Journal of Magnetism and Magnetic Materials 324 (2012) 3727–3730

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Journal of Magnetism and Magnetic Materials

0304-88

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n Corr

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

Effect of monovalent metal substitution on the magnetocaloric effect ofperovskite manganites Pr0.5Sr0.3M0.2MnO3 (M¼Na, Li, K and Ag)

Hangfu Yang, Pengyue Zhang n, Qiong Wu, Hongliang Ge, Minxiang Pan

College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China

a r t i c l e i n f o

Article history:

Received 11 November 2011

Received in revised form

9 February 2012Available online 15 June 2012

Keywords:

Magnetocaloric

Curie temperature

Manganites

53/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jmmm.2012.06.004

esponding author. Fax: þ86 571 28889526.

ail address: Zhang_pengyue@cjlu.edu.cn (P. Z

a b s t r a c t

The influence of monovalent doping on the magnetocaloric effect (MCE) and refrigerant capacity or

relative cooling power (RCP) of Pr0.5Sr0.3M0.2MnO3 (M¼Na, Li, K and Ag) materials has been

investigated. A large magnetocaloric effect was inferred over a wide range of temperature around

the second order paramagnetic–ferromagnetic transition. The maximum magnetic entropy changes

(DSM) reached 1.8, 2.2, 1.6 and 2.1 J/kg K and the relative cooling power (RCP) approached 58.9, 59.3,

69.6 and 54.6 J/kg for Na, Li, K and Ag doped materials in the magnetic change of 15 kOe, respectively.

According to the results determined by the Maxwell relation, the magnetic entropy change fits well

with the Landau theory of phase transition above TC for Pr0.5Sr0.3Li0.2MnO3. The large magnetic entropy

change induced by low magnetic field suggested that these materials are beneficial for practical

applications.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Magnetic refrigeration (MR), which is a technology used towarm and cool in response to application and removal of anexternal magnetic field, gets comprehensive attention due to itsseveral apparent advantages over vapor compression refrigera-tion: high cooling efficiency, environmentally friendly technologyand convenient for miniaturization [1]. MR is based on themagnetocaloric effect which is determined by the isothermalmagnetic entropy change and adiabatic temperature change. Tofind an active magnetic refrigerant (AMR) working at roomtemperature, most researchers focus on the metal alloys. Theserefrigerants, e.g. Gd [2], Gd5(SixGe1�x)4 [3], LaFeSi [4], undergoingthe first-order magnetic transition, generally have large magneticentropy changes. It should be noticed that although the first-ordertransition is able to concentrate the MCE in a narrow temperaturerange and this produces large magnetic entropy changes, the RCPbecomes small. Furthermore, material with a large MCE under-going first order transition is always in a high magnetic field andhas considerable hysteresis.

Refrigerants of manganites undergoing second-order magnetictransition with a large MCE take a resurgence of interest due totheir low hysteresis, affluent metamagnetic transition and cou-pling between charge and lattice. The MCE of Pr1�xSrxMnO3

(0.3oxo0.5) polycrystalline manganites was reported by Chen

ll rights reserved.

hang).

et al. [5], who found the maximum of DSM of 7.1 J/kg K at 161 Kfor 10 kOe for the x¼ 0.5 samples. However, the Curie tempera-ture (TC) is far from room temperature, limiting their practicalapplications. Koubaa et al. [6]., investigated the MCE properties ofLa0.8Ag0.1Na0.1MnO3 and La0.8Ag0.1K0.1MnO3 that reached 4.39and 4.92 J/kg K and the Curie temperatures reached 320 K and310 K for Na and Ag for 50 kOe magnetic field changes, respec-tively. Das and Dey [7,8]. also did a lot of work on monovalentelements doped perovskite systems; it was found that the Curietemperature of La1�xKxMnO3 strongly depends on K content andthe enhancement of magnetic entropy change was observed also.Thus, materials with monovalent elements doping at A-sitesincrease the Curie temperature without distinct decrease inthe MCE.

In the present work, we investigate the effect of the samemonovalent metal content of Na, Li, K and Ag substitution on thestructural, the magnetocaloric properties and the Curie tempera-ture of Pr0.5Sr0.3M0.2MnO3 (M¼Na, Li, K and Ag).

