Journal of Nuclear and Radiochemical Sciences, Vol. 9, No. 2, pp. R1-R9, 2008

Reviews

Mössbauer Studies for Exploring CMR and TMR Perovskites
Z. Németha, c,*, Z. Klencsárb, A. Vértesa, and K. Nomurac,*aLaboratory of Nuclear Chemistry, Institute of Chemistry, Eötvös Loránd University and Chemical ResearchCenter, Hungarian Academy of Sciences, Pázmány P. s. 1/A, Budapest, HungarybDepartment of Surface Modifications and Nanostructures, Chemical Research Center, Hungarian Academy of
Sciences, Pusztaszeri út 59-67, Budapest, 1025, HungarycSchool of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received: November 14, 2008
In the present review we show a series of recent studies with the aim of exploring the electronic and magnetic
structure of materials with perovskite structure and strong magnetoresistance effect. The examined samples
include Sr2-x A xFeMO6 (A = Ca, Ba and M = Mo, Re, Ru) double perovskites with x = 0 and 0.1 and iron doped
La0.8Sr 0.2FeyCo1-yO3-δ cobaltate perovskites with 0 ≤ y ≤ 0.3. Besides understanding the basic electronic and magnetic interactions presented by these compounds, we have tried to trace nanoscale phase separation, which has
been introduced as a key feature for both colossal and intrinsic tunneling magnetoresistance recently.

1. Introduction
Doped transition metal compounds with complex electronic
and magnetic structures show a wide range of new physicalphenomena like high-temperature superconductivity, room
temperature magnetic semiconductivity, or colossal magnetoresistance (CMR).1-3 Following the discovery of the effect ofcolossal negative magnetoresistance in manganese based
perovskites,4-7 several different classes of transition metal compounds such as Sr2FeMoO6 double perovskites8 or La1-xSrxCoO3
perovskites9-12 were found to exhibit unusually high magnetoresistance (MR) partly as a peak around their Curie-temperature(CMR effect), partly as an increasing feature with decreasing
temperature (tunneling-type magnetoresistance, TMR)(Table1).
The most important difference between these two types of MR
is their temperature dependence: while CMR effect manifests

only around the magnetic temperature as a peak in the MR vs.
T plot, TMR increases monotonically with decreasing temperature. Due to the complexity of the underlying physical andchemical processes in these materials, the understanding of
their electronic and magnetic structure is one of the most vital
topics in condensed matter physics nowadays.
The first models aiming to shed light on the CMR effectfound in manganite perovskites, and to explain the unusuallystrong correlation between the magnetic state of the material
and its electric transport properties were based on the theories
of double exchange model13 and strong electron-lattice interactions.14 The former was based on the fact that in La1-xCa xMnO3
perovskites the doping divalent ions (usually Ca) introduce anumber of x extra electrons to the system, either oxidizing Mn3+
ions into Mn4+ or creating oxygen vacancies, although for low
doping rates the latter effect was found to be negligible. As a

TABLE 1: Magnetoresistance of perovskites and double perovskites of different composition
Chemical formulaLa0.75Ca0.25MnO3
La0.63Ca0.37FexMn1-xO3
La0.8Sr0.2FexCo1-xO3-δ

Sr2FeMoO6
Sr2-xCa xFeMoO6
Sr2-xBa xFeMoO6
Sr2FeReO6
Sr2FeCoO6
Sr2FeRuO6*

Doping levelx = 0.12x=0
x = 0.025
x = 0.05
x = 0.15
x = 0.2
x = 0.3
x = 0.1
x = 1.6

TMR at RT
5%
11%
11%
7%
1%

CMR at Tm
80%
99.75%
6%
8%
9%-

TMR ≤ 10 K
40%
99.9%
13%
84%
71%
37%
29%
10%
42%
?
?
21%
15%
25%*

Measured at 110 K* C o r r e sp ond i ng a ut ho r. E -m a i l : h e nt e s @ ch em .elt e. hu ,k-nomura@t-adm.t.u-tokyo.ac.jp© 2008 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 02/10/2009

Ref.
(6)
(7)
(78)
(79)
(80)
(81)
(81)
(81)
(8)
(39)
(39)
(26)
(48)
(59)

