Radiation Physics and Chemistry 89 (2013) 28–32

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Radiation Physics and Chemistry

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Effect of gamma radiation on the structural, dielectric andmagnetoelectric properties of nanostructured hexagonal YMnO3

B. Raneesh a, A. Saha b, Nandakumar Kalarikkal a,c,n

a School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686560, Indiab UGC-DAE Consortium for Scientific Research, Kolkata 700098, Indiac Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala 686560, India

H I G H L I G H T S

� Multiferroic YMnO3 nanoparticles have been synthesized by using the sol–gel method.

� Effect of γ-irradiation on the physical and structural properties of the samples is investigated.� γ-Radiation results significant change on the dielectric constant and magnetoelectric coefficient.

a r t i c l e i n f o

Article history:Received 24 December 2012Accepted 22 March 2013Available online 2 April 2013

Keywords:MultiferroicSol–gel processIrradiationDielectric propertiesYMnO3

6X/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.radphyschem.2013.03.040

esponding author at: School of Pure and Applity, Kottayam, Kerala 686560, India. Tel.: +91481 2731669.ail address: nkkalarikkal@mgu.ac.in (N. Kalari

a b s t r a c t

Nanocrystalline YMnO3 ceramic powders have been synthesized by using the standard sol–gel method.The samples were irradiated with 60Co γ-radiation at a dose rate of 4.7 kGy/h for different time intervals.The effects of irradiation on surface morphology viz. grain size, porosity and also the existence of a fewimpurity phases have been studied by scanning electron microscopy, X-ray diffraction and far-infraredreflectivity spectroscopy. Irradiation in air produced changes in the dielectric spectrum in the frequencyrange between 100 Hz and 2 MHz. Subjection of γ-radiation brings about key change on the properties asa consequence of structural transformations. The attained data shows that the electrical conductivity anddielectric constant is actually so much dependent on radiation dose. A quantitative magnetoelectriccoefficient measurement in YMnO3 nanosystem was performed by using the dynamic lock-in amplifiertechnique. The response of the magneto-electric coefficient shows perfect anisotropy in the irradiatedsamples.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The hexagonal RMnO3 (R¼Ho, Lu, Sc and Y) compounds are thecurrent options for the industrial applications because of theirspecial characteristics regarding multiferroism. It is establishedthat YMnO3 is simultaneously ferroelectric with the Curie tem-perature TC is ∼900 K and antiferromagnetic with the Néeltemperature TN is ∼77 K (Van Aken et al., 2004). It has also beenshown earlier that there exists a coupling between the ferro-electric and antiferromagnetic orders in YMnO3 (Zhong et al.,2011; Filippetti and Hill, 2002; Lee et al., 2005). Furthermore,YMnO3 has a single spontaneous polarization axis (Kim et al.,2000) and does not contain any volatile elements such as Pb andBi. These outstanding properties make YMnO3, a promising

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ied Physics, Mahatma Gandhi9447671962;

kkal).

candidate for applications to nonvolatile ferroelectric randomaccess memories, magnetoelectric sensors and ferroelectric-gatefield-effect transistors (Fujimura et al., 1996; Choi et al., 2001; Itoet al., 2003).

Hexagonal YMnO3 consists of layers of Y3+ ions separatinglayers of corner-shared MO5 trigonal bipyramids where eachmanganese ion is surrounded by three in-planes and two apicaloxygen ion. A net electric polarization in YMnO3 is due to bucklingof the layered MnO5 polyhedra, accompanied by displacements ofthe Y ions (Vajk et al., 2005; Fennie and Rabe, 2005). Below theNéel temperature (77 K) the magnetic structure of YMnO3 can bedescribed as frustrated antiferromagnetism on a triangular latticein the ab-plane (Munoz et al., 2000; Sharma et al., 2004; Fiebiget al., 2002; Chen et al., 2005). The lower TN is the main alimitation for the multiferroic applications of YMnO3, and it is animportant issue to improve room temperature magnetic proper-ties of the present material.

In particular, the recent work on the hexagonal YMnO3 com-pounds was focused on the magnetic phases and the magnetic

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Inte

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nts

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2Θ

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Fig. 1. XRD patterns of YMnO3 samples: (a) as formed and exposed to γ-radiationto an accumulated dose of (b) 100 kGy (c) 500 kGy, and (d) 1000 kGy.

