Europium valence state in europium monoxide-iron (Cobalt) magnetic composites

Article abstract of DOI:10.1134/S0036023609100027

Composite magnetic powders EuO/Fe(Co) were synthesized. These composites are candidates for use as materials for spintronics devices operating at room temperature. The composites synthesized by carbothermal reduction of oxide mixtures Eu₂O

Full text of DOI:10.1134/S0036023609100027

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2009, Vol. 54, No. 10, pp. 1517–1521. © Pleiades Publishing, Inc., 2009.
Original Russian Text © N.I. Ignat’eva, A.S. Shkvarin, V.I. Osotov, L.D. Finkel’shtein, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 10, pp. 1591–1595.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Europium Valence State in Europium Monoxide–Iron (Cobalt)
Magnetic Composites
N. I. Ignat’evaa, A. S. Shkvarinb, V. I. Osotovb, and L. D. Finkel’shteinb
a Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences, ul. Pervomaiskaya 91, 620219 Russia
b Institute of Metal Physics, Ural Division, Russian Academy of Sciences, ul. S. Kovalevskoi 18, Yekaterinburg, 620219 Russia
Received May 14, 2008
Abstract—Composite magnetic powders EuO/Fe(Co) were synthesized. These composites are candidates for
use as materials for spintronics devices operating at room temperature. The composites synthesized by carbo-
thermal reduction of oxide mixtures Eu2O3 and Fe2O3 (or Co3O4) contain not only europium in the main
valence state (2+) but also europium in the state 3+, its content increasing with the content of the d metal and
depending on technological parameters of the reduction process. The content of the trivalent europium in the
composites was estimated by chemical analysis and L111-edge X-ray absorption spectroscopy (XAS). The
results obtained by these methods correlate with each other. XAS is considered to be a quantitative express
method of certification of materials containing europium in various valence states.
DOI: 10.1134/S0036023609100027
The development of a new field in microelectron- metals is determined. In the present work, in addition to
traditional chemical analysis methods, we used L111-
edge X-ray absorption spectroscopy (XAS) developed
at the Institute of Metal Physics, Ural Division of the
RAS, which makes it possible to determine, to a rather
high accuracy, the quantitative ratio of the Eu2+ and Eu3+
ions in EuO/Fe(Co) composites of different composi-
tions.
ics—spin electronics—is based on realization of elec-
tron transport between the working elements of elec-
tronic devices in which the information carrier is the
spin of an electron. In solid heterostructures, the source
(injector) of spin-polarized electrons can be wide-gap
ferromagnetic semiconductors. These conditions are
completely met by composites based on europium
monoxide, a ferromagnetic semiconductor (ρ = 108–
1010 Ω cm, the bandgap Ög = 1.2 eV, Tc = 69 K, chemi-
cally stable at ambient temperatures) [1, 2]. It is pre-
cisely europium monoxide that is currently believed to
be the best candidate for film heterostructures of the
FS/S type (ferromagnetic semiconductor/nonmagnetic
semiconductor) in which both solid solutions based on
europium monoxide and its composites with 3d- and 4f
metals can act as spin injectors [3, 4]. Composite mate-
rials EuO/Fe(Co), composed of two phases differing in
ferromagnetic ordering temperatures and conductivity,
meet the requirements of operation of spin devices at
room temperature. To realize spin transport in such sys-
tems, the content of the metal phase should fall within
the range where direct M–M interaction is absent and
the material retains the properties of the ferromagnetic
semiconductor at a rather high magnetic moment (up to
25 wt % metal). On the other hand, the content of triva-
lent europium in the material should be controlled since
the presence of paramagnetic Eu3+ ions sharply deterio-
EXPERIMENTAL
To obtain composite materials with phases contain-
ing low valence states of metals (Fe0, Co0, and Eu2+)—
iron or cobalt metals and europium monoxide, the
known methods of reduction of their highest oxides
with carbon (carbothermy) can be used. The reduction
of Fe2O3 to the metal is a well-documented physico-
chemical process. It proceeds by the indirect reduction
mechanism involving the gas phase (carbon monoxide
CO) at temperatures on the order of 1100°ë. The high-
est europium oxide Eu2O3, in contrast to 3d-metal
oxides, is not reduced by CO. The main role in the
reduction of Eu2O3 with carbon is played by oxygen dif-
fusion from the bulk of the sample to its surface while
the reagents are in direct contact, and the synthesis of
lower europium oxides occurs at ~1300°ë producing
CO.
Taking into account the differences in the reduction
rates the ferromagnetic characteristics of the compos- mechanisms and temperatures of the highest iron,
cobalt, and europium oxides by carbon, we selected
high-temperature carbothermal reduction of a mixture
of Eu2O3, Fe2O3, and Co3O4 as the simplest and most
available method of synthesis of composites [5, 6]. The
ite. Chemical methods of quantification of valence
states of rare earth metals (Ce, Sm, Eu, Yb) are rather
complicated even when voltammetry, polarography,
EPR, and other methods are invoked. As a rule, in nat-
ural and technical objects, the total content of these synthesis was carried out at reduced pressures of the
1517

