SYNTHESIS OF SODIUM-ZINC SPINEL FERRITES

The synthesis of spinel ferrites with composition Zn 1-2x Na x Fe 2+x O 4 has been performed and the composition range in which single phase samples are obtained has been defined. The characterization of the samples has been carried out from atomic absorption and X-ray fluorescence analyses, X-ray diffraction patterns, Mossbauer spectroscopy and thermomagnetic measurements. It is show that significant loss of Na does exist when the synthesis is performed at high temperatures. When the Na volatilization is avoided spinel oxides with Na content up to 0.25 atoms per unit formula can be obtained. In this case the increase of the interatomic distances leads to differing fundamental magnetic properties as compared to the equivalent lithium-zinc ferrites.

Lithium-zinc spinel ferrites are a wide class of ferrimagnetic materials with interesting magnetic characteristics in view of microvawe applications 111.In the same way, sodium-zinc spinel ferrites could be good candidates for these technological applications.As a matter of fact.due to the different ionic radii of Na and Li atoms the distances between magnetic ions will be very different in both cases.In this way, according to pre-vious studies on spinel ferrites 121. a change of the fundamental magnetic properties may be expected.sodium.zincspinel ferrites was done by Mones  and Banks [ 3 ] .From this study it seemed clear that a wide range of solid solutions could exist between ZnFe O4 and the hypothetical Na0,5Fe2.504spine?.However. the composition and magnetic properties of their samples remain unclear.Therefore, we decided to undertake a full study of the synthesis conditions of Znl-2xNaxFe2+x04 spinels in or-.der to define their composition range.Some preliminary results concerning their magnetic properties are presented as well.

Experimental
The syntheses of all compounds, with nominal composition Znl-2xNaxFe2+x04, were performed in air from a high temperature solid state reaction of OC-Fe203, ZnO and Na2C03 stoichiometric mixtures.Before being introduced in the furnace the samples were homogenized in an agate mortar and pelletized.Different firing temperatures and cooling rates were used.X-ray diffraction patterns were obtained with a Siemens D-500 powder diffractometer using Cu K a as incident radiation and a graphite monochromator.Lattice parameters were computed with the aid of a least squares program from X-ray patterns containing Si powder as internal standard.The Na content of the samples was determined by the atomic absorption technique while the Fe and Zncontents were determined by X-ray fluorescence.Mijssbauer spectra were obtained with a conventional spectrometer by using a 10 mCi 57CO:Rh single line source.Velocity calibrations were made via a-Fe spectra.Isothermal magnetization measurements were performed by the axial extraction technique in fields up to 55 KOe by using a superconductor scjlenoid and up to 150 KOe with a water cooled Bitter magnet of the Service Nationaldes Champs Intenses at^ Grenoble.

Results and discussion
The firing temperatures during the synthesis, together with the results of the chemical analyses for all the single phase samples obtained are indicated in Table 1.The samples corresponding to the M and A series have been slowly cooled at a rate of 50 K/hour from the synthesis temperature up to 473 K and then removed from the furnace and allowed to cool until room temperature.The B samples, instead, were cooled by immersing them in liquid nitrogen from the synthesis temperature.
been not posskble to obtain single phase samples with higher Na contents than those indicated in Table 1.The samples with higher nominal Na content showed after the synthesis process additional Bragg reflections, corresponding to =-Fe2O3, in the X-ray patterns.The composition dependence of the cubic lattice parameters in the different series compounds are show in figure 1. becomes clear that an important loss of Na does exist in the M series compounds which allows to understand the invariancy of their lattice parameters (fig.1).As a consequence of the Na volatilization, and taking into accound that only the spinel phase is observed in the X-ray patterns, we can conclude that a mixture or a solid solution of different spinels, such as ZnFe204 and Fe304, does exist For example, in lithium spinel ferrite theloss of lithium or oxygen results in the formatton of Fe304 or 8-Fe2O3, depending on the synthesis temperature and oxygen pressure [4].Due to the similarity of the lattice parameters of these solid solutions it is very difficult to Under our experimental procedure it as From the results reported in  distinguish them in the X-ray diffraction patterns.Nevertheless, the different magnetic characteristics of these spinels enable us to distinguish them by using Mijssbauer spectroscopy and thermomagnetic measurements.A typical Mijssbauer spectrum of a M series compound is shown in figure 2 where the coexistence of a magnetic and a paramagnetic component is evidenced.The hyperfine parameters of the magnetic phase observed in the Mijssbauer spectra are indicated in Table 2.The best fit of the x = 0.1 spectra has been obtained by introducing two separate hyperfine fields while the spectra of the x = 0.05 compound have been fitted to an unique hyperfine field.The spectra obtained at T = 80K show that the intensity ratio of the magnetic to the paramagnetic componentisinvariantwith temperature; thus any interpretation of these spectra in terms of a superparamagnetic relaxation can beexcluded.The values the hyperfine fields correspond roughly to those found in Fe304 or Fe3-xZnx04 spinels [5], supporting our conclusion about the formation of solid solutions or mixtures of ZnFe204 and Fe304.
The isothermal magnetization curves of the M series compounds have been measured in fields up to 55 KOe at temperatures ranging from 4.2K until 300K.Above saturation the curves M (H) can be fitted by a law M(H) = Mo + X H.In figure 3  In what it concerns the A and B series compounds, both the chemical analyses (Na, Zn and Fe contents) an the linear increase of the lattice parameters with the Na content (Vegard's law) show that the Na ions are effectively incorporated into the lattice.Because of the high ionic radius of Na ions (rvl = 1.02A [8]) we may expeotthat they will occupy the octahedral sites of the spinel structure.In this way, the increase of the lattice parameters with x can be understood as an overall rise of the size of the spinel polyhedra.This effect is put in evidence in figure 4, where for comparison purposes, the values corresponding to lithium-zino ferrites are included as well.
Some preliminary high field (150 KOe) magnetic measurements of these samples show that saturation magnetizations as high as 72 emulg (2.9, f l /f.u.) can be obtained (x = 0.25, T = 4.2K).This magnetization curve shows as well a superposed high field differential sisceptibility of x = 13.7'10-5emu/g which can be interpreted, as for lithium-zing spinel ferrites [ 9 ] , as arising from localized random canting.of the spins., Nevertheless, if we compare these values with those representing the lithium compound with the same Zn content, we find that higher Mo values (4.3 p B /.f.u.) and lower x values(7.10emu/g)are obtained in the Li-Zn spinels, indicating that a higher degree of canting does exist in the Na-Zn spinels.Finally, the room temperature Mijssbauer spectra of the A and B series samples show a progressive evolution from a typical quadrupole splitted paramagnetic spectra to spectra showing features characteristics of the spin-spin relaxation phenomenon which is well known to exist in Zn susbstituted spinels [lo, 111 .
Fig.l.Lattice parameter as a function of the Na content, x, for the different series.