We have prepared crystallographically ordered and disorder specimens of the
double perovskite, Sr$_2$FeMoO$_6$ and investigated their magnetoresistance
behaviour. The extent of ordering between the Fe and Mo sites in the two
samples is determined by Rietveld analysis of powder x-ray diffraction patterns
and reconfirmed by M\"{o}ssbauer studies. While the ordered sample exhibits the
sharp low-field response, followed by moderate changes in the magnetoresistance
at higher fields, the disordered sample is characterised by the absence of the
spectacular low-field response. We argue that the low field response depends
crucially on the half-metallic ferromagnetism, while the high-field response
follows from the overall magnetic nature of the sample, even in absence of the
half-metallic state.Comment: accepted in Solid State Communicatio

Ashwani Kumar

D.D. Sarma

E.V. Sampathkumaran

G. Nalini

Kobayashi

Mathur

Millis

Nakagawa

R. Nagarajan

Subham Majumdar

Sugata Ray

T.N. Guru Row

English

arXiv

We have prepared crystallographically ordered and disordered specimens of the double perovskite, $Sr_2FeMoO_6$, and investigated their magnetoresistance (MR) behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder X-ray diffraction patterns and reconfirmed by Mössbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate changes in the MR at higher fields, the disordered sample is characterised by the absence of the spectacular low-field response. We argue that the low field response depends crucially on the half-metallic ferromagnetism, while the high-field response follows from the overall magnetic nature of the sample, even in absence of the half-metallic state

Sarma, DD

Sampathkumaran, EV

Ray, Sugata

Nagarajan, R

Majumdar, Subham

Kumar, Ashwani

Nalini, G

Row, Guru TN

Open Access Repository of IISc Research Publications

Magnetoresistance in ordered and disordered double perovskite oxide, $Sr_2FeMoO_6$

We have prepared crystallographically ordered and disordered specimens of the double perovskite, Sr2FeMoO6, and investigated their magnetoresistance (MR) behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder X-ray diffraction patterns and reconfirmed by Mossbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate changes in the MR at higher fields, the disordered sample is characterised by the absence of the spectacular low-field response. We argue that the low field response depends crucially on the half-metallic ferromagnetism, while the high-field response follows from the overall magnetic nature of the sample, even in absence of the half-metallic state

Sarma, D. D.

Sampathkumaran, E. V.

Nagarajan, R.

Nalini, G.

Guru Row, T. N.

