The mapon technique and recent developments

Typical linewidths observed in NMRON on dilute impurities in ferromagnetic metals are of order 1 MHz, making difficult the observation of structure in the resonance with splitting Δv much less than this value. The technique of Modulated Adiabatic Passage on Oriented Nuclei (MAPON) was recently developed as a means of measuring the weak electric quadrupole splitting Δv_Q of the nuclear hyperfine interaction due to an electric field gradient V_zz. MAPON has successfully been applied to measure Δv_Q as low as 4 kHz, i.e. less than 0.5% of the inhomogeneously broadened NMRON CWFM resonance line. The isotopes⁵⁶Co,⁵⁷Co,⁵⁸Co and⁶⁰Co have been studied in iron single crystal hosts, yielding Δv_Q consistent with known and estimated quadrupole moments. In addition the results to date give striking confirmation of analyses based on the single impurity relaxation model. Following a brief summary of the theoretical development of MAPON a review of experimental data is given for the CoFe<100> system. The variation of Δv_Q with direction of magnetization, measured in⁵⁸CoFe and⁶⁰CoFe single crystal samples, is also described. Further MAPON measurements are described for a⁵⁶CoFe polycrystalline sample, for which the most probable value and width of the distribution of V_zz can be described simply in terms of the single crystal principal axis results. The application of the MAPON technique to the measurement of nuclear electric quadrupole moments in implanted and diffused samples is discussed.     

Recent experiments with mapon spectroscopy

Electric quadrupole interactions of impurity nuclei in single crystals of⁵⁴MnNi,¹²⁵SbNi and¹²⁵SbFe have been studied as a function of crystal field direction and applied magnetic field using MAPON. Distributions are in all cases broad compared with the mode values of the EQI's. For⁵⁴MnNi the mode efg is isotropic to better than 5% between the easy <111> axis and a hard <100> axis. The efg is +0.88 (15)x 10¹⁹ Vm⁻². The mode efg for¹²⁵SbFe along its easy <100> axis is one half of that along a hard <112> axis, and one third of that measured along the easy <111> axis of¹²⁵SbNi. The much larger efg mode and distribution seen in¹²⁵SbNi, for four to eight times greater dilution than for¹²⁵SbFe, suggests intrinsic contributions due to valence screening effects in the more itinerant nickel host.     

NMRON spin echo in a slowly varying magnetic field

Nuclear spin-spin relaxation of⁶⁰Co and⁵⁶Co in iron single crystals has been studied, using the three-pulse NMRON spin echo. A previously reported rapidT ₂ in⁶⁰CoFe is shown to have arisen from a modulation of the echo amplitude, caused by variations in the phase of the Larmor precession relative to the applied rf field. A lower limit ofT ₂∼0.2s is found in⁵⁶Co⁵⁶ Fe. Extension of this result to other CoFe samples is discussed.     

Specific features of magnetic states of impurity iron ions in the perovskite La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript> <Superscript>57</Superscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript>

Single-phase polycrystalline La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ samples have been prepared by solidstate ceramic technology. The samples have the rhombohedral structure (space group \(R\bar 3c\)). The studies of perovskite La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ by Mössbauer spectroscopy on impurity ⁵⁷Fe nuclei in the temperature range of 5–293 K have revealed the existence of a superparamagnetic relaxation in the temperature range of 100–210 K. The parameters of hyperfine interactions (hyperfine magnetic fields, line shifts, and quadrupole shifts) and the anisotropy energy have been measured, and the frequencies of magnetic moment relaxation of iron ions have been estimated.     

Nuclear magnetic relaxation in ferrimagnetic Y3Fe5−x SixO12 films

The effect of silicon impurities on the damping of spin-echo signals from the ⁵⁷Fe nuclei of tetrahedral Fe³⁺ ions in epitaxial yttrium-iron-garnet films was investigated. It was found that for silicon concentrations 0.015⩽x⩽0.037 the damping of the spin echo is a two-component process, which made it possible to separate nuclei into two types, differing by both the longitudinal and transverse magnetic relaxation times. For silicon concentrations 0.044⩽x⩽0.073 the decay of the echo can be described by one exponential and all nuclei in the sample have the same transverse relaxation times and the same longitudinal relaxation times. The experimental results are interpreted on the basis of the supposition that impurity “macromolecules” form around the Si⁴⁺ ions. The relaxation times of the iron nuclei in a “macromolecule” are much shorter than the relaxation times of iron nuclei belonging to the matrix ions. The radius of a “macromolecule” is estimated on the basis of percolation theory. Fiz. Tverd. Tela (St. Petersburg) 40, 1494–1497 (August 1998)     

