Magnetocaloric effect in RMnO3 (R¼Gd, Tb and Dy) multiferroics

N. Pavan Kumar, P. Venugopal Reddy n

Department of Physics, Osmania University, Hyderabad 500007, India

a r t i c l e i n f o

Article history:Received 1 January 2014Accepted 13 February 2014Available online 20 February 2014

Keywords:MultiferroicsOrthorhombic manganitesSpecific heatMagnetocaloric effectEntropy

a b s t r a c t

Multiferroic materials with the compositional formula RMnO3 (where R¼Gd, Tb and Dy), were preparedby the citrate gel technique. After characterizing the samples structurally, a systematic investigation ofspecific heat has been undertaken, over a temperature range 4–300 K at different magnetic fields. All thesamples display interesting specific heat behavior exhibiting transitions due to the long range ordering ofMn3þ and R3þ moments. Magnetic field change generates an entropy change of �1 J/mol K when amagnetic field changes from 0 to 5 T in the vicinity of antiferromagnetic ordering temperature of R3þ .The value of adiabatic temperature change �3 K for a field of 5 T is also appreciable to consider thesematerials as possible magnetic refrigerants at low temperatures.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

In recent times, the magnetic refrigeration, based on themagneto-caloric effect (MCE), has attracted considerable interestdue to its potential advantage of the higher energy efficiency andenvironmental friendliness over the conventional vapor compres-sion refrigeration. Previous studies on MCE were concentratedmainly on rare earth elements and their alloys and intermetalliccompounds with high total angular momentum quantum number(J) such as Gd, Gd5Si2Ge2, RAl2 (R¼Dy, Ho, and Er) [1–3] etc. LargeMCE has also been observed in some Mn-based alloys andcompounds [4–6] and in some ferromagnetic colossal magneto-resistive manganites [1,7,8]. Research work is in progress to findnew large MCE materials at low fields and close to room tem-perature for domestic and other technological applications.

On the other hand, systems showing large MCE in the low-temperature region from about 30 K down to sub-Kelvin are alsoimportant for basic research as well as for specific technologicalapplications such as space science, liquefaction of hydrogen in fuelindustry [1,9] etc. Recently, the magnetocaloric nature of rareearth manganites exhibiting multiferroic behavior has attractedconsiderable attention due to their potential applications inmagnetic refrigeration industry [10–12]. In view of this, an efforthas been made to investigate the MCE in rare earth manganites,with compositional formula RMnO3 (R¼Gd, Tb and Dy), usingspecific heat studies and the results of such an investigation arepresented here.

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/matlet

Materials Letters

Inte

nsity

(a.u

.)

GdMnO3

TbMnO3

DyMnO3

20 30 40 50 60 70

Inte

nsity

(a.u

.)

2Theta (degrees)

IobsIcalIobs- Ical

GdMnO3

Fig. 1. (a) XRD patterns of RMnO3 manganites. (b) Best fit from the Rietveldrefinement for GdMnO3 sample.

http://dx.doi.org/10.1016/j.matlet.2014.02.0450167-577X & 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ91 40 27682287; fax: þ91 40 27009002.E-mail address: paduruvenugopalreddy@gmail.com (P.V. Reddy).

Materials Letters 122 (2014) 292–295

2. Experimental

Multiferroic materials with the compositional formula, RMnO3

(R¼Gd, Tb and Dy) were prepared by the sol–gel method and thedetails are given in an earlier publication [13]. The structuralcharacterization of the samples was carried out by powder X-raydiffraction (XRD) technique using a Bruker AXS D8 Advanceddiffractometer and the data were analyzed by Rietveld refinementusing DBWS 486 software in MS-DOS mode. The specific heatmeasurements were performed by the semi-adiabatic heat pulse

method in the presence of 0 T, 2 T and 5 T fields with an accuracyof 0.5%.

3. Results and discussions

The structural investigations of the samples were carried out byXRD studies and powder diffraction patterns are shown in Fig. 1(a).The XRD data were analyzed using Rietveld refinement techniqueassuming orthorhombic structure with Pnma space group. Thepseudo-Voigt function was used to fit the peak profiles. Theexperimentally observed and calculated Rietveld refined patternsof GdMnO3 along with difference in both the patterns are shownin Fig. 1(b) and the refined crystallographic data are given inTable 1. It is clear from the XRD data that all the samples arehaving single phase, without any detectable impurity. The latticeparameters along with other crystallographic data are in agree-ment with those in literature [14].

