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Journal of Magnetism and Magnetic Materials 270 (2004) 244–246
Reply to the ‘‘Comment on papers ‘Effect of Ag substitution on the transport property and magnetoresistance of LaMnO3’ [J. Magn. Magn. Mater. 248 (2002) 26] and ‘Possible magnetic phase separation in Ru doped La0.67Ca0.33’ [J. Magn. Magn. Mater. 257 (2003) 195]’’$ S.L. Yea,b, W.H. Songa,b, J.M. Daia,b, K.Y. Wanga,b, S.G. Wanga,b, C.L. Zhanga,b, J.J. Dua,b, Y.P. Suna,c,*, J. Fangb a
Institute of Solid State Physics, Key Lab of Internal Friction and Defects in Solids, Chinese Academy of Sciences, PO Box 1129, 230031 Hefei, People’s Republic of China b National High Magnetic Field Laboratory, 230031 Hefei, People’s Republic of China c Key Lab of Structure Research, University of Science and Technology of China, 230026 Hefei, People’s Republic of China Received 23 June 2003
Abstract The author of the comment is too unilateral to associate double-peak behaviour (broad insulator–metal (I–M) transition below the Curie temperature (Tc )) and a sharp one near (Tc ) in rare-earth manganese perovskites only with the grain boundaries (GB) effect. Moreover, it is especially confused that the grain-size effect should be necessary to relate with this electronic transport behaviour. In this reply, we provide an argument in support of the statement that inhomogeneous electronic/magnetic phase separation is one of the most important reasons for the double-peak behaviour in the rðTÞ curve. r 2003 Elsevier B.V. All rights reserved. PACS: 71.70.Ej; 71.30.+h; 75.30.Vn Keywords: Ag-doping; Insulator–metal transition; Inhomogeneity; Phase separation (PS); Grain boundaries (GB) effect
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PII of original articles: S0304-8853(02)00017-3 and S03048853(02)01171-X. doi of comment: 10.1016/j.mmm.2003.08. 021. *Corresponding author. Institute of Solid State Physics, Key Lab of Internal Friction and Defects in Solids, Chinese Academy of Sciences, PO Box 1129, 230031 Hefei, People’s Republic of China. Tel.: +86-551-559-1436; fax: +86-551-5591434. E-mail address:
[email protected] (Y.P. Sun).
This is in reply to the comment by E. Rozenberg on our paper ‘‘Effect of Ag substitution on the transport property and magnetoresistance of LaMnO3’’ (J. Magn. Magn. Mater. 248 (2002) 26). Perovskite-type (ABO3) doped manganese oxides have generated a considerable interest because of their many electronic, magnetic and structural properties and potential applications. Extensive
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.08.022
ARTICLE IN PRESS S.L. Ye et al. / Journal of Magnetism and Magnetic Materials 270 (2004) 244–246
theoretical and experimental efforts have been made to understand their complicated mechanisms [1]. It has revealed that the local inhomogeneity of the electronic/magnetic/lattice structure often plays a crucial role in their properties. Moreover, it has found that the competition between different phases is very sensitive to the particular kind of disorder in the compound, such as A- or B-site disorder, grain-size effects or intragranular strain, which can induce a rich variety in the electronic transport property [2,3]. One notable phenomenon shown by the electronic property is the doublepeak behaviour, i.e., an additional broad I–M transition besides the sharp one below TC on rðTÞ curve in some polycrystalline manganese oxides [4]. Some studies show that the modification of the grain-size has an obvious effect on the I–M transition. However, it is also found that other sources of defects, such as oxygen content and its spatial distribution can produce similar phenomenon even with the same grain-size [5]. Recently, more and more works find that the really crucial reason is the spatial distribution of the coexisting phases, i.e., the coexistence of the antiferromagnetic insulating (AFI) phases, ferromagnetic insulating (FMI) phases and ferromagnetic metallic (FMM) phases. In our studied La1xAgxMnO3 (0:05pxp0:5) bulk samples, to offer a complementary understanding on the complicated mechanisms in distorted perovskite manganates, we focused on the monovalent metal doping effects on the electronic transport properties [6]. In the earlier paper, Fig. 3 shows the samples with xo0:2 exhibit another broad I–M transition at a lower temperature TP2 besides the sharp I–M transition at TP1 accompanying the para-ferromagnetic transition. Moreover, it shows that the broad I–M transition at TP2 shifts to TP1 and gradually disappears with the increase of Ag-doped level. In order to analyse the grain boundary effect, the size of this system has been evaluated by the formula in Rietveld program as shown in Table 2 in our earlier paper. The calculated results obviously present no appreciable differences in grain size for all Ag-doped samples. However, from Fig. 3, it can be easily seen that the double-peak behaviour disappears with increase in the Ag-doped level. So it is natural
