Solid State Communications 131 (2004) 779–783 www.elsevier.com/locate/ssc
Lanthanum deficiency effect in magnetic transition and transport property of La0.8KdCa0.2MnO3 Chang Seop Honga, Nam Hwi Hurb,*, Yong Nam Choic a
Department of Chemistry and Center for Electro- and Photo-Responsive Molecules, Korea University, Seoul 136-701, South Korea Center for CMR Materials, Korea Research Institute of Standards and Science, Yusong, PO Box 102, Daejeon 305-600, South Korea c Neutron Physics Department, HANARO center, Korea Atomic Energy Research Institute, Yusong, PO Box 105, Daejeon 305-600, South Korea b
Received 26 February 2004; accepted 30 June 2004 by H. Akai Available online 29 July 2004
Abstract The evolution of magnetic and electrical phases in La0.8KdCa0.2MnO3 was investigated in terms of La deficiency. We found that the increase of the La deficiency tends to raise the Curie temperature (TC) in La0.8KdCa0.2MnO3. The FM clusters formed in compounds with large La deficiency provide percolation paths above TC. With increasing the La defect, the transport property changes from insulating to metallic state, which is in association with the crossover from a second order to a first order magnetic phase transition in the vicinity of TC. q 2004 Elsevier Ltd. All rights reserved. PACS: 71.30.Ch; 75.30.Kz; 75.40.Cx Keywords: A. Magnetically ordered materials; A. Perovskite manganites; D. Colossal magnetoresistance
Perovskite manganite has received much attention in recent year largely due to its rich physics and fascinating transport phenomenon called as colossal magneto resistence (CMR) [1]. In general, the double exchange model has described the underlying physics of the manganite exhibiting CMR. Recent results, however, show that phase separation should be involved to explain the basic physics of the doped manganite. For instance, an optimally doped La0.7Ca0.3MnO3 showed the coexistence of metallic ferromagnetic (FM) and insulating paramagnetic (PM) domains below TC, which was elucidated by scanning tunneling spectroscopy [2]. The CMR phenomenon found in this material was interpreted by a percolation of metallic FM phases. Moreover, De Teresa et al. provided the evidence for magnetic polaron even above TC by using volume thermal
* Corresponding author. Tel.: C82-428-685-233; fax: C82-428685-475. E-mail address:
[email protected] (N.H. Hur). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.06.036
expansion, magnetic susceptibility, and small angle scattering measurements [3]. On the other hand, the correlation between charge and lattice distortion suggests that the possibility of the first order phase transition near TC is aroused in the doped manganite [4]. Along this line, a first order metal–insulator (MI) transition in Ln0.7A0.3MnO3, where Ln is a trivelent rare earth ion and A is a divalent alkali earth ion was reported in view of the temperature hysteresis in resistivity and the reduction in cell volume [5]. Archibald et al. explained the observation of the discontinuous transition as a first order decrease in the mean Mn–O bond length [6]. In contrast, Mira et al. argued that the first order character is not universal for CMR materials since La2/3Sr1/3MnO3 exhibits a second order transition [7]. Our recent result revealed that the nature of magnetic phase transition is sensitive to Ca concentration in the La1Kx CaxMnO3 system [8]. To understand the correlation between magnetic phase transition and CMR behavior in more detail, we have studied the 20% doped sample La0.8Ca0.2MnO3 that lies in
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the boundary between metallic and insulating states below TC [9]. The Mn valence state of this material, which is crucial for determining the magnetic and electrical properties, can be easily tuned by creating vacancies in the La site. Accordingly, the percolation scenario can be examined in the La defect samples having inhomogeneous phases. In this report, we present the magnetic transition and transport property in the La0.8KdCa0.2MnO3 samples. We found that the La deficiency induces the enhancement of TC, the formation of FM clusters, and the transition from insulating to metallic states below TC. The crossover from second order to first order magnetic phase transition near TC is also observed. Interestingly, the high MR is found in the first order compound rather than in the second order one. Polycrystalline samples of La0.8(1Kx)Ca0.2MnO3 (xZ0.0, 0.05, 0.1, 0.2, and 0.3) were prepared by solid state reaction of appropriate mixtures of La2O3, CaCO3, and MnCO3 in air at 1000 8C for 20 h. The calcined samples were pulverized, pelletized, and sintered at 1300 8C for 48 h with intermediate regrinding. Initial structural characterization of the resulting products was made by powder X-ray diffraction using a Rigaku RAD diffractometer (Cu Ka radiation). Neutron diffraction data were collected at 300 K over the 2q range of 08 to 1608 with a step size of 0.058 on a highresolution powder diffractometer at HANARO Center in KAERI. The diffraction data were refined by the Rietveld method, using