Journal of Nuclear and Radiochemical Sciences, Vol. 5, No. 1, pp. R1-R8, 2004

Reviews
Colossal Magnetoresistance in Focus: Studies of Different CMR Materials by Mössbauer
Spectroscopy
Z. Klencsár,a Z. Németh,b Z. Homonnay,b E. Kuzmann,a G. Gritzner,c Á. Cziráki,d I. Kotsis,eM. Nagy,e and A. Vértes*, a,baResearch Group for Nuclear Methods in Structural Chemistry, Hungarian Academy of Sciences, Roosevelt t. 9.,
Budapest 1051, HungarybDepartment of Nuclear Chemistry, Eötvös Loránd University, Pázmány P. s. 1/a, Budapest 1117, HungarycInstitut für Chemische Technologie Anorganischer Stoffe, Johannes Kepler Universität, A-4040 Linz, Austriad
Department of Solid State Physics, Eötvös Loránd University, POB 32, 1518 Budapest, HungaryeDepartment of Silicate- and Materials Engineering, University of Veszprém, Hungary
Received: December 13, 2003
The study of transition metal based materials displaying colossal magnetoresistance (CMR) is one of today’s most
exciting fields of research mainly due to the scientific challenge of finding the possibly common root (if thatexists) of the CMR effect in the various materials. In this review we present novel results obtained by 57Fe
Mössbauer studies done on three different types of CMR materials, namely the cobalt based perovskites
La 0.8 Sr 0.2 Fe x Co 1−x O 3−d , the Sr 2 FeMoO 6 double perovskite, and copper containing chalcogenide spinels(Fe1−xCuxCr2S4).

100

Magnetoresistance (MR) is the change of the electricalresistivity due to an applied external magnetic field. It is
usually expressed as MR = [R(H) − R(0)]/R(0) where R(0) andR(H) denote the electrical resistivity measured without andwith H external magnetic field applied, respectively. In thecase of negative magnetoresistance the value of MR isrestricted to the range of −100% to 0%. Although magnetoresistance was known long ago, the technological applicability of
the MR effect was only available when giant negative magnetoresistance (GMR) was found in magnetic multilayers even atroom temperature. These materials are used nowadays in theread-heads of several magnetic storage devices. The discoveryof an even higher MR in manganese perovskites (calledcolossal magnetoresistance, CMR)1 raised renewed interest in
the transition metal based materials with anomalous MR properties.
In the present article we report the results of some recent
investigations on members of three different classes of CMR
materials: La 0.8 Sr 0.2 Fe x Co 1−x O 3−d perovskites, Sr 2 FeMoO 6
double perovskite, and Fe1−xCuxCr2S4 chalcogenide spinels.
The magnetoresistance properties of the investigated materials
are demonstrated in Figure 1. The figure illustrates well thevariety of magnetoresistance phenomena in materials withdifferent structure. While the FexCu1−xCr2S4 spinels show a CMR
effect2 similar to that observed originally in the manganeseperovskites, in the double perovskite system Sr2FeMoO6, MR
decreases near linearly with temperature,3 and the iron doped
cobalt perovskites display huge negative MR below ∼ 50 K as
well as above ∼ 150 K.4
Iron is thought to play a key role in the realization ofmagnetoresistance in each of the investigated compounds.
Consequently, the study of the local state of iron as a functionof temperature, sample composition, etc. may reveal the microscopic background of the magnetoresistance effect in thesesystems. As 57Fe Mössbauer spectroscopy can reflect even80

*Corresponding author. E-mail: vertesa@ludens.elte.hu. FAX:
+36-1-209-06-02.

MR / %

1. Introduction

x=0
x = 0.025
x = 0.050
x = 0.15
x = 0.30

60
40
20
0
0

50

100

150

200

250

300

T/K

10

MR / %

0.1

0.2 0.3 0.4
0.5

0.05

0
0

100

200

300

T/K
30

MR / %

20

10

0

0

100

200

300

T/K
Figure 1. Magnetoresistance properties of La 0.8 Sr 0.2 Fe x Co 1−x O 3−dperovskites4 (top, the magnetic field was 7.5 T for x > 0 and 7 T for x= 0), Fe1−xCuxCr2S4 chalcogenide spinels2 (in the middle, the numbers
inside the picture correspond to the copper concentration, x, theapplied magnetic field was 5 T), and Sr2FeMoO6 double perovskite3(bottom, measured in a magnetic field of 0.4 T).