R2

NémethJ. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008
3+

3+

result, beside the Mn –Mn antiferromagnetic superexchangeinteractions, Mn3+ –Mn4+ ferromagnetic coupling manifests due
to electrons being itinerant between the manganese ions. It was
shown that for a pair of cations two electrons move at the sametime, one from the trivalent manganese to the intermediateoxygen, while the other from the oxygen to the Mn4+ (hence thename double exchange). This phenomenon (in contrast withsuper exchange) requires the ferromagnetic ordering of thetransition metal ions, so the ferromagnetism and the itinerancyof the electrons are strongly correlated.13 Later on, it was suggested that an unusually strong effect of the distortion of thecrystal lattice on the local trapping of electrons should be taken
into consideration, as well.14 However, a high number of compounds were found to show CMR effect, which has neither
double exchange nor Jahn-Teller distortions, which made thenecessity of these phenomena for the realization of colossal
magnetoresistance doubtful.2
Recently it was suggested that the doping at the rare earth
site in these materials can lead to electronic phase separation,
which is essential in the appearance of the CMR effect.2,15,16
This phase separation in manganese perovskites was attributed
to the locally increased density of Mn4+ ions in Sr rich regions,and so the local occurrence of the double exchange effect. It is
worth to express that the exact meaning of the term ‘local’ usedhere is still debated, ranging from few tens of nanometers to
micrometers.17-20 This model suggests that magnetic clusters
form even just above the magnetic ordering temperature (Tm),where they cannot build up long range magnetic order.
However, an external magnetic field (Hext) can have a huge
effect on enhancing the long range magnetism at this small
temperature range. As magnetism and electric conductivity areconnected to each other strongly in CMR materials, the appearance of bulk magnetism above Tm due to the external magnetic
field results in a colossal increase of bulk conductivity, whichis the magnetoresistance effect (Figure 1).
The tunneling type magnetoresistance is usually explainedby the polarization dependent hopping of electrons between
magnetically ordered regions. In a simplified picture, the probability of electron transfer between ferromagnetically ordered
areas is the highest if the magnetization of the two clusters isaligned. However, spin glass like systems, where the magnetic
regions are connected via several different magnetic interactions, are in a frustrated state with randomly aligned magnetizations. An external magnetic field can inf luence thisalignment to enhance the electron hopping, if the distance
between the conducting regions is small enough. The lower thetemperature, the higher the effect of Hext , i.e. the higher the
TMR ratio. This theory requires either a magnetic phase separation (as in the case of CMR effect) or a TMR effect between
crystal grains (Figure 2).
It follows then that the close investigation of the electronic
and magnetic phase separation phenomenon seems to be thekey point in the study of peculiar magnetoresistance effects. In
this review we show some recent studies on perovskite structured CMR and TMR materials concerning their electronic andmagnetic structure. Besides the numerous bulk investigation
techniques, applying Mössbauer spectrometry gives a uniquepossibility to discover the local state of iron, and to distinguishbetween electronically dissimilar regions even on a nanometerscale.

H���

H���

Figure 1. Schematic draw of the phase separation based theory ofcolossal magnetoresistance. The grey areas represent magneticallyordered clusters with an arrow indicating their magnetization. The
dashed arrows symbolize the path of electrons. The more scattered
the path, the less the electric conductivity. The white arrows indicate
the absence or presence of external magnetic field. The left panel
shows the magnetic state above the bulk magnetic ordering temperature with some small magnetic clusters in a nonmagnetic matrix inthe absence of an external magnetic field (Hext). The right panel
shows the effect of Hext: the magnetic clusters grow to coalesce, which
results in the increase of bulk electric conductivity.

��

��

����

����

Figure 2. Schematic picturing of the phase separation based theoryof tunneling type magnetoresistance. The grey areas represent magnetically ordered clusters with an arrow indicating their magnetization. The white arrows indicate the absence or presence of external
magnetic field. The left panel shows the glassy magnetic state withthe frozen magnetization vectors of magnetic clusters in the absenceof an external magnetic field (Hext). The right panel shows the effectof Hext: the magnetization of magnetic clusters tend to align with Hext,
making the electron hopping more probable and thus increase bulk
electric conductivity.

2. Sr2-xAxFeMO6 (A = Ca, Ba and M = Mo, Re, Ru) double
perovskites with x = 0, 0.1
Double perovskite systems consist of divalent cations in thefirst crystallographic site and two transition metal ions occupying alternatively the other metal sites of the perovskite structure (Figure 3). The two transition metal elements are usuallyhigh spin 3d and low spin 4d or 5d atoms. In principle, theFigure 3. Scheme of the basic ABO3 perovskite structure. In a cubic
cell representation the A cations occupy the center of the cube (whitesphere), while the B cations are on the corners (grey spheres).
Oxygen atoms (black spheres), surrounding octahedral the B ions, areonly indicated for one B. In a double perovskite, B sites are filled
alternatively (by e.g. with Fe and Mo).

