NEUTRON DIFFRACTION AND MÖSSBAUER STUDIES OF ZINC FERRITE

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NEUTRON DIFFRACTION AND MÖSSBAUER STUDIES OF ZINC FERRITE

U. König, E. Bertaut, Y. Gros, G. Chol

To cite this version:

U. König, E. Bertaut, Y. Gros, G. Chol. NEUTRON DIFFRACTION AND MÖSSBAUER STUDIES OF ZINC FERRITE. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-320-C1-321.

�10.1051/jphyscol:19711105�. �jpa-00213920�

JOURNAL DE PHYSIQUE

Colloque C I , suppliment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 320

by U. KONIG (*), E. F. BERTAUT (**), Y. GROS (***) and G. CHOL (****)

RBsum6.

On montre par diffraction neutronique et spectromktrie Mossbauer que la formation d'ordre magn6- tique a Iongue distance dans le ferrite de zinc d6pend de f a ~ o n sensible de la teneur en fer bivalent. Les donnkes expkri- mentales sont en faveur d'un modele de structure magnktique non colinkaire.

Abstract.

It is shown by neutron diffraction and Mossbauer spectrometry that the formation of a long range magnetic order in zinc ferrite depends sensitivily on the content of divalent iron. The experimental data favour a non collinear model of the magnetic structure.

Zinc ferrite is a normal spinel with the tetrahedral A sites occupied by Zn2+ ions and the octahedral B sites occupied by Fe3+ ions. Any magnetic order therefore is due to the nearest or more distant B-B interactions of the Fe3' ions.

Measurements of the susceptibility [I 1: of the specific heat [Z], studies by neutror diffraction [3] and Moss- bauer spectrometry [4] have shown that below 10 OK an antiferromagnetic order appears. The neutron diffraction patterns of J. M. Hastings and L. M. Corliss [3] and recently also one of M. K. Fayek et al. [5]

have only shown broad magnetic lines, typical for short range order. From these experiments there was some doubt that a long range order can exist in ZnFez04 [6].

We show first that the formation of the magnetic order depends very sensitively on the stoichiometry of the sample, especially from the Fez+ content.

Figure 1 (left) shows Mossbauer spectra of a sample with a content of 0.002 Fez+ per molecule. We observe well shaped Zeeman spectra, indicating a long range

FIG. 1.

Mossbauer spectra of two samples of ZnFezO4

different Fez+-content. 57Co source embedded in

at room temperature.

(*)

Zentralinstitut fiir Forschung und Entwickiung der Fried.

Krupp GmbH, Essen.

(**)

CNRS and CEN-G., Grenoble.

(***)

Laboratoire de Spectromktrie Physique, Grenoble.

(****) Laboratoire Central des TkICcommunications, Paris.

magnetic order up to temperatures just below the Nee1 point.

Figure 1 (right) also shows Mossbauer spectra of ZnFez04 with a content of 0.01 Fe2+ per molecule.

In this sample line broadenings occur and at 10 OK we observe a superparamagnetic behaviour.

This is in agreement with neutron diffraction results (Fig. 2). The sample with small ~ e ' + content shows

FIG.

Liquid helium temperature neutron diffraction pat- tern of two samples of ZnFezO4

different Fez+-contents.

The indices are based on the cubic unit cell (a

8.43 A).

Neutron count rates in arbitrary units.

well resolved and strong magnetic peaks, whereas the sample with a content of 0.01 Fez+ shows only broad lines, thus indicating short range order.

These observations may explain the published results mentioned above.

Recently, B. Boucher et al. [7] independently confir- med our experimental results. Their neutron diffraction pattern of a sample with small Fez+ content agrees very well with our measurements.

It may be mentioned that the neutron diffraction patterns of the two samples of figure 2 show no difference at room temperature.

The results of our investigation of the sample with low Fe2+ content have been reported in [B]. The Mossbauer measurements have shown a deviation of the Hi(T) vs. T-plot from the Brillouin B (5) behaviour and the occurrence of a quadrupolar shift below the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711105

NEUTRON DIFFRACTION AND M~SSBAUER STUDIES OF ZINC FERRITE C

- 321 Neel point, indicating a non-collinear magnetic struc-

ture.

The magnetic peaks of the liquid helium temperature neutron diffraction diagram (Fig. 2) can be indexed in a tetragonal cell a = b = 8.43 A and c = 16.86 A,

as suggested by M. K. Fayek et al. 151. The selection rule of the magnetic lines h,

h,

2 n

1 and h, ⁼ 2 n -t 1 indicates a propagation vector of the magnetic mode k = [lo 41. In order to determine the magnetic structure, we have used the theory of magnetic modes as given by representation analysis [9]. If we look .for the symmetry operation of highest order which leaves k invariant modulo a vector of the reciprocal F-centered spinel lattice, we find the rotation inversion 4. By inspection of the irreducible represen- tations of the space groupe F a, we find a non-collinear arrangement as shown in figure 3. Collinear spin arrangements non parallel to Oz would require a spin Hamiltonian of an order higher than two.

An essential feature of the model are parallel z-components of groups of four neighbouring iron spins. Our model agrees with the first of the three models given by B. Boucher et al., but they predict zero z-components from the experimental evidence.

We obtained the best agreement between observed and calculated magnetic intensities for s, = s,,. An arrangement along the c-axis can be ruled out, but for s, = 0 the R-factor increases only slightly, so that a clear decision between the model s , = s , , and s, = 0 cannot be made on the basis of the present experimental data.

We observed a low magnetic moment of 4.2 pB,

FIG. 3. - Magnetic structure of ZnFe204. Open circles have

+ components and full circles - components along z-axis.

Arab number indicate the height in eights of the chemical unit cell

and -& 5 0 are antitranslations.

extrapolated to OOK, which may be explained by covalency effects and partially by competitions of short range order scattering (Fig. 2). Evidence that the electron configuration of Fe3

changes when crossing the Neel temperature is also given by the observation of a discontinuity of the isomeric shift of - 0.03 mm/s.

X-ray investigations have shown that any detectable tetragonal distorsion should be smaller than 0.01 A.

References

[I] ARROT (A.) and GOLDMAN (J. E.), Phys. Rev., 1955, [6] ANDERSON (P. W.), Phys. Rev., 1956, 102, 1008.

98, 1201. [7] BOUCHER (B.), BUHL (R.) and PERRIN (M.), Phys.

[2] FRIEDBERG (S. A.) and BURK (D. L.), Phys. Rev., Stat. Sol., 1970, 40, 171.

1955, 98, 1200. [8] KONIG (U.), BERTAUT (E. F.), GROS (Y.), MITRIKOV [3] HASTINGS (J. M.) and CORLISS (L. M.), Phys. Rev., (M.) and CHOL (G.), Solid State Communica-

1956, 102, 1460. tions, 1970, 8, 759.

[4] SAWICKI (J.), Czech. J. Phys., 1967, B 17, 371. [9] BERTAUT (E. F.), Acta Crystallogr., 1968, 24A, 217.

[5] FAYEK (M. K.), LECIEJEWIECZ (J.), MURASIK (A.)

and YAMSIN (I. I.), f'hy~. Stat. Sol., 1970, 37,843.