© 2004 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 02/28/2004

Z. Klencsár

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

minute changes in the local electronic, magnetic, and vibrational state of iron, it is an especially useful tool in the study ofthese materials.
2. La0.8Sr0.2FexCo1–xO3–d Perovskites
While in manganese perovskites the colossal magnetoresistance effect is observed only at temperatures in the neighborhood of the Curie temperature, iron doped La0.8Sr0.2FexCo1−xO3−d(0.025 < x < 0.3) display negative magnetoresistance that
increases monotonically with temperature in the temperatureinterval 150 K ≤ T ≤ 300 K. Additionally, La0.8Sr0.2FexCo1−xO3−dalso displays strong negative MR below T ≈ 50 K.4 The negative
MR observed above T≈150 K is particularly interesting because,
unlike in the case of manganites, in La0.8Sr0.2FexCo1−xO3−d themagnitude of MR increases continuously even in the paramagnetic phase. Namely, the Curie-temperature of the iron free
end member La0.8Sr0.2CoO3−d is TC ≈ 180 K,5 and TC decreases
with the x iron concentration.6
Above T ≈ 150 K, the amplitude of negative magnetoresistance in La 0 . 8 Sr 0 . 2 Fe x Co 1 − x O 3 − d perovskites was found to
decrease with increasing iron concentration in the range of
0.025 to 0.3. At the same time, the iron free La0.8Sr0.2CoO3−dperovskite displays only a modest magnetoresistance effectaround its Curie-temperature. This suggests that iron plays a
key role in the realization of the considerable negative magnetoresistance observed in La0.8Sr0.2FexCo1−xO3−d perovskites.4, 7
One of the main questions regarding these perovskites is how
already a low amount of iron substituted for Co manages tochange dramatically the MR properties of the mother compound
La 0.8Sr 0.2 CoO 3−d . To collect information in this regard, we
decided to take a closer look on the temperature dependence ofthe local state of iron in La0.8Sr0.257Fe0.05Co0.95O3−d. The local
electronic, magnetic and vibrational state of iron can be advantageously studied by the help of 5 7 Fe transmission and57
Co(57Fe) emission Mössbauer spectroscopy (in the followings
referred to as TMS and EMS, respectively).
The La0.8Sr0.257Fe0.05Co0.95O3−d sample was prepared by thefollowing method. At first, La0.8Sr0.2CoO3 samples were fabricated via the citric acid-ethylene glycol method. The stoichiometric amounts of La(NO3)3 ·6H2O, SrCO3 and Co(NO3)2 ·6H2O
were dissolved in 50 mL of concentrated nitric acid. 57Fe metal
was dissolved in nitric acid and the respective aliquot wasadded to the nitric acid solution. The solution was diluted with
200 mL water. Thirty grams of citric acid and 8 g of ethylene
glycol were added. The aqueous solution was evaporated until
a brown gel was obtained. The gels were dried under vacuum
at 125˚C. The residues were ground, first heated to 500˚C,
kept this temperature for two hours and then heated to 800˚C.
The heating rates were 5 K min−1. The samples were kept at
800˚C for five hours. Upon ballistic cooling the samples wereground and compacted at 750 MPa into discs of 10 mm diameter. The specimens were then sintered in air at 1200˚C for 24hours. Heating rates were 3 K min−1, cooling rates 10 K min−1.
57
Fe transmission Mössbauer spectroscopy measurements
were carried out on powdered samples. The temperaturedependent 57 Fe TMS measurements were performed in atemperature controlled flow-through type liquid nitrogen cryostat (Leybold) and an APD type liquid He cooled cryostat.
During the measurements the temperature of the samples was
kept constant with a precision of ∆T ≈ ±0.5 K. The γ-rays
were provided by a 57Co(Rh) source with 109 Bq activity.
For the purpose of 57Co(57Fe) emission Mössbauer measurements, the La 0.8 Sr 0.2 57 Fe 0.05 Co 0.95 O 3−d sample was doped by
57
Co. Ethanolic solution of 100 MBq carrier-free 57Co-nitrate
was transferred onto the La-Sr-cobaltate pellet drop-by-drop,
and after the evaporation of the alcohol, the pellet was treatedfor 3 hours in dry oxygen flow at 1000˚C for a perfect isotopic
exchange. XRD measurement confirmed that the perovskite

structure and the composition of the original sample were notchanged during the associated heat treatment. Two Mössbauer
spectra using the conventional emission geometry8 wererecorded at 4.2 K (in a helium bath cryostat) and at room
temperature.
57
Fe isomer shift values are given relative to α-iron at room
temperature. Mössbauer spectra were analyzed with version
3.0 of the MossWinn program.9
57
Fe transmission Mössbauer spectra of La0.8Sr0.257Fe0.05Co0.95O3−d
at different temperatures are shown in Figure 2. Above T ≈ 45
K the spectrum consists of a quadrupole doublet with a small
quadrupole splitting (∆) of ∆ ≈ 0.2 mm s−1. The isomer shift(δ ) of the doublet increases from δ ≈ 0.33 mm s−1 at T = 290 K
to δ ≈ 0.4 mm s−1 at T = 80 K. These δ and ∆ values indicatethat the electronic state of iron is high spin Fe3+ (t2g3eg2) in thiscompound, and in contrast to some assumptions (see, e.g.,
Reference 10) it does not support the existence of Fe4+ in thissystem. The Lorentzian curves of the quadrupole doublet are
rather broad; their line width (Γ ) is around Γ ≈ 0.35 mm s−1 at
T = 290 K and it increases with decreasing temperature.
In the spectrum taken at T = 45 K the quadrupole doublet issuperimposed on a strongly broadened absorption curve(Figure 2). This latter broadened subspectrum originates fromiron cations whose magnetic moment displays a slowed-down
relaxation rate compared to the iron cations contributing to theparamagnetic spectrum component still observable in the
Mössbauer spectrum. Namely, when the relaxation time of themagnetic moment of iron becomes comparable to the Larmor
precession time ( τ L ≈ 10 −7 s) of the 57 Fe nucleus, then themagnetic hyperfine interaction, although still being averagedout to some extent, becomes observable in the 57Fe Mössbauer
spectrum. A further slow-down of magnetic relaxation withdecreasing temperature results in the appearance of a fulldeveloped sextet as clearly observed in the spectrum taken at T
= 10 K.
The detailed temperature dependence of the δ, ∆, and Γ 57Fe
Mössbauer parameters of La0.8Sr0.257Fe0.05Co0.95O3−d are shown6

3,17×10

6

3,15×1010 K
64,35×10

6

4,20×10

6

4,05×10counts

R2

45 K

5

4,50×1078 K

54,00×105
4,50×10
54,00×10150 K
55,00×10

5

4,50×10290 K

54,00×10

-15

-10

-5

0

5

10

15

-1

v / mm s

Figure 2. 57Fe transmission Mössbauer spectra of La0.8Sr0.257Fe0.05Co0.95O3−d
at selected temperatures.

