Synthesis, thermal, spectroscopic and magnetic studies of the Mn(SeO3)·2H2O and Fe2(SeO3)3·3H2O selenites

Article abstract of DOI:10.1016/j.saa.2007.06.019

Mn(SeO₃)·2H₂O (1) and Fe₂(SeO₃)₃·3H₂O (2) have been synthesized by slow evaporation from an aqueous solution in the case of (1) and using mild hydrothermal conditions for (2). The crystal structures of both phases have been refined by the Rietveld method. The compounds crystallize in different spatial groups, the P2₁/n monoclinic one with parameters a = 6.649(1) A, b = 6.542(1) A, c = 10.890(1) A and β = 103.85(1)° being Z = 4 for (1) and the R3c trigonal space group with parameters a = 9.361(1) A, c = 20.276(1) A and Z = 6 for (2). The crystal structure of compound (1) consists of a three-dimensional framework formed by MnO₆ octahedra and (SeO₃)^2- oxoanions with trigonal pyramidal geometry, which gives rise to Mn₂O₁₀ dimers of edge-sharing octahedra. The crystal structure of phase (2) can be described as a three-dimensional framework formed by MnO₆ octahedra and (SeO₃)^2- oxoanions with trigonal pyramidal geometry. In this phase the octahedral entities are linked along the three crystallographic axes through the selenite anions. Diffuse reflectance spectrum and luminescent measurements for (1) indicate the existence of Mn²⁺ cations in a slightly distorted octahedral environment. Diffuse reflectance spectrum and Moessbauer spectroscopy, in the paramagnetic region, for (2) show the existence of Fe³⁺ cations in slightly distorted octahedral symmetry. ESR spectra of both compounds are isotropic with a g-value of 1.99(1) and 2.00(1), respectively. Magnetic measurements of both phases indicate an antiferromagnetic behavior. For phase (2), both, the ESR and magnetic measurements suggest a spin change from Fe³⁺ (S = 5/2) to Fe²⁺ (S = 2) at low temperatures.

Full text of DOI:10.1016/j.saa.2007.06.019

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Spectrochimica Acta Part A 69 (2008) 1020–1026
Synthesis, thermal, spectroscopic and magnetic studies of the
Mn(SeO3)·2H2O and Fe2(SeO3)3·3H2O selenites
a
b,∗
a
b
˜
´
´
Aitor Larranaga , Jose L. Mesa , Jose L. Pizarro , Luis Lezama ,
a
b
´
´
Marıa I. Arriortua , Teofilo Rojo
a Departamento de Mineralog´ıa y Petrolog´ıa, Facultad de Ciencia y Tecnolog´ıa, Apdo. 644, 48080 Bilbao, Spain
b Departamento de Qu´ımica Inorga´nica, Facultad de Ciencia y Tecnolog´ıa, Apdo. 644, 48080 Bilbao, Spain
Received 12 December 2006; accepted 2 June 2007
Abstract
Mn(SeO3)·2H2O (1) and Fe2(SeO3)3·3H2O (2) have been synthesized by slow evaporation from an aqueous solution in the case of (1) and using
mild hydrothermal conditions for (2). The crystal structures of both phases have been refined by the Rietveld method. The compounds crystallize in
◦
˚
˚
˚
different spatial groups, the P21/n monoclinic one with parameters a = 6.649(1) A, b = 6.542(1) A, c = 10.890(1) A and β = 103.85(1) being Z = 4
˚
˚
for (1) and the R3c trigonal space group with parameters a = 9.361(1) A, c = 20.276(1) A and Z = 6 for (2). The crystal structure of compound (1)
consists of a three-dimensional framework formed by MnO6 octahedra and (SeO3)2− oxoanions with trigonal pyramidal geometry, which gives
rise to Mn2O10 dimers of edge-sharing octahedra. The crystal structure of phase (2) can be described as a three-dimensional framework formed
by MnO6 octahedra and (SeO3)2− oxoanions with trigonal pyramidal geometry. In this phase the octahedral entities are linked along the three
crystallographic axes through the selenite anions. Diffuse reflectance spectrum and luminescent measurements for (1) indicate the existence of
Mn2+ cations in a slightly distorted octahedral environment. Diffuse reflectance spectrum and Mo¨ssbauer spectroscopy, in the paramagnetic region,
for (2) show the existence of Fe3+ cations in slightly distorted octahedral symmetry. ESR spectra of both compounds are isotropic with a g-value
of 1.99(1) and 2.00(1), respectively. Magnetic measurements of both phases indicate an antiferromagnetic behavior. For phase (2), both, the ESR
and magnetic measurements suggest a spin change from Fe3+ (S = 5/2) to Fe2+ (S = 2) at low temperatures.
