DSpace at VNU: Some properties of La-deficient La0.54Ca0.32MnO3-delta

Journal of Magnetism and Magnetic Materials 262 (2003) 502–507 Some properties of La-deficient La0.54Ca0.32MnO3Àd N.H Sinh*, N.P Thuy Cryogenics Laboratory, Faculty of Physics, College of Natural Science, Hanoi National University, 334 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet Nam Abstract A La-deficient sample of La0.54Ca0.32MnO3Àd was prepared by the solid-state reaction method The Curie temperature TC equals 300 K, which is significantly higher than those of the La1ÀxCaxMnO3Àd system The magneticentropy change reaches a maximum value of ÀDSMD5.5 J/kg K at the Curie temperature upon a T magnetic field variation A saturation magnetic moment sS ¼ 2:99 mB/f.u at K has been derived from the magnetization data Values of 0.0230 and 0.441 for the oxygen deficiency d and the ratio of Mn3+/Mn4+, respectively, have been determined From our study, it is suggested that this compound is a suitable candidate for application as a working substance in magnetic refrigeration r 2003 Elsevier Science B.V All rights reserved PACS: 75.30.Sg; 75.47.Lx Keywords: La-deficient La0.54Ca0.32MnO3Àd, Magnetic-entropy change; Oxidation; Ratio Mn3+/Mn4+; Saturation moment Introduction Without doping, LaMnO3 is an insulator at all temperatures The insulating nature of this parent compound as well as its anisotropic magnetic interaction is related to the structure, in particular to the Jahn–Teller (J–T) distortion around Mn3+ ions When this insulator is hole-doped, the Mn4+ ions decrease the cooperative J–T distortion The structure plays a crucial role in determining the electron transport and magnetic properties of this oxide [1] LaMnO3 with a small proportion of Mn4+ (p0.05) becomes antiferromagnetically ordered at low temperatures (TN E150 K) When *Corresponding author Tel.: +84-4-8585281; fax: +84-48584438 E-mail address: nhsinh@netnam.vn (N.H Sinh) La3+ in LaMnO3 is progressively replaced by a divalent cation, as in La1ÀxAxMnO3 (A=Ca, Sr, Ba), the proportion of Mn4+ increases and the orthorhombic distortion decreases The material becomes ferromagnetic with a well-defined Curie temperature at a finite x, and metallic below TC The saturation moment is typically 3.8 mB, which is close to the theoretical estimate based on localized spin-only moment This suggests that the conduction electrons are fully spin-polarized Recently, attention was focused on the magnetic-refrigeration possibilities of La–Ca–Mn–O compounds, because of the large magnetocaloric effect (MCE) in this system [2–5] Up to now, MCE has been extensively studied in other ferromagnetic substances Experimentally much attention has been paid to find refrigerants that have large magneticentropy change under a magnetic-field change, 0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved doi:10.1016/S0304-8853(03)00085-4 N.H Sinh, N.P Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507 specially, to those that can be used at room temperature Many studies have been concentrated on intermetallic compounds and alloys of rare earth, which provide a comparatively large magneticentropy change at the Curie temperature Among them, the perovskites La0.67Ca0.33MnO3 and La2/3Ca1/3MnO3 are the most attractive, because their TC and magnetic-entropy changes are 257 K and 4.37 J/kg K at 1.5 T and 267 K and 6.4 J/kg K at 1.5 T, respectively [6,7] However, it is still lower than room temperature Xu et al [8] have found TC to be 272 K and a magnetic-entropy change of 2.9 J/kg K at a field change of 0.9 T for La0.54Ca0.32MnO3 In this work, we report some properties of Ladeficient La0.54Ca0.32MnO3Àd, which have been obtained by measurements of X-ray powder diffraction, magnetization, magnetocaloric effect, susceptibility, oxygen deficiency (d), ratio of Mn3+/Mn4+ and SEM Sample preparation La0.54Ca0.32MnO3Àd sample was prepared by a conventional solid-state reaction method Stoichiometric compositions of La2O3, CaCO3 and MnO were mixed for h The mixed powders were dried at 200 C for h and pressed into pellets The pellets were first presintered at 1000 C for 20 h and then cooled down to room temperature by a turning off the furnace After that, the pellets were ground and Mastersize Microplus measured to collect particles smaller than 100 mm The powders were pelletized using a cold isostatic press A multi-step procedure is applied for the heat treatment of the sample First the sample is heated up to 1100 C and sintered for 24 h, then subsequently heated to 1250 C and sintered for further 15 h at this temperature The sintering procedure is stopped by lowering the sample temperature to 1150 C and kept at this level for 15 h A subsequent second annealing at 1050 C for 15 h is followed by the third annealing at 650 C for 24 h After this annealing, the sample was furnace-cooled by simply switching off the supply to the furnace The structure of the sample was 503 inspected by X-ray powder diffraction (XPD), using Cu Ka radiation at room temperature The magnetization curves (from to 300 K) were measured with a vibrating sample magnetometer Resistivity versus temperature