Ossbauer Spectroscopy Research Articles

Feasibility study of a nuclear exciton laser

Nuclear excitons known from M\"ossbauer spectroscopy describe coherent excitations of a large number of nuclei---analogous to Dicke states (or Dicke super-radiance) in quantum optics. In this paper, we study the possibility of constructing a laser based on these coherent excitations. In contrast to the free-electron laser (in its usual design), such a device would be based on stimulated emission and thus might offer certain advantages, e.g., regarding energy-momentum accuracy. Unfortunately, inserting realistic parameters, the window of operability is probably not open (yet) to present-day technology; but our design should be feasible in the UV regime, for example.

Iron-based Heusler compounds Fe2YZ: Comparison with theoretical predictions of the crystal structure and magnetic properties

The present work reports on the new soft ferromagnetic Heusler phases Fe2NiGe, Fe2CuGa, and Fe2CuAl, which in previous theoretical studies have been predicted to exist in a tetragonal regular Heusler structure. Together with the known phases Fe2CoGe and Fe2NiGa these materials have been synthesized and characterized by powder XRD, 57 Fe M\"ossbauer spectroscopy, SQUID and EDX measurements. In particular M\"ossbauer spectroscopy was used to monitor the degree of local atomic order/disorder and to estimate magnetic moments at the Fe sites from the hyperfine fields. It is shown that in contrast to the previous predictions all the materials except Fe2NiGa basically adopt the inverse cubic Heusler (X-) structure with differing degrees of disorder. The disorder is more enhanced in case of Fe2NiGa, which was predicted as an inverse Heusler phase. The experimental data are compared with results from ab-inito electronic structure calculations on LDA level incorporating the effects of atomic disorder by using the coherent potential approximation (CPA). A good agreement between calculated and experimental magnetic moments is found for the cubic inverse Heusler phases. Model calculations on various atomic configurations demonstrate that antisite disorder tends to enhance the stability of the X-structure. Given the fundamental scientific and technological importance of tetragonal Heusler phases the present results call for further investigations to unravel the factors stabilizing tetragonal Heusler materials.

Phase transitions in Sr2YRuO6investigated by Mössbauer spectroscopy, magnetization, and thermodynamic measurements

Recent specific-heat data measured in Sr${}_{2}$YRuO${}_{6}$ have revealed two transition features at $\ensuremath{\sim}$26 and $\ensuremath{\sim}$30 K, where only one transition could be inferred before from magnetic susceptibility and transport measurements. We have investigated this temperature region using ${}^{99}$Ru M\"ossbauer spectroscopy, magnetization, and thermodynamic measurements in order to elucidate the unusual properties in this temperature region. Below 25 K, fits to the M\"ossbauer spectra show that there is a unique value of the hyperfine magnetic field at each Ru site indicating static long-range magnetic order. Beyond 25 K, this static long-range magnetic order rapidly deteriorates. We have found that the temperature dependence of the M\"ossbauer spectra between $\ensuremath{\sim}$25 and $\ensuremath{\sim}$28 K can be described as due to either motional narrowing or to a temperature-dependent distribution of hyperfine magnetic fields. The M\"ossbauer spectra collapse to a single peak in this narrow temperature interval so that there is no evidence of static magnetic order by $\ensuremath{\sim}$30 K. As a result, no evidence of the transition at 30 K is seen in the M\"ossbauer spectra.

Magnetic behavior of the NixFe1−xNb2O6quasi-one-dimensional system: Isolation of Ising chains by frustration

Physical properties of the Ni${}_{x}$Fe${}_{1\ensuremath{-}x}$Nb${}_{2}$O${}_{6}$ compounds are investigated combining x-ray and neutron powder diffraction with magnetic and calorimetry measurements as well as ${}^{57}$Fe M\ossbauer spectroscopy. This system is known to present quasi-one-dimensional magnetism with the magnetic moments arranged along weakly interacting Ising chains. Partial substitution of the magnetic ion tends to suppress the magnetic ordering observed in the end members of the series. When this happens, the low-temperature magnetic specific heat agrees well with what is expected for isolated Ising chains. The lowest temperature powder neutron-diffraction patterns exhibit evidence for the occurrence of short-range order, and analysis of these diffuse neutron-scattering patterns allow us to obtain information on the magnetic correlations. The suppression of magnetism is consistently interpreted as resulting from the magnetic-cation disorder induced by substitution, which enhances the system's tendency for frustration of geometrical origin.