the Fullprof program. Magnetization and resistivity measurements were performed on a SQUID magnetometer (Quantum Design). Powder neutron diffraction data for xZ0.0, 0.1, 0.2, and 0.3 show that they are iso-structural and single phase. All of the four samples were successfully indexed with an orthorhombic symmetry space group Pnma, which provided satisfactory refinements as listed in Table 1. Changing the La content, while the Ca and Mn doping levels are held constant, results in systematic variations in lattice constants without any structural change. With increasing the La defect, all the three lattice parameters monotonously decrease. Fig. 1 displays the normalized field-cooled (FC) magnetization curves (M/M5K) measured at 100 G for La0.8(1Kx)Ca0.2MnO3. For xZ0.0 and 0.05, PM to FM transitions are observed at 165 and 170 K, respectively. Interestingly, additional magnetic transitions are also found below TC mainly due to magnetic inhomogeneity [10]. A notable feature is that the FM transition temperature increases with increasing x. This feature is likely associated with the fact that the La deficiency in the lattice produces the Mn4C ions on account of self-doping and consequently enhances the double exchange interaction [11]. The increase of the Mn4C concentration with increasing the La deficiency is also indirectly supported by the M(H) data given in the inset of Fig. 1. As anticipated, the saturated magnetization values of the La-deficient samples decrease with increasing x. For xZ0.2 and 0.3, remarkably, magnetic tails were
Fig. 1. Temperature dependence of normalized magnetization for La0.8(1Kx)Ca0.2MnO3, where applied field is 100 G. The inset shows the field dependence of magnetization for La0.8(1Kx)Ca0.2MnO3 at 10 K.
observed above TC in the M(T) plots. The tails are survived up to w250 K, which are clearly seen in the inverse magnetic susceptibility curves in the top panel of Fig. 2. The cK1(T) data of xZ0.2 and 0.3 exhibit sudden drops around 250 K. On the other hand, any anomalous transition is absent in the cK1(T) curve of xZ0.05. In order to understand the nature of magnetic tails, we have taken the M(H) data for the three samples at constant temperatures, which are given in the bottom panel of Fig. 2. Here the measured temperatures are about 10 K higher than their corresponding Curie temperatures. Namely, magnetization data were collected in the PM range. The M(H) curves of xZ0.2 and 0.3 exhibit the presence of FM component even above TC, indicating that FM clusters are evolved in the PM region. Presumably, the FM clusters are related to the occurrence of the magnetic tails. Due to the La deficiency, the neighboring site around the La defect has higher Mn4C content than the other area, which leads to the formation of the FM clusters in the La-deficient sites and eventually promotes a double-exchange mediated FM coupling. Moreover, the magnetic tails in xZ0.2 and 0.3 subsist up to about 250 K, which is much higher than their Curie temperatures. This clearly suggests that the FM clusters are embedded in the PM matrix. It is worth noting that xZ0.05 has a typical PM character, which also supports that the magnetic tails are ascribed to the La deficiency. To assess the effect of the La deficiency on magneto transport property, we have measured the temperature dependence of resistivity with and without a magnetic field, shown in the top panel of Fig. 3. With increasing x, the
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Table 1 Structural parameters for La0.8(1Kx)Ca0.2MnO3 at 300 K based on the neutron powder diffraction data x
˚) a (A
˚) b (A
˚) c (A
˚ 3) V (A
c2
0.0 0.1 0.2 0.3
5.5024(4) 5.4859(2) 5.4853(3) 5.4771(3)
7.7720(4) 7.7563(3) 7.7551(5) 7.7433(4)
5.5083(3) 5.5023(2) 5.4955(4) 5.4906(3)
235.56(2) 234.12(2) 233.77(3) 232.86(2)
2.79 1.85 2.23 2.35
magnitude in resistivity is diminished, which is due to the enhanced double-exchange contribution to the electrical conduction created by the generation of Mn4C ions near the La-deficiency sites. For the xZ0.0 and 0.05 samples noticeable MI transitions (TMI) were not observed near TC. The resistivity in the PM region was well fitted with the small polaron hopping model [12]. For samples with large La defect, MI transitions were clearly seen in the resistivity curves near TC [13]. A notable feature is that the metallic transitions (TMI) of xZ0.2 and 0.3 occur at temperatures much higher than TC. It is thus evident that the FM clusters embedded in the PM matrix, as seen in the M(T) and M(H) curves, reach a percolation threshold at which electrons can easily hop from one FM cluster to neighboring FM sites. Apparently, the FM clusters found in xZ0.2 and 0.3 play an essential role in the percolation process far above TC. It is worth mentioning that the MI transition in xZ0.1, where no FM clusters are discernible above TC in the M(T) and M(H) plots, coincides with the Curie temperature. This is
Fig. 2. Temperature dependence of inverse magnetic susceptibility (top) and field dependence of magnetization in low field region (bottom) for La0.8(1Kx)Ca0.2MnO3.