Colossal Magnetoresistance in Focus:

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

in Figure 3.
The 57Fe isomer shift decreases linearly with temperaturefrom 78 K to ∼ 150 K and from ∼ 160 K to room temperature,
but at ∼ 150 K the slope of the decline changes.
dδ (T)The gradientis inversely proportional to the effectivedTvibrating mass (Mvibr) of the iron cations.11 The magnitude ofdδ (T)
increases suddenly at ∼ 150 K, which is equivalent to adT
sudden decrease in Mvibr with increasing temperature. Such a
decrease is to be expected when the vibrational motion of ironand that of the neighboring cations becomes less correlated due
to a decrease in the force that made them move in a more correlated manner previously. Thus a sudden decrease in the strengthof the chemical bonding or in the strength of the magnetic interaction between iron and its surroundings may well result in asudden decrease of Mvibr. The ratio Mvibr(T > 150 K)/Mvibr(T <150 K) ≈ 2/3 indicates that the vibrational state of iron changes
considerably at T ≈ 150 K in La0.8Sr0.257Fe0.05Co0.95O3−d.
The Mössbauer line width of the quadrupole doublet starts
to rise (with decreasing temperature) at around the same
temperature (T ≈ 150 K) where the effective vibrating masschanges suddenly (Figure 3). The evaluated 57Fe quadrupole
splitting (Figure 3) also increases with decreasing temperaturein correlation with the line width.
The temperature T ≈150 K at the same time coincides with the ferromagnetic ordering temperature (TC) of La0.8Sr0.257Fe0.05Co0.95O3−d.6
Thus it is plausible to assume that the sudden decrease of Mvibr
at T ≈ 150 K is a consequence of the FM → PM transition of
La 0.8Sr 0.257 Fe 0.05Co 0.95O 3−d. Similarly, the rather anomalous
increase of Γ and ∆ Mössbauer parameters with decreasing
temperature below T ≈ 150 K is likely to be of magnetic origin.
The increase of the Mössbauer line width reflects that therelaxation time of the iron magnetic moment increases toaround 10−8 s when the Co host matrix orders magnetically.
Such an increased relaxation time is expected to occur whena

δ / mm s

-1

0,40

0,35

0,30

0,50

Γ / mm s

-1

b

R3

iron magnetic moments are relaxing not individually like in aparamagnet, but they are part of nanosized magnetic clusters as
is the case, e.g., in superparamagnetic materials. When therelaxation time increases over the Larmor precession time of
the 57Fe nucleus, then a full-developed magnetic spectrum isobserved as displayed in Figure 2. At this point one shouldnote that the apparent increase in the ∆ parameter withdecreasing temperature (Figure 3) below T ≈ 150 K is not necessarily connected to a change in the quadrupole interaction,
but it may also be the result of the slowing down of magnetic
relaxation that appears in the spectrum as an apparent increasein the separation of the two broadened lines that we fit as a
doublet.
Figure 4 shows the 5 7 Co( 5 7 Fe) EMS spectrum of
La0.8Sr0.257Fe0.05Co0.95O3−d at temperatures 4.2 K and 290 K.
The results obtained in this way are basically the same as
obtained by the TMS technique (Figure 2). This shows that inthis compound the electronic relaxation time of the nucleogenic iron is much shorter than the lifetime of the excited
Mössbauer nuclear level of the 57Fe nucleus (∼ 10−7 s), and as aconsequence the Mössbauer parameters observed via the EMS
measurement also reflect the equilibrium chemical state of iron
cations. In this case, however, as the nucleogenic iron isformed at the site of a Co cation, the values we obtain for the57
Fe Mössbauer parameters represent the state of iron cations
situated at Co sites in La0.8Sr0.257Fe0.05Co0.95O3−d. The good
agreement between the 57Fe Mössbauer parameters obtained via
TMS and EMS techniques shows that in La0.8Sr0.257Fe0.05Co0.95O3−dthe substituted 5 mol% 57Fe does not form iron clusters, but it
is distributed (probably near uniformly) in the Co host matrix.
Thus, considering also the relatively low amount of 57 Fe
present in the system, the magnetic state of iron has to be
determined solely by Fe-Co magnetic exchange interactions.
The observation of magnetic relaxation in the 5 7 Fe
Mössbauer spectra below the magnetic ordering temperature
indicates that the magnetic moment of iron is strongly
connected to and relaxes together with magnetically ordered
nanosized Co clusters. This indicates that the magnetic
exchange interaction between Fe and Co is strong in thissystem, and consequently when the material gets orderedmagnetically at T ≈ 150 K iron magnetic moments get alsoordered (or are already ordered) with respect to neighboring
Co magnetic moments. As a consequence, the relaxation
behavior we observe in the 5 7 Fe Mössbauer spectra of
La 0.8 Sr 0.2 57 Fe 0.05 Co 0.95 O 3−d must correspond to the magnetic
relaxation of nanosized Co clusters existing in this perovskite.
This result further confirms the idea 12 that the existence ofnanoscale magnetic clusters is a prerequisite of intrinsic negative magnetoresistivity.

0,45
0,40
8,70×10

5

8,65×105∆ / mm s

-1

c
0,24

counts

0,35

4.2 K
5

6,8×100,21

5

6,6×10

0,18
50

100

150

200

250

300

290 K
5

6,4×10-10

T/K
Figure 3. The temperature dependence of 57Fe Mössbauer isomer
shift ( δ ), line width ( Γ ), and quadrupole splitting ( ∆ ) of
La0.8Sr0.257Fe0.05Co0.95O3−d.

