Magnetic behavior of MnCo2O4+δ spinel obtained by thermal decomposition of binary oxalates

Article abstract of DOI:10.1016/j.jmmm.2008.04.123

Manganese cobaltites MnCo₂O_4.62 and MnCo₂O_4.275 having a spinel structure were studied by measuring magnetization, AC susceptibility and by XANES spectroscopy. These compounds were synthesized by decomposition of the binary oxalate Mn_1/3Co_2/3C₂O₄· 2H₂O in air at 220 and 500 °C, respectively. It was found that the differences in magnetic characteristics of these cation-deficient spinels are due mainly to variations in the degree of oxidation of manganese. It was shown that the complex oxide MnCo₂O_4.62 formed right after decomposition of the binary oxalate contains about 5×10^-4 mass% metallic cobalt, which determines the dependence of magnetic susceptibility χ on the magnetic field at 300 K. The magnetic transition peculiar to the stoichiometric spinel MnCo₂O₄ at 183 K decreases to 167.5 K for MnCo₂O_4.275 and 67.5 K for MnCo₂O_4.62.

Full text of DOI:10.1016/j.jmmm.2008.04.123

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) 2262– 2268
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Magnetic behavior of MnCo2O4+d spinel obtained by thermal decomposition
of binary oxalates
Ã
G.V. Bazuev a, , A.V. Korolyov b
a Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 620041, Ekaterinburg GSP-145 620041, Russia
b Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg GSP-145 620041, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Manganese cobaltites MnCo2O4.62 and MnCo2O4.275 having
a spinel structure were studied by
Received 11 February 2008
Available online 22 April 2008
measuring magnetization, AC susceptibility and by XANES spectroscopy. These compounds were
synthesized by decomposition of the binary oxalate Mn1/3Co2/3C2O4 ꢀ 2H2O in air at 220 and 500 1C,
respectively. It was found that the differences in magnetic characteristics of these cation-deficient
spinels are due mainly to variations in the degree of oxidation of manganese. It was shown that the
complex oxide MnCo2O4.62 formed right after decomposition of the binary oxalate contains about
5 ꢁ10ꢂ4 mass% metallic cobalt, which determines the dependence of magnetic susceptibility w on the
magnetic field at 300 K. The magnetic transition peculiar to the stoichiometric spinel MnCo2O4 at 183 K
PACS:
75.50.Tt
75.50.Dd
61.50.Nw
61.66.Fn
decreases to 167.5 K for MnCo2O4.275 and 67.5 K for MnCo2O4.62
.
Keywords:
& 2008 Elsevier B.V. All rights reserved.
Oxide
Chemical synthesis
Magnetic property
1. Introduction
depending on the temperature of decomposition of oxalates
1/3Co2/3C2O4 ꢀ 2H2O. The cubic spinel begins to form at about
A
200 1C. That phase has an extremely low crystallinity and an
excessive content of oxygen: ACo2O4+d (d40). Therefore cobalt or
manganese (or a part of these elements) in this phase have a
higher valence as compared with the stoichiometric spinel
ACo2O4. As the temperature rises further, this phase gradually
loses oxygen and turns to the ACo2O4 spinel. An increase of
temperature to 700 1C results in improved crystallinity of the
cubic phase and an enhanced parameter of the cubic cell. The
increase of the unit cell parameter with temperature is due to
variations in the chemical composition, i.e. reduction of some
cobalt or manganese. These findings are in agreement with data
(Ref. [2] and references therein), according to which the
manganese cobaltite MnCo2O4+d synthesized at low temperatures
has the structure of the cation-deficient spinel with enhanced
oxygen content.
The foregoing suggests that the magnetic characteristics of the
spinel MnCo2O4+d depend on the value of d determined by the
conditions of synthesis. The magnetic properties of MnCo2O4 were
reported in a number of studies. The ferromagnetism of this spinel
was first reported by Lotgering [7] who examined a specimen
synthesized by decomposition of oxalates at 500 1C. Wickham and
Croft [8] have investigated solid solutions Co3ꢂxMnxO4 with a
cubic spinel structure (0pxp1.2) obtained by solid-phase sinter-
ing of oxide mixtures in air at 1000 1C (x40.5) and 800 1C (xp0.5).
