MAGNETIC PROPERTIES OF AMORPHOUS FERRIC HYDROXIDE GELS

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MAGNETIC PROPERTIES OF AMORPHOUS FERRIC HYDROXIDE GELS

S. Okamoto, H. Sekizawa

To cite this version:

S. Okamoto, H. Sekizawa. MAGNETIC PROPERTIES OF AMORPHOUS FERRIC HYDROXIDE GELS. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-137-C2-139. �10.1051/jphyscol:1979247�.

�jpa-00218645�

JOURNAL DE PHYSIQUE Colloque C2, suppldment au n ^O 3, Tome 40, mars 1979, page C2-137

MAGNETIC PROPERTIES OF AMORPHOUS FERRIC HYDROXIDE GELS

S. Okamoto and H. Sekizawa

The I n s t i t u t e of PhysicaZ and CkemicaZ Research Wako-shi, S a i t m a , 351 Japan

Rdsum6.- Les propridtds magndtiques de gels amorphes d'hydroxides de fer prgpards de diffdrentes ma- niSres ont dtd Qtudides par spectromdtrie MEssbauer. I1 apparaft que l'influence du vieillissement se traduit par une augmentation du nombre de ~ e contenu dans les particules de gel, et un accrois- ~ + sement consid6rable du moment magndtique par ~ e ~ + . Cela signifie que le modsle de Ndel pour un anti- ferromagndtique perturbs n'est pas applicable aux gels amorphes d'hydroxide ferrique. Un modSle de petits amas ferrimagndtiques est proposd.

Abstract.- Magnetic properties of amorphous ferric hydroxide gels prepared in different conditions are investigated with the aid of Mijssbauer spectroscopy. It was revealed that when the ageing pro- ceeded, the number of Fe3+ contained in the gel particles increased, while the resultant magnetic moment per ~ e increased remarkably. This means that the Nee1 model of perturbed antiferromagnet ~ + at least, not applicable to the amorphous ferric hydroxide gels. Tentatively, ferrimagnetic sub- cluster model is proposed.

In previous studies /I/, we reported on the preparation and magnetic properties of amorphous ferric hydroxide gels. A complication of the gels has been found in the facts that many properties in- volving magnetic ones are strongly affected by the preparation conditions. Further, the gels were ob- tained as very small particles which led to a super- paramagnetic behavior superimposed on more intrinsic properties which seem to be reflecting the distri- bution of magnetic ferric ions in the almost amor- phous structures. In this paper, we will discuss the intrinsic magnetic natures and the amorphous struc- tures with the aid of assbauer spectra and X-ray diffraction patterns.

In table I are shown the characteristic data of three samples representative to ferric hydroxide gels.

The sample 0.1N and ION were freshly precipitated in dilute (0.1N) and strong (ION) NaOH solution, and the sample ION-A was precipitated in ION NaOH solu- tion followed by the ageing in IN NaOH solution for 30min, at 100~~. These results together with the X-ray diffraction patterns reveal that the chemical and magnetic properties of the gels change remarka- bly as the ageing proceed, though almost amorphous nature is kept unchanged.

X-ray diffraction patterns of all the samples examined were very diffuse ones suggesting practi- cally amorphous structure, but careful examination of the patterns taken by step counting techniques revealed several broad and weak lines on very high background. They were indexed taking hexagonal unit cell with a = 5.881 and c = 9.4/L, which correspond to approximately twice of each of the cell dimensions

Table I : Characteristic data of the ferric hydroxide gels Chem.Comp. 91 as (77K) particle sizex2 Mijssbauer data at 4.2K

Samples Fen03 (G. cm3 /g> (x1021 cm3) I.S.(s.s.) AE Hi Line width

I

¹

^{5 . 5}

I

1 1

All the samples contained 0.5 wt% of NanO. Trace of NO; was also detected.

"2~stimated by magnetic analysis.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979247

c2-138 JOURNAL DE PHYSIQUE

a = 2.951 and c = 4.521 of f errimagnetic 6-FeOOH, and also are compared with hexagonal unit cell of fer- ritin iron core 121 with a = 11.79 ( = 5.88x2)A and c = 9.91. It was proved also from the patterns that no trace amount (less than about 0.5 wt%) of crys- talline phases, such as a- or y-Fe203, or a-, 6- or y-FeOOH was mixed in the samples.