-50v / mm s

5

10

-1

Figure 4. Co( Fe) emission Mössbauer spectra of La0.8Sr0.257Fe0.05Co0.95O3−d
at temperatures 4.2 K and 290 K.
57

57

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

Z. Klencsár

3. Sr2FeMoO6: A Double Perovskite System
The double perovskites Sr 2 FeMoO 6 has been brought
recently into the center of scientific interest because of theirconsiderable magnetoresistance observed already in relatively
low magnetic fields even at and above room temperature.13−19
The double perovskite Sr2FeMoO6 has been reported to havethe cubic ABO 3 structure that contains Fe and Mo cations
ordered alternately at the B site.14
Sr 2 FeMoO 6 has long been known as a conducting ferrimagnet with a relatively high magnetic transition temperatureof T c ≈ 450 K. 20 Recently, however, neutron diffraction
measurements revealed zero magnetic moment on Mo cations,and consequently the ferrimagnetic structure of Sr2FeMoO6
was disputed. 1 4 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 thehybridization of 3d (Fe) and 4d (Mo) electronic states.14, 16−18
At the same time, in Sr 2 FeMoO 6 saturation magnetic
moments of 3.0 µB,13 3.6 µB,3 3.18 µB,20 and 3.08 µB21 wereobserved at low temperatures on the basis of magnetization
measurements. These values are below the range from 4 µB to
5 µB expected for Fe+(3−δ), where δ ≤ 1 denotes the contributionof the delocalized electron to the Fe minority spin t2g level.
The decreased value of saturation magnetic moment has been
attributed to a deviation from the perfectly ordered structure by
the replacement of a certain amount of Mo by Fe and viceversa. Indeed, it was shown that iron substituting Mo aligns
antiferromagnetically to neighboring iron cations in
Sr2FeMoO6.22 Recently it was suggested that this kind of antisite defects might be responsible for low field magnetoresistance observed in polycrystalline Sr2FeMoO6.23, 24
In order to gain insight into the effects of cation disorder on
the local electronic structure, in this article we studied a double
perovskite Sr2FeMoO6 that contains a considerable amount ofmisplaced cations.
The studied Sr 2 FeMoO 6 sample was prepared in thefollowing way. Stoichiometric mixture of SrCO3, Fe2O3, andMoO3 was homogenized and pressed into pellet under a pressure of 100 MPa. The pellet was calcined in air at 900˚C for 4
hours. The precursor obtained in this way, consisting of
SrMoO4 and SrFeO3, was grinded and pressed again into apellet. This procedure was followed by a heat treatment at
900˚C for 2 hours under nitrogen atmosphere containing 5
120(V/V)% H 2 . The heat treatment was repeated once after
regrinding and repressing the sample.
Figure 5 shows the X-ray diffractogram of the investigatedSr2FeMoO6 sample. The diffractogram can be decomposed
into two components. The crystal structure of the main
component can be identified as a double perovskite with atetragonal I4/m space group and a lattice parameter of a =0.5586 nm and c = 0.7888 nm. This result is in agreementwith the earlier neutron diffraction investigations19 indicatingthat below 420˚C the I4/m tetragonal and above this temperature the cubic Fm3m structure is stable.
The Bragg peaks not belonging to the double perovskite
structure originate from a simple tetragonal minority phasewith I4/a space group and with cell parameters of a = 0.53943
nm and c = 1.20436 nm. The volume fraction of this simple
tetragonal phase, calculated on the basis of the ratio of integrated Bragg line intensities, was estimated to be around 6%.
The cell parameters of this minority phase are very close tothat of the precursor material SrMoO4 (I4/a tetragonal, a =0.53944 nm, c = 1.2020 nm).
Figure 6 shows the 57Fe Mössbauer spectrum of Sr2FeMoO6
measured at T = 4.2 K. The spectrum can be decomposed into
four different magnetically split subspectra reflecting magnetically ordered iron cations with different electronic configurations on their 3d level. In the followings, these subspectra will
be referred to as S1 (61%), S2 (13%), S3 (15%), and S4(11%). The 57Fe isomer shift and hyperfine magnetic field (B)parameters belonging to the observed subspectra are visualized
in Figure 7 together with the corresponding electronic configu1340000
1330000
1320000

CountsR4

1280000
1270000
-15

-10

-5

0

5

v / mm s

0,75(0,0,4)20(0,1,3)(0,1,2)*(0,1,1)(1,1,0)010

20(0,2,3)(1,3,0)(0,2,2)

*

30

*

*

40

(1,3,2)(2,2,4)

Fe Isomer Shift / mm s-1

40

57

60

15

Figure 6. 57Fe Mössbauer spectrum of Sr2FeMoO6 at T = 4.2 K, andits decomposition into four magnetically split subspectra.

T = 4.2 K

61 %

80

10

-1

* Minority phase

Relative amplitude

1300000
1290000(1,1,2)100

1310000

0,70

S1, 3d

5+δ 1

0,65

13 %
0,60

S2, 3d5+δ2

S4, 3d0,55

S3, 3d
0,50

5+δ 3

5

11%

15 %

*

50

60

70

2θ / deg
Figure 5. X-ray diffractogram of Sr2FeMoO6 powder measured with
Cu Kα (λ = 0.154184 nm) radiation at room temperature. The (hkl)
indexing of the peaks is shown for the I4/m tetragonal structure. The
peaks belonging to the minority phase are marked by * sign. The
strongest line of the minority phase coincides with the (012) line of
the main phase.

0,45
47

48

49

50

51

52

53

54

55

56

57

Hyperfine Magnetic Field / T
Figure 7. 57Fe isomer shift and hyperfine magnetic field Mössbauer
parameters obtained for the four magnetic subcomponents observed in
the 57 Fe Mössbauer spectrum of Sr 2 FeMoO 6 at T = 4.2 K. The
suggested electronic configurations of the corresponding iron cations
are indicated next to the data points. It is assumed that δ3< δ2 < δ1.

