Influence of sr doping on magnetic and magnetocaloric properties of Nd₀,₆Sr₀,₄MnO₃

VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 20-27 20 Original Article  Influence of Sr Doping on Magnetic and Magnetocaloric Properties of Nd0.6Sr0.4MnO3 Ho Thi Anh, Nguyen Ngoc Huyen, Pham Duc Thang* Faculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam Received 11 October 2019 Revised 02 December 2019; Accepted 30 December 2019 Abstract: Nd0.6Sr0.4MnO3 sample was fabricated by a solid-state reaction method and its magnetic, magnetocaloric properties were investigated. The Curie temperature, TC, at which a ferromagnetic- paramagnetic transition occurred was found to be of about 270 K. An analysis using the Banejee’s criterion of the experiment results for magnetization as a function of temperature and magnetic field and the universal curves of the normalized entropy change versus reduced temperature indicated that the sample undergo the second-order magnetic phase transition. Furthermore, the maximum magnetic entropy change that occurred near TC, measured at a magnetic field span of 50 kOe was found to be of about 6.0 J/kg.K, corresponding to a relative cooling power of 250 J/kg. These values are comparable to those of other manganites. Keywords: Perovskite manganites; Magnetocaloric effect; Universal entropy 1. Introduction Hole-doped perovskite manganites with a chemical formula of R1-xAxMnO3 (R = La, Nd, Pr; A = Ca, Ba, Sr) have received a lot of attention due to their intriguing physical properties, and their applicability in the magnetic refrigeration technology based upon the magnetocaloric effect (MCE) [1- 4]. This effect is associated with the temperature change of the suitable magnetic material under an applied magnetic field. Particularly, if the magnetic field is applied adiabatically, the temperature of the materials increases, and if the magnetic field is removed, the temperature decreases. Additionally, this effect is often expected to get maximum value at magnetic phase transition. It is well known that the ________ Corresponding author. Email address: thangducpham@yahoo.com https//doi.org/ 10.25073/2588-1124/vnumap.4403 H.T. Anh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 20-27 21 properties of perovskite manganites is directly related to ferromagnetic (FM) or anti-ferromagnetic (AFM) ordering, charge ordering and orbital ordering [5-7]. The strength of FM and AFM interactions is associated with the double-exchange pair of Mn3+ - Mn4+ and super-exchange pair of Mn3+ - Mn3+ and Mn4+ - Mn4+, respectively, depends on Mn3+ and Mn4+ concentrations and the structure parameters. The FM interaction becomes strongest when the concentration ratio of Mn3+/Mn4+ is about 7/3, corresponding to an A-doping content x ≈ 0.3. Accordingly, the giant MCE and colossal magnetoresistance effects are usually obtained in the region 0.2 ≤ x≤ 0.4 for most R1-xAxMnO3 systems [8]. The strength of double-exchange interactions in Nd-based manganites is usually weaker in comparation with La-based manganites, their magnetic properites are more interesting and complicated due to a larger lattice distortion would happen for smaller Nd ion. It is well known that the parent compound NdMnO3 is an insulating antiferromagnet with TN ≈ 78 K [9] , with a small partial substitution 0<x< 0.1 of Sr for Mn, the system becomes ferromagnetic but still insulating [1] while a ferromagnetic metallic phase appears in the region 0.2≤ x ≤ 0.4. Venkatesh et. al [10] showed that a tricritical point was observed in the Nd1-xSrxMnO3 (x = 0.3, 0.33, and 0.4) at x = 0.33 which separates the first-order transition in compound with x = 