Publications of the IAS Fellows

ar
X
iv
:c
on
d-
m
at
/0
00
32
37
v1
  [
co
nd
-m
at.
str
-el
]  
14
 M
ar 
20
00
Magnetoresistance in ordered and disordered double perovskite
oxide, Sr2FeMoO6
D. D. Sarmaa, E.V. Sampathkumaranb, Sugata Raya, R. Nagarajanb, Subham Majumdarb,
Ashwani Kumara, G. Nalinia and T.N. Guru Rowa
aSolid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
bTata Institute of Fundamental Research, Mumbai - 400 005, India
(February 1, 2008)
Abstract
We have prepared crystallographically ordered and disorder specimens of
the double perovskite, Sr2FeMoO6 and investigated their magnetoresistance
behaviour. The extent of ordering between the Fe and Mo sites in the two sam-
ples is determined by Rietveld analysis of powder x-ray diffraction patterns
and reconfirmed by Mo¨ssbauer studies. While the ordered sample exhibits
the sharp low-field response, followed by moderate changes in the magnetore-
sistance at higher fields, the disordered sample is characterised by the absence
of the spectacular low-field response. We argue that the low field response
depends crucially on the half-metallic ferromagnetism, while the high-field re-
sponse follows from the overall magnetic nature of the sample, even in absence
of the half-metallic state.
Keywords: A. magnetically ordered materials, D. electronic transport, order-disorder ef-
fects, E. x-ray and γ-ray spectroscopies.
1
I. INTRODUCTION
Colossal magnetoresistance (CMR) is a property that is of great technological potential,
since the large change in the resistance (R) with the application of a magnetic field can be
used effectively for mass storage magnetic devices. However, the well-known CMR materi-
als, Mn based oxides, have significant CMR effect only at low temperatures and, therefore,
are not suitable for room temperature applications. In their recent work, Kobayashi et
al. [1] have pointed out that the fully-ordered double perovskite Sr2FeMoO6 with alternat-
ing Fe3+ (3d5, S = 5/2) and Mo5+ (4d1, S = 1/2) ferrimagnetically coupled ions exhibit
substantial CMR even at room temperature. It is suggested that the half-metallic ferro-
magnetic (HMFM) state below Tc is responsible for the magnetoresistance (MR) behavior
via spin-dependent carrier scattering processes. While the spin-dependent scattering due to
the intergrain tunneling effects enables one to construct manganite based devices [2], the
intra-grain properties of the manganites are generally believed to be intimately connected
with the intrinsic instability of Mn3+ 3d4 states arising from strong electron-phonon interac-
tion as well as double-exchange interaction [3,4]. Mo5+ is also a Jahn-Teller ion with its 4d1
2D degenerate ground state. Thus, a priori it is not possible to rule out any contribution
to the magnetoresistance of Sr2FeMoO6 from effects other than intergrain spin-dependent
carrier scatterings. Considering the complexity of the mechanism of magnetoresistance in
the manganites, in which in addition to half-metallic ferromagnetism, various instabilities
play important role, it is essential to address the factors responsible for the novel magne-
toresistance properties of Sr2FeMoO6.
Central to the HMFM state in Sr2FeMoO6 as deduced by the band structure calcula-
tion [1] is the long-range ordering of FeO6 and MoO6 octahedra alternatingly along three
cubic axes. Disorder is expected to be detrimental to the half-metallic nature, giving rise
to finite up- and down-spin densities of states at the Fermi energy, destroying the HMFM
state; preliminary super-cell band structure calculations in our group indeed suggest that
HMFM state is easily destroyed by a positional disorder at the Fe/Mo sites, though an over-
2
all ferrimagnetic order persists in the system. Keeping this in mind we have prepared for the
first time Sr2FeMoO6 by melt-quenching, where Fe and Mo sites are heavily disordered. A
comparative study of disordered and ordered samples using x-ray diffraction, 57Fe Mo¨ssbauer
and the magnetoresistive properties allows us to delineate the effects arising from long range
crystallographic order and those from factors other than half-metallic ferromagnetism in
controlling the MR properties.
II. EXPERIMENTAL
We prepared ordered Sr2FeMoO6 following ref. 1 by hydrogen reduction of a mixture
of SrCO3, MoO3 and Fe2O3. The disordered sample was prepared by melt-quenching of a
mixture of SrCO3, MoO3, Fe2O3 and Mo-metal in an argon atmosphere. Mo¨ssbauer study
was carried out in transmission geometry using a 57Co/Rh matrix source. High resolution
powder diffraction data were collected on a STOE STADI/P diffractometer. The resistivity
measurements with and without an applied field were carried out using the standard four-
probe dc measurements.
III. RESULTS AND DISCUSSION
We show the powder x-ray diffraction patterns of the two samples in Fig. 1; Fig. 1(a)
shows the pattern of the sample prepared by the conventional solid state techniques [1],
while Fig. 1(b) shows the same for the melt-quenched sample prepared by the arc melting.
The main diffraction peaks are the same in both the panels of Fig. 1 and these are readily
ascribed to the perovskite lattice. The only difference between the two diffraction patterns
is in terms of the existence of two weak peaks, at 19.6◦ and 38◦ in Fig. 1(a), but absence in
Fig. 1(b). We have expanded the vicinity of these two angles in order to illustrate this point
clearly in the respective panels. The existence of these two peaks in Fig. 1(a) establishes the
presence of the supercell arising from the ordering of Fe and Mo sites alternatingly, giving
rise to the double perovskite structure. Total or near-absence of these two order-related
3
peaks in Fig. 1(b) suggests that the melt-quenched sample obtained from the arc furnace
gives rise to heavy disrodering between the Fe and the Mo sites The extent of ordering at the
Fe/Mo sites is easily estimated from x-ray diffraction (XRD) of ordered (A) [Fig. 1(a)] and
disordered (B) [Fig. 1(b)] samples of Sr2FeMoO6. Rietveld refinement for both converged
to the same cell dimensions (a = b = 5.566 A˚ and c = 7.858 A˚) with Rwp ≤ 8.0%. The
ordering at the Fe and Mo sites in sample A was found to be ∼ 91%, slightly larger than
the previously reported [1] value of 87%. Near-absence of the long-range order-related peaks
within the noise level of the data [see insets I and II to Fig. 1(b)] for sample B confirms the
extensive disorder in this case. Rietveld analysis suggests approximately 31% ordering in
this case.
The effect of disorder at the Fe-sites is also probed by 57Fe Mo¨ssbauer spectra of these
two samples at 300 K and 4.2 K (Fig. 2); a comparison of the raw spectra for the two
samples at these temperatures is sufficient to establish the relative degrees of disorder in
