Cooling by adiabatic pressure application in La0.7Ca0.3MnO3 magnetocaloric effect material

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 1589–1591 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Cooling by adiabatic pressure application in La0.7Ca0.3MnO3 magnetocaloric effect material R. Szymczak a, R. Kolano b, A. Kolano-Burian b, J. Pietosa a, H. Szymczak a, a b Institute of Physics, Polish Academy of Sciences, al.Lotnikow 32/46, Warsaw, Poland  Institute of Non-Ferrous Metals, Sowinskiego 5, 44-101 Gliwice, Poland a r t i c l e in fo abstract Available online 9 September 2009 The effect of hydrostatic pressure on Curie temperature, the critical exponents and entropy change in La0.7Ca0.3MnO3 are determined. The pressure increases the Curie temperature. The combined magnetic entropy change, due to both magnetic field and pressure application, is found to be significant. It suggests that manganites are suitable candidates as barocaloric refrigerants near room-temperature region. & 2009 Elsevier B.V. All rights reserved. Keywords: Magnetocaloric effect Magnetic refrigeration Manganite 1. Introduction Recently, there has been an upsurge of interest in the development of a new magnetic refrigeration technology, based on magnetocaloric effect, as a promising alternative to the conventional gas compression technique. A number of new materials with large magnetocaloric effect have been discovered but the search is still on materials with even higher values of the magnetocaloric effect to use in magnetic refrigerators. Recently, it has been shown that manganites are good candidates to work as magnetic refrigerants at room-temperature region [1]. We have shown [2] that the changes in entropy near Curie temperature (TC) depend strongly on various extrinsic factors. These results suggest that the magnitude of the magnetocaloric effect should depend strongly on methods of sample preparation. We have performed detailed studies on the magnetocaloric effect for La1  xCaxMnO3 with x= 0.3, 0.35 and 0.4 prepared by nonstandard ceramic method. It was shown that in this case the sharpness of the paramagnetic to ferromagnetic transition increases with Ca doping. In all studied samples paramagnetic–ferromagnetic transition is very narrow but no hysteresis was observed and the transitions were identified as second-order ones. Accordingly, the values of the critical exponents were determined for these transitions. The maximum entropy change detected in La0.7Ca0.3MnO3 for a field of 2 T reaches 8 J/kg K, which exceeds that of gadolinium. It is the highest value ever observed for doped LaMnO3 manganites. The values of the critical exponents of La0.7Ca0.3MnO3 (b =0.5 and g =1) are satisfactorily described in frames of the mean field model.  Corresponding author. E-mail address: szymh@ifpan.edu.pl (H. Szymczak). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.09.020 An alternative technique for clean cooling devices was proposed by Müller et al. [3]. This technique is based on the barocaloric effect. The basic principle is analogous to cooling by the magnetocaloric effect; however, for the barocaloric effect an entropy change is realized by the application of external pressure. In this paper we report on first measurements of the impact of hydrostatic pressure on magnetocaloric effect in La0.7Ca0.3MnO3 manganite. Since the paramagnetic–ferromagnetic transition in this manganite is very narrow the effect of pressure on Curie temperature was expected to be high. Though the pressure dependence of magnetic properties has been studied in manganites [4–12], to the best of our knowledge this is the first report on the influence of pressure on magnetocaloric effect and on the barocaloric effect in this group of materials. 2. Experimental Details on the preparation and characterization of La0.7Ca0.3MnO3 manganite can be found in Ref. [2]. A miniature hydrostatic pressure cell was used for magnetization measurements in a commercial (Quantum Design Ltd.) SQUID magnetometer. The pressure value was determined at low temperatures using the known pressure dependence of the critical temperature of the superconducting state of a Pb sensor placed inside the cell. The magnetization has been measured under pressure up to 11 kbar at temperatures from 4.2 to 300 K and in magnetic fields up to 50 kOe. Fig. 1 shows the temperature dependence of the zero field cooled (ZFC) and field cooled (FC) magnetization of La0.7Ca0.3MnO3 in a field of 20 Oe at ambient pressure and for an applied pressure of 11 kbar. As seen in the figure, the paramagnetic to ferromagnetic transition is very sharp and the sharpness increases with pressure. The Curie temperature, TC, determined by the inflection point in the low-field (H=20 Oe) M–T curve, was found to be 250 and ARTICLE IN PRESS 1590 R. Szymczak et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 1589–1591 La0.7 Ca0.3 MnO3 P = 11 kbar 50 kO e H = 20 Oe ZFC FC 0.1 P = 11 kbar P=0 −ΔS mag [J/(kg K)] M/H [emu/(g Oe)] 8 La0.7 Ca0.3 MnO3 0.2 6 20 4 10 2 0.0 200 250 5 300 2 T [K] 0 Fig. 1. Temperature dependence of the ZFC and FC magnetization in a field of 20 Oe at ambient pressure and for applied pressure of 11 kbar for the La0.7Ca0.3MnO3. 275 270 280 T [K] Fig. 3. Magnetic entropy changes for La0.7Ca0.3MnO3 under magnetic field variation DH for applied pressure of 11 kbar. 8 La0.7 Ca0.3 MnO3 P=0 relations to isothermal magnetization M(H) measurements Z H  @M DSmag ðT; HÞ ¼ dH @T 0 -ΔS mag [J/(kg K)] 20 kO e 6 10 4 5 2 2 0 240 245 250 255 260 T [K] performed at ambient pressure and under pressure of 11 kbar. As expected, |DSmag(T, H)| becomes the maximum at TC where the magnetization drops rapidly. Magnetic entropy change in La0.7Ca0.3MnO3 is reported in Figs. 2 and 3 and shows a decrease in |DSmag| under pressure. At the same time the width of the peak in DSmag(T, H) dependence considerably increases under pressure. It means that pressure increases the refrigerant capacity RC of the system defined as RC = DS DT, where DT is the operating temperature range. This effect is important for possible applications of manganites as magnetic refrigerants. Fig. 2. Magnetic entropy changes for La0.7Ca0.3MnO3 under magnetic field variation DH at ambient pressure. 