The charge states of iron in insulators implanted with57Co and57Fe

Single crystals of α-Al₂O₃ and LiNbO₃ were implanted with⁵⁷Co (dose: up to 2×10¹⁵ atoms/cm²) and with⁵⁷Fe (dose: 2×10¹⁵ atoms/cm²) ions. The Mössbauer spectra revealed the disordered atomic environment. Fe²⁺ and Fe³⁺ charge states were observed. The spectra were compared to the spectra of crystals doped with⁵⁷Co. It was remarkable that in the doped α-Al₂O₃ Fe³⁺ states with slow spin-spin relaxation have appeared. The CEMS study of the samples implanted with⁵⁷Fe resulted in Fe²⁺ ionic states indicating that a fraction of Co atoms can also be in Co²⁺ state.     

Thermal evolution of high dose iron implanted sintered alumina

Sintered plates of alumina have been implanted at room temperature with 110 keV⁵⁷Fe⁺ at a dose of 1.2×10¹⁷ ions.cm^?2. The analysis of the Conversion Electron Mössbauer Spectrum indicated that implantation introduces iron in alumina in three charge state: Fe²⁺ (two components), Fe⁴⁺ and Fe⁰ (metallic clusters). The evolution of the iron depth distribution during annealings in oxiding or in neutral atmosphere has been followed using the Rutherford backscattering spectroscopy. Up to 800°C the profile as well as the charge states of iron evolve very slowly. A drastic change occurs' for annealing temperature around 1000°C. The total amount of iron is distributed among α-Fe₂O₃ and α-(Fe_1?x Al _x )₂O₃ precipitates. Some scanning electron micrographs have allowed to locate these precipitates. For highest temperature anneals, up to 1600°C, only substitutional iron remain.     

Perovskites Bi0.8La0.2FeO3 and Bi0.8La0.2Fe0.95Cr0.05O3: Crystal structure and magnetic and charge states of iron ions

Perovskites of the Bi_0.8La_0.2Fe_{1 ? x} Cr _x O₃ system (x = 0, 0.05) were investigated by Mössbauer spectroscopy in the temperature range of 298–800 K. The samples were fabricated by solid-state synthesis and had a rhombic structure. Iron ions in Bi_0.8La_0.2FeO₃ and Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ are situated in trivalent states. The magnetic transition temperatures (the Néel temperatures T _N ) T _N = 677.5 ± 2.5 K for Bi_0.8La_0.2FeO₃ and T _N = 647.6 ± 2.5 K for Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ are measured. The substitution of trivalent iron ions from trivalent chromium ions in the amount x = 0.05 in Bi_0.8La_0.2Fe_0.95Cr_0.05O₃ perovskite decreases the hyperfine magnetic field at nuclei ⁵⁷Fe in Fe⁺³-O-Cr⁺³ chains by 30 kOe.     

Mössbauer study of iron in sp-metals and semimetals

Bi, In, Pb, Sb, α- and β-Sn were implanted with⁵⁷Co and⁵⁷Fe. The results of the analysis suggested strongly hybridized bonds between the iron and the host atoms. An upturn of the isomer shift (δ) values relative to the δ values in hosts with completed d-shell was not justified. Charge states of Fe²⁺ or Fe⁺ also appeared. The possible role of the native surface oxide layers in the formation of these states was investigated.     

Mössbauer study on Zn1 − x Fe x O semiconductors prepared by high energy ball milling

We present the preparation of massive Zn_1???x Fe _x O ternary oxides using the mechanical mill. The Fe atom is a particular dopant since it presents two different oxidation states which allow us to vary the starting materials: Fe₂O₃, $\upalpha $ -Fe or FeO. Parameters such as initial concentrations, atmosphere and milling times were varied. X-ray diffraction and ⁵⁷Fe Mössbauer spectrometry (MS) were applied in order to analyze the structure evolution and iron incorporation in the wurtzite crystalline structure with milling time. At final stages, Fe atoms seem to be incorporated in the ZnO structure for those samples milled under Ar atmosphere. In all cases, two paramagnetic components, attributed to Fe atoms in both valence states, were observed by MS.     

