IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 2502504 Magnetic Properties and Magnetocaloric Effect in Pb-Doped La0.9Dy0.1MnO3 Manganites T A Ho1 , Tran Dang Thanh1,2, P D Thang3, Jong Suk Lee4 , T L Phan1, and Seong Cho Yu1 Department of Physics, Chungbuk National University, Cheongju 361-763, Korea of Materials Science, Vietnam Academy of Science and Technology, 18-Hoang Quoc Viet, Hanoi, Vietnam Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi, Vietnam Department of Precision Mechanical Engineering, Gangneung-Wonju National University, Gangwon 220-711, Korea Institute We have studied the magnetic properties and magnetocaloric effect of rhombohedral (La0.9 Dy0.1 )1-x Pb x MnO3 (x = 0.1, 0.2, and 0.3) fabricated by solid-state reaction Thermomagnetization curve reveals that the increase of Pb doping in (La0.9 Dy0.1 )1-x Pb x MnO3 shifts the ferromagnetic (FM)–paramagnetic (PM) phase-transition temperature (T C ) toward room temperature; the T C values determined for x = 0.1, 0.2, and 0.3 are about 172, 249, and 322 K, respectively Based on magnetic-field dependencies of magnetization, M(H), the magnetic entropy changes ( S m ) of the samples were calculated Under an applied magnetic field of H = 10 kOe, the | Smax | value slightly increases from 0.74 J/kg · K for x = 0.1 to about 1.1 J/kg · K for x = 0.2 and 0.3 These results correspond to a relative cooling power of ∼50 J/kg, which is comparable with some perovskite manganites, indicating the applicability of (La0.9 Dy0.1 )1-x Pb x MnO3 in magnetic refrigeration devices working around room temperature Particularly, we find that the variation of | Smax | versus the magnetic field H can be described by a power law | Smax | ∝ H n , where the magnetic-ordering parameter (n) decreases from 0.861 for x = 0.1–0.834 for x = 0.3 A deviation of the n values estimated from the mean-field theory (n = 2/3) demonstrates an existence of short-range FM order in the samples Index Terms— Magnetic properties, magnetocaloric (MC) effect, perovskite manganites I I NTRODUCTION T HE crystal structure and magnetic properties of perovskite manganites were first reported in 1950 [1] These materials exhibit not only magnetoresistance (MR), magnetic phase transition, and transport properties [2], [3], but also the magnetocaloric (MC) effect [4]–[6] Recently, doped lanthanum manganites with a generalized chemical formula La1-x A x MnO3 ( A = Ca, Ba, Sr, Pb, etc.) have attracted much interest due to a number of unique features, such as colossal MR and MC effects that are controllable toward room temperature [7], [8] The richness in physical properties of these materials is generated from the interplay of charge, spin, orbital, and lattice degrees of freedom of 3d electrons [9], [10] For the parent compound of lanthanum manganite LaMnO3, it is known as an antiferromagnetic insulator at room temperature, in which Mn ions are trivalent (Mn3+ ) A partial substitution of La3+ ions by a divalent ion (for example: Ca, Ba, Sr, Pb, etc.) changes electrical conductivity, and leads to strong ferromagnetic interaction and notable physical effects Basically, the explanation of these properties is based on the double-exchange (DE) mechanism, where electrons’ exchange interactions between Mn3+ and Mn4+ ions play an important role DE interactions result from the localized t2g spins and itinerant eg electrons To fully understand the various phenomena (such as phase separation and magnetotransport properties) Manuscript received November 22, 2013; accepted January 14, 2014 Date of current version June 6, 2014 Corresponding author: S C Yu (e-mail: scyu@chungbuk.ac.kr) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2014.2301842 taking place near the ferromagnetic–paramagnetic (FM–PM) phase-transition temperature, a strong electron–phonon coupling (known as the Jahn–Teller effect [11]) also needs to be considered The FM interaction between Mn3+ and Mn4+ ions in manganites depends on the bond length Mn–O , and the bond angle Mn–O–Mn It has also been found that the colossal MR and MC effects are tightly related to the cation type located at the A site in the perovskite-type structure of ABO3 [12], the mean A-cation radius ( R A ), and the size variance (σ ) caused by the difference in the A-cation radii [13] Hwang et al [14] observed that the R A increase is associated with a decrease in the distortion, and thus the DE interaction is facilitated Concerning our work, Chen et al [15] reported a large MC effect in (La1-x Rx )2/3 Ca1/3 MnO3 (R = Gd, Dy, Tb, and Ce) materials They concluded that all the samples doped with a rare-earth metal with x = 0.1 offer the largest MC effect For example, polycrystalline (La0.9 Dy0.1 )2/3 Ca1/3 MnO3 has a maximum magnetic entropy change (| Smax |) of ∼6.06 J/kg · K for an applied field of H = 15 kOe However, its TC value of 176 K is too far from room temperature