B. Raneesh et al. / Radiation Physics and Chemistry 89 (2013) 28–32 29

symmetry at low temperatures (Gao et al., 2011; Kristin et al., 2011),the coupling between the magnetic and FE orderings (Sugie et al.,2002; Zhou et al., 2007), and the strong spin–lattice interaction ofthe geometrically frustrated Mn-spin system. The studies onYMnO3, HoMnO3 and LuMnO3 indicated that the values of orderingtemperatures are associated with the size of R3+ ion. The studies ofthe modification in multiferroic material due to irradiation are ofboth technological importance and scientific interest. Generally, theexposure of material to high-energy particles (γ-rays) results indisplacement lattice damage and ionization damage in materials.These effects are of much relevance when we realize devices whichare likely to be exposed to such high energy particles. Such damagescan have an impact on the electrical properties of the devices andconsequently on their performances. Recently, we observed thatradiation could result in particle-aggregation in case of the Fe3O4

system, whereas no particle-coalescence occurred in the nanoscaleGd2O3 system (Paul et al., 2012; Devi et al., 2012). Thus, the presentinvestigation was carried out in an attempt to understand the effectof γ-radiation on the structural, dielectric and magneto-electricproperties of multiferroic YMnO3 nanoparticles. To the best ofour knowledge, this is the first ever attempt made to study theγ-irradiation effects on the properties of multiferroic YMnO3

nanoparticles.

2. Experimental

The YMnO3 nanoparticles were prepared by the sol–gel tech-nique. The starting solution was stoichiometric mixtures ofyttrium nitrate Y(NO3)3 �9H2O and manganese nitrate Mn(NO3)3 �6H2O were dissolved in an aqueous solution of PVA whichwas used here as a chelating agent in making the gel. The mixtureswere stirred continuously, at temperature of 80 1C until theformation of a gel was observed and dried in an oven for another24 h at 120 1C. The dry gels were then heat treated in air at 850 1Cfor 5 h. The samples were characterized by powder XRD measure-ments recorded using a diffractometer (PANalyticalX'pert PRO).Scanning electron microscopic (JEOL Model JSM-6390LV) photo-graphs of the sample were taken to understand the distribution ofthe nanoclusters.

The synthesized YMnO3 samples of size (6075 nm) wereirradiated with 60Co γ-source that is capable of emitting photonswith an average energy of 1.25 MeV at dose rate of 4.7 kGy/h withan accumulated dose of 100, 500, and 1000 kGy. 60Co gammairradiation experiment (cylindrical irradiation chamber withlength 14 cm and diameter 10 cm) was conducted at UGC-DAEConsortium for Scientific Research, Kolkata Centre. All the struc-tural, dielectric and magneto-electric properties were investigatedbefore and after gamma radiation. The UV–visible absorptionspectra of virgin and high dose gamma irradiated YMnO3 nano-particles have been recorded by using UV–visible spectrophot-ometer (Shimadzu UV 2550) in the wavelength range of 220–900 nm. The dielectric measurements were performed by using anAgilent impedance analyzer E4980A before and after 60Co γ-rayirradiation at the room temperature. Far-infrared reflectivityspectra were measured at room temperature with a Perkin ElmerSpectrum GX in the range of 50–1200 cm−1.

3. Results and discussion

Fig. 1 shows the XRD patterns of unirradiated and irradiatedYMnO3 nanoparticles sintered at a temperature of 850 1C. Usingthe standard reference (JCPDS no. 25-1079), each of the seen peakscan be indexed on the basis of a hexagonal unit cell of space groupP63cm, suggesting that all samples are pure of phases without any

impurity. Further, on irradiation it is evident from Fig. 1 that thepeak positions are shifted to a little higher 2θ values and the peakintensities are decreased. Further, the width of the peaks isincreased from that of the unirradiated samples. It may be dueto irradiation produced the compressive strain and also generatedsome disorder in the lattice structure.

The particle size reduction after irradiation may be due to highenergy γ-rays penetrate the sample and lead to material modifica-tion by pushing the atoms from their normal sites and splitting themolecules into small fragments (Parvatheeswara Rao et al., 2006).In this process, the high energy γ-rays lose their energy which isused to generate defects and partial amorphization in the YMnO3

samples resulting in a decrease in the average particle size. Theelectron micrographs of the unirradiated and the irradiatedsamples at the highest dose have no prominent structural differ-ence (Fig. 2a–d). The reduced grain size and pronounced porositycan be seen in the irradiated samples. Conversely, the presence ofless cracks settled in the inter grain links within the samplesexhibits a small increase in the normal state resistivity.

