Journal of Alloys and Compounds 551 (2013) 654–659

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Journal of Alloys and Compounds

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Structural and magnetic properties of geometrically frustrated multiferroicErMnO3 nanoparticles

B. Raneesh a, A. Saha b, D. Das b, Nandakumar Kalarikkal a,c,⇑a School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686 560, Indiab UGC-DAE Consortium for Scientific Research, Kolkata 700 098, Indiac Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala 686 560, India

a r t i c l e i n f o

Article history:Received 15 May 2012Received in revised form 29 October 2012Accepted 2 November 2012Available online 16 November 2012

Keywords:MultiferroicSol–gel processNanostructuresMagnetic measurements

0925-8388/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jallcom.2012.11.019

⇑ Corresponding author at: School of Pure and ApplUniversity, Kottayam, Kerala 686 560, India. Tel.: +912731669.

E-mail address: nkkalarikkal@mgu.ac.in (N. Kalarik

a b s t r a c t

Single phase hexagonal nanocrystalline ErMnO3 has been synthesized by the sol–gel method to studytheir structural and magnetic properties. Rietveld refinement of the X-ray powder diffraction data showsthat the sample is single phase and crystallized in a hexagonal structure belonging to the P63cm spacegroup. The magnetic characterization indicates that antiferromagnetic ordering of the Mn sublatticeoccurs at TN = 65 K. Indications of weak ferromagnetism in the form of a narrow hysteresis loop at 5 Kand a very weak magnetoelectric (ME) coupling in ErMnO3 nanoparticles are attributed to intrinsiceffects.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Magnetic material studies for modern electronics have focusedon multifunctional properties aimed toward developing high-density, nonvolatile, and low energy consuming mass-informationstorage devices [1]. In particular, multiferroic materials, whichsimultaneously exhibit ferroelectricity and ferromagnetism, or atleast some kind of magnetic ordering, have been brought to thefore, because of the possibility of controlling magnetic propertieswith electric fields and dielectric properties by magnetic fields[2–4]. The renewed interest in multiferroics has led to intensivestudy in different classes of compounds and especially in therare-earth manganites RMnO3 (R-rare earth) in which the sponta-neous polarization is coupled to the magnetic order through themagnetoelectric effect [5–9]. The ground state of the ternary man-ganites RMnO3 is a polar hexagonal structure with space groupP63cm for R = Y, Er, Ho, Tm, Yb and Lu, which displays a high ferro-electric transition of TC > 500 K, and a low Neel temperature TN ofthe order of 70–120 K [10–12]. RMnO3 with R = Dy, Tb and Gd crys-tallize in a distorted orthorhombic perovskite structure with spacegroup Pbnm at ambient conditions [13,14].

ErMnO3 is a outstanding multiferroic material that belongs tothe hexagonal rare earth manganites RMnO3 characterized by atypical triangular magnetic structure with considerable geometrical

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kal).

frustration. ErMnO3 shows a ferroelectric (FE) transition at 588 Kand an antiferromagnetic (AFM) transition at 77 K [15]. Themagneto electric coupling between the FE and AFM orderings isconfirmed by a vibrant anomaly in the dielectric constant aroundthe Neel temperature TN [16]. A significant spin-phonon couplinghas also been inferred from heat transport measurements per-formed in the presence of a magnetic field. The crystal structurecomprises layers of corner-sharing MnO5 trigonal bipyramids withtwo apical oxygen atoms and three in-plane oxygen atoms. Theother layer consists of erbium atoms, located between these twotrigonal bipyramids. The AFM ordering in ErMnO3 due to theorientation Mn3+ ions within the ab plane and form a triangularlattice with the moments aligned in the same MnO plane to makean angle of 120� with each other [17].

However, there are no reports on the synthesis and relatedcharacterization of ErMnO3 nanoparticles. The combination of ahigh surface area, flexibility, and superior directionality makesnano structure suitable for many applications. But the size effectson the multiferroism is still not much clear, and its understandingrequires more experimental evidences [12]. In this paper, the re-sults of our investigations on the structural and magnetic proper-ties of ErMnO3 nanoparticles prepared through a citrate sol–gelmethod are reported.

