57Fe NMR Research Articles

NMR and Mossbauer studies of core–shell FeCo@C ferromagnetic nanoparticles near the superparamagnetic transition

A series of nanoparticles of the bimetallic core and carbon shell type FexCo1-x@C (x = 0.4 – 0.8) have been obtained by the gas condensation synthesis method. The microstructural peculiarities and magnetic behaviour of the nanoparticles is analysed depending on the composition and thermal treatment. Collateral phases, which are difficult to detect by traditional methods, are identified. The 59Co,57Fe NMR and 57Fe Mossbauer spectroscopies have shown that homogeneity of the composition and removal of carbon from the metallic core can be reached by annealing. The combination of methods has quantified the fraction of superparamagnetic particles, the proportion of paramagnetic inclusions. A simplified visual interpretation of the 59Co NMR spectra is proposed for binary FeCo alloys.

Magnetism and the Trimeron Bond.

A review of progress in understanding the Verwey transition in magnetite (Fe3O4) over the past decade is presented. This electronic and structural transition at TV ≈ 125 K was reported in 1939 and has since been a contentious issue in magnetism. Long range Fe2+/Fe3+ charge ordering has been confirmed below the transition from crystal structure refinement, and Fe2+ orbital ordering and formation of trimerons through weak bonding of Fe2+ states to two Fe neighbors has been discovered. This model has accounted for many spectroscopic observations such as the 57Fe NMR frequencies. The trimeron lifetime has been measured, and trimeron soft modes have been observed. The origin of the first to second order crossover of Verwey transitions in doped magnetites has been revealed by a nanoparticle study. Electronic and structural fluctuations are found to persist to temperatures far above TV and local structural distortions track the bulk magnetization, disappearing at the 850 K Curie transition. New binary mixed-valent iron oxides discovered at high pressure are found to have electronic transitions and orbital molecule ground states similar to those of magnetite.

Zero-field 57Fe NMR in BiFeO3 based compounds: Problems, solutions and application to Bi1-xSrxFeO3

In this paper, we present a protocol designed to avoid typical problems occurring in the process of the spatial spin modulated structure (SSMS) investigation in BFO-based (BiFeO3) materials by zero-field nuclear magnetic resonance (ZF NMR) spectroscopy on 57Fe nuclei. We have analyzed the significant discrepancies in the parameters of the spectra in the literature and developed the measurement protocol to improve the quality and unify the obtained spectra. The study of the 57Fe spectra of Sr-doped BFO at 4.2 K samples was carried out to illustrate the protocol. The obtained spectra demonstrate suppression of the SSMS by heterovalent substitution. The Sr-concentration dependent evolution of the SSMS anharmonicity parameter was observed as well.

Quantitative phase analysis of magnetic Fe@C nanoparticles

Substantial progress in the area of nanotechnology requires special quality of magnetic nanoparticles. Conventional methods of analysis are not always efficient to the study of nanoparticles. This paper is dedicated to the problem of quantitative analysis of the phase composition of ferromagnetic nanoparticles and the fundamental issue of transition to the single domain magnetic state. A comprehensive study of Fe@C nanoparticles has been carried out, including local 57Fe NMR and Mössbauer spectroscopy methods. It has been found that the cores of nanoparticles contain phases of α-iron, iron carbides, ferromagnetic and paramagnetic phases of iron-carbon FeCx solid solutions. Annealing of the nanoparticles leads to a significant redistribution of the phase composition and leads to an increase in magnetization. Also, a nanoscale effect has been found. It consists in observation of two lines in NMR spectra corresponding to multidomain and single-domain state. The hyperfine fields obtained by NMR and Mössbauer spectroscopy methods are discussed.

Ambiguity of NRPS Structure Predictions: Four Bidentate Chelating Groups in the Siderophore Pacifibactin.

Identified through a bioinformatics approach, a nonribosomal peptide synthetase gene cluster in Alcanivorax pacificus encodes the biosynthesis of the new siderophore pacifibactin. The structure of pacifibactin differs markedly from the bioinformatic prediction and contains four bidentate metal chelation sites, atypical for siderophores. Genome mining and structural characterization of pacifibactin is reported herein, as well as characterization of pacifibactin variants accessible due to a lack of adenylation domain fidelity during biosynthesis. A spectrophotometric titration of pacifibactin with Fe(III) and 13C NMR spectroscopy of the Ga(III)-pacifibactin complex establish 1:1 metal:pacifibactin coordination and reveal which of the bidentate binding groups are coordinated to the metal. The photoreaction of Fe(III)-pacifibactin, resulting from Fe(III) coordination of the β-hydroxyaspartic acid ligands, is reported.

