Magnetism in Re-based ferrimagnetic double perovskites

Abstract

We have investigated spin and orbital magnetic moments of the Re 5d ion in the double perovskites A2FeReO6 (A = Ba, Sr, Ca) by X-ray magnetic circular dichroism (XMCD) at the Re L(2,3) edges. In these ferrimagnetic compounds an unusually large negative spin and positive orbital magnetic moment at the Re atoms was detected. The presence of a finite spin magnetic moment in a 'non-magnetic' double perovskite as observed in the double perovskite Sr2ScReO6 proves that Re has also a small, but finite intrinsic magnetic moment. We further show for the examples of Ba and Ca that the usually neglected alkaline earth ions undoubtedly also contribute to the magnetism in the ferrimagnetic double perovskites.

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arXiv:0909.0209v1 [cond-mat.mtrl-sci] 1 Sep 2009
Magnetism in Re-based ferrimagnetic double perovskites
A. Winkler,1N. Narayanan,1D. Mikhailova,1K. G. Bramnik,1H. Ehrenberg,1H. Fuess,1
G. Vaitheeswaran,2,3V. Kanchana,2F. Wilhelm,4A. Rogalev,4A. Kolchinskaya,1and L. Alff1, ∗
1Technische Universit¨ at Darmstadt, Petersenstr. 23, 64287 Darmstadt, Germany
2Royal Institute of Technology(KTH), Brinellv¨ agen 23, 10044 Stockholm, Sweden
3ACRHEM, University of Hyderabad, Hyderabad 500 046, India
4European Synchrotron Radiation Facility (ESRF),
6 Rue Jules Horowitz, BP 220, 38043 Grenoble Cedex 9, France
(Dated: received 29 April 2009)
We have investigated spin and orbital magnetic moments of the Re 5d ion in the double perovskites
A2FeReO6 (A = Ba, Sr, Ca) by X-ray magnetic circular dichroism (XMCD) at the Re L2,3 edges.
In these ferrimagnetic compounds an unusually large negative spin and positive orbital magnetic
moment at the Re atoms was detected. The presence of a finite spin magnetic moment in a ’non-
magnetic’ double perovskite as observed in the double perovskite Sr2ScReO6 proves that Re has
also a small, but finite intrinsic magnetic moment. We further show for the examples of Ba and Ca
that the usually neglected alkaline earth ions undoubtedly also contribute to the magnetism in the
ferrimagnetic double perovskites.
PACS numbers:75.25.+z, 75.30.-m, 75.50.-y
I.INTRODUCTION
Ordered
A2MNO6(with A an alkaline earth, M a magnetic tran-
sition metal ion, and N a non-magnetic ion) have come
again into the focus of research because of their inter-
esting magnetic properties. First, in Sr2FeMoO6a large
room-temperature magnetoresistance was observed [1].
Second, within the group of ferrimagnetic double per-
ovskites materials with higher Curie-temperatures, TC,
than in the simple perovskites (e.g. doped manganites)
can be obtained. At the moment the highest TCvalues
have been reported for Sr2CrReO6(TC≈ 635K) [2, 3, 4]
and Sr2CrOsO6(TC≈ 725K) [5, 6, 7]. Third, the mech-
anism leading to magnetic coupling is believed to be as-
sociated with a strong tendency to a half-metallic nature
of the charge carriers at the Fermi level [8, 9, 10]. Clearly,
these materials are interesting candidates for spintronic
applications [11], in particular when having in mind fully
epitaxial structures based on perovskite materials.
Recently, Majewski et al.
proposed a simple scaling law between the Curie-
temperature and the induced magnetic moment at the
non-magnetic site in the double perovskite structure
[4, 12, 13]. Philipp et al. have discussed that a high
Curie-temperature is associated with a tolerance factor
close to one for the corresponding crystal [14]. The only
exception for this rule is found in the series A2FeReO6
(A = Ba, Sr, Ca).In this particular FeRe-system it
is the strongly monoclinically distorted Ca-based com-
pound having an anomally high TC, namely about 540K
[15, 16, 17] (comparing to about 400K for Sr2FeReO6
[16] and 325K for Ba2FeReO6 [18, 19]).
doubleperovskites ofthe composition
and Sikora et al.
have
The dimen-
∗Electronic address: alff@oxide.tu-darmstadt.de
sionless tolerance factor, f, in A2FeReO6 whose devia-
tion from unity implies structural distortion varies from
about f = 1.057 for A = Ba over f = 0.997 for A =
Sr to f = 0.943 for A = Ca [14].
