Hyperfine fields in dilute Cr-Rh systems

The hyperfine fields on rhodium nuclei in a chromium matrix have been investigated for rhodium concentrations ranging from 39 to 1000 at. p.p.m. For dilute alloys the hyperfine fields measured at the maximum of the spin density wave of chromium areH _o=28.9±0.4 kOe at 308 K just below the Néel temperature andH _o=45.2±1.0 kOe at 272.6 K. It is shown, in a simple model, that the hyperfine field is proportional to the local magnetization of chromium. Concentration effects have been found which prove the existence of long-range interactions between the impurities in the chromium matrix.     

Hyperfine field and isomer shift in ferromagnetic Fe−Cr alloys

The valence contact spin and charge densities at Fe sites in ferromagnetic Fe−Cr alloys are calculated using the discrete variational method. The hyperfine field at Fe nucleus is expressed as a linear sum of a core term, that is proportional to the local 3d moment, and a valence term, which is proportional to the valence spin density. The dependence of the hyperfine field, the contact charge density and the 4s magnetic moment on the number and orientation of chromium atoms in the first and second shells is studied. Comparison to experimental data is made.     

The magnetic properties across the martensitic transition in the Co38Ni34Al28 alloy

The magnetic properties of the Co₃₈Ni₃₄Al₂₈ alloy have been studied. The alloy exhibits a first order austenite-martensite phase transition in the temperature region between 155 and 247 K. A strain of 0.07% is produced across this phase transition. The Arrott plots obtained from the isothermal magnetic field dependence of magnetization indicate the presence of spontaneous magnetization both in the austenite and martensite phases, confirming the ferromagnetic character of the alloy up to room temperature. The temperature dependence of the high field magnetization indicates the presence of spin wave excitations, spin wave excitation gap and spin wave-spin wave interactions in the martensite phase. The magnetic anisotropy energy constant for the Co₃₈Ni₃₄Al₂₈ alloy is estimated both with the help of the standard law of approach to saturation of magnetization, and also from the field dependence of magnetization using the field for technical saturation of magnetization. The temperature dependences of these energy terms are compared. The estimated values of the magnetic anisotropy constant seem to be in agreement with the magnitude of the spin wave excitation gap estimated from the temperature dependence of high field magnetization.     

Short-range magnetic order in a spin density wave in Cr-based multilayers

A mechanism of the formation of the short antiferromagnetic order with a spin density wave (SDW) in the vicinity of the interfaces in the Fe/Cr type multilayers is proposed. The main reason behind the emergence of magnetic ordering with SDWs is the redistribution of charge (and, hence, spin) density in the vicinity of Fe/Cr interfaces, which leads to the paramagnetic phase instability at a temperature considerably higher than the Néel temperature in chromium. The Ginzburg-Landau expansion for the free energy of the system is used for determining the inhomogeneous collinear structures of CDWs and for constructing the phase diagram (the dependence of the transition temperature on the thickness of the antiferromagnetic interlayer). The obtained results are used for discussing the experimental data on neutron scattering and tunnel microscopy.     

Pulsed EPR and ENDOR investigation of hydrogen atoms in silsesquioxane cages

High sensitivity and spectral resolution provided by pulsed electron paramagnetic resonance and electron nuclear double resonance techniques at high Larmor frequencies open the way for a study of atoms in chemical traps. As an example we studied deuteron atoms encased in silsesquioxane cages to probe the cage symmetry as function of temperature. An analysis of the temperature dependence showed that the system undergoes a structural phase transition near 100 K. At this temperature the character of distortion of the ideal cubic symmetry changes from oblate to prolate (or vice versa). With quantum chemical methods, a model for cage escape of the encased atom could be derived. The calculated escape barrier of 0.9 eV is close to the experimental value derived by thermal release experiments. Although the encased deuterium atom exhibits an isotropic hyperfine coupling constant nearly identical with the free atom value, a spin population analysis revealed that approximately 10% of the spin density is transferred to the cage. We therefore conclude that confinement of the hydrogen atom leads to a compression of its wave function compensating the decrease of spin density. In this respect the system falls somewhat short of the properties of an ideal cage, being defined by well decoupled atomic and molecular wave functions.     