2. Experimental procedure

According to the formula ErMnO3, stoichiometric amounts of Er(NO3)36H2O(Sigma Aldrich, 99.99% pure) and Mn(NO3)24H2O (Sigma Aldrich, 99.99% pure) weredissolved in distilled water. After the solution was clear without residue, citric acid

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nsity

Cou

nts

(a.u

)

2θ

Fig. 1. The X-ray diffraction data (black circles), rietveld refinement and the deviation from data (red and blue lines respectively) of ErMnO3. The vertical bars representallowed reflections of the P63cm structure. (For interpretation of color in this figure, the reader is referred to the web version of this article.)

Table 1Room-temperature structural parameters for ErMnO3 nanoparticles.

Atoms site x y z

Er1 0.33330 0.66670 0.77499Er2 0.00000 0.00000 0.75316Mn 0.32880 0.00000 0.49735O1 0.56691 0.00000 1.15812O2 1.36816 0.00000 1.35431O3 0.33330 0.66660 0.33253O4 0.00000 0.00000 1.03800Unit cell: a = b = 6.123361, c = 11.387633RP = 14.25, RWP = 12.4, v2 = 3.62

Fig. 2. (a) SEM image and (b) EDX spectrum of ErMnO3 nanoparticles.

B. Raneesh et al. / Journal of Alloys and Compounds 551 (2013) 654–659 655

(C6H8O7) was added to the solution with nitrates to citric acid molar ratio of 1:1 andthen pH of the resultant solution was adjusted to a value of seven by adding ammo-nia drop by drop under constant stirring condition to obtain homogeneous solution.The stable transparent sol was dried at 90 �C to form a gel and then obtained gelwas burned until the combustion process was finished [12]. Finally, sample wassintered at temperature of 850 �C for 3 h.

The crystalline structure of the sample were examined using an X-raydiffractometer (PANalytical X’pert PRO) with a monochromatized Cu Ka radiation.The structure refinement was carried out by Rietveld analysis of the X-ray powderdiffraction data using the FullProf software. Morphology of the sample was ana-lyzed using a scanning electron microscope (SEM JEOL JSM 6390). Energy dispersiveX-ray analysis (EDX) was used to check the overall chemical homogeneity and com-position of the sample. The magnetic properties were measured with a QuantumDesign superconducting quantum interference device (SQUID, Quantum DesignMPMS XL 7) at various temperatures.

3. Results and discussion

Fig. 1 shows Rietveld fitted XRD pattern of the ErMnO3 nanopar-ticles at room temperature. So as to determine the quality of therefinement, reliability factors are defined to compare the differencebetween the observed pattern and the calculated pattern, includ-ing the weighted profile reliability factor. The entire data set fromroom temperature was refined as a single phase of P63cm with noimpurity phases and each refining parameter showed as support-ing materials. The lattice parameters and atom positions of ErMnO3

nanoparticles from the refinement are listed in Table 1. Accordingto the calculations using Debye–Scherrer equation, the mean sizeof the particles calcined at 850 �C is 55 nm. Fig. 2a shows theSEM images of the as-prepared ErMnO3 sample, revealing thatthe particles are nearly spherical in shape with a narrow diameter

distribution. The SEM observation result shows particle size of thesample consistent with that derived from XRD patterns. In order tocheck the existence of all elements in the ErMnO3 compounds,

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FC ZFC

a = 1 kOeb = 5 kOec = 20 kOe

a

(a)

(b)

Fig. 3. (a) Temperature dependence of magnetization for ErMnO3 nanoparticles under different magnetic fields; the inset shows enlarged portions at bifurcation region. (b)temperature variation of reciprocal dc magnetic susceptibility.

