Hyperfine interactions and electronic structures of BaFe 2 As 2 : a first-principles LDA+U study

The hyperfine parameters of hyperfine fields, electric field gradients and isomer shifts at the Fe site are investigated based on the first-principles calculations of the electronic structures using LDA (GGA)+U method in the low-temperature orthorhombic antiferromagnetic phase of undoped BaFe₂As₂. It is fond that the electric field gradient of Fe nucleus is highly related with the electronic structures close to the Fermi level. Though the addition of negative on-site Coulomb interaction to Fe-3d states improves the calculated magnetic moment of Fe atom and the hyperfine parameters of Fe nucleus when U = ?0.1 Ry (?0.08 Ry) for GGA+U (LDA+U) method, a negative U correction does not capture the right physics of this system. The calculations prove the strong coupling between the magnetic, structural and electronic properties in antiferromagnetic BaFe₂As₂ parent.     

Magnetic phase transitions and iron valence in the double perovskite Sr 2 FeOsO 6

The insulating and antiferromagnetic double perovskite Sr₂FeOsO₆ has been studied by ⁵⁷Fe Mössbauer spectroscopy between 5 and 295 K. The iron atoms are essentially in the Fe^3?+? high spin $( {t_{2\mathrm{g}}{^3} e_\mathrm{g}{^2} } )$ and thus the osmium atoms in the Os $^{5+} ( {t_{2\mathrm{g}}{^3} } )$ state. Two magnetic phase transitions, which according to neutron diffraction studies occur below T _N?= 140 K and T ₂?= 67 K, give rise to magnetic hyperfine patterns, which differ considerably in the hyperfine fields and thus, in the corresponding ordered magnetic moments. The evolution of hyperfine field distributions, average values of the hyperfine fields, and magnetic moments with temperature suggests that the magnetic state formed below T _N is strongly frustrated. The frustration is released by a magneto-structural transition which below T ₂ leads to a different spin sequence along the c-direction of the tetragonal crystal structure.     

A M?ssbauer study of the magneto-structural coupling effect in SrFe2As2 and SrFeAsF

In the present paper, we report a comparison study of SrFe₂As₂ and SrFeAsF using M?ssbauer spectroscopy. The temperature dependence of the magnetic hyperfine field is fitted with a modified Bean–Rodbell model. The results give much smaller magnetic moment and magneto-structural coupling effect for SrFeAsF, which may be understood as due to different inter-layer properties of the two compounds.     

Mössbauer spectroscopy of 57Fe in high Tc superconductors YBa2Fe3xCu3(1−x)O7−δ

Mossbauer spectroscopy of ⁵⁷Fe in both tetragonal and othorhombic phases of YBa₂(Fe_xCu_1−x)₃O_7−δ, with x = 0.01, 0.02 and 0.10, at temperatures 4.2 K, 75 K, 90 K, and 300 K have been performed. In all samples three major subspectra corresponding to iron in different local environments are observed. It is concluded that Fe substitutes mainly Cul. At 4.2 K, samples with x=0.01 in the “quenched” tetragonal phase exhibit magnetic hyperfine structure, due to slow spin relaxation rates, whereas in the orthorhombic superconducting phase, only samples with x=0.1 exhibit magnetic hyperfine structure, in this case probably due to spin glass magnetic order.     

Hyperfine magnetic interactions of 57Fe nuclei in NaFeAs arsenide

The results of the Mössbauer effect studies of layered NaFeAs arsenide in a wide temperature range are presented. The measurements at T > T _N demonstrate that the main part (～90%) of iron atoms are in the low-spin state Fe²⁺. The other atoms can be attributed to the impurity NaFe₂As₂ phase or to the extended defects in NaFeAs. The structural phase transition (at T _S ≈ 55 K) does not produce any effect on hyperfine parameters (δ, Δ) of iron atoms. At T < T _N, the spectra exhibit the existence of a certain distribution of the hyperfine magnetic field (H _Fe) at ⁵⁷Fe nuclei, indicating the inhomogeneity of the magnetic environment around iron cations. The analysis of the temperature behavior of the distribution function p(H _Fe) allows us to determine the temperature of the magnetic phase transition (T _N = 46 ± 2 K). It has been found that the magnetic ordering in the iron sublattice has a two-dimensional type. The analysis of the H _Fe(T) dependence in the framework of the Bean-Rodbell model reveals a first-order magnetic phase transition accompanied by a drastic change in the electron contributions to the main component (V _ZZ ) and the asymmetry parameter (η) of the tensor describing the electric field gradient at ⁵⁷Fe nuclei.     

