Effect of Monovalent Doping on the Structural, Magnetic and Magnetocaloric Properties in La0.7M0.2M ′ 0.1MnO3 Manganese Oxides (M=Sr, Ba and M ′ =Na, Ag, K) Academic research paper on "Physical sciences"

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ELSEVIER Physics Procedia 2 (2009) 989-996

www.elsevier.com/locate/procedia

Proceedings of the JMSM 2008 Conference

Effect of Monovalent Doping on the Structural, Magnetic and Magnetocaloric Properties in La0.7M0.2M' 0.1 MnO3 Manganese Oxides (M=Sr, Ba and M'=Na, Ag, K)

W. Cheikh-Rouhou Koubaaa *, M. Koubaaa, A. Cheikhrouhoua,b

aLaboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, B. P. 1171, 3000 Sfax, Tunisia bInstitut NEEL, CNRS, B.P.166, 38042 Grenoble cedex9, France

Received 1 January 2009; received in revised form 31 July 2009; accepted 31 August 2009

Abstract

Structural, magnetic and magnetocaloric effects of powder perovskite manganites La0.7Mo.2M'o.iMnO3 (M=Sr, Ba and M'=Na, Ag and K) have been investigated. Our samples have been elaborated using the conventional solid state reaction at high temperature. X-Ray diffraction characterizations showed that all our synthesized samples crystallize in the distorted rhombohedral system with R3c space group. Magnetization measurements versus temperature in a magnetic applied field of 50mT showed that all our samples are ferromagnetic above room temperature. From the measured magnetization data of our synthesized samples as a function of magnetic applied field, the associated magnetic entropy change close to their respective Curie temperature TC and the relative cooling power RCP have been determined. A maximum magnetic entropy change, |A5^axJ, of 4.07Jkg-1K-1 around 345K was obtained in La0 7Sr0 2Na01MnO3 sample upon a magnetic field change of 5T. The |asM| values of La07Ba02M'01MnO3 are smaller in magnitude compared to La07Sr02M'01MnO3 samples and occur at lower temperatures.

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Manganites; ferromagnetism; Curie temperature; Magnetocaloric effect

1. Introduction

The ABO3-type manganese oxides with general formula RE1-xAExMnO3 where RE and AE, respectively, being a trivalent rare earth element and a divalent alkali-earth one have been extensively studied since the discovery in 1994 of a large magnetoresistance effect in these systems [1,2]. These materials exhibit many significant properties like metal-insulator transition, ferromagnetic-paramagnetic phase change, charge and orbital ordering, etc..., depending on several parameters as charge density, temperature, atomic structure, average ionic radius <rA> of the A-site cations and Mn3+/Mn4+ ratio. Moreover, novel properties have been observed ten years ago in the perovskite-type

* Corresponding author. Tel./fax :+216 74 67 66 07. E-mail address: wissem.koubaa@yahoo.fr.

doi:10.1016/j.phpro.2009.11.054

ferromagnetic manganese oxides related to magneto-caloric effect [3,4]. The magnetocaloric effect was developed as a technology by Giauque and MacDougall [5] in 1933 in order to obtain very low temperatures (tenths of a Kelvin) using paramagnetic salt as a magnetic refrigerant. However, this cooling method has remained a low-temperature technique. Recently, there has been interest in extending the magnetic refrigeration technique to higher temperatures because of the desire to eliminate the chlorofluorocarbons present in high-temperature gas-cycle systems. Refrigeration in the sub-room temperature range is of particular interest due to the potential impact on energy savings as well as environmental concerns. Materials with large magnetocaloric effects are needed to improve energy efficiency. Pecharsky and Gschneidner [6,7] discovered a giant MCE in the pseudo-binary alloy Gd5(SixGe1-x)4 in the range of temperature from 50 to 280K. This is a great piece of work with enormous importance for both physics and technology. More recently, giant MCE at about 300 K has been measured in MnFeP (O0 45As0 55) [8]. Several studies have been published on magnetic perovskites for high-temperature magnetocaloric effect applications [9,10]. In the present work, we synthesized La0i7M0.2M'0.1MnO3 (M=Sr, Ba and M'=Na, Ag and K) powder perovskite manganites by the conventional solid state reaction at high temperature and studied the crystallographic, magnetic and magnetocaloric effects due to the substitution of 10% of strontium or barium by monovalent element Na, Ag and K.