Mössbauer studies for exploring CMR and TMR perovskites

1.9×10-4

1.8×10-4

1.7×10-4

1.6×10

-4

R3

1 kHz
10 kHz3

� ' / m kg

-1

J. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

0

50

100

150

200

250

T/K

Figure 5. AC magnetic susceptibility of Sr2FeMoO6 recorded withdifferent frequencies.
Figure 4. Magnetoresistance of Sr2FeMoO6 at 4.2 K. Figure takenfrom Reference 8.

1.000

relative transmission

structure and physicochemical properties of double perovskites
are then controlled by the size and valences of the different
cations. From the magnetic point of view, double perovskites
exhibit a variety of magnetic regimes, a result of the numerous
ferromagnetic and antiferromagnetic interactions between
nearest-neighbor and next-nearest-neighbor cations. Sr2FeMO6(M = Mo, Re, Ru etc.) double perovskites and their doped
derivatives have usually high magnetic ordering temperaturesand consequently high tunneling type magnetoresistance values even at room temperature (see e.g. Figure 4),8,21-27 making
them potential candidates for next generation MR devices. On
the way to explore the source of magnetoresistance of thesesystems, intensive research was conducted on several memberof this family of compounds. As one of the significant results,
cation disorder and the resulting nanometer scale electronic
phase separation were explained as the possible source of theunusually strong magnetoresistance.28-31 However, it was alsosuggested that the TMR effect of double perovskites measured
with low external magnetic fields originates from the tunnelingof electrons through the grain boundaries. 2 2 Anyhow,
Cham issem et a l have found two iron components in
Sr2FeMoO6, one with the expected 6 Mo neighbors, maintaining strong electron delocalization between Fe and Mo, and one
with a lower degree of delocalization. 30% of the iron ionsbelonged to the latter type, and they were attributed being next
to Mo vacancies.32 This effect was modeled successfully withthe help of Mössbauer investigations.33 It seemed to be interesting to investigate these double perovskites focusing on theoccurrence of nanosize phase separation.
Effect of cation disorder. Sr2FeMoO6 has been known as aconducting ferrimagnet with a relatively high magnetic transition temperature of TC ≈ 410 K.34 Recently, however, neutron
diffraction measurements revealed zero magnetic moment on
Mo cations, and consequently the ferrimagnetic structure of
Sr2FeMoO6 was disputed.35 It was suggested that while localized magnetic moment can be found only on ferromagneticallyarranged Fe3+ cations, the 4d1 electron of Mo is delocalized aswell as spin polarized due to the involvement of Fe3+ t2g levelsin the conduction band, the latter being formed by the hybridization of 3d (Fe) and 4d (Mo) electronic states.35-37 Inspiredby the high ratio of the second iron component (corresponding
to disordered iron cations),32 we prepared a Sr2FeMoO6 sample
with higher cation disorder in order to examine the expected
local separation of phases via the different iron environments.
The temperature and frequency dependence of the real com-

0.995

300 K

0.990

1.00

0.98

0.96

-15

4.2 K
-10

-5

0

v / mm s

5

10

15

-1

Figure 6. 57Fe Mössbauer spectra of Sr2FeMoO6 at 300 K and 4.2 K.
While one of the four components found at 4.2 K transformed to adoublet at room temperature, the other three sextets merged into a
broad distribution. (Ref. 36)

ponent (χ’) in AC susceptibility of the Sr2FeMoO6 sample withhigh cation disorder indicates spin glass or spin cluster glasslike behavior with a glass transition temperature above thehighest temperatures used (Figure 5).38 Figure 6 shows the 57Fe
Mössbauer spectra of Sr2FeMoO6 measured at T = 300 K and
4.2 K.38 The latter can be decomposed into four different magnetically split subspectra reflecting magnetically ordered iron
cations with different electronic configurations on the 3d level.
It was shown, that the two major components reflect iron ionsin the ideal, perfectly ordered regions of the double perovskite(showing a high degree of hybridization between 3d(Fe) andR4

NémethJ. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008
1.0
0.1
0.8 a )
1.0

a)

0.0
0.1

b)

0.0
0.1

P(Bhf / T)

relative transmission

0.8 b )
1.0

0.8 c )
1.0

0.8

d)

c)

0.0
0.1

d)

0.0

1.0

0.1
0.8
e)

1.0

e)

0.0

e)

0.1

0.8
f)

-6

-4

-2

0

2
-1

v / mm s

Tc (K)

Figure 7. The change of room temperature magnetoresistance of
Sr2-x-yBayCa xFeMoO6 due to Ba or Ca doping (y and x, respectively)
measured with the indicated external magnetic fields. (Ref. 39)

Figure 8. The change of Curie-temperature of Sr2-x-yBayCa xFeMoO6
with Ba or Ca doping (y and x, respectively). (Ref. 39)

