Effect of annealing on polycrystalline La1−xNaxMnOz ceramics

Physics Letters A 277 (2000) 56–60 www.elsevier.nl/locate/pla

Effect of annealing on polycrystalline La1−x Nax MnOz ceramics Z.Q. Li a,b,∗ , E.Y. Jiang a , D.X. Zhang b , D.L. Hou a , W.C. Li a , H.L. Bai a a Department of Applied Physics, Tianjin University, Tianjin 300072, China b Department of Physics, Hebei University of Technology, Tianjin 300130, China

Received 28 July 2000; accepted 16 October 2000 Communicated by J. Flouquet

Abstract The structure, magnetic and transport properties of La1−x Nax MnOz manganese perovskite prepared by Pechini process were investigated. Low field magnetoresistance in temperature range from 77 K to Curie temperature was obtained and two obvious peaks were observed in the resistivity versus temperature curves. Differential scanning calorimetry results show that the broad bumps far below Curie temperature does not originate from anomalous phase transition. The effect of annealing is carefully investigated and the results indicate that oxygen inhomogeneous distribution may be the main reason for the curious behaviors.  2000 Elsevier Science B.V. All rights reserved. PACS: 75.30.V; 72.15.G; 72.20.M Keywords: Magnetoresistance; Transport properties; Annealing

Mixed manganese oxides R1−x Ax MnO3 (R stands for rare earth metals and A for divalent metals like Ca, Sr and Ba) with perovskite structure have been extensively studied due to the challenge of the theoretical understanding, as well as the application potentiality of the so-called colossal magnetoresistance (CMR) effect [1–3]. In generic CMR materials, the field need to exhibit the CMR effect is typically of the order of 1 to 5 T. Grain boundary effect has been studied by several groups [4–7] and perhaps this is a solution to the issue of high field and small temperature range. The basic interactions of the CMR materials include the so-called double exchange interaction between Mn–O–Mn nearest neighbors [8], the strong coupling between lattice degrees of freedom and the Mn eg electron configuration [9], and the Jahn–Teller

* Corresponding author.

E-mail address: [email protected] (Z.Q. Li).

distortion of the Mn–O octahedron [10]. However, it has recently been reported that much complex magnetic and transport behaviors, such as charge ordering of the Mn eg electrons [11,12] and electronic phase separation [13,14], could occur in the manganese oxides. There are still not so much experimental data on manganites doped with alkali metals up to now though some work had been done [15–18]. In the present letter, we report the structure, magnetic and transport properties of Na doped Lax Na1−x MnOz manganese perovskites prepared by Pechini process [19]. Low field magnetoresistance (MR) in the temperature range from 77 K to Curie temperature was observed. Additionally, in the curves of the resistivity versus temperature, two obvious peaks, one near Curie temperature and the other below Curie temperature, were also observed. The effect of annealing is carefully investigated and the results indicate that oxygen inhomogeneous distribution may be the main reason for the curious behaviors.

0375-9601/00/$ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 6 0 1 ( 0 0 ) 0 0 6 7 9 - 4

Z.Q. Li et al. / Physics Letters A 277 (2000) 56–60

57

Fig. 1. X-ray diffraction pattern of the as-prepared sample (No. 1).

Polycrystalline La1−x Nax MnOz samples were prepared as follows. Stoichiometric metal nitrates were dissolved in a mixture of citric acid and ethylene glycol. A clear solution was then acquired by heating the mixture at 363 K. The resulting solution was further heated at 413 K to induce esterification and distil out ethylene glycol. The viscous solution was then vacuum dried at 453 K to yield an organic polymer foam. Fine crystalline powders were produced by calcining the precursors in air at 1073 K for 6 h and furnace cooled. Finally the powders were pressed into discs and sintered at 1373 K for 10 h and then furnace cooled. The atomic fraction of elements was determined by inductive coupled plasma emission spectroscopy (ICP). Since directly determining oxygen content is difficult, the oxygen content measurement is not carried out in the present letter. The phase characterization and determination of lattice parameters were performed with X-ray diffraction (XRD) analysis using a Rigaku X-ray diffractometer (D/max-2500×). Scanning electron microscopy (SEM) was used to check grain size. Magnetic properties were measured by using vibrating sample magnetometer (VSM). Differential scanning calorimeter (DSC) was used to further determine the phase transition of the sample. Resistivity measurements were carried out by standard four probe method. Since the results taken from all

samples are similar, only the results taken from sample No. 1 are presented. The X-ray diffraction pattern is shown in Fig. 1 indicating a pure rhombohedral perovskite structure. One can deduce that the Na substitutes onto the La site in the rhombohedral perovskite lattice from the structure and the compositions [16–18]. Figs. 2(a) and (b) show the relations of the magnetization M and constant pressure thermal capacity CP as a function of temperature T , respectively. One can see that the Curie temperature TC determined from DSC is nearly equal to that obtained by VSM. From Fig. 2(b), it can also be seen that a second order phase transition occurs at TC [20]. The nominal compositions, lattice parameters, cell volumes and Curie temperatures of the samples are listed in Table 1, where each lattice parameter is obtained by fitting thirteen diffraction peaks. ¯ For rhombohedral LaMnO3+δ , the space group R 3c was deduced from powder neutron diffraction studies [21,22]. ah , ch and Vh express the lattice parame¯ space ters and unit cell volume in the hexagonal R 3c group and ar , α express the lattice parameter and an¯ space group. From Tagle in the rhombohedral R 3c ble 1, one can see that the Curie temperature increase monotonically with x and the lattice parameter ar and cell volume Vh decrease monotonically with increasing x except for sample No. 4.

