Spectroscopic evidence of electron doping in La0.7Ce0.3MnO3

Journal of Magnetism and Magnetic Materials 282 (2004) 237–239

Spectroscopic evidence of electron doping in La

_0.7

Ce

_0.3

MnO

₃

J.-Y. Lin

*, W.J. Chang

, J.Y. Juang

, T.M. Wen

, K.H. Wu

, Y.S. Gou

,

J.M. Lee

, J.M. Chen

a_{Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC} b_{Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC}

c_{National Synchrotron Research Center, Hsinchu 300, Taiwan, ROC}

Available online 10 May 2004

Abstract

X-ray absorption near edge spectroscopy was used to investigate the hole states in the La0.7Ce0.3MnO (LCeMO).A substantial decrease in the spectral weight of the egorbital was observed in LCeMO compared to that in the hole-doped

La0.7Ca0.3MnO.This directly proves that electron doping occurs in LCeMO. r2004 Elsevier B.V. All rights reserved.

PACS: 75.47; 85.75.
d; 75.70

Keywords: CMR; XANES; Electron doping

The colossal magnetoresistance (CMR) phe-nomenon in hole-doped rare-earth manganites R1
xAxMnO3(R: trivalent rare-earth element; A:

divalent alkaline earth ions) has attracted enthu-siasm for its dramatic MR effects near the Curie temperature (TC) and the generous phase diagram.

The doping of divalent cations leads Mn3+of the undoped manganite to form a mixture of Mn3+ and Mn4+, and drives a ferromagnetic metallic transition.In electron-doped manganites, a mix-ture of Mn2+ and Mn3+, the analogy to hole-doped manganites are still not clear.Unfortu-nately, the cerium-doped manganese oxide is usually mixed with unbinding Ce4+, which are in CeO2state.For polycrystalline samples, the effects

of CMR are from hole doping (lanthanum deficient) rather than electron doping.Very recently, the single-phase La0.7Ce0.3MnO3

(LCe-MO) epitaxial thin films have been successfully grown by pulsed laser deposition, using KrF excimer laser [1,2].This breakthrough could provide a novel opportunity to study the physical properties of electron-doped manganites and an alternative to reach CMR.

Although the single-phase LCeMO in the thin film form has been confirmed, its electronic structure has to be further investigated.Manga-nese has been verified to be in a mixture of Mn2+ and Mn3+ valence states [3].However, whether the electron doping occurs in the electronic structure of LCeMO is crucial to further studies of the properties and applications of this material. X-ray absorption near edge spectroscopy (XANES) has proven to be an effective tool to

ARTICLE IN PRESS

*Corresponding author.Tel.:573-1653; fax: +886-3-572-0728.

E-mail address:[email protected] (J.-Y. Lin).

0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.04.053

investigate the hole states in the complex com-pounds.In this paper, XANES clearly shows the evidence of the electron doping in the eg states of

Mn in LCeMO.

Epitaxial LCeMO films were grown on SrTiO3

(1 0 0) (STO) substrates by pulsed laser deposition (PLD), using a KrF (248 nm) excimer laser with a repetition rate of 5 Hz.For this study, the oxygen pressure of 0.35 Torr was set for all experiments as it gave the best results.X-ray diffraction data demonstrated the formation of single-phase LCe-MO in the thin films.The O K-edge and Mn L-edge X-ray absorption spectra were carried out at T ¼ 300 K using linear polarized synchrotron radiation from 6-m high-energy spherical grating monochromator beamline located at NSRRC in Taiwan.The energy resolution of the monochro-mator was about 0.2 and 0.1 eV for the O K-edge and Mn L-edge energy range, respectively.Details of XANES experiments can be found in Refs.

[4,5].The O K-edge and Mn L-edge spectra were in the fluorescence and the total electron mode, respectively.The saturation (or ‘‘self-absorption’’) effects were corrected for all measured O K-edge spectra.