4d(Mo) electronic levels) and one with decreased electron density at the minority spin 3d5+δ level of iron, respectively.32 The
decreased minority spin electron density can be interpreted asa reduced degree of delocalization of the 4d1 electron of Mo
ions. The two extra subspectra with lower isomer shifts andhigher hyperfine magnetic fields refer to iron ions in strongly
disordered regions of the studied sample, where the delocalization of Mo electrons is suppressed.38 Increasing the temperature of the sample, one observes a considerable broadening of
the absorption lines (Figure 6), implying that with the higher
mobility of electrons, due to the increasing thermal energy, a
distribution in the level of electron-delocalization develops. In
overall, the evaluation of the Mössbauer spectra was in a very
good accordance with the theory of small size phase separation, occurring due to the cation disorder in Sr2FeMoO6 double
perovskite.
Effect of doping on Sr site. Doping Sr2FeMoO6 with a small
amount of Ba or Ca on the Sr sites increases the magnitude ofmagnetoresistance significantly.39 It was shown that room temperature MR doubled when 5% of strontium was exchanged to
Ca or when 80% of strontium was replaced by Ba ions (Figure7) along with the change of the Curie-temperature (Figure 8).
These phenomena were explained by the chemical pressure
effect of doping, as the major source of MR in these double
perovskites is associated with the tunneling of polarized electrons.39 Our recent 57Fe Mössbauer study of Ca and Ba doped
Sr 2 FeMoO 6 double perovskite has shown that Ba doping
increases the isomer shift of the main component (being a

4

6

0.0

f)

0

10

20

30

40

Bhf / T

Figure 9. 57Fe Mössbauer spectra (left panel) of Sr2-xBa xFeMoO6 (0 ≤x ≤ 2) at room temperature and the corresponding distributions of thehyperfine magnetic fields of the major component. a)–f) represent x= 0, 0.2, 0.4, 1, 1.8, and 2, respectively.

broad sextet, best fitted with a distribution of magnetic hyperfine field) and decreases the maximal magnetic field (Figure9).40 These indicate clearly the decreased level of hybridization
between Mo and Fe atoms occurring due to increasing the Bacontent. While the root of this phenomenon lies in the different cationic size of Sr and Ba ions and thus in the change oflattice parameters (often called as chemical pressure), the result
is a higher electric resistance and a lower TC. Besides, it was
found that chemical pressure affects the phonon DOS (densityof states), determined by nuclear inelastic scattering, of 5% Ba
and Ca doped Sr2FeMoO6, too.40 While in the case of Ba doping the phonon peak edges shifted to lower energies, indicating
softening of the lattice due to the expansion of the lattice cell,
Ca doping result shows the opposite tendency (Figure 10).
On the other hand, in the Re based analogue, Sr 2 FeReO 6
resistivity and Curie temperature increases with Ca doping as aconsequence of changing the level of hybridization of Fe 3d
and Re 5d electrons.41 The strong hybridization between Fe
and Re was shown in many studies, and a model for cation disorder was stated.42,43 Investigating the effect of Ba doping at
the crystallographic site of Sr in Sr 2 -x Ba x FeReO 6 double
perovskites with x = 0 and 0.1, three iron components were
found due to cation disorder (Figure 11).44 Although the saturation magnetization of Sr2FeReO6 was drastically decreasedby the introduction of x = 0.1 barium ions (Figure 12), thehyperfine parameters obtained from the Mössbauer spectra at
low temperatures show only slight changes. However, above
the magnetic transition temperature the observed intensity ratioof the singlet and the doublet, associated with the ordered iron
ions with 6 Re neighbors and the disordered iron ions, respectively, is surprisingly low. This phenomenon was explained by
the low crystalline size of the samples (being between 80 nm
and 800 nm) due to the sol-gel preparation, resulting in a higher
number of disordered iron ions.44 Moreover, significant dependence of the Mössbauer-Lamb factor for the different iron
species was also pointed out, which contributed to the low area
ratio of the singlet subspectrum.44 These results suggest that insamples prepared to have small crystalline size the ratio ofdisordered and ordered regions can be tuned in a great extent,
resulting in significant changes of bulk magnetic properties
and so of their magnetotransport.
The cobalt based double perovskite, Sr2FeCoO6-δ, has a spinpolarized conduction band similar to that of the Sr 2 FeMoO6
relatives, the former having Co 4+/Co3+ ion pairs with itinerant
electron holes instead of Mo5+/Mo6+ with itinerant electrons on
the t 2g level.45,46 Both materials show no sign of either Jahn-

Mössbauer studies for exploring CMR and TMR perovskites

J. Nucl. Radiochem. Sci., Vol. 9, No. 2, 20082

Sr1.9Ca0.1FeMoO6

0.08

1

0.06

M / (�B/ f.u.)

