NMR of Fe57 in lithium ferrite

An apparatus is described for studying NMR of Fe⁵⁷ and Ni⁶¹ in various ferromagnets and ferrimagnets. The NMR of Fe⁵⁷ was observed in lithium ferrite at room temperature. The magnitude of the NMR signal was studied as a function of the external magnetic field, and the signal was found to fall off more rapidly than the permeability with the applied field. The NMR signal of Fe⁵⁷ in lithium ferrite is apparently amplified by domain-boundary motion.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 10, pp. 62–66, October, 1970.     

57Fe Mössbauer effect study of Mn-substituted goethite and hematite

⁵⁷Fe Mössbauer spectroscopy is performed on synthetic Mn-substituted goethites, α-(Fe_1?cMn_c)OOH, and hematites, α-(Fe_1?cMn_c)₂O₃, with c up to 0.08. The hyperfine parameters and in particular the magnetic hyperfine field are found to be less influenced by Mn substitution than in the case of Al. On the other hand, Mn substitution suppresses drastically the Morin transition in hematite which results in a weakly-ferromagnetic state at 80K for compositions with c>0.04.     

NMR of57Fe in erbium-yttrium iron garnets

NMR of 57Fe was studied by the two pulse spin echo method in systems Er(x)Y(3−x)Fe5012. For small (x<0.3) or large (x>2.7) concentrations of Er the spectra of 57Fe exhibit resolved satellite lines. The splitting between the satellite and the parent line reflects the change of the dipolar field on the Fe nuclei (diamagnetic yttrium is replaced by magnetic erbium ion) and the change which the substitution causes in the electronic structure of the ferric ion. Relative importance of these effects is discussed.     

A study of the nuclear magnetic resonance of isotope Fe57 in hematite

The results of studying the nuclear magnetic resonance of isotope Fe⁵⁷ in weak ferromagnetic hematite -Fe₂O₃ for different types of samples are presented. Measurements of the temperature dependence of the nuclear magnetic resonance frequency in the range of 250–820°K, as well as the dependence of the signal intensity on temperature, the orientation and intensity of the static and h.f. field were carried out. The interpretation of the experimental results is based on the assumption that the signal originates from the nuclei inside the domain walls. The abnormally high value of the enhancement factor of the h.f. field is explained by the closeness of the nuclear magnetic resonance frequency and the resonance frequency of the domain walls.In conclusion the author expresses his thanks to Prof. J. Bro for his valuable advice and comments when preparing this paper; to Prof. R. Rost, Dr. Z Hauptman and M. Vichr for providing the samples; to Dr. V. Houdek for measuring the Néel temperature and to J. Pavlíek for substantial help in the measurements     

57Fe NMR in Y3Fe4.5Ga0.5O12 single crystal

The angular dependence of the⁵⁷Fe NMR spectra in a single crystal of Ga substituted YIG is studied with special attention paid to the satellite lines of the octahedral iron. These satellites correspond to ferric ions in the neighbourhood of which the Ga³⁺ ion is located. The anisotropy of the hyperfine field of these satellites is in accord with the prediction of the independent bond model. A simple analysis indicates that the main source of the anisotropy, besides the dipolar interaction, is the change of the supertransferred part of the hyperfine field.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.     

NMR spectra of57Fe in hexagonal ferrites with magnetoplumbite structure

The NMR spectra of⁵⁷Fe in domains of MFe₁₂O₁₉ (M=Ba, Sr, Pb) were measured by spin echo technique at 4.2 K. The change of the heavy ion causes frequency shifts of lines corresponding to Fe³⁺ ions in 2b and 4f₂ sites while leaving other lines essentially unchanged; the significant role of different Fe³⁺−M²⁺ bonding was found. The dipolar broadening of lines in BaFe₁₂O₁₉ caused by random and static displacement of bipyramidal Fe³⁺ ions from the mirror plane is calculated and the results are compared with the experiment.     

Cation distribution and exchange interactions in CuCr2S4:57Fe

The character of the distribution of Fe ions over tetrahedral and octahedral positions of the spinel structure was investigated in the magnetic-hardening range at 290 and 78 K in CuCr₂S₄:⁵⁷Fe with different degrees of doping with the isotope⁵⁷Fe. It was established that for degrees of substitution not greater than 0.1 per formula unit the Fe ions do not have a definite preference for either positions. The dependence of the effective magnetic field at the⁵⁷Fe nuclei on the degree and character of the embedding of Fe ions in the crystal lattice was determined. The features found are explained by the change in the indirect exchange of Cr ions via current carriers and the degree of delocalization of the 3d-electrons of Cr and Fe ions in the process of cationic substitution.A. N. Tupolev Aviation Institute, Kazan. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 71–74, October, 1992.     

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.     

57Fe NMR study of a spatially modulated magnetic structure in BiFeO3

The ⁵⁷ Fe spin echo spectra were studied in local magnetic fields of a BiFeO₃ ferroelectric antiferromagnet over the temperature range 77–304 K. The line shape analysis confirmed the presence of a spatially modulated, incommensurate cycloidal spin structure in BiFeO₃ and allowed the actual spin distribution to be reproduced throughout the cycloid length. The distribution was found to be essentially anharmonic. The modulated structure is stable over the whole temperature range studied. The cycloid wave becomes more harmonic with temperature elevation.     