Fig. 2(a–c) shows the variation of specific heat with tempera-ture of all the samples under various magnetic fields. It can beseen from the figure that all the samples are found to exhibit apeak in the temperature range, 40–45 K, hereafter designatedas TNMn3þ and is attributed to the transition of antiferromagneticphase into the sinusoidal incommensurate phase. In addition tothis transition, all the samples are also found to exhibit anothertransition, known as the lock-in-transition (Tlock), in the tempera-ture range 20–29 K and the behavior is in conformity with those of

Table 1Crystallographic data of RMnO3 based multiferroics.

Sample GdMnO3 TbMnO3 DyMnO3

a (Å) 5.83 5.81 5.8b (Å) 7.44 7.41 7.39c (Å) 5.31 5.29 5.27V (Å)3 230.32 227.74 225.88RP 12.1 8.25 12.26RWP 16.23 11.05 15.74REXP 12.17 8.33 12.61S(goodness of fit) 1.33 1.32 1.24

0

10

20

30

Cp (J

/mol

K)

0 T2 T5 T

TbMnO3

0

5

10

15

20

25

30

Cp (

J/ m

ol K

)

0T2T5T

GdMnO3

0 10 20 30 40 50 60

10

20

Cp (J

/ mol

K)

T (K)

0T 2T 5T

Fig. 2. Temperature dependences of specific heat (Cp) for RMnO3 manganites atdifferent magnetic fields.

0

4

8

12

TbMnO3

0T 2T 5T

S (J

/ mol

K)

0 5 10 15 20 25 30

0

5

10

15

20

0T 2T 5T

DyMnO3

S (J

/ mol

K)

T (K)

0

3

6

9

12

S ( J

/ mol

K)

0T 2T 5T

GdMnO3

Fig. 3. (a–c) Total entropy as a function of temperature for different magneticfields.

N.P. Kumar, P.V. Reddy / Materials Letters 122 (2014) 292–295 293

reported ones [14–16]. Finally, all the samples are found to exhibitanother transition, below 10 K indicating the ordering of rare earthion spins.

Although, with increasing field no observable change in thespecific heat behavior has been observed in the case of first twotransitions, the third transition is found to shift not only to highertemperatures but also broadens under 5 T magnetic field. In fact,the broadening of this peak with increasing field indicates theferromagnetic like ordering of rare earth ions [17].

4. Magnetocaloric effect

In order to calculate magnetocaloric effect (MCE) values, thefollowing procedure has been used. In this method, first the totalentropy of the manganite, S(T,H) can be calculated using theexperimental values of capacity C(T,H) using the well- knownrelation,

SðT ;HÞ ¼Z T

0

CðT ;HÞT

dT ð1Þ

The variation of total entropy with temperature for all the threesamples is shown in Fig. 3. It can be seen from Fig. 3(a) and (b) thatthe entropy decreases with magnetic field and all the plots merge

into a single one at temperatures 410 K. However, in the case ofDyMnO3 (Fig. 3(c)) although there is decrease of entropy in theinitial stages, the entropy of 5 T is found to increase continuouslywithout merging at higher temperatures. In fact, there is a cleardifference of 2.6461 J/mol K, between the 0 T and 5 T entropies.In view of this typical behavior, for this sample an amount of2.6461 J/mol K entropy was added to the zero Tesla entropy tocalculate the magnetocaloric effect at 5 T.

From the calculated total entropies S(0,T) and S(H,T), theisothermal magnetic entropy change ΔSM(H,T) has been calculatedat a given temperature, T

ΔSðH; TÞ ¼ SðH; TÞ–Sð0; TÞ ð2Þ

Later, the adiabatic temperature change caused by the magneticfield change, i.e. magnetocaloric effect, was obtained by anequation,

ΔT ðH; TÞ ¼ TðS;HÞ�TðS;0Þ ð3Þ

where T(S,H) and T(S,0) are the temperatures in the field H andH¼0 at constant total entropy.

The variation of entropy change (ΔS) with temperature at twofields (2 T and 5 T) is shown in Fig. 4(a–c). It can be seen from theplots that the broad transition observed around 10 K in a fieldof 2 T is found to sharpen when the field is increased to 5 T.

0

1

2

3GdMnO3

ΔT

( K )

2T 5T

0.0

0.6

1.2

1.8

2.4Δ

T ( K

) 2T

5T

TbMnO3

0

2

4

ΔT

( K )

T ( K )

2T5T

DyMnO3

0.0

0.4

0.8

1.2

-ΔS

(J/m

ol K

)

2T5T

GdMnO3

0.0

0.4

0.8

1.2

-ΔS

(J/m

ol K

)

2T 5TTbMnO3

5 10 15 20 25 305 10 15 20 25 30

0

1

2

3

4

-ΔS

(J/m

ol K

)

T ( K )

2T5TDyMnO

3

Fig. 4. (a–c) Entropy change (ΔS) as a function of temperature for different magnetic fields. (d–f) Temperature dependence of the adiabatic temperature change (ΔT).

