40 research outputs found
Fe-rich border of miscibility gap and activation energy of phase decomposition in a Fe-Cr alloy
Concentration of Cr in the Fe-rich alpha-phase, x, resulted from a phase
decomposition caused by an isothermal annealing at T = 415 and 450 C of a
non-irradiated (NR) Fe-Cr14 EFDA sample and that of a He-ions irradiated (IR)
one annealed at 415 C was determined with M\"ossbauer spectroscopy. The x-value
in the latter was by ~3 at% higher than the one in the NR-counterpart. The
activation energy for the phase decomposition in the NR-sample was 122 kJ/mol.
In the IR-sample its value was by 12 kJ/mol lower. Avrami exponents for the
NR-samples were close to 0.5, and that for the IR-sample had a value of about
1.Comment: 5 pages 3 figures 1 Tabl
Effect of 0.25 and 2.0 MeV He-ion irradiation on short-range ordering in model (EFDA) Fe-Cr alloys
The effect of He+ ion irradiation on a distribution of Cr atoms in model
(EFDA) Fe(100-x)Cr(x) (x=5.8, 10.75, 15.15) was studied by means of conversion
electrons Mossbauer spectroscopy. The alloys were irradiated to the dose of
1.2E16 ions per cm2 (7.5 dpa) with He+ of 0.25 and 2.0 MeV. The distribution of
Cr atoms within the first two coordination shells around the probe Fe atoms was
expressed in terms of the Warren-Cowley short-range order parameters alpha1
(first-neighbour shell, 1NN), alpha2 (second-neighbour shell, 2NN) and alpha12
(1NN+2NN. In non-irradiated alloys alpha1 was positive and alpha2 was negative
for all three samples which indicates an ordering of Cr atoms. Yet, the value
of alpha12 were close to zero, i.e. the distribution of Cr atoms averaged over
the first two coordination shells around the probe Fe atoms, was random. The
effect of the irradiation of the Fe5Cr sample is similar for the two energies
of the He+ projectiles viz. increase of number of Cr atoms in the 1NN and
decrease in the 2NN shell. Consequently, the degree of the ordering increased.
For the other two samples, the effect of the irradiation depends on the
composition, and it is stronger for the less energetic ions where, in the case
of Fe10Cr, the disordering disappeared and some traces of Cr clustering
(alpha12 weakly positive) can be seen. In the Fe15Cr sample the clustering is
clear. In the samples irradiated with 2.0 MeV ions the ordering also survived
in the samples with x=10.75 and 15.15, although its degree became smaller than
in the Fe5Cr sample. Small changes in the magnetic texture were revealed.Comment: 19 pages, 11 figures, 41 references. arXiv admin note: text overlap
with arXiv:1402.561
Moessbauer and magnetic measurements of superconducting LiFeP
The LiFeP sample has been prepared by means of high-temperature solid state
reaction. A transition temperature to the superconducting state was found as
about 5.5 K by measurements of magnetization versus temperature. The second
critical field was found above 1 kOe. Moessbauer spectra of the LiFeP phase are
characterized by the quadrupole split doublet having splitting of about 0.10
mm/s at room temperature and 0.12 mm/s at 4.2 K. No magnetic order occurs down
to 4.2 K.Comment: 6 pages, 5 figures, 1 tabl
Unusual dynamics of Fe atoms in chromium matrix
57Fe site Mossbauer Spectroscopy (MS) was used to investigate a dynamics of
57Fe atoms embedded into chromium lattice as impurities. From the Mossbauer
spectra recorded in the temperature range of 80 to 350 K, a temperature
dependence of the Lamb-Mossbauer factor, f, was determined. The latter revealed
an unusual dynamics of 57Fe atoms viz. harmonic mode below T = 145 K with a
characteristic effective Debye temperature, Qeff = 185 K and anharmonic one
above T o 145 K. The latter mode exists in two clearly separated temperature
intervals with slightly different Qeff - values viz. (i) 156 K for 145 K < T <
240 K and the record-high anharmonic coefficient e = 18 * 10-4 K-1 and (ii) 152
K for T > 240 K and e = 13.6 * 10-4 K-1. Based on the Visscher's theory, the
record-low values of relative binding force constants for Fe atoms were
determined as 0.0945, 0.0673 and 0.0634, respectively. It is suggested that the
unusual dynamics observed in this study might be related to the underlying