Magnetic behaviour of NpAs2 from Mössbauer spectroscopy

The Mössbauer hyperfine spectra of the 60 keV resonance of ²³⁷Np in powder and single crystal absorbers of NpAs₂ were measured between 4.2 and 60 K. Below 18 K a simple magnetic plus quadrupole pattern is seen in accordance with a ferromagnetic spin structure in tetragonal NpAs₂. The isomer shift favors the 4+ charge state, the hyperfine field of 288 T implies a moment of 1.5μ_B at the Np ion. The large reduction compared to the free ion values points towards a strong mixing of the electronic ground state by crystalline field interactions. Above 18 K the spectrum changes into a complex hyperfine pattern indicating a sinusoidally modulated spin structure. Near 54 K a transition into the paramagnetic state is observed. Both magnetic transitions (18 and 54 K) exhibit a feature typical for a first-order character.     

Hyperfine interactions in Zr(Fe1−x Al x )2 measured forx≤0.20 by TDPAC and Mössbauer spectroscopy

Mössbauer (⁵⁷Fe) and TDPAC spectroscopy (¹⁸¹Hf) have been used to study quasibinary compounds Zr(Fe_1?x Al _x )₂ forx≤0.20. It has been found that the dependence of the mean values of the hyperfine magnetic field, quadrupole splitting and isomer shift on the Al concentrationx is strong. The dependence of the hyperfine magnetic field on the number of Al atoms as nearest and next-nearest neighbours of⁵⁷Fe has been established. The TDPAC results also indicate a dependence of the hyperfine field on¹⁸¹Ta on Al concentration.     

57Fe NMR spectra in domain walls of hexagonal ferrites with magnetoplumbite structure

The domain wall NMR spectra of⁵⁷Fe were measured on polycrystalline samples of BaFe₁₂O₁₉ and SrFe₁₂O₁₉ at 4·2 K. We have calculated the anisotropy of the hyperfine field dipolar component. The measured NMR spectra were interpreted supposing that the hyperfine field anisotropy is caused only by the dipolar field anisotropy.     

Etude par effet Mössbauer de la briartite (Cu2FeGeS4)

Mössbauer experiments on Cu₂FeGeS₄ have shown that the ground orbital level of the Fe²⁺ ions is |L_z=0>, which corresponds to the existence of an easy plane of magnetization perpendicular to the tetragonal axis 0z; in the antiferromagnetic phase the magnetic moments actually lie inside this plane. The tetragonal splitting of the ground orbital doublet ₃ is found to be about 1430 cm^?1, and the Néel temperature is 12,3±0,3 K. A. self consistent calculation of molecular field is used to explain the order of magnitude of the hyperfine field observed at low temperature (H_hf=167±2 kOe at 4,2 K).     

Band structure of (Sr<Subscript>3</Subscript>Sc<Subscript>2</Subscript>O<Subscript>5</Subscript>)Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> as a possible basis phase of new FeAs superconductors

The results of the ab initio FLAPW-GGA computations of the band structure of the recently synthesized layered tetragonal (space group I4/mmm) arsenide (Sr₃Sc₂O₅)Fe₂As₂ as a possible basis phase of a new group of FeAs superconductors are presented. For (Sr₃Sc₂O₅)Fe₂As₂, the energy bands, electron state density distributions, Fermi surface topology, low-temperature electron specific heat, molar Pauli paramagnetic susceptibility, and effective atomic charges have been determined. These results are discussed compared to similar data for the layered tetragonal crystals LaFeAsO, SrFeAsF, SrFe₂As₂, and LiFeAs that are the basis phases of the recently discovered high-temperature (T _C ～ 26–56 K) 《1111》, 《122》, and 《111》 FeAs superconductors.     