2. Experimental details

Powder samples of Pr0.5Sr0.3M0.2MnO3 (M¼Na, Li, K and Ag)have been synthesized by using the standard solid state reactionmethod at high temperature. The starting materials (Pr6O11,SrCO3, MnCO3, Na2CO3) were mixed in an agate mortar. After ballmilling for 2 h, the mixture was heated in air up to 1273 K for24 h. The obtained powders were pressed into pellets of about2 mm thickness and 13 mm diameter and then sintered at 1573 K.

H. Yang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3727–37303728

Finally, these pellets were slowly cooled down to room tempera-ture. Phase purity and homogeneity were determined by powderX-ray diffraction at room temperature. Magnetization measure-ments versus temperature in the range 80–350 K and versusapplied magnetic field up to 20 Oe were carried out using aVibrating Sample Magnetometer (VSM). The MCE was calculatedfrom the magnetization measurements versus applied magneticfield up to 15 kOe at various temperatures.

3. Results

The analysis of X-ray pattern confirms the single phase ofPr0.5Sr0.3M0.2MnO3 (M¼Na, Li, K and Ag) with orthorhombicstructure (space group pnma). No detectable second phase wasfound. The temperature dependence of magnetization ofPr0.5Sr0.3Na0.2MnO3 measured in the magnetic field of 20 Oedisplays large divergence between zero-field cooled (ZFC) andfield cooled magnetization (FC) curves below 285 K (Fig. 1), whichis indicative of magnetic glassy behavior [11]. The divergencebetween ZFC and FC could also result from an anisotropy effect[12], and competition between ferromagnetic and antiferromag-netic coupling. Spin glasses as a result of competing ferromag-netic double exchange and antiferromagnetic super-exchangeinteractions are not new [13,14]. The divergence behavior canbe interpreted by facts that at low temperature the antiferromag-netic phase is more stable while application of a magnetic fieldtends to stabilize the ferromagnetic phase. However, there is nocharge-order transition found, which has been extensivelyreported in half doped PSMO systems [15–17]. Tomioka et al.[13] showed that the charge order becomes actively favorable fordoped electrons to get localized and crystallized. are largerTolerance factors of 0.95 (Na), 0.97 (K) and 0.95 (Ag) are largercompared to 0.93 of Pr0.5Sr0.3M0.2MnO3. [18]. In this case doubleexchange interaction is enhanced, subsequently with an increasein the transfer of eg holes (electrons) which also may gain becauseof metallicity of the monovalent element. Thus it suppresses thecharge order.

The Curie temperatures (TC) are determined at the inflectionpoint of dM/dT curves of the samples. The para–ferromagnetictransition temperatures are 285, 290, 290 and 290 K for Na, Li, Kand Ag doped samples respectively. In the double exchangemechanism, TC was estimated [19] as TC� J�t2

dp�S2dp�cos2 y (J:

effective exchange between adjacent Mn ions; t2dp: Mnd–O2p

transfer integral; S2dp: overlap and y: angle of Mn–O). Thus almost

the same values of TC could be ascribed to the similar electronic

Fig. 1. Temperature dependence of zero-field cooled (ZFC) and field cooled (FC)

magnetization curves measured at 20 Oe for Pr0.5Sr0.3M0.2MnO3 (Na, Li, K and Ag).

configuration and tolerance factor doped at the A-site of manga-nites. The tolerance factor approaching 1 increases the angle ofMn–O which is beneficial to get a high value of Curie temperature.Das and Dey [8], explained that the increase in the TC was fromthe larger K1þ ion than La3þ ion but in our case Li1þ ion (ionicradii �0.92 A) is smaller than Pr3þ (ionic radii �1.126 A). There-fore it is suggested that the similar electronic configuration maymainly give rise to the observed increase in the phase transitiontemperature from 161 K [5] to room temperature. The magneticentropy change DSM which is associated with the MCE could becalculated from isothermal magnetization data (Fig. 2) using thethermodynamic Maxwell relation

DSMðT ,HÞ ¼

Z Hmax

0

@M

@T

� �H

dH ð1Þ

where Hmax is the maximum value of the applied magnetic field.Thus the maximum values of DSM are 1.8, 2.2, 1.6 and 2.1 J/kg Kfor Na, Li, K and Ag, respectively, in the magnetic field change of15 kOe (Fig. 3). The magnetic entropy change of 2.2 J/kg K forPr0.5Sr0.3Li0.2MnO3 is about 52% of that of pure Gd, [20] and islarger than the DSM values of Pr0.6Sr0.4MnO3 [21,22]. For commonhousehold, automotive, and practical magnetic cooling applica-tions in general the magnetic fields induced by permanentmagnets are about 20 kOe [23]. Consequently our materials withlarge magnetic entropy changes generated by low magnetic fieldare beneficial for practical applications. The relative coolingpower (RCP) is the most important parameter to determine thecooling efficiency or the usefulness of a magnetic refrigerantmaterial. The RCP is defined as

