Spin freezing properties in LaNi5-xFex

We report magnetic and transport measurements in the metallic pseudobinary compound LaNi_5-xFe_x, which show a Fe spin freezing. The magnetic phase diagram in the x-T plane is derived in the range 0 ⩽ x 1.2, 20 K ⩽ T ⩽ 300 K. Above a critical Fe concentration x_c ≈ 0.3, the system undergoes a superparamagnetic to long range ferromagnetic ordering at a finite temperature T. At lower concentrations, no long range spin ordering is observed, which suggests that x is the percolation threshold. Instead, a transition to a phase characterized by strong irreversibilities is observed at temperature T_f(x). Very strong ferromagnetic coupling interactions between Fe atoms are observed at low concentrations (x < x_c), which contrast with small values of T observed for x ∼ x_c. The results, together with the nature of the spin freezing are discussed in relation to the 2d character of the magnetic lattice.     

Superconducting and magnetic properties of the oxide superconductors La1.85Sr0.15Cu1−xFexO4

Mössbauer and magnetic measurements have been made in the perovskite superconductors La_1.85Sr_0.15Cu_1−x ⁵⁷Fe_xO₄ for x=0.0025, 0.005, 0.01, 0.05 and 0.10. The superconducting transition temperature is rapidly depressed as x increases, reaching T_c=0 for x≈0.03. Mössbauer data for x=0.05 show behavior below ≈15 K which indicates magnetic ordering. Magnetic susceptibility data give an effective moment of 3–4 μ_B/Fe atom, and a hand-like susceptibility which increases with x.     

Magnetic ordering of interstitial iron in Fe1+xSb alloys

Mössbauer and magnetization measurements have been used to observe the magnetic ordering of interstitial iron atoms in the Fe_1+xSb phase in seven Fe:FeSb alloys. Short range magnetic ordering of the interstitials occurs only at temperatures well below the FeSb lattice Néel temperature. Magnetization measurements show that the interstitials form magnetic clusters which freeze in typical mictomagnetic fashion around 30 K. Field cooled alloys show an irreversible magnetization better described as classical superparamagnetism. Variations in the cluster freezing temperature with applied field are observed.     

Fabrication and characterization of iron pnictide wires and bulk materials through the powder-in-tube method

The recent discovery of superconductivity in the iron-based superconductors with very high upper critical fields presents a new possibility for practical applications, but fabricating fine-wire is a challenge because of mechanically hard and brittle powders and the toxicity and volatility of arsenic. In this paper, we report the synthesis and the physical characterization of iron pnictide wires and bulks prepared by the powder-in-tube method (PIT). A new class of high-T_c iron pnictide composite wires, such as LaFeAsO_1?xF_x, SmFeAsO_1?xF_x and Sr_1?xK_xFeAs, has been fabricated by the in situ PIT technique using Fe, Ta and Nb tubes. Microscopy and X-ray analysis show that the superconducting core is continuous, and retains phase composition after wire drawing and heat treatment. Furthermore, the wires exhibit a very weak J_c-field dependence behavior even at high temperatures. The upper critical field H_c2(0) value can exceed 100 T, surpassing those of MgB₂ and all the low temperature superconductors and indicating a strong potential for applications requiring very high field. These results demonstrate the feasibility of producing superconducting pnictide composite wire. We also applied the one-step PIT method to synthesize the iron-based bulks, due to its convenience and safety. In fact, by using this technique, we have successfully discovered superconductivity at 35 K and 15 K in Eu_0.7Na_0.3Fe₂As₂ and SmCoFeAsO compounds, respectively. These clearly suggest that the one-step PIT technique is unique and versatile and hence can be tailored easily for other rare earth derivatives of novel iron-based superconductors.     

First-principle electronic structure calculations for magnetic moment in iron-based superconductors: An LSDA + negative U study

In order to resolve a discrepancy of the magnetic moment on Fe between the experimental and calculation results, we perform first-principle electronic structure calculations for iron-based superconductors LaFeAsO_1-xF_x in which x=0.0 and x=0.125 by using the LSDA + U framework. Consequently, we confirm in both the mother and doped compounds that negative U correction is crucial in matching the calculated magnetic moment with the observed one. A reason of the negative correction is that the Coulomb interaction on Fe orbitals is unexpectedly screened than LSDA’s expectation. We discuss which type of situation emerges when the negative U is a good correction in these compounds.     