Intrinsic magnetic relaxation in goethite

The intrinsic magnetic relaxation properties of goethite are elucidated via a comparative study of the magnetism of a stoichiometric and a nonstoichiometric sample, \ensuremath{\alpha}-FeOOH and \ensuremath{\alpha}-Fe${}_{0.92}$\ensuremath{\square}${}_{0.08}$O${}_{0.76}$OH${}_{0.24}$, where \ensuremath{\square} denotes a vacancy site. It is proposed that both goethite samples exhibit ``mode superparamagnetism'' at low temperatures, wherein the iron moments undergo thermally induced fluctuations over an energy barrier separating two closely related geometrical arrangements for the sublattice spins. This is an intrinsic relaxation mechanism that was previously predicted from magnetic structure refinements, which we have identified here in ac susceptibility and thermal decay of remanence data. An additional magnetic relaxation mode occurs in the nonstoichiometric goethite. This mode is characterized by relatively slow fluctuations, observable on the microsecond timescale of muon spin relaxation experiments, but not on the fast timescale of M\ossbauer spectroscopy, and appearing in ac susceptibility and dc magnetization data as blocking transitions in the range 70 to 100 K. These data are best described as the result of ``cluster ordering'': the formation and relaxation of magnetic clusters in the disturbed environment of the nonstoichiometric lattice.

Synchrotron x-ray spectroscopy studies of valence and magnetic state in europium metal to extreme pressures

In order to probe the changes in the valence state and magnetic properties of Eu metal under extreme pressure, x-ray absorption near-edge spectroscopy, x-ray magnetic circular dichroism, and synchrotron M\"ossbauer spectroscopy experiments were carried out. The M\"ossbauer isomer shift exhibits anomalous pressure dependence, passing through a maximum near 20 GPa. Density functional theory has been applied to give insight into the pressure-induced changes in both Eu's electronic structure and M\"ossbauer isomer shift. Contrary to previous reports, Eu is found to remain nearly divalent to the highest pressures reached (87 GPa) with magnetic order persisting to at least 50 GPa. These results should lead to a better understanding of the nature of the superconducting state found above 75 GPa and of the sequence of structural phase transitions observed to 92 GPa.

Top and bottom interfaces in Fe-B multilayers investigated by Mössbauer spectroscopy

M\"ossbauer spectroscopy is frequently applied to gain information on the interfaces in multilayers, and recently the layer sequence permutation of multitrilayers was suggested to enhance its capability to characterize bottom and top interfaces of a given layer. The sequence permutation, however, in the investigated Fe-B-Ag multitrilayers affected the waviness of the layers as well, and the results hinted at the possibility that although Ag does not mix with Fe and B, the Fe-B amorphous alloy formation is influenced by the layer-sequence-dependent morphology of the Ag layers in the multitrilayers. Now we examine the interface mixing of Fe and B in the case of a fixed layer sequence and varying thickness of the Ag layers in [2 nm B/2 nm Fe/$x$ nm Ag]${}_{4}$, $0.2\ensuremath{\le}x\ensuremath{\le}10$, multitrilayer samples and in B/Fe/B and B/Fe/Ag trilayers. Below $x=5$ nm, both the ratio of the nonalloyed Fe layer and the average hyperfine field of the amorphous Fe-B interface compound change. The variation is attributed to thickness-dependent discontinuities of the Ag layers. Ag acts as a barrier to the diffusion of B from the top side and thus the discontinuities lead to a varying ratio of top and bottom interfaces. The evaluation based on this model shows that the ``B on top of Fe'' interface has an average B concentration about 11 at. % higher than the ``Fe on top of B'' interface. A slightly smaller difference, 6 at. %, is deduced from low-temperature conversion electron M\"ossbauer spectroscopy measurements of the trilayers.