RCP¼ 9DSM9dTFWHM ð2Þ

where dTFWHM is the full width at the half maximum value of DSM

versus T curves. dTFWHM is relatively large, reaching 27 and 45 Kfor Li and K doped samples respectively. The wide temperaturerange for dTFWHM gives rise to a high value of RCP. In general,better the RCP values the better the materials for magneticrefrigeration because materials with higher RCP would supportthe transport of a greater amount of heat in a practical refrig-erator. In our work, the RCP values reach 58.9, 59.3, 69.6 and54.6 J/kg for Na, Li, K and Ag doped samples respectively. Thesevalues are comparable to those reported for other manganites [2],and high enough to propose our manganite materials as activemagnetic refrigerants working around room temperature. The

Fig. 2. Isothermal magnetization curves of Pr0.5Sr0.3Li0.2MnO3 measured at differ-

ent temperatures.

Fig. 3. Temperature dependence of magnetic entropy change for the samples.

Fig. 4. Arrott plots (H/M versus M2) of Pr0.5Sr0.3Li0.2MnO3 at different

temperatures.

Fig. 5. Temperature dependence of DSM of Pr0.5Sr0.3Li0.2MnO3 sample along with

theoretical curve (solid line).

H. Yang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3727–3730 3729

Banerjee criterion (H/M versus M2) is generally used to determinethe nature of magnetic phase transition [24,25]. According to thiscriterion, the linear fit to the high M2 values of the Arrott plotintercept on the ordinate (1/w) at zero for T¼TC since it is bydefinition 1/w¼0 at TC. Furthermore, the positive or negativeslope of H/M vs. M2 curves also indicates whether the magneticphase transition is second order or first order, respectively. As wecan see from Fig. 4, the paramagnetic–ferromagnetic transitions(TC) are obtained from H/M versus M2 curves when the interceptsare zero. The obtained value is about 290 K which agrees with thevalue obtained from dM/dT curves. Arrott plots show a clearlypositive slope in the whole range of M2, which confirms thesecond order nature of phase transition. From the applicationpoint of view, it should be noted that there is large thermal andfield hysteresis for the first order transitions while the hysteresiscould be neglected for second order transitions. Furthermore,materials with a large MCE undergoing first order transition arealways in a high magnetic field, and this limits their potential ofcommercial application [1,2]. The H/M versus M2 curves show aseries of parallel lines at various temperatures inferring that themean field theory is valid for the samples.

To deeply understand the temperature dependence ofmagnetic entropy change, DSM, of the sample, we applied the

Landau theory of phase transition, which is represented by therelation [26]

DSMðT ,HÞ ¼ �1

2

@A

@TM2�

1

4

@B

@TM4

ð3Þ

We selected Li doped manganites as an example. The DSM

curves of this manganite calculated using the Landau theory agreewell with plots determined by the thermodynamic Maxwelltheory above TC. However at low temperature, below TC, thesetheories are not well fit(Fig. 5). It is supposed that when thetemperature is above TC the magnetoelastic coupling and electroninteraction could account for the magnetic entropy change and itstemperature dependence [27]. Similar results were reported inRef. [7–10] by Das, however when the temperature is below TC

there are some other factors which have major effects on themagnetic properties of manganites such as the John–Teller effect,exchange interactions and micromagnetism. Thus still furtherresearch is needed to understand this phenomenon.

4. Summary

The large magnetic entropy changes and relative coolingpower of Pr0.5Sr0.3M0.2MnO3 (M¼Na, Li, K and Ag) materials wereinvestigated. All the samples show second order para–ferro phasetransition with large magnetic entropy changes induced by thelow magnetic field over a wide range of temperature. The mainadvantages of these compounds are their low cost, chemicalstablity and ease of synthesis [8], which is indicative of thesematerials being desirable candidates for refrigeration applicationsat the near room temperature [9]. The observed temperaturedependence of DSM is in accordance with the Landau theoryabove Curie temperature, but does not fit well below TC forPr0.5Sr0.3Li0.2MnO3 materials.

