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Jurnal Fisika ISSN 0854-3046 Himpunan Fisika Indonesia Akreditasi: No. 242/Akred-LIPI/P2MBI/05/2010 Vol. 10 - No. 2 - Desember 2010 __________________________________________________________________________________________ MAGNETIC AND MAGNETOTRANSPORT PROPERTIES OF La0.7Ca0.3Mn1-x(Zn,Cu)xO3 P. Q. Thanh1, N. H. Sinh1, N. A. Tuan1, V. V. Khai1, K. Tarigan2,3, M. Ginting4, T. L. Phan2 1 Department of Physics, College of Science, Vietnam National University, Hanoi, Vietnam 2 Department of Physics, Chungbuk National University, Cheongju 361-763, Korea 3 Department of Electrical Engineering, Indonesia Institute of Technology, Serpong, Tangerang Selatan 15320, Indonesia 4 Research and Development Center for Applied Physics, LIPI, Serpong, Tangerang Selatan 15314, Indonesia E-mail: ptlong2512@yahoo.com ABSTRACT Magnetic and magnetotransport properties of two perovskite manganite samples of La0.7Ca0.3Mn0.9Zn0.1O3 and La0.7Ca0.3Mn0.95Cu0.05O3 prepared by conventional solid-state reaction have been studied in detail. Experimental results revealed that the temperature dependences of magnetization and resistance varied strongly around the phase-transition temperature. Maximum magnetoresistance (MR) values of La0.7Ca0.3Mn0.9Zn0.1O3 and La0.7Ca0.3Mn0.95Cu0.05O3 under an applied field of 400 Oe were about 21.4 % and 11.0 %, respectively. The maximum magnetic-entropy change (ΔSM) was 2.73 J/kg.K for La0.7Ca0.3Mn0.9Zn0.1O3, and 3.34 J/kg.K for La0.7Ca0.3Mn0.95Cu0.05O3 when the applied field was 45 kOe. Both the MR and ΔSM values obtained from two samples were smaller than those of the parent compound La0.7Ca0.3MnO3. This was due to the change in the Mn3+/Mn4+ ratio caused by Zn and Cu dopants, which led to a change in the type of the ferromagneticparamagnetic phase transition. Keywords: Perovskite manganites, magnetic entropy, magnetotransport property. ABSTRAK Sifat-sifat magnetik dan magnetotransport dari dua sampel manganite perovskit yakni La0.7Ca0.3Mn0.9Zn0.1O3 dan La0.7Ca0.3Mn0.95Cu0.05O3 yang disiapkan dengan reaksi solid-state konvensional telah dipelajari secara mendetail. Hasil penelitian menunjukkan bahwa pengaruh suhu terhadap variasi magnetisasi dan resistensi sangat kuat di sekitar suhu fase-transisi. Nilai magnetoresistance (MR) maksimum La0.7Ca0.3Mn0.9Zn0.1O3 dan La0.7Ca0.3Mn0.95Cu0.05O3 dalam medan magnet sebesar 400 Oe masing-masing sekitar 21,4% dan 11,0%. Perubahan entropi magnetik maksimum (ΔSM) ketika menggunakan medan sebesar 45 kOe masing-masing sebesar 2,73 J/kg.K untuk La0.7Ca0.3Mn0.9Zn0.1O3, dan 3,34 J / kg.K untuk La0.7Ca0.3Mn0.95Cu0.05O3. Nilai MR dan ΔSM yang diperoleh dari ke dua sampel itu lebih kecil dari pada senyawa induk La0.7Ca0.3MnO3. Hal ini karena perubahan harga rasio Mn3+ / Mn4+ sebagai akibat dopan Zn dan Cu, yang menyebabkan perubahan fase transisi feromagnetik ke paramagnetik. Kata kunci: Perovskite manganites, magnetic entropy, magnetotransport property. _______________________________________________________________________________________ Magnetic and Magnetotransport Properties of La0.7Ca0.3Mn1-x(Zn,Cu)xO3 95 (P.Q. Thanh et.al.) Jurnal Fisika ISSN 0854-3046 Himpunan Fisika Indonesia Akreditasi: No. 242/Akred-LIPI/P2MBI/05/2010 Vol. 10 - No. 2 - Desember 2010 __________________________________________________________________________________________ INTRODUCTION Recently, many works have been made to study magnetic and magnetotransport properties of perovskite-type manganites La1-yCa yMnO3 [1-7]. This material system is applicable in recording/reading heads, magnetic sensitive sensors, and magnetic refrigerators working around room temperature. By the Ca-concentration