On the transferred hyperfine interaction in substituted yig

NMR of⁵⁷Fe is studied in a number of (M_xY_3–x) Fe₅O₁₂ garnets for small concentrations of M (M is either trivalent RE ion –Ho 3+, Gd 3+, Nd3+, Pr 3+, La 3+ or Bi 3+ ion). Beside the main resonance lines, the satellites were observed, which correspond to those Fe, in vicinity of which the impurity M is located. After correcting for the dipolar field, the field corresponding to the change of the transferred hyperfine interaction in M³⁺–O^2–-Fe³⁺ vs. Y³⁺–O^2–-Fe³⁺ triad was deduced from the satellites splitting. The analysis of the results indicates that the observed change in the transferred hyperfine field is mainly connected with the transfer of electrons between M³⁺ and Fe³⁺ ions and not with the local deformation around the impurity.     

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.     

High-frequency EPR of Tb<Superscript>3+</Superscript>-doped KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystal

High-frequency electron paramagnetic resonance (EPR) spectra of the KPb₂Cl₅:Tb³⁺ crystal have been investigated. Three types of spectra were observed in the frequency range of 74–200 GHz. The most intensive spectrum with the resolved hyperfine structure corresponded to transitions between sublevels of the¹⁵⁹Tb³⁺ ground quasi-doublet with the zero-field splitting (ZFS) close to 48 GHz. Experimental results were analyzed by the exchange charge model of the crystal field affecting terbium ions in low-symmetry Pb²⁺ positions with the chlorine sevenfold coordination and the charge compensating vacancy in the nearest potassium site. The calculated values ofg-factors and ZFS were in agreement with the experimental data. The nature of a broad EPR line with ZFS of about 180 GHz and of additional weak EPR lines observed as satellites of the main Tb³⁺ lines was discussed.     

Electron magnetic resonance of gadolinium-doped calcium fluoride

Electron magnetic resonance (EMR) spectra of gadolinium-doped calcium fluoride have been studied at room temperature for Gd concentrations between 0.01 and 2.00 mol%. Gd³⁺ ions in sites with two different symmetries were observed. One of the sites, with cubic symmetry, is unstable at room temperature and decays with a time constant of 2.2 day⁻¹. The other site, with tetragonal symmetry, is stable and is attributed to Gd³⁺ ions in substitutional sites next to a charge-compensating F⁻ interstitial ion. The linewidth and intensity of the EMR spectrum with tetragonal symmetry increase with increasing Gd concentration. A theoretical calculation based on the concentration dependence of the EMR linewidth yields an effective range of the exchange interaction between Gd³⁺ ions in CaF₂ of 0.774 nm, of the same order as that of Gd³⁺ ions in other cubic ionic compounds.     

Mössbauer isomer shift and hyperfine fields of ordered and disordered Fe3Al alloys

The Mössbauer technique was used to measure the hyperfine magnetic field and isomer shift of⁵⁷Fe nucleus in the ordered and disordered Fe₃Al alloys. The Mössbauer spectra were analyzed to yeld the hyperfine magnetic field distribution curves. A linear correlation has been revealed between the average hyperfine field and the average number of Al atoms in the first two nearest neighbour shells of⁵⁷Fe nucleus,N _Al, for both ordered and disordered alloys. The measured values of the mean isomer shift agree very well with the values expected from the Miedema-Van der Woude model.     

Umbrella structure in Ho substituted YIG - NMR study

The NMR of ⁵⁷Fe in Ho: YIG and Gd: YIG was measured on polycrystalline samples at 4.2 and 77K. In spectra of both tetrahedral and octahedral Fe³⁺ ions satellite lines appear which were ascribed to Fe³⁺ with a rare earth ion in their vicinity. From the position of the satellites the magnitude and direction of the Ho³⁺ magnetic moment were deduced. The results for Ho³⁺ ion are in good agreement with the crystal field calculations and with other magnetic data, but disagree with the neutron scattering results in HoIG garnet. The results on Gd: YIG were used to check the correctness of the analysis.     

Temperature features of the EPR spectrum of GdMnO3: Single crystal and thin film GdMnO3/LaAlO3

Electron paramagnetic resonance (EPR) spectra of GdMnO₃ single crystal and GdMnO₃/LaAlO₃ thin film are measured at Q-band frequency in the temperature range of 4.2 to 300 K. The temperature dependence of parameters of the spin Hamiltonian corresponding to the monoclinic symmetry of such paramagnetic centers as Gd³⁺ ions in the GdMnO₃/LaAlO₃ film is determined.     

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.     

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.     