656 B. Raneesh et al. / Journal of Alloys and Compounds 551 (2013) 654–659

energy dispersive X-ray analysis was performed. The EDX spec-trum (Fig. 2b) reveals the presence of Er, Mn, and O which confirmsthat there is no loss of any integrated element during sintering.

In an effort to understand the magnetically obtained state sig-nificantly better, the magnetization (M) was measured underzero-field-cooled (ZFC) and field-cooled (FC) conditions at threedifferent fields (1 kOe, 5 kOe, 20 kOe). Fig. 3a shows that particularbehaviors regarding ZFC and FC are quite similar for different ap-plied field strengths. As the strength of H increases, M increasesin magnitude and a dramatic rise observed below 10 K which isassociated with the arrangement of Er3+ magnetic moments. It isinteresting to note from the figure that the sample is found toexhibit bifurcation in FC/ZFC cycles (T split) at 65 K which corre-sponds to the Neel temperature TN. The data regarding M–T underH = 1 kOe are similar to 5 kOe as well as 20 kOe field strengths, andthe shift of TN is negligible with increase of H. The behavior couldbe similar to the situation associated with YMnO3, recommendingthat this AFM phase is actually inflexible with regard to appliedmagnetic fields [18]. The small bifurcation in FC/ZFC cycles isattributed to the fact that magnetic moments of Mn3+ (4.9 lB) ions

are much smaller than the magnetic moments of Er3+ (9.6 lB) ions,causing the paramagnetism of Er3+ ions to dominate the M–T curve[19].

Temperature dependence of the reverse dc magnetic suscepti-bility (Fig. 3b) shows linear behavior above 70 K. From the linearpart above TN in 1/v–T curve, Curie–Weiss temperature (hCW)and effective magnetic moment (leff) were calculated. Then byusing Curie–Weiss law, linear part of the reverse dc susceptibilitywas fitted. The negative value of Curie–Weiss temperature showspredominant antiferromagnetic interactions in the ErMnO3 sam-ple. It is found that nano crystalline ErMnO3 shows Curie–Weisstemperature of �22 K and an effective magnetic moment, leff, of9.6 lB which is consistent with the corresponding theoretical value10.6 lB [15]. The observed hCW is considerably lower than the TN

reported for similar RMnO3 systems.The antiferromagnetism of ErMnO3 originates from the triangu-

lar long-range arrangement of the spins of Mn3+ ions in the abplane, wherein neighboring Mn moments are aligned at 120� angle[14,20]. Compared to typical data for bulk sample [7,8], theordering temperature TN of the ErMnO3 nanoparticles decreased

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Fig. 4. Room temperature M–H curves of ErMnO3 nanoparticles; the inset shows M–H at 65 K.

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Fig. 5. M–H curves of ErMnO3 nanoparticles at 5 K; the inset shows enlarged central portion.

B. Raneesh et al. / Journal of Alloys and Compounds 551 (2013) 654–659 657

by 12 K. The decrease in TN could be attributed to the decrease inspontaneous polarization and the decrease in the number of anti-ferromagnetic interactions with decreasing grain size [21]. TheFC anomaly is sharper for the fine powder and shows a small in-crease compared to ZFC data which might be attributed to theoccurrence of a ferromagnetic component.

The magnetization versus magnetic field (M–H) curve recordedat room temperature is shown in Fig. 4. The room temperature M–H curve of the sample ErMnO3 shows linear variation of magneti-zation with magnetic field, suggesting paramagnetic behavior ofthis sample. We have also recorded M–H curves at 65 K for ErMnO3

sample (Fig. 4 inset). The 65 K data also shows a paramagnetic re-sponse, indicating that magnetic ordering occurred below 65 K.This temperature corresponded to the AFM ordering temperature

of Mn3+ ions. Although no saturated magnetization at high fieldwas reached, ferromagnetic hysteresis was observed in the 5 Kdata (Fig. 5), originating from the ferromagnetic ordering of theEr3+ spins.

