Raising the Curie temperature in Sr
2
FeMoO
6
double perovskites by electron doping
J. Navarro, C. Frontera, Ll. Balcells, B. Martı
´
nez, and J. Fontcuberta
*
Institut de Cie `ncia de Materials de Barcelona (CSIC), Campus Universitat Auto `noma de Barcelona,
E-08193 Bellaterra, Catalunya, Spain
Received 30 January 2001; published 15 August 2001
Electron doping in the ferromagnetic and metallic double perovskites Sr
2
FeMoO
6
is achieved via the partial
substitution of Sr
2+
by La
3+
. We show that this doping promotes an extraordinary rising of the Curie tem-
perature of about 70 K above that of the pristine compound. This finding reveals that the ferromagnetic
coupling is mediated by itinerant carriers, thus providing solid experimental support to the double-exchange
picture for the magnetic interactions. The observation that T
C
can be substantially enhanced may be of
relevance for technological applications of these materials in advanced spin devices.
DOI: 10.1103/PhysRevB.64.092411 PACS numbers: 72.15.Gd, 73.23.Hk, 73.40.Gk, 75.50.Tt
Carrier doping in strongly correlated electron systems has
been a very successful tool to disclose some of the most
exciting new phenomena and materials in the last few years.
High-T
C
superconducting cuprates and the so-called manga-
nese perovskites are among the most celebrated examples. In
this last case, the parent compound LaMnO
3
is insulating
and antiferromagnetic; by suitable hole doping, such as
La
1 -x
Sr
x
MnO
3
, metallic and ferromagnetic behavior is pro-
moted. In ferromagnetic manganese perovskites, the itinerant
charge carriers holes via the so-called double-exchange
DE mechanism mediate the ferromagnetic coupling.
1
Within this framework, the strength of the ferromagnetic
coupling and, thus, the Curie temperature ( T
C
) is controlled
mainly by the width of the conduction band WRefs. 2 and
3 and the itinerant charge density n ( x ).
4
In L
1 -x
A
x
MnO
3
( L is a lanthanide and A is an alkaline-earth ion, the band-
width W is determined by the bending of the Mn-O-Mn
bonding angle, which varies with the ionic radius of the
( L , A ) ions.
2,3
Recently, double-perovskite oxides of the type
A
2
FeMoO
6
( A =Ca,Sr,Ba), being also half-metallic
ferromagnets,
5
have received a great deal of attention be-
cause they display a substantial magnetoresistance and their
Curie temperature is well above that of the optimally doped
La
2/3
Sr
1/3
MnO
3
( T
C
360 K) simple perovskite. Indeed,
Curie temperatures about 400–430 K have been reported for
Sr
2
FeMoO
6
Ref. 5 and even higher ( T
C
500 K) for
Sr
2
FeReO
6
.
6
This fact pushes the operation range for even-
tual applications well above room temperature.
It has been recently shown that the dominant magnetic
interaction in A
2
FeMoO
6
double perovskites is
ferromagnetic,
7
thus implying that the ferrimagneticlike or-
dering does not come from a nearest-neighbor antiferromag-
netic interaction as commonly found in other oxides in
spinells, for instance. Instead, it signals that the magnetic
interactions are not of superexchange origin but more likely
of DE type as found in the manganese perovskites. Indeed,
the reported correlation between the electrical resistivity and
the Curie temperature
8
as well as the correlation discovered
between T
C
and the Fe-O-Mo bonding angle
9
also points in
this direction. Unfortunately, strategies to increase T
C
based
on size effects for instance, by substitution of Sr by Ba or
Ca invariably lead to a reduction of T
C
.
10
In this paper we will show that charge doping, i.e., modi-
fications of the density of charge carriers in the conduction
band, provides an efficient way to increase the Curie tem-
perature of these metallic oxides. Electron doping is
achieved by appropriate substitution of the divalent alkaline
earth by a trivalent lanthanide. We will show that
Sr
2 -x
La
x
FeMoO
6
(0 x 1) oxides can be prepared with
T
C
rising from about 400 K for x 0 to about T
C
490 K
for x 1.
Ceramic samples have been prepared by solid-state reac-
tion at high temperature. SrCO
3
, La
2
O
3
, Fe
2
O
3
, and MoO
3
oxides have been mixed at stoichiometric ratios appropriate
to obtain Sr
2 -x
La
x
FeMoO
6
( x =0 –1). In the first step, oxide
mixtures are prereacted at 900 °C under air. After grinding,
the samples are pelletized and sintered at T
S
=1200 °C under
an Ar/H
2
(5%) atmosphere for 12 h. The process is repeated
several times in order to complete the reaction. Samples are
heated and cooled at a rate of 5 °C/min under the same at-
mosphere. Results from different batches B1 and B2 pre-
pared under nominally identical conditions will be presented.
Energy-dispersive x-ray analysis and inductively coupled
plasma ICP methods indicate that the ratio of concentra-
tions of Sr/La/Mo/Fe atomic species in the samples vary ac-
cording to the expected ratio within the typical experimental
accuracy ( 2%). Room-temperature x-ray powder diffrac-
tion XRPD experiments have been performed using a
Rigaku Ru-200B diffractometer and k
1
,
2
Cu radiation.
The obtained profiles have been analyzed using the
FULLPROF program.
11
The magnetization and magnetotrans-
port properties has been measured by using a Quantum De-
sign superconducting quantum interference device SQUID
magnetometer up to 5.5 T and in the 10–700 K temperature
range. The magnetic- and temperature-dependent resistivity
has been measured in a PPMS Quantum Design.
XRPD patterns of samples with 0 x 0.4 can be well
refined (
2
=1.8, R
wp
=11.2–11.1 %) using the tetragonal
T I 4/mmm space group SG. Patterns of samples with x
0.4 present orthorhombic O distortions. For instance,
(2 0 2) Bragg reflection of an x =0.2 compound Fig. 1a
splits into (2 0 2) and (0 2 2) in an x =0.6 compound Fig.
1b. For 0.4x 0.8 successful refinements (
2
=2.0,
R
wp
=11.0–11.6 %) can be achieved using the monoclinic
PHYSICAL REVIEW B, VOLUME 64, 092411
0163-1829/2001/649/0924114/$20.00 ©2001 The American Physical Society 64 092411-1