Ba-based ferrimagnetic double perovskites are close to
a structural transition into a hexagonal lattice where
ferro(i)magnetism is not allowed for symmetry reasons;
the Sr-based compounds are always close to a perfect cu-
bic structure with maximal TC, and the Ca-based double
perovskites are orthorhombically or monoclinically dis-
torted, with still a large but - due to the reduced ex-
change - clearly reduced ferrimagnetic transition temper-
ature. The exceptional large TCof Ca2FeReO6is accom-
panied by an insulating state at low temperatures, in
contrast to Sr2FeReO6 or even the similarly monoclini-
cally distorted Ca2FeMoO6which both are metallic [20].
The metal-insulator transition in Ca2FeReO6 has been
reported to occur between 100 and 150K [15, 16, 20].
This behavior has been attributed to strongly enhanced
electron-electron correlations on the Re site due to a re-
duced transfer integral between Fe and Re corresponding
to an extremely large effective Coulomb repulsion, Ueff,
of about 4eV on both ions [20]. This, however, is in some
contradiction to the observed high Curie-temperature,
which is believed to be a consequence of a kinetic energy
gain due to the hybridization of the Fe 3d and Re 5d t2g-
orbitals. The prediction that a decreased band-filling is
favorable for TC[21], which could be used to conciliate a
high TCwith a reduced Re-Re overlap, has turned out to
be not valid: an increased band-filling actually leads to
a strong TCenhancement for both the FeMo-system [22]
and the CrW-system [23]. Within the kinetically driven
exchange model [8, 9, 10], the increase of TCis more natu-
rally explained as a consequence of increased band-filling.
Note, that the cases of Ca2FeReO6and Sr2CrOsO6, both
being insulating and having a high TCat the same time,
are completely different: In the case of Sr2CrOsO6having
In general, the

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Table I: Summary of sample properties from x-ray diffrac-
tion at 300K (calculated by Rietfeld-refinement) and SQUID
magnetometry.
material
TC symm.
(K)
lattice
(˚ A)
antisites
(%)
Ba2FeReO6 317 Fm3m a = 8.0571(2)
Sr2FeReO6 418 Fm3m a = 7.8752(4)
0.9
2.6
a = 5.3992(2)
Ca2FeReO6 556 P21/n b = 5.5269(2)3.6
c = 7.6826(3)
a = 5.6760(2)
Sr2ScReO6
- P21/n b = 5.6534(2)
c = 7.9862(3)
7
only a tiny rhombohedral distortion, the Os 5d t2gband is
completely filled, while for Ca2FeReO6it is the structural
distortion that drives the metal-insulator transition. Re-
cently, it was suggested that in double perovskites with
heavy ions as Re a large orbital contribution to the mag-
netic moment leads to an enhanced total magnetization
above the integer value that is expected for a half-metallic
material [25]. This elsewhere predicted and calculated
[26] strong influence of spin-orbit coupling leads to a
quasi-half metallicity which still is from the view point of
applications in spintronics very high (above 90%). An-
other point of interest is the possibility of an intrinsic
enhancement of the Re spin magnetic moment due to
the peculiar Re5+state in the ferrimagnetic double per-
ovskites. In this study, we present the XMCD analysis
of the system A2FeReO6(A = Ba, Sr, Ca), compare the
experimental data to theoretical predictions calculated
within the full-potential linear muffin-tin orbital method
(FP-LMTO) [27] with included spin-orbit coupling, and
complete the so far suggested scaling law [4] by using the
identical method to extract separately spin and orbital
magnetic moments. Furthermore, we search for a con-
tribution of the alkaline earth element to the magnetic
behavior, and also look for an intrinsic Re moment in
a suited double perovskite compound with M being a
non-magnetic ion: Sr2ScReO6.
II. EXPERIMENTAL
A summary of the sample properties is given in Ta-
ble I. All values where a comparison can be made to
literature values are in good agreement with these data
[2, 15]. Note that the small amount of antisite disorder
does not affect our results. The XMCD measurements on
the Re L2,3edges were performed at the European Syn-
chrotron Radiation Facility (ESRF) at beam line ID12
[29]. The spectra were recorded within the total fluores-
cence yield detection mode. The XMCD spectra were ob-
tained as direct difference between consecutive XANES
??
?
?
?
???
?
?
?
?
Figure 1: XMCD spectra for A2FeReO6(A =Ba, Sr, Ca).
scans (X-ray Absorption Near Edge Spectrum) recorded
with opposite helicities of the incoming x-ray beam. To
ensure that the XMCD spectra are free from any exper-
imental artefacts the data were collected for both direc-
tions of the applied magnetic field of 6T (parallel and
antiparallel to the x-ray beam). The degree of circular
polarization of the monochromatic x-ray beam was 98%.