58

Z.Q. Li et al. / Physics Letters A 277 (2000) 56–60

Table 1 Composition, lattice parameters, cell volume and Curie temperature of the samples No.

Composition

ah (Å)

ch (Å)

Vh (Å3 )

ar (Å)

α (degree)

TC (K)

1

La0.93 Na0.075 MnOz

5.5193

13.3663

352.612

5.4777

60.5032

264

2

La0.9 Na0.11 MnOz

5.5060

13.3043

349.287

5.4564

60.6024

301

3

La0.85 Na0.17 MnOz

5.5064

13.2659

348.329

5.4462

60.7336

323

4

La0.83 Na0.19 MnOz

5.4887

13.3637

348.645

5.4667

60.2660

325

1a

La0.93 Na0.075 MnOz0

5.5116

13.3358

350.826

5.4668

60.5424

267

3a

La0.85 Na0.17 MnOz0

5.4845

13.3048

346.577

5.4493

60.4274

325

a Sample annealed in air.

Fig. 2. (a) Magnetization vs. temperature. (b) Constant pressure heat capacity vs. temperature.

Figs. 3(a) and (b) illustrate the temperature-dependent resistivity and MR (MR = 1ρ/ρ0 , where ρ0 is the resistivity at zero field), respectively. From Fig. 3(a), one can clearly see two pronounced peaks, a sharp one at a higher temperature and a broad one at a lower temperature. The higher temperature peak just occurs at about TC , indicating that it originates from the phase transition from ferromagnetic to paramagnetic. The ‘double exchange’ model can qualitatively explain the simultaneous electronic and

Fig. 3. (a) Resistivity ρ vs. temperature T . (b) MR vs. temperature T for the as-prepared sample; dotted line: variation of MR with T for air annealed sample in 7.5 kOe.

magnetic transition. The second peak, which occurs at lower temperature, is surprising. Similar phenomenon was once observed by several groups [15–17, 23–25], but there is still no generally accepted explanation though some hypotheses have been given. The temperature-dependent CP curve shown in Fig. 2(b) indicates that no other new phase transition occurs around this temperature. This means that the broad bump is not caused by anomalous phase transition.

Z.Q. Li et al. / Physics Letters A 277 (2000) 56–60

To determine whether the phenomena are related to oxygen incorporation, the sample was annealed in air at 1173 K for 30 h and the relation between resistivity and temperature after annealing is also shown in Fig. 2(a). From the figure, one can see that the resistivity is considerably reduced and the bump at lower temperature is suppressed after annealing. (A slight shifting of the resistivity peak to higher temperature is observed.) It is well known that annealing in air at 1173 K for 30 h may improve oxygen content, modify the grain boundaries and increase particle size of samples. According to reports [26,27], oxygen deficiency can decrease metallic-semiconducting transition temperature and Curie temperature, lower the magnetization and increase resistivity, hence some oxygen deficiency grains (even if more smaller unit) mixing in a rich oxygen sample may lead to two bumps in the resistivity versus temperature curves. For a oxygendeficient sample and a rich oxygen sample, the magnitude changes of resistivity (up to 107 times [26,27]) are far larger than that of magnetization, and therefore the changes of the magnetization and Curies temperature is negligible for a sample which involves very small percentage of oxygen-deficient particles. This may be the case in the present study. This conclusion is consistent with the result that the cell volume is decreased after annealing [26] (Table 1). In a word, the sharp peak at TC is attributed to the rich oxygen parts of the sample and the lower temperature peak is attributed to the oxygen-deficient parts. The oxygen content of the oxygen-deficient domains is improved after air annealing and the peak at low temperature moves to higher temperature and overlaps the transition at Curie temperature. Rubinstein et al. [28] once used similar scenario to explain two MR bumps occurring in granular films. The dotted line in Fig. 2(a) shows the magnetization versus temperature of the sample annealed in air, which indicates that the Curie temperature is slight increased after annealing in air. (A slight increase of the magnetization in lower temperature is also observed.) Some recent research show that marked particle size reduction can also decrease the metallic-semiconducting transition temperature [29,30], hence particle size extreme inhomogeneity in sample may also produce double peaks in the of resistivity versus temperature curve. Figs. 4(a) and (b) show the SEM photographs of the as-prepared and the air annealed samples

59

Fig. 4. SEM photographs: (a) as-prepared sample, (b) sample annealed in air.