0.3 and second-order transition in compound with x = 0.4. Additionally, the complicated behavior at low fields in which the order of the transition could not be fixed and a second-order-like behavior at high fields was observed in Nd0.7Sr0.3MnO3[11]. The complex ferromagnetic state in this compound due to a competition between the double exchange mechanism and correlations arising from coupled spin and lattice degrees of freedom. The MCE in this compound was also studied in two ways indirectly by estimating the isothermal change in entropy, ΔSm, and directly by measuring the field indiced adiabatic temperature change. However, up to now, the effect of Sr doping on magnetic and MC properties in Nd0.6Sr0.4MnO3 have been studied not much. To get more insight into these problems, we prepared Nd0.6Sr0.4MnO3 samples and carried out the magnetization measurements versus temperature and magnetic field. By combining the Banejee’s criterion and the universal curves of the normalized entropy change versus reduced temperature, we assessed the magnetic order existing in this compound. We also studied the MCE by estimating ΔSm from the magnetization data in different applied magnetic field. 2. Experimental details Polycrystalline Nd0.6Sr0.4MnO3 was prepared by a conventional solid-state reaction. High-purity (99.9%) powdered precursors of Nd2O3, SrCO3 and MnO2 with a specific composition, were well mixed, carefully ground, and then annealed at 900oC in air for 12h. The annealed samples were re-ground, pressed into pellets and calcinated at 1100oC in air for 24h. This process was done for serval times and finally the pellets were sintered in air at 1300oC for 10h. The single phase in an orthorhombic structure of the final sample was checked by an X-ray diffractometer using Cu-Kα radiation over the 2Ɵ range of 20 ÷ 70o. The dependence of magnetization on temperature and applied magnetic field was measured by using a superconducting quantum interference device up to 50 kOe in the temperature range of 5 ÷ 300 K. 3. Results and discussion Figure 1(a) shows temperature dependences of zero-field-cooled (ZFC) and field-cooled (FC) magnetization, MZFC/FC(T), at such a small H as 100 Oe for Nd0.6Sr0.4MnO3 sample. For the MZFC(T) curve, M increases with the increase of T, and reaches a maximum at 250 K. This phenomenon is explained due to the existence of FM/AFM clusters and/or magnetic inhomogeneity [1]. Munoz et al. [9] have indicated the coexistence of FM and AFM interactions in polycrystalline NdMnO3. H.T. Anh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 20-27 22 Additionally, with further increasing of temperature, above 250 K, the magnetization was rapidly decreased due to the FM-PM phase transition, where magnetic moments became disorder under the impact of thermal energy. The Curie temperature, TC, determined from the minima of the first derivative of MZFC(T)/dT versus T curve (not shown here) is about 270 K. Similar behavior of MZFC(T) and TC values were found in the previous works [1, 10, 12]. Additionally, it is notable that the large difference between MZFC(T) and MFC(T) was observed. In figure 1(b), this difference, ΔM, is plotted with respect to T for this sample at H = 100 Oe. It is well known that ΔM is small and independent on temperature for good ferromagnets. The large ΔM implies a FM order and a rather high coercivity comparing to the applied field below TC. When applied field is smaller than the magnetic anisotropy which locked the magnetic moment of the Mn ions in the random direction, ΔM is becoms larger. Moreover, the magnetic anisotropy was strong at temperature below TC, ΔM increased as T