Multifunctionality from coexistence of large magnetoresistance and magnetocaloric effect in La0.7Ca0.3MnO3

Transcription

1 University of Wollongong Research Online Faculty of Engineering - Papers (Archive) Faculty of Engineering and Information Sciences 2011 Multifunctionality from coexistence of large magnetoresistance and magnetocaloric effect in La0.7Ca0.3MnO3 J. C. Debnath University of Wollongong R. Zeng University of Wollongong, rzeng@uow.edu.au J. H. Kim University of Wollongong, jhk@uow.edu.au S. X. Dou University of Wollongong, shi@uow.edu.au Publication Details Debnath, J. C., Zeng, R., Kim, J. H. Dou, S. X. (2011). Multifunctionality from coexistence of large magnetoresistance and magnetocaloric effect in La 0.7 Ca 0.3 MnO 3. International Conference on Magnetic Materials, ICMM-2010 (pp ). Australia: American Institute of Physics. Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au

2 Multifunctionality From Coexistence Of Large Magnetoresistance And Magnetocaloric Effect In La 0.7 Ca 0.3 MnO 3 J. C. Debnath*, R. Zeng, J. H. Kim, S. X. Dou Institute for Superconducting and Electronic Materials, University of Wollongong, Northfields Ave., Wollongong, NSW 2522, Australia Abstract. We report the existence of multifunctionality involving large magnetoresistance (MR) and magnetocaloric effect (MCE) near room temperature in La 0.7 Ca 0.3 MnO 3. A peak in the temperature dependence of the magnetoresistance and a change in the magnetic entropy ( S M ) are observed at 251 K, with a maximum magnetoresistance MR (defined as (0) / (H) 1) of about 206 % and a maximum S M of approximately 7.54 J/kgK, at a field of 5 T, where the Curie temperature (T C ) and metal-insulator transition coexist at 251 K. The magnetic entropy change is measured on the basis of resistive measurements, which are close to the values obtained from the M-H curves in the intermediate temperature region, as well as close to T C. The large MR and large MCE suggest that the material is interesting for multiple applications. Keywords: Magnetoresistance; Magnetocaloric effect; Curie temperature. PACS: Replace this text with PACS numbers; choose from this list: INTRODUCTION Large magnetoresistance (MR) is the phenomenon where resistance of a material drops drastically on application of magnetic field. It has tremendous applications in recording information on hard disks, magnetic sensors, and spin-electronic devices [1, 2]. On the other hand, materials with a large magnetocaloric effect (MCE) have been recognized as a promising alternative to conventional gas refrigeration [3, 4]. The coexistence of large magnetoresistance (MR) and a large magnetocaloric effect in a compound should attract considerable attention for promising technological applications. According to the double exchange theory, electrons tend to hop between Mn ions of different valences while keeping their spins unchanged. Therefore, when the arrangement of the spins of the Mn ions is modified by external field, resistivity changes simultaneously. In this picture, the colossal magnetoresistance in the manganites can be qualitatively understood [5-6]. This explanation actually suggests a magnetic-resistive interplay in the manganites. Mills et al. have suggested the presence of electron-phonon interaction, which leads to the formation of lattice polarons that influence the magnetoresistive property of the oxides [7, 8]. In this letter, we report large MCE and MR in La 0.7 Ca 0.3 MnO 3.We also report the correlation between resistivity and change in the magnetic entropy ( S M ) in a specific region around T C. International Conference on Magnetic Materials (ICMM-2010) AIP Conf. Proc. 1347, (2011); doi: / American Institute of Physics /$

3 EXPERIMENTAL The manganite La 0.7 Ca 0.3 MnO 3 was prepared by the conventional solid state reaction method. Powder X-ray diffraction (XRD) patterns, revealed the single-phase orthorhombic perovskite structure. Magnetization and resistivity measurements were performed using a physical properties measurement system (PPMS). RESULT AND DISCUSSION The temperature dependence (T) of (0) is shown in the inset of Figure 1 (a), where a sharp peak indicating the metal to insulator transition (T MI ) is observed at 251 K. FIGURE 1. (a) Temperature (T) dependence of (0) - (1.5 T). Resistivity in zero field, (0), against temperature is shown in the top inset. (b) Temperature dependence of the resistivity,, measured under different magnetic fields. A significant change in was noticed when it was measured in a low field with H = 1.5 T. (0) - (1.5 T) is plotted against T in Figure 1 (a), where a typical feature of colossal MR, with a sharp peak in MR, is observed at 251 K, below which, MR shows a decreasing trend with decreasing temperature. We note that the Curie temperature, T C is observed at 251 K (not in figure), close to T MI. Resistance was measured using the conventional four-probe method. Figure 1 (b) illustrates the temperature dependence of resistivity under different magnetic fields. The sample is metallic at low temperatures and insulating at high temperatures. The metal-insulator (MI) transition takes place at T C for zero field, but shifts to higher temperature with applied filed. Due to the instability of magnetic polarons in an external magnetic field, the resistivity peak shifts to higher temperatures, and its magnitude drastically diminishes as one applies magnetic field, which also can be explained by the double exchange model. When field is applied, the angle between the Mn 3+ - O- Mn 4+ bonds is flattened, which is favourable for hopping. In addition to driving the MI transition to higher temperatures, an external field greatly depresses the resistivity. The MR ratio, defined as MR = ( (0)/ (H) 1), exhibits a maximum of ~206 % at T C under magnetic field of 5 T, as shown in Figure 2 (a). As expected, the MR and S M peaks appear simultaneously (inset in Fig. 2a). However, the detailed temperature 279