Colossal Magnetoresistance in Focus:

3

Apart from perovskites, recently chalcogenide spinels werealso found showing remarkable CMR properties. 2 , 2 5 − 2 7
Although Mössbauer studies of some iron containing chalcogenides were already performed several decades ago (see, e.g.,
Reference 28), we present some new 57Fe Mössbauer measurements in order to gain a deeper insight into the structure of
Fe1−xCuxCr2S4 type chalcogenides.
The Fe1−xCuxCr2S4 (x = 0, 0.25, 0.5, and 0.75) specimens
were prepared by solid state reactions. The stoichiometrically
mixed samples sealed in an evacuated quarz tube were fired
for 12 h at 850˚C, and then they were ground and pressed into
pellets. Sample purity was checked by X-ray diffraction andby scanning electron microscopy in combination with energy
dispersive X-ray fluorescence spectroscopy.29 57Fe Mössbauer
spectroscopy measurements were carried out on the Fe1−xCuxCr2S4
samples in transmission geometry with a conventional constantacceleration type spectrometer. 57Fe isomer shifts are given
relative to α-iron at room temperature.
The temperature dependence of the 57Fe Mössbauer parameters characteristic for iron at the tetrahedral site in the sample
FeCr2S4 (x = 0) was published earlier.30, 31 The Curie temperature (Tc) was found to be about 170 K and the magnetic field
was 20 T in this sample at 76 K. The isomer shift (δ ) was 0.72
mm s −1 at 76 K and it decreased to 0.6 mm s −1 when thetemperature was increased to room temperature. These parameters confirm the oxidation level of iron(II).30
Figure 8a and b show the 57 Fe Mössbauer spectra of thesamples Fe1−xCuxCr2S4 (for x = 0, 0.25, 0.5, and 0.75) recordedat room temperature. As clearly reflected by the 5 7 Fe
1,91×10counts2

4. Fe1–xCuxCr2S4 Type Spinels

7

1,90×107

9,30×10

6

9,20×106

9,10×106

x =0.75

x =0.25
-8

-4

0

4

8 -8

1,38×106

1,36×106

x =0.5 1,34×10

6

1,30×10

6

1,20×10

6

x =0
-4

v / mm s-1

0

4

8

v / mm s-1

Figure 8a. 57Fe Mössbauer spectra of Fe1−xCuxCr2S4 (x = 0, 0.25, 0.5,
and 0.75) at room temperature (only the envelope of the fitting curves
are shown).

counts1

R5

countsration of the iron cations. The main sextet S1 can be attributed
to the ideal, perfectly ordered regions of the double perovskite.
Its Mössbauer parameters agree well with those reported in
Reference 19 for Sr2FeMoO6. While the corresponding isomer
shift value is relatively high, the hyperfine magnetic field isrelatively low compared to the values expected for high spin
Fe3+ at T = 4.2 K (δ ≈ 0.5 mm s−1, B ≈ 55 T). This points to aconsiderable electron density on the minority spin t2g level ofiron, which is in agreement with a high degree of hybridization
between 3d(Fe) and 4d(Mo) electronic levels. Furthermore,
the intermediate value of isomer shift and hyperfine magnetic
field Mössbauer parameters, i.e. between values that would be
representative for Fe3+ and Fe2+, indicates that the δ1 minority
spin electron density at the 3d5+δ level of iron (Figure 7) arises
from delocalized electrons. The S2 magnetic component in
the 57 Fe Mössbauer spectrum was also detected earlier in
Sr2FeMoO6.19 This component displays a decreased isomer
shift and an increased hyperfine magnetic field compared tothe case of the perfectly ordered structure represented by S1(Figure 7). This corresponds to a decreased electron density(δ2) at the minority spin 3d 5+δ level of iron. The decreased
minority spin electron density can be interpreted as a reduced
degree of delocalization of the 4d1 electron of Mo.
In contrast with earlier 57 Fe Mössbauer studies done on
Sr 2 FeMoO 6 with moderate levels of cation disorder, in the
Mössbauer spectrum of our Sr2FeMoO6 we observe two more
subspectra, S3 and S4, whose hyperfine magnetic field ishigher and whose 57Fe isomer shift is lower than the corresponding values representing S1 and S2. Thus, S3 and S4 canbe attributed to an electronic state of iron where the degree ofdelocalization of the minority spin electron is further reduced
compared to the case of S1 and S2. Actually, the 5 7 Fe
Mössbauer parameters of the component S4 (Figure 7) alreadyrefer to a fully localized state of high spin Fe 3+ with the 3d5
electronic configuration. Although the isomer shift of thespecies S3 equals that of S4 within the experimental uncertainty, its decreased hyperfine magnetic field already refers to
the existence of moderate minority spin electron density on the
3d5+δ level of the corresponding iron cations (Figure 7). The
less pronounced difference between the isomer shift of S3 andS4 can be explained by assuming that beside the 3d(Fe) and4d(Mo) electronic states the 4s(Fe) electronic state of iron is
also involved in some extent in the hybridization of electroniclevels forming the conduction band.
The relatively high extent of cation disorder expected to bepresent in our sample is thought to be responsible for theappearance of the two additional sextets, S3 and S4. Thismeans that cation disorder results in a decreased delocalizationof the 4d1 electron of Mo. Misplaced cations are expected toform barriers to delocalized electrons. For example, an antisite
iron is expected to repel itinerant electrons of spin polarized
nature because of its antiferromagnetic alignment relative toneighboring iron cations. Thus, cation disorder leads to electrically insulating domains in the otherwise half metallic
double perovskite. The degree of electronic insulation at thesame time is expected to depend on the size of the insulating
domains. While the electrons may enter into the domainboundaries and may also pass through thin insulating domains,
they are completely excluded from the inside of bulkier
domains. Accordingly, in the presence of domains withconsiderable volume fraction of cation disorder, iron cations
belonging to different levels of minority spin electron density
are expected to exist in the double perovskite. This explains
the appearance of the magnetic subcomponents S2, S3, and S4
in the 57Fe Mössbauer spectrum of Sr2FeMoO6. Namely, while
S2 can be attributed to moderate disorder levels (e.g. isolated
antisite defects), S3 and S4 may arise from iron cations locatedin the inside of bulkier insulating domains.