Pulsed fm NMR/on studies of single crystal54MnNi and125SbFe

Precision field shift studies of B_appl-8T using pulsed FM NMR/ON along hard directions (eg <100>⁵⁴MnNi, <112>¹²⁵SbFe and the easy axis (<111>⁵⁴MnNi, <100>¹²⁵SbFe) yield no measurable Knight shifts within the uncertainty allowed by current accuracy of the nuclear moments. This is in striking contrast to earlier measurements with the same apparatus that yielded K=+1.5 (4)% for⁶⁰CoFe. The modulation frequency dependences of the pulsed FMNMR/ON signals are investigated for a variety of rf parameters and compared with model predictions.     

NMR-on study of60Co in Fe−Si single crystal

NMR-ON measurements have been performed for⁶⁰CoFe−Si. The samples of Fe−Si (6 at. %) single crystal dish with the (110) surface were used. Rosonance measurements have been carried out with the magnetization direction along the <100> and the <111> axes. Five prominent resonances were found at 166, 162, 151, 147 and 135 Mllz. The resonance at 166 Mllz has been known to be due to⁶⁰Co without neighbourt Si atoms. The resonances at 151 and 135 Mllz are due to⁶⁰Co nuclei with one and two Si atom(s) in the 1st neighbour site, respectively. The resonances at 162 and 147 Mllz would be due to the contribution of the 3rd <111> and the <100> magnetization axes, the differences of the resonance width have been qualitatively explained using the dipolar field.     

Mössbauer investigation of iron disulphide (marcasite)

Mössbauer absorption of Fe⁵⁷ in naturally occuring and synthetic crystals of FeS₂, marcasite, has been studied in the ideal absorber thickness in the transmission geometry from a Co⁵⁷/Rh and Co⁵⁷/LiNbO₃ source. The recoilles fraction at room temperature, 298 K, has been obtained to be 0.2 and the mean square displacement < r² > was found to be 13×10^-19 cm² at room temperature, 298 K. Because of the small size of marcasite crystals it has not been possible to record good spectra of the monocrystals as a function of the orientation of the incident gamma rays from Co⁵⁷/LiNbO₃ source.     

Mössbauer study of spin-spin relaxation of Fe3+ ions in the presence of other paramagnetic ions

Mössbauer spectroscopy has been used to study the influence of paramagnetic ions, viz. Cu²⁺, Cr³⁺, Co²⁺ Mn²⁺, Gd³⁺ and Dy³⁺ on the spin-spin relaxation time of Fe³⁺ ions in amorphous frozen aqueous solutions. It is found that these ions shorten the relaxation time, but the effect is much smaller than suggested earlier on the basis of measurements of relaxation of Fe³⁺ in an α-Al₂O₃ matrix. It is also found that S-state ions have a greater influence on the relaxation time than other paramagnetic ions. The spectra obtained in presence of S-state impurity ions could only be fitted by allowing the individual transition probabilities to vary independently.     

Investigations of EPR parameters and impurity structure for Co<Superscript>2+</Superscript> in Ca(OH)<Subscript>2</Subscript> crystal

The electron paramagnetic resonance (EPR) parameters (g _‖ andg _⊥ factors and hyperfine structure constantsA _‖,A _⊥) for Co²⁺ in Ca(OH)₂ are studied from the second-order perturbation formulas on the basis of the cluster approach. In these formulas, the contributions to EPR parameters from the state interactions and covalency effects are considered and the parameters related to both effects are obtained from the optical spectra and impurity structure of the studied system. From the study, it is found that the β angle between the metal-ligand bond and the C₃ axis changes from 61° in a pure crystal to 53.68(26)° in the impurity center of a Co²⁺-doped Ca(OH)₂ crystal because of the impurity-induced local lattice relaxation. The reduction of the angle β in the impurity center is also supported by the result obtained by analyzing the EPR zero-field splitting for Mn²⁺ in the same Ca(OH)₂ crystal. The EPR parameters of Ca(OH)₂:Co²⁺ are also reasonably explained by considering the suitable local lattice relaxation.     