3. Discussion and conclusions 268.5 K for samples at ambient pressure and for applied pressure of 11 kbar, respectively. Although the sharpness of the transition indicates a first-order character there is no thermal hysteresis characteristic of this type of transition. It is possible to resolve the above ambiguity applying the Banerjee criterion [13], according to which the negative slope of the H/M versus M2 plot implies that the system possesses a first-order phase transition, whereas a positive slope corresponds to a second-order magnetic phase transition. Examining the slope of the H/M versus M2 plots (not shown here) indicates the second-order character of this transition. Therefore, it may be characterized by critical exponents. The critical exponents at ambient pressure were determined to be b =0.2 and g =1.7 and for applied pressure of 11 kbar, b =0.1 and g =2. Such a small value of b indicates that this phase transition can be characterized as nearly first order [14]. The La0.7Ca0.3MnO3 manganites show a large difference between ZFC and FC magnetization curves, enhanced by applied pressure. The difference between ZFC and FC magnetization curves arises due to domain structure and the observed enhancement of this effect is due to the pressure-induced suppression of magnetic anisotropy observed also in other manganites [15]. Magnetocaloric effect was determined in this paper by magnetic entropy change DSmag, obtained by applying the Maxwell It is commonly accepted that double-exchange interactions between Mn3 + and Mn4 + ions are responsible for ferromagnetism in the metallic phase of manganites. The effect of hydrostatic pressure on the ferromagnetic manganites with strong double exchange is, at least qualitatively, well understood. As it results from Fig. 1 the hydrostatic pressure increases TC and consequently stabilizes ferromagnetic state. The application of pressure induces the increase in the electronic transfer through the change in the Mn–O–Mn bond angles and bond lengths. As a result, the increase in the eg electron bandwidth W, described by empirical formula WEcos Y/[Mn–O]3.5 where Y = 12(p-/Mn–O–MnS), /Mn–O–MnS) is the Mn–O–Mn bond angle and [Mn–O] is the bond length, favors charge delocalization. This should lead to more pronounced ferromagnetic and metallic properties and stabilization of the ferromagnetic metal state through an enhancement of the double-exchange interactions and finally to an increase in TC under pressure. Nevertheless, although pressure increases volume of the ferromagnetic phase (determined by the spontaneous moment) the system under pressure remains inhomogeneous. It is confirmed by magnetization measurements presented in Figs. 4 and 5 (for comparison). Although the magnetization reaches a high value indicating on ferromagnetic ordering, the presence of unsaturated ARTICLE IN PRESS R. Szymczak et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 1589–1591 60 M [emu/g] 50 La0.7 Ca0.3 MnO3 P = 11 kbar 40 266 k 268 270 272 274 276 278 280 30 20 10 0 0 10 20 30 40 50 H [kOe] Fig. 4. Magnetization as a function of temperature for La0.7Ca0.3MnO3 under applied pressure of 11 kbar. 1591 where N is the number of magnetic spins, S is average spin momentum at magnetic sites, M and TC are magnetization and Curie temperature for sample under pressure, respectively, whereas MS is M for T= 0. The calculations give DSmag = 24.3 J/kg K. This value is rather high in comparison with the magnetic entropy change due to the application of magnetic field. To our knowledge, this is the first time the value of pure barocaloric effect in manganites has been reported. In conclusion, the magnetocaloric effect in La0.7Ca0.3MnO3 manganite under hydrostatic pressure has been investigated. It was shown that the applied pressure itself can be used to change the entropy of a solid in a significant way, without the need for magnetic field variation. The results obtained suggest that the combined effect of magnetic field and applied hydrostatic pressure on the caloric properties of manganites is very promising for constructing magnetic refrigerators. Although practical realization of this suggestion is not simple, one should nevertheless consider manganites to be suitable candidates as barocaloric refrigerants. Acknowledgements This work was partly supported by Ministry of Science and Higher Education of Poland Project PBZ-KBN-115/T08/2004. References Fig. 5. Magnetization as a function of magnetic-field and temperature for La0.7Ca0.3MnO3 under ambient pressure. magnetization even at 5 T is an indication of a phase separation into ferromagnetic and antiferromagnetic domains. Using the above results one can calculate the magnetic entropy change DSmag for H= 0 due to the hydrostatic pressure. These calculations are performed under the assumption that the pressure has negligible effect on phononic and electronic contribution to the entropy of the studied system. It seems that the mean-field approximation is a reasonable method to perform such estimation. The magnetic entropy change DSmag is calculated (following [15]) for T= TC, where TC is taken for La0.7Ca0.3MnO3 at ambient pressure (TC =250 K) DSmag ¼ Smag ðp; TC Þ  Smag ðp ¼ 0; TC Þ Smag ¼ NkB ½lnð2S þ 1Þ  3=2ðS=S þ1Þs2 9=20½ð2S þ 1Þ4  1=16ðS þ1Þ4 s4  s ¼ M=MS ¼ tan h½3SsTC =ðS þ 1ÞT [1] M.-H. Phan, S.-C. Yu, J. Magn. Magn. Mater. 308 (2007) 325. [2] R. Szymczak, M. Czepelak, R. 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