Evidence of oxygen-dependent modulation in LuFe2O4

A polycrystalline sample of LuFe${}_{2}$O${}_{4}$ has been investigated by means of powder synchrotron x-ray and neutron diffraction and transmission electron microscopy (TEM), along with M\"ossbauer spectroscopy and transport and magnetic properties. A monoclinic distortion is unambiguously evidenced, and the crystal structure is refined in the monoclinic $C$2/$m$ space group [${a}_{M}$ $=$ 5.9563(1) \AA{}, ${b}_{M}$ $=$ 3.4372(1) \AA{}, ${c}_{M}$ $=$ 8.6431(1) \AA{}, \ensuremath{\beta} $=$ 103.24(1)\ifmmode^\circ\else\textdegree\fi{}]. Along with the previously reported modulations distinctive of the charge-ordering (CO) of the iron species, a new type of incommensurate order is observed, characterized by a vector $\stackrel{P\vec}{q}$${}_{1}$ $=$ ${\ensuremath{\alpha}}_{1}$${\stackrel{P\vec}{\mathrm{a}}}_{M}^{*}$ + ${\ensuremath{\gamma}}_{1}$${\stackrel{P\vec}{c}}_{M}^{*}$ (with ${\ensuremath{\alpha}}_{1}$ \ensuremath{\cong} 0.55, ${\ensuremath{\gamma}}_{1}$ \ensuremath{\cong} 0.13). In situ heating TEM observations from 300 to 773 K confirm that the satellites associated with $\stackrel{P\vec}{q}$${}_{1}$ vanish completely, only at a temperature significantly higher than the CO temperature. This incommensurate modulation has a displacive character and corresponds primarily to a transverse displacive modulation wave of the Lu cations position, as revealed by the high resolution, high angle annular dark field scanning TEM images and in agreement with synchrotron data refinements. Analyses of vacuum-annealed samples converge toward the hypothesis of a new ordering mechanism, associated with a tiny oxygen deviation from the O${}_{4}$ stoichiometry.

Mechanism of stress relaxation in nanocrystalline Fe-N thin films

The mechanism of stress relaxation in nanocrystalline Fe-N thin film has been studied. The as-deposited film possesses a strong in-plane compressive stress which relaxes with thermal annealing. Precise diffusion measurements using nuclear resonance reflectivity show that stress relaxation does not involve any long-range diffusion of Fe atoms. Rather, a redistribution of nitrogen atoms at various interstitial sites, as evidenced by conversion electron M\"ossbauer spectroscopy, is responsible for the relaxation of internal stresses. On the other hand, formation of the ${\ensuremath{\gamma}}^{\ensuremath{'}}$-Fe${}_{4}$N phase at temperatures above 523 K involves long-range rearrangement of Fe atoms. The activation energy for Fe self-diffusion is found to be $0.38\ifmmode\pm\else\textpm\fi{}0.04$ eV.

