8
70
V. Pandey et al. / Solid State Communications 149 (2009) 869–873
was used for magnetization measurements and its temperature
variations. Magnetoresistance (MR) and resistivity measurements
were conducted using the four-probe method.
3. Results and discussions
3
.1. X-ray results
Fig. 1 shows the XRD patterns of different samples; the inset
shows the Fe–Mo ordering peak (311) along with a peak (220) for
the parent compound. The XRD patterns of the samples have been
indexed assuming a cubic structure (space group Fm3m). All the
samples are single phase except x = 0.4 and 0.5. The impurity
phase BaMoO4 is detected in these samples. The ordering peaks
(
111) and (311) are present in the samples though the intensity
is lower for higher Nd containing samples, revealing a decrease
in Fe–Mo ordering in higher Nd containing samples. To get a
clear picture of the substitution of Nd on the structural changes,
we have done a structural refinement of the XRD patterns using
Rietveld analysis. The peak profile was fitted using a pseudo-
Voigt function. The Rietveld refinement was performed for the unit
cell parameters, atomic positions, the Fe–Mo ordering (antisite
disorder (ASD) defects) and the isotropic displacement parameters.
The refined results are shown in Table 1. The analysis shows
a decrease in the lattice parameter value, the average distance
between Fe and Mo ions and the average distance of the A site
cation from the oxygen ion a, dhFe−O−Moi and dhA−Oi respectively
where A = Ba, Sr, Nd, while O represents the oxygen ion, as
the Nd doping is increased. More ASD defects are observed with
increased Nd doping, which is indicative of decreased Fe–Mo
ordering in the sample. The decrease in lattice parameters is due
Fig. 1. X-ray diffraction patterns for different compositions of (Ba0.8Sr0.2
)
2−xNdx
FeMoO6. The inset shows the ordering peak (311) with peak (220) for the parent
compound (Ba0.8Sr0.2)2FeMoO6.
Fe3 3d5 core electron spin and delocalized Mo
spin couples antiferromagnetically, resulting in a net magnetic
moment 4 µ /f.u. [17]. The Fe ions can be supposed to couple
+
5+
4d1 electron
B
ferromagnetically in this arrangement where delocalized carriers
stabilize the ferromagnetic interaction between Fe ions. The
antisite disorder defects appear in the form of antiferromagnetic
Fe–O–Fe patches and reduce the saturation magnetic moment of
the double perovskite system. As already pointed out, an electron
added through trivalent substitution at the A site preferably
occupies the Mo site (forming a conduction band) which couples
antiferromagnetically to Fe core spins. Thus the electrons added
to the system contribute to decrease the saturation magnetic
moment value. As described in [18], the decrease in saturation
magnetic moment value in the case of trivalent doping can be given
by following equation:
3+
to smaller size Nd
ASD defects in these double perovskite (A2BB O6) systems depends
on the relative atomic size and valence state of the B and B
ion entering at the A site. The amount of
0
0
0
cations. If the atoms B and B have similar size and valence state,
the probability of cations occupying disordered site will be high.
In A2FeMoO6 systems, an electron added through trivalent ion
substitution at the A site remains at the Mo site and decreases
the valence state of Mo6 towards Mo
+
5+
while the Fe valence
remains unchanged [13–15]. This decreased charge difference of
Fe and Mo cations enhances the disorder defects as observed in
the present study. Ritter et al. [16] pointed out that with electron
doping there is a slight increase in MoO6 octahedral volume which
Ms(AS, x) = 4(1 − 2AS) µ
B
− x(1 − 2AS) µ
B
where the first term corresponds to the decrease of magnetization
due to the presence of antisite defects and the second term shows
the contribution of electrons added to the system. Thus the antisite
disorder defects and electron doping effect together contribute to
decrease the saturation magnetization with increasing trivalent
doping as observed experimentally in the present study. A
continuous increase in coercivity value is observed both at low
temperature and room temperature; it may be due to the increase
of ASD defects which act as pinning centres for magnetic domains
and ultimately increase the coercivity of the samples.
then approaches the FeO6 octahedral volume. Due to the increase
0
in MoO6 octahedral volume, the B and B crystallographic sites
approach being homogeneous, which also results in enhanced
Fe/Mo disorder.
The SEM images of some selected samples are shown in
Fig. 2(a, b, c and d). The grain size of the samples was observed to
decrease with increase in Nd doping. The parent compound shows
a well connected grain structure with average grain size more than
1
µm, while for the Nd doped sample the average grain size is less
Fig. 4 depicts a magnetization versus temperature plot for the
samples. The variation of lattice parameter and Curie temperature
versus Nd doping concentration is shown in the inset. An increase
than 1 µm and the grain connectivity is also reduced compared to
parent compound.
in Curie temperature from 350 K for x
=
0.0 to 402 K for
3
.2. Magnetization measurements
x = 0.5 is observed. Considering the impurity free compounds
only (x ≤ 0.3), the increase in Curie temperature is about 3 K
per % of Nd. This increase in Curie temperature is very high
compared to observed data in double perovskite Sr FeMoO (1.6 K
Fig. 3 shows the magnetization curves at 85 K. All samples show
good ferromagnetic response with low saturation fields and small
coercivity values. The saturation magnetic moment value at room
temperature and at low temperature decreases with increasing
Nd content [inset in Fig. 3]. A decrease in saturation magnetic
2
6
per % of Nd) and Ba FeMoO (2 K per % of La) [18,19]. The Curie
2
6
temperature of such double perovskite systems mainly depends
on the strength of antiferromagnetic coupling between the Fe and
Mo sublattices [17] which in turn depends on Fe–Mo hybridization
and the concentration of electrons at Fermi level. An electron
added to the system mainly resides at the delocalized Mo t2g
moment from 1.81 µ
300 K) and 3.75 µ /f.u. to 2.11 µ
was recorded. In these double perovskite systems the localized
B
/f.u. to 1.16 µ
B
/f.u. at room temperature
(
B
B
/f.u. at low temperature (85 K)