Electronic structures of Nd1−xSrxMnO3, La1−xSrxMnO3 and La1−x CaxMnO3 studied by photoemission and inverse photoemission spectroscopy

JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopyand Related Phenomena 88-91 (1998) 281-285

Electronic structures of Ndl_xSrxMnO3, Lal_xSrxMnO3 and Lal_x CaxMnO3 studied by photoemission and inverse photoemission spectroscopy Y. Kuwata a, S. Suga a'*, S. Imada a, A. Sekiyama a, S. Ueda a, T. Iwasaki a, H. Harada a, T. Muro a, T. Fukawa a, K. Ashida a, H. Yoshioka a, T. Terauchi a, J. Sameshima a, H. Kuwahara b, Y. Moritomo lb, Y. T o k u r a b'c aDepartment of Material Physics, Faculty of Engineering Science, Osaka University, 1-3, Machikaneyama, Toyonaka 560, Japan bJoint Research Centerfor Atom Technology (JRCAT), Tsukuba 305, Japan CDepartment of Applied Physics, University of Tokyo, Tokyo 113, Japan

Abstract

We have investigated the electronic structures of perovskite manganese oxides, Nd0.sSr0.sMnO3, (Nd0.062Sm0.938)0.sSro.sMnO3, La0.84Sro.16MnO3and La0.TCao.3MnO3,by X-ray photoemission (XPS) and X-ray bremsstrahlung isochromat spectroscopy in addition to vacuum ultraviolet inverse photoemission. It is found that the VUV-IPES spectra seem to be well explained by the band-structure calculation. Mn 3p-3d resonance photoemission measurements have also been performed, where the contributions of the occupied Mn 3d states are extracted. We observed hybridization between the Mn 3d and O 2p states from the Mn 2p and 3p spectra of all samples, and the Mn spin state from the Mn 3s XPS spectra of Nd0.sSr0.sMnO3 and (Nd0.062Sm0.938)0.sSr0.sMnO3. © Elsevier Science B.V. © 1998 Elsevier Science B.V. Keywords: Manganite; Inverse-photoemission; Resonance photoemission; Magnetism

1. Introduction

Recently, there has been a surge of interest in perovskite manganese oxides because of the discovery of colossal magnetoresistance [1,2]. It has gradually been clarified that the effects of electron correlation and the interplay of spin, charge and lattice derive complex properties. In this system, the Mn ion is octahedrally coordinated by the O atoms and the Mn 3d states are split into the upper eg and lower t2g * Corresponding author. Tel.: + 81 68506420;fax: + 81 68502845; e-mail: [email protected] t Present address: Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-01, Japan.

components. A nominal d 4 (t3ge~) compound, LaMnO3, is an A-type antiferromagnetic Mott insulator due to strong correlation effect in a half-filled band [3]. In the case of carrier (hole) doped systems, however, the eg state strongly hybridized with the O 2p orbitals could be rather itinerant. On the other hand, the t2g electrons are localized and in the high spin state of S = 3/2 according to the weak hybridization with the O 2p states. The eg electrons are ferromagnetically coupled with the t2g electrons due to the Hund rule. In the present work, we have investigated perovskite manganese oxides Nd0.sSr0.sMnO3 (hereafter termed NSMO), (Nd0.062Sm0.938)0.sSr0.sMnO 3 (NSSMO), La0.84Sr0.16MnO3 (LSMO) and La0.TCa0.3MnO3 (LCMO) by resonant photoemission

0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0368-2048(97)00140-0

282

Y. Kuwata et aL/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 281-285

spectroscopy (RPES), X-ray photoemission spectroscopy (XPS), X-ray Bremsstrahlung isochromat spectroscopy (X-BIS), and vacuum ultraviolet inverse photoemission spectroscopy (VUV-IPES). NSMO is a ferromagnetic metal below --250 K and undergoes a transition to a charge-ordered antiferromagnetic insulator below 158 K [1]. NSSMO is a ferromagnetic metal below 113K after once showing chargeordering instability and semiconductor behavior between --113 and --200 K [2]. The crystal structure of NSMO and NSSMO is the same, and the difference in the electronic properties between these two samples is derived from the change of the tolerance factor, which is rather equivalent to the eg band width, and the electron correlation. LSMO and LCMO become ferromagnetic below - 2 6 0 and - 2 4 0 K.

i

La

i

Sr

0.84

\ ~

Lasd- '. ._.,...,...-,,

''".:...'-':.'-" ~.,," ~,~,j..~.,-. :,. .,:,.,.:~ X-BIS

.~

,

"-" .~

Nd

I

,

Figure 1 shows the XPS, X-BIS and VUV-IPES spectra of NSMO and LSMO. A spectrum taken at hv = 55 eV is also shown in the figure. In LSMO [4], the prominent peak at 8.6 eV above the Fermi level (EF) in the X-BIS spectrum is exclusively assigned to the La 4f I final state. In the VUV-IPES spectrum of LSMO, a broad structure from --5 to

I

,

I

,

I

Sr MnO 0.5 0.5 3

,

i

-15

,

VUV-IPES .,.~".,. . ~

hv=55eV _ ...".''"".'.

