Mossbauer time window

K, where the electronic relaxation rate becomes fast with respect to the Mossbauer time window and the limit of fast relaxation applies. Spin spin interaction in microcrystals can also enhance the electronic relaxation rate and therefore some low-spin iron(in) compounds show quadmpole doublets even at 4.2 K. ... 

Fig. 2.36 Mixed valence biferrocene with R = Et shows temperature dependent electron fluctuation between the two iron centers. At low temperatures the fluctuation rate is comparatively slow (less than the reciprocal of the Mossbauer time window of ca. 100 ns) and the Mossbauer spectra show two subspectra indicative of localized ferrocene (grey) and ferrocinium (dark grey), respectively. At higher temperatures the fluctuation rate becomes so fast that the Mossbauer spectrum reflects a time-averaged delocalized species (black) which is neither ferrocene nor ferrocenium [66, 67]...

The value of Tq determines the temperature (volume) interval required to span the Mossbauer time window. For ferromagnetic particles, is normally of the order of Because this is not very... 

Question 4 Ultrasmall superparamagnetic particles can relax faster than the time window of Mossbauer spectroscopy even at 4.2 K and such ferric nanoparticles also show similar 5 and AEq values. 

Mossbauer spectroscopy was the distinct difiference in dynamical properties of the Dy + moments in cr- and am-DyAg with the fluctuation rate being at least one order of magnitude lower in the amorphous compoimd. The static limit (on the Mossbauer time scale) was reached in am-DyAg well above 7c- M-SR is a complementary technique to study spin dynamical properties with a quite different time window. It is usually possible to follow the temperature dependence of spin fluctuations deep inside the paramagnetic regime (see the examples discussed in sects. 4 and 5.3). 

Third, except for the methods employing nuclear reactions, any host material can be implanted with Mossbauer probes, regardless of their solubility limits. That is, measurements can be performed on all condensed matter. In addition, the dynamic behavior of single isolated atoms, such as atomic jumping, thermal stability, and defect formation and recovery, can be investigated by applying an appropriate time window. In situ characterization by in-beam Mossbauer... 

Mossbauer spectroscopy, when the fluctuation frequency of the magnetic moments matches the time window of the Mossbauer effect. This means that the temperature dependence of the linewidth in the paramagnetic states of... 

The techniques currently used to study the superparamagnetic relaxation are dc susceptibility is not well defined, estimated to be around 100 s, but it depends on the type of magnetometer and on the measuring procedure), ac susceptiblity (t , = 10 -10 s for experiments at very low frequencies t = 10 -10 s for classical experiments t = 10 —l0 s for measurements at very high frequencies, very difficult to realize, so far), Mossbauer spectroscopy (time window, 10 -10 s for Fe), ferromagnetic resonance (t , = 10 s), and neutron diffraction (time window, 10 -10 s, depending on the type of experiments). 

Mossbauer Spectral Analysis and Analog Measurements. Mossbauer spectra were obtained in the temperature range between 200 and 270 K and in two different energy windows (14.4 and 6.4 keV), which provide depth selective information about a sample [346]. To compensate for low counting statistics due to limited integration time, all available spectra were summed for the integrations on the undisturbed and brushed surface, respectively. In addition to kamacite (a-(Fe,Ni)) ( 85%) and small amounts of ferric oxide (see Fig. 8.38), all spectra exhibit features indicative for an additional mineral phase. Based on analog measurements... 

F.6.4.3. Modeling of Experimental Spectra. At present, the model combining the two-state relaxation model [Eq. (F.6.8)] and Morup s model for the low-temperature limit [Eq. (F.6.12)] is the only amenable model for fitting experimental spectra, and also the only model for which the results can be compared with the results from other techniques. Such a model may be useful only if the observed spectra are little affected by relaxation phenomena, that is, when the temperature set of spectra can essentially be described in terms of two spectral components, a magnetic component and a superparamagnetic component, which coexist in varying amounts as a function of the temperature. This indeed implies that particles with a relaxation time in the Mossbauer window are in a relatively small amount at any temperature. 

A key factor in the design of electrochemical cells for in situ transmission Mossbauer measurements is to decrease the attenuation of the y-ray beam, so as to reduce the time required for spectra acquisition. This may be accomplished by selecting low-absorption materials for windows and electrode supports and by minimizing the amount of electrolyte in the beam path. Radiation in the keV range penetrates rather deeply into matter, and therefore small amounts of rather high-Z elements can be tolerated without seriously comprising the overall cell transmission. As a means of illustration, the half-absorption length for 14.4-keV X-rays in water is about 3.5 mm, which is approximately... 

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