© 2007 Elsevier B.V. All rights reserved.
Keywords: UV–vis; Thermal behavior; Mo¨ssbauer; ESR; Magnetism
1. Introduction
The transition metal–selenium–oxygen system has been the
subject of several previous investigations [2]. Several phases
have been reported in which the selenium is found in an oxi-
dation state +IV. This way, transition metal compounds with
the (HSeO3)−, (SeO3)2− and (Se2O5)2− oxoanions are known
[3]. Although, a great number of crystal structures related to
the selenite compounds have been resolved, studies on the
physical properties of the synthesized phases are scarce. The
thermal behavior and IR spectroscopy of several phases have
been reported [4]. The M(Se2O5) compound with a chain struc-
ture shows the existence of antiferromagnetic interaction [4].
Mild hydrothermal techniques under autogeneous pressure have
allowed the attainment of an astonishing variety of inorganic net-
works templated by organic species [5]. The preparation of two
organically templated zinc and iron(III) open framework selen-
ites has been reported [6]. Recently, we have reported the crystal
structure of the Mn4(H2O)(SeO3)4 and Mn3(H2O)(SeO3) [7],
synthesized by mild hydrothermal techniques, and the study
An important area in material science is the design of com-
pounds with a condensed framework, which can give rise to
original physical properties due to the great number of different
cation arrangements that they can exhibit [1]. It is noteworthy
that the ubiquitous presence of a stereochemically active lone-
pair of electrons on the Se(IV) centers can play an important role
inthecrystalline architectureof thesefamiliesof compounds [2].
The entire structure is affected by the requirement for empty
space to accommodate the selenium lone-pair electron. Thus, it
can be predicted that the combination of the inherently asym-
metric (SeO3)2− group with various transition metals will result
in a rich structural chemistry in transition metal selenites.
∗
Corresponding author. Tel.: +34 946015523; fax: +34 946013500.
E-mail address: joseluis.mesa@ehu.es (J.L. Mesa).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.06.019

A. Larran˜aga et al. / Spectrochimica Acta Part A 69 (2008) 1020–1026
1021
of their physical properties. We have also studied the crystal
2. Experimental
structure and the physical behavior of two polymorphs with the
composition Mn(SeO3), obtained under supercritical conditions
[8].
2.1. Synthesis and characterization
The crystal structures of (1) and (2) were solved from X-
ray single crystal data by Koskenlinna et al. [9] and Giester
et al. [10], respectively. The crystal structure of compound (1)
consists of a three-dimensional framework constructed from
(MnO6) octahedra and (SeO3)2− selenite groups with trigo-
nal pyramidal geometry. The structure is formed from isolated
Mn2O10 edge-sharing octahedral dimers, extended along the
[1 0 1] direction. These dimeric entities are connected by the
(SeO3)2− oxoanions (Fig. 1a). The crystal structure of phase (2)
can be described as a three-dimensional framework composed
of (FeO6) octahedra and (SeO3)2− selenite anions with trigonal
pyramidal geometry. The octahedral entities are linked along
the three-crystallographic axes through the selenite oxoanions
(Fig. 1b).
Mn(SeO3)·2H2Ohasbeensynthesizedinwatersolutionstart-
ing from the SeO2 (9 mmol) and MnCl2·4H2O (5.05 mmol).
The pH of the resulting solution was increased up to 1.5
by addition of NH4OH. Single-crystals with light pink color
were obtained by slow evaporation after 20 days, yield
85%. Fe2(SeO3)3·3H2O has been obtained by using mild
hydrothermal conditions under autogeneous pressure. The start-
ing reagents were SeO2 (1.44 mmol), FeCl3·6H2O (0.74 mmol),
piperazinium (0.075 mmol) and finally HF (0.025 mmol) to
decrease the pH down to 1.5. The resulting mixture was dis-
solved in water and maintained in a politetrafluoroetylene
reactor at 110 ◦C during 5 days. Yellow single-crystals were
obtained. Yield 80%.