....

I

f,...... Mn 3d

2. Experimental

3. Results and discussion

VUV-IPES

~4~ /

3

x~s

XPS

Single crystal samples were grown by the floating zone method [1,2]. In order to obtain clean surfaces, the samples were scraped in situ with a diamond file at respective temperature. RPES spectra were taken at BL-3B of the Photon Factory using a SCIENTA SES200 analyzer. Sample temperatures were set to - 2 7 0 , 160 and 100 K. XPS measurements were performed by use of a twin-anode Mg/AI X-ray source and a double pass cylindrical mirror analyzer. X-BIS spectra were taken at the detecting photon energy of hv = 1486.6 eV by use of a home built instrument. Both X-ray measurements were done at --130K. The VUV-IPES spectra were measured at room temperature for the detecting photon energy of hv = 9.4 eV. The resolutions of these measurements were better than 50 meV for RPES, 1.0 eV for XPS, 0.7 eV for X-BIS and VUV-IPES.

MnO

0.16

- 2/

f"

}

.'~

".

i

i

-10

-5

i

"% +-Sr-4d "2"-

.."

r

i

i

0

5

i

i

10

relative energy to EF (eV) Fig. 1. Inverse photoemission spectra (VUV-IPES and X-BIS) and valence-band XPS spectra of La084Sro.16MnO3 and Nd05Sr0.5MnO> A spectrum taken at hv = 55 eV is also shown.

--8 eV and a shoulder structure centered around 1.8 eV with a tail toward EF are observed. The former is mostly due to the La 5d band and the latter originates from the Mn 3d states. In the case of NSMO, the prominent peak at --6.8 eV in the X-BIS is assigned to the Nd 4f 4 final state, which is rather broad compared with the La 4f peak. The structure in 5-12 eV in the VUV-IPES spectrum is mainly due to the Nd 5d and Sr 4d states. The shoulder structure centered around 1.8 eV in the VUV-IPES is attributed to the Mn 3d states, which is also found in the X-BIS spectrum. Previously, X-BIS spectra of a series of polycrystalline Lal_,SrxMnO3 were reported by Chainani et al. [5]. In their spectra, the peak derived from the Mn 3d states was observed at --2.4 eV, whose line shapes are somewhat different from the 1.8 eV shoulders in our spectra. The origin of the discrepancy is not clear. As for the photoemission spectra, there is a broad peak from EF to --9 eV for each XPS spectrum. The XPS spectrum of LSMO is similar to the spectra reported by Saitoh et al. [6]. If we compare the XPS spectrum

283

Y. Kuwata et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 281-285

of NSMO with that of LSMO, the intensity from - 6 to - 3 eV is rather enhanced, indicating that the Nd 4f components are located there (such a result is confirmed by our Nd 3 d - 4 f resonance photoemission). The intensity of a high-resolution spectrum (hv = 55 eV: thin solid curve) taken at 270 K rapidly decreases toward EF. According to Sarma et al., the band-structure calculation using the local-spin density approximation (LSDA) well reproduces the XPS spectrum of LaMnO3 [7]. However, the LSDA calculation for LaMnO3 predicts that the unoccupied Mn 3d components make a strong peak centered at - 1 . 0 eV [8], which does not well explain the 1.8 eV structure in our VUV-IPES spectra. One may consider that the discrepancy is attributed to a possible chemical shift due to hole-doping, but it is experimentally known that the energy shift of the eg states due to hole-doping is < 0.4 eV for the photoemission side [6] and --0.0eV for the inverse-photoemission side [5]. Furthermore, the 1.8 eV structure in our spectra is almost identical between the LSMO and NSMO, although the amounts of hole-doping are remarkably different. On the other hand, band calculation using the LDA + U1 method, where an additional Coulomb interaction between the 3d electrons is taken into account compared with the LSDA method, shows that the unoccupied eg up (majority spin) and t2g down (minority spin) states are distributed from - 1 to - 3 eV and above 4 eV, respectively [9]. If we assume that the observed 1.8 eV structure is mainly due to the eg up states, the LDA + U1 calculation explains our spectra better than the LSDA calculation. As for the t2g down states, they are not observed as distinct structures. Two possible reasons are considered. One is that the LDA + UI correctly gives the energy distribution of the t2g down states but the overlap with the rare earth 5d states broadens the whole structure. The other possibility is that the dynamical effect of the self-energy may shift the peak energy of the t2g down states to --2 eV as reported in a photoemission spectrum of FeSi [10]. In order to further clarify the unoccupied Mn 3d states, resonance inverse-photoemission and high-resolution inversephotoemission studies are required. Valence-band spectra of NSSMO (A), NSMO (B), LSMO (C) and LCMO (D) are shown in Fig. 2. There is a peak at --37, --37 and --34 eV in A, B and C,

