THE OBSERVATION OF DIFFERENCES BETWEEN SCATTERING AND TRANSMISSION MÖSSBAUER SPECTRA NEAR THE CURIE TEMPERATURE

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THE OBSERVATION OF DIFFERENCES BETWEEN SCATTERING AND TRANSMISSION MÖSSBAUER

SPECTRA NEAR THE CURIE TEMPERATURE

R. Mirzababayev, R. Mcgrath, J. Walker

To cite this version:

R. Mirzababayev, R. Mcgrath, J. Walker. THE OBSERVATION OF DIFFERENCES BETWEEN SCATTERING AND TRANSMISSION MÖSSBAUER SPECTRA NEAR THE CURIE TEMPERA- TURE. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-216-C2-217. �10.1051/jphyscol:1979277�.

�jpa-00218675�

JOURNAL DE PHYSIQUE Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-216

THE OBSERVATION OF DIFFERENCES BETWEEN SCATTERING AND TRANSMISSION MOSSBAUER SPECTRA NEAR THE CURIE TEMPERATURE*

R.M. Mirzababayev'i", R.D. McGrath and J.C. Walker

The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.

Résumé.- Nous avons utilisé la spectroscopie Mossbauer pour étudier les alliages de PdFe près de la température Curie, en géométrie normale de transmission et également en retour. Les spectres en retour, dans la région de température critique, ont des profils plus étroits que ceux obtenus en géométrie de transmission, et ont des formes anormales. Les températures de Curie mesurées diffèrent d'environ 2 K.

Abstract.- We have used 57Fe Mossbauer spectroscopy both in transmission and scattering geometry to observe a PdFe alloy near its Curie temperature. We have observed different lineshapes and narrower linewidths in the scattering spectra than in the transmission spectra. The effective Curie temperature differs by as much as 2 K in the two different geometries.

The Mossbauer effect can be observed by seve- ral different experimental techniques /l/. In parti- cular, with 57Fe one can perform a transmission ex- periment to detect the resonant absorption of 14.4 keV y-rays. Or one can perform a scattering experi- ment to detect the resonantly scattered 14.4 keV y-rays directly or either the 6.3 keV X-rays or the electrons produced by the internal conversion of readmitted resonant 14.4 keV y-rays. We have made the striking observation that near the magnetic or- dering temperature of a PdFe alloy, the spectra obtained by these different techniques appear to show different behavior with changing temperature.

Two PdFe alloy foils, of thickness 10 Um and 25 yrn, were used. They were annealed after rolling to remove strains. Each was fabricated with iron isotopically enriched to 50 % Fe and has a nominal 14.5 at.% iron concentration. PdFe alloys of this composition have a convenient magnetic ordering temperature 12/ making possible simple experimental setups for both scattering and transmission geome- tries and minimizing thermal gradients.

We performed experiments in two configura- tions. In the first /3/, a PdFe foil was mounted at 45° to the incident 14.4 keV y-rays. Two proportio- nal counters were arranged so that spectra could be obtained simultaneously in scattering and transmis- sion geometry. The scattering detector, which detec- ted the 6.3 keV internal conversion X-rays, was si- tuated at 90° to the incident y-rays. The transmis- sion detector was set up for a conventional 14.4 keV y-ray experiment.

A series of runs on the 25 um thick PdFe foil were made to compare scattering and transmission spectra at various temperatures. Below 309 K, the scattering and transmission spectra, each of which showed magnetic splitting, had similar behavior.

However, between 309 K and 313 K, where the hyper- fine splitting collapses and the individual spectral lines are not resolved, the character of the scatte- ring and transmission spectra differ significantly

(Fig. 1).

Fig. 1 : Mossbauer spectra of a PdFe alloy at 310. IK observed in a) scattering with 6.3 keV X-rays and b) transmission with 14.4 keV y-rays.

The scattering spectra show narrower linewidths than the corresponding transmission spectra. While the 'Stork supported by NSF Grant DMR 77-07-127.