Structural and magnetic phase formation in nanophase brass—iron electron compounds

Starting with Cu0.65Zn0.35 with an e/a ratio of 1.35 we studied the phase formation in nanophase (Cu_0.65Zn_0.35)_1?x Fe _x alloys in the concentration range 0.1 ≤x ≤0.7 to see the effect of altering the electron concentration. The evolution of bcc phase from the fcc phase as a function of Fe concentration was investigated by Mössbauer spectroscopy and X-ray diffraction. The grain size, lattice parameters, and average hyperfine magnetic field distributions were estimated for the nanophase alloys. The fcc phase was observed to persist up to 40 atomic per cent Fe substitutions, a mixed (fcc + bcc) phase region up to 70 atomic per cent Fe and bcc phase beyond 70 atomic per cent Fe. The magnetic state of the alloys changed from nonmagnetic forx ≤0.3 to magnetically ordered state at room temperature forx ≤0.33, which lies in the fcc phase region. The fcc phase alloys of Fe with non-magnetic metals have very low magnetic transition temperatures. However, in this system the room temperature state is unusually magnetic     

Atomic and magnetic ordering in Fe3−xVxSi alloys

Alloys of composition Fe_3?xV_xSi with x = 0.25, 0.50, 0.75, 0.90 and 1.00 were studied by neutron diffraction to obtain site distributions and information on magnetic ordering. Vanadium atoms were found to substitute almost completely on the Fe site with 8Fe near neighbors, with little evidence for other types of disorder. Ferromagnetic scattering was observed for x = 0.25, 0.50 and 0.75 in an amount consistent with the observed magnetization, but the sample with x = 0.90 showed no evidence of magnetic ordering down to 4K.     

Mössbauer measurements on lattice and interstitial iron atoms in Fe1+xSb alloys

Mössbauer measurements on the Fe_1+xSb phase in seven samples of Fe : FeSb have been used to observe the independent magnetic ordering of lattice and interstitial iron atoms. An ordering temperature of (55 ± 5) K is deduced for the interstitial iron atoms. The anomalous behaviour of the lattice iron hyperfine fields at about the interstitial ordering temperature is attributed to the perturbation of the superexchange interaction between lattice atoms by the magnetic ordering of the interstitials.     

57Fe Mössbauer study of hydrides and deuterides of manganese

Hydrides and deuterides of manganese with hydrogen-to-metal ratiosx between 0.65 and 0.93 were studiéd by Mössbauer spectroscopy on dilute substitutional⁵⁷Fe probes. No traces of magnetic ordering were found. Experiments on⁵⁷Fe in MnD_0.77 in external magnetic fields show that the iron has no measurable magnetic moment. The shapes of the Mössbauer patterns are compatible with superstructure ordering of the interstitials of the anti-Cdl₂ type nearx=0.5, while the dependence of the isomer shifts onx shows that there are no substantial iron-hydrogen interactions.     

Spin, charge, and orbital orderings in iron-based superconductors

In this article, we briefly review spin, charge, and orbital orderings in iron-based superconductors, as well as the multi-orbital models. The interplay of spin, charge, and orbital orderings is a key to understand the high temperature superconductivity. As an illustration, we use the two-orbital model to show the spin and charge orderings in iron-based superconductors based on the mean-field approximation in real space. The typical spin and charge orderings are shown by choosing appropriate parameters, which are in good agreement with experiments. We also show the effect of Fe vacancies, which can introduce the nematic phase and interesting magnetic ground states. The orbital ordering is also discussed in iron-based superconductors. It is found that disorder may play a role to produce the superconductivity.     

Mössbauer studies of Fe doped superconducting oxides: Present understanding

The information obtained from⁵⁷Fe Mössbauer spectroscopy for samples of YBa₂(Cu_1?x Fe _x )₃O_7±δ covering a large range ofx values is discussed concerning the structural aspects, iron site occupation, local oxygen coordination, magnetic ordering, sensitivity to oxygen content and aging effects.     

Antiferromagnetic stripe ordering at the tetragonal-orthorhombic transition in LnOFeAs probed by X-ray absorption spectroscopy

Measurements of the Fe K-edge X-ray absorption near edge (XANES) spectra of LnOFeAs high T_c superconductors exhibit small but measurable changes across the tetragonal to orthorhombic phase structural phase transition. Calculations, based on the local structure centered at the Fe site obtained by crystallographic investigations, reproduce the observed changes in the spectra under the change of the crystal structure, indicating variations in the Fe local unoccupied electronic states. Calculated Fe density of states including stripe spin ordering in the orthorhombic phase show significant modifications with respect to Fe 3d density of states in the tetragonal phase or the density of states in the orthorhombic phase calculated ignoring spin ordering. This implies that the striped spin ordering in the orthorhombic phase is necessary to describe its electronic properties not only in the ground state but also for excited states above the Fermi energy.     

Mössbauer 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 Mössbauer 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 arranges in an anti-ferromagnetical order at a much lower temperature as compared with the onset of SDW. These two magnetic systems remain almost uncoupled one to another. For the non-stoichiometric Fe_1+xTe 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 the compounds studied, but it has still a metallic character.     

Structural and superconductivity study on α-FeSex

We have successfully synthesized the α-FeSe_x binary tetragonal superconductors with nominal composition of FeSe_x (x=0.6-1.0) via conventional solid state reactions between Fe and Se sealed in quartz tubes. Fe and β-FeSe are the most commonly seen impurities in this binary system. A low-temperature annealing at 400 °C is found to be crucial to remove β-FeSe, which is the thermodynamic stable phase with hexagonal symmetry. For all the samples of FeSe_x, superconductivity is confirmed by magnetic measurements as well as resistivity measurements with their T_c at around 8 K. We noticed that their T_c does not vary with the different nominal Se amount. High-resolution synchrotron X-ray diffraction analysis revealed that the unit cell parameters of all these samples do not change within the error range, and their structure only tolerate the same very small amount of Se deficiency. Based on this study, we concluded that the α-FeSe_x superconductor only exist in a very narrow deficiency range.     