The measurements were performed at about 10K for all
samples (T ≪ TC), if not indicated otherwise. Since the
samples measured in backscattering geometry were very
thick, the spectra were first normalized to the edge jump
of unity and then corrected from self-absorption effects.
The edge jump intensity ratio L3/L2was then normal-
ized to 2.19/1 [30]. This is different from the statistical
2:1 branching ratio due to the difference in the radial ma-
trix elements of the 2p1/2to 5d(L2) and 2p3/2to 5d(L3)
transitions. The XMCD measurement as a function of
applied field suggests that our samples are closer to sat-
uration at 6T as is concluded by de Teresa at al. [25]
from high-field SQUID measurements. This issue has to
be clarified in future by high-field XMCD measurements.
III.RESULTS AND DISCUSSION
In this paper, the XANES spectra themselves are
not further discussed.As shown in Fig. 1, for FeRe-
compounds at both absorption edges we find a rather
intense XMCD signal. This is a clear evidence for the
existence of a magnetic moment at the Re 5d shell. For
all three compounds, the XMCD spectra at the L2edge
are largest (as expected for m = 1 orbitals) and similar
in shape. In Ca2FeReO6 the size of the XMCD signal
is by a factor of 2 smaller compared to the two other
FeRe compounds. At the L3edge, the Ca-based double
perovskite again stands out by a pronounced peak with
negative XMCD signal which is absent for Sr2FeReO6
and Ba2FeReO6. The data at the L3edge look slightly
different in amplitude as compared to previously pub-
lished data [13]. However, the data are consistent in that
the integrated XMCD intensity at the L3edge is nega-
tive only in the case of Ca2FeReO6. In this sense, all
data support the unusual behavior of Ca2FeReO6, which
cannot only be attributed to the different ionic size of the

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Table II: Measured (exp., normalized to 5K) and calculated
(th., calculated within the generalized gradient approxima-
tion including spin-orbit coupling (GGA+SO)) magnetic mo-
ments at the Re site for different double perovskites at about
10K. For a detailed discussion of the applied band-structure
calculation see e.g. [26, 28]. Calculation in [31] is GGA with
spin-orbit coupling. The number of d-holes was taken from
the band-structure calculation. In our case this number was
around 5.3. The error of the measured values is estimated as
2.5%.
materialmS(µB/f.u.) mL(µB/f.u.) |mL/mS|
−0.560.15
−0.740.21
−0.470.16
−0.680.25
0.013−0.002
-0.65 0.19
-0.680.15
-0.850.23
-0.85 0.18
exp. Ba2FeReO6
Sr2FeReO6
Ca2FeReO6
Sr2CrReO6[4]
Sr2ScReO6(80K)
th.Ba2FeReO6
Sr2FeReO6
Sr2FeReO6[31]
Sr2CrReO6[26]
0.27
0.28
0.34
0.37
0.15
0.29
0.22
0.27
0.21
???
?
?
?
?
?
?
?
Figure 2: XANES and derived XMCD spectra at the Re L2
and L3 edges of Sr2ScReO6.
A site ions.
In Fig. 2 we show XANES and XMCD spectra for the
compound Sr2ScReO6. This compound is important be-
cause the absence of any free electrons at Sc3+which
has a 3d0configuration will lead to a complete break-
down of the induced magnetic moment at the Re site.
This compound therefore allows the measurement of the
intrinsic magnetic moment of Re5+(also in contrast to
Re6+compounds as Sr2MgReO6). Previously, Kato et
al. have calculated from a Curie-Weiss fit to the suscep-
tibility an effective magnetic moment of Re in Sr2ScReO6
of about 1.1µB/f.u., as expected within the ionic picture
[32]. In contrast, our data show the existence of a much
smaller, but finite intrinsic moment at the Re site, in-
dicating an increased tendency to magnetic ordering of
Re5+. Since this moment is present above the antiferro-
magnetic transition temperature, it is not related to spin
glass behavior. The spin magnetic moment is about 50
times smaller than corresponding induced moments on
???
?
?
?
?
?
?
???
?
?
??
?
?
?
Figure 3: XANES and derived XMCD spectra at the a) Ba
L2 and L3 edges of Ba2FeReO6 and b) at the Ca K edge of
Ca2FeReO6.
