ON THE ANISOTROPY OF THE RECOILLESS FRACTION IN FeCO3 AT 4.2 K

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ON THE ANISOTROPY OF THE RECOILLESS FRACTION IN FeCO3 AT 4.2 K

D. Nagy, K. Kulcsár, H. Spiering, R. Zimmerman

To cite this version:

D. Nagy, K. Kulcsár, H. Spiering, R. Zimmerman. ON THE ANISOTROPY OF THE RECOILLESS FRACTION IN FeCO3 AT 4.2 K. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-385-C6-387.

�10.1051/jphyscol:1974672�. �jpa-00215829�

JOURNAL DE PHYSIQUE Colloque C6, supplément au no 12, Tome 35, Décembre 1974, page C6-385

ON THE ANISOTROPY OF THE RECOILLESS FRACTION IN FeC0, AT 4.2 K

D. L. NAGY (*), K. KULCSAR (*), H. SPIERING and R. ZIMMERMANN Physikalisches Institut II der Universitat Erlangen-Nürnberg, 8520 Erlangen, BRD

Résumé. - On met en doute la valeur anormalement élevée de I'effet Goldanskii-Karyagin reportée par Kündig et al. dans la phase antiferromagnétique de FeCO3. Contrairement à la dévia- tion de 55 % par rapport au cas isotrope observée par ces auteurs pour les intensités des raies 2 et 5,

nous n'avons trouvé qu'une déviation d'environ 10 %. Cette valeur n'implique pas une anisotropie déraisonnablement élevée du facteur de Debye-Waller.

Abstract.

The abnormally large Goldanskii-Karyagin Effect in the antiferromagnetic phase of FeCO

reported by Kündig et al. is questioned. In contrast to the 55 % deviation of the 2nd and 5th line intensities from the isotropic case observed by them we found a deviation only of about 10 %. This value does not invoke an unrealistically high anisotropy of the Debye-Waller factor.

1. Introduction. - In the ideal case, a polycrystal- line absorber of a Fe compound with a quadrupole interaction should show an exactly symmetric Moss- bauer doublet. The angular dependence of the recoilless fraction f' relative to the crystal axes may, however, violate this symmetry. This phenomenon [l, 21 is known as the Goldanskii-Karyagin Effect (GKE).

FeCO,, the minera1 form of which is siderite, belongs to the space group R3c ( ~ 3 ~ ) [3]. The Fe2 ' ions are located on collinear threefold axes and have a point symmetry 3. If no external field is applied, al1 the Fe2+

sites are equivalent.

FeCO, is paramagnetic above its Néel-temperature which lies, according to most data, between 38 and 39 K. Below TN, FeC0, is antiferromagnetic ; the Fe2*

moments point along the c-axis [4].

FeCO, was a likely choice for the observation of the GKE on 57Fe nuclei in its paramagnetic phase [5, 61.

In spite of some contradictory results 15-71, however, this effect could not be observed, at least at room temperature. The observed asymmetries turned out to be caused by a preferred orientation of the crystallites (texture) 18-11]. An analysis of the single crystal measurements of Brümmer et al. [12] predicts a very small asymmetry R, ⁼ Z,/I, ⁼ 0.98 f 0.01 at room temperature for an extremely thin absorber. Here, I, and I, denote the intensities of the n (+ 3 ⁺ 5 3)

and the ^o (f 2 + + 2) lines. On the other hand, at lower temperatures a slight increase of R, has been observed (R, = 1.07 ) 0.03 at 90 K) by Vinogradov et al. [7].

(*) On leave of absence from the Central Research Institute for Physics, Budapest, Hungary.

In the antiferromagnetic phase the hyperfine field is parallel to the principal axis V,, of the axial EFG ten- sor. Therefore, an anisotropy of the recoilless fraction should be manifested as a deviation from the expected 3 : 2 : 1 intensity ratio. To describe this deviation, two quantities are used. First, the ratio R, of the 2nd (and 5th) to the 3rd (and 4th) line intensities : R,

12,,/Z3,,.

Secondly, the ratio IJI2 where the line intensities are described as 3 Il : 2 1, : 1,. Obviously, Zl/12 = 2/R,.

Recently, an abnormally large deviation from the expected 3 : 2 : 1 ratio has been reported by Kündig et al. at 6 K [13, 141. Their value I1/I2 ⁼ 1.55 f 0.05 requires an anisotropy of the recoilless fraction as high as f hlf; ^N 7 where the subscripts denote the directions parallel and perpendicular to the trigonal axis, respec- tively.