EPR of Gd3+ in single crystal colquiriite and analysis of the spin Hamiltonian tensorsB 4 andB 6

In single crystal colquiriite LiCaAlF₆ doped with Gd³⁺ ions two EPR spectra of the Gd³⁺ ions with the Laue site-symmetry groups C_i and C_3i were observed. The spectrum angular dependence for trigonal Gd³⁺ centre was investigated in detail and corresponding spin Hamiltonian parameters were fitted. From analysis of the spin Hamiltonian tensorsB ₄ andB ₆ it was established that Gd³⁺ with the Laue group C_3i substitutes at Ca²⁺ site with the excess charge compensation by an ion located along the threefold axis from this site. The transformation formulas for a sixth-rank irreducible Hermitian tensor under coordinate rotation are tabulated in an explicit form. By using the EPR data for Gd³⁺ substituted in a variety of host crystals, the fourth-rank and sixth-rank tensors of Gd³⁺ spin Hamiltonians were tabulated and correlated with structures of the coordination polyhedra at substitution sites. The results suppose a predominance of quadratic crystal field contributions into the spin Hamiltonian tensorB ₄ of Gd³⁺.     

EPR study of Gd3+ in a ferroelastic BiVO4 single crystal

Electron paramagnetic resonance of the Gd³⁺ ion in a ferroelastic BiVO₄ single crystal with a single domain, grown by the Czochralski method, has been investigated at room temperature using a Q-band spectrometer. The rotation patterns of the resonance fields measured in the crystallographic planes are analyzed using a monoclinic spin Hamiltonian. The principal Z-axis of the second-order zero-field splitting tensorD is found to be along the crystallographicb-axis. Spin Hamiltonian parameters together with the principal axes ofg andD tensors in the monoclinic plane show that the local site symmetry of Gd³⁺ ion in BiVO₄ crystal is monoclinic and that the Gd³⁺ ion substitutes for Bi³⁺ ion.     

Influence of gadolinium concentration on the EMR spectrum of Gd in zircon

Electron magnetic resonance (EMR) spectra of gadolinium-doped zircon (ZrSiO₄) powders have been studied at room temperature for gadolinium concentrations between 0.20 and 1.0 mol%. The results suggest that Gd³⁺ ions occupy substitutional sites in the zircon lattice, that the electron magnetic resonance linewidth increases with increasing gadolinium concentration and that the range of the exchange interaction between Gd³⁺ ions is about 1.17 nm, larger than that of the same ion in other host lattices, such as ceria (CeO₂), strontium oxide (SrO) and calcium oxide (CaO). The fact that the electron magnetic resonance linewidth of the Gd³⁺ ion in polycrystalline zircon increases, regularly and predictably, with Gd concentration, shows that the Gd³⁺ ion can be used as a probe to study, rapidly and non-destructively, the crystallinity and degradation of ZrSiO₄.     

57Fe Emission Mössbauer Study on Gd3Ga5O12 implanted with dilute57Mn

⁵⁷Fe emission Mössbauer spectroscopy has been applied to study the lattice location and properties of Fe in gadolinium gallium garnet Gd₃Ga₅O₁₂ (GGG) single crystals in the temperature interval 300 – 563 K within the extremely dilute (<10^?4 at.%) regime following the implantation of⁵⁷Mn (T¹_/2= 1.5 min.) at ISOLDE/CERN. These results are compared with earlier Mössbauer spectroscopy study of Fe-doped gadolinium gallium garnet Gd₃Ga₅O₁₂(GGG), with implantation fluences between 8×10¹⁵ and 6×10¹⁶ atoms cm^?2. Three Fe components are observed in the emission Mössbauer spectra: (i) high spin Fe²⁺ located at damage sites due to the implantation process, (ii) high spin Fe³⁺ at substitutional tetrahedral Ga sites, and (iii) interstitial Fe, probably due to the recoil imparted on the daughter^57?Fe nucleus in the β^? decay of⁵⁷Mn. In contrast to high fluence⁵⁷Fe implantation studies the Fe³⁺ ions are found to prefer the tetrahedral Ga site over the octahedral Ga site. No annealing stages are evident in the temperature range investigated. Despite the very low concentration, high-spin Fe³⁺ shows fast spin relaxation, presumably due to an indirect interaction between nearby gadolinium atoms.     

Electron paramagnetic resonance of Gd3+ ions in Ca1 − x − y Y x Gd y F2 + x + y crystals

Electron paramagnetic resonance of Ca_{1 ? x ? y} Y _x Gd _y F_{2 + x + y} single crystals has revealed spectra that are not typical of gadolinium-doped CaF₂ crystals. These spectra have a nearly tetragonal symmetry and are most probably caused by Gd³⁺ ions localized in yttrium clusters. Weak spectra of tetragonal Gd³⁺ centers, whose parameters are close to those of a cubic gadolinium center caused by an isolated Gd³⁺ ion, have been also detected. These centers are attributed to isolated Gd³⁺ ions localized near octahedral rare-earth clusters or their associations.     

