailFigure 4. Temperature dependence of magnetization for the YMnO3 samples with different grain sizes. Open symbols are the data with the ZFC while the solid ones with FC mode.

thumbnailFigure 5. Magnetic hysteresis loops at 5 K for the YMnO3 samples with different grain sizes. Inset: magnetic hysteresis curve at 55 K for the sample with grain size of 25 nm.

Figure 6 shows the temperature-dependent dielectric permittivity ε(T) and loss tangent (tanδ) at 100 kHz for all measured YMnO3 samples. In Figure 6a, the dielectric anomalies are observed at T* which is defined as the crossing point of two slopes as indicated by arrows. It shows that the T* shifts from 55 to 74 K with increasing grain sizes from 25 to 200 nm. As clearly apparent in Figure 6b, the positions of the broad peaks for the YMnO3 samples with different grains sizes are near their T*. Moreover, the enhanced dielectric response observed for YMnO3 with larger grains is similar to previously reported results for BaTiO3 dielectrics [19]. The observed systematic shift in the temperatures of magnetic transition and dielectric anomaly demonstrates a strong correlation between magnetic ordering and electric polarization in nanosized hexagonal YMnO3 ceramics. As to the coupling between antiferromagnetism and dielectric property, Katsufuji et al. [20] suggested that the dielectric anomaly was caused by the magnetic-ordering-dependent electronic excitation gap Eg in ab-plane. According to this model, the change of AFM ordering pattern can induce dielectric anomaly via the change of Eg, in a formula of ε = 1/Eg2. Therefore, one can understand that the shift in the temperature of dielectric anomalies is related to the AFM interaction through the variation of Mn-O bond length with change the lattice parameters. In addition, the systematical change in the lattice constant a plays an important role since the strength of AFM interactions strongly depends on the bond length of Mn-O. In general, the strength of AFM interaction can be written as [21]:

thumbnailFigure 6. Temperature-dependent (a) dielectric constant (ε), and (b) loss tangent for the YMnO3 samples. Samples have different grain sizes (25 nm to 200 nm).

(1)

where the sum is over the nearest neighbors and is a spin operator. The parameter J is proportional to the inverse of the distance between two nearest spins. Therefore, the reduction in a-parameter leads to the enhancement of J and hence to the rising of AFM transition temperature.

To further probe the electrical leakage effect, the leakage current were measured for all the samples at room temperature as shown in Figure 7. The leakage current density is large (> 100 μA/cm2) for the sample with grain size of 25 nm. On the other hand, the leakage currents are much decreased by about four orders of magnitude for the samples with grain size larger than 45 nm. In addition, it is not expected that the sample with larger grain size of 200 nm is not the less leaky sample. As for the improvement of the leakage properties, it should be associated with the high denseness of the ceramics [22].

thumbnailFigure 7. Leakage current as function of applied electric field for the YMnO3 samples. Samples have different grain sizes (25 nm to 200 nm).

Conclusions

In summary, a series of hexagonal YMnO3 samples with different grain sizes are synthesized by a modified Pechini method. The magnetic susceptibility indicates that with increasing grain size from 25 to 200 nm, the AFM transition temperature increases from 52 to 74 K. At the same time, a corresponding shift of the dielectric anomalies is observed, which suggests a strong correlation between the magnetic ordering and the electric polarization. Since the electronic excitation gap is inversely proportional to the dielectric permittivity and the spin structure influences the electronic excitation gap, we propose that the coherent shift in the magnetic ordering and the dielectric anomalies to high temperature with increasing grain size is related to the suppression of the in-plane lattice parameter.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

The work presented here was carried out in collaboration between all authors. TH defined the research theme and designed methods and experiments, carried out the laboratory experiments, analyzed the data, interpreted the results and wrote the paper. WH and WL prepared the samples, helped to carry out the laboratory experiments and discussed analyses. All authors read and approved the final manuscript.

Acknowledgements

The financial support of this work is from the National Science Council of Taiwan under the grant nos. NSC96-2112-M-390-003-MY3, 99-2112-M-390-005-MY3 and 98-2815-C-390-015-M.

References

  1. Van Aken BB, Palstra TTM, Filippetti A, Spaldin NA: The origin of ferroelectricity in magnetoelectric YMnO3.

    Nat Mater 2004, 3:164. PubMed Abstract | Publisher Full Text OpenURL

  2. Fiebig M: Revival of the magnetoelectric effect.

    J Appl D: Appl Phys 2005, 38:R123. Publisher Full Text OpenURL

  3. Bertaut EF, Forrat EF, Fang P: A new class of ferroelectric: rare earth and yttrium manganites.

    Acad Sci 1963, 256:1958. OpenURL

  4. Choi T, Horibe Y, Yi HT, Choi YJ, Wu W, Cheong SW: Insulating interlocked ferroelectric and structural antiphase domain walls in multiferroic YMnO3.