State of iron atoms in SiO2·xFe2O3 gels

Mössbauer studies are made of ⁵⁷Fe nuclei in dry SiO₂·xFe₂O₃ gels at low temperatures in external magnetic fields. It is shown that the state of the iron atoms in the gels depends significantly on their concentration. In dry gels with an iron content exceeding 0.01Fe₂O₃, the Fe atoms belong to finely dispersed particles of the oxide with maximum sizes of ～80 Å in a superparamagnetic state. For lower iron contents, the Fe atoms are in the form of a highly diluted paramagnetic impurity of trivalent ions in a diamagnetic matrix. It is assumed that clusters develop from a small number of iron and oxygen ions dispersed in the gel matrix. Here spin-lattice relaxation of the magnetic moments of the iron atoms is observed.     

Hyperfine interactions in Sc<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript>x</Subscript>Fe<Subscript>2</Subscript> cubic Laves alloys

Effects of hybridization of 3d bands of iron with 3d bands of scandium and 4d bands of yttrium in Sc_1?xY_xFe₂ cubic Laves alloys (0≤_x≤1) are studied by the nuclear magnetic resonance method. The concentration dependences of the lattice parameters a, saturation magnetization σ, and hyperfine fields at the ⁵⁷Fe, ⁴⁵Sc, and ⁸⁹Y nuclei—as well as the ²⁷Al impurity nuclei, whose atoms substitute iron atoms in the lattices of these alloys—are measured. The “local” and “induced” contributions to hyperfine fields at the ⁵⁷Fe nuclei are separated and the magnetic moments at iron atoms are estimated. It is found that the hybridization effect leads to the formation of magnetic moments at Sc and Y atoms (whose direction is opposite to the direction of the magnetic moment at iron atoms) and is responsible for the ferrimagnetic structure in Sc_1?xY_xFe₂ alloys.     

57Fe,119Sn and151Eu Mössbauer study of EuBa2(Cu1−x−y 119Sn x 57Fe y )3O7−δ superconductor

¹⁵¹Eu,¹¹⁹Sn and⁵⁷Fe Mössbauer spectroscopy was used to study a EuBa₂(Cu_1?x?y Sn _x Fe _v )₃O_7?δ superconductor in which sites were replaced by all three Mössbauer isotopes in the same sample in order to get information about the site preferences and structural changes. We have found changes in the Mössbauer parameters of the⁵⁷Fe,¹¹⁹Sn and¹⁵¹Eu spectra compared to those recorded in the separately substituted EuBa₂Cu₃O_7?δ, EuBa₂(Cu_1?x Sn _x )₃O_7?δ, YBaCu_3?x Fe _x O_7?δ YBaCu_3?x Sn _x O_7?δ materials. These results can be interpreted as a consequence of the site preferences and the changes in the structure due to the presence of other additional substitutional elements.     

YIG formation in annealed iron implanted YAG

YAG and YIG crystals implanted respectively with 100 keV⁵⁷Fe²⁺ ions (1 × 10¹⁷ ions.cm^?2) and 50 keV²⁷Al ions (1.1 × 10¹⁷ ions.cm^?2) have been studied by conversion electron Mössbauer spectroscopy (CEMS) directly after implantation and after annealings in air at temperatures up to 1100°C. In both as-implanted samples iron is found mainly in three states: Fe²⁺, Fe³⁺ and small metallic precipitates. Annealing behaviour is divided into two stages: (i) up to 400°C the iron has become completely oxidized and (ii) between 400 and 850°C the epitaxial regrowth of the implanted layer takes place. During this process a part of iron ions are incorporated into octahedral and tetrahedral sites, thus making a Y₃ (Al Fe)₅ O₁₂ compound. The remaining iron part precipitates in the form of Fe₂O₃ particles.     

