Effect of spatial spin modulation on the relaxation and NMR frequencies of 57Fe nuclei in a ferroelectric antiferromagnet BiFeO3

The ⁵⁷Fe NMR spectra of the ⁵⁷Fe-enriched (95.43%) ceramics of a ferroelectric antiferromagnet BiFeO₃ with a spatially modulated magnetic structure have been studied. It is established that a cycloidal spin modulation in BiFeO₃ causes a spatial modulation of the spin-spin relaxation rate along the magnetic cycloid period and results in inhomogeneous broadening of the local NMR lineshape. It is also found that the local magnetic moments of Fe ions in various parts of the cycloid depend differently on the temperature, which is indicative of a difference in the spin wave excitation. The observed phenomena can be explained in terms of the Shul-Nakamura indirect nuclear interaction which becomes effective at high concentrations of the magnetoactive nuclei and low temperatures. Similarity of the obtained experimental results to the regularities of NMR in the Bloch walls is discussed.     

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.     

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 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.     

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.     

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.     

Nonlinear Landau-Zener tunneling of Bose-Einstein condensate in a spatially magnetic modulated trap

We study tunneling of a Bose-Einstein condensates confined in a effective double-well potential (a single well with a spatially magnetic modulated scattering length, actually), called pseudo double-well trap, in which the interaction of atoms characterized by the s-wave scattering length a _s can be widely tuned with a magnetic-field Feshbach resonance. As a result, corresponding to different nonlinear parameters, the energy levels of the nonlinear Schrödinger equation can have complex structures in their dependence on the bias between the wells. We discuss the emergence of looped levels, which lead to a breakdown of adiabaticity that the Landau-Zener transition probability does not vanish even in the adiabatic limit. Moreover, we also find that the Landau-Zener tunneling in the pseudo trap show many striking properties distinguished from that of the real trap model (where the barrier is created by the external potential). Possible experimental observation in an opticallyinduced photonic lattices in a photorefractive material is suggested.     

Study of the magnetic structure and57Fe hyperfine interaction in FeMnP

The⁵⁷Fe Mössbauer spectra of FeMnP show a remarkable magnetic distribution for T 175K, although all iron atoms are crystallographically equivalent. It seems possible to explain this distribution by a spin modulation described by coexistence of an antiferro — and a helimagnetic probe. A theoretical calculation performed, justifies this explanation.     

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.     

BiFeO3电子结构的第一性原理研究

采用广义梯度近似下的密度泛函理论计算了铁电相BiFeO3以及顺电相BiFeO3的轨道占据数、电子云重叠布居数、净电荷分布和态密度.结果表明,BiFeO3晶体现铁电性的主要原因是:Fe原子的3d轨道和O原子的2p轨道杂化.而且由于两相晶体结构上的差异,铁电相BiFeO3原子间的共价性和离子性相对顺电相BiFeO3增强了,态密度图在价带区基本相同,但是在导带区具有一定的差异性.     

57Fe NMR and exchange integrals in Al-substituted hematite

The room temperature NMR spectra of⁵⁷Fe in hematite substituted by varying content of Al were measured and exchange integrals of the nearest four pairs Fe³⁺−Fe³⁺ were estimated: The values 261.9, −109.5, 146.6, 49.6 [10⁻²²T²/J] for the first, second, third and fourth pair respectively were obtained.     

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     

Hyperfine structure of LiNbO3:57Fe(III)

Lithium niobate monocrystals exhibit many interesting physical phenomena such as ferro-, piezo- and pyroelectricity, low acoustical losses, small optical absorption in the visible region and, especialy after doping with small amounts of transition metal ions, strong photorefraction and photovoltaic effects [1–3]. According to the multitude of the properties there are numerous applications of LiNbO₃ for e.g. light modulation, Q-switch, second harmonic generation, fibre optics, acoustic transducers, pyroelectric detectors and holographic data storage. In order to understand in detail the microscopic mechanisms of the bulk photovoltaic effect and the photorefraction, which are strongly enhanced by doping the crystals with iron, an exact knowledge of the not yet unambigously known lattice site and the charge compensation [4,5] as well as the electronic structure of the iron impurities is indispensable. Here the Mössbauer investigation of the hyperfine interaction of⁵⁷Fe ions probing the crystal field may contribute to clarification.As can be seen from the isomer shift iron enters the LiNbO₃-lattice as Fe(II) or Fe(III) [5]. By annealing the samples in oxydizing atmospheres (e.g. p24 hours in air p600C) all iron is transformed to Fe(III).     

Hyperfine interactions of 57Fe impurity nuclei in TmNiO3 and YbNiO3 nickelates in the range of magnetic and structure phase transitions

Hyperfine interactions of ⁵⁷Fe impurity nuclei are studied by probe Mössbauer spectroscopy in TmNiO₃ and YbNiO₃ perovskite-like nickelates in the ranges of temperature transitions of an insulator (T < T _IM ) ? metal (T > T _IM ) and antiferromagnetic (T < T _N ) ? paramagnetic (T > T _N ). The changing behavior of hyperfine interaction parameters of ⁵⁷Fe nuclei in the ranges of phase transition temperatures (T _IM and T _N ) is analyzed. The results are interpreted in the context of the charge disproportionation of Ni³⁺ cations associated with the electronic localization in monoclinic-distorted nickelates at T < T _IM .     

The three-line magnetic hyperfine spectrum of57Fe

Longitudinal magnetization of a⁵⁷Co in iron metal foil source and an iron metal foil absorber in a uniform external magnetic field results in a simple three-line magnetic hyperfine absorption spectrum. Measurement of the spectral splitting as a function of applied magnetic field yields the⁵⁷Fe excited-and ground-state,g-factors.     

Incommensurate modulated magnetic structure in orthorhombic EuPdSb

Orthorhombic EuPdSb is known to undergo two magnetic transitions, at 12 K and at T _N≃ 18 K, and in phase III (T < 12 K), single crystal magnetisation data have shown that the spin structure is collinear antiferromagnetic, with magnetic moments along the crystal a axis. From a ¹⁵¹Eu M?ssbauer absorption study, we show that, at any temperature within phase III, all the moments have equal sizes, and that in phase II (12 K< T <18 K) the magnetic structure is modulated and incommensurate with the lattice spacings. The modulation is close to a pure sine-wave just below T _N = 18 K, and it squares up as temperature is lowered. We measured the thermal variations of the first and third harmonics of the moment modulation, and we could determine the first and third harmonics of the exchange coupling. We furthermore show that the antiferromagnetic-incommensurate transition at 12 K is strongly first order, with a hysteresis of 0.05 K, and that the incommensurate-paramagnetic transition at 18 K is weakly first order. Finally, we present an explanation of the spin-flop transition observed in the single crystal magnetisation data in phase III when || in terms of an anisotropic molecular field tensor. Received 17 January 2001 and Received in final form 20 March 2001     

Spontaneous phase transitions in magnetic films with a modulated structure

The influence of monoperiodic and biperiodic bias fields on the nucleation of domain structures in quasi-uniaxial magnetic films near the Curie point has been studied experimentally. The main types of observed nonuniform magnetic moment distributions have been established and chains of a devil’s staircase phase transitions are shown to be realized when the films are slowly cooled.     

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.