Precision of integrals in quantitative NMR

Quantitative applications of NMR use the integral of a line. This random variable has an expectation value (the area of interest) and a standard deviation due to incomplete noise averaging by the integration process. The presence of such fluctuations adversely affects the precision with which integrals can be determined. The autocorrelation function of the noise in the frequency domain has been computed. An expression of the standard deviation of the integral has been derived. The effect of an exponential apodization was investigated. It was found that the precision of the integral decreases roughly as the inverse square root of the integration range, and that it is hardly improved by exponential multiplication. Sine-bell apodization is also shown to be ineffective in improving precision, while zero-filling has no influence. These conclusions are validated by computer simulations.     

Internal magnetic fields in submicron particles of barium hexaferrite detected by 57Fe NMR

⁵⁷Fe nuclear magnetic resonance (NMR) spectra of hexaferrite BaFe₁₂O₁₉ powder samples prepared by glass crystallization method were measured at 4.2 K and analyzed in comparison to spectra of single crystals. Samples with various mean particle dimensions were tested. NMR spectral lines corresponding to individual iron sublattices showed pronounced frequency shifts of their positions and a significant line broadening compared to single crystals. The significant contribution to the line shifts and line shape had a uniform macroscopic origin giving identical absolute value of shifts and the same line shapes for all measured lines of a particular sample. Estimations of demagnetization fields based on mean particle dimensions reasonably corresponded to the observed frequency shifts for particle mean diameter 67 nm, or had a higher value for a sample with mean diameter of 340 nm, for which a presence of domain walls was detected by NMR. In the spectrum of a sample with the smallest particles (～16 nm), an additional contribution having broader lines and faster spin-spin relaxations was found. It could be assigned to weaker exchange interactions or deviations of magnetic moment directions from the hexagonal axis in a surface layer.     

Intermediate coupling model description of55Fe and57Fe

The properties of the negative parity states of⁵⁵Fe and⁵⁷Fe are investigated in the framework of the intermediate coupling model. In the model, a neutron or a quasineutron is coupled to anharmonic vibrations of the core. Anharmonicities of the vibrations are estimated through the observed properties of the core. Energy levels, spectroscopic factors and electromagnetic properties have been calculated. The results of the present calculations are also compared with available experimental results and other theoretical results. The model reasonably accounts for many of the properties of the low-lying states.     

Maxwellian Averaged Neutron Capture Cross Sections in 56Fe and 57Fe

Measurements of the keV-neutron capture cross sections and radiative γ-ray spectrum of 56Fe and 57Fe are performed based on a 7 Li（p,n）7 Be reaction neutron source. The incident neutron spectrum on a capture sample is measured by means of a time-of-flight （TOF） method with a 6Li-glass detector. The radiative capture 7-rays emitted from an iron CS Fe or 57 Fe） or standard gold （197Au） sample are detected by a large anti-Compton NaI（TI） spectrometer covered with a heavy shield. The capture yields of samples are obtained by applying a pulse-height weighting technique to the corresponding capture γ-ray pulse-height spectrum. The Maxwellian averaged neutron capture cross sections of 56Fe and 5T Fe are derived according to the present capture cross section results.     

Performance-comparison of57Fe DCEMS and UHV-ICEMS

In UHV-applications of DCEMS or ICEMS the optimization of the experimental performance is of utmost importance. For the characterization of the “statistical quality” of the Mössbauer-effect experiment, the statistical utility rateu is defined. The measuring time gain valuesG are determined by comparing values ofu. It is shown with⁵⁷Fe DCEMS and UHV ICEMS measurements performed on various⁵⁷Fe samples applying a channeltron and an extremely low-noise scintillation e^?-counter as electron detector that preferable experimental conditions for ICEMS and DCEMS measurements are different.     

57Fe Mössbauer study of Fe nitrides

Summary Magnetic properties of Fe nitrides have been re-examined by⁵⁷Fe M?ssbauer spectroscopy. Hyperfine magnetic fields for α″-Fe₁₆N₂ are 30, 31 and 39T at 298K, but the averaged hyperfine field is 33T and nearly equal to the value of pure α-Fe. σ-Fe₂ N is an antiferromagnet below 9K having a small magnetic moment less than 0.1 μ_B, although γ′-Fe₄N and ε-Fe_3–2N are ferromagnets. ZnS-type FeN is non-magnetic at 4.2K. M?ssbauer spectra obtained from NaCl-type FeN are complex and some Fe atoms in this nitride show a surprisingly large hyperfine magnetic field of 49T. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.     

Nature of the high-pressure transition in Fe2O3 hematite

We present a new method to separate the crystallographic and electronic phase transitions in hematite using x-ray emission spectroscopy and x-ray diffraction. Our observations, based on the behavior of a metastable high-pressure phase in the stability domain of the low-pressure phase, show that the electronic transition is preempted by the crystallographic transition. The former occurs only afterwards in the high-pressure phase, possibly as a result of a Mott transition. The idea that the electronic transition drives the transition in hematite is therefore invalidated. Such methods should help elucidate the mechanics and the driving forces behind a number of first-order high-pressure phase transitions.     

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.     

α-57Fe spectra only

It is the intention of this contribution to demonstrate how polarization-and thickness effects-are reflected on the most common Mössbauer material: α-Fe.     

Implantation of Coulomb excited57Fe

The method of in-beam Mössbauer spectroscopy after recoil implantation of Coulomb excited nuclei is discussed and new setups are described. Particular aspects of time differential measurements are considered. Results obtained by implantation studies of Fe into Si and Al₂O₃ are presented. In silicon the long range diffusion of interstitially implanted iron atoms has been observed between 500 K and 650 K via diffusional linebroadening. The time differential spectra of Si and of Al₂O₃ do not show significant dynamic effects.