The single-crystals obtained for both phases were filtered out,
washed with water and acetone and dried over P2O5 during 6 h.
The metal ion and selenium contents were confirmed by induc-
tively coupled plasma atomic emission spectroscopy (ICP-AES)
analysis, performed with a RL Fisons 3410 spectrometer. For
(1), it was found: Se, 35.8; Mn, 24.8. Mn(SeO3)·2H2O requires:
Se, 36.2; Mn, 25.2. For (2), it was found: Se, 14.5; Fe, 10.0.
Fe2(SeO3)3·3H2O requires: Se, 14.9; Fe, 10.2. The density of
phase (1), 3.1(1) g cm−3, was measured by flotation in a mix-
ture of CH2I2/CCl4. In the case of compound (2), the density was
measured by picnometry using kerosene. The value obtained is
In this work, we report the synthesis from aqueous solu-
tion and the hydrothermal method of the Mn(SeO3)·2H2O and
Fe2(SeO3)3·3H2O compounds, respectively. The thermal, lumi-
¨
nescent, Mossbauer and magnetic studies are carried out for both
phases.
3.4(1) g cm−3
.
2.2. X-ray diffraction
X-ray powder diffraction patterns of (1) and (2), used for
the Rietveld analysis, have been recordered with a PHILIPS
X’PERT automatic diffractometer (Cu K␣ radiation). The spec-
tra were scanned in steps of 0.02◦ in 2θ for 12 s/step. Rietveld
refinements have been performed with the FULLPROF program
[11]. Details of the process are given in Table 1. The unit cell
parameters obtained after the refinement of both compounds are
Table 1
Details of the Rietveld refinement for Mn(SeO3)·2H2O and Fe2(SeO3)3·3H2O
Empical formula
Formula weight (g/mol)
Crystal system
Mn(SeO3)·2H2O
Fe2(SeO3)3·3H2O
546.61
Trigonal
R3c (161)
9.361(1)
9.361(1)
20.276
90
90
120
1538.8(2)
6
217.92
Monoclinic
P21/n (14)
6.649(1)
6.542(1)
10.890(1)
90.0
103.85(1)
90.0
459.9(1)
4
Space group (Nu.)
˚
a (A)
˚
b (A)
˚
c (A)
α (◦)
β (◦)
γ (◦)
Volume (A )
Z
3
˚
ρ (calc.)
3.14
3.54
RBragg, Rf (%)
Rp, Rwp (%)
χ2 (%)
11.1, 7.82
13.6, 18.2
4.52
8.26, 4.38
10.9, 14.1
2.28
Fig. 1. Polyhedral representation of the three-dimensional crystal structure of
(a) compound (1) and (b) compound (2).

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A. Larran˜aga et al. / Spectrochimica Acta Part A 69 (2008) 1020–1026
spectra from room temperature up to 4.2 K. The temperature
was stabilized by an Oxford Instrument (ITC 4) regulator. The
magnetic field was measured with a Bruker BNM 200 gauss-
meter and the frequency inside the cavity was determined using
a Hewlett–Packard 5352 microwave frequency counter. Mag-
netic measurements for powdered sample were performed in the
temperature range 5–300 K, using a Quantum Desing MPMS-7
SQUID magnetometer. The magnetic field was 1000 Gauss, a
value lying within the range of linear dependence of magnetiza-
tion versus magnetic field even a 5.0 K.
3. Results and discussion
3.1. Thermal behavior
Compounds (1) and (2) are stable up to, approximately, 75
and 200 ◦C, respectively. The elimination of the water molecules
(experimental 18.0 and 10.2%, calculated 16.6 and 9.9%, for
phases (1) and (2), respectively) takes place in the 75–300 and
200–350 ◦C ranges for (1) and (2), respectively. Above 475
and 460 ◦C in the case of compounds (1) and (2), respectively,
the decomposition of the (SeO3)2− anions occurs (experimen-
tal 46.5 and 60.5%, calculated 47.2 and 60.9% for (1) and (2),
respectively) giving rise to SeO2(g). At 700 ◦C, the inorganic
residues are the Mn2O3 and Fe2O3 oxides for every compound
[12].