Structural, electronic, and magnetic characteristics of Np2Co17

A previously unknown neptunium-transition-metal binary compound Np${}_{2}$Co${}_{17}$ has been synthesized and characterized by means of powder x-ray diffraction, ${}^{237}$Np M\"ossbauer spectroscopy, superconducting-quantum-interference-device magnetometry, and x-ray magnetic circular dichroism (XMCD). The compound crystallizes in a Th${}_{2}$Ni${}_{17}$-type hexagonal structure with room-temperature lattice parameters $a=8.3107(1)$ $\AA{}$ and $c=8.1058(1)$ \AA{}. Magnetization curves indicate the occurrence of ferromagnetic order below ${T}_{C}>350$ K. M\"ossbauer spectra suggest a Np${}^{3+}$ oxidation state and give an ordered moment of ${\ensuremath{\mu}}_{\mathrm{Np}}=1.57(4)$ ${\ensuremath{\mu}}_{B}$ and ${\ensuremath{\mu}}_{\mathrm{Np}}=1.63(4)$ ${\ensuremath{\mu}}_{B}$ for the Np atoms located, respectively, at the $2b$ and $2d$ crystallographic positions of the $P{6}_{3}/mmc$ space group. Combining these values with a sum-rule analysis of the XMCD spectra measured at the neptunium ${M}_{4,5}$ absorption edges, one obtains the spin and orbital contributions to the site-averaged Np moment [${\ensuremath{\mu}}_{S}=\ensuremath{-}1.88(9)$ ${\ensuremath{\mu}}_{B}$, ${\ensuremath{\mu}}_{L}=3.48(9)$ ${\ensuremath{\mu}}_{B}$]. The ratio between the expectation value of the magnetic-dipole moment and the spin magnetic moment (${m}_{\mathrm{md}}/{\ensuremath{\mu}}_{S}=+1.36$) is positive as predicted for localized $5f$ electrons and lies between the values calculated in intermediate-coupling (IC) and $jj$ approximations. The expectation value of the angular part of the spin-orbit-interaction operator is in excellent agreement with the IC estimate. The ordered moment averaged over the four inequivalent Co sites, as obtained from the saturation value of the magnetization, is ${\ensuremath{\mu}}_{\mathrm{Co}}\ensuremath{\simeq}1.6$ ${\ensuremath{\mu}}_{B}$. The experimental results are discussed against the predictions of first-principles electronic-structure calculations based on the spin-polarized local-spin-density approximation plus the Hubbard interaction.

Noncollinear Fe spin structure in (Sm-Co)/Fe exchange-spring bilayers: Layer-resolved57Fe Mössbauer spectroscopy and electronic structure calculations

Magnetization reversal in nanoscale (Sm-Co)/Fe (hard/soft) bilayer exchange-spring magnets with in-plane uniaxial magnetic anisotropy was investigated by magnetometry, conversion-electron M\ossbauer spectroscopy (CEMS) and atomistic Fe spin-structure calculations. Magnetization loops along the easy direction exhibit signatures typical of exchange-spring magnets. In-field CEMS at inclined \ensuremath{\gamma}-ray incidence onto thin (2 nm) ${}^{57}$Fe probe layers embedded at various depths in the 20-nm-thick natural (soft) Fe layer provides depth-dependent information (via the line-intensity ratio ${R}_{23}$ as a function of the applied field $H$) about the in-plane rotation of Fe spins. A minimum in the ${R}_{23}$-vs-$H$ dependence at (${H}_{\mathrm{min}}$, ${R}_{\mathrm{min}}$) determines the field where Fe magnetic moments roughly adopt an average perpendicular orientation during their reversal from positive to negative easy-axis orientation. A monotonic decrease of ${H}_{\mathrm{min}}$ with distance from the hard/soft interface is observed. Rotation of Fe spins takes place even in the interface region in applied fields far below the field of irreversible switching, ${H}_{\mathrm{irr}}$, of the hard phase. Formation of an Fe-Co alloy is detected in the interface region. For comparison, the noncollinear Fe spin structure during reversal and the resulting ${R}_{23}$ ratio were obtained by electronic-structure calculations based on a quantum-mechanical Hamiltonian for itinerant electrons. The coupling at the hard/soft interface is described by the uniaxial exchange-anisotropy field, ${h}_{\mathrm{int}}$, as a parameter. Our calculated ${R}_{23}$ ratios as a function of the (reduced) applied field $h$ exhibit similar features as observed in the experiment, in particular a minimum at (${h}_{\mathrm{min}}$, ${R}_{\mathrm{min}}$). ${R}_{\mathrm{min}}$ is found to increase with ${h}_{\mathrm{int}}$, thus providing a measure of the interface coupling. Evidence is provided for the existence of fluctuations of the interface coupling. The calculations also show that the Fe spin spiral formed during reversal is highly inhomogeneous. In general, our simulation of the Fe spin structure is applicable for the interpretation of experimental results on layered exchange-spring magnets.