decreased. This behavior is clearly seen in Fig. 1(b). This result indicates that our sample was highly anisotropic and the applied field of 100 Oe was not strong enough to align the magnetic moments, resulting in a larger ΔM at low T. Figure 1. (a) MZFC(T) and MFC(T) curves for Nd0.6Sr0.4MnO3 measured at H = 100 Oe. (b) MFC – MZFC with respect to T for Nd0.6Sr0.4MnO3. To understand the Sr-doping influence on magnetic and magnetocaloric properties of Nd0.6Sr0.4MnO3, series of isothermal magnetization curves, M(H), were recorded at temperatures around TC in the interval of 2K. Figure 2(a) shows the typical curves of M(H) data for this sample. Clearly, like M(T) data, at a given H, M decreased with increasing temperature. In the FM-PM phase transition region, M increased with H increasing up to a magnetic field as large as 50 kOe. This probably is due to the coexistence of the FM and PM phase and magnetic inhomogeneities. This behavior was also observed for other manganite compounds [1, 13-16]. As temperature increased, the nonlinear M(H) curves in FM region become linear that is the characteristic property of the PM state. The nature of FM-PM phase transition can be clearly observed in the Arrott plots [17], the M2 dependence of H/M curves are shown in Fig. 2(b). At high H values, the plots near TC exhibit series of parallel straight lines, the straight line at T = TC did not pass through the origin of ordinates. According to the mean-field theory applied for the long-range FM order, the existence of short-range FM order in our sample can be suggested. 0 50 100 150 200 250 300 0 5 10 15 0 50 100 150 200 250 300 0 5 10 15 T (K) M ( e m u /g ) T (K) FC ZFC H = 100 Oe (a) (b)H = 100 Oe M F C - M Z F C ( e m u /g ) H.T. Anh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 20-27 23 In order to check the nature of magnetic transition we have used the Banerjee’s criterion [18]. By plotting M2 vs. H/M in the magnetic phase transition region, the slope of the resulting curves indicates whether a magnetic transition is of first or second order. From a thermodynamic point of view, one can deduce following, if some of the M2 vs. H/M curves near TC have negative slope, the magnetic transition in the sample is a first-order magnetic transition. Otherwise, the entire slope is positive, the magnetic transition is a second-order magnetic transition (SOMT). The result shown in Fig. 2(b) indicates that the slope of the Arrott-plot curves near TC is positive, this means that the existence of SOMT in our sample can be considered according to the Banerjee’s criterion. Figure 2. (a)–(b) M(H) curves and (c)–(d) Arrott plots (M2 vs. H/M) for Nd0.6Sr0.4MnO3. To evaluate the MCE in the sample, we assessed the magnetic entropy change ΔSm, which can be calculated by using Maxwell’s relation in the H range of 0 to Hmax [19] 𝑆𝑚(𝑇, 𝐻) = − ∫ 𝐻𝑚𝑎𝑥 0 ( 𝜕𝑀 𝜕𝑇 ) 𝐻 𝑑𝐻 (1) Obviously, the slope of ( 𝜕𝑀 𝜕𝑇 ) 𝐻 contribute the value of ΔSm, indicating that an abrupt magnetic transition leading to large ΔSm and this value reach maximum around TC where magnetization decays most rapidly[20]. For magnetization measurements with small, discrete field and temperature intervals, ΔSm could be numerically calculated using the following equation |𝑆𝑚| = ∑ 𝑀𝑖−𝑀𝑖+1 𝑇𝑖−𝑇𝑖+1 𝐻𝑖 (2) Where Mi and Mi+1 are magnetization values measured in a magnetic field H at temperature Ti and Ti+1, respectively. Figure 3(a) show the dependence of -ΔSm on temperature, -ΔSm(T), for different field variations at interval of 5 kOe. Clearly, at a specific temperature, -ΔSm increases with increasing ΔH from 5 kOe to 50 kOe. As expected from Eq (1), ΔSm was maximized, denoted as | ΔSmax|, near TC and increased with ΔH. Additionaly, |ΔSmax| point shifts gradually towards high temperature with increasing ΔH. The |ΔSmax| value measured at ΔH = 50 kOe was the largest which is 0 10 20 30 40 50 0 10 20 30 40 50 60 0 200 400 600 800 1000 1200 1400 0 1000 2000 3000 4000  T = 4 K 290K M ( e m u /g ) H (kOe) 250K (a) (b) T= 4K M 2 ( e m u /g )2 H/M (Oe.g/emu) 290K 250K H.T. Anh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 20-27 24 6.0 J/kg.K at T ≈ 270 K. The |ΔSmax| value measured at ΔH = 50 kOe for this sample is comparable to those of La0.7Ca0.2Ba0.1MnO3[15], La0.87Sr0.13MnO3[21], and La0.7Sr0.3Mn0.9Cu0.1O3 [22]. In addition to the assessment based on ΔSm, in order to evaluate the MCE, the other parameter could be considered that is relative cooling power (RCP). This parameter can be calculated according to the magnitude of ΔSm and its full width at half maximum, as RCP = |ΔSmax| × FWHM that is shown in [4]. Figure 3(b) shows the dependence of RCP on applied field H. Clearly, the RCP increased with H and under applied field of ΔH = 50 kOe, its value is 250 J/kg. This value is large than those of La0.67Sr0.33Mn0.9M0.1O3 with M = Cr, Sn, Ti [23] and La0.67Ba0.33MnO3 [24]. Figure 3. -ΔSm(T, H) and RCP curves at various ∆H values for Nd0.6Sr0.4MnO3. Within the framework of second order phase, the field dependence of the magnetic entropy change follows a power law of the filed ΔSm α Hn with an exponent, n, which depends on temperature and field. It can be locally calculated as n = 𝑑𝑙𝑛|∆𝑆𝑚| 𝑑𝑙𝑛𝐻 (3) For temperature well below TC, n = 1; well above TC, n = 2 and at TC, n = 2/3 [25, 26]. Figure 4(a) shows experimental data of n(T, H) which calculated from the – ΔSm(T, H). Its minimum n value at TC is changed from 0.8 (for ΔH = 5 kOe) to 0.55 (for ΔH = 50 kOe), which are much different from the mean-field-theory value n = 2/3. This can be due to the absence of long-range magnetic order and the existence of magnetic inhomogeneities in this sample. Recently, Franco et al.[26] have proposed a new criterion based on the entropy change curves to distinguish the order of magnetic transition by means of magnetic measurements. According to them, for ferromagnets undergoing SOMT, the universal curve consists in the collapse ΔSm(T) curves 240 250 260 270 280 290 300 0 2 4 6 0 10 20 30 40 50 0 50 100 150 200 250 50 kOe - S m (J /k g .K ) T (K) 5 kOe (a) (b) R C P (J /k g ) H (kOe) H.T. Anh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 20-27 25 measured with different maximum applied magnetic fields after a scaling process. Therefore, it is natural to predict a breakdown of the universal curve for ferromagnets undergoing FOMT. The universal curve could be done by normalize all the ΔSm(T) curves with their respective peak entropy change, ΔS’(T,Hmax) = ΔSm(T,Hmax)/ ΔSpeak(Hmax) and then rescaling the temperature axis defining a new variable Ɵ, θ = { −(𝑇 − 𝑇𝑐)/(𝑇𝑟1 − 𝑇𝑐) 𝑓𝑜𝑟 𝑇 ≤ 𝑇𝑐 (𝑇 − 𝑇𝑐)/(𝑇𝑟2 − 𝑇𝑐) 𝑓𝑜𝑟 𝑇 > 𝑇𝑐 (4) The two reference temperatures Tr1 and Tr2 satisfy Tr1<TC<Tr2. According to previous report, by using two reference temperatures, it can be avoided the effect of a minority magnetic phase and the demagnetization factor leading to the breakdown of the ΔSm curve collapse. These reference temperatures can be selected for each curve from temperature corresponding to ΔSm(Tr, Hmax)/ ΔSpeak(Hmax) = a (0<a<1). The choice of a does not affect the actual construction of the universal curve. In this study, we choose value of a is 0.5. Figure 4(b) shows the universal curve constructions for the sample by plotting ΔS’ versus Ɵ. Clearly, all the experimental points collapse onto the master curve, which is consistent with the characteristics of SOMT. Additionally, the Arrott plot constructions confirm that our sample undergo SOMT. This result demonstrates the usefulness of using the universal curve, besides Banerjee’s criterion, in assessing the order of magnetic phase transition of perovskite manganites. Figure 4. n(T, H) and -ΔS’ curves at various ∆H values for Nd0.6Sr0.4MnO3. 