Nucleogenic iron studied by emission Mössbauer spectroscopy in Co-metal

Results obtained by means of the emission Mössbauer spectroscopy in metallic cobalt are reported. The Mössbauer line of the 14.4-keV energy connecting the first excited state of the stable ⁵⁷Fe nucleus with its ground state was used. Radioactive ⁵⁷Co was used as the precursor of the above nuclear state. It was dissolved in the natural metallic cobalt with the concentration of about 40 at. ppm including nucleogenic iron generated during decay of the cobalt precursor. Mössbauer spectra were collected in the temperature range between room temperature (RT) and 1075 K with the sample kept under vacuum. A transition from the low temperature hexagonal phase to the face centered cubic high temperature phase at about 690 K has no influence on the iron magnetic hyperfine field arising due to the ferromagnetic ordering of the host. On the other hand, the electron charge density on the iron nucleus has some relatively narrow maximum in the vicinity of the transition temperature. There is some discontinuity in the recoilless fraction as well indicating that the high temperature cubic phase provides somewhat stronger bonds for the isolated iron impurity. The anharmonic behavior of the lattice vibrations could be seen in the cubic phase well above the transition point. No measurable electric quadrupole interaction was found in the hexagonal phase. The evolution of the magnetic hyperfine field with the temperature is reasonably described by the spin wave formalism provided strong magnon-magnon scattering is allowed for. On the other hand, charge density on the iron nucleus follows thermal expansion except some singularity in the vicinity of the transition point.     

Spin density wave in oxypnictide superconductors in a three-band model

The spin density wave and its temperature dependence in oxypnictide are studied in a three-band model. The spin susceptibilities with various interactions are calculated in the random phase approximation (PPA). It is found that the spin susceptibility peaks around the M point show a spin density wave (SDW) with momentum (0, π) and a clear stripe-like spin configuration. The intra-band Coulomb repulsion enhances remarkably the SDW but the Hund’s coupling weakens it. It is shown that a new resonance appears at higher temperatures at the Γ point indicating the formation of a paramagnetic phase. There is a clear transition from the SDW phase to the paramagnetic phase.     

Nonequilibrium Phase Transition in Spin-S Ising Ferromagnet Driven by Propagating and Standing Magnetic Field Wave

The dynamical response of spin-S(S=1, 3/2, 2, 3) Ising ferromagnet to the plane propagating wave, standing magnetic field wave and uniformly oscillating field with constant frequency are studied separately in two dimensions by extensive Monte Carlo simulation. Depending upon the strength of the magnetic field and the value of the spin state of the Ising spin lattice two different dynamical phases are observed. For a fixed value of S and the amplitude of the propagating magnetic field wave the system undergoes a dynamical phase transition from propagating phase to pinned phase as the temperature of the system is cooled down. Similarly in case with standing magnetic wave the system undergoes dynamical phase transition from high temperature phase where spins oscillate coherently in alternate bands of half wavelength of the standing magnetic wave to the low temperature pinned or spin frozen phase. For a fixed value of the amplitude of magnetic field oscillation the transition temperature is observed to decrease to a limiting value as the value of spin S is increased. The time averaged magnetisation over a full cycle of the magnetic field oscillation plays the role of the dynamic order parameter. A comprehensive phase boundary is drawn in the plane of magnetic field amplitude and dynamic transition temperature. It is found that the phase boundary shrinks inwards for high value of spin state S.Also in the low temperature(and high field) region the phase boundaries are closely spaced.     

Temperature investigations of the spatial spin-modulated structure of multiferroic BiFeO<Subscript>3</Subscript> by means of Mössbauer spectroscopy

The results from ⁵⁷Fe Mössbauer spectroscopic studies of multiferroic BiFeO₃ in a range of tem-peratures including that of the magnetic phase transition are presented. The Mössbauer spectra are processed and analyzed by reconstructing the hyperfine magnetic field distributions and interpreting the spectra with a cycloid-type spatial spin-modulated structure model. The temperature dependences of the hyperfine spectrum parameters (the Mössbauer line shift, the quadrupole shift, and the isotropic and anisotropic contributions to the hyperfine magnetic field) are obtained, along with the anharmonicity parameter of an incommensurate spin wave.     

Study of combined magnetic and electric hyperfine interactions for cadmium in Dysprosium by time differential perturbed angular correlations

The combined magnetic and electric hyperfine interaction at the site of a¹¹¹Cd impurity in magnetically ordered Dysprosium has been investigated as a function of temperature by time differential perturbed angular correlation measurements. Three different phases have been found in metallic Dy with transition temperatures of 85 and 179 °K in agreement with the results of bulk material measurements. In the paramagnetic phase above 179 °K a pure electric quadrupole interaction has been observed. The various contributions to the electric fieldgradient are analyzed and it is shown, that the dominant contribution comes from the conduction electrons. In the ferromagnetic phase which extends from 0 to 85 °K the magnetic hyperfine field at the site of¹¹¹Cd has the same temperature dependence as the spontaneous magnetization. The value of the hyperfine field at 4.2 °K is ¦H _eff¦=(221 ± 4) kG. At 85 °K a transition to the antiferromagnetic phase of Dy occurs, which shows a hysteresis of the transition temperature. In the antiferromagnetic phase the temperature dependence of the hyperfine field deviates considerably from the magnetization curve. It is suggested that this deviation might be due to a temperature dependence of thes-f exchange interaction.     