site occupancies. The spectra for the sample B exhibits relatively larger line-widths in
contrast to those of sample A, implying a larger disorder in sample B. All the spectra are
magnetic hyperfine split, indicating magnetic ordering of the materials. The spectrum at
300 K for sample A favorably compares with that reported earlier [5]. Our analysis of the
data at 4.2 K for sample A yields a saturation hyperfine field of about 480 kOe, with a
small distribution of about 10 kOe. This value is somewhat smaller than that known for
Fe2O3. The hyperfine field for sample B has a much larger distribution (490 ± 60 kOe),
quantitatively confirming a strong variation in the chemical environment for Fe from site to
site. The chemical composition was however found to be identical between the two samples
by energy dispersive x-ray analysis. Moreover, the grain size and morphology were also
found to be similar between the two samples by scanning electron microscopy (SEM). The
above results establish that it is indeed possible to obtain highly ordered and extensively
disordered samples with respect to Fe and Mo occupancies without otherwise disturbing the
structural integrity of this double perovskite system.
We show the magnetoresistance, MR(H, T ) = {R(H, T )− R(0, T )}/R(0, T ), of ordered
4
and disordered samples as a function of applied magnetic field, H, at 300 K and 4.2 K in
Fig 3; in the same figure we reproduce the results from Kobayashi et al. [1]. At both the
temperatures, the ordered sample is characterised by a sharp magnetoresistive response in
the low-field region, though the magnitude of the magnetoresistance is considerably larger at
the lower temperature, in agreement with the published literature [1]. The present ordered
sample has a sharper low-field response and a larger magnitude, particularly at the low
temperature compared to the previous result [1]. This improved magnetoresistance response
is possibly related to somewhat higher degree of ordering in the present sample. This
improvement in the low magnetic field response suggests that a fully (100%) ordered sample
is likely to be a very good candidate for real-life device applications. Beyond about 1 T, the
magnetoresistance of the ordered sample exhibits a slower change at higher field strengths,
without showing any signs of saturation upto the highest magnetic field probed in the present
experiments. The MR changes significantly (by about 6.5% at 4.2 K and 3% at 300 K) in
the larger field regime between 1 and 7 T.
In contrast, the MR of the disordered sample does not exhibit the low-field sharp mag-
netic response below 1 T. Since the two samples differ only in the extent of Fe/Mo ordering,
the low-field rapid variation in MR of sample A evidently arises from the long-range or-
der, leading to the half metallic ferromagnetic state of sample A and the consequent strong
intergrain spin-dependent scattering. It is, however, intriguing to note that there is a sub-
stantial negative magnetoresistance in the disordered sample and the high field behaviors
of the MR in the two samples are similar, the two curves being approximately parallel in
the high-field region at both the temperatures shown in Fig. 3. This implies that there is a
common intragrain origin of this high-field MR behavior in the disordered and the ordered
samples.
We have carried out band structure calculations, simulating the disordered system within
a supercell approach where the Fe and Mo atoms were allowed to occupy the other sites in
contrast to the perfectly ordered system. Preliminary results from these calculations show
that in the disordered state, both up- and down-spin states exist at the Fermi energy; this
5
indicates that the half-metallic ferromagnetic ground state is not realised for any disordering
of the Fe and the Mo sublattices, though the system remains to be a ferrimagnet. Thus,
the high-field, intragrain contribution to the negative magnetoresistance is independent of
the long range ordering of Fe and Mo atoms and the cosnequent existence of the HMFM
state. Instead, it is to be associated with the usual negative magnetoresistance observed
for ferromagnetic substances, arising from a suppression of the spin-fluctuations with the
application of an external magnetic field.
In conclusion, we have prepared highly ordered and disordered samples of the per-
ovskite oxide, Sr2FeMoO6. We characterised the extent of ordering by x-ray diffraction
and Mo¨ssbauer studies of these samples. Rietveld analysis of the powder diffraction pattern
shows that the extent of ordering in the Fe/Mo sites is about 91% for the ordered sample,
while it is about 31% for the disordered sample. We point out that the disordered sample
is not a half-metallic ferromagnet, though it is magnetic even at the room temperature. We
showed that the magnetoresistance of the two samples are distinguished by the existence
of the sharp changes in the magnetoresistance at low fields for the ordered sample. In the
case of the disordered sample, there is no such sharp changes in the MR, though a negative
MR is seen at all fields for the disordered sample also. The MR of the disordered sample is
very similar to that of the ordered sample in the high-field regime, indicating a common ori-
gin. Thus, we conclude that the low-field magnetoresistance is dominated by the intergrain
spin-dependent scattering of the highly spin polarized charge-carriers of the half-metallic
ferromagnet, the high-field negative magnetoresistance behaviours in both the samples are
intrinsic, intragrain property and arise from the suppression of spin-fluctuations under the
application of a magnetic field on a magnetic system.
Acknowledgements: The authors thank Prof. B. Sriram Shastry and Dr. Tanusri
Saha Dasgupta for useful discussions.
6
REFERENCES
[1] K. -I. Kobayashi, T. Kimura, H. Sawada, K. Terakura and Y. Tokura, Nature 395 (1998)
677.
[2] N.D. Mathur, Burnell G., Isaac S.P., Jackson T.J., Teo B.S., MacManusDriscoll J.L.,
Cohen L.F., Evetts J.E., Blamire M.G., Nature 387 (1997) 266.
[3] A.J. Millis, P. B. Littlewood and B. I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144.
[4] A.J. Millis, B. I. Shraiman and R. Mueller, Phys. Rev. Lett. 77 (1996) 175.
[5] T. Nakagawa, J. Phys. Soc. Japan 24 (1968) 806.
7
IV. FIGURE CAPTIONS
Fig. 1. X-ray diffraction patterns of (a) ordered and (b) disordered Sr2FeMoO6. Order-
related peaks, appear at 19.6◦ and 38◦, shown in the insets on an expanded scale.
Fig. 2. 57Fe Mo¨ssbauer spectra of the disordered and ordered Sr2FeMoO6 at 300 and 4.2
K.
Fig. 3. The comparisons of percentage magnetoresistance between the ordered and dis-
ordered samples as well as the result presented by Kobayashi et al. [1] at (a) 4.2 K and (b)
300 K.
8
01000
2000
3000
4000
5000
 