4. Conclusions The fabricated ceramic sample Nd0.6Sr0.4MnO3 was crystallized in an orthorhombic structure. Detailed analyses of M(T,H) and -ΔSm(T,H) data based on the Banerjee’s criterion and phenomenological method of constructing the universal entropy curve, demonstrated the existence of second-order magnetic phase transition in this sample. Additionally, we found large MC effect in this sample, particularly |ΔSmax| ≈ 6.0 J/kg.K and RCP ≈ 250 J/K at 270 K under an applied field of 50 kOe. Such features indicate a potential application of Nd0.6Sr0.4MnO3 in magnetic refrigeration close to room temperature. -3 -2 -1 0 1 2 3 4 5 6 0.0 0.2 0.4 0.6 0.8 1.0 240 250 260 270 280 290 300 0.5 1.0 1.5 2.0 (b)  S '  50 kOe n T (K) 5 kOe(a) H.T. Anh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 20-27 26 Acknowledgements This research is funded by Asia Research Center (ARC) under grant number CA.19.05A and is funded by the Domestic Master/PhD Scholarship Programme of Vingroup Innovation Foundation References [1] D.N.H. Nam, R. Mathieu, P. Nordblad, N.V. Khiem, N.X. Phuc, Ferromagnetism and frustration in Nd0.7Sr0.3MnO3, Phys. Rev. B 62(2000) 1027-1032. https://doi.org/10.1103/PhysRevB.62.1027. [2] X. Moya, L.E. Hueso, F. Maccherozzi, A.I. Tovstolytkin, D.I. Podyalovskii, C. Ducati, et al., Giant and reversible extrinsic magnetocaloric effects in La0.7Ca0.3MnO3 films due to strain, Nat. Mater. 2(2013) 52-58. https://doi.org/10.1038/nmat3463. [3] A. Dhahri, F.I.H. Rhouma, S. Mnefgui, J. Dhahri, E.K. Hlil, Room temperature critical behavior and magnetocaloric properties of La0.6Nd0.1(CaSr)0.3Mn0.9V0.1O3, Ceram. Int. 40(2014) 459-464. https://doi.org/10.1016/j.ceramint.2013.06.024. [4] M.-H. Phan, S.-C. Yu, Review of the magnetocaloric effect in manganite materials, J. Magn. Magn. Mater 308(2007) 325-340. https://doi.org/10.1016/j.jmmm.2006.07.025. [5] C. Zener, Interaction between the d-shells in the transition metals. II. ferromagnetic compounds of manganese with perovskite structure, Phys. Rev. B 82(1951) 403-408. https://doi.org/10.1103/PhysRev.82.403. [6] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Double exchange alone does not explain the resistivity of La1- xSrxMnO3, Phys. Rev. Lett. 74(1995) 5144-5147. https://doi.org/10.1103/PhysRevLett.74.5144. [7] M.B. Salamon, P. Lin, S.H. Chun, Colossal magnetoresistance is a griffiths singularity, Phys. Rev. Lett. 88 (2002) 197203-197206. https://doi.org/10.1103/PhysRevLett.88.197203. [8] L. Demkó, I. Kézsmárki, G. Mihály, N. Takeshita, Y. Tomioka, and Y. Tokura, Multicritical end point of the first- order ferromagnetic transition in colossal magnetoresistive manganites, Phys. Rev. Lett. 101(2008) 037206- 037209. https://doi.org/10.1103/PhysRevLett.101.037206. [9] A. Munoz, J.A. Alonso, M.J. Martinez-Lope, J.L. Garcia-Munoz, M.T. Fernandez-Diaz, Magnetic structure evolution of NdMnO3 derived from neutron diffraction data, J. Phys.: Condens. Matter 12 (2000) 1361-1368. https://doi.org/10.1088/0953-8984/12/7/319. [10] R. Venkatesh, M. Pattabiraman, K. Sethupathi, G. Rangarajan, S. Angappane, and J.