2. Experimental techniques

Polycrystalline samples of La0.7A0.2M0.1MnO3 (A=Sr, Ba and M= Na, Ag and K) were synthesized using the solid state reaction method at high temperature. The starting materials were intimately mixed in an agate mortar and then heated in air up to 1000°C for 60h. The obtained powders were then pressed into pellets (of about 1mm thickness) and sintered at 1100°C in air for 60h with intermediate regrinding and repelling. Finally, these pellets were rapidly quenched to room temperature in air in order to freeze the structure at the annealed temperature. Phase purity, homogeneity and cell dimensions were determined by X-ray powder diffraction (XRD) at room temperature (diffractometer using Cu-Ka radiation). Structural analysis was carried out using the standard Rietveld Method [1112]. The Mn4+ ions amount has been quantitatively checked by iodometric titration. Magnetization measurements versus temperature in the range 20-400 K and versus magnetic applied field up to 7 T were carried out using a vibrating sample magnetometer. The |AS„| induced by the magnetic field change have been determined from magnetization measurements versus magnetic applied field according to the classical thermodynamic theory based on Maxwell's relations.

3. Results and discussion

In our studied samples, the Mn4+ amount remains constant equal to 40%. Monovalent element doping leads to a change in the average ionic radius <rA> and the mismatch size a2 of the A-cation site. The X-ray diffraction (XRD) patterns of all samples were recorded at room temperature. The XRD data refined using the Rietveld technique show that all our samples are single phase and can be indexed in the rhombohedral structure with R3c space group. A good fit between the observed and the calculated profiles was obtained, as shown in Fig. 1 for La0.7Sr0.2Na0.1MnO3 sample.

Fig. 2 displays the unit cell volume evolution versus <rA> for both series. For La0.7Sr0.2M0.1MnO3 samples, we can observe that, with increasing <rA>, the unit cell volume is found to slightly decrease from 352.5A for M=Na to 351.1A for M=K, which can be explained by the increase of a2. Whereas, in La0.7Ba0.2M0.1MnO3 compounds, the unit cell evolution is rather governed by <rA> than a2, which explain the observed increase of the unit cell volume. It should be noted that although both the A-site ionic radius and the mismatch size are similar for both La0.7Sr0.2K0.1MnO3 and La0.7Ba0.2Na0.1MnO3 samples, the unit cell volume presents different values. This behaviour emphasizes the effects of electronegativity difference between ions in A-site in our compounds [13]. With increasing <rA>, the Mn-O-Mn bond angle decreases from 166.7° for La0.7Sr0.2Na0.1MnO3 sample to 165.9° for La0.7Ba0.2Na0.1MnO3 sample while the Mn-O bond length increases from 1.957A to 1.968A, which influences the double exchange strength as will be discussed below.

li 1 lobs _ Itealc - t I l.l.i

: 1 1 1 II II 1 II llll II III li ; i l-l -

20 27 J4 41 li SS 62 6? 7S S3 SO

Fig. 1. X-Ray diffraction patterns at room temperature of La0.7Sr0.2Na0.1MnO3 sample. Circles indicate the experimental data and the calculated data is the continuous line overlapping them. The lowest curve shows the difference between experimental and calculated patterns. The vertical

bars indicate the expected reflection positions.

g 354-

-■- M=Sr —ffl—M=Ba ______ffi M'=Ag — M'=K j®" / / ©' M'=Na

■--------«...

M'=Ag M'=K

1,18 1,19

<rA>(A)

Fig. 2. Unit cell volume versus <rA> for La0.7M0.2M'0.1MnO3 (M=Sr, Ba and M'=Na, Ag and K) samples.

Magnetization measurements recorded versus temperature in the range 20-350K in a magnetic applied field of 50mT showed that all our substituted samples exhibit a sharp transition from paramagnetic to ferromagnetic phase with decreasing temperature, as illustrated in Fig. 3 for La0.7M0.2K0.1MnO3 (M=Sr, Ba) samples.

-■- M=Ba M=Sr

Fig. 3. Temperature dependence of magnetization at 50mT for Lao.7Mo.2Ko.iMnO3 (M=Sr, Ba) samples.

The Curie temperature TC has been taken as the position of the inflexion point in the M(T) curves. As can be seen in Fig. 4, the Curie temperature shifts to lower values with <rA> and varies from 340K for <rA>=1.182 A to 311.5 K for <rA>=1.247 A.

M'=Na ■ LaSrM'

■ a LaBaM'

M'=Ag B

M'=Na a

M'=K ffl

1,18 1,19 1,20 1,21

1,22 1,23 1,24 1,25

<rA>(A)

Fig. 4. Curie temperature versus <a> for Lao.7Mo.2M'o.iMnO3 (M=Sr, Ba and M'= Na, Ag and K) samples.