Magnetic exchange interactions and supertransferred hyperfine fields at119Sn probe atoms in CaCu3Mn4O12

The double manganite CaCu${}_{3}$Mn${}_{4}$O${}_{12}$ doped with ${}^{119}$Sn atoms (\ensuremath{\sim}1 at.$%$ with respect to manganese atoms) was studied by use of M\"ossbauer spectroscopy. Formally tetravalent Sn${}^{4+}$ ions substitute for isovalent manganese ions in the octahedral (Mn${}^{4+}$O${}_{6}$) polyhedra. The covalency effects on the magnetic interactions like superexchange in Cu${}^{2+}$-O-Mn${}^{4+}$ and Mn${}^{4+}$-O-Mn${}^{4+}$ bonds and supertransferred hyperfine interactions of the ${}^{119}$Sn probe atoms in the manganite structure are discussed. Using a semiquantitative nearest-neighbor cluster model relating the hyperfine magnetic field on the ${}^{119}$Sn nuclei (${H}_{\mathrm{Sn}}$ $=$ 105 kOe at $T$ $=$ 77 K) to covalency parameters and angle characterizing the Sn-O-$M$ ($M$ $=$ Cu, Mn) bonds, it has been shown how such an analysis of supertransferred hyperfine interactions of tin probe ions can get fruitful information about strength and sign of the superexchange interactions between Mn${}^{4+}$ and Cu${}^{2+}$ magnetic ions. A consistent description of the results in the framework of the Weiss molecular field model considering the specific local environment of tin atoms has made it possible to estimate exchange integrals: ${J}_{\mathrm{CuMn}}$ $=$ \ensuremath{-}51.1 \ifmmode\pm\else\textpm\fi{} 0.3 K and ${J}_{\mathrm{MnMn}}$ $=$ \ensuremath{-}0.6 \ifmmode\pm\else\textpm\fi{} 0.2 K.

Intricate relationship between pressure-induced electronic and structural transformations in FeCr2S4

Electrical-transport, magnetic and structural properties of the ferrimagnetic semiconductor FeCr${}_{2}$S${}_{4}$ (${T}_{\mathrm{N}}$ $=$ 170 K) have been studied by electrical resistance, $R$($P$, $T$), ${}^{57}$Fe M\"ossbauer spectroscopy (MS), and synchrotron x-ray diffraction to 20 GPa using diamond anvil cells. It was found that the local maximum, ${R}_{\mathrm{max}}$($P$) on the $R$($T$) curve, corresponding to the colossal magnetoresistance effect, is substantially reduced and broadened with pressure increase accompanied by a shift to higher temperatures and finally disappears at \ensuremath{\sim}7 GPa, the highest pressure of the single, high-spin spinel phase designated as LP1. Suppression of ${R}_{\mathrm{max}}$($P$) precedes a gap closure leading to metallization at \ensuremath{\sim}7 GPa. The 7--10 GPa range is a coexistence pressure zone composed of three phases: (i) LP1, a paramagnetic spinel (SG $Fd$3$m$); (ii) LP2, a nonmagnetic isostructural spinel; and (iii) HP1, a high-spin Cr${}_{3}$S${}_{4}$ (SG $I$2/$m$) type structure. Based on MS and $R$($P$, $T$) studies it was concluded that the Mott transition is responsible for the onset of metallization (correlation breakdown) coinciding with the collapse of Fe${}^{2+}$ moments. The shortening of the Fe-O bond length due to the electronic transition leads to a volume decrease of the low pressure (LP) phase by \ensuremath{\sim}1$%$. This electronic transition initiates a structural instability of the spinel structure resulting in a first-order phase transition into HP1, a post-spinel with Cr${}_{3}$S${}_{4}$-like structure. The onset of HP1 is accompanied by the Fe${}^{2+}$ 4 \ensuremath{\rightarrow} 6 coordination number increase resulting in an additional \ensuremath{\sim}12$%$ volume reduction. In the coexistence zone the post-spinel phase is paramagnetic, but at $P$ > 10 GPa an isostructural transition takes place and Fe${}^{2+}$ becomes nonmagnetic, as evidenced from the large drop of the isomer shift and of the quadrupole splitting. The structural transition is irreversible with the isothermal pressure decrease, and the Cr${}_{3}$S${}_{4}$-like structure remains upon full release of pressure at 300 K. Interestingly at decompression the high pressure (HP) phase undergoes a reverse noncorrelated \ensuremath{\rightarrow} correlated transition recovering its localization features, e.g., insulating state and paramagnetism with ${T}_{\mathrm{N}}\ensuremath{\le}$ 6 K. The original room temperature (RT), LP spinel phase is finally recovered following heat treatment at 400 \ifmmode^\circ\else\textdegree\fi{}C.