Atomic short-range order in Fe1−xCrx alloys studied by 57Fe Mössbauer spectroscopy

The room temperature Mössbauer spectra of ⁵⁷Fe were measured for iron-based solid solutions Fe_1?xCr_x with x in the range of 0.01≤x≤0.10. Analysis of the obtained spectra gave clear evidence that the distribution of impurity atoms in the first coordination spheres of ⁵⁷Fe nuclei is not homogeneous and it cannot be described in terms of binomial distribution. Quantitatively, the effects were described in terms of the short-range order parameters.     

Relaxation of magnetic clusters in Sr and Fe doped cobaltate perovskites

Mössbauer study of La_1???x Sr _x ⁵⁷Fe_0.025Co_0.975O₃ (0.15 ≤?x?≤ 0.25) perovskites below their magnetic ordering temperature shows cooperative electronic and magnetic phase separation for all compositions investigated. It is shown that the increase of the Sr concentration results in a transition from superparamagnetism to long range ferromagnetism of Co clusters between x = 0.18 and 0.2. In-field Mössbauer measurements revealed the existence of frustrated magnetic interactions at x = 0.22.     

Mechanism of magnetic ordering in Dy<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-Ni bilayer films

A mechanism for the magnetic ordering of dysprosium in Dy_1?x Ni _x -Ni bilayer films is proposed. This ordering was discovered earlier by the authors when studying magnetic circular dichroism. For x exceeding a threshold value (～0.05), the contribution from the Dy_1?x Ni _x layer in a bilayer film to the magnetic circular dichroism over the temperature range 80–300 K is approximately equal in magnitude to the magnetic circular dichroism observed in a single-layer Dy film at temperatures below the ferromagnetic phase transition temperature of Dy (～100 K). Since magnetic circular dichroism is an effect linear in magnetization, the observed effect is associated with magnetic ordering of the Dy_1?x Ni _x layer in bilayer films due to the simultaneous influence of two factors: the incorporation of Ni into the Dy layer and the influence of the continuous Ni sublayer. The ferromagnetic ordering of a dysprosium layer doped with nickel (under conditions of an atomic contact with a continuous nickel layer) was confirmed by the field dependences of the polar and longitudinal Kerr effects. It was shown that both layers in the bilayer structure are magnetized in the same direction and characterized by an anisotropy of the easy-plane type. The magnetic ordering is assumed to be due to the change in the density of states of the Dy_1?x Ni _x alloy caused by hybridization with the narrow peaks near the Fermi level characteristic of nickel.     

Inelastic X-ray scattering investigations of lattice dynamics in SmFeAsO1−xFysuperconductors

We report measurements of the phonon density of states as probed with inelastic X-ray scattering in SmFeAsO_1−xF_y powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO_1−xF_y single crystals allow us to identify the 21 meV A_1g in-phase (Sm,As) and the 26 meV B_1g (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors.     

On the two-phase magnetic state in a CuxZn1−x Cr2Se4 cation-substituted chalcogenide spinel

The region of the concentration magnetic phase transition in a Cu_xZn_1?x Cr₂Se₄ chalcogenide spinel in which the substituted ion concentration x smoothly varies from 0.1 to 0.2 with a step of 0.02 is thoroughly investigated. An anomalous behavior of the Curie temperature is observed. This anomaly is associated with the nucleation of ferromagnetically ordered microregions in the vicinity of Cr ions with an intermediate valence and subsequent long-range magnetic ordering at a critical Cu concentration in the range 0.12<x _c <0.14. The possible coexistence of ferro-and antiferromagnetism in the same temperature range is revealed.     

Magnetic structure and phase diagram of the frustrated spinel Cd1−xZnxCr2Se4

In attempt to characterise the magnetic ordering in the whole composition range of the Cd_1−xZn_xCr₂Se₄ system, various magnetic measurements were performed on both crystalline and polycrystalline samples with 0?x?1. The magnetic properties of the system are typical of a ferromagnet below x=0.4 and of a complex antiferromagnet one above x=0.6. In this work the intermediate region was carefully studied. The variations of both M(T) and χ_ac at low fields suggest that transitions from ferromagnetic to Gabay–Toulouse ferromagnetic-spin-glass mixed phase at low temperature occur in the range 0.41?x?0.58. The high-temperature susceptibility measurements show that for the whole concentration range the system obeys Curie–Weiss laws. The results can be explained by the coexistence of competing interactions (ferromagnetic between nearest neighbours and antiferromagnetic between higher order neighbours) and disorder due to the random substitution between zinc and cadmium ions in the tetrahedral sites of the spinel lattice. An experimental magnetic phase diagram of the system is established.