N.P. Kumar, P.V. Reddy / Materials Letters 122 (2014) 292–295294

The typical values ΔSmax, which increase with increasing magneticfield are, 1.27, 1.08 and 4.0726 J/mol K for GdMnO3, TbMnO3 andDyMnO3 respectively. Fig. 4(d–f) shows the estimation of adiabatictemperature difference obtained from the isentropic curves using thespecific heat data. For a field change of 5 T, ΔTad values of GdMnO3,TbMnO3 and DyMnO3 are found to be 2.8, 2.4 and 4.89 K respectively.

Further, ΔS and ΔT values of these three samples whencompared with those of manganites exhibiting CMR behavior arehigher [1], indicating that the samples of the present investigationare better candidates for magnetic refrigeration applications. Infact, both these values are comparable with those of manganitebased multiferroics obtained from magnetization studies [18–21].

5. Conclusions

Polycrystalline RMnO3 orthorhombic multiferroic sampleswere prepared by the citrate gel method. A systematic investiga-tion of specific heat studies over a temperature range 4–300 Kindicate that all the samples exhibit transitions in three differenttemperature regions, namely 40 K, 20–26 K and below 10 K, andare attributed to the sinusoidal ordering of Mn moments intoincommensurate phase, the lock-in transition and the ordering ofthe rare-earth moments respectively. It has been concluded that asthese materials exhibit a large magnetic entropy change of �1 J/mol K, they may be exploited for possible magnetic refrigerantapplications at low temperatures.

Acknowledgments

One of the authors thank CSIR for providing fellowship. Theauthors are grateful to the Director and Dr. Rajeev Rawat of UGC-DAE

Consortium for Scientific Research (CSR), Indore, India, for providingfacilities to undertake heat capacity measurements and for theirvaluable discussions.

References

[1] Gschneidner Jr. KA, Pecharsky VK, Tsokol AO. Rep Prog Phys 2005;68:1479(and references therein).

[2] Pecharsky VK, Gschneidner Jr. KA. Phys Rev Lett 1997;78:4494.[3] Hashimoto T, Kuzuhara T, Sahashi M, Inomata K, Tomokiyo A, Yayama H. J Appl

Phys 1987;62:3873.[4] Wada H, Tanabe Y. Appl Phys Lett 2001;79:3302.[5] Hu F, Shen B, Sun J. Appl Phys Lett 2000;76:3460.[6] Tegus O, Brück E, Buschow KHJ, deBoer FR. Nature (London) 2002;415:150.[7] Phan M, Yu S. J Magn Magn Mater 2007;308:325 (and references therein).[8] Sarkar P, Mandal P, Choudhury P. Appl Phys Lett 2008;92:182506.[9] Provenzano V, Li J, King T, Canavan E, Shirron P, DiPirro M, et al. J Magn Magn

Mater 2003;266:185.[10] Eerenstein W, Mathur ND, Scott JF. Nature (London) 2006;442:759.[11] Cheong SW, Mostovoy M. Nat Mater 2007;6:13.[12] Fiebig M. J Phys D 2005;38:R123.[13] Pavan Kumar N, Lalitha G, Venugopal Reddy P. Phys Scr 2011;83:045701.[14] Harikrishnan S, Robler S, Naveen Kumar CM, Bhat HL, Robler UK, Wirth S, et al.

Phys Condens Matter 2009;2(1):096002.[15] Kimura T, Ishihara S, Shintani H, Arima T, Takahashi KT, Ishizaka K, et al. Phys

Rev B 2003;68:060403.[16] Kajimoto R, Yoshizawa H, Shintani H, Kimura T, Tokura Y. Phys Rev B

2004;70:012401.[17] Rawat R, Das IJ. Phys Condens Matter 2006;18:1051.[18] Shao Mingjie, Cao Shixun, Yuan Shujuan, Shang Jin, Kang Baojuan, Lu Bo, et al.

Appl Phys Lett 2012;100:222404.[19] Midya A, Mandal P, Das S, Banerjee S, Sharath Chandra LS, Ganesan V, et al.

Appl Phys Lett 2010;96:142514.[20] Midya A, Das SN, Mandal P. Phys Rev B 2011;84:235127.[21] Wolff Fabris F, Pekala M, Drozd V, Fagnard J-F, Vanderbemden Ph, Liu Ru-Shi,

et al. J Appl Phys 2007;101:103904.

N.P. Kumar, P.V. Reddy / Materials Letters 122 (2014) 292–295 295