-G. Park, Tricritical point and magnetocaloric effect of Nd1-xSrxMnO3, J. Appl. Phys. 103(2008) 07B319-07B322. https://doi.org/10.1063/1.2832412. [11] R. Venkatesh, M. Pattabiraman, S. Angappane, G. Rangarajan, K. Sethupathi, J. Karatha, Complex ferromagnetic state and magnetocaloric effect in single crystalline Nd0.7Sr0.3MnO3, Phys. Rev. B 75 (2007) 224415-224418. https://doi.org/10.1103/PhysRevB.75.224415. [12] R.S. Freitas, L. Ghivelder, F. Damay, F. Dias, L.F. Cohen, Magnetic relaxation phenomena and cluster glass properties of La0.7−xYxCa0.3MnO3 manganites, Phys. Rev. B 64 (2001) 144404-144407. https://doi.org/10.1103/PhysRevB.64.144404. [13] T.A. Ho, S.H. Lim, C.M. Kim, M.H. Jung, T.O. Ho, P.T. Tho, Magnetic and magnetocaloric properties of La0.6Ca0.4- xCexMnO3, J. Magn. Magn. Mater 438(2017) 52-59. https://doi.org/10.1016/j.jmmm.2017.04.038. [14] T.A. Ho, S.H. Lim, P.T. Tho, T.L. Phan, S.C. Yu, Magnetic and magnetocaloric properties of La0.7Ca0.3Mn1−xZnxO3, J. Magn. Magn. Mater 426(2017) 18-24. https://doi.org/10.1016/j.jmmm.2016.11.050. [15] T.-L. Phan, N.T. Dang, T.A. Ho, T.V. Manh, T.D. Thanh, C.U. Jung, First to second order magnetic phase transition La0.7Ca0.3-xBaxMnO3 exhibiting large magnetocaloric effect, J. Alloys Compd. 657(2016) 818-834. https://doi.org/10.1016/j.jallcom.2015.10.162. [16] T.A. Ho, D.-T. Quach, T.D. Thanh, T.O. Ho, M.H. Phan, T.L. Phan, Magnetocaloric effect and critical behavior in a disordered ferromagnet La0.7Sr0.3Mn0.9Ti0.1O3, IEEE Trans. Magn. 51(2015) 2501304-2501307. https://doi.org/10.1109/TMAG.2015.2436383. H.T. Anh et al. / VNU Journal of Science: Mathematics – Physics, Vol. 36, No. 1 (2020) 20-27 27 [17] A. Arrott., Criterion for ferromagnetism from observations of magnetic isotherms, Phys. Rev. 108(1957) 1394- 1397. https://doi.org/10.1103/PhysRev.108.1394. [18] S.K. Banerjee, On a generalised approach to first and second order magnetic transitions, Phys. Lett.12 (1964) 16- 22. https://doi.org/10.1016/0031-9163(64)91158-8. [19] V. Franco, J. S. Blázquez, A. Conde, Field dependence of the magnetocaloric effect in materials with a second order phase transition: A master curve for the magnetic entropy change, Appl. Phys. Lett. 89 (2006) 222512- 222517. https://doi.org/10.1063/1.2399361. [20] N.S. Bingham, M.H. Phan, H. Srikanth, T.M.A., C. Leighton, Magnetocaloric effect and refrigerant capacity in charge-ordered manganites, J. Appl. Phys. 106(2009) 023909-023914. https://doi.org/10.1063/1.3174396. [21] A. Szewczyk, H. Szymczak, A. Wisniewski, K. Piotrowski, R. Kartaszynski, B. Dabrowski, et al., Magnetocaloric effect in La1−xSrxMnO3 for x=0.13 and 0.16, Appl. Phys. Lett. 77(2000) 1026-1029. https://doi.org/10.1063/1.1288671. [22] M.H. Phan, H.X. Peng, S.C. Yu, N. Tho, N. Chau, Large magnetic entropy change in Cu-doped manganites, J. Magn. Magn. Mater. 285(2005) 199-203. https://doi.org/10.1016/j.jmmm.2004.07.041. [23] B. Arayedh, S.Kallel, N. Kallel, O.Pena, Influence of non-magnetic and magnetic ions on the magnetocaloric properties of La0.7Sr0.3Mn0.9M0.1O3, J. Magn. Magn. Mater 361(2014) 68-73. https://doi.org/10.1016/j.jmmm.2014.02.075. [24] D.T. Morelli, A.M. Mance, J.V. Mantese, A.L. Micheli, Magnetocaloric properties of doped lanthanum manganite films, J. Appl. Phys. 79 (1996) 373-376. https://doi.org/10.1063/1.360840. [25] V. Franco, A. Conde, Scaling laws for the magnetocaloric effect in second order phase transitions: From physics to applications for the characterization of materials, Int. J. Refri. 33(2010) 465-473. https://doi.org/10.1016/j.ijrefrig.2009.12.019. [26] V. Franco, J.S. Blazquez., A. Conde, Field dependence of the magnetocaloric effect in materials with a second order phase transition: A master curve for the magnetic entropy change, Appl. Phys. Lett. 89(2006) 222512-222515. https://doi.org/10.1063/1.2399361.