change, leading to a change in the Mn3+/Mn4+ ratio and lattice parameters, one can control easily their magnetic and electrical properties. Together with these studies, the replacement of metals (such as Ni, Al, Co and so forth) for Mn in La1-yCa yMnO3 has been also carried out [3, 8]. This would lead to a remarkable change in the bond length 〈Mn-O〉 and the bond angle 〈Mn-O-Mn〉, which influence directly on the magnetic interactions in the system. In the previous work [3], we studied influences of Ni- and Co-substitutions on the magnetotransport property of La0.7Ca0.3Mn1-x(Ni,Co)xO3. It was observed at the Curie temperature (TC) that the maximum magnetoresistance (MR) value under an applied field of 400 Oe for the Ni- and Codoped samples was about 17.0% and 8.0%, respectively, which was smaller than those of La0.7Ca0.3MnO3. The coexistence of antiferromagnetic interactions, due to exchange pairs Ni-Mn and Co-Mn, besides the ferromagnetic Mn3+-Mn4+ interaction was proposed to explain the MR data obtained. To gain more insight into this material system, we prepared La0.7Ca0.3Mn1-x(Cu,Zn)xO3 compounds (where Mn ions was substituted partly by Cu and Zn ions) and studied their magnetic and magnetotransport properties. EXPERIMENT Two polycrystalline samples of La0.7Ca0.3Mn0.9Zn0.1O3 and La0.7Ca0.3Mn0.95Cu0.05O3 were prepared by the solid-state reaction method, used commercial powders MnCO3, CaCO3, La2O3 ZnO and CuO (99.9 % purity) as the precursors. These powders combined with appropriate masses were well-mixed, pressed into pellets, and then pre-sintered at 900 oC for 2 hrs. After several times of the intermediate grinding and heat treatment, the pellets were annealed at 1050 oC for 24 hrs in air. The process is shown in Figure 1. The crystallographic structure of the final products was examined by an X-ray diffractometer (Brucker D5005). The magnetic field dependence of resistance in the temperature range of 100 - 300 K was investigated by the standard four-probe method. Magnetic measurements with respect to temperature and the magnetic field were performed on a superconducting quantum interference device (SQUID). _______________________________________________________________________________________ Magnetic and Magnetotransport Properties of La0.7Ca0.3Mn1-x(Zn,Cu)xO3 96 (P.Q. Thanh et.al.) Jurnal Fisika ISSN 0854-3046 Himpunan Fisika Indonesia Akreditasi: No. 242/Akred-LIPI/P2MBI/05/2010 Vol. 10 - No. 2 - Desember 2010 __________________________________________________________________________________________ Figure 1. Diagram shows preparation of La0.7Ca0.3Mn1-x(Zn,Cu)xO3 bulks. RESULTS AND DISCUSSION Before investigating the magnetic and magnetotransport properties of the samples, we checked their quality based on an X-ray diffractometer. It was revealed that the samples were the single phase in an orthorhombic structure (belonging to the space group Pnma), see Figure 2. There was no trace of secondary phases related to the starting powders. Based on the XRD data, we determined the lattice parameters of La0.67Ca0.33Mn0.9Zn0.1O3 to be a = 5.441 Å, b = 7.697 Å, and c = 5.434 Å, and of La0.67Ca0.33Mn0.95Cu0.05O3 to be a = 5.467 Å, b = 7.707 Å, and c = 5.447 Å. Meanwhile, those for the parent compound La0.7Ca0.3MnO3 were a = 5.453 Å, b = 7.728 Å, and c = 5.471 Å [3]. One can see clearly that the lattice parameters of the samples La0.67Ca0.33Mn0.9Zn0.1O3 and La0.67Ca0.33Mn0.95Cu0.05O3 were slightly decreased as comparing to those of La0.7Ca0.3MnO3. This is due to the substitution of Zn2+ and Cu2+ ions for the Mn3+ and Mn4+ ions in the manganite structure; because the ionic radii of Zn2+ (0.74 Ǻ) and Cu2+ (0.73 Ǻ) are greater than those of Mn3+ (0.66 Ǻ) and Mn4+ (0.60 