•

]130K ..... Sm 5s

Sr4p O 2s f

~

'~

.

i

,

i

(a)

~• Nd 5p

La5p

: '. : : : : J

.=o

i

',~s.~5p

I

: I : ', : : : ,

~

Nd 0 5Sro 5MnO3

. (1~

v

270K

I ............ , eV

/'~

'i "~ . // i satellite

4s 4 0 3 ' 5 3 0 ' 2 ' s ' 2 b ' l ' 5

lo

I

i1

s

o

binding energy (eV) Fig. 2. XPS (a) spectra of (Nd0.062Smo.938)0.sSr0.sMnO3 (A), Nd0.sSr0.sMnO3 (B), La0.84Sr0.16MnO3 (C) and La0.TCa0.3MnO3 (D). (b) RPES spectra. The difference spectrum between one measured at hu = 55 eV and one at hu = 48 eV is also shown. assigned to the Sm 5s, Nd 5s and La 5s states, respectively. The strong and broad peaks around --20 eV observed in all spectra show different line shapes from each other. In the case of NSSMO (NSMO), the peak at 19 eV and the shoulder at 21.5 eV are mainly due to the Sm 5p (Nd 5p) and O 2s electrons, which are also observed in the high-resolution spectra of NSMO as shown in Fig. 2(b). As for LSMO and LCMO, the peaks at 17 eV and 19.5 eV are assigned to the La 5p states, while the O 2s contributions also appear at 21.5 eV. In addition, the peak at 25 eV seen only for LCMO is attributed to the Ca 3p orbitals. It is noticed that the peak due to the O 2s states is clearly seen only for LCMO. This result suggests that the Sr 4p states are located between 19.5 and 21.5 eV. Comparing the broad peaks from EF to 9 eV in the XPS spectra, the peak for NSSMO seems to relatively shift toward higher binding energy. The photoionization cross sections for the Sm 4f orbitals are relatively larger than the others at hu = 1253.6 and 1486.6 eV [11]. Moreover, the Sm 4f final states are observed from - 3 eV to - 9 eV, which have been confirmed by

284

Y. Kuwata et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 281-285

the Sm 4 d - 4 f resonance photoemission for NSSMO [12]. Therefore, the energy shift originates from the Sm 4f contribution overlapping with the O 2p and Mn 3d states. Figure 2(b) shows the Mn 3p-3d resonance photoemission spectra of NSMO measured at hv = 55 eV and 48 eV, respectively, and at 270 K, where the integral backgrounds have been already subtracted. The structure around 19 eV derived from the Nd 5p, Sr 4p and O 2s states is stronger for hv = 48 eV than for 55 eV since the photoionization cross sections of these orbitals decrease remarkably with increasing photon energy [11]. On the other hand, a satellite structure in 11-17 eV is clearly seen for hv = 55 eV. This satellite is attributed to the resonance enhancement of the Mn 3d components. Such an enhancement has also been observed in LaMnO3 and in SrMnO3 by Saitoh et al. [6]. The difference spectrum between 55 and 48 eV is added in the figure. A significant enhancement is also observed at 1-4 eV in the spectrum taken at hv = 55 eV as judged from the difference spectrum with a strong peak at 2.5 eV. Such structures are ascribed to the Mn 3d (t2g) states. We also note that the enhanced component includes a small amount of the Nd 4f contributions because of the difference of the photoionization cross section for the Nd 4f orbitals between these photon energies. The contribution from the eg states, whose binding energy is expected to be lower than that of the t2g states, is very weak even in the difference spectrum. In the case of Mn 2 p - 3 d resonance photoemission spectra, however, the eg contribution has been clearly seen from EF to --1.5eV as a shoulder structure [13] due to the much stronger resonance for the 2p core excitation. The resonance behavior for the 2p and 3p core excitation is generally different between the t2g and eg states. This effect may also be contributing to the present result. In Fig. 3(a), the Mn 2p XPS spectra of NSSMO, NSMO, LSMO and LCMO are shown. All the spectra display 2p3/2 and 2p]/2 spin-orbit doublet peaks located at - 6 4 2 and - 6 5 4 e V , respectively. As shown in Fig. 3(a), the Mn 2p spectrum of NSSMO overlaps with the Sm MNN Auger structures. These Mn 2p spectra show the satellite structures at around ~664 eV ( ~ 10 eV above the main peak). According to Saitoh et al. [6], such structures indicate that the Mn 3d states are strongly hybridized with the O 2p