0

0.04

phonon DOS; g (E ) /(meV)-1

0.02
0.00
0.08

Sr2FeMoO6-1

Sr2FeReO6

-2
2
1
0

0.06

-1

0.04

Sr1.9Ba 0.1FeReO6

-2

0.02

-60

-40

-20

0

20

40

60

H / kOe

0.00
0.08

Sr1.9Ba0.1FeMoO6

Figure 12. Magnetic hysteresis curves of Sr 2 FeReO 6 (top) andSr1.9Ba0.1FeReO6 (bottom) measured at 10 K.

0.06
0.04
0.02
0.00
0

20

40

60

E / meVFigure 10. Phonon DOS distributions of undoped and 5% Ca or Ba
doped Sr2FeMoO6 double perovskite measured by nuclear inelastic
scattering.

1.00

0.95

relative transmission

R5

0.90

Sr2FeReO6

1.00

0.95

0.90

Sr1.9Ba0.1FeReO6
-10

-5

0

v / mm s

5

10

-1

Figure 11. 57Fe Mössbauer spectra of Sr2FeReO6 (top) and Sr1.9Ba0.1
FeReO6 (bottom) double perovskites at 10 K.

Teller distortions or double exchange effects.47 However, theirhigh magnetoresistance48 urged for a detailed study of the elec-

tronic structure of double perovskites to understand the basicroots of their MR. The study of these perovskites with the resonant nuclear inelastic scattering method revealed significant
changes in the phonon spectra of iron at the magnetic ordering
temperature (TC ≈ 150 K), such as narrowing and softening of
the low energy peaks near 12–14 meV, which are well separated from the main energy bands at 20–45 meV. The latter
softens below TC, too, which feature was attributed to the effectof the lattice vibrations on the formation of the spin-polarizedband.49
Recently, ruthenium based double perovskites became extensively studied due to their rich magnetic phase diagram andvarious magnetic exchange interactions. As the most popular
example, CaRuO3 and SrRuO3 perovskites revealed similar
chemical and structural properties, although their magnetic
characteristics differ significantly.50-56 While SrRuO3 is a ferromagnet with TC = 160 K, CaRuO3 is a spin glass with shortrange antiferromagnetic interactions. Ca xSr1−xRuO3 ruthenates
show a gradual transition between the magnetic states of thetwo extreme compositions.57-60 As for the Fe-rich, Sr2FeRuO6
ruthenate, beside its TMR effect59 it indicates a high degree of
Fe/Ru disorder and spin-glass structure at 4.2 K as reflected
through the wide distributions of the hyperfine interactions in
57
Fe Mössbauer spectra.61 Later on, the spin-glass behavior of
Sr2FeRuO6 was proved by combined magnetic and Mössbauer
data and simulation studies, too, with a transition temperatureof 60 K.62-64 We have studied the effect of cation substitution at
the strontium site of Sr 2 FeRuO 6 double perovskite, too.
According to the transmission electron microscopic and scanning electron microscopic studies, the average particle size ofthe samples produced (Sr 2 FeRuO 6 , Sr1.9 Ba 0.1 FeRuO 6 , andSr1.9Ca0.1FeRuO6) were between 15 nm and 70 nm, as a consequence of the sol-gel preparation method.65 The small level of
doping resulted in very small changes in the cell parameters forboth Ca and Ba, but they altered the local and bulk magnetic
properties considerably. Magnetization and in-field Mössbauer
spectroscopy measurements fortified the spin glass state below
the magnetic transition temperature for these materials, themagnetic freezing point being 64 K, 66 K, and 86 K for the
calcium, barium, and undoped sample, respectively. 6 5Moreover, Ca doping was found to strengthen the antiferromagnetic interactions, as the Weiss temperature was found to
be more negative with about 50 K in Sr1.9Ca0.1FeRuO6 than in
the parent Sr 2 FeRuO 6 .65 This shows again that doping thestrontium site with different sized divalent cations affects longrange magnetic interactions in a great extent. That is, the magnetic properties and so the magnetoresistance of double

R6

NémethJ. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

perovskites can be tuned effectively with optimizing the composition.