Sarma et al., Fig. 1
(b)
In
te
n
sit
y 
(ar
bi
tr
a
ry
 u
n
its
)
2θ
Disordered
10 20 30 40 50 60 70 80
0
1000
2000
3000
4000
5000
 
(a)
Ordered
18 19 20 21
340
390
440
490
540  
 
I
38 39
300
325
350
375
400
425
 
 
II
18 19 20 21
340
390
440
490
540  
 
I
38 39
300
325
350
375
400
425
 
  
II
-12 -6 0 6 12
 
 
Sarma et al., Fig. 2
4.2 K
4.2 K
300 K
300 K
Ordered
Disordered
Ordered
Disordered
In
te
n
sit
y 
(ar
bi
tr
a
ry
 u
n
its
)
Velocity (mm/sec)
0 1 2 3 4 5 6 7
-40
-30
-20
-10
0
 
(a)
4.2 K
 Disordered
 Kobayashi et al.
 Ordered
%
M
R
-10
-8
-6
-4
-2
0
2
 
 
H(T)
(b)
Sarma et al., Fig. 3
300 K
 Disordered
 Kobayashi et al.
 Ordered
%
M
R


Magnetoresistance in ordered and disordered double perovskite oxide, Sr<SUB>2</SUB>FeMoO<SUB>6</SUB>