curves were measured on cooling from 300 to 77 without an external magnetic field by the four-point probe technique The magnetocaloric effect measurement was performed in a pulse field SEM measurements were also carried out Results and discussions The XPD pattern shown in Fig reveals that the sample is of a single-phase orthorhombicperovskite structure without any impurity phase Lattice parameters that have been determined ( b ¼ 7:709 A ( from XPD pattern are a ¼ 5:446 A, ( and c ¼ 5:445 A which is identified with the Pnma structure in comparison with the crystal structure of the parent compound LaMnO3 ( b ¼ 5:742 A, ( c ¼ 7:728 A) ( So (with a ¼ 5:532 A, it is found that the crystal structure of La0.54Ca0.32MnO3Àd has been distorted by the La-deficiency Fig 2(a–c) shows the temperature dependence of the magnetization measured in fields of H ¼ 100 1000 and 10000 Oe, respectively, obtained under zero-field (ZFC) and field cooled (FC) conditions It is found that the magnetic moments of the sample are almost the same in the ZFC and FC curves at 1000 and 10000 Oe At 100 Oe, it shows only a very slight difference This suggests that the spin order does not strongly depend on external magnetic fields Furthermore, a clear anomaly at 50 K is seen This may be related to a crystal structure phase transition, which must be further elaborated The Curie temperature TC is determined as 300 K, being the temperature of the maximum dM/dT This value is much higher than that of La0.67Ca0.33MnO3 and La2/3Ca1/3MnO3 (by 30 K) In the La1ÀxCaxMnO3 system, with a surplus of Mn, both the anionic and cationic vacancies arise in the actual structure of the oxides as a result of an oxidation–reduction process created via the heating and cooling procedure in 504 N.H Sinh, N.P Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507 Fig XPD result of La0.54Ca0.32MnO2.977 The pattern was obtained from powder of a sintered pellet type sample and measured at room temperature the sample preparation It is closely related to changes of manganese valency from Mn4+ to Mn3+ on heating and from Mn3+ to Mn4+ on cooling Thus, the real structure contains Mn3+ and Mn4+ ions as well as the anionic and cationic vacancies Therefore, the increase of the Curie temperature of the La0.54Ca0.32MnO3Àd sample may originate from this structure The result of Chen et al [9] showed that TC increases to its highest value of 314.5 K in the La-deficient system La1ÀxMnO3Àd at x ¼ 0:30: This result also indicates that decreasing the La-content causes a marked increase of the Curie temperature Magnetization as a function of applied magnetic field up to T, at K and 77 K, is shown in Fig From these curves, the saturation magnetic moments have been calculated as sS ¼ 2:99 mB/f.u in La0.54Ca0.32MnO2.977 It is in good agreement with the magnetic moment value of Mn3+ in this compound Magnetization in the dependence on applied fields up to T was measured at various temperatures, ranging from 200 to 300 K From the MðHÞ curves with various temperature intervals, the magnetic-entropy change DSmag can be approximately calculated by the following expression: DSmag Ti ; Hmax ị ẳ X Mi Miẳ1 ị DHi : Ti Tiẳ1 ị 1ị Here, Mi and Miỵ1 are experimental magnetization values obtained at temperature Ti and Tiỵ1 ; respectively, in a magnetic field Hi : The temperature change DT of the sample is related to the total entropy change by DT ¼ À TDSmag : CP;H ð2Þ Here, CP;H is the (field dependent) heat capacity of the sample depending on the applied magnetic field The obtained magnetic-entropy change DSmag is shown in Fig as a function of temperature The maximum magnetic-entropy change of La0.54Ca0.32MnO3Àd is reached at its Curie temperature, where the change of the magnetization with temperature is the fastest The maximum entropy change, corresponding to a magnetic-field change of 1, and T, is 1.81, 3.92 and 5.50 J/ kg K, respectively It is clear that the large magnetic-entropy change in this compound originates from the considerable change of the magnetization near TC The obtained entropy change shows that these values are interesting with both increasing magnetic field and doping concentration It is a possible reason that at higher magnetic fields, the magnetic moments are orientated better than at lower magnetic fields On the other hand, an amount of Ca2+ substituted for La3+ induces a N.H Sinh, N.P Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507 505 100 50 80 30 B =1000 Oe M (emu/g) M (emu/g) 40 20 ZFC FC 10 0 60 La0.54Ca0.32MnO3-δ 40 5K 77 K 20 50 100 150 200 T (K) 250 300 M (emu/g) 90 80 70 60 50 40 30 20 10 0 B (T) Fig Magnetization plotted as a function of magnetic field at and 77 K for La0.54Ca0.32MnO2.977 sample From these curves, a saturation magnetic moment of 2.99 mB/f.u has been calculated B = 10000 Oe ZFC FC 50 100 150 200 250 300 ∆SM (J/kg.K) T (K) 16 14 12 M (emu/g) 10 B = 100 Oe ZFC FC 0 50 100 200 240 280 320 T (K) 150 200 250 300 Fig The entropy change as a function of temperature for La0.54Ca0.32MnO2.977 calculated for field variation 1, and T T (K) Fig The temperature dependence of the magnetization for La0.54Ca0.32MnO2.977 in