for some applications In this paper, we present the influence of the Pb substitution for La/Dy on the magnetic properties and MC effect of polycrystalline (La0.9 Dy0.1 )1-x Pbx MnO3 ceramics We point out that an increase in Pb-doping concentration leads to a shift of the TC toward room temperature Under the field of H = 10 kOe, the | Smax | value achieved around TC slightly increases from 0.74 J/kg · K for x = 0.1–1.1 and 1.06 J/kg · K for x = 0.2 and 0.3, respectively Their relative cooling power (RCP) is accordingly ∼50 J/kg, and comparable with that of some MC manganites 0018-9464 © 2014 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information 2502504 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 TABLE I VALUES OF E XPERIMENTAL PARAMETERS , S UCH AS THE C URIE T EMPERATURE (TC ), THE M EAN R ADIUS OF C ATIONS L OCATED AT THE A S ITE ( R A ), THE T OLERANCE FACTOR ( T ), AND VARIANCE OF THE A-S ITE C ATION R ADIUS THE D ISTRIBUTION (σ = r 2A − r A ) Fig (a) M(T ) curves with H = 100 Oe (b) Magnetic hysteresis loops recorded at 100 K for (La0.9 Dy0.1 )1-x Pbx MnO3 Inset: their dM/dT versus T curves II E XPERIMENTAL D ETAILS Three ceramic compounds of (La0.9 Dy0.1 )1-x Pbx MnO3 with x = 0.1, 0.2, and 0.3 were prepared by conventional solidstate reaction, used high-purity powders La2 O3 , PbO2 , Dy2 O3 , and Mn (3N) as precursors These powders combined with stoichiometric quantities were well mixed, and calcined at 1000 °C for 24 h in air Several times of intermediate grinding and calcining processes with the same annealing conditions were carried out These mixtures were then pressed into pellets, and sintered in air at 1300 °C for 48 h The crystal structure checked by an X-ray diffractometer (Brucker AXS, D8 Discover) revealed the final products exhibiting a single phase in a rhombohedral structure (space group R-3c) Magnetic measurements versus temperature (with temperature increments of K) and the magnetic field (in the range 0–10 kOe) were performed on a vibrating sample magnetometer III R ESULTS AND D ISCUSSION Fig shows temperature dependencies of zero-field-cooled magnetization, M(T ), for three (La0.9 Dy0.1 )1-x Pbx MnO3 samples under an applied field of H = 100 Oe With increasing temperature T , the results indicate that magnetization M values decrease rapidly as T increases above 150, 250, and 310 K for x = 0.1, 0.2, and 0.3, respectively This is related to the FM–PM phase transition (taking place at the Curie temperature, TC ), where magnetic moments are disordered by thermal energy By performing dM/dT versus T curves (the inset of Fig 1), TC values obtained from the minima of the curves are ∼172, 249, and 322 K for x = 0.1, 0.2, and 0.3, respectively Clearly, the TC of (La0.9 Dy0.1 )1-x Pbx MnO3 compounds increases if Pb concentration (x) is increased However, M values slightly decrease with the addition of Pb, as can be observed in Fig Such the results are attributed to crystallographic distortion induced by the substitution of Pb2+ with the ionic radius (1.19 Å) larger than ionic radii of La3+ (1.032 Å) and Dy3+ (0.912 Å) In other words, the Pb2+ substitution for La3+ /Dy3+ leads to an expansion of the lattice structure, which is characteristic of the structural parameters, see Table I, such √ as: 1) R A ; 2) the tolerance factor t = ( R A + R O )/ 2( R B + R O ), where R A and R B are the average radii of cations located at the A and B sites, respectively, and R O is the radius of oxygen anion; and 3) the variance of the A-site cation radius distribution defined by σ = r A2 − r A To further understand the magnetic properties and phase transition of (La0.9 Dy0.1 )1-x Pbx MnO3 compounds, their magnetic hysteresis loops at T = 100 K, see Fig 1(b), and field dependencies of magnetization, M(H ), at various temperatures around TC [see Fig 2(a)–(c)] were investigated The coercivity, Hc ≈ 290 Oe, of these samples of x = 0.1–0.3 indicates their soft magnetic behavior For the M(H ) data shown in Fig 2(a)–(c), one can observe clearly that nonlinear M(H ) curves at low temperatures become linear as T > TC , due to the FM–PM phase transition [16] Similar to the M(T ) data, at a given magnetic field, magnetization gradually decreases with increasing temperature This magnetic-phase separation can be observed more easily if performing Arrott plots of M versus H /M (denoted as M –H /M) [17], as shown in Fig 2(d)–(f) In the low-field region, the nonlinear M –H /M parts at temperatures below and above TC are driven toward two opposite directions Around TC , they are nearly linear and pass through the original coordinate, revealing the FM–PM separation Fundamentally, if it is a system with long-range magnetic order, M –H /M curves in the vicinity of TC are parallel straight lines The straight line at the critical point TC passes through the origin However, such criteria at low magnetic fields are absent