The thermal behavior of compounds (1) and (2) was also stud-
ied by time resolved X-ray thermodiffractometry. A PHILIPS
X’PERT automatic diffractometer (Cu K␣ radiation) equipped
with a variable temperature stage (Anton Paar HTK16) and a
Pt sample holder was used. The data have been recordered in
2θ steps of 0.02◦ in the range 5 ≤ 2θ ≤ 50◦, counting for 1s per
step and increasing the temperature at 5 ◦C min−1 from room
temperature to 650 ◦C. The results are reported in Fig. 3. The
compounds are stable up to ca. 125 and 325 ◦C, respectively and
the intensity of the monitored (0 1 2) peak at, approximately,
2θ = 22◦ remains practically unchanged. For compound (1), a
phase, which cannot be identified on the PDF files, appears in
the 125–425 ◦C range. Above 425 ◦C, the Mn2O3 oxide [12] is
formed in good agreement with the thermogravimetric data. For
phase (2), in the 325–475 ◦C range an amorphous compound is
obtained. At higher temperatures a residue of Fe2O3 is found
[12], in accordance with the thermogravimetric data.
Fig. 2. Observed, calculated and difference X-ray powder diffraction patterns
of the (a) compound (1) and (b) compound (2). The observed data are shown
by the dots, the calculated pattern is denoted by the solid line and the difference
between the spectra is presented in the lower region.
˚
similar to those described in the literature [P21/c, a = 6.655(1) A,
◦
˚
˚
b = 6.542(1) A, c = 10.900(1) A and β = 103.84(1) ] for (1) [9]
˚
˚
and [R3c, a = 9.36(1) A, c = 20.30(1) A] [10] for (2), respec-
tively. The results of the Rietveld refinement are illustrated in
Fig. 2.
2.3. Physical measurements
Thermogravimetric measurements were carried out heating
the samples at 5 ◦C min−1 under synthetic air in the 30–800 ◦C
temperature range using a SDC 2960 Simultaneous DSC-TA
instrument. TheIRspectra(KBrpellets)havebeenobtainedwith
a Nicolet FT-IR 740 spectrophotometer in the 400–4000 cm−1
range. The diffuse reflectance spectra were registered at room
temperature on a Varian Cary 5000 UV-VIS-NIR spectrometer
in the 210–2000 nm range. The excitation and emission lumines-
cent spectra of the manganese compound were recordered with
a SPEX Fluorolog 212 spectrofluorometer. Excitation spectrum
was corrected for the variation of the incident flux as well as
emission spectra for the transmission of the monochromator and
the response of the photomultiplier. A Bruker ESP 300 spec-
trometer was used to record the X-band ESR polycrystalline
3.2. IR, UV–vis and Mo¨ssbauer spectroscopies
In the IR spectra of both phases, the bands belonging to the
stretching and deformation modes of the water molecules can
be observed at 3450–2590 and 1625, 1545 cm−1, respectively.
Three groups of bands have been assigned to the vibrational
modes of the (SeO3)2− oxoanions. The symmetric stretching
mode, νs(SeO3)2− is located at 910 cm−1, the antisymmetric
stretching, νas(SeO3)2−, splitted at 810, 725 and 590 cm−1, and
finally the bending mode, δ(SeO3)2− is peaking at 480 cm−1
[13].
The diffuse reflectance spectrum of the phase (1) shows sev-
eral weak spin forbidden transitions at 18,410, 22,210, 24,400,

A. Larran˜aga et al. / Spectrochimica Acta Part A 69 (2008) 1020–1026
1023
Fig. 3. (a–b) Thermodiffractogram characteristic of the phases (1) and (2).
28,980 and 31,750 cm−1. The emission spectrum recordered at
6 K presents a band centered at 685 nm (Fig. 4(a)). This result
indicates the existence of Mn2+ cations in slightly distorted
octahedra environments. The excitation spectrum (Fig. 4(b))
shows a spectral distribution of five bands, which correspond
to transitions from the 6A1g(6S) ground state, to the 4T1g(4G);
Fig. 4. (a) Luminescent emission spectrum (λ excitation 412 nm) at 6 K (b)
Luminescent excitation spectrum (λ emission 630 nm) at 6 K for (1).
4T2g(4G); A1g(4G), Eg(4G); T2g(4D) and Eg(4D) excited
levels at the following frequencies 18,420, 22,220, 24,390,
28,985 and 31,745 cm−1, respectively. The positions of these
bands are coherent with those observed on the diffuse reflectance
spectrum (continuous line in Fig. 4(b)). The calculated Dq
parameter is 790 cm−1 and B and C-Racah parameters are
690 and 3500 cm−1, respectively. The reduction of the B-
parameter from that of the free Mn2+ cation (960 cm−1) is
72%, which indicates a significant covalent character in the
Mn–O chemical bonds. All these results are in good agree-
ment with the octahedral coordination of the Mn2+cations
[14].