The following distribution of cations in the structural sites was
Transition metal cobaltites having a spinel structure ACo2O4
(AQMn, Ni, Zn) possess various functional properties defining
their application as materials for sensors, fuel cell electrodes,
electrical catalysts, etc. [1–4]. Those compounds are currently
synthesized by low-temperature techniques allowing to obtain
the final products in the nanocrystalline or ultradisperse states.
Spinels synthesized in this way have a very developed surface and
electrocatalysts possess an extremely high specific surface energy.
The low-temperature methods of synthesis are based on decom-
position of precursors—preliminarily deposited hydroxides or
salts of cobalt and other transition metals—at 400 1C or lower
temperatures (Ref. [2] and references therein). Nitrates, oxalates,
acetates of the corresponding metals, etc., are usually used
as salts.
In the works [5,6] we studied the conditions of formation of
cobaltites ACo2O4 (AQMn, Zn) in the form of wire-like whiskers
or spheroids by thermal decomposition of presynthesized binary
oxalates of cobalt and manganese (zinc) A1/3Co2/3C2O4 ꢀ 2H2O in
air. The chemical composition and the microstructure of the
decomposition products were examined. It was found in parti-
cular that spinel phases with different composition are formed
Ã
Corresponding author. Tel.: +7343 374 4943; fax: +7343374 4495.
E-mail address: bazuev@ihim.uran.ru (G.V. Bazuev).
0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2008.04.123

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2263
3+
2ꢂx
deduced based on magnetic measurements: Co2+[Co Mn3+x]O4.
3. Results and discussion
For MnCo2O4, the Neel temperature (TN) is ꢃ187 K. Using data
Lotgering [7], Blasse [9] made another conclusion about the
distribution of cations: Co2+[Co2+Mn4+]O4. Vainshtein et al. [10]
also interred based on X-ray absorption spectra that the charge of
manganese in MnCo2O4 is close to +4. Mixed oxidation states of
cobalt (Co2+ and Co3+) and manganese (Mn3+ and Mn4+) in solid
solutions Co3ꢂxMnxO4 (0pxp1) obtained by thermal decomposi-
tion of nitrates at 400 1C were reported [1].
3.1. Synthesis and characterization
As was noted in the Introduction, the content of oxygen in the
manganese cobaltite MnCo2O4+d depends essentially on the
temperature, at which the specimens are synthesized. Three
specimens of manganese cobaltite MnCo2O4+d with different
values of d were prepared in this work. The first specimen was
obtained by decomposition of Mn1/3Co2/3C2O4 ꢀ 2H2O in air at
220 1C for 6 h, the second specimen—at 500 1C for 4 h, and the
third—at 750 1C for 4 h. X-ray diffraction patterns of all these
cobaltites are given in Fig. 1. It is seen that all the decomposition
products are characterized by the cubic spinel structure and a
different degree of crystallinity. The cubic cell parameter a of
At present, the magnetic properties of the Co3ꢂxMnxO4
system attract particular interest owing to unusual hysteresis
phenomena and magnetization processes, as well as mag-
netic behavior of MnCo2O4 in the nanocrystalline and thin-
film states [11–14]. The information on the influence of
methods of synthesis and especially the temperature of synthesis
on the magnetic properties of nonstoichiometric specimens
MnCo2O4+d is not available. At the same time, for spinels of
other compositions, in particular, NiMn2O47d [15], oxygen
stoichiometry was found to exert a certain effect on magnetic
characteristics.
In this work, we studied the magnetic properties of two
manganese cobaltites MnCo2O4+d having different composition
(d ¼ 0.275 and 0.62). To evaluate the degrees of oxidation of Mn
and Co, we examined the near-edge fine structure of X-ray
absorption using XANES spectroscopy.
˚
MnCo2O4 (750) was found to be 8.224(2) A. It is difficult to
determine precise parameters for specimens obtained at 220 and
500 1C. Considering that X-ray diffraction peaks of these com-
pounds are shifted towards greater angles in comparison with
MnCo2O4 (800), it may be asserted that the unit cell sizes of the
spinel decrease when the temperature of synthesis lowers.
