Discussion

In the following, we try to put the experimental observations presented above together to see if it can be reconciled to the outstanding issues about the CMR effects on Sn-doped manganites that we set forth to resolve at the beginning. The very first question to be answered is whether or not Sn-ions can substitute La-sites? The results obtained from the as-deposited films indicated that the substitution of the tetravalent Sn into the La site does prevail and result in a ferromagnetic-insulating manganite. The unexpected insulating behavior thus suggests either the lack of itinerant carriers or a fundamentally different exchange mechanism in the electron-doped regime. In the former scenario, similar double-exchange mechanism as in the hole-doped manganites gives rise to the paramagnetic-ferromagnetic transition and the existence of Mn²⁺ due to tetravalent doping of Sn⁴⁺ should result in enhancement of saturation magnetization below T_c owing to contributions from extra eg electrons. However, due to the charge localization effects associated with strain-induced lattice distortion, the lack of itinerant electrons not only has hindered effective transport but also is responsible for the absence of long range FM order as manifested in the dramatic reduction of saturation magnetization and marked spin-glass-like behavior displayed in Fig. 3.1－3.3. The large epitaxial strain originally existent in the AD-film as revealed by the x-ray and X-TEM analyses seemed to give consistent support. On the other hand, the existence of Mn²⁺ in as-deposited and prolonged air exposure LCMO films was found to yield charge localization, and hence increasing resistivity and reducing magnetization, as well.²⁵ However, the effects of Mn²⁺-induced charge localization seemed too modest to drive it into ferromagnetic

insulator. Another possibility for explaining why our AD LSnMO films present as a ferromagnetic insulator is that the doping level (x＝0.3) is too low to drive the LSnMO into n-type manganite. Chang et al. [12] indicated that the one of the main effects of doping Ce (up to x ＝ 0.3) into LaMnO3 was to dramatically shrink the hole-pockets near the Fermi level, instead of providing itinerant electrons and driving LCeMO into a n-type manganite. We believe that, from the evidence exhibited by XAS (especially on Sn ion) and EELS of XTEM analyses, for AD-LSnMO films, effective doping is obtained.

However, the tetravalent- doping-induced hole-pocket shrinkage effects similar to that observed in Ce-doped manganites [12] may also happen in the present AD-LSnMO films.

Together with the influences of the coherent strain and, possibly larger ion-size effect, thus resulted in the ferromagnetic insulating as well as spin-glass-like behaviors displayed by the AD-LSnMO films.

On the other hand, although post annealing under oxygen or argon is usually practiced in the fabrication of the “electron-doped” manganites, our experimental results demonstrate that it may not work as expected. According the XAS of Sn, we are tempted to rule out the existence of Sn²⁺ ions in the LSnMO films. The XAS of Sn for the PA-LSnMO film evidently shows the characteristics of Sn⁴⁺ valence. On the contrary, the same spectrum for the Ar-annealed film does not show the characteristics of either Sn²⁺ or Sn⁴⁺, suggesting that the doped Sn in the initial target material may have been largely missing in the ArPA-LSnMO film. The question is how do these spectrum results correlated to the magneto-transport properties displayed by the respective films? The TEM microstructure analyses and the EELS elemental map of PA-LSnMO film evidently showed that there is some kind of Sn-O compound formed during the post-annealing. The

unambiguous Sn⁴⁺ feature of XAS results strongly implies that within the oxygen annealing affected regions it may have indeed induced the formation of the La-deficient manganite with SnO2 clusters. The significant improvement of the CMR effects, including high Tc, TIM, low resistivity and relatively large magnetization at low temperatures, is naturally explained within the context of this scenario. Finally, for the ArPA-LSnMO films, although also exhibit transitions from paramagnetic insulator to ferromagnetic metal albeit at a significantly lower temperature with higher resistivity, the detailed mechanism may be fundamentally different from the oxygen-annealed PA-LSnMO films. In particular, the XAS of Sn for this film showed most of the Sn was missing after annealing. Thus, although the reduced average unit-cell volume for the ArPA- and PA-LSnMO films is indicative of strain relaxation, it is doubtful to attribute it as the sole reason for the obtained enhancement on the magneto-transport properties. It is quite possible that the change of average lattice parameters upon annealing is also coupled with change of composition, or even segregation of second phases. In this scenario, the excessive oxygen presumably present in the PA-LSnMO films would provide more itinerant carriers and Mn⁴⁺ content³², and is responsible for the improvement of electrical transport and magnetic properties. While Ar-annealing, though may induce the loss of Sn and hence result in La-deficient manganite, it may also provoke the formation of non-ferromagnetic Mn²⁺ component and give rise to charge localization-induced increase in resistivity and reduction in saturated magnetization [25].