Estimation of structural composition of the inverse spinel ferrites using 57Fe-Zero Field Nuclear Magnetic Resonance

We have demonstrated 57Fe Zero Field Nuclear Magnetic Resonance (ZFNMR) as a powerful tool in determining the structural composition of nickel-cadmium spinel ferrites of various compositions of Ni1-x Cdx Fe2O4 from x = 0 to1, which are synthesized via one-step auto combustion technique. The XRD measurements confirm the phase purity of all the samples. Vibrating Sample Magnetometry (VSM) measurements show that saturation magnetization (MS) increases initially (up to x = 0.3) and then decreases for higher concentrations of cadmium. The Fe3+ ions in the inverted spinel ferrite distribute equally among two possible sites (tetrahedral A and octahedral B) with different hyperfine fields. Therefore for x = 0 under the assumption that Ni enters B sites, 57Fe NMR of Fe3+ ions yield two signals of equal integral intensities in spectral lines corresponding to these sites. Thus, for the sample series Ni1-x Cdx Fe2O4, the contribution of Fe3+ nuclei varies for A and B sub-spectra with the substitution of a non-magnetic Cd2+ ion. By measuring the Fe3+ distribution on A and B sites which is determined from relative spectral areas of A and B NMR sub-spectra the cation distribution is estimated and has been verified by the binomial distribution. Further, XRD Rietveld refinement results are also in good agreement with the composition estimated by NMR technique and the ideal composition. We have demonstrated the usefulness of NMR technique to quantify the accurate composition of the mixed spinel ferrite systems using Ni-Cd ferrite as a test case. Further, the estimated inversion parameter (at around x = 0.4), for the studied system, obtained from ZFNMR, XRD, and VSM techniques are in excellent agreement with each other.

Giant thermal hysteresis in Verwey transition of single domain Fe3O4 nanoparticles

Most interesting phenomena of condensed matter physics originate from interactions among different degrees of freedom, making it a very intriguing yet challenging question how certain ground states emerge from only a limited number of atoms in assembly. This is especially the case for strongly correlated electron systems with overwhelming complexity. The Verwey transition of Fe3O4 is a classic example of this category, of which the origin is still elusive 80 years after the first report. Here we report, for the first time, that the Verwey transition of Fe3O4 nanoparticles exhibits size-dependent thermal hysteresis in magnetization, 57Fe NMR, and XRD measurements. The hysteresis width passes a maximum of 11 K when the size is 120 nm while dropping to only 1 K for the bulk sample. This behavior is very similar to that of magnetic coercivity and the critical sizes of the hysteresis and the magnetic single domain are identical. We interpret it as a manifestation of charge ordering and spin ordering correlation in a single domain. This work paves a new way of undertaking researches in the vibrant field of strongly correlated electron physics combined with nanoscience.

57Fe NMR characterisation of pyrrhotite

The detection of pyrrhotite (Fe7S8) by 57Fe NMR spectroscopy in zero applied field is reported. The 57Fe signal of pyrrhotite is detected in zero field using two different methods, with both yielding peak signal at 42.03MHz. The technique and theory of 57Fe NMR spectroscopy of ferromagnetic iron compounds in zero applied field has recently been reviewed, and a zero field NMR ore detection and separation method has been recently described. The method presented here can be suitably adapted for detection and selection of pyrrhotite-containing ore fragments enriched in Fe7S8 in mining applications. The association of pyrrhotite with the Ni-rich mineral pentlandite (Fe,Ni)9S8 enables peak signal detection at 42.03MHzasa proxy for nickel-rich material in ultramafic nickel sulphide deposits.

Oxidative Transformations of Biosourced Alcohols Catalyzed by Earth‐Abundant Transition Metals

AbstractThe catalytic acceptorless dehydrogenative oxidation of biosourced alcohols into carboxylic acid salts was achieved using earth‐abundant Fe and Mn complexes that feature aliphatic PNP pincer ligands in good to excellent yields. The Fe derivatives were characterized by using 57Fe NMR spectroscopy. Mn pincer catalysts are catalytically more efficient than their Fe counterparts thanks to their robustness under basic conditions. Attempts to generate aldehydes from alcohols were not successful using the Fe and Mn species, but a commercially available Ru analogue achieves this transformation selectively under very mild conditions in the presence of a large excess of acetone as a hydrogen acceptor.