4
4
4
4
¨
The Mossbauer spectrum of (2) in the paramagnetic state
at 300 K shows two singlets corresponding to the pure elec-
trostatic interactions of the two crystallographic independent
Fe3+ cations, located in a very regular octahedral environment
(Fig. 5). The values of the isomer shift obtained from the fit of
the spectrum by using the NORMOS program [15] are 0.36(2)
and 0.46(2) mm s−1 for Fe(1) and Fe(2) sites, respectively.
The diffuse reflectance spectrum of the compound (2) shows
the characteristic features of the Fe3+ d5-high spin cation. The
weak forbidden electronic transitions from the 6A1g(6S) ground
state, to the 4T1g(4G); 4T2g(4G); 4A1g(4G), 4Eg(4G); 4T2g(4D)
4
and Eg(4D) excited levels are observed at the following fre-
quencies 12,130, 15,615, 20,385, 22,225 and 29,575 cm−1
,
respectively. The calculated Dq parameter is 1015 cm−1 and
B- and C-Racah parameters are 615 and 2850 cm−1, respec-
tively. The reduction of the B-parameter from that of the free
Fe3+ cation (1150 cm−1) is 55%, which indicates a signifi-
cant covalent character in the Fe–O chemical bonds. These
results are in good agreement with the existence of octahedri-
cally coordinated Fe3+ cations [14] and confirm the structural
results.
Fig. 5. Mossbauer spectrum at 300 K of the compound (2).
¨

1024
A. Larran˜aga et al. / Spectrochimica Acta Part A 69 (2008) 1020–1026
Fig. 7. Temperature dependence of the intensity and the line-width of the ESR
signals for (a) compound (1) and (b) compound (2).
Fig. 6. Powder X-band ESR spectra of (a) compound (1) and (b) compound (2).
3.3. ESR and magnetic properties
The ESR spectra of compounds (1) and (2) remain isotropic
from 4.2 K to room temperature for (1) and from 40 K to room
temperature for (2) (Fig. 6). At approximately 40 K the sig-
nal disappears in (2) probably due to a spin change in the iron
cations (5/2 to 2). The ESR signals of both phases were fitted
to Lorentzian curves and then calculated the value of the g-
gyromagnetic tensor, 1.99(1) and 2.00(1), respectively, in good
agreement with the existence of Mn(II) and Fe(III) d5-cations,
respectively.
For (1), the thermal evolution of the intensity of the
signals increases with decreasing temperature and shows a
maximum at approximately 25 K. This result suggests the exis-
tence of antiferromagnetic interactions. After this temperature
the intensity decreases down to 4.2 K (Fig. 7(a)). The ther-
mal variation of the line-width of the ESR signals increases
slowly up to 50 K and below this temperature the line-width
increases dramatically due to a phenomenon of spin correlation
[16].
down to, approximately, 40 K. This effect could be associated
to a change in the spin of the iron atoms, from 5/2 to 2. Finally,
the ESR signal disappears below 40 K. These results are in good
agreement with the existence of antiferromagnetic interactions.
The line-width of the signals shows a slow increase with decreas-
ing temperature up to 80 K. Below this temperature it increases
dramatically up to 40 K due to a phenomenon of spin correlation
[16].
The molar magnetic susceptibility, χm, of the compound
(1) exhibits a sharp maximum at approximately 8 K, indicat-
ing that a three-dimensional magnetic ordering is established
around this temperature, taking into account the structural
features of this phase (Fig. 8(a)). The compound follows a
Curie–Weiss law in the 15–300 K range, with values of the
Curie and Curie–Weiss constants of Cm = 4.38 cm3 K/mol and
θ = −15.1 K. The χmT versus T curve decreases at room tem-
perature from 4.17 cm3 K/mol down to 0.70 cm3 K/mol at 5 K.