4 dependences of MR and S M are apparently different. This result reveals the absence of a simple relation between these two quantities. FIGURE 2. (a) Magnetoresistance plotted vs. temperature. The MR and magnetic entropy changes under a field of 5 T are shown in the top inset. (b) Magnetic entropy change as a function of temperature and magnetic field. Magnetic entropy change can be calculated from the magnetic data [9]. Figure 2 (b) presents S M as a function of temperature. As expected, the effect of external field on magnetic order is strongly temperature dependent, and the maximum variation of magnetic entropy occurs near T C, where S M shows a rapid decrease, resulting in an asymmetric peak. The maximum S M under a field of 5 T is ~ 7.54 J/kgK. It is well known that the MR of La 0.7 Ca 0.3 MnO 3 also undergoes a great change near the magnetic transition temperature due to the magnetic order. So, it is an interesting question whether a quantitative relation can be found between S M and MR. However, from the top inset in Figure 2(a), it is clear that a simple relation between these two quantities is absent. FIGURE 3. (a) Comparison of the magnetic entropy change vs. temperature ( S M - T relation) calculated from the magnetic data and the resistivity data. (b) Isothermal magnetization measured under the magnetic fields from 0 to 5 T. 280

5 The entropy changes determined by the resistivity measurement S M = - H δ ln( ρ) dh T and the Maxwell relation are shown in Figure 3(a), where the 0 δ H resistivity results agree quite well with the fitting parameter = 9.98 emu/g in the intermediate temperature range. It can be seen that deviation of the magnetic results from the resistivity measurements occurs in the temperature range below 245 K, where the system is in a nearly complete ferromagnetic state, and above 270 K, where the paramagnetic state prevails (Fig. 3b). This result implies that the resistivity measurements are in agreement with the magnetic data only in the intermediate temperature range, i.e., during the establishment of perfect ferromagnetic order. This also remind us of magnetic polarons, which prevail when magnetic disorder exists. It is obvious that magnetic disorder, characterized by S M, affects the magnetic polarons, while the magnetic polarons influence the electronic transport properties, which may be the underlying reason for the occurrence of the S M relation. In the low temperature region, where the system is in a relatively ordered magnetic state, the magnetic polarons are depressed significantly, which causes the deviation from the resistivity measurements. It is obvious that a larger means a more sensitive dependence of S M on. On the other hand, the stronger the magnetic scattering is, the smaller the will be. So it is expected that the smaller value of is due to the stronger magnetic scattering. CONCLUSION Large magnetoresistance (MR) and a large magnetocaloric effect (MCE) are observed in La 0.7 Ca 0.3 MnO 3. The maximum MR and MCE are, in magnetic field of 5 T, 206 % and ~7.54 J/kgK, respectively. A strong correlation between S M and is observed in the intermediate temperature range around T C. The exhibition of large MR and MCE near room temperature suggests the utility of the compound for multiple applications in magnetoresistive devices as well as magnetic refrigeration. ACKNOWLEDGMENTS This work was supported by the Australian Research Council (DP ). REFERENCES 1. M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas, Phys. Rev. Lett. 61, 2472, (1988). 2. G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn, Phys. Rev. B 39, 4828, (1989). 3. P. Debye, Ann. Phys. 386, 1154 (1926). 4. W. F. Giauque, J. Am. Chem. Soc. 49, 1864 (1927). 5. T. Y. Koo, S. H. Park, K.-B. Lee, and Y. H. Jeong, Appl. Phys. Lett. 71, 977 (1997). 6. H. L. Ju, C. Kwon, Q. Li, R. L. Greene, and T. Venkatesan, Appl. Phys. Lett. 65, 2108 (1994). 7. A. J. Millis, P. B. Littlewood, and B. J. Shraiman, Phys. Rev. Lett. 74, 5144 (1995) 8. Y. Yamada, O. Hino, S. Nohdo, R. Kanao, T. Lnami, and S. Katano, Phys. Rev. Lett. 77, 904 (1995). 9. V. K. Pecharsky and K. A. Gschneidner, Jr., Phys. Rev. Lett. 78, 4494 (1997). 281