Electron paramagnetic resonance of Gd3+ ions in powders of LaF3:Gd3+ nanocrystals

The observation of electron paramagnetic resonance of Gd³⁺ ions in nanosized powders of rare-earth fluorides LaF₃:Gd³⁺ has been reported. The measurements have been performed on a single crystal and micro- and nanosized powders at room temperature. Electron paramagnetic resonance spectra and spin-Hamiltonian parameters of Gd³⁺ ions have been obtained. A qualitative difference of spectra in nano- and micropowders due to the increase in the spread of the crystal field parameters with the decrease in the particle size has been found. The relationship between the single-crystal domain size and the hydrothermal treatment time has been established.     

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.     

Temperature and pressure dependences of EPR spectra of Gd ion doped in the EuAl3(BO3)4 monocrystal

The ground state of Gd³⁺ ions substituting for trivalent europium in the EuAl₃(BO₃)₄ single crystal was studied by electron paramagnetic resonance (EPR) over the temperature range of 300-4.2 K and at pressures up to 9 kbar. The EPR spectra were analysed using the spin Hamiltonian of axial symmetry. The following parameters are reported: g=1.981±0.002, b₂⁰=280.18±0.12, b₄⁰=−12.95±0.08 and b₆⁰=0.61±0.12 (at Т=298 K). The distortions of the nearest environment of Gd³⁺ ion were analysed within the framework of the superposition model of crystal field.     

Effects of averaging of spin packets of interacting resonances in Gd<Superscript>3+</Superscript> EPR spectra of scheelites

The EPR spectra of Gd³⁺ tetragonal centers in crystals with a scheelite structure are analyzed. It is found that the EPR spectra exhibit additional signals in the vicinity of coincidence of the resonance lines attributed to Gd³⁺ EPR transitions. It is shown that these signals are caused by averaging (due to relaxation spin-lattice transitions between the resonance doublets) of the internal part of the quasi-symmetric system of spin packets corresponding to the inhomogeneously broadened initial EPR lines. The quasi-symmetric arrangement of the spin packets is associated with the mosaic structure of the studied crystals.     

<Superscript>57</Superscript>Fe NMR study of amorphous and rapidly quenched crystalline Fe-B alloys

Amorphous and crystalline Fe-B alloys (5–25 at % B) were studied using pulsed ⁵⁷Fe nuclear magneticr esonance at 4.2 K. The alloy samples were prepared from a mixture of the ⁵⁷Fe and ¹⁰B isotopes by rapid quenching from the melt. In the microcrystalline Fe-(5–12 at %) B alloys, the resonance frequencies were measured for local states of ⁵⁷Fe nuclei in the tetragonal and orthorhombic Fe₃B phases and also in α-Fe. The resonance frequencies characteristic of ⁵⁷Fe nuclei in α-Fe crystallites with substitutional impurity boron atoms in the nearest neighborhood were also revealed. In the resonance frequency distribution P(f) in the amorphous Fe-(18–25) at % B alloys, there are frequencies corresponding to local Fe atom states with short-range order of the tetragonal and orthorhombic Fe₃B phases. As the boron content decreases below 18 at %, the P(f) distributions are shifted to higher frequencies corresponding to ⁵⁷Fe NMR for atoms exhibiting a short-range order of the α-Fe type. The local magnetic structure of the amorphous Fe-B alloys is also considered.     

Electron–nucleus interactions in highly doped rare earth phosphate glasses

³¹P NMR measurements have been used to study the dynamics of magnetic rare earth ions in the stable metaphosphate systems (R₂O₃)_x(X₂O₃)_0.25-x(P₂O₅)_0.75, with R≡Er³⁺, Nd³⁺ and Gd³⁺, for varying concentrations x, where X denotes a nonmagnetic rare earth buffer. Broad EPR spectra are observable at liquid helium temperatures but it is not possible to measure the paramagnetic relaxation time directly. Electronic relaxation parameters and crystal field splittings have been inferred from the NMR relaxation data. Nuclei situated near paramagnetic ion sites experience shifts in resonance frequency due to the local field produced by the ion. The nuclear resonance line is inhomogeneously broadened with decreasing temperature and disappears abruptly at temperatures which depend on the magnetic ion present, its concentration and the magnitude of the applied magnetic field. A discussion of the line broadening process is presented. This revised version was published online in August 2006 with corrections to the Cover Date.