zero field cooled (’) and field cooled (&) regimes in (a) 100 Oe; (b) 1000 Oe and (c) 10 000 Oe change of the Mn3+/Mn4+ ratio, increasing the competition between the double-exchange (DE) and the superexchange (SE) interaction, where in this case the SE interaction will be dominated by the interaction of the Mn3+ and Mn4+ ions and by the increase of Mn4+ ions in the compound By the dichromate method, the oxygen concentration in La0.54Ca0.32MnO3Àd has been determined The obtained value is d ¼ 0:0230: Thus, the actual composition of the sample is La0.54Ca0.32MnO2.977 From the oxygen deficiency d; the ratio of Mn3+/Mn4+ was estimated to be 0.3060/0.6940 = 0.441 Fig shows the temperature dependence of the susceptibility From this curve, a transition temperature near 300 K is also revealed for the ferromagnetism to paramagnetism transition The temperature dependence of the resistance of the sample is shown in Fig The data exhibit a maximum in the electrical resistivity as the temperature decreases Indeed, most La1ÀxAxMnO3 compounds show an insulator– N.H Sinh, N.P Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507 506 χac (arb.u) 60 40 20 100 150 200 250 300 350 T (K) Fig Susceptibility curve of La0.54Ca0.32MnO2.977 TC has been determined by dw/dT decreases TC The structural parameters, in particular the oxygen thermal parameters, show significant changes across the I–M phase transition Thus, clearly, Mn4+ plays a crucial role in this material The surface structure of the sample obtained by SEM measurement is shown in Fig It is found that the size, shape and distribution of the grains on the surface of the sample are homogeneous Table presents data on the MCE for several compounds for comparison As can be seen in Table 1, La0.54Ca0.32MnO2.977 is suitable for application in magnetic refrigeration Besides the ease of production and the high chemical stability, its Curie temperature is at room temperature range and the material exhibits a large magnetic-entropy change R (T)/R(300 K) 1.0 0.9 0.8 100 150 200 250 300 350 T (K) Fig The resistance curve of La0.54Ca0.32MnO2.977 The maximum value on this curve is corresponding to the insulator–metal transition at TC metal (I–M) transition around TC This (I–M) transition is associated with a peak in the resistivity curve at a so-called TIM; generally, TIM is somewhat lower than TC In our case we estimated TIM ETC : The nature of the I–M transition can be understood that, in manganates, Jahn–Teller distortion due to the Mn3+ ions plays a key role The creation of Mn4+ ions removes the distortion leading to more cubic structures Therefore, across the I–M transition occurring around TC, the J–T distortion decreases, and the distortion becomes more prominent in the insulating phase Increasing the static coherent MnO6 distortion favors the insulating behavior and Fig SEM of La0.54Ca0.32MnO2.977 showing a homogeneous distribution of grains with the same size and shape over the surface of the sample Table Curie-temperature and maximum entropy change (ÀDSmag) for several typical magnetic refrigeration materials Sample TC (K) ÀDSM (J/kg K) Hmax (T) Ref La0.54Ca0.32MnO2.977 La0.54Ca0.32MnO3Àd La2/3Ca1/3MnO3 La0.67Ca0.33MnO3 La0.8Ca0.2MnO3 La0.6Ca0.4MnO3 La0.9Ca0.1MnO3 La0.8Ca0.2MnO3 300 272 267 255 230 263 255 260 5.5 2.9 6.4 4.47 5.5 5.0 5.93 7.75 0.9 1.5 1.5 3 Ours [9] [8] [7] [10] [11] [12] [12] N.H Sinh, N.P Thuy / Journal of Magnetism and Magnetic Materials 262 (2003) 502–507 In conclusion, we have studied some properties of La-deficient La0.54Ca0.32MnO2.977 The obtained results on the oxygen deficiency d and the ratio of Mn3+ and Mn4+ ions revealed intrinsic processes in the material It is found that the ferromagnetism-paramagnetism and I–M transitions occur near the same temperature TC The Curie temperature TC is as high as room temperature Moreover, large magneticentropy changes around TC have been observed With these advantages, the La0.54Ca0.32MnO2.977 compound can be considered as a suitable candidate for application as a working substance in magnetic refrigeration technology at room temperature Acknowledgements The authors would like to thank Ph.D student Nguyen Phuc Duong for help in magnetization measurement This work was supported by the National project 421101/2002 of Vietnam and National University Project QGTD-00-01 507 References [1] C.N.R Rao, A.K Raychaudhuri, Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific Publishing Co Pte Ltd., Singapore, 1998 p [2] A.P Ramirez, P Schiffer, S.W Cheong, C.H Chen, W Bao, T.T.M Palstra, P.L Gammel, D.J Bishop, B Zegarski, Phys Rev Lett 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K, is shown in Fig From these curves, the saturation magnetic moments have been calculated... Fig as a function of temperature The maximum magnetic-entropy change of La0.54Ca0.32MnO3Àd is reached at its Curie temperature, where the change of the magnetization with temperature is the fastest... La0.54Ca0.32MnO2.977 The pattern was obtained from powder of a sintered pellet type sample and measured at room temperature the sample preparation It is closely related to changes of manganese valency