To determine the content of oxygen, TG studies of specimens
obtained at 220 and 500 1C were carried out. Fig. 2 presents data
of TG examination of the MnCo2O4 (220) specimen in air. The
reduction in the mass of the specimen in the range 80–200 1C is
indicative of isolation of H2O absorbed during storage, and at
280–650 1C—of isolation of oxygen. Based on the variation of the
2. Experimental
(311)
The binary dihydrate of Mn and Co oxalate with the com-
position Mn1/3Co2/3C2O4 ꢀ 2H2O was prepared from aqueous
0.2 M solutions of cobalt and manganese sulfates ASO4 by
deposition with oxalic acid H2C2O4. Element analysis for
the content of Mn and Co in the examined specimens was
carried out employing atomic adsorption spectroscopy in ‘‘acet-
ylene-air’’ flame on a Perkin-Elmer setup and atomic emission
on an inductive-plasma spectrum analyzer JY-48. The oxygen
content of the compounds was established with the use of
thermogravimetric (TG) analysis based on the loss of mass in the
process of reduction of compounds during calcination in air to
800 1C.
(220)
(440)
(511)
(422)
(400)
(111)
(222)
3
2
The microstructure and the sizes of particles of the Mn1/3Co2/
3C2O4 ꢀ 2H2O oxalate and manganese cobaltites MnCo2O4+d were
determined with a scanning electron microscope Tesla BS-301.
Images were obtained by surveying in inversely mapped electrons
by SEM with 1000 and 5000 power magnification.
1
10
20
30
40
2Θ, degree
50
60
70
Magnetic measurements were performed on a SQUID magnet-
ometer MPMS-XL-5 produced by QUANTUM DESIGN. Tempera-
ture was varied from 2 to 300 K. Controlled magnetic field
strength H was set to 50 kOe. Measurements were carried out
when a specimen was cooled in zero (zero-field cooling (ZFC)) and
measured (field cooling (FC)) magnetic field. A specimen was first
cooled in zero field to 2 K, then magnetization M was measured
when it was heated to 300 K (ZFC mode). After that magnetization
was measured when the specimen was cooled from 300 to 2 K (FC
mode). From these measurements we found the static magnetic
susceptibility w ¼ M/H, whereas the AC susceptibility measure-
ment technique was used to determine the real w0 and imaginary
w00 components of dynamic susceptibility for the amplitude value
of variable magnetic field 4 Oe.
Fig. 1. X-ray diffraction patterns of MnCo2O4+d: (1) (220), (2) (500), (3) 750 1C. 3 is
indexed as a cubic spinel Fd3m (227).
0
2
4
6
XANES spectra of Co–K and Mn–K were measured in the Center
of Synchrotron Radiation at the Institute of Nuclear Physics SB RAS
(Novosibirsk). A monoblock cutting monocrystal Si(III) was used
as a two-crystal X-ray monochromator, and an Ar/He ionization
chamber served as a monitoring detector.
100
300
500
700
T°C
Fig. 2. TG curve of MnCo2O4 (220) in air at a heating rate of 10 1C minꢂ1
.

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specimen’s mass in the interval 280–650 1C it was calculated that
the composition of the cobaltite corresponds to the formula
MnCo2O4.62. Excessive oxygen results in the formation of va-
cancies in the cationic subsystem and in enhanced mean degree of
oxidation of Mn and Co (+3.07). The chemical composition of this
spinel phase without allowance for the distribution of cations in
structural sites may be expressed by the formula Mn0.87Co1.732O4.
According to an analogous study of the specimen synthesized at
5001C, the content of oxygen in it corresponds to the formula
MnCo2O4.275 (Mn0.936Co1.871O4) and the mean degree of oxidation
of Mn and Co is +2.85.
The binary oxalate Mn1/3Co2/3C2O4 ꢀ 2H2O prepared by deposi-
tion with oxalic acid under the described above technique has the
form of fibers (whiskers). In conformity with SEM studies (Fig. 3),
the products of decomposition of Mn1/3Co2/3C2O4.2H2O retain this
microstructure during heating in air in spite of changes in the
chemical composition:
Mn1=3Co2=3C2O4:2H2O ! Mn1=3Co2=3C2O4
! MnCo2O4þd ! MnCo2O4
These MnCo2O4 fibers are 2–4 mm in diameter, 15–40 mm in
length and do not change considerably with temperature. The
particles exhibit a porous structure reflecting the procss of
isolation of H2O, CO and CO2 from the binary dihydrate of Mn–
Co oxalate, which is well seen on the specimen upon sintering at
800 1C (Fig. 3d).