Re5+, and the orbital magnetic moments even by a fac-
tor of 100. However, due to the high sensitivity of the
set-up at ESRF, one can unambiguously prove the exis-
tence of this moment. In contrast to the opposite sign of
the induced magnetic moment with respect to the applied
field, the spin magnetic moment at the Re in Sr2ScReO6
is aligned with the field. This is expected because the
kinetic exchange via fully polarized spin down is not at
work. This intrinsic moment of Re5+therefore has to be
considered as an indicator of the tendency to unusually
high magnetization of Re based double perovskites.
As a last point, we address magnetism in the earth al-
kaline ions itself, which usually are completely neglected
in the magnetic scenario. The XANES and XMCD spec-
tra of the Ba L2and L3edges of Ba2FeReO6and of the
Ca K-edge of Ca2FeReO6 are shown in Fig. 3. The 5d
spin magnetic moment (calculated with 9 as the number
of d-holes corresponding to the band-structure calcula-
tion) of Ba is µS= −0.0065 and the 5d orbital magnetic
moment µL= −0.0013 (both in µB/f.u.), |µL/µS| ≈ 0.2.
The theoretical predictions calculated as described else-
where [26, 28] are µS = −0.0084 and µL = −0.0014
which is in fair agreement with our experimental data.
For Ca2FeReO6we can only qualitatively say that a fi-
nite magnetic moment is observed, because the K-edge
probes only the 4p orbital magnetism. Since the L edges
are experimentally not accessible, a quantative analysis
cannot be done. The observation of a magnetically polar-
ized density of states gives clear evidence for a magnetic
interaction of the earth alkaline ions with the other ions.
The magnetic contribution of Ba in this case is a fac-
tor of 2 smaller than the contribution of the intrinsic Re
moment. Naturally, one expects that the magnetic con-

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tribution increases with ionic size due to the increased
exchange with the neighboring ions. The clear orbital
contribution in Ba2FeReO6is not unexpected due to the
heavy ionic mass. Our data provide a test for a detailed
theoretical study of the magnetism in the double per-
ovskites, and underlines the importance of taking spin-
orbit coupling into account. Note, that for example in
CrO2, where the importance of oxygen in the magnetic
mechanism is undoubted, comparable values of spin and
orbital moments of the oxygen ion have been measured
[33] as compared to our results on Ba in Ba2FeReO6.
In Table II we summarize our results for the spin and
orbital magnetic moments at the Re site as derived from
the XMCD measurements by applying the standard sum
rules [34, 35] and compare them to theoretical values.
Also, the ratio | mL/mS| is calculated, since this quan-
tity is not affected by possible uncertainties in the calcu-
lated number of holes. In general, the calculated data are
in surprisingly good agreement with the measured data.
One of the main reasons certainly is, that spin-orbit cou-
pling is taken into account from the beginning. Note,
that in the hard x-ray range the sum rules apply with
high validity due to the large spin-orbit splitting of the
core level.
Let us finally discuss again our data for the three FeRe-
based compounds. Our data are in good qualitative and
quantitative agreement with literature data with one ex-
ception: Ca2FeReO6. While Sikora et al. [13] find, that
the spin magnetic moment of Re in Ca2FeReO6 scales
with the high TC, in our case it has the lowest spin mag-
netic moment, letting Ca2FeReO6 stand out from the
scaling law [4, 12, 13] which so far holds in all other
cases. This behavior is certainly more natural, since one
expects that a reduced exchange will also lead to a re-
duced spin magnetic moment on the Re site. Note that
the ratio of orbital and spin magnetic moments are con-
sistent with the previous data. As suggested previously
by Kato et al. [16], a Re t2gorbital ordered state or its
glass-state analog associated with the monoclinic lattice
distortion occurs, pointing out the importance of corre-
lation effects in this compound. Recently, Sikora et al.
[36] proposed a scenario with a complex competition be-
tween two phases with different electronic and crystallo-
graphic structure. Our data give further indication that
Ca2FeReO6is exceptional among the double perovskites
due to the strong octahedral-site distortions.
IV.SUMMARY
In summary, we have elucidated the Re magnetic mo-
ments in the FeRe-based series of double perovskites as a
function of the earth alkaline ion, confirming the excep-
tional position of Ca2FeReO6. We have measured a finite
intrinsic magnetic moment at the Re5+site in Sr2ScReO6
indicating the tendency to enhanced magnetic moments
observed in Re based double perovskites. Furthermore,
for the first time we were able to measure by XMCD the
magnetic moments directly at the alkaline earth site it-
self. Our result shows that the usually neglected Ca and
Ba ions play a role in the magnetic scenario of the kinet-
ically driven exchange model, comparable in size to the
role of oxygen.
Thiswork was supported
2114/2115/2379).
bytheESRF(HE-
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