Ǻ). Under such the circumstance, the concentration of Mn3+ and Mn4+ ions was varied, which influenced magnetic and magnetotransport properties of the samples as being presented below. Following the structural investigations, we studied the magnetic properties and magnetocaloric effect of these samples. Figure 3 shows the temperature dependence of the zero-field cooled magnetization, M(T), under an applied field of 50 Oe. _______________________________________________________________________________________ Magnetic and Magnetotransport Properties of La0.7Ca0.3Mn1-x(Zn,Cu)xO3 97 (P.Q. Thanh et.al.) Jurnal Fisika ISSN 0854-3046 Himpunan Fisika Indonesia Akreditasi: No. 242/Akred-LIPI/P2MBI/05/2010 Vol. 10 - No. 2 - Desember 2010 __________________________________________________________________________________________ Figure 2. XRD patterns of La0.7Ca0.3Mn0.9Zn0.1O3 and La0.7Ca0.3Mn0.95Cu0.05O3. The samples are the single phase in an orthorhombic structure (the space group Pnma). Figure 3. Temperature dependence of the zero-field cooled magnetization for La0.7Ca0.3Mn0.9Zn0.1O3 and La0.7Ca0.3Mn0.95Cu0.05O3 under an applied field of 50 Oe. It appears from Figure 3 that with increasing temperature from 80 to 300 K, there is a rapid decrease of magnetization at a temperature value called Curie temperature, TC. The TC values of La0.7Ca0.3Mn0.9Zn0.1O3 and La0.7Ca0.3Mn0.95Cu0.05O3 are about 210 and 200 K, respectively. Having compared to La0.7Ca0.3MnO3 with TC ≈ 220 K [3, 5, 6] or 260 K [4, 7], it is seen that TC of our samples _______________________________________________________________________________________ Magnetic and Magnetotransport Properties of La0.7Ca0.3Mn1-x(Zn,Cu)xO3 98 (P.Q. Thanh et.al.) Jurnal Fisika ISSN 0854-3046 Himpunan Fisika Indonesia Akreditasi: No. 242/Akred-LIPI/P2MBI/05/2010 Vol. 10 - No. 2 - Desember 2010 __________________________________________________________________________________________ is slightly decreased. This is due to the fact that the presence of Zn2+ and Cu2+ ions in the manganite host lattice reduces the coupling possibility between Mn3+ and Mn4+, leading to the decrease of TC compared to the La0.7Ca0.3MnO3 compound. Notably, Zn2+ does not contribute the magnetic behavior to La0.7Ca0.3Mn0.9Zn0.1O3, because it is nonmagnetic ion with the full-filled electron configuration of 3d10. In contrast, the presence of Cu2+ (with the unfulfilled electron configuration of 3d9) in La0.7Ca0.3Mn0.95Cu0.05O3 could introduce antiferromagnetic interactions between Cu2+ and Mn3+,4+ ions [9], which compete with the pre-existing ferromagnetic Mn3+-Mn4+ interaction. Such situations explain why the TC and magnetization values of these two samples are different, as can be seen in Figure 3. Next, we measured isothermal magnetization curves around their phase transition temperature TC. The applied field was scanned from 0 to 45 kOe, and the temperature increment was 1-2 K. Based on these data, we could assess the magnetocaloric effect through magnetic-entropy change (ΔSM). Theoretically, the entropy change can be calculated by means of [10]: H2 ⎛ ∂M ⎞ ΔS M (T , H ) = ∫ ⎜ ⎟ dH . ∂T ⎠ H H 1⎝ (1) It is integrated numerically in the desired range of temperatures and magnetic fields on the basis of the set of magnetic isotherms M(H) at different temperatures. This equation usually works well for material systems with the second-order magnetic phase transition, because at the first-order magnetic phase transition the derivative ∂M/∂T becomes infinity. Recently, McMichael et al. [11] proposed another simple way to calculate ΔSM based on the following formula: | ΔS M |= ∑ i M i − M i +1 Ti +1 − Ti ΔH i (2) where Mi and Mi+1 are isothermal magnetization