3.2. Magnetic properties of MnCo2O4.275
Temperature dependences of magnetic susceptibility w for the
MnCo2O4.275 spinel synthesized at 500 1C in ZFC and FC modes in
magnetic field 0.5 kOe are depicted in Fig. 4. Below 180 K, w rises
abruptly and the w(T) curves for the specimen cooled in zero field
(ZFC mode) and measured field (FC mode) diverge at Tp165 K.
Measurements in the zfc mode show that the w(T) dependence
exhibits a maximum at 157 K. The character of changes in
susceptibility in the considered magnetic transition is analogous
to that described in [12] for MnCo2O4 synthesized from oxides at
1000 1C. However the point of magnetic transformation for
MnCo2O4.275 (167.5 K) is obtained from the plot of dw/dT versus
T (Inset on Fig. 4) is somewhat lower than TN for the stoichio-
metric spinel MnCo2O4 (183 K [12]). In the region of low
temperatures, the zfc and FC curves w ¼ f(T) exhibit a number of
0
-5
-10
-15
-20
-25
6
4
2
0
FC
150160170180190200
T K
ZFC
0
100
200
300
Fig. 3. SEM micrographs of (a) Mn1/3Co2/3C2O4 ꢀ 2H2O and (b–d) MnCo2O4+d
whiskers; decomposition temperature of (b) 220, (c) 500 and (d) 800 1C.
T K
Fig. 4. Temperature dependence of w for MnCo2O4 (500) in magnetic field 0.5 kOe.
Inset: dependence of dw/dT for MnCo2O4 (500).

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peculiarities. So, the ZFC susceptibility increases below 120 K and
there is a broad maximum at ꢃ18 K on the curve in addition to
that at 157 K. The FC dependence exhibits a maximum at 120 K
appearance of Mn4+ cations in octahedral sites is the most
probable reason for the decreased TN of this cobaltite in
comparison with the stoichiometric MnCo2O4. It is noteworthy
that the absence of data on the composition and quantity of the
impurity phase found in magnetic measurements at low tem-
peratures may introduce certain corrections in the composition of
the spinel.
and
a
minimum at ꢃ15 K. We suppose that the observed
anomalous behavior of the MnCo2O4.275 spinel at low tempera-
tures may be due to the second magnetic phase formed on the
first stage of decomposition of the precursor.
The inset in Fig. 5 displays magnetization measurement results
for MnCo2O4.275 as a function of magnetic field at 2 and 14 K. The
measurements were performed using specimens cooled in the ZFC
mode when the magnetic field was decreased from 50 kOe. Both at
2 and 14 K the s versus H curves are not linear, and magnetization
does not reach saturation in the magnetic field 50 kOe. Residual
magnetization is 1.25 cm3/g at 2 K and approaches zero at 14 K.
The temperature dependence of inverse magnetic suscept-
ibility 1/w (Fig. 5) for MnCo2O4.275 above 230 K obeys the Curie-
Weiss law:
3.3. Magnetic properties of MnCo2O4.62
Magnetic susceptibility measurement results for the
MnCo2O4.62 spinel in the temperature interval 2–300 K in the FC
and ZFC modes in magnetic field 0.5 kOe are given in Fig. 6. The
inset in Fig. 6 presents dynamic magnetic susceptibility (w0) versus
temperature.
According to the data obtained, the MnCo2O4.62 spinel, as
distinct from MnCo2O4.275, contains one magnetic phase with a
much lower transition temperature. The curves measured in ZFC
and FC modes begin to diverge at 65 K (Fig. 5). In the ZFC
measurements in magnetic field 0.5 kOe, susceptibility exhibits a
maximum at 46 K. In measurements in magnetic field 10 kOe, the
w ¼ C=ðT ꢂ yÞ,
where w is the molar susceptibility, C is the Curie constant
(Cexp ¼ 4.07 cm3 K/mole), and
y is the Weiss temperature
(y ¼ ꢂ329.5 K). The molar magnetic susceptibility wmol and the
Curie constant Cexp are related to the formula Mn0.936Co1.872O4
reflecting the cation-deficient structure of the spinel formed at
500 1C.
8
55 K
3
In order to evaluate the valence state of Co and Mn and their
distribution in tetrahedral and octahedral sites, we compared the
experimental Cexp and calculated Ccalc values of the Curie constant.