TC
Tg

200

3. La0.8Sr0.2FeyCo1-yO3-δ perovskites with 0 ≤ y ≤ 0.3
PM

T/ K

150

100

PM + FM

50

SG

SCG
0
0.00

0.05

0.10

0.15

0.20

y
Figure 13. Magnetic phase diagram of La0.8Sr 0.2FeyCo1-yO3-δ (0 ≤ y ≤0.3) perovskites. PM denotes paramagnetic, FM ferromagnetic, SCG
spin-cluster glass, while SG spin-glass phases.
y=0

0.0

0.0

y = 0.025

6.0×10-5-50.0

y = 0.05

1.0×10-4
5.0×10y = 0.15

1.0×10-5

� ' / m3 kg - 1

� ' / m3 kg- 1

2.0×10-5

3.0×10-5

3.0×10y = 0.1

4.0×10-5

6.0×10-5-5

5.0×10-6
0.0

y = 0.2

1.0×10-5
5.0×10

0.0

-6

0.0

0

50

100

150

200

250

y = 0.3

1.0×10-5

T/K

5.0×10-6
0.0

0

50

100

150

200

250

T/K
Figure 14. Real part of AC magnetic susceptibility of La 0.8 Sr 0.2
FeyCo1-yO3-δ (0 ≤ y ≤ 0.3) perovskites as a function of temperature.

1,000

1,000

0,980

0,995

relative transmission

relative transmission

In the case of the LaCoO3 derived perovskites, doping withdivalent ions (e.g. Ca, Sr) at the rare-earth site results in converting some Co3+ ions into Co4+ (and thus introducing electron
holes in the transition metal network) as well as increasing the
average spin state of cobalt ions.12 Strontium substitution also
alters the electronic transport, the magnetic, as well as the MR
properties of these materials.9-12,66-74 While La1-xSrxCoO3-δ is asemiconductor for low Sr concentration levels, it becomes ferromagnetic as well as metallic above x ≥ 0.18 strontium doping.
Below the doping level x = 0.18 La1−xSrxCoO3−δ shows semiconducting characteristics accompanied by only short range magnetic correlations with a characteristic coherence length of afew nanometers. Below ~ 60 K the magnetization of these separate magnetic droplets freezes out to some locally preferred
direction, and the material enters a glassy magnetic state.
Above x ≈ 0.18, below the Curie temperature coalesced ferromagnetic clusters coexist with insulating, magnetically disordered regions on the nanometer scale. The magnetic clusters
are thought to owe their ferromagnetic and metallic characterto the double exchange effect between Co3+ and Co4+. At thesame time, in the insulating regions superexchange interaction
between nearest neighbor Co3+ ions is thought to be responsible
for the absence of long-range ferromagnetic order and diminished electrical conductivity. The simultaneous presence offerromagnetic (between Co4+ and Co3+) and antiferromagnetic(between Co3+ and Co3+) exchange interactions, as well as thepossible occurrence of Co in a diamagnetic (low-spin Co3+)
state, leads to the formation of a glassy magnetic state, which
visualizes itself as a spin-glass (for x ≤ 0.18) or as a clusterglass (for x > 0.18).12,75
Magnetoresistance of La1-xSrxCoO3-δ depends strongly on thedoping level x. While for x ≥ 0.2 a CMR peak around TC isdominant, for x ≤ 0.18 tunneling-like magnetoresistance is mostpronounced at low temperatures below the spin-glass freezing
temperature (this behavior is sometimes referred to as ‘low-TMR’). For 0.18 ≤ x ≤ 0.2 the two types of MR occur simultaneously.
Doping of La1-x Srx CoO3- δ at the transition metal site withiron (resulting in La1-xSrxFeyCo1-yO3-δ perovskites) considerablyenhances low-T magnetoresistance,76,77 while at the same time
also reversing the effect of Sr doping from the viewpoint ofbulk magnetization and electrical conductivity, and also it wassuggested that the alteration of the MR properties of the investigated iron doped cobalt perovskites can be interpreted by themodification of the magnetic field induced low-spin to highspin transition of Co3+ ions.77 However, the effect of iron doping on cobaltate perovskites on the local and bulk electronic
and magnetic properties was far from being explored or understood.
We carried out measurements on members of the La1-x
SrxFeyCo1-yO3- δ family of compounds with the aim to explore
the local electronic and magnetic structure of these perovskites
around the magnetic phase transition regime (x ≈ 0.2).
Figure 13 shows the magnetic phase diagram of La0.8Sr0.2Fey
Co1-yO3- δ (0 ≤ y ≤ 0.2) perovskites, as suggested by magnetic
susceptibility measurements (Figure 14) and 57 Fe Mössbauer
spectroscopy (Figure 15 and Figure 16).78-81 Ferromagnetism,i.e. the presence of magnetically ordered clusters displaying asubstantial effective magnetic moment and a characteristic size
well above ~ 10 nm, is observed only for y < 0.15, but even inthis iron concentration range paramagnetic (as well as superparamagnetic) regions coexist with the magnetically ordered
ones (Figure 13). As referred to also by the temperature dependence of the in-phase part of the AC magnetic susceptibility

218 K

1,000

149 K

1,000

0,990
0,995
191 K

0,980
-8

-6

-4

-2

0

2
-1

v / mm s

4

6

8

78 K

-8

-6

-4

-2

0

2

4

6

8

-1

v / mm s

Figure 15. 57Co emission Mössbauer spectra of La0.8Sr 0.2CoO3-δ measured at the temperatures indicated.

