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INVESTIGATION OF A RELAXATION
BROADENED ABSORPTION SPECTRUM BY USE OF A POLARIZED SOURCE
S. Mørup
To cite this version:
S. Mørup. INVESTIGATION OF A RELAXATION BROADENED ABSORPTION SPECTRUM BY USE OF A POLARIZED SOURCE. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-683-C6-684.
�10.1051/jphyscol:19746150�. �jpa-00215766�
JOURKAL DE PHYSIQUE Colloque C6, supplement au n° 12, Tome 35, Decembre 1974, page C6-683
INVESTIGATION OF A RELAXATION BROADENED ABSORPTION SPECTRUM BY USE OF A POLARIZED SOURCE
S. M0RUP
Laboratory of Applied Physics II, Technical University of Denmark, Lyngby, Denmark
Resume. — Les composantes d'un spectre Mossbauer qui est influe par relaxation electronique peuvent etre decomposers en utilisant une source polarisee. L'effet est montrS par application d'une source de 57Co dans le fer naturel et un absorbant de NH4Fe(SO.t)2.12 H2O avec un champ magne- tique perpendiculaire sur les rayons gamma.
Abstract. — The components of a Mossbauer spectrum which is influenced by electronic relaxa- tion can be resolved by application of a polarized source. The effect is demonstrated by use of a source, of S7Co in natural iron and an absorber of NH-iFe(S04)2.12 H2O with the source and the absorber in a magnetic field perpendicular to the gamma ray direction.
Mossbauer spectroscopy is a sensitive method for the study of electronic relaxation when the electronic relaxation time, x, is comparable to the nuclear Larmor period, xL. For x < rL, and in the absence of quadru- pole interactions the spectrum consists of a single broadened line. The line width and the line shape depend on the relaxation time and on the interaction of the ion with the crystal field, the neighbouring magnetic ions and the applied magnetic field. In the case of F e+ 3 ions in the high spin state, (S = i)the broadening is negligible when x < 10"1 2 s. The detailed interpre- tation of relaxation broadened spectra is not always straightforward. Generally the + i -*• ± i, the
+ i ~* + i and the • + £ ' - » + £ transitions are broadened to a different extent, but the components can not always be unambiguously resolved.
In cases where the crystal field splitting and the magnetic interaction between neighbouring ions are small compared to the Zeeman interaction, due to an external magnetic field, the eigenstates of theFe + 3 ions are the eigenstates of Sz. Then for T < TL, at temperatures where the magnetization is small and in the absence of a nuclear quadrupole splitting, the Mossbauer spectrum contains a broad + f -» + \ component, a narrow ± i -> + i component and a
± i -* ± i component which has an intermediate width.
The line widths of the individual components are given by [1, 2]
r(mc, mg) = T0 + v(me, m^2 < h(t)2 > av x ; (1) where T0 is the line width in the absence of relaxation broadening, v(me, m^) is the difference between the z components of the nuclear magnetic moments of the excited state and the ground state of the nucleus, h(t) is the hyperfine field, and x is the relaxation time.
We have developed a new technique by which the
Am = 0 components of the spectrum can be distin- guished from the Am = + 1 components. The method is based on the use of a 57Co source in an iron matrix.
The source as well as the absorber is placed in external magnetic fields. Then the relative intensities of the components of the spectrum depend on the angles between the gamma ray direction and the hyperfine fields acting at the source and absorber nuclei [3]. A special situation arises when the fields acting at the source and absorber nuclei are parallel and at the same time perpendicular to the gamma ray direction.
In that case the Am = 0 emission lines of the source are absorbed only by the Aw = 0 components of the absorber, whereas the Am = ± 1 emission lines are absorbed only by the Am = ± 1 components of the absorber. This means that the Am = 0 and the Am = + 1 components of the absorber can be studied separately. The calculated relative intensities of the components are given in table I.
TABLE I
Relative line areas of the components in a spectrum obtained with a polarized source and absorber
Source transition/absorber transition
(± 1 - ± i)/(± * - ± i) (± ! - ± i)/(± i - + i) (+ f - ± i)/(+ i -> + i) (± i - ± i)/(± i - + i) (+ i - + i)/(± i -> + i) (± i - ± i)/(± i - + i) (+ i - + i)/(± f - ± i) (± i -* + i)/(± i - ± i) (+ i - + i)/(± i -> + i)
area 9 0 3 0 16 0 3 0 1
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19746150
We have demonstrated the method using an absorber I I I I I I I
of ferric alum (NH4Fe(S0,), .12 H,o).- It has pre- viously been found [l, 41 that in a magnetic field larger than a few kOe the line widths of the compo- nents are given by (I). Our experimental results are shown in figure 1. The spectrum (a) was obtained by use of the magnetically split source of 57Co in iron and a single line absorber of K,Fe(CN), . 3 H,O. The source was placed in a magnetic field of 1.8 kOe perpendicular to the gamma ray direction. The spec- trum (b) was obtained using a single line source of 57Co in Pd and a ferric alum absorber at 113 K andin an external field of 8.0 kOe. The best computer fit, and its decomposition into three components are shown by solid lines. Spectrum (c) is the result of the experiment with both the magnetically split source and the ferric alum absorber in an applied field of 8.0 kOe perpendicular to the gamma ray direction.
The separation of the Am = 0 components with inter- mediate line width from the broad and narrow Am = f 1 components can be seen clearly. The solid lines represent the best computer fit with the line widths given by (1) and the relative areas given in table I.
The maximum hyperfine field for S, = 3 in ferric alum is 595 kOe [5]. Hence the relaxation time, z can be computed by use of (I). The result is z = 1.3 ns.
We have also obtained the spectrum with the absor- ber at 78 K. In this case the lines are considerably broader corresponding to a relaxation time of 2.011s.
The temperature dependence of the relaxation time demonstrates the influence of spin-lattice relaxation.
The method described here may be useful for the interpretation of spectra which cannot be decomposed into three components with line widths given by (1).
The change of the components as a function of the applied magnetic field may then give information about the size of the crystal field splitting.
I I I I I
-10 -5 0 +5 +I 0
VELOCITY ( M M I S )
FIG. I . -a) Mossbauer spectrum obtained with a source of 57C0 in natural iron and an absorber of K4Fe(CN)6.3 HzO. b) Moss- bauer spectrum of NH4Fe(S04)2.12 H z 0 at 113 K in an 8.0 kOe external field obtained with a single line source of 57C0 in Pd.
The lines show the best computer fit and the decomposition of the spectrum into the three components. c) Mossbauer spectrum obtained with a source of 57C0 in natural iron and an absorber of NH4Fe(S04)2.12 H z 0 at 113 K. The source and the absorber were placed in a magnetic field of 8.0 kOe perpendicular to the
gamma ray direction.
The method may also be applied in a modified form with a polarized source and a single crystal absorber.
In that case a rotation of the absorber may give rise t o changes in the spectrum.
References
[I] MBRUP, S. and THRANE, N., Phys. Rev. B 4 (1971) 2087. [4] MBRUP, S. and THRANE, N., in t( Hyperfine Interactions in 121 MBRUP, S. and THRANE, N., Phys. Rev. B 8 (1973) 1020. Excited Nuclei D, edited by G. Goldring and R. Kalish [3] WEGENER, H., Der Miissbauer-Effekt und seine Anwendung (Gordon and Breach, New York) 1971 p. 827.
in Physik und Chemie, 2. edition (Hochschultaschen- [5] BRUCKNER, W., RITTER, G. and WEGENER, H., Z. Phys. 236
biiher-Verlag, Mannheim) 1966. (1970) 52.