Mössbauer and infrared spectroscopic studies of novel mixed valence states in cobaltous ferrocyanides and ferricyanides

Novel mixed valence states have been obtained by the treatment of cobaltous ferrocyanides (Co⁺²Fe^II) and ferricyanides (Co⁺²Fe^III) in an ozone flow. The CN stretching bands occur at 2085 cm^–1 for Co⁺²Fe^II and at 2160 cm^–1 for Co⁺²Fe^III. After the ozonization process of Co⁺²Fe^II, an intense band approximately at 2125 cm^–1 is detected. This intermediate band must correspond to a mixed valence state of the type: Fe^II–CN–Co²⁺–NC–Fe^III Mössbauer spectra recorded in situ during the ozonization of Co⁺²Fe^II show the presence of two components: a doublet with isomer shift and quadrupole splitting values close to the cobalti ferricyanide and a very broad line for the mixed valence state. From the Mössbauer and infrared spectra of the aged samples of the Co⁺²Fe^II after ozonization, a relaxation process to the initial state of the samples is observed but the mixed valence state is stable.     

The crystal field and exchange coupling in iron group metal carbonates

The wave functions of Co²⁺ and Fe²⁺ ions near the ground state in the CaCO_3? type lattice have been calculated from EPR data in the Abragam-Pryce approximation. The orbital angular momentum contributions to the anisotropic and antisymmetric parts of exchange coupling are determined assuming that this interaction between the magnetic ions occurring in nonequivalent positions is isotropic with respect to spin orientations. It is shown that, in the given approximation, the exchange coupling components in the basal plane for such Fe²⁺?Fe²⁺ and Co²⁺? Fe²⁺ ion pairs are missing. This fact explains the uniaxial antiferromagnetic ordering in FeCO₃ and the presence of a low-lying oscillation branch for Fe²⁺ impurity ions in antiferromagnetic CoCO₃. The EPR spectra of exchange-coupled Co²⁺?Co²⁺, Fe²⁺?Fe²⁺, and Co²⁺?Fe²⁺ pairs occupying nonequivalent positions have been calculated and their parameters have been numerically estimated.     

Synthesis,structural and complex impedance spectroscopy studies of Ni0.4Co0.4Mg0.2Fe2O4 spinel ferrite

Spinel ferrite having composition Ni_0.4Co_0.4Mg_0.2Fe₂O₄ was prepared by sol-gel method. X-ray diffraction result indicates that the ferrite sample has a cubic spinel type structure. FT-IR showed two absorption bands (ν₁ and ν₂) that are attributed to the stretching vibration of tetrahedral and octahedral sites. Complex impedance properties have been investigated in 200–420 K temperature range with varying frequency between 40 and 10⁷ Hz. Frequency and temperature dependency of imaginary part of permittivity (?″) and dielectric loss (tanδ) has been discussed in terms of hopping of charge carriers between Fe²⁺ and Fe³⁺ ions. Activation energy has been estimated from both temperature dependency of dc conductivity and relaxation time data, which indicates that the relaxation process and conductivity have the same origin. Nyquist plots of impedance show semicircle arcs for sample and an electrical equivalent circuit has been proposed to explain the impedance results.     

ENDOR-investigations of iron doped GaAs and GaP

From ENDOR-measurements of GaAs:Fe³⁺ the symmetries and hyperfine parameters of three groups of neighbour nuclei have been obtained. The values of ||², r ^–3 and of the electric field gradients at the sites of the neighbour nuclei have been determined for GaAs:Fe³⁺ and GaP:Fe³⁺. The comparison of our results with those of NMR-investigations indicates the Fe³⁺-ion to be an interstitial impurity surrounded by four groupV atoms in a tetrahedral arrangement.     

Magnetic properties of Co-ferrite-doped hydroxyapatite nanoparticles having a core/shell structure

The magnetic properties of Co-ferrite-doped hydroxyapatite (HAP) nanoparticles of composition Ca_10−3xFe_2xCo_x(PO₄)₆(OH)₂ (where x=0, 0.1, 0.2, 0.3, 0.4 and 0.5% mole) are studied. Transmission electron microscope micrograms show that the 90 nm size nanoparticles annealed at 1250 °C have a core/shell structure. Their electron diffraction patterns show that the shell is composed of the hydroxyapatite and the core is composed of the Co-ferrite, CoFe₂O₄. Electron spin resonance measurements indicate that the Co²⁺ ions are being substituted into the Ca(1) sites in HAP lattice. X-ray diffraction studies show the formation of impurity phases as higher amounts of the Fe³⁺/Co²⁺ ions which are substituted into the HAP host matrix. The presence of two sextets (one for the A-site Fe³⁺ and the other for the B-site Fe³⁺) in the Mössbauer spectrum for all the doped samples clearly indicates that the CoFe₂O₄.cores are in the ferromagnetic state. Evidence of the impurity phases is seen in the appearance of doublet patterns in the Mössbauer spectrums for the heavier-doped (x=0.4 and 0.5) specimens. The decrease in the saturation magnetizations and other magnetic properties of the nanoparticles at the higher doping levels is consistent with some of the Fe³⁺ and Co²⁺ which being used to form the CoO and Fe₂O₃ impurity phase seen in the XRD patterns.     