Finally, we make a brief comparison between LSnMO and LCeMO, the two representative “electron-doped” manganites. The primary distinction of these two materials is the ionic size difference of Sn⁴⁺ (0.81Å) and Ce⁴⁺ (0.97Å) [31]. One of the

instant effects resulted from this was the low temperature saturation magnetization obtained from the as-deposited films with M_LCeMO ~ 1.4μ_B/Mn-site [13] and M_LSnMO ~ 0.4μ_B/Mn-site measured under T＝10K, H＝100Oe. This may be easily attributed to more severe structure disorder originated from the larger ionic size difference between Sn⁴⁺ and La³⁺ (0.32Å) than that between Ce⁴⁺ and La³⁺ (0.16Å). However, as the films were annealed under oxygen environment, both LSnMO and LCeMO exhibit similar magneto-transport properties. The existence of SnO2 or CeO2 [13]seemed to indicate that both films tend to turn into La-deficient manganites after annealing. However, for as-deposited PLD LCeMO films, there are still some discrepancies on the magneto-transport properties reported in the literature remained to be clarified [9-13].

4. Summary

In summary, we have presented systematic investigations on one of the highly anticipated electron-doped CMR materials. Single-phase La_0.7Sn_0.3MnO₃ (LSnMO) thin films were grown on LaAlO₃ substrates by pulsed laser deposition. The as-deposited LSnMO films are ferromagnetic insulators with typical Curie temperature around 150 K.

Both the electronic structure revealed by the x-ray absorption spectra (XAS) and the detailed TEM analyses indicate that in this case the doped Sn-element is indeed acting as the tetravalent ion uniformly distributed in the LaMnO3 parent compound. The large internal strain originated from the marked ion size difference between the doped Sn⁴⁺ and substituted La³⁺ ions, however, is believed to hinder the long-range itinerancy of the carriers, hence preventing it from becoming metallic. Unfortunately, due to the insulating nature of these as-deposited LSnMO films, it is not clear whether it is indeed “n-type”

electron-doped manganite. Ex-situ annealing in oxygen and argon both drive the films to exhibit insulator-metal transition with hole-doped characteristics when becoming ferromagnetic. The transition temperatures, however, are different for films annealed in different environments, presumably due to the final phase and compositions obtained.

From the results of magnetoresistance measurements and XAS, it is suggestive that LSnMO films annealed in argon causes the significant loss of Sn and results in La-deficient phase. Whereas those annealed in oxygen appeared to form some kind of Sn-O compound, turning the films into La-deficient manganite, albeit with some excessive oxygen. We emphasize that the existence of tetravalent Sn from x-ray absorption spectroscopy (XAS) should not be taken as the sole evidence of achieving electron-doped manganite. As being clearly demonstrated in this study, it may just reveal

the emergence of SnO₂.

References

[1] Y. Tokura and N. Nagaosa, Science 288, 462 (2000), and references therein.

[2] E. Dagotto, “Nanoscale Phase Separation and Colossal Magnetoresistance” (Springer, 2003), and references there in.

[3] C. Zener, Phys. Rev. 82, 403 (1951).

[4] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74, 5144 (1995).

[5] V. L. Joseph Joly, P. A. Joy, S. K. Date, J. Magn. Magn. Mater. 247, 316 (2002).

[6] S. Das and P. Mandal, Z. Phys. B 104, 7 (1997); P. Mandal and S. Das, Phys. Rev. B 56, 15073 (1997).

[7] A. Gupta, T. R. McGuire, P. R. Duncombe, M. Rupp, J. Z. Sun, W. J. Gallagher, and Gang Xiao, Appl. Phys. Lett. 67, 3494 (1995).

[8] S. Pingard, H. Vincent, J. P. Senateur, J. Pierre, and A. Abrutis, J. Appl. Phys, 82, 4445 (1997).

[9] C. Mitra, P. Raychaudhuri, J. John, S.K. Dhar, A.K. Nigam, and R. Pinto, J. Appl.