DC magnetization MðTÞ and the resistivity rðT Þ of the LCeMO thin film are shown inFig.1.The ferromagnetic magnetic ordering happens at TCE255 K judged by the inflection point of the

MðT Þ curve.A ‘‘metal-to-insulator’’ transition for rðTÞ follows the magnetic ordering at almost identical temperature.The same phenomenon has been observed in the well-studied La0.7Ca0.3MnO3

(LCMO) with similar TC: Beyond these superficial

similarities, it is of interest to investigate the underlying mechanism in further detail.The magnetic and transport properties in LCMO are associated with the eg orbital.The unoccupied

states in e1_g lead to the magnetic ordering and the consequent rðT Þ through the double exchange mechanism.In the following, XANES results reveal a different electronic structure of LCeMO from that of LCMO.

Fig.2shows the Mn L-edge spectra of LCeMO, LCMO, and standard samples.In comparisons, it is clearly demonstrated that the mixed valence of Mn is between +2 and +3 in LCeMO, in contrast to that between +3 and +4 in LCMO.These

ARTICLE IN PRESS

Fig.1. The DC magnetization MðT Þ and the resistivity rðT Þ of the LCeMO thin film. MðT Þ was measured under field cooling.

640 644 648 652 656 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 T o ta l E le c tr o n Y iel d (arb. units)

Photon Energy (eV) MnO Mn2O3 MnO2 La0.7Ce0.3MnO3 La0.7Ca0.3MnO3 Mn-L3 Mn-L2

Fig.2.Mn L-edge XANES spectra show that Mn is in the mixture of +2 and +3 states.

J.-Y. Lin et al. / Journal of Magnetism and Magnetic Materials 282 (2004) 237–239 238

results can be taken as an indication of electron doping in LCeMO and are consistent with the previous report[3].O K-edge spectra are shown in

Fig.3 and provide further evidence.The O 2p states are hybridized with Mn 3d and other states. In Fig.3, these assignments were labeled to the associated spectral peaks.For the Mn 3d peak, the onset in LCeMO occurs at a higher energy than in LCMO.This is consistent with the electron doping by the rigid band model.More obviously, the spectral weight of the shoulder at 529 eV in LCMO, which is assigned to the egm states [6], is

much reduced in LCeMO.Therefore, there are more occupied egm states in LCeMO.

In summary, XANES provides strong evidence of electron doping in LCeMO.Whether the doped holes reside in the e2

gm or t2gk states as proposed

by Ref. [2] remains to be further investigated. More XANES measurements at low temperatures are currently underway.

This work was supported by National Science Council of Taiwan, Republic of China, under Contract No.NSC92-2112-M-009-032.

References

[1] C. Mitra, P. Raychaudhuri, J. John, S.K. Dhar, A.K. Nigam, R.Pinto, J.Appl.Phys.89 (2001) 524.

[2] C.Mitra, P.Raychaudhuri, K.D.orr, K.-H. M.uller, L. Schultz, P.M. Oppeneer, S. Wirth, Phys. Rev. Lett. 90 (2003) 017202.

[3] C.Mitra, Z.Hu, P.Raychaudhuri, S.Wirth, S.I.Csiszar, H.H. Hsieh, H.-J. Lin, C.T. Chen, L.H. Tjeng, Phys. Rev. B 67 (2003) 092404.

[4] J.M. Chen, R.S. Liu, J.G. Lin, C.Y. Huang, J.C. Ho, Phys. Rev.B 55 (1997) 14586.

[5] I.P. Hong, J.-Y. Lin, J.M. Chem, S. Chatterjee, S.J. Liu, Y.S. Gou, H.D. Yang, Europhys. Lett. 58 (2002) 126. [6] J.-H. Park, C.T. Chen, S.-W. Cheong, W. Bao, G. Meigs, V.

Chakarian, Y.U. Idzerda, Phys. Rev. Lett. 76 (1996) 4215; J.-H. Park, unpublished.

ARTICLE IN PRESS

Photon energy (eV)

(Mbarn

/unit cell)

Fig.3.Comparisons of the O K-edge XANES spectra of LCeMO and LCMO indicate the electron doping in the eg

states of LCeMO.