Low temperature behavior of hyperfine fields in amorphous and nanocrystalline FeMoCuB

Low temperature (4.2 K) magnetic behavior of Fe76Mo8Cu1B15 metallic glass was studied by 57Fe Mössbauer spectrometry (MS) and 57Fe NMR. Distributions of hyperfine magnetic fields P(B) were determined for as-quenched and annealed (nanocrystalline) samples with relative fraction of the grains about 43%. P(B) distributions were derived for both the amorphous matrix and nanocrystalline grains. NMR of alloys with natural and 57Fe enriched Fe enabled to assess the contribution of 11B to the total NMR signal. P(B) distribution of the as-quenched alloy derived from MS matches reasonably well the one from NMR of the enriched sample. NMR signal from the sample with natural Fe exhibits contributions from 11B nuclei. The principal NMR lines of the annealed alloys at 47 MHz correspond to bcc Fe nanocrystals. Small asymmetry of the lines towards higher frequencies might be an indication of possible impurity atoms in the bcc structure. The observed differences between natural and enriched samples are attributed to higher sensitivity in the latter. Positions of the lines attributed to bcc Fe nanocrystals obtained from MS and NMR are in perfect agreement.

Hexagonal ferrites of X-, W-, and M-type in the system Sr–Fe–O: A comparative study

Three hexagonal ferrites from the Fe-rich part of the pseudo-binary system SrO–Fe2O3 were investigated. Besides the well-known M-type SrFe12O19 two other ferrites were found to exist at high temperatures: W-type SrFe22+Fe163+O27 and X-type Sr2Fe22+Fe283+O46 ferrites. A detailed characterization of the X-type Sr-ferrite is reported here for the first time using XRD, HRTEM, magnetization measurements, Mössbauer and 57Fe NMR spectroscopies and ab initio calculations of the electronic structure. The results are compared to those of W- and M-type Sr ferrites. Mössbauer spectra were analyzed with six Fe contributions in the case of the X-type, seven Fe-sites for the W-type and five Fe sites for the M-type in agreement with crystal structure arrangements. Based on a detailed analysis of the NMR spectra in comparison with ab initio calculations the NMR lines were assigned to individual crystal sites. A preferential occupation of ferrous ions in the S blocks of the X- and W-type ferrites was elucidated from Mössbauer and NMR data as well as ab initio calculations.

Structure of Iron Oxide Nanoparticles Studied by ^{57}Fe NMR

Structure of Iron Oxide Nanoparticles Studied by ^{57}Fe NMR

Effects of Al substitution and thermal annealing on magnetoelectric Ba0.5Sr1.5Zn2Fe12O22 investigated by the enhancement factor of 57Fe nuclear magnetic resonance

The magnetoelectric properties of hexaferrite Ba0.5Sr1.5Zn2Fe12O22 are significantly improved by Al substitution and thermal annealing. Measuring the enhancement factor of 57Fe NMR, we found direct microscopic evidence that the magnetic moments of the L and S blocks are rotated by a magnetic field in such a way as to increase the net magnetic moment of a magnetic unit, even after the field is removed. Al substitution makes magnetoelectric property arise easily by suppressing the easy-plane anisotropy. The effect of thermal annealing is to stabilize the multiferroic state by reducing the number of pinning sites and the electron spin fluctuation. The transverse conic structure gradually changes to the alternating longitudinal conic structure where spins fluctuate more severely.

Effect of oxygen nonstoichiometry on the structure, 55Mn and 57Fe NMR, electromagnetic properties, and magnetoresistance of manganese zinc ferrites

Detailed studies of manganese zinc ferrites for electronic applications are used to assess the effect of oxygen nonstoichiometry on their structural, electrical, and magnetic properties. The optimal synthesis conditions and nonstoichiometry are determined which ensure the preparation of high-permeability manganese zinc ferrites with low electromagnetic energy losses. The manganese zinc ferrites are shown to be attractive as materials for high-performance magnetoresistive electric current and magnetic field sensors and spintronic devices.

