Mössbauer spectroscopy with polarized monochromatic radiation

Solid State Communications, Vol. 5, PP. 701-703, 1967. Pergamon Press Ltd. Printed in Great Britain

MÔSSBAUER SPECTROSCOPY WITH POLARIZED MONOCHROMATIC RADIATION S. Shtrikman Department of Electronics, The Weizmann Institute of Science, Rehovoth, Israel (Received 26 June 1967 by E. Burstein)

The possibility of carrying out Mössbauer experiments with monochromatic linearly polarized radiation is demonstrated. Mössbauer spectra of a transversely magnetized iron foil for radiation polarized parallel and perpendicular to the magnetization are given.

MOSSBAUER experiments using polarized radiation have been carried out in many cases, 1 and it has been shown, as was expected, that such experiments yield a considerable amount of information not or hardly obtainable by means of unpolarized radiation. Since, however, the polarization in these experiments was obtained by magnetizing the emitter, the radiation obtained was not monochromatic. It instead consisted of several monochromatic rays, each with a definite polarization,

If we tune the polarizer by Doppler shtfting its energy so that line 2 of the polarizer (lines are numbered from left to right) coincides with the source energy (Spectrum II, Fig. 1) the intensity of lines 2 and 5 should be zero. In fact, since the polarizer is not “black” these lines are not completely washed out. However, with this arrangement, we have obtained, as shown in Spectrum U, a Mössbauer spectrum of iron in which the polarization of the incident y rays is essentially, parallel to the magnetic field. -

The advantages of replacing the polychromatic source with a monochromatic one, becomes very clear when one compares the Fe67 spectra obtained in the early days of Mössbauer spectroscopy,2 before the introduction of the monochromatic source currently in standard use, with those obtained today. ~

To obtain a spectrum wherein the incident rays are we polarized magnetic field, tune theperpendicular polarizer so to thatthe line 1 is equal to the energy of the stationary source. In this case (see Fig. 1, Spectrum Ill) the absorption spectrum consists, as expected, of essentially lines 2 and 5 with only slight remnants of the other lines due to the incomplete absorption of the unwanted polarization by the polarizer. y

We have obtained monochromatic polarized radiation by using the following arrangement. We start with a stationary monochromatic source of Co~in Pd in front of which an iron foil (3. 8 mg/ cm2 enriched to 90% in Fe~)to be referred to as the polarizer is mounted on an “Elron”4 constant velocity transducer. The absorber to be studied, here also an iron foil, is mounted behind the polarizer on a regular “Elron”4 constant acceleration spectrometer. Both the polarizer and absorber are magnetized to saturation transversely in the present case. With the polarizer stationary, spectrum I of Fig. 1 is obtained. (The polarization of the various absorption lines is indicated by arrows). 701

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One can improve the situation by taking spectra under different conditions. For example, first a spectrum can be taken with the polarizer stationary. From this we then substract a spectrum taken with the polarizer tuned so as to absorb a chosen polarization. This can be done in practice by using a multichannel analyzer and by collecting counts with the polarizer stationary for a certain time, then substracting counts for the same period of time with the polarizer moving. Clearly, as shown in traces I-U and i-rn, one gets the spectra which correspond to irradiation with the polarization that was absorbed by the

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POLARIZED MONOCHROMATIC RADIATION

SOURCE POLARIZER POLARIZATION

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POLARIZER POLARIZATiON

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57 in Pd SOURCE — Co POLARIZER—Enflched Fe~Foil ~ TI ABSORBER— Iron Foil JMAGNE ZED MAGNETIC FIELD DIRECTION Ct) II To Foils

FIG. 1 ______

Mossbauer spectra using linearly polarized monochromatic y rays. For explanation see text. -

ABSORBER VELOCITY

polarizer. As one would expect, this procedure results in a better “wanted to unwanted” signal ratio, but one loses in statistics. We have here demonstrated the possibility of carrying out Mössbauer experiments with monochromatic polarized radiatIon. 6 Only the case of plane polarized v rays has been considered but it is possible to apply the same techniques to the more interesting case of circularly polarized rays. The research potential of

spectroscopy using monochromatic polarized rays is enormous and we only want to mention here such possibilities as determining the direction of the sub-lattice magnetization In antiferromagnets, measuring the asymmetry parameter in the electric field gradient, and studying the polarization of scattered Mössbauer radiation. -

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Acknowledgements It is a pleasure to acknowledge the help of Mr. S. Somech in setting up this experiment. -

References 1.

JOHNSON C. E., MARSHALL W. and PERLOW G. T., Phys. Rev. GONSER U. etal. Appi. Phys. Lett. 9, 18 (1966).

126,

1503 (1962);

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2.

HANNA S.S. et al. Phys. Rev. Left. 4, Rev. Sci. InstE31, 580 (1960).

177 (1960); RUBY S. L., EPSTEIN L.M. and SUN K.H.

3.

WERTHEIM G. K., Introduction to Mössbauer Spectroscopy, Academic Press, New York (1963).

4.

Mössbauer Effect Spectrometers, produced by “Elron”, Electronic Industries, Haifa, Israel.

5.

The use of a stationary source as described here is not essential. With some complication in the electronics a stationary absorber may be used, as preferable when studies of temperature dependence are desired.

La possibilitée de faire des experiments Mössbauer avec des rayons monochromatiques polarizées linearement est demontrée. On present les spèctres Mössbauer d’une feulile de fer magnetisée transversallement pour des rayonement polarisés parallèle et perpendiculaire.