The observed discrepancy between La0i7Sr0i2K01MnO3 and La0i7Ba02Na()i1MnO3 compounds is also related to the effects of electronegativity difference between ions in A-site for the two samples. In order to confirm the ferromagnetic behavior of our samples at low temperatures, we performed magnetization measurements versus magnetic applied field up to 7T at several temperatures. A typical M(H) curves for La0i7Sr0i2Na0i1MnO3 sample are plotted in Fig, 5,a),

H0H(T) T (K)

Figi 5i a) Plot of magnetization M versus magnetic applied field ^qH up to 7T at several temperatures and b) Temperature dependence of the spontaneous magnetization Msp and 1/% for Lao^Sro^NaojMnOs samplei

Below TC, the magnetization M increases sharply with magnetic applied field for H<0i5T and then saturates above 1Ti The saturation magnetization shifts to higher values with decreasing temperaturei This result confirms the ferromagnetic behavior of our sample at low temperatures! Figi 5b shows the temperature dependence of the spontaneous magnetization Msp and 1/% for La0i7Sr0i2Na01MnO3 samplei The experimental value of the spontaneous magnetization Msp(exp), deduced from the M(H) curves is found to be 348^^^ The magnitude of the Msp(exp) is comparable to the theoretical value of 3i6^B/mole calculated for full spin alignment The critical exponent y defined by

m.CD = M_(0)

and deduced from the fit of the Msp(T) curve is found to be 0.31, which confirms the ferromagnetic behavior of our sample at low temperatures. The values of the spontaneous magnetization as well as the critical exponent are comparable for all our samples [14-16].

From the thermodynamical theory, the magnetic entropy change produced by the change of the magnetic field from 0 to H is given by [17]:

ASM (T, H) = SM (T, H) - SM (T,0) = [ ÍJH-] dH (2)

with Maxwell's relation

f M) = f dS.

1 dT J h ~ I dH

one can obtain the following expression:

ASM (T, H) = dH (4)

According to (4) the maximum of the magnetic entropy change is attained at the Curie temperature TC where the ferromagnetic-paramagnetic phase transition takes place. Using isothermal magnetization measurements in small discrete fields and temperature intervals, the magnetic entropy change can approximately be calculated using the numerical formula [18]:

ASM (T, H) = ^ ^ (5)

where Mi and Mi+1 are the experimental magnetization values obtained at the temperatures Ti and Ti+1, respectively, in a magnetic applied field Hi. We plot in Fig. 6a the magnetic entropy change, |ASm| , of Lao.7Sro.2Nao.1MnO3 sample as a function of temperature under several magnetic applied field changes. We observe that |ASm| increases and reaches its maximum around the Curie temperature TC. We can notice that the larger the magnetic applied field is, the larger the magnetic entropy change is obtained. The values of the maximum of the magnetic entropy change, ASM™ , are 1.58, 2.34, 2.99, 4.07 and 4.99J/kgK under magnetic applied field change of 1, 2, 3, 5 and 7 T, respectively.

-■-AH=1T -•- AH=2T -a-aH=3T -▼-AH=5T —♦— AH=7T

///a ^

300 350

-■- LaSrNa LaSrAg 4- LaSrK -□— LaBaNa

LaBaAg ,

LaBaK D \_

—I—

350 400

Fig. 6. a) Magnetic entropy change -ASm evolution versus temperature at several magnetic applied field for La07Sr02Na0.iMnO3 sample and b) Magnetic entropy change -ASm evolution versus temperature at 5T for La07M02M'01MnO3 samples (M=Sr, Ba and M'=Na, Ag and K).

We plot in Fig. 6b the different curves of the magnetic entropy change for all our synthesized polycrystalline samples as a function of temperature under 5T. For each series, the IaS"^"! values are comparable but the temperatures at which occurs shift to lower values with increasing <rA>. The Lao7Sro.2Nao.iMnO3 sample

exhibits the higher value of \ASM"Z , 4.07Jkg-1K-1, around 345K. For La0.7Ba0.2Na01MnO3 sample, we observe an asymmetrical broadening of |ASM peak, which can be explained by structural inhomogenity. Although these

I ASM" I

values in our samples are smaller than that observed in Gd or Gd-based compounds, the |ASM| versus temperature curves are significantly broader. This wider temperature range with large magnetic entropy change is useful for an ideal Ericsson refrigeration cycle. Moreover, our samples are interesting in application as potential

candidates in magnetic refrigeration since they are cheap, easier to fabricate, possess tunable TC and high chemical stability.

