Journal of Magnetism and Magnetic Materials 242–245 (2002) 760–762 On the magnetic and magnetocaloric properties of perovskite La1ÀxSrxCoO3 Nguyen Hoang Luonga,*, Nguyen Chaua, Phan Manh Huonga, Dang Le Minhb, Nguyen Ngoc Chaua, Bach Thanh Congb, M Kurisuc a Faculty of Physics, Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam b Department of Solid State Physics, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam c School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan Abstract Structural, magnetic and magnetocaloric studies on La1ÀxSrxCoO3 (0.20pxp0.50) were reported The crystal structure of the samples is rhombohedral with x ¼ 0:2020:45 and is cubic with x ¼ 0:50: Magnetization, Curie temperature (Tc ) and maximum magnetic-entropy change (DSmax ) depending on the Sr content in the samples are discussed La0.55Sr0.45CoO3 exhibits the largest value for DSmax and Tc among the compounds investigated r 2002 Elsevier Science B.V All rights reserved Keywords: Magnetization; Magnetocaloric effect; Magnetic oxides; Perovskite structure The perovskite compounds La1ÀxSrxCoO3 are believed to exhibit spin glass (SG)-like properties at low temperatures for xo0:20 and cluster glass (CG)-like properties at higher Sr concentraion [1] In high Sr concentration range, x ¼ 0:50 is a typical concentration corresponding to CG behavior [2] Wang and Zhang [3] suggested that the double-exchange interaction played an important role in the ferromagnetic metallic conductivity of La1ÀxSrxCoO3 Takahashi et al [4] have studied the electronic structures of La1ÀxSrxCoO3 and showed that La1ÀxSrxCoO3 becomes metallic near x ¼ 0:20 because of the ferromagnetic coupling between O 2p and Co 3d electrons Ibarra et al [5] have found a huge anisotropic magnetostriction in La1ÀxSrxCoO3 (x ¼ 0:30; 0.50) Recently, Chaudhary et al [6] have found moderately large magnetocaloric effect in La1ÀxSrxCoO3 (0.05pxp0.40) with maximum entropy change in La0.6Sr0.4CoO3 among samples studied In the present work, we report on our study of magnetic and magnetocaloric properties of La1ÀxSrxCoO3 with 0.20pxp0.50 *Corresponding author Fax: 84-4-858-94-96 E-mail address: luong@cms.edu.vn (N.H Luong) Polycrystalline La1ÀxSrxCoO3 samples with x ¼ 0:20; 0.25, 0.30, 0.35, 0.40, 0.45, and 0.50 were prepared by the conventional solid-state reaction technique from a stoichiometric mixture of La2O3, SrCO3 and CoO at pre-sintering temperature of 10001C for 15 h After regrinding and pressing for parallelepipeds, the samples were again heat treated to 9501C for 12 h and then to 10501C for 10 h The structure of the samples was examined by Bruker X-ray diffractometer D5005 The thermal stability associated with crystallization and melting was determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) using TA Instruments apparatus SDT 2960 with a heating rate of 201C/min The magnetization measurements were performed with an SQUID magnetometer in the temperature range between 1.7 and 300 K and in the magnetic fields up to 70 kOe as well as with vibrating sample magnetometer (VSM) DMS 880 in the fields up to 13.5 kOe X-ray diffraction analysis revealed that all the samples are of single phase The crystal structure of the samples is rhombohedral with x ¼ 0:2020:45 and is cubic with x ¼ 0:50: These results are in good agreement with those obtained by Ibarra et al [5], who reported that the 0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V All rights reserved PII: S - 8 ( ) 1 - N.H Luong et al / Journal of Magnetism and Magnetic Materials 242–245 (2002) 760–762 crystal structures of x ¼ 0:30 and 0.50 are rhombohedral and cubic, respectively The DSC curves for all the samples show three sharp endothermic peaks The DSC curves for the samples with x ¼ 0:25 and 0.45 are displayed in Fig 1, as examples, together with DSC and TGA curves for starting material La2O3 It is suggested that, the first two endothermic peaks are due to the decomposition of La2O3 into intermediate oxide phases It is interesting to note that the third endothermic peak of both samples around 931–9321C corresponds to the sharp endothermic peaks in the DSC curves for the starting materials CoO and SrCO3 (not shown in the figure) This third peak can be considered as corresponding to the reaction forming a perovskite structure Probably because of this reason, several research groups have sintered La1ÀxSrxCoO3 at temperatures lower than those for La1ÀxSrx MnO3 [5,6] The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements were performed for all the samples investigated Representative results obtained for x ¼ 0:30 and 0.50 at H=100 Oe are displayed in Fig The Curie temperature, Tc ; of the samples determined from these measurements is presented in Table It can be seen from Table that Tc slightly increases with Sr content up to x ¼ 0:45 then becomes somewhat lower