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to conclude that the double-peak behaviour in the transport property of the La1xAgxMnO3 system have no direct relation with the grain-size effect. In fact, many studies have shown that the effect of the A- or B-site disorder on the electronic transport is closely related to the size of A or B cations (average cationic radius and size effect mismatch) and to the Mn3+/Mn4+ ratio [7], which can induce the redistribution of the AFI, FMM, FMI phases associated with the percolative transport through the ferromagnetic phases and has an obvious influence on the I–M transition. In the studied La1xAgxMnO3 system, the large difference in valences of La3+ and Ag1+ ions and the random distribution of La3+ and Ag1+ ions in the A-site are probably causing the electronic/magnetic inhomogeneity which can induce phase or domain separation. As we know, I–M transition is observed when the metallic phases are connected in a percolation manner [8]. Therefore, with decrease of the temperature, sometimes for some systems there are no metallic phases sufficiently in number to connect each other even when the systems display paramagnetic to ferromagnetic transition, which leads to no I–M transition or another broad I–M transition with lower temperature shown in the electronic transport behaviour. This phenomenon is what we observed in our low-doped Ag level with xo0:2: There is no basic contradiction between Ref. [9] and our experimental results. In the former, the author himself suggests that the difference of the inhomogeneity of the samples is responsible for the difference in the behaviour on rðTÞ curve of the samples with different sintering temperatures. In the meanwhile, we agree that the oxygen content and the grain surface can also play a distinctive role in the spatial distribution of coexisting phases. On the other hand, the preparing procedures such as time and temperature of thermal treatments, the reactive atmosphere, even the grinding time also have effect on the distribution of the phases in the bulk samples, no matter what the grain size is. In a word, the grain boundary is not the only reason for the appearance of the broad I–M transition, specially the variation of the grain size is not the only reason for
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the disappearance of the broad I–M transition. The doubt of the author of the comment regarding the conclusion that the transport property of the La1xAgxMnO3 system has no direct relation with the grain-size effect can also be solved with these interpretations. As to question coming from the author of the comment, our explanation is as follows: (i) The author of the comment insists that the low-temperature transport properties in manganites must be attributed to the grain boundaries effect. However, the interpretation for this point is still being disputed [10,11]. In fact, we have not discussed the low-temperature character in our earlier paper and the real reason needs to be investigated further. (ii) The effect of the applied magnetic field is to influence the content of the ferromagnetic metallic phases, which leads to magnetoresistance in manganites. It is not reasonable to just associate the magnetic field effect with grain boundaries. (iii) The AC susceptibility versus temperature dependence in our earlier paper is to show the paramagnetic-to-ferromagnetic transition. The author of the comment confuses magnetic transition with distribution of AFI phases and FMI phases. In conclusion, it is unilateral to attribute all the inhomogeneity of the polycrystalline-doped manganites to the presence of the grain boundaries regions. We suggest the author of the comment to make physical picture about the origin of the I–M transition in perovskite manganates
and the essential effect of the grain boundaries clear.
Acknowledgements This work was supported by the National Key Research under contract No. 001CB610604, and the National Nature Science Foundation of China under contract No. 10174085, 10074066, Anhui Province NSF Grant Nos. 00046115, 03046201 and the Fundamental Bureau Chinese Academy of Sciences.
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