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

9,30×10

6

9,25×10

6

9,20×10

6

9,15×106

9,10×10

6

-6

-4

-2

0

2

4

6

v / mm s-1
Figure 8b. 57Fe Mössbauer spectrum of Fe0.75Cu0.25Cr2S4 at room
temperature showing the decomposition of the absorption curve into
two singlets belonging to paramagnetic Fe3+ and Fe2+.

R6

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

Z. Klencsár

Mössbauer spectrum shown in Figure 8b, already a substitutionof 25% of iron with Cu results in considerable differences in
the oxidation state of iron in these compounds. While in
FeCr2S4 all iron cations display the same (Fe2+) oxidation state,
the 57Fe Mössbauer spectrum of Fe0.75Cu0.25Cr2S4 consists of
two clearly distinguishable subspectra each revealing a
different oxidation state of iron in the chalcogenide. The
singlet with Mössbauer isomer shift δ = 0.66(1) mm s−1 can be
assigned to Fe2+ state that has been reported in FeCr2S4.30 The
second singlet with isomer shift δ = 0.40(1) mm s−1, however,undoubtedly refers to the existence of Fe 3+ cations in the
Fe 0.75 Cu 0.25 Cr 2 S 4 sample. Considering the requirement ofcharge neutrality, and assuming that the concentration of sulfur
vacancies is negligible compared to the concentration of Cu, itfollows that at least part of the Cu being present in the system
must be in the Cu+ state. If all the copper in the sample would
be in the Cu+ state, then one would expect the relative amountof Fe2+ and Fe3+ to be Fe2+ : Fe3+ = 2 : 1. The room temperature
57
Fe Mössbauer spectrum of Fe0.75Cu0.25Cr2S4 is, however, notin agreement with this expectation (Figure 8b). Namely, bycomparing the peak areas belonging to Fe 2+ and Fe 3+ one
observes a ratio of Fe2+ : Fe3+ ≈ 1 : 2.2. One should consider,however, that the ratio of Mössbauer peak areas reflects faithfully the relative occurrence of the corresponding iron species
only if the recoilless fraction (the probability of the Mössbauer
effect, also called the Mössbauer-Lamb factor) is the same forthe two species. As the recoilless fraction should have atendency to approach 100% (from below) with decreasing
temperature for all species, a Mössbauer measurement carriedout at a lower temperature should result in a Mössbauer spectrum in which the area ratio of the peaks reflects the relative
occurrence of the corresponding iron species more faithfully
than it does in the case of the measurement done at room
temperature (Figure 8a). For this reason we also carried out a57
Fe Mössbauer measurement of Fe0.75Cu0.25Cr2S4 at T = 77 K,
which resulted in the Mössbauer spectrum shown in Figure 9.
The Mössbauer spectrum at T = 77 K can be decomposed into
three different components. The component belonging to Fe2+
is now a magnetically split sextet (δ = 0.67(2) mm s−1, B =2+

22.9(2) T) showing that at T = 77 K Fe cations are alreadyordered magnetically in agreement with results obtained earlier
for FeCr2S4.30, 31 At the same time we observe two different
subspectra with parameters representative for Fe 3+. One ofthem is a magnetically split sextet (δ = 0.52(1) mm s−1, B =33.1(1) T) showing that there are also Fe3+ cations magnetically ordered at T = 77 K, whereas the other is a quadrupole
doublet (δ = 0.55(2) mm s−1, ∆ = 1.19(3) mm s−1) referring toFe3+ cations in the paramagnetic state. The relative area fraction of the latter component is ∼ 9%. The nearly equal isomer
shift of the two different Fe3+ components suggests that thecorresponding cations may be situated in the same crystallographic position in Fe 0.75 Cu 0.25 Cr 2 S 4 , but the iron cations
belonging to the doublet are probably separated from themagnetically ordered regions (e.g. by sulfur vacancies) andconsequently they display paramagnetic behavior at T = 77 K.
At the same time it is also not entirely excluded that the paramagnetic Fe3+ subspectrum at T = 77 K originates from some
FeSx impurities in the sample.
By analyzing the area ratio of subspectra belonging to Fe2+
and Fe3+ cations in the Mössbauer spectrum taken at T = 77 K
one observes Fe 2+ : Fe 3+ ≈ 3 : 2 which ratio is closer to the
expected ratio 2 : 1 than that observed at room temperature (1 :2.2). The significant difference between the apparent Fe2+ :Fe3+ ratios observed at T = 290 K and at T = 77 K indicates thatin Fe0.75Cu0.25Cr2S4 at room temperature the Mössbauer-Lambfactor belonging to Fe3+ is at least three times greater than thatbelonging to Fe2+. This result shows that by replacing ironwith copper in Fe1−xCuxCr2S4 it is not only the oxidation stateof iron that changes, but simultaneously with the transition
Fe2+ → Fe3+ the local vibrational state of iron cations is alteredconsiderably, too.
In contrast with Fe0.75Cu0.25Cr2S4, the compounds Fe0.5Cu0.5Cr2S4
and Fe0.25Cu0.75Cr2S4 are magnetically ordered already at room
temperature as revealed by the corresponding 57Fe Mössbauer
spectra in Figure 8.
The Mössbauer spectrum of Fe0.5Cu0.5Cr2S4 can be decomposed into a magnetically split sextet (with parameters δ =0.31(1) mm s−1, B = 24.5(1) T) and a quadrupole doublet (with

6

6,30×10

2,05×106

x =0.5x =0.75

2,00×106

6

5,72×10

1,42×10

6

5,68×10

1,40×10x =0

x =0.25
-15

-10

-5

0

5
-1

v / mm s

10

15 -15

-10

-5

0

5
-1

v / mm s

Figure 9. 57Fe Mössbauer spectra of Fe1−xCuxCr2S4 (x = 0, 0.25, 0.5, and 0.75) at T ≈ 77 K.