(Figure 14), magnetic glass-freezing transition was observed in
all the studied samples (y ≤ 0.3): below Tg ≈ 53 K the magnetic
state of La0.8Sr0.2FeyCo1-yO3-δ (y ≤ 0.3) resembles that of a spinglass (for y ≥ 0.15) or spin-cluster glass (for y < 0.15).
Interestingly, the temperature at which this glass-freezing transition occurs does not seem to depend on the iron concentrationy.
The magnitude of χ’ shows a decreasing tendency withincreasing iron concentration (Figure 14), showing that ironsubstitution gradually suppresses ferromagnetic correlations.

Mössbauer studies for exploring CMR and TMR perovskites

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J. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

1.00
1.00
0.95

doublet:y=0
y = 0.025
sextet:y=0
y = 0.025

0.5

0.90
0.99
125 K

1.00

0.95
100 K

1.00

67 K

1.00
0.99
0.98

40 K

0.4

� / mm s- 1

relative transmission

relative transmission

0.85

0.3

1.00

0.2
0.98

0.99

0.98

-6

-4

-2

0

2
-1

v / mm s

4

6

8-10 -8

0.1

4.2 K

-6

-4

-2

0

2

4

6

8

100

10

150

200

-1

v / mm s

Figure 16. 57Fe Mössbauer spectra of La0.8Sr 0.257Fe0.05Co0.95O3-δ measured at the temperatures indicated.

Indeed, compa r ing to the Curie temperature value of
La 0.8Sr 0.2CoO3- δ (being TC = 220 K), for the sample with y =0.05 (by applying the Curie-Weiss formula for the high temperature part, T > 185 K, of the AC susceptibility) TC = 120 K canbe identified as Curie temperature, along with 3.2 µB/f.u. effective magnetic moment.80 In contrast, the in-phase part of AC
magnetic susceptibility displays no ferromagnetic transition forLa0.8Sr 0.2Co1−yFeyO3 with y ≥ 0.15.81 However, as the temperature dependence of the inverse of χ’ revealed,81 above the glassfreezing temperature the seemingly paramagnetic state consistsof magnetically ordered atom clusters in which the magnetic
moment of a substantial fraction of magnetic ions is oriented
opposite to that of the cluster, which in turn results in clusters
with a rather small effective magnetic moment. The formationof this state is thought to be mediated by the strong antiferromagnetic exchange interaction between nearest neighbor Fe3+ –Fe3+ ion pairs, which are expected to be rather abundantly
present in the samples with y ≥ 0.15.
In the samples with low (y ≤ 0.025) Fe concentrations iron
may also be regarded as a probe of the magnetic structure of
the host material (i.e. La0.8Sr 0.2CoO3-δ): while above the Curie
temperature of the ferromagnetic cobalt clusters only a paramagnetic signal can be seen in the corresponding Mössbauer
spectra, indicating paramagnetic nucleogenic (in the case of57
Co doping) or aboriginal (in the case of y > 0) Fe3+ ions, thespectra measured below TC show a magnetic subspectrum in
addition to the existing paramagnetic doublet (Figure 15).
Whereas the magnetic subspectrum can be identified as originating from iron ions connected by magnetic exchange interaction to Co ions in hole-rich clusters of La 0.8 Sr 0.2 CoO3- δ , theparamagnetic component signalize the presence of iron in holepoor paramagnetic regions. In the case of La0.8Sr0.257Fe0.05Co0.95
O3- δ iron doping already results in a substantially decreased
Curie temperature78 as well as in a fragmentation of the ferromagnetic regions into nanosized clusters, which latter is visualized in the 57 Fe Mössbauer spectra as a magnetic relaxation
effect (most apparent at around 70 K, see Figure 16). For 0.15
≤ y ≤ 0.3 La0.8Sr 0.2FeyCo1-yO3- δ magnetic relaxation signalizes
the formation of the glassy magnetic state by becoming apparent just above the magnetic freezing temperature Tg.80
The Mössbauer isomer shifts of the sextets (δs) and doublets( δ d ) d i f fer expl icit ly i n each measu re d sp e ct r u m of
La0.8Sr0.2FeyCo1-yO3-δ with y ≤ 0.025: at a given temperature one
observes an average gap of about 0.15 mm/s (Figure 17).79
While the higher δd refers to high-spin trivalent state, the lower
δs values indicate a small decrease of the population of 3d electrons, which can arise from e.g. delocalization of valence electrons. This result is in good agreement with the presumption