spin- and charge- and strain-density waves of chromium.Comment: 11 pages, 5 figure
Anomalous binding of Fe atoms in chromium
Binding of 57Fe atoms in a metallic chromium was investigated in a Cr-Fe
alloy, containing less than 0.1 at% Fe enriched to ~95 % in 57Fe isotope, using
57Fe-site Mossbauer spectroscopy. The binding force was derived from the Debye
temperature, T_D, that, in turn, was calculated from the temperature dependence
of the central shift of the Mossbauer spectra recorded in the range of 80 to
330 K. Following a temperature dependence of the line width that shows a
minimum at ~155 K, two temperature intervals were considered: a low temperature
one (LT) ranging from 80 to 155 K, and the T_D - value of 292 (12) K or 279
(34) K, and a high temperature one (HT) ranging from 155 to 330 K with the T_D
- value of 399 (15) or 399 (25) K, depending on the fitting procedure. The
corresponding values of the harmonic force (spring) constant are: 35.4 N/m and
66.1 N/m or 33.8 N/m and 66.1 N/m for the LT and HT, respectively. This means
that in the HT range the binding force of 57Fe atoms by the Cr matrix is by a
factor of ~1.9 - 2 stronger than that in the LT range. This anomaly is possibly
related with a different polarization of the spin-density waves in the LT and
HT "phases".Comment: 3 figures, 9 pages, 28 reference
M\"ossbauer studies of the peculiar magnetism in parent compounds of the iron-based superconductors
A review of the magnetism in the parent compounds of the iron-based
superconductors is given based on the transmission Moessbauer spectroscopy of
57Fe and 151Eu. It was found that the 3d magnetism is of the itinerant
character with varying admixture of the spin-polarized covalent bonds. For the
122 compounds a longitudinal spin density wave (SDW) develops. In the case of
the EuFe2As2 a divalent europium orders antiferromagnetically at much lower
temperature as compared to the onset of SDW. These two magnetic systems remain
almost uncoupled one to another. For the non-stoichiometric Fe(1+x)Te parent of
the 11 family one has a transversal SDW and magnetic order of the interstitial
iron with relatively high and localized magnetic moments. These two systems are
strongly coupled one to another. For the grand parent of the iron-based
superconductors FeAs one observes two mutually orthogonal phase-related
transversal SDW on the iron sites. There are two sets of such spin arrangements
due to two crystallographic iron sites. The FeAs exhibits the highest covalency
among compounds studied, but it has still a metallic character.Comment: A contribution to XVI National Conference on Superconductivity,
Zakopane, Poland, 7-12 October 201
Interplay between magnetism and superconductivity in EuFe(2-x)Co(x)As2 studied by 57Fe and 151Eu M\"{o}ssbauer spectroscopy
The compound EuFe(2-x)Co(x)As2 was investigated by means of the 57Fe and
151Eu Moessbauer spectroscopy versus temperature (4.2 - 300 K) for x=0
(parent), x=0.34 - 0.39 (superconductor) and x=0.58 (overdoped). It was found
that spin density wave (SDW) is suppressed by Co-substitution, however it
survives in the region of superconductivity, but iron spectra exhibit some
non-magnetic component in the superconducting region. Europium orders
anti-ferromagnetically regardless of the Co concentration with the spin
re-orientation from the a-axis in the parent compound toward c-axis with the
increasing replacement of iron by cobalt. The re-orientation takes place close
to the a-c plane. Some trivalent europium appears in EuFe(2-x)Co(x)As2 versus
substitution due to the chemical pressure induced by Co-atoms and it
experiences some transferred hyperfine field from Eu2+. Iron experiences some
transferred field due to the europium ordering for substituted samples in the
SDW and non-magnetic state both, while the transferred field is undetectable in
the parent compound. Superconductivity coexists with the 4f-europium magnetic
order within the same volume. It seems that superconductivity has some
filamentary character in EuFe(2-x)Co(x)As2 and it is confined to the
non-magnetic component seen by the iron Moessbauer spectroscopy.Comment: 13 pages, 8 figures, 2 table