obviously associated with the absence of FM clusters above TC in xZ0.1. As shown in the top panel of Fig. 3, large reduction of resistance was found in xZ0.1, 0.2, 0.3 near TC upon applying an external field. The MR curves, defined as (rHKr0)/r0, where r0 stands for the resistivity at a zero magnetic field and rH at 5 T, are illustrated in the bottom panel of Fig. 3. The MR peaks are observed near TC for xZ0.1 but above TC for xZ0.2 and 0.3 due to the presence of FM clusters. In order to elucidate the nature of the magnetic transition, we have carried out magnetization measurement as a function of magnetic field. Fig. 4 represents H/M vs M2 isotherm curves from which the order of the transition can be obtained. Provided that the magnetic free energy (G) around TC is expressed as G ¼ G0 þ a2 ðT K TC ÞM 2 þ a4 M 4 þ .; second order transition should have the positive coefficient of a4 [14]. Based on this criterion, the transition in xZ0.05 is second order since slopes are positive in all M2 range. This result is consistent with that of La0.8Ca0.2MnO3 [8]. For xZ0.1, the slope becomes negative, implying that the transition changes from second order to first order. The
Fig. 3. Resistivity (top) and MR (bottom) for La0.8(1Kx)Ca0.2MnO3 versus temperature.
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Fig. 4. Isotherms of H/M vs M2. Numbers denote temperatures of the isotherms.
transition in xZ0.3 also belongs to first order. For xZ0.2, however, an intriguing feature is found in that the slope is positive in the low M2 range but the drastic slope variation occurs in the high M2 region. The slope change is reminiscent of the first order feature as observed in xZ 0.3. This mixed phase transition may be attributed to the presence of the FM clusters in the PM matrix. Earlier study on the La1KzCazMnO3 system revealed that the order of transition depends on the valence state of the Mn ion [8], where zZ0.2 and 0.3 are second order and first order, respectively. This implicates that the first order character is
associated with the FM clusters and the second order is originated from the PM–FM transition. This conjecture seems reasonable in the sense that the major background phase has a TC of 190 K, which is close to that of xZ0.05 with the second order character. A noteworthy feature is that the second order samples are insulating across temperature while the first order ones are varied from insulating to metallic state with decreasing temperature. Therefore, the crossover from polaronic electrons to itinerant conduction carriers may be responsible for the first order phase transition [6]. This discontinuity in the phase transition
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appears to affect MR more significantly as evidenced in MR(T) plots. In summary, we have investigated the magnetic transition and transport property of La0.8(1Kx)Ca0.2MnO3. An important finding is that the magnetic tails are observed in the M(T) data of xZ0.2 and 0.3. Based on the inverse magnetic susceptibility and M(H) results, the FM clusters are developed around the La-deficient sites and are embedded in the PM background, opening percolation paths and generating a MI transition above TC. The La deficiency leads to the change of the magnetic transition from second order to first order around TC. The first order transition, associated with the crossover from polaronic to itinerant electrons, has a higher MR value than the second order one in the La-deficient system.
Acknowledgements The Creative Research Initiative Program financially sponsored this work.
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