There were no appreciable differences of the

~Zssbauer spectra of various samples taken at room temperature. A typical doubly split absorption peak characteristic of a paramagnetic (or superpararnagne- tic) ferric ion in an axially symmetric electric field gradient was observed (AE = 0.60 mm/s)

.

77K, a broad absorption pattern with hardly appre- ciable structures was observed, suggesting a relaxa- tion time of the electronic magnetic moment at that temperature roughly comparable with the reciprocal of the Larmor frequency of the nuclear magnetic mo- ment. At 4.2K,a~well-resolved six-line pattern cha- racteristic of an ordered magnetic arrangement of the electron spin system was observed. The parame- ters observed are shown in table I. Most significant conclusions deduced from inspection of the Mijssbauer spectra are that the gels consists of only Fe06 octahedra, and that the blocking of the magnetic mo- ment is almost complete at 4.2K.

The almost negligible quadrupole coupling at 4.2K presents a striking contrast to the room tempe- rature value which is approximately 1.2mm/s. As has been stated by van Diepen and Pompa in their paper on an amorphous Fez03 131, such a difference is rather characteristic of amorphous magnetic mate- rials. In short, the quadrupole coupling effect su- perficially vanishes leaving wide apparent line widths. It was observed that the line widths of six

lines at 4.2K are much broader than the natural width. This should be caused partly by the above mentioned quadrupole coupling effect. However, as

the width of each line increases with increase in the distance from the center of the spectrum, an appreciable distribution of the hyperfine magnetic fields also contributes to the broadening of the widths.

A spectrum was taken with a magnetic field of 40kOe applied parallel to the direction of the y-ray.

As already has been shown in our previous report 111, the integrated intensities of the second and fifth peaks do not change very much compared with those of zero field spectrum. This means that the spins within the gel particles are very difficult to align parallel or perpendicular to the strong magne-

tic field. According to Coey and Readman, the spin direction within the gel particlesis almost statisti- cally distributed even in the magnetically ordered state relative to the direction of the resultant mo- ment (speromagnetic)/4/.

Ndel suggested that for the ultra-fine and very fine antiferromagnetic particles, the resultant ma- gnetic moment per Fe3*, M, decreases with increasing N as is shown in figure 1, (a) and (b), where N is the number of magnetic ions within the antiferroma- gnetic particles. On the contrary,observed values of the resultant magnetic moments per ~ e increased ~ + with increasing number of ~ e ~ + , as is shown in the

same figure.

Fig. 1 : Resultant magnetic moment per ~e~', M, ver- sus number of Fe3+, N, within ferric hydroxide gel particles.

(a) for ultra-fine antiferromagnetic particles.

(b) for very fine antiferromagnetic particles having odd number of special net planes on which all the magnetic ions belong to the same subblattice.

M=O, if number of the special net planes is even, (+ denotes the values observed by the present autbors

a by Coey and Readman 141, and

.

This means that the Ndel model of the perturbed antiferromagnet is not applicable, at least to the amorphous hydroxide gels, and some new models should be sought which can represent the real situation in

the amorphous solids.

The chemical and magnetic circumstances around FeOs octahedron which constitutes the gel particle may differ from each other Eecause each octahedron is linked with the adjacent octahedra differing in number, and in bonding manners sharing corners, edges or faces. That is, some of the octahedra are bound to the adjacent octahedra loosely with weak magnetic interactions, but the others are bound tightly with strong magnetic interactions forming a sort of sub-cluster. If the sub-cluster is ferrima- gnetic in nature, the resultant magnetic moment in- creases as the number of the sub-cluster in the gel

particle increases. The increase in the number of the sub-cluster is reasonably considered to be indu- ced by the ageing. It is well known that when the ageing proceeds, the gels become sparingly soluble against HC1 solutions. This implies that the FeO6 octahedra become more tightly bound resulting in the increase in the number of the sub-cluster in the gel particles

.

References

/I/ Okamoto, S., Sekizawa, H. and Okamoto, S.I.,

"Reactivity of Solids" (Chapman and Hall) 1972, p. 341.

/ 2 / Girardet, J.L. and Lawrence, J .J., Bull. S0c.F.

Mineral Cristallogr.

41

/3/ Van Diepen, A.M. and Popma, Th. J.A., J. Physique Colloq.

37

/4/ Coey, J.M.D. and Readman, P.W., Earth and Plane- tary Sci. Lett.

21

/5/ v.d. Giessen, A.A., J. Phys. Chem. Solids

28

(1967) 343.

/6/ Nlel, L., J. Phys. Soc. Japan

17