Insensitivity of the band structure of substituted SrRuO
3
as probed
by Seebeck coefficient measurements
Y. Klein, S. Hébert, and A. Maignan
Laboratoire CRISMAT, UMR 6508 CNRS ENSICaen, 6 boulevard Maréchal Juin, 14050 Caen, Cedex 4, France
S. Kolesnik, T. Maxwell, and B. Dabrowski
Physics Department, Northern Illinois University, DeKalb, Illinois 60115, USA
Received 29 November 2005; revised manuscript received 23 January 2006; published 28 February 2006
The influence of substitutions and vacancies in a unique 4d itinerant ferromagnet SrRuO
3
are studied by
thermoelectric power, resistivity, and magnetic measurements. Despite large changes of the Curie temperature,
ordered magnetic moment, and conductivity, the thermoelectric power Seebeck coefficient S is much less
sensitive to substitutions. Below about T 100 K, i.e., below T
C
, S is proportional to T as for other metallic
oxides indicating that S is controlled by the electronic diffusion enhanced by electronic correlations. For
T T
C
, the S values are much less dependent on T as if only the spin degeneracy term in the high-T expression
of the Heikes formula is controlling the behavior. For all the compounds with calculated Ru valency between
3.8 and 4, the values of S at 300 K are approximately constant, but they decrease when the valency
exceeds 4. The practically constant value of S at 300 K shows that the density of states of SrRuO
3
is
relatively unaffected by the various substitutions and defects studied here. Furthermore, the peculiar role of Cr,
which enhances the ferromagnetism, is confirmed by thermopower measurements.
DOI: 10.1103/PhysRevB.73.052412 PACS numbers: 72.80.Ga, 73.50.Lw, 71.27.+a, 71.28.+d
Correlated electron systems exhibit a wide range of un-
usual physical properties. This is well illustrated by the ru-
thenates with superconductivity observed for Sr
2
RuO
4
,
1
metal-insulator transition for Ca
2
RuO
4
,
2
and itinerant ferro-
magnetism below T
C
160 K for SrRuO
3
.
3
Many studies
have been devoted to the last compound to better understand
the origin of its unique properties among 4d perovskites.
From a chemical viewpoint, SrRuO
3
ferromagnetism is very
sensitive to substitution at either the A or B crystal site. The
isovalent Ca
2+
for Sr
2+
substitution, which increases the
structural distortion and decreases T
C
, results in a weakly
magnetic end member CaRuO
3
.
4
The aliovalent La
3+
for Sr
2+
substitution leads also to a T
C
decrease as Ru
3+
species are
created.
5
Also, substitution by M cations at the Ru site ac-
cording to the formula SrRu
1-x
M
x
O
3
as well as formation of
vacancies in SrRu
1-v
O
3
usually cause rapid decrease of T
C
.
6
However, there is one major exception to this trend as Cr
substitution for Ru was observed to increase T
C
.
7
This in-
crease, ascribed to the double-exchange interaction between
Cr
3+/4+
and Ru
4+/5+
,
8
is in marked contrast with the T
C
de-
crease reported for other magnetic M =Mn
3+
, Ni
2+
, Co
2+
Ref. 7; Rh
4+
Ref. 9 and nonmagnetic M =Ti
4+
Ref. 10,
Zn
2+
Ref. 7 cations.
Tetravalent ruthenium assumes a 4d
4
configuration when
octahedrally coordinated to oxygen in the perovskite struc-
ture with large crystal field splitting of the e
g
and t
2g
orbitals
that favors a low-spin state S =1 with a strong Ru-O cova-
lent character.
11
The size and charge differences between Ru
and the M cations in SrRu
1-x
M
x
O
3
induce modifications of
the average structural distortion and the ruthenium oxidation
state, respectively. The local structural distortion and the M
cation electronic configuration may also contribute to the
narrow t
2g
band structure of SrRu
4+
O
3
. In order to probe the
effect of the M cations with different electronic configura-
tions on the transport properties, we have performed Seebeck
coefficient measurements for SrRu
1-x
M
x
O
3
compounds with
M =Mn
3+/4+
, Cr
3+/4+
and for Sr
0.8
La
0.2
Ru
3.8+
O
3
. The crucial
role of the Ru valency in the transport properties has been
observed. Furthermore, the effect of disorder on the Ru lat-
tice has been investigated: the properties of two Ru deficient
perovskites, SrRu
1-v
O
3-
containing mixed Ru
4+
/Ru
5+
and
Ru
4+
will be presented.
Pure and ruthenium-deficient SrRu
1-v
O
3-
, Ru-substituted
SrRu
1-x
M
x
O
3
, and Ru
3+
-containing Sr
0.8
La
0.2
RuO
3
samples
have been prepared using the standard ceramic method.
6–8
Calcination of the starting mixtures was done for short peri-
ods of time at 800 ° C to avoid the conspicuous volatility of
RuO
2
at elevated temperatures. The intimately mixed pow-
ders were then pressed into pellets and fired in air at increas-
ing temperatures with several intermediate grindings up to
the following final synthesis conditions: SrRu
1-x
M
x
O
3
samples were obtained in air at 1100 and 1340 °C for x =0
Ref. 6 and M = Cr, x = 0.025 Ref. 8 and M = Mn, x = 0.1,
respectively. The Sr
0.8
La
0.2
RuO
3
sample was prepared in Ar
at 1200 ° C. The ruthenium-deficient SrRu
1-v
O
3
samples v
= 0.03 and 0.08 were obtained following the process de-
scribed in Ref. 6 from 600 atm O
2
at 1100 °C. The oxygen-
and ruthenium-deficient sample SrRu
0.97
O
2.94
was obtained
by subsequently annealing the v = 0.03 =0 sample in
flowing Ar at 1100 ° C. The vacancy concentration and oxy-
gen stoichiometry have been detemined by careful energy-
dispersive x-ray spectroscopy and thermogravimetric analy-
sis measurements and x-ray and neutron diffractions.
6
All
samples were single phase and the chemical formulas have
been obtained and verified using these techniques. The list of
these samples is given in Table I together with the calculated
valency of Ru.
ac magnetic susceptibility and dc magnetization M
PHYSICAL REVIEW B 73, 052412 2006
1098-0121/2006/735/0524124/$23.00 ©2006 The American Physical Society 052412-1