    Nat Mater 2010, 9:253. PubMed Abstract | Publisher Full Text OpenURL

  5. Fiebig M, Lottermoser Th, Pisarev RV: Spin-rotation phenomena and magnetic phase diagrams of hexagonal RMnO3.

    J Appl Phys 2003, 93:8194. Publisher Full Text OpenURL

  6. Floros N, Rijssenbeek JT, Martinson AB, Poeppelmeier KR: Structural study of A2CuTiO6 (A = Y, Tb-Lu) compounds.

    Solid State Sci 2002, 4:1495. Publisher Full Text OpenURL

  7. Malo S, Maignan A, Marinel S, Hervieu M, Poeppelmeier KR, Raveau B: Structural and magnetic properties of the solid solution (0 ≤ × ≤ 1) YMn1-x(Cu3/4Mo1/4)xO3.

    Solid State Sci 2005, 7:1492. Publisher Full Text OpenURL

  8. Fiebig M, Fröhlich D, Kohn K, Leute St, Lottermoser Th, Pavlov VV, Pisarev RV: Determination of the magnetic symmetry of hexagonal manganites by second harmonic generation.

    Phys Rev Lett 2000, 84:5620. PubMed Abstract | Publisher Full Text OpenURL

  9. Muñoz A, Alonso JA, Martínez-Lope MJ, Casáis MT, Martínez JL, Fernández-Díaz MT: Evolution of the magnetic structure of hexagonal HoMnO3 from neutron powder diffraction data.

    Chem Mater 2001, 13:1497. OpenURL

  10. Huang ZJ, Cao Y, Sun YY, Xue YY, Chu CW: Coupling between the ferroelectric and antiferromagnetic orders in YMnO3.

    Phys Rev B 1997, 56:2623. Publisher Full Text OpenURL

  11. Sugie H, Iwata N, Kohn K: Magnetic ordering of rare earth ions and magnetic-electric interaction of hexagonal RMnO3 (R = Ho, Er, Yb or Lu).

    J Phys Soc Jpn 2002, 71:1558. Publisher Full Text OpenURL

  12. Zhou HD, Lu J, Vasic R, Vogt BW, Janik JA, Brooks JS, Wiebe CR: Relief of frustration through spin disorder in multiferroic Ho1-xYxMnO3.

    Phys Rev B 2007, 75:132406. Publisher Full Text OpenURL

  13. Zhang MF, Liu JM, Liu ZG: Microstructural characterization of nanosized YMnO3 powders: the size effect.

    Appl Phys A 2004, 79:1753. OpenURL

  14. Zheng HW, Liu YF, Zhang WY, Liu SJ, Zhang HR, Wang KF: Spin-glassy behavior and exchange bias effect of hexagonal YMnO3 nanoparticles fabricated by hydrothermal process.

    J Appl Phys 2010, 107:053901. Publisher Full Text OpenURL

  15. Selbach SM, Tybell T, Einarsrud MA, Grande T: Size-dependent properties of multiferroic BiFeO3 nanoparticles.

    Chem Mater 2007, 19:6478. Publisher Full Text OpenURL

  16. Ma C, Yan JQ, Dennis KW, McCallum RW, Tan X: Size-dependent magnetic properties of high oxygen content YMn2O5 ± δ multiferroic nanoparticles.

    J Appl Phys 2009, 105:033908. Publisher Full Text OpenURL

  17. Bi H, Li SD, Zhang YC, Du YW: Ferromagnetic-like behavior induced by lattice distortion of ultrafine NiO nanocrystallites.

    J Magn Magn Mater 2004, 277:363. Publisher Full Text OpenURL

  18. Bañdobre-Lopez M, Vázquez-Vázquez C, Rivas J, López-Quintela MA: Magnetic properties of chromium (III) oxide nanoparticles.

    Nanotechnology 2003, 14:318. OpenURL

  19. Ihlefeld JF, Vodnick AM, Baker SP, Borland WJ, Maria JP: Extrinsic scaling effects on the dielectric response of ferroelectric thin films.

    J Appl Phys 2008, 103:074112. Publisher Full Text OpenURL

  20. Katsufuji T, Mori S, Masaki M, Moritomo Y, Yamamoto N, Takagi H: Dielectric and magnetic anomalies and spin frustration in hexagonal RmnO3 (R = Y, Yb, and Lu).

    Phys Rev B 2001, 64:104419. Publisher Full Text OpenURL

  21. Munawar I, Curnoe SH: Theory of magnetic phases of hexagonal rare earth manganites.

    J Phys: Condens Matter 2006, 18:9575. Publisher Full Text OpenURL

  22. Chen F, Zhang QF, Li JH, Qi YJ, Lu CJ, Chen XB, Ren XM, Zhao Y: Sol-gel derived multiferroic BiFeO3 ceramics with large polarization and weak ferromagnetism.

    Appl Phys Lett 2006, 89:092910. Publisher Full Text OpenURL