The coexistence of the ferroelectric and magnetic phases in thepresent samples brings about the magneto electric (ME) effect,which is characterized by the magneto electric voltage coefficient,a = dE/dH [22–24]. A conventional ME measurement had been car-ried out using the lock-in amplifier method [25,26]. There is no anymethodical review on the primary way of measuring polarizationdifference of an ME nanostructured material under a magneticfield. This imposes serious limitations on our understanding ofthe reversible ME affect in ErMnO3 nanoparticle at room tempera-ture. We tackle this problem within our existing research it will be

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Fig. 6. ME voltage vs. ac magnetic field with constant dc bias magnetic field.

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Fig. 7. Variation of ME voltage as a function of the dc magnetic field.

658 B. Raneesh et al. / Journal of Alloys and Compounds 551 (2013) 654–659

applied both in lab as well as commercial configurations in order toquantitatively figure out the actual magnitude of a.

The linear ME coupling coefficient (a) generally familiarizedelectrically (dM/dH) or magnetically (dP/dH). In this work, theME output voltage is measured across the sample applying themagnetic field (H). The measurement system uses the techniquewhich allows direct measurement of the polarization (P) behaviorcorresponding to the applied field (H). The sample was placed atthe center of a Helmholtz coil which is excited with an ac magneticfield Hac from the drive output of the tester. The actual reorienta-tion from the electric dipoles within the small sample through acmagnetic field induces an ac voltage on top as well as bottomsurface areas of the sample via ME coupling, that was calculatedutilizing the lock-in amplifier within the differential setting to

get rid of the actual mistakes due to induction voltage through Far-aday effect. For dc bias field, an electromagnet (EM) is used to pro-vide a maximum of 5 kOe with the pole spacing of 10 mm. Moredetails of these experimental measurements have been publishedelsewhere by Naik and Mahendiran [27].

The ac magnetic field (Hac) dependence of ME voltage at roomtemperature is shown in Fig. 6. at a fixed frequency of 850 Hz withconstant dc bias field of 3500 Oe. The response of the magnetoelectric coefficient (a) shows a linear path and value of a deter-mined from the slope of the ME voltage vs. Hac. The calculated valueof a is �0.02157 mV/cm/kOe. A comparison of our results withthose of other similar systems has been made to suggest the MEoutput detected in the ErMnO3 nanoparticles is mainly due to theferroelectric and magnetic interaction in the lattice [22,28]. Fig. 7

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shows the ME coupling coefficient of the ErMnO3 nanoparticle var-iation as a function of the dc bias magnetic field. The maximummagnetoelectric coefficient is attained at optimized dc magneticfield [29]. In this case, peak observed near 2287 Oe in ME outputdata (Fig. 7) may be attributed to the geometrical frustration[30,31]. Based on the previous symmetry concept report, originsregarding room temperature ME effect in ErMnO3 might also causeby the point defects and also mixed valance of Mn ion [28].

4. Conclusion

We have successfully prepared hexagonal ErMnO3 nanoparti-cles using sol–gel method and performed structural characteriza-tion by means of Rietveld refinements of XRD data. Significantsize effect of antiferromagnetic transition was demonstrated bythe (M–T) magnetic measurements. In addition, Curie–Weiss tem-perature (�22 K) and effective magnetic moment leff (9.6 lB) werefound to be in agreement with theoretical results. As-synthesized55 nm crystallites showed indications of weak ferromagnetism at5 K. We have also observed a very weak magneto electric effectin ErMnO3 nanoparticles, detected by a dynamic lock-in amplifiertechnique.

Acknowledgements

The authors B.R. and N.K. are thankful to UGC-DAE Consortiumfor Scientific Research, Kolkata Centre for financial support and forproviding the SQUID facility. The authors are thankful to Mr. P.V.Rajesh for magnetic measurements. The financial support fromUGC-Govt. of India through SAP and DST-Govt. of India throughFIST program to SPAP is also gratefully acknowledged. One of theauthors NKK also acknowledges DST-Govt. of India for financialsupport through Nanomission program.

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