10

15

6

6

countscounts6

6,25×10

Colossal Magnetoresistance in Focus:

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

R7

TABLE 1: 57Fe Mössbauer Parameters of Fe1–xCuxCr2S4 (x = 0, 0.25, 0.5, and 0.75) Chalcogenide
Spinels
SampleT/K

FeCr2S4
75 K 290 K

Fe0.75Cu0.25Cr2S4
78 K 290 K

Fe0.5Cu0.5Cr2S4
76 K 290 K

Fe0.25Cu0.75Cr2S4
77 K 290 K

Fe2+ / %δ / mm s−1
B /T

100
0.71
20.0

100
0.59
–

59
0.65
22.6

31
0.66
–

–
–
–

–
–
–

–
–
–

–
–
–

Fe3+ (M) / %δ / mm s−1
B /T

–
–
–

–
–
–

32
0.51
33.0

–
–
–

88
0.43
37.2

90
0.31
24.5

94
0.42
37.3

82
0.30
25.3

Fe3+ (PM) / %δ / mm s−1∆ / mm s−1

–
–
–

–
–
–

9
0.55
(1.2)

69
0.40
–

12
0.45
(0.5)

10
0.3
(1.0)

6
0.50
(0.29)

18
0.3
(0.9)

‘Fe3+ (M)’ and ‘Fe3+ (PM)’ denote magnetically ordered Fe3+ and Fe3+ in a paramagnetic state, respectively.
Quadrupole splitting (∆) values given in parentheses are uncertain because of the strong overlap of the absorption
lines. Isomer shift (δ ) and magnetic field (B) Mössbauer parameters concerning Fe2+ in FeCr2S4 at T = 75 K are
average values, see Reference 30 for details.

parameters δ = 0.31(3) mm s−1, ∆ = 0.9(1) mm s−1).
The relative area fraction of the doublet is ∼ 10%. While atroom temperature the two peaks of the doublet overlap with
the inner two lines of the sextet, at T ≈ 76 K the existence ofthe doublet becomes more obvious (Figure 9). The origin of
this doublet is likely to be the same as that found in the case of
Fe0.75Cu0.25Cr2S4 at T = 77 K, namely Fe3+ in the neighborhoodof a sulfur vacancy. This assignment is supported by the 57Fe
Mössbauer spectrum of Fe0.25Cu0.75Cr2S4 at T = 290 K (Figure8) which also contains a doublet with the same isomer shift
and quadrupole splitting parameters, but the relative area fraction of this doublet is 18% which is a considerably increased
value compared to the case of Fe0.5Cu0.5Cr2S4. This increase
can be understood again considering the requirement of macroscopic charge neutrality. Namely, the isomer shift of the main
magnetic component in the 5 7 Fe Mössbauer spectra of
Fe0.5Cu0.5Cr2S4 and Fe0.25Cu0.75Cr2S4 undoubtedly refers to Fe3+,
which further supports that copper is present mainly in the Cu+
state in this system. It follows that without any sulfur vacancies in Fe0.5Cu0.5Cr2S4 the capability of the iron subsystem tocompensate for the presence of Cu+ cations by the Fe2+ → Fe3+transition (in order to maintain charge neutrality) is alreadyfully exhausted. Therefore, if we replace more iron with Cu+
in the system, the missing positive charge has to be compensated by the appearance of sulfur vacancies (i.e. missing negative charges). And, assuming that the relative area fraction of
the observed doublet component is really proportional to thenumber of sulfur vacancies, this is exactly what we observe inthe 57Fe Mössbauer spectra. (It should be noted here, that ourreasoning remains valid even if a certain amount of copper isin the Cu2+ state in Fe1−xCuxCr2S4 with x ≥ 0.5.)
Although the 57Fe isomer shift δ ≈ 0.3 mm s−1 found for thechalcogenides with x ≥ 0.5 clearly refers to Fe3+, the hyperfine
magnetic field reflected by the main magnetic component
in the 5 7 Fe Mössbauer spectrum of Fe 0 . 5 Cu 0 . 5 Cr 2 S 4 andFe 0.25 Cu 0.75 Cr 2 S 4 (Table 1) is much lower than the typicalmagnetic fields in the high spin iron(III) compounds (45–55 T
at 76 K). This contradiction between the measured isomer
shift and magnetic field values can be resolved if we take into
account that there is a covalent bond between sulphur and ironand suppose that the electronic structure of iron in this sample
is: 3d5+x4sy with x > 0, y > 0.
5. Conclusions
Iron is thought to play a decisive role in determining themagnetic state and the MR properties of the studied perovskite(La0.8Sr0.257Fe0.05Co0.95O3−d), double perovskite (Sr2FeMoO6),