Spin correlations in the extended kagome system YBaCo3FeO7

The transition metal based oxide YBaCo3FeO7 is structurally related to the mineral Swedenborgite SbNaBe4O7, a polar non-centrosymmetric crystal system. The magnetic Co3Fe sublattice consists of a tetrahedral network containing kagome-like layers with trigonal interlayer sites. This geometry causes frustration effects for magnetic ordering, which were investigated by magnetization measurements, M\"ossbauer spectroscopy, polarized neutron diffraction, and neutron spectroscopy. Magnetization measurement and neutron diffraction do not show long range ordering even at low temperature (1 K) although a strong antiferromagnetic coupling (~2000 K) is deduced from the magnetic susceptibility. Below 590 K, we observe two features, a spontaneous weak anisotropic magnetization hysteresis along the polar crystallographic axis and a hyperfine field on the Fe kagome sites, whereas the Fe spins on the interlayer sites remain idle. Below ~50 K, the onset of a hyperfine field shows the development of moments static on the M\"ossbauer time scale also for the Fe interlayer sites. Simultaneously, an increase of spin correlations is found by polarized neutron diffraction. The relaxation part of the dynamic response has been further investigated by high-resolution neutron spectroscopy, which reveals that the spin correlations start to freeze in below ~50 K. Monte Carlo simulations show that the neutron scattering results at lower temperatures are compatible with a recent proposal that the particular geometric frustration in the Swedenborgite structure promotes quasi one dimensional partial order.

57Fe Mössbauer spectroscopy studies of CaFe4As3

We report on the ${}^{57}$Fe M\"ossbauer spectroscopy and direct-current magnetization studies of orthorhombic CaFe${}_{4}$As${}_{3}$, which undergoes two magnetic transitions with the formation of spin density wave at ${T}_{\mathrm{N}1}=$ 88 K (incommensurate) and ${T}_{\mathrm{N}2}=$ 26 K (commensurate). The magnetic M\"ossbauer spectroscopy spectra below ${T}_{\mathrm{N}1}$ are composed of four subspectra attributed to the four in-equivalent Fe crystallographic sites: three Fe ions in the divalent state (Fe${}^{2+}$) and one as Fe${}^{1+}$. However, the magnetic lines are much broader than that below ${T}_{\mathrm{N}2}$, indicating an incommensurate magnetic state. In the paramagnetic state, the M\"ossbauer spectroscopy spectra are composed of three doublets; one of them is related to Fe${}^{1+}$. Evidence for spin fluctuations above ${T}_{\mathrm{N}1}$ is observed.

Phase separation in superconducting and antiferromagneticRb0.8Fe1.6Se2probed by Mössbauer spectroscopy