Electrical conductivity of pulverulent iron-cobalt molybdates Co1−xFexMoO4: evidence for hopping conduction

Different mixed iron-cobalt molybdates Co_1−xFe_xMoO₄ (0 < x ≤ 1) were prepared by means of a ceramic process. The influence of the isostructural substitution of Co²⁺ by Fe²⁺ and Fe³⁺ on the electrical conductivity of CoMoO₄ was studied in the temperature range (50–600°C). The results show that the iron substitution increases the electrical conductivity and changes the conduction mechanism of CoMoO₄. From a band conduction mechanism with an activation energy higher than 0.8 eV the conduction mode transforms into a hopping mechanism between the Fe²⁺ and Fe³⁺ ions in the octahedrally coordinated divalent cation sublattice. The activation energy is lower (0.4 eV) and does not alter around the polymorphic transition temperature. Owing to careful oxidations of the samples into cation deficient phases it was shown that the conductivity is proportional to the [Fe²⁺]/[Fe³⁺] ratio. These mild oxidations confirm the hopping mechanism. The presence of Co²⁺/Co³⁺ pairs has a minor contribution to the overall conductivity process. Paper presented at the 2nd Euroconference, Funchal, Madeira, Portugal, 10 – 16 Sept. 1995     

Positive muons in single crystal sodium fluoride: A simple spin system

We report on the relaxation of positive muons (μ ⁺) stopped in a single crystal of sodium fluoride at 15±0.2K. At this temperature theμ ⁺ is believed to be static, and the observed time dependence of theμ ⁺ spin polarization is seen to deviate from the familiar Kubo-Toyabe form at late times. Specifically these data exhibit oscillations in the long time tail, which are attributed to hyperfine transitions between theμ ⁺ and neighboring spins. Quantitative comparison of these data to exact quantum mechanical calculations indicates that most of the time dependence can be explained by considering only interactions with the first shell (i.e., two fluorine and two sodium nuclei), and suggests strongly that the muon occupies a site along the <110> axis, midway between two fluorine nuclei.     

Hyperfine field relaxations in the Fe−Co−B amorphous alloys

A systematic increase of the average hyperfine magnetic induction at⁵⁷Fe measured at room temperature during interrupted isothermal annealing was found to be inherent to the irreversible relaxation processes in the Fe−B based soft magnetic alloys at moderately elevated temperatures. Assuming superimposed asymptotic exponential field vs. time dependences. several processes can be distinguished, their relaxation times determined and from the Arrhenius-like log τ vs. 1/T plots average activation enthalpies estimated. Results on the Fe₇₀Co₁₀B₂₀ and Fe_85−x Co _x B₁₅ (x=17, 19 and 21 at. % Co) amorphous ribbons between 100 and 200°C are compared and discussed in terms of possible stress relief, free volume annihilation and short range ordering mechanisms.     

Anisotropy of the Hyperfine Field on 57Fe in Pure and Substituted Yttrium Iron Garnet

Using NMR, anisotropy of the hyperfine field on ⁵⁷Fe nuclei was measured in Y₃Fe₅O₁₂ with nonmagnetic, trivalent substitutions on d (Ga³⁺) and a (Y³⁺ - yttrium antisite defect) sites which are occupied by ferric ions in a pure, stoichiometric system. The measurements were performed by the spin-echo method at liquid helium temperature. Thorough analysis of the hyperfine field anisotropy on the ⁵⁷Fe nuclei in an ideal environment and in an environment where one of Fe³⁺ nearest cation neighbours is replaced by the nonmagnetic impurity is given. When the Fe³⁺ on which the NMR is measured is on the a site and the impurity is Ga³⁺(d), the results may be interpreted in terms of the superposition model. On the other hand, the results for Fe³⁺ on the d site and Y³⁺(a) are in clear disagreement with the predictions of this model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Recent experiments with transient NMRON

High resolution magnetic and electric hyperfine interaction distributions are determined for single crystal⁵⁴MnNi with diffused impurity concentrations of 0.14 and 0.40 at%, and for a 0.1 µm layer of⁶⁰CoFe co-plated onto single crystal Fe. High quality gamma detected nuclear spin echoes recorded for the⁶⁰CoFe specimen demonstrate that a reduced influence of the RF skin effect far outweighs any magnetic hyperfine field interaction broadening associated with the co-plating process.