from the performance shown in Fig 2(d)–(f) This reflects an existence of short-range FM order in the samples Notably, slopes of the H /M versus M curves, reversed axes in Fig 2(d)–(f), not shown, are positive This demonstrates HO et al.: MAGNETIC PROPERTIES AND MC EFFECT 2502504 Fig (a)–(c) M(H ) curves (d)–(f) Arrott plots of M versus H /M for (La0.9 Dy0.1 )1-x Pbx MnO3 compounds with x = 0.1, 0.2, and 0.3 that the magnetic-phase transition in (La0.9 Dy0.1 )1-x Pbx MnO3 compounds belong to the second-order type, according to Banerjee’s criteria [18] From the M(H ) data of the samples, we can assess their MC effect based on the magnetic entropy change ( Sm ) [6] For a ferromagnet undergoing a second-order magnetic phase transition, Sm can be calculated from the following relation [15], [19]: Hmax | Sm (T, H )| = ∂M ∂T dH (1) H where Hmax is the maximum applied field According to this equation, Sm depends on the temperature gradient of magnetization, and thus reaches a maximum value around TC at which magnetic moments become disordered Fig shows the temperature dependencies of − Sm , for the samples under applied field intervals ranging from to 10 kOe For each applied field, the − Sm (T ) curve of each sample exhibits a maximum value (| Smax |) at TC Under an applied field interval of H = 10 kOe, | Smax | values are ∼0.74, 1.1, and 1.06 J/kg · K for x = 0.1, 0.2, and 0.3, respectively Clearly, the Pb doping is not only to shift TC toward room temperature, but also to increase | Sm | Particularly, the peak position of − Sm (T ) curves is not shifted by the field change because of the second-order nature of the FM–PM transition If taking account for the full-width at half maximum of the − Sm (T ) Fig − Sm (T ) curves with applied magnetic-field intervals ranging from to 10 kOe for (La0.9 Dy0.1 )1-x Pbx MnO3 with x = (a) 0.1, (b) 0.2, and (c) 0.3 curve (denoted as δTFWHM ), the RCP can be calculated from the relation RCP = | Smax | × δTFWHM [20] Under an applied field of 10 kOe, δTFWHM values are ∼70, 45, and 40 K, and RCP values are ∼52, 50, and 43 J/kg for x = 0.1, 0.2, and 0.3, respectively These RCP values are comparable with those determined from some perovskite manganites [6], [21], [22], indicating the applicability of (La0.9 Dy0.1 )1-x Pbx MnO3 (with x ≥ 0.2) in magnetic refrigeration systems operating at room temperature Theoretically, magnetic-field dependencies of | Smax | in the second-order magnetic transition obey a power law | Smax | = α H n , where α is a coefficient and n is the field exponent and known as magnetic-ordering parameter [23] We found that the | Smax (H )| data of our samples can be explained by this power law, as shown in Fig 4, for two typical samples with x = 0.1 and 0.2 It has been suggest that n is the field invariant at TC In this paper, n values for the samples with x = 0.1, 0.2, and 0.3 are 0.861, 0.848, and 0.834, respectively These values are very different from the value of the mean-field theory (n = 2/3) for long-range FM order We believe that the existence of short-range FM order associated with magnetic inhomogeneities in our samples caused 2502504 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 Fig Magnetic-field dependencies of | Smax | at TC for two typical samples with x = 0.1 and 0.2 are fitted to the power law of | Smax | = α H n The data of the sample x = 0.3 are quite similar to those of x = 0.2, not shown here such deviation of the n values This is in good agreement with the above discussion for the M(H ) data IV C ONCLUSION Magnetic properties and MC effect of (La0.9 Dy0.1 )1-x Pbx MnO3 (x = 0.1–0.3) were investigated in detail The experimental result revealed that the ability of controlling the TC point from 172 K toward the room temperature region by increasing Pb-doping concentration from 0.1 to 0.3 This gradually enhances the | Smax | values around TC , and still keeps RCP values (of ∼50 J/kg) quite stable Such features indicate a potential application of (La0.9 Dy0.1 )1-x Pbx MnO3 compounds with x ≥ 0.2 in magnetic refrigeration around room temperature We also found magnetic-field dependencies of | Smax | at TC obey the power law of | Smax | = α H n , with the field exponent n = 0.861, 0.848, and 0.834 for x = 0.1, 0.2, and 0.3, respectively The deviation of our n values from the value expected for the mean-field theory proves the existence of short-range FM order in the samples ACKNOWLEDGMENT This research was supported by the Converging Research Center program through the Ministry of Science, ICT and Future Planning, Korea (2013K000405) R EFERENCES [1] G H Jonker and J H Van Santen, “Ferromagnetic compounds of manganese with perovskite structure,” Physica, vol 16, no 3, pp 337–349, 1950 [2] K I Chahara, T Ohno, M Kasai, Y Kanke, and Y Kozono, “Magnetoresistance effect of La0.72 Ca0.25 MnOz /YBa2 Cu3 O y 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