Materials Letters 132 (2014) 82–85
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Investigation of magnetocaloric effect in Dy doped TbMnO3 N. Pavan Kumar, P. Venugopal Reddy n Department of Physics, Osmania University, Hyderabad 500007, India
art ic l e i nf o
a b s t r a c t
Article history: Received 28 April 2014 Accepted 5 June 2014 Available online 13 June 2014
Multiferroic materials with the compositional formula, Tb1 xDyxMnO3 (x ¼ 0.1, 0.2, 0.3 and 0.4) were prepared by the sol gel technique. After characterizing the samples structurally, a systematic investigation of specific heat has been undertaken, over a temperature range 4–300 K in 0 T and 5 T magnetic fields. All the samples display interesting specific heat behavior exhibiting transitions due to the long range ordering of Mn3 þ as well as R3 þ magnetic moments. Magnetic field change generates an entropy change of 1 J/mol K when a magnetic field changes from 0 to 5 T in the vicinity of antiferromagnetic ordering temperature of R3 þ . The values of adiabatic temperature change is also appreciable to consider these materials as possible magnetic refrigerants at low temperatures. & 2014 Elsevier B.V. All rights reserved.
Keywords: Multiferroics Orthorhombic manganites Specific heat Magnetocaloric effect Entropy
1. Introduction Conventional magnetic materials heat up when they are placed in a magnetic field and cool down when they are removed and the phenomenon is known as the magnetocaloric effect (MCE). Magnetic refrigeration based on the MCE was first applied for lowtemperature Physics applications by using paramagnetic salts to reach sub-Kelvin temperatures [1]. The discovery of roomtemperature giant magnetocaloric compounds, such as Gd5Si4 xGex [2], LaFe13 xSix [3] and MnFeP(As, Ge) [4] has driven development of magnetic refrigeration close to ambient conditions. The new technology is considered to be an environmental friendly and energy-efficient with the potential to outperform conventional gas-compression refrigeration for everyday applications. In recent times, the large magnetic entropy change found in perovskite manganites suggests that these materials might be exploited for magnetic refrigeration applications. Apart from this, large MCE has also been observed in some manganite based multiferroics [5–7]. Although research on both these group of materials was initiated almost two decades back, there has been a renewed interest after the discovery of large MCE. In fact, the large MCE observed in these materials opened a new window for potential applications in magneto-electronic devices as well as in magnetic refrigeration mainly due to the fact that they are less expensive than others especially, compounds based on Gd. In view of this, efforts have been going on in this laboratory for the last few years to investigate the magnetocaloric behavior of some of manganites. For this purpose, a series of manganites
doped with various dopents and manganite based multiferroics have been chosen and investigated [8,9]. In the present investigation, the MCE studies on a material system with compositional formula, Tb1 xDyxMnO3 (x¼ 0.1–0.4) using specific heat results have been taken up and the results of such an investigation are presented in this paper.
2. Experimental Multiferroic materials with the compositional formula, Tb1 xDyxMnO3 (x¼0.1, 0.2, 0.3 and 0.4) were prepared by sol–gel method. In this method, highly pure (99.99% ) rare earth oxides (Tb4 O7 and Dy2 O3) and freshly prepared MnCO3 were taken in stoichiometric ratio. The precursor samples after calcining at 900 1C were sintered at 1350 1C for 4 h. More details about the sample preparation are given in an earlier publication [10]. The structural characterization of the samples was carried out by powder X-ray diffraction (XRD) technique using Bruker D8 Advanced diffractometer and the data were analyzed by Rietveld refinement using Fullprof software. The specific heat (Cp) measurements were performed by the semi adiabatic heat pulse method using indigenous set up [11] along with 8-Tesla Oxford superconducting magnet system in the presence of 0 T and 5 T fields.
3. Results and discussions n
Corresponding author. E-mail address:
[email protected] (P. Venugopal Reddy).
http://dx.doi.org/10.1016/j.matlet.2014.06.012 0167-577X/& 2014 Elsevier B.V. All rights reserved.