Since optical properties of a material give evidence about theelectronic structures, localized states and the optical transitions,the investigation on the optical behavior is very significant for theunderstanding of the material (Sharma and Maity, 2008; Balboulet al., 2012). So, the UV–visible absorption spectra of virgin andgamma irradiated (100–1000 kGy) YMnO3 samples were recordedand are shown in Fig. 3. The recorded spectra show a shift ofabsorption edge toward shorter wavelength with increasinggamma dose. The broadening of absorption peak with increasinggamma dose is also observed in the spectra. The observed changesin spectral features may occur due to decreased density of themedium and out-diffusion of oxygen causing disorder in the MnO5

oxide planes.In order to facilitate the characterization, the infrared reflectiv-

ity spectra were measured in YMnO3 nanomaterials in the phononfrequency range, at room temperature, before and after irradiation.Phonon mode assignment is usually a big task in anisotropicpolycrystalline samples like YMnO3, since optical responses ofsamples in different crystallographic directions are mixed(Zaghrioui and Phuoc, 2011). Polarized IR reflectivity spectra ofan YMnO3 single crystal were published by Zaghrioui et al., 2008.Nearly 23 infrared active phonon modes are predicted for hex-agonal YMnO3 with space group of P63cm on the basis of thegroup theory at the Brillouin-zone center. Finally all these modessplit into 9 A1 and 14 E1 modes with the electrical dipole momentaligned along and perpendicularly to the c-axis.

From the experimental results, we can see that intensityof vibration mode decreased after irradiation, which may be due

Fig. 2. SEM images of YMnO3 samples: (a) before irradiation and after irradiation at doses of (b) 100 kGy, (c) 500 kGy, and (d) 1000 kGy.

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Fig. 3. The absorption spectra for unirradiated and irradiated YMnO3 samples.

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Fig. 4. Infrared reflectivity spectra of YMnO3 nanomaterials before and aftergamma irradiation.

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Fig. 5. The variation of the real part of the dielectric constant as a function offrequencies before and after γ-ray irradiation for YMnO3 samples.

B. Raneesh et al. / Radiation Physics and Chemistry 89 (2013) 28–3230

to small structural change occurred in the multiferroic system.The irradiation affects the shape, position and intensity of thebands. Only changes in the shape, position and intensity of theband resulting from the Y–O and Mn–O–Mn stretching bond havebeen studied, because the in- and out-diffusion of oxygen mostlytakes place in the Mn oxide layer. Among them two major changesare in the 469 cm−1 and 299 cm−1 vibration mode regions. Afterthe irradiation (Fig. 4), intensity of the vibration mode located at469 cm−1 which is associated to Y and O atom displacements alongthe c-axis (Iliev et al., 1997), drastically reduced and becamenarrow. From our results, it is strongly suggested that Y–O bonddistances decrease with irradiation in the same way as anotherprominent change occurs at 299 cm−1 associated with M–O dis-placement in the trigonal bipyramid. Irradiation deteriorates thevibration mode of Mn–O bonding and leads to reduction in thecorresponding absorption energy.

Dielectric constant in yttrium manganite is contributed byseveral structural and micro-structural factors. The frequencydependence of the dielectric constant for all the samples has beenstudied at room temperature. Figs. 5 and 6, depict variation of realand complex part of dielectric constant with frequency. It is clearthat all the samples studied exhibit the dielectric dispersion whereboth real (ε′) and imaginary part (ε″) decrease as the frequency

increases from 100 Hz to 2 MHz. It is observed that dielectricconstant increases as radiation dose increases from 100 to1000 kGy. The larger value (ε′) acquired with regard to irradiatedexamples, often described within interaction of γ-rays with thematter. This particular interaction is actually succinct with two

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Fig. 6. The variation of the imaginary part of the dielectric constant as a function offrequencies before and after γ-ray irradiation for YMnO3 samples.

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Fig. 7. Variation of ac conductivity with frequency before and after γ-rayirradiation.

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Fig. 8. ME voltage as a function of the ac field at different gamma radiation doses.

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Fig. 9. ME voltage as a function of the dc magnetic field at different gammaradiation doses.