Acknowledgments

This work was supported by the Key Project of the Interna-tional Cooperation and Exchanges of the Zhejiang Province (no.2006C14014), the Science and Technology Project (no.2009C21010), the Provincial Natural Science Foundation (no.Y6100640), National Science Foundation (no. 51001092) and the

H. Yang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3727–37303730

Foundation for University Young Teachers from the Ministry ofEducation of Zhejiang province.

References

[1] A. Rostamnejadi, M. Venkatesan, P. Kameli, H. Salamati, J.M.D. Coey, Journalof Magnetism and Magnetic Materials 323 (2011) 2214.

[2] M.H. Phan, S.C. Yu, Journal of Magnetism and Magnetic Materials 308 (2007)325.

[3] V.K. Pecharsky, K.A. Gschneidner, Applied Physics Letters 70 (1997) 3299.[4] S. Fujieda, A. Fujita, K. Fukamichi, Applied Physics Letters 81 (2002) 1276.[5] P. Chen, Y.W. Du, G. Ni, Europhysics Letters 52 (2000) 589.[6] M. Koubaa, Y. Regaieg, W.C. Koubaa, A. Cheikhrouhou, S. Ammar-Merah,

F. Herbst, Journal of Magnetism and Magnetic Materials 323 (2011) 252.[7] S. Das, T.K. Dey, Journal of Physics: Condensed Matter 18 (2006) 7629.[8] S. Das, T.K. Dey, Journal of Alloys and Compounds 440 (2007) 30.[9] S. Das, T.K. Dey, Journal of Physics D: Applied Physics 40 (2007) 1855.

[10] S. Das, T.K. Dey, Materials Chemistry and Physics 108 (2008) 220.[11] D.N.H. Nam, R. Mathieu, P. Nordblad, N.V. Khiem, N.X. Phuc, Physical Review

B 62 (2000) 1027.[12] R.N. Mahato, K. Sethupathi, V. Sankaranarayanan, R. Nirmala, A.K. Nigam,

S.K. Malik, Journal of Applied Physics 109 (2011) 07E319.[13] Y. Tomioka, A. Asamitsu, Y. Moritomo, H. Kuwahara, Y. Tokura, Physical

Review Letters 74 (1995) 5108.

[14] Y. Tokura, H. Kuwahara, Y. Moritomo, Y. Tomioka, A. Asamitsu, PhysicalReview Letters 76 (1996) 3184.

[15] N.S. Bingham, M.H. Phan, H. Srikanth, M.A. Torija, C. Leighton, Journal of

Applied Physics 106 (2009) 023909.[16] R. Venkatesh, M. Pattabiraman, K. Sethupathi, G. Rangarajan,

S.N. Jammalamadaka, Journal of Applied Physics 101 (2007) 09C506.[17] A. Biswas, T. Samanta, S. Banerjee, I. Das, Applied Physics Letters 92 (2008)

012502.[18] G.H. Jonker, J.H. van Santen, Physica 16 (1950) 337.[19] A. Sadoc, B. Mercey, C. Simon, D. Grebille, W. Prellier, M.B. Lepetit, Physical

Review Letters 104 (2010) 046804.[20] A. Biswas, T. Samanta, S. Banerjee, I. Das, Applied Physics Letters 92 (2008)

212502.[21] S. Zemni, M. Baazaoui, J. Dhahri, H. Vincent, M. Oumezzine, Materials Letters

63 (2009) 489.[22] N. Chau, P.Q. Niem, H.N. Nhat, N.H. Luong, N.D. Tho, Physica B 327 (2003)

214.[23] J. Lyubina, Journal of Applied Physics 109 (2011) 07A902.[24] L.K. Leung, A.H. Morrish, B.J. Evans, Physical Review B 13 (1976) 4069.[25] L.E. Hueso, P. Sande, D.R. Miguens, J. Rivas, F. Rivadulla, M.A. Lopez-Quintela,

Journal of Applied Physics 91 (2002) 9943.[26] V.S. Amaral, J.S. Amaral, Journal of Magnetism and Magnetic Materials 272

(2004) 2104.[27] R.N. Mahato, K. Sethupathi, V. Sankaranarayanan, R. Nirmala, Journal of

Applied Physics 107 (2010) 09A943.