${}^{57}$Fe-M\"ossbauer studies of superconducting Rb${}_{0.8}$Fe${}_{1.6}$Se${}_{2.0}$ with ${T}_{C}$ $=$ 32.4 K were performed on single-crystalline and polycrystalline samples in the temperature range 4.2--295 K. They reveal the presence of 88% magnetic and 12% nonmagnetic Fe${}^{2+}$ species with the same polarization dependence of their hyperfine spectra. The magnetic species are attributed to the 16$i$ sites of the $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}\ifmmode\times\else\texttimes\fi{}1$ superstructure and the nonmagnetic Fe species to a nanosized phase observed in recent structural studies of superconducting K${}_{x}$Fe${}_{2\ensuremath{-}}$${}_{y}$Se${}_{2}$ systems rather than to the vacant 4$d$ sites in the $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}\ifmmode\times\else\texttimes\fi{}1$ superstructure. The ${}^{57}$Fe spectrum of a single-crystalline sample in an external field of 50 kOe applied parallel to the crystallographic $c$ axis confirms the antiferromagnetic order between the fourfold ferromagnetic Fe(16$i$) supermoments and the absence of a magnetic moment at the Fe sites in the minority phase. A discussion of all spectral information and comparison with superconducting FeSe provides convincing evidence that the nanoscale phase separation is monitored by M\"ossbauer spectroscopy in Rb${}_{0.8}$Fe${}_{1.6}$Se${}_{2.0}$.

Interplay between magnetism and superconductivity in EuFe2−xCoxAs2studied by57Fe and151Eu Mössbauer spectroscopy

The compound EuFe${}_{2\ensuremath{-}x}$Co${}_{x}$As${}_{2}$ was investigated by means of ${}^{57}$Fe and ${}^{151}$Eu M\"ossbauer 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 the spin density wave (SDW) is suppressed by Co substitution; however, it survives in the region of superconductivity, but iron spectra exhibit some nonmagnetic components in the superconducting region. Europium orders magnetically, regardless of the cobalt concentration, with the spin reorientation from the $a$-axis in the parent compound toward the $c$-axis with increasing replacement of iron by cobalt. The reorientation takes place close to the $a$-$c$ plane. Some trivalent europium appears in EuFe${}_{2\ensuremath{-}x}$Co${}_{x}$As${}_{2}$ versus substitution due to the chemical pressure induced by Co atoms, and it experiences some transferred hyperfine field from Eu${}^{2+}$. Iron experiences some transferred field due to the europium ordering for substituted samples in the SDW and nonmagnetic 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${}_{2\ensuremath{-}x}$Co${}_{x}$As${}_{2}$, and it is confined to the nonmagnetic component seen by the iron M\"ossbauer spectroscopy.

Intriguing sequence of GaFeO3structures and electronic states to 70 GPa

Structural studies of the ferrimagnetic (${T}_{\mathrm{N}}$ = 200 K) Mott insulator GaFeO${}_{3}$ (SG Pc2${}_{1}n$) to 70 GPa, complemented by ${}^{57}$Fe M\"ossbauer spectroscopy and resistance (R) measurements at compression, decompression, and recompression, reveal a fascinating sequence of structures. Starting at \ensuremath{\sim}25 GPa a new structure, an orthorhombic perovskite (Pv) (SG Pbnm), is sluggishly formed followed by a volume V(P) drop of 5.4%. The complete formation of the Pv occurs at 42 GPa. In the 0--33 GPa range ${T}_{\mathrm{N}}$ reaches 300 K and R(P) decreases by one order of magnitude. At 53 GPa an isostructural transition is detected, characterized by a discontinuous drop of V(P) by \ensuremath{\sim}3%. M\"ossbauer spectra (MS) reveal a nonmagnetic component coexisting with the magnetic one at \ensuremath{\sim}60 GPa. Its abundance increases and above 77 GPa no sign of a magnetic hyperfine interaction is detected down to 5 K. Concurrently, one observes a continuous yet precipitous decrease in R(P) taking place in the 53--68 GPa range, leading to an onset of the metallic state at P = 68 GPa. These electronic/magnetic features of the high pressure (HP) Pv are consistent with a Mott transition. With pressure decrease below 50 GPa, the insulating Pv is recovered, and at \ensuremath{\sim}24 GPa a 1st-order structural transition takes place to a LiNbO${}_{3}$-type structure with SG R3c. This structure remains stable down to ambient pressure and with recompression it is stable up to 50 GPa, afterwards it transforms back to the HP Pv structure. It is noteworthy that this transition occurs at the same pressure, regardless of the preceding structures: Pbnm or R3c. The results are compared with hematite (Fe${}_{2}$O${}_{3}$, SG $R\overline{3}c$) and other ferric oxides. The mechanisms of the transitions are discussed.