Indian Academy of Sciences

Name not available

Crossref

Solid State Communications

Magnetoresistance in ordered and disordered double perovskite oxide, Sr2FeMoO6

ar
X
iv
:c
on
d-
m
at
/0
00
32
37
v1
  [
co
nd
-m
at
.s
tr
-e
l]
  1
4 
M
ar
 2
00
0
Magnetoresistance in ordered and disordered double perovskite
oxide, Sr2FeMoO6
D. D. Sarmaa, E.V. Sampathkumaranb, Sugata Raya, R. Nagarajanb, Subham Majumdarb,
Ashwani Kumara, G. Nalinia and T.N. Guru Rowa
aSolid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
bTata Institute of Fundamental Research, Mumbai - 400 005, India
(February 1, 2008)
Abstract
We have prepared crystallographically ordered and disorder specimens of
the double perovskite, Sr2FeMoO6 and investigated their magnetoresistance
behaviour. The extent of ordering between the Fe and Mo sites in the two sam-
ples is determined by Rietveld analysis of powder x-ray diffraction patterns
and reconfirmed by Mössbauer studies. While the ordered sample exhibits
the sharp low-field response, followed by moderate changes in the magnetore-
sistance at higher fields, the disordered sample is characterised by the absence
of the spectacular low-field response. We argue that the low field response
depends crucially on the half-metallic ferromagnetism, while the high-field re-
sponse follows from the overall magnetic nature of the sample, even in absence
of the half-metallic state.
Keywords: A. magnetically ordered materials, D. electronic transport, order-disorder ef-
fects, E. x-ray and γ-ray spectroscopies.
1
I. INTRODUCTION
Colossal magnetoresistance (CMR) is a property that is of great technological potential,
since the large change in the resistance (R) with the application of a magnetic field can be
used effectively for mass storage magnetic devices. However, the well-known CMR materi-
als, Mn based oxides, have significant CMR effect only at low temperatures and, therefore,
are not suitable for room temperature applications. In their recent work, Kobayashi et
al. [1] have pointed out that the fully-ordered double perovskite Sr2FeMoO6 with alternat-
ing Fe3+ (3d5, S = 5/2) and Mo5+ (4d1, S = 1/2) ferrimagnetically coupled ions exhibit
substantial CMR even at room temperature. It is suggested that the half-metallic ferro-
magnetic (HMFM) state below Tc is responsible for the magnetoresistance (MR) behavior
via spin-dependent carrier scattering processes. While the spin-dependent scattering due to
the intergrain tunneling effects enables one to construct manganite based devices [2], the
intra-grain properties of the manganites are generally believed to be intimately connected
with the intrinsic instability of Mn3+ 3d4 states arising from strong electron-phonon interac-
tion as well as double-exchange interaction [3,4]. Mo5+ is also a Jahn-Teller ion with its 4d1
2D degenerate ground state. Thus, a priori it is not possible to rule out any contribution
to the magnetoresistance of Sr2FeMoO6 from effects other than intergrain spin-dependent
carrier scatterings. Considering the complexity of the mechanism of magnetoresistance in
the manganites, in which in addition to half-metallic ferromagnetism, various instabilities
play important role, it is essential to address the factors responsible for the novel magne-
toresistance properties of Sr2FeMoO6.
Central to the HMFM state in Sr2FeMoO6 as deduced by the band structure calcula-
tion [1] is the long-range ordering of FeO6 and MoO6 octahedra alternatingly along three
cubic axes. Disorder is expected to be detrimental to the half-metallic nature, giving rise
to finite up- and down-spin densities of states at the Fermi energy, destroying the HMFM
state; preliminary super-cell band structure calculations in our group indeed suggest that
HMFM state is easily destroyed by a positional disorder at the Fe/Mo sites, though an over-
2
all ferrimagnetic order persists in the system. Keeping this in mind we have prepared for the
first time Sr2FeMoO6 by melt-quenching, where Fe and Mo sites are heavily disordered. A
comparative study of disordered and ordered samples using x-ray diffraction, 57Fe Mössbauer
and the magnetoresistive properties allows us to delineate the effects arising from long range
crystallographic order and those from factors other than half-metallic ferromagnetism in
controlling the MR properties.
II. EXPERIMENTAL
We prepared ordered Sr2FeMoO6 following ref. 1 by hydrogen reduction of a mixture
of SrCO3, MoO3 and Fe2O3. The disordered sample was prepared by melt-quenching of a
mixture of SrCO3, MoO3, Fe2O3 and Mo-metal in an argon atmosphere. Mössbauer study