The structural investigations of the samples were carried out by XRD studies and powder diffraction patterns are shown in Fig. 1(a).
Intensity (a.u.)
N. Pavan Kumar, P. Venugopal Reddy / Materials Letters 132 (2014) 82–85
technique assuming orthorhombic structure with Pnma space group. The pseudo-voigt function was used to fit the peak profiles. The experimentally observed and calculated Rietveld refined patterns for the sample x ¼0.2 along with difference in both the patterns are shown in Fig. 1(b) and the refined crystallographic data are given in Table 1. One can see in Fig. 1(c) that both the lattice parameters viz., a and c are found to decrease continuously with increasing dopent concentration. In fact, the linear variation of both the lattice parameters with varying dopent concentration is in conformity with the Vegard's law. Fig. 2 shows the specific heat (Cp) data over a temperature range, 3–60 K for both 0 and 5 T. It can be seen from the figure that all the samples are found to exhibit a peak in the temperature
x=0.4 x=0.3 x=0.2
x=0.1 Iobs Ical Iobs- Ical
Intensity (a.u.)
83
Tb0.8Dy0.2MnO3
20
30
40
50
60
70
2 Theta (deg)
5.301
5.298 5.8455
5.295
5.8446
Cp ( J/mol-K )
5.8464
Lattice parameter c (A°)
Lattice parameter a (A°)
5.8473
40 30 20 10 0 40
0.3
0.4
x (Concentration) Fig. 1. (a) XRD patterns of Tb1 xDyxMnO3 manganites. (b) The best fit from the Rietveld refinement for Tb0.8Dy0.2MnO3 sample. (c) Shows the variation of lattice parameter ‘a and c’ with composition. Table 1 Crystallographic data and MCE valuesresults (for 5 T magnetic field change) of Tb1 xDyxMnO3 multiferroics. Sample (x)
0.1
0.2
0.3
0.4
a (Å) b (Å) c (Å) V (Å)3 RP RWP REXP S (goodness of fit) ΔSmax (J/mol/K) ΔTmax (K) RCP
5.846 7.4086 5.3 229.5 11.12 14.84 11.01 1.34 0.9 2.1 9.9
5.845 7.407 5.299 229.414 10.5 14.67 11.38 1.28 0.8 1.77 8.9
5.8439 7.401 5.295 229.012 11.42 14.97 12 1.24 0.91 2 9.6
5.8436 7.399 5.293 228.85 11.09 14.35 11.02 1.3 0.83 1.8 10
30 20 10 0
Cp ( J/mol-K )
0.2
30 20 10 0 40
Cp ( J/mol-K )
5.292 0.1
Cp ( J/mol-K )
5.8437
30 20 10 0 10
20
30
40
50
T(K)
It is clear from the XRD data that all the samples are having single phase, without any detectable impurity. It is also observed that the peaks are found to shift towards high 2θ side with increasing x, indicating the contraction of lattice with increasing x. This variation is reasonable because of slightly smaller ionic radius of Dy3 þ than Tb3 þ . The XRD data were analyzed using Rietveld refinement
Fig. 2. (a) Schematic diagram of temperature dependence of Tb0.9Dy0.1MnO3 on specific heat at 0 T magnetic field. The color regions indicating the different transition regions, exhibited by the sample. The black arrows represent Mn spins and red arrows represents R3 þ moments. (b)–(e) Variation of specific heat (Cp) with temperature of Tb1 xDyxMnO3 manganites under 0 T and 5 T magnetic fields. Inset shows the total Entropy as a function of temperature under 0 T and 5 T magnetic fields. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
N. Pavan Kumar, P. Venugopal Reddy / Materials Letters 132 (2014) 82–85
2.0
1.6
1.6
Tb0.9Dy0.1MnO3
1.2
1.2
0.8
0.8
0.4
0.4
0.0
0.0 5
10
15
20
1.6 Tb0.8Dy0.2MnO3
1.2
1.2
0.8
0.8
0.4
0.4
0.0
25
0.0 5
10
T(K)
15
ΔS ΔT
2.0
1.2
ΔT ( K )
Tb0.7Dy0.3MnO3
−ΔS ( J/mol K)
−ΔS ( J/mol K)
1.6 1.6
0.8
0.4
0.4
0.4
0.0
0.0
20
25
1.6
0.8
0.4
15
Tb0.6Dy0.4MnO3
1.2
0.8
0.0
2.0
1.2
0.8
10
25
ΔS ΔT
2.0
1.6
5
20
T(K)
2.0
1.2
2.0
ΔS ΔT
ΔT ( K )
1.6
2.0
ΔT ( K )
2.0
−ΔS ( J/mol K)
2.4
ΔS ΔT
ΔT ( K )
2.4
−ΔS ( J/mol K)
84
0.0 5
T(K)
10
15
20
25
T(K)
Fig. 3. Entropy change (ΔS) and adiabatic temperature change (ΔT) as a function of temperature.