The spectra of polycrystalline antiferromagnetic FeCO, published so far [15, 161 seem to contradict the above observation. The present work is concerned with an examination of this anomaly.

2. Experimental. - For the measurements natural siderite from Dobsina, ~zechoslovakia has been used.

The minera1 was ground in an agate mortar under alcohol to prevent oxidation of the probe which might result in a contribution to the asymmetry [9]. To achieve a random distribution of the crystal axes in the sample, the FeCO, powder (average grain size about 2 pm) was mixed with active carbon powder. When the mixture was placed in the sample holder the powder was not exposed to any pressure at ail. It can be readily visualised that in a porous mixture of crystallites of non-regular shape a rather high texture can be induced even by a small pressure.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974672

PIERING AND R. ZIMMERMANN The thickness of the absorber was 8 mg Fe/cm2. The

spectra were recorded by a conventional constant acce- leration spectrometer using a 5 7 C o ( C ~ ) source at 4.2 K.

To check the quality of the sample, a spectrum was recorded at room temperature. The asymmetry of the quadrupole doublet was R, = 0.99 f 0.01 in excellent agreement with the single crystal data of Brümmer et al. [12] and with the similar measurement of Kündig et al. [13]. To prevent as far as possible the sample from orientation on being cooled the room tempera- ture measurement was recorded in the cryostat.

The spectrum recorded at 4.2 K is shown in figure la.

The anomaly reported by Kündig et al. [13, 141 cannot be seen on the spectrum. (Note that, because of the strong quadrupole and weak magnetic interaction, the order of the 1st and 2nd lines is interchanged.)

velocity (rnrnls) FIG. 1. - Mossbauer spectrum of polycristalline FeC03 at 4.2 K : a) raw data ; b and c : data after blackness correction ; b) Gauss-filter ; c) Fermi-filter (see text). The velocities are

relative to metallic iron at 298 K.

3. Discussion. -The solid line of figure 1 was obtained by a least squares fit with the plausible

constraint that the intensity ratio of the 1st (and 6th) to the 3rd (and 4th) line is 3 : 1. The analysis yields 11/12 = 1.15 f 0.02.

Though the absorber was rather thin, a blackness correction of the spectrum was performed using the Fourier technique [17]. In this technique an arbitrary factor is the shape of the filter function which is used to reduce the noise. For spectra of relatively high quality, however, no drastic damping is needed and therefore the actual shape of the filter turns out to be immaterial.

This can be seen in figure Ib and c. For b a Gaus- sian 1171 and for c a Fermi-function [18] were used. The width of the filters was chosen so that false oscillations did not replace the noise [18].

The transformed spectra were c o m p t e r ana- lyzed also. As a result I1/I2 = 1.10 f 0.03 (b) and I,/I2 = 1.1 1 f 0.03 (c) were obtained. The former value is probably the better one, since this was obtained by applying a somewhat lower damping.

Theivalue 1,/12 = 1.10 + 0.03 (which corresponds to R, = 1.82 f 0.05 or to a quadrupole doublet asymmetry R, = 1.06 f 0.02) is much less than the value reported by Kündig et al. (1.55 f 0.05) [13]. It is, however in accordance with the data of Vinogradov et al. [7] and thus the small anomaly seems to be caused in fact by a GKE. To check this point, the angular dependence of the asymmetry should be measured. In the absence of such measurements the deviation of 11/12 from unity being due to the appea- rance of a small texture as the sample is cooled cannot be unequivocably excluded. In any event, the value 11/12 ⁼ 1.10 ^f 0.03 could be explained by a ratio f ;,/fi = 1.56 tr: 0.19 which may be realistic.

4. Conclusions. - The present study hasnot confirm the existence of a very large GKE in the antiferro- magnetic phase of FeCO,. Without measuring the angular dependence of the line intensities one can never be sure that the observed anomaly is really due to a GKE and not to a texture effect. A marked texture can be obtained by small pressures or - according to Our very recent observations - by relatively weak external magnetic fields.

Acknowledgments. - The authors are pleased to

thank Professors U. Gonser and H. Wegener and to

Drs. 1. Dézsi, G. Ritter and 1. P. Suzdalev for their very

stimulating discussions. One of the authors (D. L. N.)

is indebted to the Deutscher Akademischer Austausch-

dienst for a fellowship at the Universitat Erlangen-

Nürnberg. Additionally, the Bundesministerium für

Forschung und Technologie and the Deutsche For-

schungsgemeinschaft are gratefully acknowledged for

their financial assistance.

ON THE ANISOTROPY OF THE RECOILLESS FRACTION IN FeC03 AT 4.2 K C6-387

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