Relation between anisotropy and crystal structure in rare-earth (3d) transition compounds (R2M17)

The ⁵⁷Fe Mössbauer effect in Fe-rich compounds of the series Tm₂Fe_17?xCo_x was studied, using enriched iron. It was found that there exists a preference for occupancy by the Fe atoms for the f site. This fact proves useful to explain the changes in sign and magnitude of the anisotropy constant observed in other series of rare-earth transition compounds like Y₂Fe_17?xCo_x.     

Bismuth-induced enhancement of magneto-optical effects in iron garnets: A theoretical analysis

A semiquantitative model of circular magneto-optical effects in iron garnets is constructed within the concept of charge-transfer transitions and the existing qualitative notions. In the framework of the proposed model, the drastic enhancement of circular magneto-optical effects in R₃Fe₅O₁₂ iron garnets containing impurities of Bi³⁺ or Pb²⁺ ions is explained by the increase in the oxygen contribution to the spin-orbit coupling constant of the (FeO₆)^9? and (FeO₄)^5? complexes (the main magneto-optically active centers in iron garnets). This increase is associated with the covalent admixture of the Bi³⁺ (or Pb²⁺) 6p orbitals (with a giant one-electron spin-orbit coupling constant) to the oxygen 2p orbitals. The influence of the substitution does not reduce to an enhancement of the oxygen spin-orbit interaction alone but also leads to the appearance of the effective anisotropic tensor contributions to the spin-orbit interaction and circular magneto-optical effects. These contributions to the magneto-optical effects in garnets are estimated. The influence of an inhomogeneous bismuth distribution on the magneto-optical effects in Y_3?xBi_xFe₅O₁₂ garnets is investigated using computer simulation. Analysis of the available experimental data on the magneto-optical effects in garnets confirms the validity of the theoretical model proposed.     

Possible oxygen hopping in the YBa2Cu3O7 system observed by Mössbauer spectroscopy

In situ high temperature⁵⁷Fe Mössbauer measurements have been performed on samples of YBa₂ (Cu_1?xFe_x)₃O_7?δ withx=0.005 andx=0.010, in pure oxygen or in air, in the range 295 K≤T≤640 K. TheT-dependence of Fe species, of the Debye-Waller factorf(T) as well as of the hyperfine parameters is analysed.     

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron

The core and valence level XPS spectra of Fe_xO (x ~ 0.90–0.95); Fe₂O₃ (α and γ); Fe₃O₄; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). Fe^II and Fe^III states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below E_F, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for Fe_xO and Fe₃O₄. Fe surfaces (films, foils, (100) face) yield predominantly Fe^III species when exposed to high exposures of oxygen or air, though there is evidence for some Fe^II also. At low exposures the Fe^II/Fe^III ratio increases.     

Iron-57 Mössbauer spectroscopy study of phases in the CaZrTi2−2x Nb x Fe x O7 zirconolite system

Materials of composition CaZrTi_2?2x Nb _x Fe _x O₇ with the fluorite-related zirconolite structure have been prepared. The ⁵⁷Fe Mössbauer spectra show that iron is initially located in the five co-ordinate cation sites. As the iron content increases the iron enters the octahedral sites until, at a composition CaZrTi_0.4Nb_0.8Fe_0.8O₇, ca. 50% of the iron is five co-ordinate and the remainder is located in the octahedral sites.     

Role of holes states on metal to insulator transition and collapse of magnetic ordering in LaFe1−xNixO3 (x = 0.0–0.5)

Polycrystalline LaFe_1?xNi_xO₃ (x = 0.0, 0.1, 0.3 and 0.5) have been prepared by the standard solid state reactions method. The phase formation has been confirmed by the powerful synchrotron X-ray diffraction experiment. In order to investigate the effects of Ni doping on the oxidation state, spin state and the magnetic ordering of the iron cations, ⁵⁷Fe Mössbauer Spectroscopy has been carried out at room temperature. Iron is present as Fe³⁺ in high spin state in LaFeO₃. Ni doping has no effect on the spin state of the Fe³⁺ cations. However, a progressive increase in the concentration of Fe⁴⁺ cations has been inferred. Relatively stronger covalent character of the Fe⁴⁺–O^?2 bond causes a progressive collapse in the magnetic ordering and delocalization of the hole states.     