Magnetic anisotropy and lattice dynamics in FeAs studied by M\"ossbauer spectroscopy
Iron mono-arsenide in the powder form has been investigated by transmission
57Fe Moessbauer spectroscopy in the temperature range 4.2 - 1000 K. Additional
spectra have been obtained at 20 K and 100 K applying external magnetic field
of 7 T. It was found that the spin spiral propagating along the c-axis leads to
the complex variation of the hyperfine magnetic field amplitude with the spin
orientation varying in the a-b plane. The magnitude of the hyperfine field
pointing in the direction of the local magnetic moment depends on the
orientation of this moment in the a-b plane. Patterns are vastly different for
iron located in the [0 k 0] positions and for iron in the [0 k+1/2 0] positions
within the orthorhombic cell set to the Pnma symmetry. Lattice softens upon
transition to the paramagnetic state at 69.2 K primarily in the a-c plane as
seen by iron atoms. This effect is quite large considering lack of the
structural transition. Two previously mentioned iron sites are discernible in
the paramagnetic region till 300 K by different electron densities on the iron
nuclei. The anisotropy of the iron vibrations developed at the transition to
the paramagnetic state increases with the temperature in accordance with the
harmonic approximation, albeit tends to saturation at high temperatures
indicating gradual onset of the quasi-harmonic conditions. It seems that
neither hyperfine fields nor magnetic moments are correct order parameters in
light of the determined static critical exponents. Sample starts to loose
arsenic at about 1000 K and under vacuum
M\"ossbauer study of Eu0.57Ca0.43Fe2As2 and Eu0.73Ca0.27(Fe0.87Co0.13)2As2: A comparison to 122 iron-based superconductors parent compounds EuFe2As2 and CaFe2As2
57Fe and 151Eu Moessbauer spectra were obtained versus temperature for
Eu0.57Ca0.43Fe2As2 compound with 3d and 4f magnetic order and
Eu0.73Ca0.27(Fe0.87Co0.13)2As2 re-entrant superconductor, where the finite
resistivity reappears while approaching the ground state. They were compared
with previously obtained spectra for parent compounds EuFe2As2 and CaFe2As2. It
was found that substitution beyond the Fe-As layers does not lead to the
rotation (canting) of the Eu2+ magnetic moments and does not generate Eu3+
states. On the other hand, re-entrant superconductor exhibits rotation
(canting) of the Eu2+ moments on the c-axis of the unit cell leading to the
transferred hyperfine magnetic field on iron nuclei. Divalent europium orders
magnetically within the bulk of the re-entrant superconducting phase. The
re-entrant superconductor remains in the inhomogeneous state close to the
ground state with about 27 % of the volume being free of 3d magnetism, while
the remainder exhibits weak spin density wave. Those two regions slightly
differ by the electric field gradient and electron density on iron nuclei
Magnetism of BaFe2Se3 studied by M\"ossbauer spectroscopy
The compound BaFe2Se3 (Pnma) has been synthesized in the form of single
crystals with the average composition Ba0.992Fe1.998Se3. The Moessbauer
spectroscopy used for investigation of the valence states of Fe in this
compound at temperature ranging from 4.2 K till room temperature revealed the
occurrence of mixed-valence state for iron. The spectrum is characterized by
sharply defined electric quadrupole doublet above magnetic ordering at about
250 K. For the magnetically ordered state one sees four iron sites at least and
each of them is described by separate axially symmetric electric field gradient
tensor with the principal component making some angle with the hyperfine
magnetic field. They form two groups occurring in equal abundances. It is
likely that each group belongs to separate spin ladder with various tilts of
the FeSe4 tetrahedral units along the ladder. Two impurity phases are found,
i.e., superconducting FeSe and some other unidentified iron-bearing phase being
magnetically disordered above 80 K. Powder form of BaFe2Se3 is unstable in
contact with the air and decomposes slowly to this unidentified phase
exhibiting almost the same quadrupole doublet as BaFe2Se3 above magnetic
transition temperature