and chalcogenide spinel (Fe 1 − x Cu x Cr 2 S 4 ) systems. 5 7 Fe
Mössbauer spectroscopy provided us with detailed informationon the local vibrational, electronic and magnetic state of iron in
the studied compounds, which enabled us to draw novel
conclusions concerning the magnetic, electronic or defectstructure of these systems. The further detailed exploration of
the temperature dependence of the local state of iron in thesecompounds is promising because it may reveal correlations
between the macroscopic MR properties and the local electronic and/or magnetic state of iron, which hopefully will bring
us closer to the understanding of the possible microscopic
backgrounds of the CMR effect in general.
Acknowledgements. This work was supported by the
Hungarian Science Foundation OTKA (F 034837, T 034839,
T02474, T 043687, T 043565) and by the Austrian Hungarian
Action Foundation. Z. Klencsár acknowledges the support fromthe Hungarian Bolyai János research grant. The authors arethankful to Imre Vincze (KFKI, Budapest) for making available
his facilities to record the Mössbauer spectrum of Sr2FeMoO6
at T = 4.2 K, and also to G. Molnar and A. Bousseksou(Laboratoire de Chimie de Coordination, CNRS, France) for the
T = 10 K Mössbauer spectrum of La0.8Sr0.257Fe0.05Co0.95O3−d.
References(1) S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R.
Ramesh, and L. H. Chen, Science 264, 413 (1994).
(2) V. Fritsch, J. Deisenhofer, R. Fichtl, J. Hemberger, H.-A.
Krug von Nidda, M. Mücksch, M. Nicklas, D. Samusi, J.
D. Thompson, R. Tidecks, V. Tsurkan, and A. Loidl,
Phys. Rev. B67, 144419 (2003).
(3) T.H. Kim, M. Uehara, S-W. Cheong, and S. Lee, Appl.
Phys. Lett. 74, 1737 (1999).
(4) A. Barman, M. Ghosh, S. Biswas, S. K. De, and S.
Chatterjee, Appl. Phys. Lett. 71, 3150 (1997).
(5) V. Golovanov, L. Mihály, and A. R. Moodenbaugh, Phys.
Rev. B53, 8207 (1996).
(6) Z. Németh, Z. Klencsár, E. Kuzmann, Z. Homonnay, A.
Vértes, B. Lackner, K. Kellner, G. Gritzner, J. Hakl, S.
Mészáros, K. Vad, G. Molnár, and A. Bousseksou, to be
published.
(7) Á. Cziráki, I. Ger"o cs, M. Köteles, A. Gábris, L. Pogány, I.
Bakonyi, Z. Klencsár, A. Vértes, S.K. De, A. Barman, M.
Ghosh, S. Biswas, S. Chatterjee, B. Arnold, H. D. Bauer,
K. Wetzig, C. Ulhaq-Bouillet, and V. Pierron-Bohnes,
Eur. Phys. J. B21, 521 (2001).
(8) A. Vértes, S. Nagy, and Z. Klencsár, Handbook of Nuclear

R8

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

Chemistry (Kluwer Academic Publishers, Amsterdam,
2003).
(9) Z. Klencsár, E. Kuzmann, and A. Vértes, J. Radioanal.
Nucl. Chem. 210, 105 (1996).
(10) L. W. Tai, M. M. Nasrallah, H. U. Anderson, D. M. Sparlin,
and S. R. Sehlin, Solid State Ionics 76, 259 (1995).
(11) R. H. Herber, Chemical Mössbauer Spectroscopy (Plenum
Press, New York, 1984).
(12) A. Nath, Z. Klencsár, E. Kuzmann, Z. Homonnay, A.
Vértes, A. Simopoulos, E. Devlin, G. Kallias, A. P. Ramirez,
and R. J. Cava, Phys. Rev. B66, 212401 (2002).
(13) K.-I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, andY. Tokura, Nature 395, 677 (1998).
(14) B. Garcia-Landa, C. Ritter, M. R. Ibarra, J. Blasco, P. A.
Algarabel, R. Mahendiran, and J. Garcia, Solid State
Comm. 110, 435 (1999).
(15) T. Manako, M. Izumi, Y. Konishi, K. Kobayashi, M.
Kawasaki, and Y. Tokura, Appl. Phys. Lett. 74, 2215 (1999).
(16) Z. Fang, K. Terakura, and J. Kanamori, Phys. Rev. B63,
180407 (2001).
(17) J. Kanamori, Trans. Magn. Soc. Jpn. 1, 1 (2001).
(18) J. Kanamori and K. Terakura, J. Phys. Soc. Jpn. 70, 1433
(2001).
(19) O. Chmaissem, R. Kruk, B. Dabrowski, D. E. Brown, X.
Xiong, S. Kolesnik, J. D. Jorgensen, and C. W. Kimball,Phys. Rev. B62, 14197 (2000).

Z. Klencsár(20) T. Nakagawa, J. Phys. Soc. Jpn. 24, 806 (1968).
(21) M. Itoh, I. Ohta, and Y. Inaguma, Mater. Sci. Eng. B41,
55 (1996).
(22) J. Linden, T. Yamamoto, M. Karppinen, H. Yamauchi,
and T. Pietari, Appl. Phys. Lett. 76, 2925 (2000).
(23) T. Saha-Dasgupta and D. D. Sarma, Phys. Rev. B64, 064408(2001).
(24) M. García-Hernández, J. L. Martínez, M. J. MartínezLope, M. T. Casais, and J. A. Alonso, Phys. Rev. Lett. 86,
2443 (2001).
(25) K. Ando, Y. Nishihara, T. Okuda, and T. Tsushima, J.
Appl. Phys. 50(3), 1917 (1979).
(26) A. P. Ramirez, R. J. Cava, and J. Krajewski, Nature 386,
156 (1997).
(27) Z. Yang, S. Tan, Z. Chen, and Y. Zhang, Phys. Rev. B62,
13872 (2000).
(28) G. R. Hoy and K. P. Singh, Phys. Rev. 172, 514 (1968)(29) M. Kühberger, Z. Klencsár, E. Kuzmann, Z. Homonnay,
Z. Németh, G. Gritzner, and A. Vértes, to be published.
(30) Z. Klencsár, E. Kuzmann, Z. Homonnay, A. Vértes, A.
Simopoulos, E. Devlin, and G. Kallias, J. Phys. Chem.
Solids 64, 325 (2003).
(31) Z. Klencsár, E. Kuzmann, Z. Homonnay, A. Vértes, A.
Simopoulos, E. Devlin, and G. Kallias, Hyperfine Interact.
144/145, 261 (2002).