The Ccalc constant was found in accordance with the combination
of Co and Mn cations described in [8]. According to [8],
the tetrahedral sites of the Co3ꢂxMnxO4 spinel are occupied by
cobalt in the form of Co2+ cations, whereas in the octahedral
site there are trivalent cations of cobalt and manganese:
Co2+[Co2ꢂxIIIMnx3+]O4. With account for these data, we deduced
6
4
2
0
FC
2
1
0
0
100
200
300
300
T K
2+
III
3+
the following formula for MnCo2O4.275
:
Co
[Co
Mn
0.936
0.936 0.421
ZFC
Mn
]O4, where CoIII is a low-spin cobalt cation (S ¼ 0), and
4+
0.515
Co2+ and Mn3+ are high-spin cations (spins S are 1.875 and
3 cm3 K/mole, respectively). The calculated Curie constant Ccalc for
this composition is 3.984 cm3 K/mole, which is close to the
experimental value (4.07 cm3 K/mole). This favors the inference
that the MnCo2O4.275 phase is formed due to oxidation of
manganese when the content of oxygen increases. The average
degree of oxidation of cations in this phase is +2.85, i.e. it is higher
than in the stoichiometric compound MnCo2O4 (+2.67). The
0
100
200
T K
Fig. 6. Temperature dependence of field-cooled (FC) and zero field cooled (ZFC)
susceptibility
of MnCo2O4 (220). Inset: temperature dependence of w0 for
MnCo2O4 (220).
w
40
10
2 K
8
14 K
30
20
10
0
6
4
2
0
0
10
20
H, kOe
30
40
50
-300
-200
-100
0
100
200
300
T K
Fig. 5. Dependence of 1/w ¼ f(T) for MnCo2O4 (500) in the magnetic field 0.5 kOe. Inset: dependence of magnetization s on magnetic field for MnCo2O4 (500) at 2 and 14 K.

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40
12
2
1
0.45 μB
8
30
0
4
-1
-2
-3
-4
0
20
10
0
0
10 20 30 40 50
H, kOe
-200
-100
0
100
200
300
0
50
100
150
T K
200
250
300
T K
Fig. 7. Temperature dependence of dw0/dT for MnCo2O4 (220).
Fig. 8. Dependence of 1/w ¼ f(T) in the magnetic field 10 kOe for MnCo2O4 (220).
Inset: dependence of magnetization s on magnetic field for MnCo2O4 (500) at 2 K.
maximum is displaced to 20 K (not shown). When temperature
decreases, susceptibility measured during the FC procedure
increases and below 20 K reaches a plateau. Attention is drawn
to the large difference between field-cooled and zero-field-cooled
300 K
magnetization for MnCo2O4.62
, which is indicative of high
25.7
magnetocrystalline anisotropy of this compound. The maximum
on the temperature AC susceptibility w0 (inset in Fig. 6) cor-
responds to 55 K. The plot of dw0/dT versus T exhibits sharp
minimum at 67.5 K (Fig. 7). This temperature is assumed to be TN
of this cobaltite.
25.6
25.5
The dependence between magnetization
M and applied
magnetic field H at 2 K (inset in Fig. 8) does not reach saturation
in the field 50 kOe (measurements were performed on a ZFC
specimen when the field was decreased from 50 to 0 kOe).
Above 230 K, w of MnCo2O4.62 follows the Curie-Weiss law with
the Curie constant Cexp ¼ 3.04 cm3 K/mole and the Weiss tem-
perature y ¼ ꢂ250 K (Fig. 8). Based on the chemical composition
of Mn0.866Co1.732O4, we derived the formula reflecting the
oxidation states of cations and their distribution in the structural
0
10
20
30
40
50
H, kOe
4+
0.866
III
1.072
sites of the spinel: Co20.+66[Mn
Co
]O4. The calculated Curie
Fig. 9. Dependence of w on magnetic field for MnCo2O4 (220) at 300 K.