Confirmation of the giant hyperfine anomaly of the system184W2+-183W g by new spin-echo experiments with183WFe and remarks on possible explanations

Spin-echo measurements on¹⁸³W were performed with dilute alloys of W (0.1, 0.25, 0.5 and 1 at %) in Fe. The result for the hyperfine field:B _4.2K ^hf =(?)62 · 54 (3) T agrees with the old data of Kontani and Itoh, but the accuracy is much better. The giant hyperfine anomaly of¹⁸³W with respect to¹⁸⁴W₂₊ observed by Alzner et al. is thus confirmed and the errors are reduced to:¹⁸⁴W₂₊Δ¹⁸³W_g=+0.150(31). This is the first case of a very large hyperfine anomaly in electronic hyperfine fields which is not caused by the pathological cancellation of orbital and spin magnetism in jackknifep _1/2 ord _3/2 single proton configurations. A detailed discussion shows that the large hyperfine anomaly may be related to the anomalously small magnetic dipole moment of¹⁸³W. Our result should stimulate further theoretical work with the aim to understand this magnetic moment as well as the giant hyperfine anomaly.     

Atomically-flat,chemically-stable,superconducting epitaxial thin film of iron-based superconductor,cobalt-doped BaFe2As2

The epitaxial growth of Fe-based superconductors such as Co-doped SrFe₂As₂ (SrFe₂As₂:Co) was reported recently, but has still insufficient properties for a device application because they have rough surfaces and are decomposed by reactions with water vapor in an ambient atmosphere. This paper reports that epitaxial films of Co-doped BaFe₂As₂ grown at 700 ^°C show the onset superconducting transition temperature of ～20 K. The transition is sharper than those observed on the SrFe₂As₂:Co films, which would originate from their improved crystallinity. These films also have atomically-flat surfaces with step-and-terrace structures and exhibit chemical stability against exposure to water vapor.     

Mössbauer studies of YBa2Cu3O7−δ-type high-T c superconductors

The local magnetic, electronic, and structural properties of (RE)Ba₂Cu₃O_7?δ supercondcutors (RE=Gd, Dy, and Eu) were studied by Mössbauer spectroscopy using the resonances of¹⁵⁵Gd,¹⁶¹Dy,¹⁵¹Eu, and⁵⁷Eu. In GdBa₂Cu₃O_7?δ, different magnetic ordering behaviors of the Gd sublattice are found for the orthorhombic (superconducting) and tetragonal (non-superconducting) phases. In DyBa₂Cu₃O_7?δ, the magnetic moments of the respective CEF ground states in the orthorhombic and tetragonal phases are derived from paramagnetic hyperfine splittings at 1.4 K. In both DyBa₂Cu₃O_7?δ and EuBa₂Cu₃O_7?δ, anomalies connected with the superconducting transitions in isomer shift, recoil-free fraction, and relaxation behavior were looked for, but not found. The electric-quadrupole splittings observed for both systems are discussed in connection with the lattice EFGs derived for the Gd system. In GdBa₂ (Cu_0.995Fe_0.005)₃O_7?δ, the local properties of the various Fe sites are investigated over a wide temperature range in both the orthorhombic and tetragonal phase. The magnetic ordering of the Gd sublattice in the orthorhombic phase and of the Cu(2) sublattice in the tetragonal phase, respectively, is monitored via the magnetic splittings at the various Fe sites. Possible assignments of Cu(1) and Cu(2) sites as well as different oxygen configurations around the substituted Fe ions are discussed.     