1518
I, %
IGNAT’EVA et al.
on a DRON-2 diffractometer (CuKαradiation). The
(a)
35
weight percentage of Eu, Fe, and Co was determined by
gravimetric and volumetric chemical analysis: from
hydrochloric acid solutions, europium was isolated as
oxalate; iron was determined trilonometrically and
cobalt, by atomic emission spectroscopy. Carbon was
determined by burning a sample in oxygen. The content
of oxygen was determined by calculation. The valence
state of europium in the oxide phase of composites and
the quantitative ratio between the Eu2+ and Eu3+ ions
were determined from chemical analysis data and from
the results of processing the Eu absorption spectra
obtained by the XAS method [7, 8]. This method is
based on excitation by X-rays of the electronic transi-
100
80
60
40
20
30.2
50.3
59.7
0
–200
30
40
50
60
tion 2p3/2
5d in atoms of rare earth elements, in par-
ticular, in europium atoms. The energies of this transi-
tion for the Eu2+ and Eu3+ ions differ by 7 eV, which
makes it possible to determine the content of these
europium ions from the intensities of two signals. An
advantage of this method is its selectivity; i.e., the result
is independent of the presence of other chemical ele-
ments.
100
80
60
40
20
30.2
(b)
35
α-Fe
50.2
59.7
The Eu L111 X-ray absorption spectra were recorded
on an ARS-KD-2 vacuum X-ray spectrometer with the
44.8
use of a coordinate detector. The (134 0) plane of a bent
quartz single crystal with a curvature radius of 1.940 m
was used as the crystal analyzer. Dispersion near the
0
L111 edge was 2.6 eV/mm, the spectrometer resolution
–200
30
40
2θ, deg
50
60
was ~8000. To determine the valence, a composite
spectrum was routinely decomposed into a combina-
tion of Lorentzian and arctangent functions in order to
model the process of electron transfer, after X-ray
quantum absorption, to coupled states and continuous
spectrum, respectively. The areas under the curves
described by Lorentzian functions (SEu2+ and SEu3+) are
proportional to the number of atoms with different
valences. The europium valence (VEu) was calculated
by the formula VEu = 2 + [SEu2+/(SEu2+ + SEu3+)].
Fig. 1. X-ray powder diffraction patterns of (a) EuO and
(b) the composite EuO/Fe.
gas phase, which is necessary for stabilization of the
divalent europium by the reactions
Eu2O3 + Fe2O3 + 4C = 2EuO + 2Fe + 4CO↑, (1)
Eu2O3 + Co3O4 + 5C = 2EuO + 3Co + 5CO↑. (2)
RESULTS AND DISCUSSION
Reactions (1) and (2) are basic for calculation of the
weights of corresponding oxides and carbon required to
obtain a specified amount of a composite with an iron
(or cobalt) content within 5–40 wt %. The initial mix-
ture was prepared from 3d metal oxides (chemically
pure grade), Eu2O3 of class 0 (99.999% purity), and
acetylene black (pure for analysis). The powder blend
prepared by thorough mixing was placed into a molyb-
denum crucible and heated in two stages (at 1100 and
1300°ë) in an SNV-1500 vacuum furnace, while the
pressure of the gas phase was maintained at 102–10–1 Pa.
The first stage of the reduction process (1100°ë, pres-
sure in the system 102 Pa) leads to the appearance of Fe
and Co metal particles in the system. At the second
stage (1300°C, 10–1Pa), Eu2O3 is reduced to europium
The products of high-temperature reduction of mix-
tures of europium and 3d-metal oxides by carbon are
fine powders composed of the oxide matrix of EuO (an
NaCl structure) and α-Fe or Co metal particles 200–
300 nm in size uniformly distributed over the matrix.
Figure 1 shows the X-ray powder diffraction patterns of
the composite (b) in which, in addition to the EuO line
(a), the lie of Fe metal is observed (2θ 44.8 °). Accord-
ing to our magnetic measurements, the composites con-
sist of two ferromagnetic phases that differ in magneti-
zation and Curie temperature (Fig. 2).
The valence state of europium and the quantitative
ratio of the Eu2+ and Eu3+ ions in the europium monox-
ide phase were estimated from the results of chemical
monoxide. The phase composition of the reduction analysis of the EuO/Fe and EuO/Co composites. With
products was monitored by X-ray powder diffraction allowance for the fact that all oxygen in the composite
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 10 2009

