Electronic structure of SrRuO
3
K. Fujioka, J. Okamoto, T. Mizokawa, and A. Fujimori*
Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
I. Hase
Electrotechnical Laboratory, Umezono, Tsukuba 305, Japan
M. Abbate
Laboratorio Nacional de Luz Sincrotron, Campinas, Caixa Postal 6192, Campinas 13081-970 SP, Brazil
H. J. Lin and C. T. Chen
Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan, Republic of China
Y. Takeda
Department of Chemistry, Faculty of Engineering, Mie University, Tsu 514, Japan
M. Takano
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan
Received 25 February 1997
We have measured photoemission and oxygen 1 s x-ray absorption spectra of the ferromagnetic metal
SrRuO
3
and compared them with a first-principles band-structure calculation. The overall distribution of Ru 4 d
and O 2 p spectral weight is in good agreement with that predicted by the band-structure calculation. However,
the observed spectral line shape of the Ru 4 d band is spread over a wide energy range and the emission
intensity at the Fermi level is weakened compared to the band-structure calculation. This implies the impor-
tance of electron correlation in the Ru oxide. S0163-18299705736-6
Stimulated by the discovery of high T
c
copper oxides,
much work has been done on various 3 d transition-metal
oxides. In these oxides, electron correlation is important and
gives rise to a variety of physical properties. On the other
hand, it is generally believed that d electrons in 4 d
transition-metal oxides are less localized and therefore that
electron correlation is less important than in the 3 d oxides.
So far, relatively few works have been reported on 4 d
transition-metal oxides and little quantitative information has
been obtained about the strength of electron-electron interac-
tion. In fact, Ru oxides exhibit various types of electrical and
magnetic properties.
1
For example, rutile-type RuO
2
is a nor-
mal metal and its density of states DOS derived from a
band-structure calculation is in good agreement with photo-
emission spectra.
2
Pyrochlore-type Y
2
Ru
2
O
7
is an insulator
and exhibits a local-moment behavior while Bi
2
Ru
2
O
7
is a
Pauli-paramagnetic metal.
3
Recently, Sr
2
RuO
4
was found to
be superconducting below T
c
1 K.
4
In this paper, we present photoemission and x-ray absorp-
tion spectroscopic studies of SrRuO
3
, which is metallic
5
and
shows ferromagnetism below T
c
160 K.
6–8
We examine
the effect of electron correlation by comparing the spectra
with first-principles band-structure calculations. SrRuO
3
crystallizes in a GdFeO
3
-type distorted perovskite structure.
9
The saturation magnetization is 1.1– 1.3
B
/Ru
7
and a neu-
tron diffraction study has given 1.40.4
B
/Ru.
7
According
to the ionic picture, the Ru ion is in the tetravalent state and
has the electronic configuration of d
4
: the Ru
4+
ion is
thought to be in the low-spin ( S =1) state from the small
magnetic moment. The effective moment of 2.6
B
above
T
c
is close to the low-spin value of 2 S ( S +1) =2.83
B
.
6,7
Recently, the magnetotransport properties of SrRuO
3
were
studied
10
stimulated by the giant magnetoresistance in
perovskite-type Mn oxides.
11
In spite of sizable magnetore-
sistance, it seems that the magnetic and transport properties
show relatively independent behaviors.
12,13
Photoemission
spectra of SrRuO
3
have previously been reported by Cox
et al.
14
for surfaces heat-treated in an oxygen atmosphere.
Recently, Sr
2
RuO
4
has been studied by angle-integrated
15
and angle-resolved photoemission spectroscopy.
16
Polycrystalline samples of SrRuO
3
were prepared by the
following procedure: a mixture of RuO
2
and SrCO
3
was pre-
fired at 800 °C for 12 h in air. Then it was pressed into a
pellet and sintered at 1200 °C for 24 h in air. The product
was milled and pressed into a pellet again ( 2000 kg/cm
2
).
It was then fired at 1300 °C for 24 h in air. X-ray photoemis-
sion spectroscopy XPS measurements were carried out us-
ing a Mg x-ray source ( h =1253.6 eV). Ultraviolet photo-
emission UPS measurements were performed using
synchrotron radiation (50 eVh 100 eV) at beam line
BL-2 of the Synchrotron Radiation Laboratory SRL, Insti-
tute for Solid State Physics, University of Tokyo. The inten-
sities of the spectra taken at different photon energies were
normalized to the photon flux determined by the total yield
of Au. The total resolution was 0.3–0.4 eV for UPS and
1 eV for XPS. The base pressure in the spectrometers was
10
-10
Torr. In order to obtain fresh surfaces, we scraped
the sample in the ultrahigh vacuum using a diamond file. We
PHYSICAL REVIEW B 15 SEPTEMBER 1997-I VOLUME 56, NUMBER 11
56 0163-1829/97/5611/63804/$10.00 6380 © 1997 The American Physical Society