Mn 2p

hv=1486.6eV

(a) t-

.,,a~...,a...,,~. _

,

o'2 4-)

:

7:

\

-.:

; "

°

•

"g:A' " -

"°

• ~..,,'.;. , . , -_- ~ - ~ ~ )

L', , )...( o~

. . . . . . . . . . 680

670

i

'

Mn3s,

"

660

i.

i

/-.

i

640

I

(b)

J

o ~,'o;,,'o

,,

)

630

i

(c) t

.':',', ,-

O'2 O'1

650

"t

:

-

Mn3p

z 2/..:

CD

@ o

.t*.s.:t.-',.

i

,f

.%

~ -

7'0

'

6'0

'

5'0

~ "~'~,.':~-.. A 'k

40

30

binding energy (eV) Fig. 3. Mn 2p (a), 3p (b) and 3s (c) XPS spectra of (Nd0.062Sm0.938) 05Sr0.sMnO3 (A), Nd0.sSr0.sMnO3 (B), La084Sr0.,6MnO3 (C) and La0.7Ca0.3MnO3 (D).

orbitals in the ground state. The Mn 3p spectra also show satellite structures nearly --13 eV from the main peaks as shown in Fig. 3(b). Figure 3(c) shows the Mn 3s spectra of NSSMO and NSMO. In each spectrum, an exchange splitting is observed, whose magnitude is --4.6 eV. This large splitting indicates that the Mn 3d states of both samples are in the high spin state [6]. Moreover the similar magnitude of splitting indicates that there is no essential difference between the spin state of NSSMO and that of NSMO.

4. Conclusion

We have studied the electronic states of the perovskite manganese oxides by RPES, XPS, X-BIS and

Y. Kuwata et aL/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 281-285

VUV-IPES. In the VUV-IPES spectra, the unoccupied M n 3d contribution has been observed near EF, which seems to be better explained by the LDA + U1 calculation than the LSDA calculation. The difference of the Mn 3d states between La0.snSr0.16MnO3 and Nd0.sSr0.sMnO3 in the VUV-IPES spectra has not been recognized within our present measurements. By valence-band XPS spectra, we determine the energy distribution of atomic orbital characters. The M n 3 p - 3 d RPES show the enhancement of the satellite and the lower binding energy part of the valence band. The latter component is identified as the M n t2g states. The Mn 2p and 3p spectra indicate that the M n 3d states are heavily mixed with the O 2p orbitals in the ground state. The Mn 3s spectra show that the spin states of NSMO and NSSMO are similar and in the high-spin state.

Acknowledgements The work was partially supported by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture, Japan and by the New Energy and Industrial Technology Development Organization (NEDO).

285

References [1] H. Kuwahara, Y. Tomioka, A. Asamitsu, Y. Moritomo, Y. Tokura, Science 270 (1995) 961. [2] H. Kuwahara, Y. Tomioka, Y. Moritomo, A. Asamitsu, M. Kasai, R. Kumai, Y. Tokura, Science 272 (1996) 80. [3] E.O. Wollan, W.C. Koehler, Phys. Rev. 100 (1955) 545. [4] S. Suga, S. Imada, T. Muro, T. Fukawa, T. Shishidou, Y. Tokura, Y. Moritomo, T. Miyahara, J. Electron Spectrosc. Relat. Phenom. 78 (1996) 283. [5] A. Chainani, M. Mathew, D.D. Sarma, Phys. Rev. B 47 (1993) 15397. [6] T. Saitoh, A.E. Bouquet, T. Mizokawa, H. Namatame, A. Fujimori, M. Abbate, Y. Takeda, M. Takano, Phys. Rev. B 51 (1995) 13942. [7] D.D. Sarma, N. Shanthi, S.R. Barman, N. Hamada, H. Sawada, K. Terakura, Phys. Rev. Lett. 75 (1995) 1126. [8] N. Hamada, H. Sawada, K. Terakura, in: A. Fujimori, Y. Tokura (Eds.), Spectroscopy of Mott Insulators and Correlated Metals, Springer, Berlin, 1995, p. 95. [9] I. Solovyev,N. Hamada, K. Terakura, Phys. Rev. B 53 (1996) 7158. [10] T. Saitoh, A. Sekiyama,T. Mizokawa, A. Fujimori,K. Ito, H. Nakamura, M. Shiga, Solid State Commun. 95 (1995) 307. [1 l] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1. [12] T. Matsushita et al., unpublished. [13] S. Suga, T. Matsushita, M. Tsunekawa,T. Iwasaki, J. Samejima, Y. Kuwata, A. Kirnura, A. Sekiyama, A. Fujimori, H. Ishii, T. Miyahara, H. Kuwahara, Y. Tokura, Physica B 237238 (1997) 413.