EUROPIUM VALENCE STATE
1519
EuO
180
160
140
120
100
80
H = 15 kOe
α-Fe
60
0
20
40
60
80
T, K
100
120
Fig. 2. Temperature dependence of the saturation ferromagnetic moment of the composite EuO/25 wt % Fe in the magnetic field
H = 15 kOe.
is bound only to europium to form EuOı phases nonsto-
ichiometric with respect to oxygen, the formula of this
phase can be calculated from the weight percentages of
oxygen and europium in the composite reduced to their
atomic weights (16 and 152, respectively). The content
of europium in the di- and trivalent state in the composite
was calculated from the derived formula of monoxide.
There is an example of calculation of the oxide for-
mula and the contents of Eu2+ and Eu3+ ions for sample
no. 1 (table): 8.35 : 16 = 0.52, 76.04 : 152 = 0.5. Hence,
the oxide formula is EuO1.04. Assuming that the EuOı
oxide with an oxygen excess is a mixture of the oxides
EuO and EuO1.5, we calculate the atomic percentage of
the Eu2+ and Eu3+ ions in the oxide EuO1.04 by the equa-
Contents of Eu2+ and Eu3+ in various composites as determined by chemical analysis and X-ray absorption spectroscopy
Content, at % of the total Eu Content, at % of the total Eu
Chemical composition, wt %
content, according
to chemical analysis
content, according
to XAS data
EuOx formula
by chemical
analysis
No.
M
Eu
C
O
Eu2+
Eu3+
Eu2+
Eu3+
EuO–Fe system
1
2
3
4
13.75
25.75
34.70
40.24
76.04
64.78
56.10
50.70
1.86
2.00
2.08
2.20
8.35
7.47
7.12
6.86
EuO1.04
92
82
60
44
8
87
79
58
45
13
21
42
55
EuO1.09
18
40
56
EuO1.20
EuO1.28
EuO–Co system
EuO1.05
5
6
7
6.28
14.97
35.00
84.32 Traces
76.23 Traces
9.40
8.80
7.90
86
80
33
14
20
64
84
77
34
16
23
66
EuO1.07
56.90
0.20
EuO1.33
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 10 2009

1520
IGNAT’EVA et al.
I
I
A
A
B
B
1
1
2
2
3
3
5
5
4
4
6960
6970
6980
Energy, eV
6990
6960
6970
6980
Energy, eV
6990
Fig. 3. Eu L -edge X-ray absorption spectra of composites
Fig. 4. Eu L -edge X-ray absorption spectra of composites
111
111
with iron: (1) EuO
+ 13.75 wt % Fe, (2) EuO
+ 25.75
with cobalt: (1) EuO
+ 6.28 wt % Co, (2) EuO
+
1.04
1.09
1.05
1.07
wt % Fe, (3) EuO
+ 40.24 wt % Fe, (4) EuS, and
14.97 wt % Co, (3) EuO
+ 35 wt % Co, (4) EuS, and
1.28
1.33
(5) Eu O .
(5) Eu O .
2
3
2 3
tion 1.04 = 1.0Û + 1.5(1 – Û), hence Û = 0.92. Thus, the dominating over the process of solid-phase reduction,
oxide EuO1.04 contains 92 at % Eu2+ and 8 at % Eu3+.
and this leads to a considerable consumption of the
reducing agent at the stage of reduction of Fe2O3 and
Co3O4 to the metal and to its deficiency at the stage of
The Eu L111 X-ray absorption spectra are shown in
Figs. 3 and 4. Europium(II) sulfide EuS and
europium(III) oxide Eu2O3 were used as the references.
phase transitions Eu2O3
Eu3O4
EuO. There-
fore, for the development of technological parameters
of synthesis (reductant consumption, temperature, gas-
phase pressure, heating and cooling regimes) and certi-
fication of magnetic composites based on lower
europium oxide, the L111-edge XAS method can be rec-
ommended as a reliable express method for quantifica-
tion of the europium valence state in the materials
under consideration.
Maxima Ä and Ç correspond to the Eu2+ and Eu3+
absorption signals, respectively.
The table summarizes the results of determination
of the europium valence state on the basis of chemical
analysis and XAS data.
Analysis of the data in the table shows the results of
determination of the content of europium in different
valence states in magnetic composites by the chemical
We found that, with the use of the suggested method
analysis and XAS methods are consistent (the discrep- of synthesis of EuO/M composites, the content of triva-
ancy is no more than 5 at %). With allowance for the lent europium in the oxide phase can be stabilized at the
fact that all used chemical analytical methods implying level of 20 at % of the total Eu content when as much
the destruction of considerable samples of a material as 25% of the metal is introduced into the composite.
are indirect methods of determination of the europium
valence state and that each of the procedures introduces
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