In the magnetic refrigeration technology, it is of utmost interest that the magnetocaloric effect extends over a large temperature range; we can than consider the relative cooling power (RCP) given by [19]

RCP = -ASM (T, H) xSTFWHM (6)

where 3Tfwhm is the full-width at half-maximum of |ASm| versus temperature and |ASM"| is the maximum of |ASM|. For Lao.7Sro.2M'o.1MnO3 samples, the RCP values for M'=Na, Ag and K are respectively 118.4, 116.6 and l21.1J/kg at 5T. These values are comparables and lower than that observed in La07Ba0i2M'0i1MnO3 samples. The RCP values are found to be 307.1J/kg, 271J/kg and 337.9J/kg at 5T for M=Na, Ag and K respectively although these samples present |Asr| values much smaller. This result confirms that the larger the STjwhm the better the cooling capacity. Fig. 7 illustrates these RCP values as a function of TC for both series upon a magnetic field change of 5T in comparison with Gd metal which is considered as the most active refrigerant near room temperature.

q. 200

Fig. 7. RCP values versus TC for Lao.65Cao.3M'o.o5MnO3, Lao.65Bao.3M'o.o5MnO3 and Gd

We can also notice that the RCP value of La0 7Ba0 2K01MnO3 sample is higher in magnitude and occurs close to room temperature compared to M'=Na and Ag samples. In addition the magnitude of the RCP is about 82% of that of pure Gd.

4. Conclusions

We investigated the effect of monovalent doping on the physical properties of La0l7M0l2M'0l1MnO3 (A=Sr, Ba and M=Na, Ag and K) powder samples. The structural study shows that all our synthesized samples crystallize in the rhombohedral structure with R3c space group. All our samples exhibit a paramagnetic-ferromagnetic transition with decreasing temperature. A large magnetocaloric effect above room temperature is observed in all our samples. In this study, we have shown that La07Sr02M'0iMnO3 compounds exhibit the larger

values above room

temperature; however they are less interesting for technological applications than La0.7Ba0.2M'0.1MnO3 samples presenting higher RCP vales near 300K. A mixture of the three latter compounds characterized by slightly different

transition temperatures could result in a more broadened magnetic entropy curve leading to an active magnetic refrigerator.

REFERENCES

[1] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413.

[2] M. McCormack, S. Jin, T.H. Tiefel, R.M. Fleming, J.M. Philips, R. Ramesh, Appl. Phys. Lett. 64 (1994) 3045.

[3] Z.B. Guo, Y.W. Du, D. Feng, Phys. Rev. Lett. 78 (1997) 1142

[4] Z.B. Guo, Y.W. Du, Appl. Phys. Lett. 70 (1997) 904

[5] W.F. Giauque, D.P. MacDougall, Phys. Rev. 43 (1933) 786

[6] V.K. Pecharsky, K.A. Gschneidner Jr., J. Magn. Magn. Mater. 167 (1997) L179-L184

[7] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 4494-4497

[8] G. Tegus, E. Brück, K.H.J. Buschow, F.R. de Boer, Nature 415 (2002) 150-152

[9] X. Bohigas, J. Tejada, E. Del Barco, X.X. Zhang, M. Sales, Appl. Phys. Lett. 73 (1998) 390

[10] X.X. Zhang, J. Tejada, Y. Xin, G.F. Sun, K.W. Wong, X. Bohigas, Appl. Phys. Lett. 69 (1996) 3596

[11] D. B. Wiles, R. A. Young, J. Appl. Crystallogr. 14 (1981) 149

[12] T. Roisnel, J. Rodriguez-Carvajal, Computer program FULLPROF, LLB-LCSIM. May 2003

[13] Z.C. Xia, B. Dong, G. Liu, D.W. Liu, L. Chen, C.H. Fang, L. Liu, S. Liu, C.Q. Tang, S.L. Yuan, Phys. Stat. Sol. (a) 202 (2005) 113

[13] W.C. Koubaa, M. Koubaa, A. Cheikhrouhou, J. Alloys Comp. 453 (2008) 42

[14] W.C. Koubaa, M. Koubaa, A. Cheikhrouhou, J. Alloys Comp. 470 (2009) 42

[15] W.C. Koubaa, M. Koubaa, A. Cheikhrouhou, J. Mater. Sci.. 44 (2009) 1780

[16] T. Hashimoto, T. Numasawa, M. Shino, T. Okada, Cryogenics 21 (1981) 647

[17] Q. Luo, D.Q. Zhao, M.X. Pan, W.H. Wang, Appl. Phys. Lett. 89 (2006) 081914

[18] K.A. Gschneidner, V.K. Pecharsky, Ann. Rev. Mater. Sci. 30 (2000) 387.