range, 40–41 K and is attributed to the transition from antiferromagnetic phase into the sinusoidal incommensurate phase. In addition to this transition, all the samples are also found to exhibit another transition, known as the lock—in- transition, (Tlock) in the temperature range, 20–29 K and the behavior is in conformity with those of reported ones [12–14]. Finally, all the samples are found to exhibit another transition, below 10 K and is attributed to the ordering of rare earth ion spins. It is interesting to note that both these transitions are found to decrease with increasing Dy content. The total mechanism of the transitions is illustrated in a schematic diagram as shown in Fig. 1(a). Although, with increasing field no observable change in the specific heat behavior has been observed in the case of first transition, the second and third transitions, however, are found to shift towards lower temperature side. Apart from this, third transition peak is found to broaden under 5 T magnetic field, and the behavior indicating the ferromagnetic like ordering of rare earth ions [15]. In order to calculate magnetocaloric effect (MCE) values, the following procedure has been used. In this method, first the total entropy of the manganite, S(T,H) can be calculated using the experimental values of heat capacity C(T,H) using the well- known relation, Z SðT; HÞ ¼ ðCp=TÞdT ð1Þ
The variation of total entropy with temperature for all the samples is shown in the inset of Fig. 2(a)–(d). From the calculated total entropy S(0, T) and S(H, T), the isothermal magnetic entropy change ΔSM (H, T) has been calculated at a given temperature, T ΔSðH; TÞ ¼ SðH; TÞ–Sð0; TÞ
ð2Þ
Later, the adiabatic temperature change caused by the magnetic field change, i.e. magnetocaloric effect, was obtained
by an equation, ΔTðH; TÞ ¼ TðS; HÞ–TðS; 0Þ
ð3Þ
where T(S,H) and T(S, 0) are the temperatures in the field H and H¼ 0 at constant total entropy. The variation of entropy change (ΔS) with temperature at 5 T magnetic field is shown in Fig. 3(a)–(d). The ΔSmax, values are around 1 J/mol K for all the samples. Similarly, the adiabatic temperature difference values obtained from the isentropic curves using the specific heat data are also shown in Fig. 3(a)–(d). It can be seen from the figure that for a 5 T field change, ΔTad values of all the samples are found to be around 2 K. Finally, the Relative cooling power (RCP), represents the amount of heat transferred between cold and hot sinks in one ideal refrigeration cycle, and basically represents the cooling efficiency of a given magnetocaloric material. It is defined as the product of ΔSmax and the full width at half maximum δTFWHM of ΔSmax curve i.e., RCP ¼ ΔSmax δT FWHM
Therefore, the RCP values of all the samples have also been evaluated and are included in Table 1 along with ΔSmax, and ΔT. It is observed that ΔSmax and ΔT are not varying linearly with the doping and the non-monotonic variation could be due to error arising from the residual entropy difference below temperature range of measurement or may be due to error arising in heat capacity due to larger time constant of the samples. Further, ΔSmax and ΔT values of these samples when compared with those of manganites exhibiting CMR behavior are higher [16], indicating that the samples of the present investigation are better candidates for magnetic refrigeration applications. In fact, both these values are comparable with those of manganite based multiferroics [7–9]. Therefore, the magnetocaloric nature of these materials satisfies some important criteria for magnetic refrigerants such as large magnetic entropy change, large adiabatic temperature change and high RCP values.