was carried out in transmission geometry using a 57Co/Rh matrix source. High resolution
powder diffraction data were collected on a STOE STADI/P diffractometer. The resistivity
measurements with and without an applied field were carried out using the standard four-
probe dc measurements.
III. RESULTS AND DISCUSSION
We show the powder x-ray diffraction patterns of the two samples in Fig. 1; Fig. 1(a)
shows the pattern of the sample prepared by the conventional solid state techniques [1],
while Fig. 1(b) shows the same for the melt-quenched sample prepared by the arc melting.
The main diffraction peaks are the same in both the panels of Fig. 1 and these are readily
ascribed to the perovskite lattice. The only difference between the two diffraction patterns
is in terms of the existence of two weak peaks, at 19.6◦ and 38◦ in Fig. 1(a), but absence in
Fig. 1(b). We have expanded the vicinity of these two angles in order to illustrate this point
clearly in the respective panels. The existence of these two peaks in Fig. 1(a) establishes the
presence of the supercell arising from the ordering of Fe and Mo sites alternatingly, giving
rise to the double perovskite structure. Total or near-absence of these two order-related
3
peaks in Fig. 1(b) suggests that the melt-quenched sample obtained from the arc furnace
gives rise to heavy disrodering between the Fe and the Mo sites The extent of ordering at the
Fe/Mo sites is easily estimated from x-ray diffraction (XRD) of ordered (A) [Fig. 1(a)] and
disordered (B) [Fig. 1(b)] samples of Sr2FeMoO6. Rietveld refinement for both converged
to the same cell dimensions (a = b = 5.566 Å and c = 7.858 Å) with Rwp ≤ 8.0%. The
ordering at the Fe and Mo sites in sample A was found to be ∼ 91%, slightly larger than
the previously reported [1] value of 87%. Near-absence of the long-range order-related peaks
within the noise level of the data [see insets I and II to Fig. 1(b)] for sample B confirms the
extensive disorder in this case. Rietveld analysis suggests approximately 31% ordering in
this case.
The effect of disorder at the Fe-sites is also probed by 57Fe Mössbauer spectra of these
two samples at 300 K and 4.2 K (Fig. 2); a comparison of the raw spectra for the two
samples at these temperatures is sufficient to establish the relative degrees of disorder in
site occupancies. The spectra for the sample B exhibits relatively larger line-widths in
contrast to those of sample A, implying a larger disorder in sample B. All the spectra are
magnetic hyperfine split, indicating magnetic ordering of the materials. The spectrum at
300 K for sample A favorably compares with that reported earlier [5]. Our analysis of the
data at 4.2 K for sample A yields a saturation hyperfine field of about 480 kOe, with a
small distribution of about 10 kOe. This value is somewhat smaller than that known for
Fe2O3. The hyperfine field for sample B has a much larger distribution (490 ± 60 kOe),
quantitatively confirming a strong variation in the chemical environment for Fe from site to
site. The chemical composition was however found to be identical between the two samples
by energy dispersive x-ray analysis. Moreover, the grain size and morphology were also
found to be similar between the two samples by scanning electron microscopy (SEM). The
above results establish that it is indeed possible to obtain highly ordered and extensively
disordered samples with respect to Fe and Mo occupancies without otherwise disturbing the
structural integrity of this double perovskite system.
We show the magnetoresistance, MR(H, T ) = {R(H, T ) − R(0, T )}/R(0, T ), of ordered
4
and disordered samples as a function of applied magnetic field, H, at 300 K and 4.2 K in
Fig 3; in the same figure we reproduce the results from Kobayashi et al. [1]. At both the
temperatures, the ordered sample is characterised by a sharp magnetoresistive response in
the low-field region, though the magnitude of the magnetoresistance is considerably larger at
the lower temperature, in agreement with the published literature [1]. The present ordered
sample has a sharper low-field response and a larger magnitude, particularly at the low
temperature compared to the previous result [1]. This improved magnetoresistance response
is possibly related to somewhat higher degree of ordering in the present sample. This
improvement in the low magnetic field response suggests that a fully (100%) ordered sample
is likely to be a very good candidate for real-life device applications. Beyond about 1 T, the