Nuclear ferromagnetic resonance on Co2B and on some dilute alloys

The NMR of Co⁵⁹ at 4.2 K in the compound Co₂ B and in the series (Co_0.99X_0.01)₂B where X = Ti, V, Cr, Mn, Fe and Ni is developed in this paper. The NMR spectrum of Co₂B shows a single narrow line which indicates that the easy magnetization direction is parallel to the tetragonal axis. In a static external field the shift of the line indicates that the hyperfine field is negative.Spin-lattice relaxation time T₁ measurements show that the density of states at the Fermi level reaches a maximum in the vicinity of X = V, Cr, in agreement with previous magnetic, resistivity and specific heat investigations. These results are interpreted by the virtual bound state concept.     

First-principles study of pressure effects on and

We have performed first-principles calculations to investigate the pressure effects on CaFe₂As₂ and BaFe₂As₂. Our calculations show that in CaFe₂As₂, the orthorhombic structure transforms to a collapsed tetragonal structure at 0.4 GPa with a volume collapse of 9.5%, which is in agreement with experiments. Together with the structural phase transition, CaFe₂As₂ undergoes a magnetic transition from the stripe antiferromagnetic ordering to the nonmagnetic state. For BaFe₂As₂, we predict that the orthorhombic structure transforms to the tetragonal structure at 9.4 GPa. Unlike CaFe₂As₂, the magnetic moments of Fe ions in BaFe₂As₂ are not zero and the stripe antiferromagnetic ordering transforms to the checkerboard antiferromagnetic ordering together with the structural phase transition. The stability of the orthorhombic structure up to 9.4 GPa suggests that superconductivity and magnetism coexist in BaFe₂As₂.     

Correlation of57Fe hyperfine interactions with Fe-Fe distance in Laves phase compounds RFe2 (R=rare earth)

The⁵⁷Fe Mössbauer isomer shifts and magnetic hyperfine fields in Laves phase compounds RFe₂ (R=Pr, Nd and Sm) are studied with particular reference to the effect of the Fe–Fe interatomic distance on the hyperfine interactions. It is shown that the charge density at the Fe nuclei scales linearly with fractional volume change up to 20%. The⁵⁷Fe hyperfine field corrected for the influence of rare-earth moment shows a systematic variation with the distance, which can be understood in terms of the Bethe-Slater curve arguments. The similarity of the atomic volume dependence of the⁵⁷Fe hyperfine interactions in Lves phase compounds to those in iron with close-packed structure is emphasized.This paper is based on a paper presented at the 5th Int. Conf. on hyperfine interactions, Berlin, July 21–25, 1980.     

As-NMR/NQR study of pressure-induced superconductivity in CaFe2As2

⁷⁵As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe₂As₂ under pressure. At P=4.7 and 10.8 kbar, the temperature dependence of nuclear-spin-lattice relaxation rate (1/T₁) measured at tetragonal phase show no coherence peak just below T_c and decrease with decreasing temperature. The superconductivity is of gapless at P=4.7 kbar but evolves to multiple gaps at P=10.8 kbar. We find that the superconductivity appears near a quantum critical point. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.     

A Mössbauer study of the quasi two-dimensional antiferromagnet TlFe2−x Se2

TlFe_2?x Se₂ compounds withx from 0.36 to 0.40 have been examined with Mössbauer spectroscopy. The compounds order antiferromagnetically with broad magnetic transition regions whose width increases with decreasing Fe content. The Néel temperature decreases with x and the temperature variation of the magnetic hyperfine field indicates that the transition is of first order. In samples without vacancy ordering the Fe nuclei experience a different temperature variation of the hyperfine field than in compounds with ordered vacancies.     

Mössbauer and magnetization studies of the NpM2Ge2(M=Cr,Fe, Co,Ni, Cu) intermetallic compounds

Magnetization and ²³⁷Np Mössbauer studies were performed on tetragonal NpM₂Ge₂ intermetallic compounds. All the compounds order antiferromagnetically. The Néel points and the magnetic hyperfine interactions of the NpM₂Ge₂ compounds are 62(3), 28(3), 36(5), 27(3), 34(3) K and 2860(40), 1800(40), 3040(40), 2600(40), 2330(90) Mc/sec for M=Cr, Fe, Co, Ni and Cu respectively.