constant Ccalc for this composition is 2.86 cm3 K/mole. A higher
value of Cexp may be explained by incomplete freezing of the
orbital component of the Co2+ cation, which leads to an enhanced
effective magnetic moment as compared with the purely spin
value. Thus, the cobaltite formed at 220 1C contains by about
10% more Co3+ cations and 100% Mn4+ cations as compared
300 K is based on possible presence of metallic cobalt in the
specimen. The emergence of cobalt in the complex oxide
MnCo2O4.62 may be attributed to the process of decomposition
of the Mn0.33Co0.67C2O4 ꢀ 2H2O oxalate in air, during which this
element is partially reduced to the metallic state. This mechanism
of destruction was established in the cause of thermal treatment
of cobalt oxalate in argon [16] and nitrogen [17] flow. We found no
available evidence in the literature for the reduction of cobalt to
metal during decomposition of oxalate in air. Apparently, this is
rather difficult to do with conventional methods since the content
of metallic cobalt in the MnCo2O4.62 specimen found from
spontaneous magnetization value at 300 1C is as small as
5.3 ꢁ10ꢂ4 mass%. In view of a small content of metallic cobalt, it
produces an insignificant effect on magnetic susceptibility in the
measured temperature range and the Curie–Weiss equation
parameters. As a result of calcination in air at higher tempera-
tures, cobalt oxidizes and the ferromagnetic component in
MnCo2O4.275 disappears.
with CoMn2O4.275
.
Compounds with
a
spinel structure belong to collinear
ferrimagnetics. For such compounds it is possible to verify rather
reliably presumed distribution of cations in structural sites by
measuring spontaneous magnetization at temperatures close
to 0 K. Provided that magnetic moments of tetrahedral high-
spin Co2+ (e t
) cations interact antiferromagnetically with
4 3
g 2g
magnetic moments of octahedral Mn4+(t23geg0) cations, the
Co20.+66[Mn
Co
]O4 spinel should have a spontaneous mo-
4+
0.866
III
1.072
ment 0.62 mB at 0 K. According to the data presented in the inset in
Fig. 8, magnetization of MnCo2O4.62 at 2 K and 50 kOe is 0.45 mB.
Taking into account that there is no saturation of magnetization in
the measured interval of magnetic field, deviations from the
calculated spontaneous moment may be considered reasonable.
The measurements performed showed that the MnCo2O4.62
spinel, as distinct from MnCo2O4.275, is characterized by a small
dependence of magnetic susceptibility w on applied magnetic field
(Fig. 9) at 300 K in the region to ꢃ10 kOe. Our version of
appearance of the ferromagnetic component in MnCo2O4.62 at
3.4. XANES spectroscopy
In order to determine the degrees of oxidation of Co and Mn,
we have carried out spectroscopic studies of the short-range fine

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2267
structure of X-ray absorption (XANES spectroscopy) of the
compounds MnCo2O4.275 and MnCo2O4.62. The fine near-threshold
structure of absorption XANES spectra is very sensitive both to the
electronic state of the absorbing atom and its local neighborhood.
This method allows to examine in particular the topology of the
atom environment and establish the degrees of oxidation of
elements by comparison with reference compounds. To evaluate
the oxidation state of cobalt and manganese in MnCo2O4, cobalt
CoO and Co3O4 and manganese MnO, Mn2O3 and MnO2 oxides
were studied under the same conditions.
Figs. 10 and 11 display Co–K and Mn–K XANES spectra of
MnCo2O4.275 and MnCo2O4.62 and reference compounds CoO,
Co3O4 and MnO, Mn2O3, MnO2, respectively. Absorption XANES
spectra of these compounds are characterized by a sharp intensive
peak and several peaks of smaller intensity. The electronic
transitions from 1s to unoccupied nd states are related to the
so-called pre-edge peaks located before the basic absorption
peaks. Their intensity and location on the energy scale speak of
the element’s coordination and state of oxidation, respectively. If
the coordination environment has a center of inversion, the
electronic 1s-nd transitions turn out to be forbidden and the
pre-edge peaks on the spectra are absent. Along with octahedral
sites, there are positions with less symmetrical tetrahedral
environment in the spinel structure. Therefore, 1s-3d transitions
are partially permitted. An increase in the degree of oxidation of
an element leads to a displacement of the pre-edge peak to the
high-energy region. If the element has a mixed valence, the pre-
edge peak widens or bifurcates.