Magnetic and Mssbauer characterization of the multiferroic fluoride K3Fe5F15

The iron potassium fluorides with the general formula K${}_{x}$Fe${}^{\mathrm{II}}$${}_{x}$Fe${}^{\mathrm{III}}$${}_{5\ensuremath{-}x}$F${}_{15}$ (2 \ensuremath{\le} $x$ \ensuremath{\le} 3) are interesting multiferroic materials, for which the coexistence of all the three ferroic orders (elastic, electric, and magnetic) is simultaneously observed below the magnetic transition. Although the phase diagram has been previously defined, in particular for the potassium-rich K${}_{3}$Fe${}_{5}$F${}_{15}$ phase, its complex magnetic behavior has not been completely understood. In this paper, the information obtained by magnetization measurements carried out on an oriented single crystal, revealing high anisotropy with coercive field reaching 14 kOe on the easy axis, are combined with M\"ossbauer spectroscopy and powder neutron diffraction data to define the magnetic structure. The ferrimagnetic behavior of K${}_{3}$Fe${}_{5}$F${}_{15}$ arises from an antiferromagnetic triangular spin system, determined by neutron diffraction, frustrated by the difference in the magnetic contributions of adjacent structural layers, originated by Fe(II)/Fe(III) charge ordering. The atomic moments corresponding to the different iron sites, which cannot be reliably refined by neutron data, were evaluated from the hyperfine fields and quadrupolar parameters derived from M\"ossbauer measurements, by taking into account the saturation magnetization measured along the $b$ easy axis. The complexity of the M(H) and M(T) measurements in the H//a case is explained by the existence of an alternative magnetic solution, originated by the pseudotetragonal character of the structure, that is thermodynamically unfavorable in spontaneous magnetization, but becomes accessible when an external magnetic field is applied along a.

Ab initiostudy of point defects in the strongly correlated system CoO

Density functional theory is applied to study the effect of substitutional Fe, Al, and In impurities on the structural, electronic, and magnetic properties of stoichiometric and nonstoichiometric CoO. Ab initiocalculated hyperfine interactions at the Fe impurity are used to determine aliovalent states of iron dopant in CoO and Co${}_{1\ensuremath{-}x}$O systems as well as to characterize the local structure around the impurity. Various Hubbard potentials describing the on-site Coulomb interactions at the Fe impurity are considered, and their influence on the calculated hyperfine parameters and electronic structure of Fe-doped CoO is analyzed. Different vacancy-impurity configurations within the Co-deficient matrix are taken into account to find the site preferences for Fe, Al, and In dopants. The hyperfine parameters at the Fe impurity in Co${}_{1\ensuremath{-}x}$O are also investigated as a function of the distance between vacancy and impurity. This study shows that divalent Fe cations are created inside the almost perfect host lattice, while the trivalent Fe cations are stabilized in the system containing cobalt vacancies. Trivalent Co ions are induced in the cobalt sublattice defected by vacancies and impurities. The overall concentration of trivalent cations in Co${}_{1\ensuremath{-}x}$O is twice as large as the concentration of cation vacancies. Trivalent cations of different ionic radii prefer residing in the second-neighbor sites of a cationic sublattice with respect to the cobalt vacancy. The energy gap of CoO with Fe${}^{2+}$ ions is comparable to that calculated for the defect-free system. The band gap of Co${}_{1\ensuremath{-}x}$O with Fe${}^{3+}$ and Co${}^{3+}$ ions is reduced due to the acceptor states introduced by both trivalent cations. Band-gap reduction arising from acceptor levels created by Co${}^{3+}$ cations is also observed for Co${}_{1\ensuremath{-}x}$O defected by nonmagnetic trivalent impurities. The present first-principles studies support and supplement results of several M\ossbauer spectroscopy experiments.