250

300

T/K
Figure 17. 57Fe Mössbauer isomer shifts of La0.8Sr 0.2FeyCo1-yO3-δ (y ≤0.025) as a function of temperature.
20

50.0

y=0

15

25.0

5

12.5

0
100

0.0
50.0

y = 0.025

75
50
25
0
100

y = 0.2

37.5
25.0
12.5
0.0
20

y = 0.05

75

10

25

5

0

50

100

150

T/K

200

250

y = 0.315500

y = 0.15

37.5

10– MR / %78 K-8– MR / %

0.96

300

0

0

50

100

150

200

250

300

T/K

Figure 18. Temperature dependence of the magnetoresistive property of La0.8Sr 0.2FeyCo1-yO3-δ (0 ≤ y ≤ 0.3) perovskites.

of the cluster model: while in the magnetically ordered conductive clusters the eg electrons of the Co3+ ions are delocalized
between the trivalent and tetravalent cobalt ions (due to the
double exchange process, resulting in metallic conduction andferromagnetic exchange interaction), in the paramagnetic
matrix they remain more localized. Thus, the isomer shiftreferring to the ions in the ferromagnetically ordered clusters
should be indeed lower (representing a mixed 3+ and 4+
valance character) than that of the doublets referring mostly tothe paramagnetic matrix. This finding points out the existenceof an electronic phase separation in the aforementioned cobaltate perovskites.
With the help of the electronic phase separation based cluster theory, the observed magnetoresistance (Figure 18)79 can be
explained satisfactorily. As long as the long-range orderedmagnetic clusters are maintained (y ≤ 0.05), a CMR peakaround TC can be seen, as a consequence of the ordering effectan external magnetic field exerts on the magnetic clusters.
However, the efficiency of this ordering effect depends on thesize, number, and interconnectivity of available clusters, which
factors are most sensitive to doping at around the percolationthreshold (x ≈ 0.18). At this threshold the introduction of ironeffectively destroys (i.e. frustrates, fragments) larger ferromagnetic clusters as well as their interconnectivity, which results in
a suppression of the CMR peak. For y ≥ 0.15 the clusters
become too separated to produce colossal magnetoresistance.
On the other hand, the low-temperature magnetoresistance isenhanced drastically with small iron doping (Figure 18). This
type of magnetoresistance of cobaltate perovskites is believed

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J. Nucl. Radiochem. Sci., Vol. 9, No. 2, 2008

to origin from spin-dependent electron transfer between isolated ferromagnetic clusters, where the external magnetic fieldorientates the spins of clusters, thus increasing hopping probability. The intensity of the resulting MR, which should dependon the size and distance of clusters, increases with decreasingstrontium content in La1-x Srx CoO3− δ perovskites. While theintense growth of MR at low temperatures as a result of y =0.025 iron doping may be explained by a break-up of FM clusters via individual iron ions, the decrease of low-T MR on further increase of y may be connected to the insulating nature ofelectron paths involving antiferromagnetic Fe3+ – Fe3+ ion pairs,
as well as to the intercluster distances becoming too far for
effective electron hopping.
4. Summary
On the search for the roots of strong magnetoresistance inperovskite structured systems, we have traced the occurrence
and also the possibility of modulation of electronic and magnetic phase separation, which is thought to be responsible forthe realization of both CMR and TMR effects. Our investigations on the local electronic and magnetic state of cobalt based
doped CMR perovskites as well as of iron based double
perovskites showing TMR effect fortify the presence of coexisting nanometer scale magnetic phases. It seems that the easiest and most reasonable way to improve magnetoresistance inthese materials is the accurate adjustment of the composition
with a distinguished attention on the small amount of doping
cations. With the fine alteration of dopants both the local magnetic interactions and the size and interconnectivity of magn e t ic clu st e r s c a n b e o p t i m i z e d t o g ive m a x i mu m
magnetoresistance. For high CMR effect easily alignable big
clusters are necessary which coalesce below Tm, while for high
TMR the optimum condition is to have isolated, frustrated
magnetic clusters with short intercluster distance.
Acknowledgement. The authors appreciate the support of
the Hungarian Science Foundation OTKA K 62691. One ofauthors, Zoltán Németh thanks Matsumae International
Foundation for inviting him to the University of Tokyo, Japan.
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