Driven spin wave modes in XY ferromagnet: non-equilibrium phase transition

The dynamical responses of XY ferromagnet driven by linearly polarised propagating and standing magnetic field wave have been studied by Monte Carlo simulation in three dimensions. In the case of propagating magnetic field wave (with specified amplitude, frequency and the wavelength), the low temperature dynamical mode is a propagating spin wave and the system becomes structureless (or random) in the high temperature. A dynamical symmetry breaking phase transition is observed at a finite (non-zero) temperature. This symmetry breaking is confirmed by studying the statistical distribution of the angle of the spin vector. The dynamic non-equilibrium transition temperature was found to decrease as the amplitude of the propagating magnetic field wave increased. A comprehensive phase boundary is drawn in the plane formed by temperature and amplitude of propagating field wave. The phase boundary was observed to shrink (in the low temperature side) for longer wavelength of the propagating magnetic wave. In the case of standing magnetic field wave, the low temperature excitation is a standing spin wave which becomes structureless (or random) in the high temperature. Here also, like the case of propagating magnetic wave, a dynamical symmetry breaking non-equilibrium phase transition was observed. A comprehensive phase boundary was drawn. Unlike the case of propagating magnetic wave, the phase boundary does not show any systematic variation with the wavelength of the standing magnetic field wave. In the limit of vanishingly small amplitude of the field, the phase boundaries approach the recent Monte Carlo estimate of equilibrium transition temperature.     

Spin excitations in a K0.84Fe1.99Se2 superconductor as studied by Mssbauer spectroscopy

Mssbauer spectroscopy was used to probe the site-specific information of a K 0.84 Fe 1.99 Se 2 superconductor. A spin excitation gap,△ E ≈ 5.5 meV, is observed by analyzing the temperature dependence of the hyperfine magnetic field (HMF) at the iron site within the spin wave theory. Using the simple model suggested in the literature, the temperature dependence of the HMF is well reproduced, suggesting that, below room temperature, the alkali metal intercalated iron–selenide superconductors can be regarded as ferromagnetically coupled spin blocks that interact with each other antiferromagnetically to form the observed checkerboard-like magnetic structure.     

Mössbauer effect study of charge and spin transfer in Fe-Cr

The influence of temperature and time of annealing on hyperfine fields and isomer shifts has been studied for a range of Fe–Cr alloys containing 1–45 at% Cr. It has been revealed that up to 15at% Cr neither time nor temperature of annealing practically does affect the hyperfine parameters. For more concentrated samples, however, both temperature and time of annealing are important. In particular, the Mössbauer spectrum of Fe–45.5 at% Cr annealed at 700°C for 5 h was a single-line indicating that the sample was paramagnetic. The observed changes of the hyperfine fields and the isomer shifts have been interpreted in terms of a spin and charge trasfer, respectively.Strong linear correlations between the following quantities have been revealed: the hyperfine field H(0,0) and the isomer shift IS(0,0); the average hyperfine field $\text{H}$ and the average isomer shift $\text{IS}$; the average hyperfine field $\text{H}$ and the average number of Cr atoms in the first two coordination spheres, $\text{N}$. It has been calculated from the first two correlations that a) a change of polarization of itinerant s-like electrons of one electron is equivalent to a change of the hyperfine field of 1602 kOe, and b) on average, a unit change of s-like electron polarization is equivalent to 3277 kOe. The two constant are very close to theoretical estimations, which can be found in literature. Correlation between the hyperfine field and the isomer shift led to a conclusion that the substitution of Fe atoms by Cr ones decreases the density of spin-up electrons on average by 0.026 electrons per one Cr atom in a unit cell. These electrons are most likely trapped by Cr atoms, because the hyperfine field at neighbouring Fe nuclei decreases and the density of charge at those nuclei increases at the rate of 0.029 electrons per one Cr atom in a unit cell.Based on the results obtained, a formula describing the dependence of the average hyperfine field on Cr contents and on the heat treatment has been postulated for Fe–Cr alloys.     

Transferred hyperfine field of Rb2CoCl4 single crystals in the ferroelectric-incommensurate-normal phase by Rb NMR

The molecular susceptibility and paramagnetic shift of Rb₂CoCl₄ single crystals grown using the slow evaporation method were measured, and from these experimental results we obtained the transferred hyperfine interaction due to the transfer of spin density from Co²⁺ ions to Rb⁺ ions. The transferred hyperfine field was obtained for the ferroelectric, incommensurate, and normal phases. In the case of Rb(I), the transferred hyperfine interaction decreases with increasing temperature in the incommensurate phase, and increases with increasing temperature in the normal phase. The value of H_hf in the incommensurate and normal phases increases abruptly with increasing temperature in the case of Rb(II). These results indicate that the effects due to the transfer of spin density from Co²⁺ ions to the Rb(I) and Rb(II) ions are large above T_i. In particular, the effect due to the transfer of spin density to Rb(II) ions in the normal phase is very large; the variations with temperature of the transferred hyperfine interactions of the Rb(I) and Rb(II) nuclei are more or less continuous in T_c1 and T_i, and are not affected by the ferroelectric-incommensurate-normal phase transitions.     