values measured in a magnetic field interval ΔH at temperatures Ti and Ti+1, respectively. By using Eq. (2), we calculated ΔSM for the samples in the magnetic field intervals 15, 30 and 45 kOe. As seen in Fig. 4, ΔSM increases with increasing the applied field. At a given ΔH, when temperature is lowered, ΔSM gradually increases and attains a maximum value at temperatures around TC. If continuously lowering down temperature below TC, ΔSM decreases. With ΔH = 45 kOe, the maximum values of ΔSM for La0.7Ca0.3Mn0.9Zn0.1O3 and La0.7Ca0.3Mn0.95Cu0.05O3 are 2.73 and 3.34 J/kg.K, respectively. These values are much smaller than the maximum ΔS value of La0.7Ca0.3MnO3 (about 6.2 J/kg.K for ΔH = 10 kOe) [12]. _______________________________________________________________________________________ Magnetic and Magnetotransport Properties of La0.7Ca0.3Mn1-x(Zn,Cu)xO3 99 (P.Q. Thanh et.al.) Jurnal Fisika ISSN 0854-3046 Himpunan Fisika Indonesia Akreditasi: No. 242/Akred-LIPI/P2MBI/05/2010 Vol. 10 - No. 2 - Desember 2010 __________________________________________________________________________________________ Figure 4. Temperature dependences of the magnetic-entropy change for (a) La0.7Ca0.3Mn0.9Zn0.1O3 and (b) La0.7Ca0.3Mn0.95Cu0.05O3 under various applied fields of 15, 30, and 45 kOe. To understand why there is such a large difference, let pay attention to the ferromagneticparamagnetic phase transitions of La0.7Ca0.3MnO3 and a representative sample of La0.7Ca0.3Mn0.95Cu0.05O3. Normally, the M(T) curve of the former shows a sharp phase transition at TC while that of the latter does not reveal this feature. Alternatively, if performing the plot of H/M versus M2 for the isotherms M(H), we can see more clearly the difference between the two compounds. Figure 5 shows H/M versus M2 plots for La0.7Ca0.3MnO3 studied by Mira et al. [4] and for our sample La0.7Ca0.3Mn0.95Cu0.05O3. For the case of La0.7Ca0.3MnO3, one can see at some temperatures that H/M versus M2 curves exhibit the negative slope, Fig. 5(a); a similar result could be met in Ref. [5]. However, only the positive slope is found in the H/M versus M2 curves for La0.7Ca0.3Mn0.95Cu0.05O3, see Fig. 5(b), and La0.7Ca0.3Mn0.9Zn0.1O3 as well (not shown). According to Banerjee’s criterion [13], the negative and positive slopes are indicators of the first- and second-order magnetic phase transitions, respectively. This means that La0.7Ca0.3MnO3 exhibits the first-order magnetic phase transition while our Zn- and Cu-doped samples exhibits second-order transition. In other words, the doping of Zn and Cu into La0.7Ca0.3MnO3 leads to a change in the magnetic phase transition from first order to the second order, and thus results in a remarkable decrease in the maximum ΔS of doped samples, as mentioned. _______________________________________________________________________________________ Magnetic and Magnetotransport Properties of La0.7Ca0.3Mn1-x(Zn,Cu)xO3 100 (P.Q. Thanh et.al.) Jurnal Fisika ISSN 0854-3046 Himpunan Fisika Indonesia Akreditasi: No. 242/Akred-LIPI/P2MBI/05/2010 Vol. 10 - No. 2 - Desember 2010 __________________________________________________________________________________________ Figure 5. H/M versus M2 plots of the isotherms for (a) La0.7Ca0.3MnO3 (after Mira et al. [4]), and (b) La0.7Ca0.3Mn0.95Cu0.05O3. While the negative slope can be found in (a) La0.7Ca0.3MnO3 at some temperatures, only the positive slope is found in (b) La0.7Ca0.3Mn0.95Cu0.05O3. Together with the study of the magnetic properties of the samples, we investigated their magnetotransport properties. Figure 6 shows the temperature dependences of resistance R under applied fields of 0 and 400 Oe. Higher peak resistance generated by the