Mossbauer CEMS Study

Room-temperature CEMS spectra of Fe/AI MLS (a) as-deposited (b) after VSM high-temperature M-T measurement (c) annealed at 623 K for 1 h (d) annealed at 723 K for 4h. (Reprinted with permission from Ref. 53. Copyright (2008) by the American Institute of Physics.) 

In this chapter, using samples of mainly three types of magnetic materials, namely, nanosized powders of ferrites, mechanically alloyed/milled Fe-Cr-AI intermetallics, and a Fe-AI multilayer system, it has been demonstrated how Mossbauer spectroscopy is a powerful tool to understand the bulk magnetic properties in such nanosized systems. This is mainly because of the extreme sensitivity of the Mossbauer probe atom to short-range effects that get modified on a 

I would like to acknowledge the financial support from the DST-FIST project of the Department of Physics, Mohanlal Sukhadia University, Udaipur, Rajasthan, India and also from the BRNS research project. I thank the UGC-DAE CSR, Indore, India, for use of many of their sophisticated facilities, especially TEM, CEMS, and low-temperature, in-field Mossbauer setup. 

Bhargava, N. Lakshmi, V. Sebastian, V.R. Reddy, K. Venugopalan, A Gupta, / Phys. D Appl. Phys. 2009, 42, 245003. 

A Poddar, C. Mazumdar, S. Banerjee, V.R. Reddy, A. Gupta, / Appl. Phys. 2010, 108, 034307. 

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Conversion electron Mossbauer spectroscopy (CEMS) measurements with back scattering geometry have the merit that spectra can be obtained from a sample with much less isotope content compared with transmission measurements. Another merit is that a sample, deposited on a thick substrate, could be measured, and that because of the limited escape depth of the conversion electrons, depth-selective surface studies are possible. The CEMS technique was found to be best applicable to specimens of 10-100 pg Au cm, i.e., about two orders of magnitudes thinner than required for measurements in transmission mode [443]. This way (1) very thin films of gold alloys, as well as laser- and in beam-modified surfaces in the submicrometers range of depth [443], and (2) metallic gold precipitates in implanted MgO crystals [444] were investigated. 

Some bioinorganic chemists may also be interested in the characterization of bioinorganic or inorganic complexes on surfaces or in thin films. Such studies are rare to date, but we refer to reviews by Shinjo and Przybylski, which deal with the characterization of iron monolayers and multilayers by CEMS. " During the last two decades Mossbauer spectroscopy with synchrotron radiation has also been developed, which gives information about the magnetization of thin iron films with a sensitivity down to the monolayer... 

The Mossbauer-effect experiment can also be applied to the study of surfaces in the variation known as conversion electron Mossbauer spectroscopy (CEMS). Here, what is monitored as a function of incident y-ray energy is not absorption, but the emission of electrons through a process of internal conversion (i.e., as a byproduct of the absorption of Mossbauer y rays). Since the conversion electrons can only escape from the surface layers of the solid, data are selectively acquired for the surface region, arising from the Mossbauer effect in the (most commonly iron) atoms of the surface layers. The monitoring of emitted electrons results in a mirror image of the usual absorption spectrum. Transmission and CEM spectra of vivianite [Ee3(P04)2-8H20] are illustrated in Fig. 2.49 (after Tricker et al., 1979]. 

Fe conversion electron Mossbauer spectroscopy (CEMS) is an interesting technique for studying reactions taking place on, or near, the surface of materials which contain iron in one form or another [9]. The decay processes following the resonant excitation of a Fe nucleus are shown in Fig. 8. The number of photons and electrons produced following the capture of 100 resonant 14.4 keV recoil-free y-ray photons is indicated in this figure. 

In Fe CEMS it is the 7.3 keV internally converted electrons which are normally detected but the 6.3 keV x-rays can also be used. The advantage of CEMS over normal Mossbauer studies is the nine-fold increse in the 7.3 keV electrons over the 14.4 keV y-rays. Secondly, these electrons are rapidly attenuated within the soUd which limits the probing depth of CEMS to 300 nm from the surface. For surface studies the back-scatter method is employed and one detects the conversion electrons back-scattered from the surface of the sample under study. However, it is possible by this technique to measure the y- and X-rays back-scattered following resonant absorption to increase probing depths to several microns. 

Chang and Wei (1990) used electrochemical and conversion-electron Mossbauer spectroscopy (CEMS) methods to study the corrosion behavior of electrodeposited Fe tZnj, wherej = 0.15-0.85,on 1010 steel immersed in a 0.1% NaCl solution at room temperature for 2 weeks. The corrosion rate measured by metal weight loss and electrochemical methods revealed that the Feo,25Zno,75 specimen was more corrosion resistant than the others. CEMS analysis showed that the corrosion product of the pure 1010 steel and the higher iron-containing Fe-Zn alloys on steel in 0.10% NaCl solution is (3-FeOOH. 

Finally, we should also mention the technique of Conversion Electron Mossbauer Spectroscopy (CEMS) [25], which uses the backscattered electrons from a surface produced by alternative decay processes of the excited nuclear state. This is particularly well suited to studying surfaces. 

In addition to the proportional counters, other types of gas-filled detectors are used in CEMS. Eirst was the parallel-plate avalanche counter. In Mossbauer spectroscopy such detectors have been used as resonance detectors and at higher counting rates. These counters have found application in surface studies and are the effective tool for the registration of low-energy ( < 1 keV) electrons, which are practically impossible to detect with the proportional counter. Because of the high electron-detection efficiency this enables the measurement of reasonable spectra in a relatively short time for Ee,... 

As was mentioned in Section 3.1, conversion electron Mossbauer spectroscopy (CEMS) provides an advantageous means of studying surface structure as it affords much greater sensitivity than measurements in the transmission... 

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