^Present address : Institute of Physics, Baku, USSR.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979277

transmission spectra each appear a a broadened line, the scattering spectra each appear to consist of a narrow line centered on a broadened line. As the temperature of the sample approaches 313 K, the area under the central line in the scattering spectra increases at the expense of the broadened component.

Inferring the hyperfine field from the overall line- width, the transmission data indicates an ordering

temperature of 313 K (Fig. 2). The more rapid col- lapse of the scattering spectra with increasing temperature suggests a ordering temperature of 310.5 K. Above 313 K, spectra for both geometries are single lines with comparable linewidths.

to consist of a central component plus a broad com- ponent. However, the linewidths for the scattered 6.3 keV X-rays observed in this configuration were much narrower than the corresponding spectra from the conversion electrons. Away from the critical temperature region, the corresponding transmission and scattering spectra had identical behavior.

The range of the conversion electron is much less than that of the 6.3 keV X-rays and the 14.4 keV Y-rays. Since the anomalous lineshapes and linewidths are much less prominent in the conversion electron spectra than in the 6.3 keV X-ray spectra, one can probably rule out the possibility that we have observed some form of surface phenomena as we

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+

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SCATTERING

This leaves a considerable puzzle as to the origin of this strange effect. One possibility, suggested to the authors by M. Blume /4/, is that the unusual scattering spectra in the hemperature

A # A region in which one observes unresolved transmission

3Dr 510 315 m

TEMPERATURE (K) spectra result from interference effects among the partially resolved hyperfine lines. Although the Fig. 2 : The temperature dependence of the magnetic

hyperfine field in a PdFe alloy obtained in scatte- details are not yet worked out, it seems possible ring geometry with 6.3 keV X-rays and in transmis- that an incident 14.4 keV Y-ray could simultaneously sion geometry with 14.4 key y-rays.

excite more than one magnetic sublevel due to the We also observed the 10 pm PdFe foil in this

configuration in transmission and in scattering with both the 6.3 keV X-ray and the 14.4 y-ray.

Again near the critical temperature, the 6.3 keV scattering spectra had signigicantly narrower line- widths than the transmission spectra. The 14.4 keV scattering spectra had a lineshape comparable to but slightly broader than the 6.3 keV scattering

spectra.

In the second configuration, the 10 pm PdFe foil was mountedin a flow proportional counter. By using a mixture of 90 % Ar-10 % methane or 90 % He-10 % methane for the counter gas, one can detect, respectively, either the 6-keV conversion X-rays or the conversion electrons. The counter was situated so that it detected the X-rays or electrons scatte- red backwards through the 2a solid angle. A propor- tional counter was situated behind this optically thin counter so that spectra could be taken simulta- neously in the transmission geometry. The whole flow counter was maintained at the desired temperature with an appropriate temperature control system.

Near the ordering temperature, the spectra from the scattered conversion electrons showed a slightly narrower linewidth than the corresponding transmis- sion spectra. Again the scattering spectra seemed

partial overlap of the lines corresponding to these levels. As this is a coherent excitation, some in- terference between these states can be expected and could lead to anomalous linewidths and shapes. This conjecture is made reasonable by the fact that in the detection of conversion electrons one is avera- ging over a much wider variety of final states so that the interference effects might be somewhat

"washed out" as the data indicate. It should be em- phasized, however, that this is a highly speculative line of reasoning at this moment.

One can conclude, however, that observing cri- tical behavior using ~Essbauer spectroscopy in the scattering geometry may be risky, at least until there is a satisfactory understanding of this ano- malous behavior.

References

/I/ S~ijkerman, J., ~Zssbauer Methodology

1

/ 2 / Nieuwehhuys, G., Adv. Phys.

2

/3/ McGrath, R., Mirzababayev, R., Walker, J., Phys.

Lett. A,

67

/4/ Presently at Brookhaven National Laboratories, New York, U.S.A.