B. Raneesh et al. / Radiation Physics and Chemistry 89 (2013) 28–32 31

actions: the first may be the increase in hopping rate between theions as a consequence of much more interaction. Second stage maybe associated with a few vacancies in various absolute depths thatwork as capturing centers. The liberation associated with chargecarriers through these types of capturing centers requires variousenergies. This particular one appears to be the primary reasonwith regard to maximum value associated with ε′ in the case ofγ-irradiated samples (Ateia, 2006). Fig. 7 shows the variation of acconductivity with frequency at room temperature. The ac con-ductivity increases with an increase of frequency. The total con-ductivity of the yttrium manganite is given by the relationship

stot ¼soðTÞ þ sðω; TÞThe first term at R.H.S. of equation is dc conductivity as a result

of the band conduction, which is a frequency independent func-tion (Ansari et al., 2012). The second term of the equation is pureac conductivity due to the electron hoping between ions in YMnO3

lattice. In the present study, it has been observed that the acconductivity gradually increases with increasing radiation dosefrom 100 to 1000 kGy. The increase in AC electrical conductivitydue to irradiation may be attributed to creation of charge centers.When an AC field of sufficiently high frequency was applied to thesamples under test, it caused a net polarization, which is out ofphase with the field.

The coexistence of the electric and magnetic phases in thepresent samples gives rise to a magneto-electric (ME) effect, whichis characterized by the magneto-electric voltage coefficient, α¼dE/dH (Kumar et al., 2011; Nan et al., 2008; Eerenstein et al., 2006).A conventional ME measurement had been carried out by usingthe lock-in amplifier technique (Dong et al., 2006; Zhai et al.,2006). There is no any methodical review on the primary way ofmeasuring polarization difference of an ME nanostructured mate-rial under a magnetic field. This imposes serious limitations on ourunderstanding of the reversible ME effect in YMnO3 nanoparticleat room temperature. Based on the previous report symmetryconcept, origin of room temperature ME effect in YMnO3 may alsoattributed to the point defects and mixed valence of Mn ion(Chandra sekhar and Venkata Prasad, 2006). The linear MEcoupling coefficient (α) can be familiarized either electrically(αE¼dM/dH) or magnetically (αH¼dP/dH). In this work, the MEoutput voltage is measured across the sample applying themagnetic field (H).

The ac magnetic field (Hac) dependence of ME voltage at roomtemperature is shown in Fig. 8 at a fixed frequency of 850 Hz. Theresponse of the magneto-electric coefficient shows a linear path.The magnetoelectric coefficient α (dE/dH) value determinedfrom the slope of the ME voltage vs. Hac. As can be seen, theγ-irradiation induced a decrease in the magneto-electric voltagecoefficient for YMnO3 sample, which became more remarkable as

the irradiation dose increased. Fig. 9 shows the ME couplingcoefficient of the YMnO3 nanoparticle variation as a function ofthe dc bias magnetic field under constant applied ac magnetic fieldof 5 Oe. From the figure it is clear that, the ME coupling coefficientdecreases as the γ-dose increases even in the case of 100 kGywhich still has low ME coupling coefficient values than theunirradiated samples. This can be explained if we consider thathigh energy gamma radiation impinged on multiferroic YMnO3

sample decreased density of the medium. It can be envisaged thatthe massive external pressure applied throughout the radiation

B. Raneesh et al. / Radiation Physics and Chemistry 89 (2013) 28–3232

process hinders the thermal expansion mismatch and thusincrease the formation of micro-cracks.

4. Conclusion

Crystalline YMnO3 ceramic powder synthesized by the sol–gelmethod exhibits a single phase hexagonal structure when calcinedat 850 1C in the nano-regime with a particle size of the order of6075 nm. The XRD data of irradiated samples experience smallerintensities and higher line broadening due to induced disorder andgrain size deduction with increasing absorbed dose of gammairradiation. The absorption coefficient of the samples in the UV–visible range decreased after gamma irradiation with doses in therange of 100–1000 kGy. Phonon mode of vibration explored fromIR reflectivity spectra shows variances at the most irradiatedsamples. Exposure of the YMnO3 nanoparticles to γ-radiationresults in significant changes in the dielectric constant andconductivity. The observed decrease in the magneto-electriccoupling after irradiation is attributed to the decreased densityof the medium and rearrangement of cations in the lattice. Theinvestigation reveals the possibility of employing the ionizingradiation to degrade the dielectric and multiferrroic properties ofthe multiferroic YMnO3.

Acknowledgment

The authors B. R and N. K are thankful to UGC-DAE Consortiumfor Scientific Research, Kolkata Centre for financial support. Thefinancial support from UGC-Government of India through SAP, BRSSchemes and DST-Government of India through FIST program toSPAP is also gratefully acknowledged. One of the authors N. K alsoacknowledges DST-Government of India for financial supportthrough Nanomission program.

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