applied field of 0 Oe. It appears from Fig. 6 that with increasing temperature the samples exhibit a metallic-semiconducting phase transition at a temperature named Tp of 190 K for La0.7Ca0.3Mn0.9Zn0.1O3 and of 150 K for La0.7Ca0.3Mn0.95Cu0.05O3. With the presence of an applied field of 400 Oe, this Tp temperature is shifted about 5 K towards higher temperatures. Concurrently, the resistance R at temperatures around Tp is decreased rapidly. Such the tendency seems popular in perovskite manganites [14, 15]. To further assess the variation of R versus the magnetic field, we based on the magnetoresistance (MR) ratio defined by : MR = R( H = 0) − R( H ≠ 0) . R( H = 0) (3) _______________________________________________________________________________________ Magnetic and Magnetotransport Properties of La0.7Ca0.3Mn1-x(Zn,Cu)xO3 101 (P.Q. Thanh et.al.) Jurnal Fisika ISSN 0854-3046 Himpunan Fisika Indonesia Akreditasi: No. 242/Akred-LIPI/P2MBI/05/2010 Vol. 10 - No. 2 - Desember 2010 __________________________________________________________________________________________ Figure 6. Temperature dependences of resistance of (a) La0.7Ca0.3Mn0.9 Zn0.1O3 and (b) La0.7Ca0.3Mn0.95Cu0.05O3 under applied fields of 0 Oe and 400 Oe. The inset in (a) shows MR of La0.7Ca0.3Mn0.9Zn0.1O3. With H = 400 Oe, it was found that MR reached maximum values of 21.4 % at 186 K for La0.7Ca0.3Mn0.9Zn0.1O3, see the inset of Fig. 6(a), and about 11.0 % at 150 K for La0.7Ca0.3Mn0.95Cu0.05O3. The decrease in the MR ratio of the Cu-doped sample compared to that of the Zn-doped one could be due to the presence of anti-ferromagnetic interaction pairs Cu2+- Mn3+,4+, which influenced the hopping process of electrons between Mn3+ and Mn4+ ions. For a La0.7Ca0.3MnO3 single crystal sample, it was found the MR ratio of about 70 % under an applied field of 500 Oe [14], much higher than those determined from our Zn- and Cu-doped samples. This is probably associated with the grain boundary effects [16] and/or the difference in the magnetic phase transition type between undoped and Zn- and Cu-doped La0.7Ca0.3MnO3 compounds as discussed above. _______________________________________________________________________________________ Magnetic and Magnetotransport Properties of La0.7Ca0.3Mn1-x(Zn,Cu)xO3 102 (P.Q. Thanh et.al.) Jurnal Fisika ISSN 0854-3046 Himpunan Fisika Indonesia Akreditasi: No. 242/Akred-LIPI/P2MBI/05/2010 Vol. 10 - No. 2 - Desember 2010 __________________________________________________________________________________________ CONCLUSION We prepared polycrystalline perovskite manganites of La0.7Ca0.3Mn0.9Zn0.1O3 and La0.7Ca0.3Mn0.95Cu0.05O3, and studied their magnetic and magnetotransport properties. It was revealed that the presence of Zn and Cu dopants reduced the Curie temperature. While the parent compound La0.7Ca0.3MnO3 exhibited the first-order magnetic phase transition, our samples exhibited the secondorder transition. Such the circumstances led to a remarkable decrease in the maximum magneticentropy change (ΔSM) and MR values compared to those of La0.7Ca0.3MnO3. The role of Zn and Cu dopants in the manganite host lattice was also discussed. REFERENCES 1. Chau, N., H. N. Nhat, N. H. Luong, D. L. Minh, N. D. Tho and N. N. Chau. Physica B 327 (2003): 270. 2. Ramirez, P. Journal of Physics: Condensed Matter 9 (1997): 8171. 3. Sinh, N.H., V. V. Khai and N. T. Thuong. Proceedings of the 5th National Conference on Solid State Physics, Vungtau, Vietnam, 12-14 Nov. (2007). 4. Mira, J., J. Rivsa, F. Rivadulla, C. V. Vazquez and M. A. L. Quintela. Physical Review B 60 (1999): 2998. 5. Shin, H.S., J. E. Lee, Y. S. Nam, H. L. Ju and C. W. Park. 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