Anisotropic O 2p-Mn 3d unoccupied states in La1-xMnO3: an X-ray absorption spectroscopy study

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Journal of Magnetism and Magnetic Materials 239 (2002) 48–50

Anisotropic O 2p-Mn 3d unoccupied states in La

_1x

MnO

₃

: an

X-ray absorption spectroscopy study

J.-Y. Lin

a,

*, C.W. Chen

a

, Y.C. Liu

a

, S.J. Liu

b

, K.H. Wu

b

, Y.S. Gou

b

, J.M. Chen

c

a

Institute of Physics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC

b

Department of Electrohysics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC

c

Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, ROC

Abstract

We have studied La1xMnO3thin ﬁlms, which show the metal–insulator transition similar to La1xCaxMnO3. The

transport and magnetic properties are measured. X-ray absorption spectroscopy is employed to investigate the electronic structure. Surprisingly, the electronic structure of La1xMnO3shows large anisotropy. Our results suggest

Keywords: CMR; XANES

Colossal magnetoresistance (CMR) manganites have generated much interest of research and potential applications recently. From scientific point of view, these compounds belong to a novel strongly correlated system in which phonons, carriers, and magnetism strongly interact with each other. For example, the coexistence of ferromagnetic transition and the metal– insulator transition is conventionally explained by the mechanism of double exchange (plus polarons). To further investigate these interesting and profound phenomena in this system, the understanding of the electronic structure is definitely indispensable. Recently, XANES studies on CMR manganites have provided valuable information of the electronic states in this system [1,2]. These studies suggest that the CMR manganites are charge-transfer materials. The transport properties are closely related to the carriers with a significant O 2p character.

Self-doped La1xMnO3 shows the metal–insulator

transition similar to the well-known La1xCaxMnO3.

This metal–insulator transition and the associated magnetic ordering are supposed to be due to the La deﬁciency in the lattice [3]. It is presumed that the

mechanism for the magnetic ordering and the metal– insulator transition is the same in both La1xMnO3and

La1xCaxMnO3. However, the electronic structure of

La1xMnO3 is actually much less investigated.

There-fore, we have conducted studies of La1xMnO3to shed

light on its electronic structure.

La0.8MnO3 and La0.7MnO3 targets with nominal

compositions were made by the standard solid state reaction method. Thin ﬁlms were deposited on LaAlO3

(1 0 0) substrates by laser pulsed ablation. A KrF excimer laser was employed. The substrate temperature was kept at 7501C during deposition and the oxygen partial pressure was 250 mTorr. Thickness of both films is about 300 nm. X-ray diffraction patterns of 2y scan in Fig. 1 show that both films are highly textured. This allows the studies of the crystalline direction dependence of the electronic structure by the polarized X-ray absorption near edge structure (XANES). The XANES in florescence mode is a powerful tool to investigate the unoccupied (hole) electronic states in complex materials and is bulk sensitive. In particular, polarized XANES can give information about the anisotropy of the electronic structure. Energy resolution of the mono-chromator is about 0.1 eV for the O K-edge energy range. The saturation (or ‘‘self-absorption’’) effects were corrected for all measured spectra. The spectra were *Corresponding author. Fax: +886-3-572-0728.

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

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taken at room temperature, and normalized to the tabulated standard absorption cross section in the energy range from 600 to 620 eV.

Both La1xMnO3thin ﬁlms show the metal–insulator

transition and the associated ferromagnetic ordering. The temperature of the maximum resistivity Tp

is 267 and 282 K for x ¼ 0:2 and 0.3, respectively. Fig. 2 shows both resistivity r and magnetization M for La0.8MnO3. It can be seen that Tp and the

Curie temperature TC is nearly identical. The same

correlation was observed in La0.7MnO3. This

identiﬁca-tion between Tpand TCis curious since La1xMnO3is

considered as a disorder system due to La deﬁciency. In disordered manganite, Tp is usually signiﬁcantly lower

than TC: [4].

The spectra of O K-edge XANES in Fig. 3 show another intriguing results. These spectra correspond to O 2p character mixed with the conduction band. The main peaks are assigned to be associated with O 2p-Mn 3d, O 2p-La 5d, and O 2p-Mn 4sp hybridization around 530, 535, and 543 eV, respectively. The spectrum with E8abor E8c provides the information of the crystalline dependence of the electronic structure. As shown in Fig. 3, the spectra show certain degree of anisotropy. Especially, the O 2p-Mn 3d band, which is presumably responsible for the transport and magnetic properties, is Fig. 1. 2y X-ray diﬀraction patterns show well oriented (0 0 1)

La1xMnO3thin ﬁlms.

Fig. 2. The metal–insulator transition in La1xMnO3is

asso-ciated with a ferromagnetic ordering. The applied ﬁeld for magnetization measurements is 10 Oe.

Fig. 3. XANES of La1xMnO3for both E8ab and E8c: (a) x ¼

0:2; (b) x ¼ 0:3:

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highly anisotropic. Although the spectra cannot resolve whether there are changes of the shape of the peak around 530 eV, the intensity for E8c is clearly larger. Naively, it can be interpreted as that there are more hybridized unoccupied states along the ‘‘c-axis’’ direc-tion. This is surprising. Though with a rhombohedral structure in the bulk form [3], La1xMnO3is thought to

be cubic when deposited into thin ﬁlms [5,6]. Actually, even for pseudocubic or rhombohedral La1xCaxMnO3

(and other manganites), the electronic structure are considered to be isotropic since the a angle for the rhombohedral structure of all these manganites is very close to 601. Detailed structure studies of our La1x

-MnO3thin ﬁlms are currently underway. X-ray

absorp-tion near edge structure was also measured for a LaMnO3thin ﬁlm (not shown). Indeed, nearly identical

spectra were observed. It is likely that the anisotropic electronic structure of La1xMnO3is due to the lattice

distortion induced by the La deﬁciency. When the lattice is distorted, the bond angle of Mn–O–Mn becomes smaller. However, the decrease in the bond angle along ab is faster than that along c; thus leads to the anisotropy of the electronic structure. It would be interesting to see if the anisotropy manifests itself in the magnetotransport anisotropy. This work is currently underway.

To conclude, we have investigated the properties of La1xMnO3 by magnetic, transport, and XANES

measurements. Especially, XANES reveals the aniso-tropic electronic structure in La1xMnO3. This

aniso-tropy, together with other properties, suggests that La1xMnO3is probably unique among manganites.

This work was supported by National Science Council of Republic of China under contract No. NSC89-2112-M-009-052.

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