Phase modulation of recoilless gamma radiation by nuclear zeeman effect

The use of an alternating magnetic field for phase modulation of recoilless gamma radiation is studied. The measurements were performed with the Mössbauer resonances of Fe-57 and Zn-67. Novel time-domain data are presented from experiments on the nuclear Zeeman effect at high modulation frequencies.     

The effect of radiofrequency modulation of 57Fe hyperfine interaction by rotating magnetic field

The effect of ⁵⁷Fe hyperfine interaction radiofrequency (rf) modulation by external rotating magnetic field was studied in thin Permalloy foil by means of Mössbauer spectroscopy. The rf effect was investigated as a function of intensity for several rf field frequencies. The experiments show that the external rotating rf field causes considerable changes in the hyperfine pattern. The obtained spectra are in disagreement with those obtained by Perlow [Phys. Rev. 172 (1968) 319]. They also are inconsistent with magnetostriction hypothesis. Proceeding from the Mössbauer spectrum analysis one may conclude that the magnetization of investigated foil changes its direction in a complex manner. However, the undertaken experiments show that the essential number of Mössbauer nuclei experience the rotating magnetic field influence.     

Narrow spectral lines of modulation harmonics of gamma radiation in unordered magnetic materials

The spectra of gamma-resonance lines excited using the amplitude modulation of the pumping field of the Mössbauer effect were studied in unordered magnetic materials with a strong spread of magnetic hyperfine fields at nuclei of Mössbauer isotopes. The analysis demonstrates that the spectra of modulation harmonics consist of narrow lines, and their form and intensity are determined by the contribution of magnetic local hyperfine fields, which are excited at a pumping frequency. The inversion of intensities for odd harmonics with the change in sign of detuning of the pumping frequency relative to the average Larmor frequency of the Mössbauer nucleus was noted. The application of this method to the spectral analysis of inhomogeneous magnetic materials is discussed.     

Time modulation of recoilless nuclear resonance fluorescence

The time dependence of the resonance fluorescence of nuclei excited by the spontaneous radiation of identical nuclei is studied. Particular attention is paid to the influence of the periodic overlap of the source on the form of the scattered radiation line and it is shown that the line may even split. A mechanical filter permitting measurement of pure radiation scattering is calculated.     

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.     

High-frequency modulation of Ar-ion laser radiation due to stable two-mode oscillation

Both the spectral and temporal behaviour of argon-ion laser radiation is observed near threshold. It is shown, that in certain ranges of excitation a stable two-mode operation in the argon-ion laser leads to a modulation of the laser radiation at frequencies between 500 MHz and 900 MHz with a very high degree of modulation. Moreover, the first observation of a hysteresis effect is reported.     

Temporal optics of resonant transmission of gamma rays through57Fe

The development in time of the transmission through⁵⁷Fe of a broad Lorenztian radiation is calculated numerically. Examples are given for=0 and/2, for the magnetic hyperfine case, and for=/2 for pure quadrupole splitting.     

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.     

Investigation of magnetic properties in 57Fe/Al multilayers

Fe/Al multilayer thin films prepared by ion beam sputtering, with an overall atomic concentration ratio of Fe/Al = 1:2 have been studied by x-ray diffraction spectroscopy (XRD), X-ray reflectivity (XRR) and D.C. Magnetization. These studies show the formation of Fe–Al intermetallic layers. Two magnetic regions and transition temperatures of 473 and 533 K are evident from magnetization studies. Conversion Electron Mössbauer Spectroscopy (CEMS) shows formation of off-stoichiometric Fe₃Al like phase and phases consisting of pure Fe and Fe-rich extended Fe–Al solutions.     

Time dependence of the source recoilless fraction for a cobalt-57 in rhodium source

The effect of source resonance self absorption (SRSA) has been measured for⁵⁷Co in a rhodium matrix by determining the ratio of the intensity of the 14.4 keV gamma transition to that of the associated 22 keV rhodium x-ray, directly counting both intensities from a stationary source. A ratio, ρ, is defined asf _s/f _so, wheref _s is the recoil-free fraction with the SRSA effect present, andf _so is in the absence of SRSA effects [1]. The value of ρ is to within an additive constant, proportional to the gamma over the X-ray intensities. In the absence of SRSA it is expected that both the 14.4 keV and the 22 keV decay rate would be identical to that of the⁵⁷Co parent, and ρ would be unity and time independent. But because of SRSA a fraction of the 14.4 keV gammas are reabsorbed by⁵⁷Fe, which is increasing as a result of⁵⁷Co decay, and hence, ρ decreases with time. By direct measurement our results confirm that the effect of SRSA can be appreciable even over short time intervals, and we find that in many cases SRSA may have a pronounced effect on Mössbauer experiments, affecting both the line width andf-value, and contrary to general practice in the field, needs to be evaluated in high intensity source experiments.     

Time-dependent absorption of gamma-radiation under high-frequency magnetic excitation of 57Fe

Summary We have studied the time dependence of M?ssbauer absorption in a magnetically soft sample of Fe₁₈Ni₈₂ permalloy under the influence of a radio-frequency (r.f.) magnetic field. Absorption was measured as a function of the r.f. field phase at specific gamma-ray energies. We have shown that measuring the time dependence of absorption makes it possible to differentiate between purely magnetic and magnetoacoustic modulation mechanisms even in cases where clear distinction is not visible in conventional energy domain spectra. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Electric field gradient sign determination in low magnetic fields perpendicular to the gamma rays in57Fe Mössbauer spectroscopy

The determination, by Mössbauer⁵⁷Fe spectroscopy, of the sign and the asymmetry parameter of the electric field gradient (EFG), in a non-magnetic powder sample, needs the use of an external magnetic field. Usually a “parallel-magnetic-field-to-the-gamma-rays” configuration is employed. In the present work, theoretically generated spectra in both configurations, parallel an perpendicular, are compared. The result is that the spectra in the perpendicular configuration show always a larger asymmetry for the same fields intensitics. In consequence, the sign determination of the EFG, is possible with magnetic fields significantly smaller in the perpendicular case than in the parallel one. Furthermore, the practical consequences are important, because EFG studies with conventional electromagnets, allowing only fields lower than 25 kGauss are feasible. As an example. Mössbauer experimental results obtained at room temperature for natural pyrite (FeS₂) in perpendicular fields of 15 and 20 kGauss, created by a conventional electromagnet, are shown here.     

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.     

High-frequency modulation of segmented-contact semiconductor lasers

Simulations of the high-frequency modulation characteristics of semiconductor lasers with segmented contacts predict a pronounced resonance-like modulation response that is beyond five times the original cut-off modulation frequency. Based on an effective multi-mode Maxwell–Bloch model, the simulations reveal the underlying phenomena. They show that the high-frequency response is a direct consequence of the particular spatio-temporal and multi-mode dynamics induced by the segmented-contact laser geometry.     

Unusual preflare modulation of microwave radiation in coronal magnetic loops

In the analysis of low-frequency modulation of the microwave radiation of coronal magnetic loops, we detected modulation whose frequency increases from two to three times just before the beginning of the flare and then decreases to the initial values as the flare proceeds. It is shown that such an effect can be evidence for a sharp increase in the loop current just before the flare process, which leads to the plasma heating and increased energy release. The flute instability can serve as a possible reason for the sharp increase in current.