Both results, the negative Weiss temperature and the con-
tinuous decreasing of the χmT versus T curve confirm the
antiferromagnetic interactions in the Mn(SeO3)·2H2O com-
pound. The “J”-exchange parameter has been calculated by
fitting the experimental magnetic data to a 3D antiferromag-
For phase (2), the thermal evolution of the intensity of the
signals decreases slightly with decreasing temperature down
to, approximately, 240 K (Fig. 7(b)). Below this temperature
the intensity remains constant and then decreases dramatically

A. Larran˜aga et al. / Spectrochimica Acta Part A 69 (2008) 1020–1026
1025
4. Concluding remarks
Compounds (1) and (2) were obtained from different
synthetic methods. The Mn(SeO3)·2H2O phase was synthe-
sized by slow evaporation from a saturated aqueous solution.
The Fe2(SeO3)3·3H2O compound was obtained using mild
hydrothermal conditions under autogeneous pressure in water
solution. The crystal structure of both phases was refined using
the Rietveld method. Both structures are three-dimensional.
The framework of (1) is constructed from MnO6 octahedra
and (SeO3)2− trigonal pyramids. The crystal structure of (2)
is formed by two FeO6 crystallographically independent octa-
hedra and (SeO3)2− groups with trigonal pyramidal geometry.
The IR spectra of both phases show the characteristic bands of
the selenite oxoanion. The compounds are stable up to 75 and
200 ◦C, respectively, after these temperatures the elimination
of the water molecules occurs and the decomposition of the
selenite oxoanions giving rise to SeO2(g). The luminescent
measurements in (1) are in good agreement with the existence
of Mn2+ cations in a slightly distorted octahedral environ-
¨
ment. The diffuse reflectance spectrum and the Mossbauer
measurements performed in the paramagnetic state of (2), are
consistent with the existence of Fe3+ d5-cations in slightly
distorted octahedral geometry. ESR spectra of both compounds
are isotropic and the thermal evolution of the intensity of the
signals suggests an antiferromagnetic behavior, which has
been confirmed by the magnetic measurements. For phase
(1) the J-exchange parameter has been calculated, obtaining
a value of −0.35 K. For (2) the thermal evolution of the
ESR signals and the magnetic susceptibility suggest a change
of spin from Fe3+ (S = 5/2) to Fe2+ (S = 2) at low tempera-
tures.
Fig. 8. Thermal variation of the χm and χmT vs. T curves for (a) compound (1)
and (b) compound (2).
netic cubic lattice by using the Rushbrooke and Wood equation
(1) [17]:
(35Ng2β2)/(12KT)
χm
=
(1)
1 + (35/x) + (221.67/x2) + (608.22/x3) + (26049.6/x4) + (210986.5/x5) + (8, 014, 980/x6
where x = (KT/J), N is the Avogadros’s number, β the Bohr’s
magneton and K is the Boltzmann’s constant. The results of the
fit are given by the solid line in Fig. 8(a). The value obtained for
the J/K exchange parameter is −0.35 K.
Acknowledgements
This work has been financially supported by the “Min-
´
isterio de Educacion y Ciencia” (MAT-2004-02071) and the
The molar magnetic susceptibility, χm, of phase (2) increases
continuously from room temperature to 5.0 K (Fig. 8(b)). The
compound follows the Curie–Weiss law in the 100–300 K
range, with values of the Curie and Curie–Weiss constants
of Cm = 8.27 cm3 K/mol and θ = −60.7 K. The χmT versus T
curve decreases continuously from 6.89 cm3 K/mol at room
temperature down to 3.8 cm3 K/mol at 20 K (see Fig. 8(b)).
Below this temperature, the curve decreases dramatically down
to 2.70 cm3 K/mol at 5.0 K. Similarly to that found in the
thermal evolution of the intensity of the ESR-signals, this
result is probably due to a change in the spin of the iron
cations, from 5/2 to 2. Both, the negative Weiss temperature
and the continuous decreasing of the ␹mT versus T curve con-
firm the antiferromagnetic interactions in the Fe2(SeO3)3·3H2O
compound.
´
“Universidad del Paıs Vasco” (UPV/EHU) (9/UPV00130.310-
15967/2004; 9/UPV00169.310-13494/2001). The authors thank
the technician of SGIker, Dr. I. Orue, financed by the “National
Program for the Promotion of Human Resources within the
National Plan of Scientific Research, Development and Inno-
vation “Ministerio de Ciencia y Tecnologıa” and “Fondo Social
Europeo (FSE)”, for magnetic measurements. Dr. A. Larranaga
´
˜
wishes to thank the Gobierno Vasco/Eusko Jaurlaritza for fund-
ing.
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