Neutron Diffraction and Mössbauer Studies of Zinc Ferrite

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 Mossbauer 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 with different Fez+-content. 57Co source embedded in Cu 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. 2. Liquid helium temperature neutron diffraction pattern of two samples of ZnFezO4 with 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 confirmed 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 1 321 Neel point, indicating a non-collinear magnetic structure. 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, f h, = 2 n f 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 representations 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 001 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.