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4. Conclusions
References
Polycrystalline Tb1 xDyxMnO3 orthorhombic multiferroic samples were prepared by the sol–gel method. A systematic investigation of specific heat studies over a temperature range 3–60 K under magnetic field of 5 T, indicate that all the samples exhibit transitions in three different temperature regions, namely 40 K, 20–26 K and below 10 K, and are attributed to the sinusoidal ordering of Mn moments into incommensurate phase, the lock-in transition and the ordering of the rare-earth moments respectively. It has been concluded that as these materials exhibit a large magnetic entropy change, adiabatic temperature change and RCP values, they may be exploited for possible magnetic refrigerant applications at low temperatures.
[1] Gschneidner Jr K A, Pecharsky V K, Tsokol A O. Rep Prog Phys 2005;68:1479. [2] Pecharsky A O, Gschneidner Jr K A, Pecharsky V K. J Appl Phys 2003;93:4722. [3] Hu F X, Qian X L, Sun J R, Wang G J, Zhang X X, Cheng Z H, et al. J Appl Phys 2002;92:3620. ̈ ck E, Zhang L, Dagula, Buschow KHJ, de Boer FR. Physica B [4] Tegus O, Bru 2002;319:174. [5] Midya A, Mandal P, Das S, Banerjee S, Sharath Chandra LS, Ganesan V, et al. Appl Phys Lett 2010;96:142514. [6] Midya A, Das S N, Mandal P. Phys Rev B: Condens Matter 2011;84:235127. [7] Pavan Kumar N, Venugopal Reddy P. Mater Lett 2014;122:292. [8] Shao Mingjie, Cao Shixun, Yuan Shujuan, Shang Jin, Kang Baojuan, Lu Bo, et al. Appl Phys Lett 2012;100:222404. [9] Wolff Fabris F, Pekala M, Drozd V, Fagnard J F, Vanderbemden Ph, Liu Ru-Shi, et al. J Appl Phys 2007;101:103904. [10] Pavan Kumar N, Lalitha G, Venugopal Reddy P. Phys Scr 2011;83:045701. [11] Rawat R, Das I. Phys Rev B: Condens Matter 2011;64:052407. [12] Harikrishnan S, Robler S, Naveen Kumar C M, Bhat H L, Robler U K, Wirth S, et al. J Phys: Condens Matter 2009;2:1096002. [13] Kimura T, Ishihara S, Shintani H, Arima T, Takahashi K T, Ishizaka K, et al. Phys Rev B: Condens Matter 2003;68:060403. [14] Kajimoto R, Yoshizawa H, Shintani H, Kimura T, Tokura Y. Phys Rev B: Condens Matter 2004;70:012401. [15] Rawat R, Das I. J Phys Condens Matter 2006;18:1051. [16] Tishin A M, Spichkin Y I The magnetocaloric effect and its applications 1st ed. 2003.
Acknowledgements The first author thanks DST and CSIR for providing fellowship during the period of the present work. UGC-DAE Consortium for Scientific Research, Indore is acknowledged for providing experimental facilities and financial support in the form of local hospitality and travel allowance. Authors thank Dr. Mukul Gupta for providing X-ray Diffraction facility. Pallab Bag and R Rawat are thanked for heat capacity measurements and discussion.