respectively, which clearly indicate that the average particle size is almost unchanged after annealing. Thus the two bumps phenomenon is not related to particle size changing. To determine whether the phenomenon are related to other thermal effects (such as modifications of grain boundary), we have annealed the as-prepared samples under argon at 1173 K for 30 h too. The resistivity dependence temperature is also shown in Fig. 3(a), which indicates that the two bumps remain, and therefore the sample is relative stable in argon atmosphere at 1173 K (which means argon annealing can not change the oxygen inhomogeneous situation). Similar effect of argon annealing was observed by Prellier et al. [31]. This also indicates that other thermal effects are negligible. Thus oxygen inhomogeneous distribution is considered to be the main reason for the two bumps. Fig. 3(b) shows the MR dependence of temperature T of the as-prepared sample. The large MR around

60

Z.Q. Li et al. / Physics Letters A 277 (2000) 56–60

Curie temperature mainly originates from the intrinsic CMR, while the large MR at lower temperature comes from spin-dependent transport across grain boundaries [5–7]. The MR versus temperature of the air annealed sample in 7.5 kOe is also shown in Fig. 3(b) (dotted line), which indicates that the magnitude of MR around Curie temperature less than that of the as-prepared sample. Contrasting the MR curves in Fig. 3(b), one can see the MR peak of the as-prepared sample around 160 K is related to the broad bump in resistivity versus temperature curve. From Fig. 3(b), one can see the wide temperature range (from 77 K to TC ) and relative low field MR in both sample. Interestingly, according to reports [15– 17] and our observation, it seems that the oxygen inhomogeneous distribution phenomenon most likely occurs in alkali-metal-doped manganites. In summary, polycrystalline alkali-metal-doped polycrystalline manganites have been prepared by Pechini process and their structure, magnetic and transport properties are investigated. The Curie temperature increase monotonically with Na doping values (0.075 6 x 6 0.19). For the as-prepared samples, the resistivity versus temperature shows two peaks, a sharp one at a higher temperature and a broad one at a lower temperature. DSC results show that no anomalous phase transition occurs at the lower temperature. The sharp peak at TC is attributed to the rich oxygen parts of the sample and the lower temperature peak is attributed to the oxygen-deficient parts.

References [1] R. Von Helmolt, B. Holzapfel et al., Phys. Rev. Lett. 71 (1993) 2331. [2] K. Chahara, T. Ohno et al., Appl. Phys. Lett. 63 (1993) 1990.

[3] S. Jin, T.H. Tiefel et al., Science 264 (1994) 413. [4] H.Y. Hwang, S.-W. Cheong et al., Phys. Rev. Lett. 77 (1996) 2041. [5] A. Gupta, G.Q. Gong et al., Phys. Rev. B 54 (1996) R15629. [6] C. Dubourdieu, M. Audier et al., J. Appl. Phys. 86 (1999) 6945. [7] N.D. Mathur, G. Burnell et al., Nature 387 (1997) 266. [8] C. Zener, Phys. Rev. 82 (1951) 403. [9] J.B. Goodenough, A. Wold et al., Phys. Rev. 124 (1961) 373. [10] J.B.A.A. Ellemans, B. Vanlarr et al., J. Solid State Chem. 3 (1971) 238. [11] A.P. Ramirez, P. Schiffer et al., Phys. Rev. Lett. 76 (1996) 3188. [12] W. Bao, C.H. Chen et al., Solid State Commun. 98 (1996) 55. [13] S. Yunoki, J. Hu et al., Phys. Rev. Lett. 80 (1998) 845. [14] G. Papavassiliou, M. Fardis et al., Phys. Rev. Lett. 84 (2000) 761. [15] M. Itoh, T. Shimura et al., Phys. Rev. B 52 (1995) 12522. [16] T. Hayashi, T. Shimura et al., J. Solid State Chem. 124 (1996) 250. [17] X.L. Wang et al., J. Appl. Phys. 83 (1998) 7177. [18] M. Sahana, R.N. Singh et al., Appl. Phys. Lett. 70 (1997) 2909. [19] W. Liu, G.C. Farrington et al., J. Electrochem. Soc. 143 (1996) 879. [20] L.E. Reichl, A Modern Course in Statistical Physics, University of Texas Press, Austin, 1980. [21] B.C. Tofied, W.R. Scott, J. Solid State Chem. 10 (1974) 183. [22] J.A.M. Van Roosmalen et al., J. Solid State Chem. 110 (1994) 100. [23] J.R. Sun et al., Appl. Phys. Lett. 72 (1998) 3208. [24] N. Zhang, W. Ding et al., Phys. Rev. B 56 (1997) 8138. [25] R. Suryanarayanan, J. Berthon et al., J. Appl. Phys. 83 (1998) 5264. [26] H.L. Ju, J. Gopalakrishnan et al., Phys. Rev. B 51 (1995) 6143. [27] A.M. De Léon-Guevara, P. Berthet et al., Phys. Rev. B 56 (1997) 6031. [28] M. Rubinstein, P.R. Broussard et al., J. Appl. Phys. 83 (1998) 7067. [29] R. Mahesh, R. Mahendiran et al., Appl. Phys. Lett. 68 (1996) 2291. [30] R.D. Sanchez, J. Rivas et al., Appl. Phys. Lett. 68 (1996) 134. [31] W. Prellier, M. Rajeswari et al., Appl. Phys. Lett. 75 (1999) 1446.