Phys. 89, 524 (2001).

[10] W. J. Chang, C. C. Hsieh, J. Y. Juang, K. H. Wu, T. M. Uen, Y. S. Gou, C. H. Hsu, and J.-Y. Lin, J. Appl. Phys. 96, 4357 (2004).

[11] T. Yanagida, T. Kanki, B. Vilquin, H. Tanaka, and T. Kawai, Solid State Comm. 129, 785 (2004).

[12] W. J. Chang, J. Y. Tsai, H.-T. Jeng, J.-Y. Lin, Kenneth Y.-J. Zhang, H. L. Liu, J. M.

Lee, J. M. Chen, K. H. Wu, T. M. Uen, Y. S. Gou, and J. Y. Juang, Phys. Rev. B 72, 132410 (2005).

[13] Takeshi Yanagida, Teruo Kanki, Bertrand Vilquin, Hidekazu Tanaka, and Tomoji

Kawai, Phys. Rev. B 70, 184437 (2004).

[14] Guotai Tan, X. Zhang and Zhenghao Chen, J. Appl. Phys. 95, 6322 (2004).

[15] Ping Duan, Zhenghao Chen, Shouyu Dai, Yueliang Zhou, Huibin Lu, Kuijuan Jin, and Bolin Cheng, Appl. Phys. Lett. 84, 4741 (2004).

[16] J. Gao, S. Y. Dai, and T. K. Li, Phys. Rev. B 67, 153403 (2003).

[17] X. Guo, S. Dai, Y. Zhou, G. Yang, and Z. Chen, Appl. Phys. Lett. 75, 3378 (1999).

[18] X. Guo, Z. Chen, S. Dai, Y. Zhou, R. Li, H. Zhang, B. Shen, and H. Cheng, J. Appl.

Phys. 88, 4758 (2000).

[19] J. A. Mydosh. Spin Galsses: an Experimental Introduction. Taylor & Francis, Lodon, 1993.

[20] S. de Brion, F. Ciorcas, G. Chouteau, P. Lejay, P. Radaelli, and C. Chaillout, Phys.

Rev. B 59, 1304 (1999).

[21] T. Y. Cheng, C. C. Hsieh, J. Y. Juang, J.-Y. Lin, K. H. Wu, T. M. Uen, Y. S. Gou, C.

H. Hsu, Physica B 365, 141 (2005).

[22] M. R. Ibarra, P. A. Algarabel, C. Marquina, J. Blasco, and J. García, Phys. Rev. Lett.

75, 3541(1995).

[23] P. G. Radaelli, D. E. Cox, M. Marezio, S.-W. Cheong, P. E. Schiffer, and A.P.

Ramirez, Phys. Rev. Lett. 75, 4488 (1995).

[24] J. M. De Teresa, M. R. Ibarra, J. García, J. Blasco, C. Ritter, P.A. Algarabel, C.

Marquina, and A. del Moral, Phys. Rev. Lett. 76, 3392 (1996).

[25] S. Valencia, A. Gaupp, W. Gudat, Ll. Abad, Ll. Balcells, A. Cavallaro, B. Martínez, and F.J. Palomares, Phys. Rev. B 73, 104402 (2006).

[26] S. J. Kim, C. S. Kim, S. I. Park, and B. W. Lee, J. Appl. Phys. 89, 7416 (2001).

[27] B. C. Hauback, H. Fjellvag, and N. Sakai, J. Solid State Chem. 124, 43 (1996).

[28] G. J. Chen, Y. H. Chang, and H. W. Hsu, J. Magn. Magn. Mater. 219, 317 (2000).

[29] R. Suryanarayanan, J. Berthon, I. Zelenay, B. Martínez, X. Obradors, S. Uma, and E.

Gemelin, J. Appl. Phys. 83, 5264 (1998).

[30] K. A. Thomas, P. S. I. P. N. de Silva, L. F. Cohen, A. Hossain, M. Rajeswari, T.

Venkateson, R. Hiskes, and J. L. MacManus-Driscoll, J. Appl. Phys. 84, 3939 (1998).

[31] R. D. Shannon, Acta Crystallogr. A 32, 752 (1976).

[32] S. Valencia, Ll. Balcells, J. Fontcuberta, and B. Martínez, Appl. Phys. Lett. 82, 4531 (2003).