magnetoresistance of the ordered sample exhibits a slower change at higher field strengths,
without showing any signs of saturation upto the highest magnetic field probed in the present
experiments. The MR changes significantly (by about 6.5% at 4.2 K and 3% at 300 K) in
the larger field regime between 1 and 7 T.
In contrast, the MR of the disordered sample does not exhibit the low-field sharp mag-
netic response below 1 T. Since the two samples differ only in the extent of Fe/Mo ordering,
the low-field rapid variation in MR of sample A evidently arises from the long-range or-
der, leading to the half metallic ferromagnetic state of sample A and the consequent strong
intergrain spin-dependent scattering. It is, however, intriguing to note that there is a sub-
stantial negative magnetoresistance in the disordered sample and the high field behaviors
of the MR in the two samples are similar, the two curves being approximately parallel in
the high-field region at both the temperatures shown in Fig. 3. This implies that there is a
common intragrain origin of this high-field MR behavior in the disordered and the ordered
samples.
We have carried out band structure calculations, simulating the disordered system within
a supercell approach where the Fe and Mo atoms were allowed to occupy the other sites in
contrast to the perfectly ordered system. Preliminary results from these calculations show
that in the disordered state, both up- and down-spin states exist at the Fermi energy; this
5
indicates that the half-metallic ferromagnetic ground state is not realised for any disordering
of the Fe and the Mo sublattices, though the system remains to be a ferrimagnet. Thus,
the high-field, intragrain contribution to the negative magnetoresistance is independent of
the long range ordering of Fe and Mo atoms and the cosnequent existence of the HMFM
state. Instead, it is to be associated with the usual negative magnetoresistance observed
for ferromagnetic substances, arising from a suppression of the spin-fluctuations with the
application of an external magnetic field.
In conclusion, we have prepared highly ordered and disordered samples of the per-
ovskite oxide, Sr2FeMoO6. We characterised the extent of ordering by x-ray diffraction
and Mössbauer studies of these samples. Rietveld analysis of the powder diffraction pattern
shows that the extent of ordering in the Fe/Mo sites is about 91% for the ordered sample,
while it is about 31% for the disordered sample. We point out that the disordered sample
is not a half-metallic ferromagnet, though it is magnetic even at the room temperature. We
showed that the magnetoresistance of the two samples are distinguished by the existence
of the sharp changes in the magnetoresistance at low fields for the ordered sample. In the
case of the disordered sample, there is no such sharp changes in the MR, though a negative
MR is seen at all fields for the disordered sample also. The MR of the disordered sample is
very similar to that of the ordered sample in the high-field regime, indicating a common ori-
gin. Thus, we conclude that the low-field magnetoresistance is dominated by the intergrain
spin-dependent scattering of the highly spin polarized charge-carriers of the half-metallic
ferromagnet, the high-field negative magnetoresistance behaviours in both the samples are
intrinsic, intragrain property and arise from the suppression of spin-fluctuations under the
application of a magnetic field on a magnetic system.
Acknowledgements: The authors thank Prof. B. Sriram Shastry and Dr. Tanusri
Saha Dasgupta for useful discussions.
6
REFERENCES
[1] K. -I. Kobayashi, T. Kimura, H. Sawada, K. Terakura and Y. Tokura, Nature 395 (1998)
677.
[2] N.D. Mathur, Burnell G., Isaac S.P., Jackson T.J., Teo B.S., MacManusDriscoll J.L.,
Cohen L.F., Evetts J.E., Blamire M.G., Nature 387 (1997) 266.
[3] A.J. Millis, P. B. Littlewood and B. I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144.
[4] A.J. Millis, B. I. Shraiman and R. Mueller, Phys. Rev. Lett. 77 (1996) 175.
[5] T. Nakagawa, J. Phys. Soc. Japan 24 (1968) 806.
7
IV. FIGURE CAPTIONS
Fig. 1. X-ray diffraction patterns of (a) ordered and (b) disordered Sr2FeMoO6. Order-
related peaks, appear at 19.6◦ and 38◦, shown in the insets on an expanded scale.
Fig. 2. 57Fe Mössbauer spectra of the disordered and ordered Sr2FeMoO6 at 300 and 4.2
K.
Fig. 3. The comparisons of percentage magnetoresistance between the ordered and dis-
ordered samples as well as the result presented by Kobayashi et al. [1] at (a) 4.2 K and (b)
300 K.
8
0
1000
2000
3000
4000
5000
 