Conclusions about the state of oxidation of cobalt in
MnCo2O4.275 and MnCo2O4.62 were made from the pre-edge
K-peak, whose location usually corresponds to the energy values
about 7709–7712 eV [18]. According to Fig. 10, the pre-edge peak
has the smallest intensity in the CoO spectrum. This is quite
natural since this oxide has the NaCl structure and cobalt is in the
symmetrical environment. The pre-edge peaks in the spectra of
other transition-metal monoxides are also of inconsiderable
intensity [19]. The greatest intensity of the pre-edge peak is
observed in the Co3O4 spectrum (Fig. 10), which is attributed to
the tetrahedral environment of Co2+cations. The spectra of Fig. 10
show that the Co K-edge XANES in Co3O4, MnCo2O4.275 and
MnCo2O4.62 have a larger width of pre-edge features as compared
with CoO. The considered peaks are characterized by a compli-
cated structure brought about by mixed valence of cobalt in all
these compounds. It follows from Fig. 10 that Co–K XANES spectra
in MnCo2O4.275 and MnCo2O4.62 are almost identical. Therefore
the states of oxidation of cobalt in these compounds are close.
Thus, the XANES spectra allow us to assert that cobalt in the two
examined spinels is in a mixed (+2 and +3) state. A somewhat
smaller intensity of the pre-edge peak for the specimen
synthesized at 220 1C is likely to be due to a greater structure
imperfection in the tetrahedral sites of the spinel.
Mn K-edges of XANES spectra of MnO, Mn2O3, MnO2,
MnCo2O4.275 and MnCo2O4.62 are depicted in Fig. 11. Attention is
drawn to a smaller intensity of pre-edge peaks of complex oxides
with a spinel structure as compared with simple manganese
oxides. This indicates that manganese in the spinel has a higher-
symmetry environment, and in particular that it occupies
octahedral sites. Mn pre-edge peaks, K-edges and the basic absor-
ption peak for MnCo2O4.62 are shifted in the high-energy region in
comparison with MnCo2O4.275, which testifies to a higher degree
of oxidation of manganese in the former compound. It is evident
from comparison with Mn2O3 and MnO2 spectra that the content
of Mn4+ cations is higher in MnCo2O4.62 than in MnCo2O4.275. This
inference supports the data obtained by interpreting magnetic
properties measurements for these two compounds. It is also
typical that a similar displacement of the basic peak in XANES
spectra of MnCo2O4 was recorded for the specimens prepared by
decomposition of nitrates and chlorides of Co and Mn [20].
1.6
1.2
0.8
MnCo2O4.62
Co3O4
0.4
CoO
MnCo2O4.275
0.0
7700
7710
7720
7730
7740
7750
Energy / eV
Fig. 10. Co–K XANES spectra of MnCo2O4.275 and MnCo2O4.62 and reference
compounds CoO and Co3O4.
1.4
1.2
1.0
0.8
4. Conclusions
In accordance with the above results, cobaltites MnCo2O4.275
and MnCo2O4.62 differ in the degrees of oxidation of manganese:
in the former cobaltite the Mn4+/Mn3+ ratio is ꢃ5/4, whereas the
latter compound contains manganese only as Mn4+. The concen-
tration of Co3+ cations with respect to Co2+ in the more oxidized
phase MnCo2O4.62 increases by approximately 10%. These findings
are crucial in analyzing the differences in the properties of these
nonstoichiometric spinels and the stoichiometric spinel MnCo2O4.
The considerable decrease in TN of MnCo2O4.62 in comparison with
MnCo2O4.275 and MnCo2O4 is associated with substantial struc-
ture imperfection in the cationic sublattice of this spinel and
substitution of Mn4+ for Mn3+ cations. The imperfect spinel
MnCo2O4.62 possesses ferrimagnetism caused by antiferromag-
netic exchange between tetrahedral Co2+ cations and octahedral
Mn4+ cations. The peculiarities on ZFC and FC dependences
w ¼ f(T) for MnCo2O4.275 below 120 K may be due to variations
0.6
MnO
MnO2
0.4
Mn2O3
MnCo2O4.275 (500°)
MnCo2O4.62 (220°)
0.2
0.0
6530
6540
6550
Energy / eV
6560
6570
6580
Fig. 11. Mn–K XANES spectra of MnCo2O4.275 and MnCo2O4.62 and reference
compounds MnO, Mn2O3 and MnO2.

ARTICLE IN PRESS
2268
G.V. Bazuev, A.V. Korolyov / Journal of Magnetism and Magnetic Materials 320 (2008) 2262–2268
in the composition of the magnetic phase in the process of heat-
treatment temperature increase and the presence of impurities.
The absence of the spontaneous moment for MnCo2O4.275 at 14 K
apparently points to the presence of the compensation point on
the temperature dependence of spontaneous magnetization.
Complete removal of the impurity phase calls for temperatures
above 600 1C.
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