in the sample with x ¼ 0:50: These results differ from those obtained by Chaudhary et al [6] who reported that the Curie temperature is almost independent of Sr concentration The results of ZFC and FC magnetization measurements obtained for the samples with xp0:40 revealed the divergence of the ZFC and FC measurements at a temperature Tir marking the appearence of irreversible magnetic behavior (see Fig 2) We observe the divergence of ZFC and FC measurements in the samples with highest Sr contents in this study (x ¼ 0:45 and 0.50) in the whole temperature range apart from the paramagnetic region 761 Fig Temperature dependence of magnetization for La1ÀxSrxCoO3 with x ¼ 0:30 and 0.50 Open squares and open circles represent the field-cooled (FC) and zero-field-cooled (ZFC) curves, respectively Table Curie temperature, Tc ; and maximum magnetic-entropy change, DSmax ; as a function of the Sr concentration in La1ÀxSrxCoO3 Fig DSC curves for La1ÀxSrxCoO3 with x ¼ 0:25 and 0.45 Included are DSC and TGA curves for La2O3 X TC (K) DSmax (J/kg K) 0.20 0.25 0.30 0.35 0.40 0.45 0.50 240 245 250 252 253 255 252 0.56 0.63 0.79 0.86 0.94 1.04 0.87 762 N.H Luong et al / Journal of Magnetism and Magnetic Materials 242–245 (2002) 760–762 Fig The magnetic-entropy change as a function of temperature for La0.55Sr0.45CoO3 Fig Magnetization curves at K for La1ÀxSrxCoO3 Fig shows the magnetization curves at K for all La1ÀxSrxCoO3 samples As can be seen from this figure, magnetization clearly increases with Sr content from x ¼ 0:20 up to x ¼ 0:45 then drops rather dramatically in x ¼ 0:50: We note that, as discussed earlier, the crystal structure is rhombohedral in the samples with x ¼ 0:2020:45; whereas it is cubic in the sample with x ¼ 0:50: We suggest that, with increasing Sr content from 0.20 up to 0.45, the ferromagnetic exchange interaction between Co3+ and Co4+ is enhanced, causing the increase of magnetization of the compounds However, with higher Sr concentration x ¼ 0:50; corresponding to higher Co4+ concentration, lower-spin states of Co3+ and Co4+ occur, leading to the decrease of magnetization The isothermal M(H) curves for all La1ÀxSrxCoO3 samples have been measured at various temperatures with small temperature intervals around the Curie temperature in magnetic fields up to 13.5 kOe The entropy change resulting from the spin-ordering, induced by the applied magnetic field, can be obtained according to the thermodynamic relation [7] Z H max DST; Hị ẳ fqMT; Hị=qTgH dH: 1ị From the isothermal M(H) curves we evaluated the entropy change, DS; as a function of temperature for all the samples Results for La0.55Sr0.45CoO3 are shown in Fig 4, as example As is evident from Fig 4, the sample exhibits a maximum in DS near Tc : Similar behavior was observed for other samples investigated The maximum magnetic-entropy change, DSmax ; has been derived for all La1ÀxSrxCoO3 samples and presented in Table As one can see from Table 1, the sample with x ¼ 0:45 displays the largest value of 1.04 J/kg K for DSmax at an applied magnetic field of 13.5 kOe With higher Sr concentration, i.e x ¼ 0:50; DSmax decreases to a value of 0.87 J/kg K Our results for Sr concentrations up to x ¼ 0:4 basically agree with those reported by Chaudhary et al [6] However, the Sr content studied in Ref [6] is limited only to x ¼ 0:4: This could be the reason that authors of Ref [6] could not observe the largest value for DSmax in x ¼ 0:45: Thus, La0.55Sr0.45CoO3 can be considered as an active magnetic refrigerant (AMR) material near 255 K The authors are grateful to the Vietnam National Program for Natural Sciences for financial support and the VNU project QGTD-00-01 for a partial support References [1] M Itoh, I Natori, S Kubota, K Motoya, J Phys Soc Jpn 63 (1994) 1486 [2] D.N.H Nam, K Jonason, P Nordblad, N.V Khiem, N.X Phuc, Phys Rev B 59 (1999) 4189 [3] Z.L Wang, J Zhang, Phys Rev B 54 (1996) 1153 [4] H Takahashi, F Munakata, M Yamanaka, Phys Rev B 57 (1998) 15211 [5] M.R Ibarra, R Mahendiran, C Marquina, B GarciaLanda, J Blasco, Phys Rev B 57 (1998) R3217 [6] S Chaudhary, V.S Kumar, S.B Roy, P Chaddah, S.R Krishnakumar, V.G Sathe, A Kumar, D.D Sarma, J Magn Magn Mater 202 (1999) 47 [7] A.M Tishin, J Magn Magn Mater 184 (1998) 62  ... reported that the Curie temperature is almost independent of Sr concentration The results of ZFC and FC magnetization measurements obtained for the samples with xp0:40 revealed the divergence of the. .. 0.45, the ferromagnetic exchange interaction between Co3+ and Co4+ is enhanced, causing the increase of magnetization of the compounds However, with higher Sr concentration x ¼ 0:50; corresponding... concentration, lower-spin states of Co3+ and Co4+ occur, leading to the decrease of magnetization The isothermal M(H) curves for all La1ÀxSrxCoO3 samples have been measured at various temperatures