Sarma et al., Fig. 1
(b)
In
te
ns
it
y 
(a
rb
it
ra
ry
 u
ni
ts
)
2θ
Disordered
10 20 30 40 50 60 70 80
0
1000
2000
3000
4000
5000
 
(a)
Ordered
18 19 20 21
340
390
440
490
540
 
 
I
38 39
300
325
350
375
400
425
 
 
II
18 19 20 21
340
390
440
490
540
 
 
I
38 39
300
325
350
375
400
425
 
  
II
-12 -6 0 6 12
 
 
Sarma et al., Fig. 2
4.2 K
4.2 K
300 K
300 K
Ordered
Disordered
Ordered
Disordered
In
te
ns
it
y 
(a
rb
it
ra
ry
 u
ni
ts
)
Velocity (mm/sec)
0 1 2 3 4 5 6 7
-40
-30
-20
-10
0
 
(a)
4.2 K
 Disordered
 Kobayashi et al.
 Ordered
%
M
R
-10
-8
-6
-4
-2
0
2
 
 
H(T)
(b)
Sarma et al., Fig. 3
300 K
 Disordered
 Kobayashi et al.
 Ordered
%
M
R


http://repository.ias.ac.in/24174/1/306.pdf

Magnetoresistance in ordered and disordered double perovskite oxide,
  Sr$_2$FeMoO$_6$

Magnetoresistance in ordered and disordered double perovskite oxide, Sr $_2$ FeMoO $_6$

Abstract

Similar works

Full text

Available Versions

Open Access Repository of IISc Research Publications

Publications of the IAS Fellows

Indian Academy of Sciences

Name not available

Crossref

Indian Academy of Sciences

Magnetoresistance in ordered and disordered double perovskite oxide, Sr2_22​FeMoO6_66​

Abstract

Similar works

Full text

Available Versions

Open Access Repository of IISc Research Publications

Publications of the IAS Fellows

Indian Academy of Sciences

Name not available

Crossref

Indian Academy of Sciences

Magnetoresistance in ordered and disordered double perovskite oxide, Sr $_2$ FeMoO $_6$