Structural, dielectric, magnetic, and nuclear magnetic resonance studies of multiferroic Y-type hexaferrites

The effect of strontium substitution on structural, magnetic, and dielectric properties of a multiferroic Y-type hexaferrite (chemical formula Ba2−xSrxMg2Fe12O22 with 0 ≤ x ≤ 2) was investigated. Y-type hexaferrite phase formation was not affected by strontium substitution for barium, in the range 0 ≤ x ≤ 1.5, confirmed by x-ray diffraction and Raman spectroscopy measured at room temperature. Two intermediate magnetic spin phase transitions (at tempertures TI and TII) and a ferrimagnetic-paramagnetic transition (at Curie temperature TC) were identified from the temperature dependence of the magnetic susceptibility. Magnetic transition temperatures (TI, TII, and TC) increased with increasing strontium content. Magnetic hysteresis measurements indicated that by increasing strontium concentration, the coercivity increases, while the saturation magnetization decreases. The 57Fe NMR spectrum of the Y-type hexaferrite measured at 5 K and in zero magnetic field showed remarkable differences compared to that of other hexaferrites due to their different number of tetrahedral and octahedral iron sites. The temperature and frequency dependence of the dielectric permittivity evidenced broad peaks with frequency dispersion in correspondence of the Curie temperature.

Evolution of spin-modulated magnetic structure in multiferroic compound Bi(1−x)SrxFeO3

BiFeO3 is a perovskite-like compound with magnetic and electrical long-range order with antiferromagnetic transition temperature about 670 K. It has space modulated magnetic structure of cycloidal type with the period λ=620±20Å incommensurate with the lattice parameter leading to specific NMR line shape. Substitution of strontium for bismuth atoms destroys the space modulated structure. Several samples with substitution of strontium for bismuth Bi1−xSrxFeO3 were studied by means of zero field NMR and Mössbauer spectroscopy. A significant broadening of 57Fe NMR spectrum was observed at x=0.10 while Mössbauer spectroscopy demonstrates this effect at x=0.07. This phenomena evidence for the destruction of the spatial modulated magnetic structure at x∼0.07–0.10 by the heterovalent substitution of strontium for bismuth without the destruction of crystal rhombohedral structure.

Identification of Spinel Iron Oxide Nanoparticles by 57Fe NMR

Identification of Spinel Iron Oxide Nanoparticles by 57Fe NMR

Spin Canting of Maghemite Studied by NMR and In-Field Mössbauer Spectrometry

Local probe techniques, 57Fe in-field Mössbauer, and 57Fe NMR spectrometries have been combined to describe the magnetic structure of maghemite nanoparticles of 39 (±5) nm in size and commercial maghemite. Maghemite nanoparticles were obtained from oxidation of magnetite nanoparticles. Commercial maghemite consists of nanostructured rods, and the size of crystalline domain fairly compares to that of nanoparticles. The structure of the two samples is a partially disordered spinel structure. Both local probe techniques agree that Fe magnetic moments of octahedral and tetrahedral sites are canted in both systems. It was concluded that the canting originates not only from surface effects but also from the bulk resulting from the disordered spinel structure and the frustrated cationic topology, giving rise to reversed Fe moments.

Superconducting state of iron arsenide [formula omitted]: 57Fe and 75As NMR studies

We report 57Fe and 75As NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba0.6K0.4Fe2As2Tc=38K. In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1/T1) demonstrate the development of wave-number (q)-dependent spin fluctuations, except at q=0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57Fe-Knight shift decreases down to zero with decreasing T, evidencing that a spin-singlet superconducting state is realized in Ba0.6K0.4Fe2As2.

Comparison of the Electrochemical Lithiation/Delitiation Mechanisms of FePx (x = 1, 2, 4) Based Electrodes in Li-Ion Batteries

The electrochemical reaction mechanisms of FePx (x = 1, 2, 4) as electrode materials in Li batteries were analyzed by 57Fe Mossbauer spectroscopy, magnetic and NMR measurements. Depending on their stoichiometry, iron phosphides react with lithium along different pathways. FeP and FeP2 are fully or partially converted to a composite electrode of Li3P and Fe. For both, the same intermediate phases, namely FeP and LixFeP, were identified in cycling and FeP is partially regenerated on charge. Surprisingly, FeP4 reacts with Li through an insertion type process previously identified for earlier transition metal phosphides (Ti, V, and Mn), and no conversion reaction to Li3P/Fe is observed. The long-term stability of the composite electrode Li3P/Fe0 and of the intermediate phase LixFeP is analyzed.