Gas,liquid and solid state studies of muon spin relaxation in organic radicals

A distinctive field dependence of longitudinal muon spin relaxation in acetone liquid and vapour suggests that modulation of the isotropic hyperfine coupling of the (CH₃)₂COMu radical is the dominant relaxation mechanism. The temperature dependent correlation time extracted from the data then corresponds to the lifetime of the states of internal libration of the molecule. The variation of relaxation rate may also be followed into the solid phase, peaking at the freezing transition.     

Study of condensed nitrogen by μSR method

The temperature dependences of parameters of the muon spin relaxation in liquid and crystalline nitrogen have been studied. It has been established that in condensed nitrogen there takes place a fast depolarization of muons. An anomalous behaviour of the amplitude and phase of muon precession is found in the vicinity of the orientation phase transition in solid nitrogen. It has been shown that muon spin relaxation parameters in nitrogen do not change at reduction of the oxygen impurity content from 0.7·10⁻⁴ to 10⁻⁶. The fast depolarization of muons in condensed nitrogen is apparently due to the formation of muonium atoms. To explain the phenomena observed, a model of the muonium chemical reaction is proposed. The initial phase of the muon precession has been measured as a function of the perpendicular magnetic field to determine the state of short-lived muonium in nitrogen. It has been determined that muonium in nitrogen is in an excited state. Consideration of the nuclear hyperfine interaction of muonium in condensed nitrogen makes it possible to give a qualitative explanation for the temperature dependence of the initial amplitude of the muon precession.     

Spin excitations in a K0.84Fe1.99Se2 superconductor as studied by Mössbauer spectroscopy

Mössbauer spectroscopy was used to probe the site-specific information of a K_0.84Fe_1.99Se₂ superconductor. A spin excitation gap, ΔE ≈5.5 meV, is observed by analyzing the temperature dependence of the hyperfine magnetic field (HMF) at the iron site within the spin wave theory. Using the simple model suggested in the literature, the temperature dependence of the HMF is well reproduced, suggesting that, below room temperature, the alkali metal intercalated iron-selenide superconductors can be regarded as ferromagnetically coupled spin blocks that interact with each other antiferromagnetically to form the observed checkerboard-like magnetic structure.     

Strain wave in pure chromium

New satellites were observed in a single crystal of pure chromium below the Néel point by X-ray diffraction at the positions apart by 2δ from the Bragg points. The observed anisotropy of the satellite intensity around the Bragg point and the temperature dependence of 2δ yield the conclusion that the lattice spacing in the direction of the wave vector of the spin density wave are modulated periodically and that the period is a half of that of the SDW. The experimental results coincide with the prediction made by a theory of the exchange striction in transition metal alloys.     

Linewidth Analysis of Spin Labels in Liquids: II. Experimental

This work demonstrates that homogeneous linewidths can be extracted from continuous wave electron paramagnetic resonance spectra and that they quantitatively agree with the predictions of existing relaxation theory. We suggest that relaxation theory can be used to predict experimental lineshapes provided that the simulations properly include sources of broadening. We have found that the rotational correlation times for spin labels in different percentages of glycerol/water mixtures are best modeled by a power law treatment for the viscosity, similar to that for translational diffusion. The translational diffusion coefficients themselves also have a power law dependence on the viscosity for glycerol/water mixtures. The linewidths were linearly dependent upon both the oxygen and the spin label concentration. The hyperfine splittings of all nuclei were observed to decrease linearly with increasing spin label concentration, completely at odds with existing theory which predicts a quadratic dependence upon concentration. The linear dependence was independent of hyperfine splitting until the magnitude of the hyperfine splitting was less than the homogeneous linewidth.     

Observation of muon level-crossing resonance in antiferromagnetic MnF2

We report the observation of μ⁺ Level-Crossing Resonances (LCR's) in the ordered phase of an antiferromagnetic material. Two LCR's were observed in MnF₂ as a function of longitudinal magnetic field in the temperature range between 10K and 65K. Both are attributed to a muon in an interstitial octahedral-like site. The low field resonance is attributed to a muon-nuclear spin flip-flop transition involving the two nearest neighbour¹⁹F nuclei. The high field resonance occurs when the applied field cancels the local hyperfine field on the muon. The positions and widths of the LCR's were seen to scale with the sublattice magnetization.