M?ssbauer study of compounds of Cu3 ? xFexSnS4 and Cu2Fe1 ?xZnxSnS4 systems

A M?ssbauer study of the structural and charge states of ⁵⁷Fe and ¹¹⁹Sn atoms in the compounds of Cu_{3 −x} Fe _x SnS₄ and Cu₂Fe_{1 − x} Zn _x SnS₄ systems was performed. It was shown that the iron atoms in the compounds of both systems were in the divalent and trivalent states occupying the tetrahedral positions of the structure. The character of the changes of the degree of covalency of the Fe²⁺-S, Fe³⁺-S and Sn⁴⁺-S bonds during the isomorphic substitution in the systems was established.     

Mossbauer, infrared and magnetic susceptibility studies of iron sodium borate glasses doped by sulphur

The affect of sulphur on the structural properties of iron sodium diborate glasses having the composition {(100−x)Na₂B₄O₇+xFe₂O₃}+yS, where x=0.05, 0.15 and 0.25 mol% and Y=0, 2.5 and 5 wt% was studied by infrared, Mossbauer spectroscopy and magnetic susceptibility measurements. It was found that, for samples having 5 mol% Fe₂O₃ and free from sulphur, the iron ions are present in both Fe²⁺ and Fe³⁺ states and also 92% of the total iron enters the glass network as a glass former. The ratio of Fe³⁺/Fe²⁺ increases with increasing the iron content for sulphur-free samples and others containing sulphur. This ratio also decreases with increasing the sulphur content. The magnetic susceptibility was found to decrease with increasing the sulphur content. Also, the increase of Fe₂O₃ content led to a less symmetrical environment of Fe³⁺ ions and vice versa for the Fe²⁺ environment.     

57Fe and 151Eu Mössbauer studies of magnetoresistive Europium based cobalt perovskites

Eu_0.8Sr_0.2Fe _x Co_1?x O_3?z CMR perovskites with different iron concentrations (x?=?0, 0.025, 0.075, 0.15, 0.3) were investigated by X-ray diffraction, AC magnetic susceptibility, magnetotransport, as well as ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectrometry. The valence state of europium ions was found to be trivalent, independently of the iron concentration. ⁵⁷Fe Mössbauer spectra and magnetic susceptibility of the investigated perovskites presented complementary results for the magnetic transitions.     

Reactions of 57Mn implanted into solid oxygen

The in-beam Mössbauer spectroscopy using a short-lived ⁵⁷Mn (T _1/2=1.5 min) beam was carried out to study the production of the exotic chemical species of ⁵⁷Fe atoms arising from ⁵⁷Mn implanted into solid oxygen. The obtained spectra can be analyzed by four components of doublets at least, which are assigned to be novel Fe species of FeO, Fe(O₂), (O₂)FeO₂, and Fe(O₂)₂, on the basis of ab initio molecular orbital calculations.     

Features of local crystallographic, magnetic, and valence states of iron ions in Bi1 − x Sr x FeO3 perovskites at x = 0–0.67

Perovskites of the Bi_{1 ? x} Sr _x FeO₃ system (x = 0–0.67) at T = 295 K and T > T _N are studied using the Mössbauer effect. When the strontium content x = 0.1–0.15, the structural transition from the rhombohedral to the cubic phase takes place. It is found that in samples of the Bi_{1 ? x} Sr _x FeO₃ system (x = 0.07–0.67), there are only two structurally nonequivalent states of iron ions that correspond to Fe³⁺ ions in octahedral and tetrahedral oxygen environments.