SIZE EFFECTS IN FINE PARTICLES OF Fe3O4

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SIZE EFFECTS IN FINE PARTICLES OF Fe3O4

Yu. Krupyanskii, I. Suzdalev

To cite this version:

Yu. Krupyanskii, I. Suzdalev. SIZE EFFECTS IN FINE PARTICLES OF Fe3O4. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-407-C6-410. �10.1051/jphyscol:1974679�. �jpa-00215836�

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no. 12, Tome 35, DCcembre 1974, page C6-407

SIZE EFFECTS IN FINE PARTICLES OF F e 3 0 4

Yu. F. KRUPYANSKII and I. P. SUZDALEV Institute of Chemical Physics Academy of Science, Moscow, USSR

Rbum6. - On a ktudie par spectroscopie Mossbauer et par des mesures magnktiques les pro- priktks klectriques et magnktiques d'echantillons microcristallins stcechiometriques de Fe304 (dimension des particules entre 200 et 4 000A). On observe une transition metal-isolant aussi bien en diminuant la taille des particules qu'en appliquant un champ magnktique externe de 3 kOe A tempkrature ambiante.

Les donnks des mesures d'aimantation rkvklent que dans l'etat isolant I'arrangement magnktique dans des particules de taille d'environ 200-300 A est diffkrent de l'ordre de Verwey du matkriau macroscopique. La transition mktal-isolant est expliquke par des effets quantiques de taille : la longueur d'onde des porteurs de courant peut devenir comparable ou m6me supkrieure a la taille des petites particules.

Abstract. - The electrical and magnetical properties of microcrystalline, stoichiometric Peso4 samples (particle size between 200-4 000 A) were studied by Mossbauer spectroscopy and by magnetization measurements. As well for decreasing particle size as upon application of an external magnetic field of 3 kOe at room temperature a metal-insulator transition is observed.

The magnetization data show that in the insulating state the order in particles with sizes on the order of 200-300A is different from the Verwey order of bulk material. The metal-insulator transition is explained by quantum size effects : The wave length of current carriers may become comparable or even larger than the size of fine particles.

1. Introduction. - In the last time the interest in the metal-dielectric problem and the transition between the metallic and the dielectric state is strongly increasing. In connection with it the study of solids with well-known structure and electrical properties becomes very important. Magnetite (Fe,04) is a solid for which Verwey observed a transition similar to a metal- dielectric transition [I].

Considerable successes were achieved with theoretical and experimental studies of magnetite above and below the Verwey temperature Tv [2-61. It has been shown, for example, that for T > Tv long wave length carries with I & c ( I = wave length, c = lattice constant) are in part responsible for the conductivity o = a(T) of Fe30,. Under these conditions one can expect that the kinetic properties of the solid depend on size and form of the solid, if its size becomes comparable with the wave length of the current carriers. This paper deals with the variations of magnetical and electrical properties of stoichiometric Fe,04 upon decreasing particle size which were studied by Mossbauer spectroscopy and magnetization measurements.

In the case of magnetite Mossbauer spectroscopy provides the unicum possibility to study conductivity since a specific hyperfine structure due to the delocalization of current carriers is observed. In the dielectric state (below Tv) this structure disappears. Similar obser- vations have been reported in the literature [3,4, 51.

This peculiarity of Mossbauer spectroscopy is very

valuable for the study of small particles, since classical conductivity measurements on these samples essentially provide information about contact effects between particles.

2. Methods and results. - Fine particles of Fe304 were prepared by thermal decomposition of Fe,(C,04), . 5 H,O in a rarified oxigen atmosphere.

The decomposition temperature was 400 OC and higher.

The mean particle sizes were drawn from measurements of the sample surfaces. For all samples a accurate X-ray analysis was done from which we conclude that the structure of Fe304 was pure without any a-Fe and FeO impurities. Particles with a diameter

dm > 350

A

consist of Fe304 with spinel structure.

Particles with dm = 200

A

have no line in the X-ray spectrum which is typical for spinel structure and show two lines which cannot be attributed in this lattice type. Probably this sample has either a disturbed spinel structure or, however, an orthorhombic structure with c = 8.36 A, a = 5.23

A

and b = 6.05

A

which is also compatible with the X-ray pattern.

Mossbauer spectra of 4 samples with different particle sizes for T ⁼300 K with and without applied magnetic field are shown in figure 1.

The spectra consist of two 6 line hyperfine patterns ; one is due to tetrahedral (A) (lines 1-6), the other is due to octahedral iron sites (B) (lines 1'-6'). The hyperfine structure lines 4, 4', 5, 5', 6, 6' coincide. The relative intensities of the lines 1, l', 2,2', 3,3' change for decreas

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974679

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C6-408 YU. F. KRUPYANSKII A N D I. P. SUZDALEV

FIG. 1. - Mossbauer spectra of samples of Fe304 : a) at T = 300 K, H = 0 ; b) at T = 300 K, H = 3 kOe.

ing particle size. Similar but smaller variations are dm = 200

A

the particles are dielectric. Localization is fo'tind upon application of an external magnetic field favoured by the application of an external magnetic

of 3 kOe. field of 3 kOe (see Fig. Ib). Note that a similar effect

For T = 300 K the effective hyperfine field at was' observed in bulk Fe304 even in larger magnetic

positions A and B are fields [3] !

These values agree with the data of bulk magiie- tite [3]. All hyperfine parameters of the sample with dm = 3 500

A

satisfy the bulk data of Fe304 a t T = 300 K and 80 K. In samples with particles of smaller size additional hyperfine lines (7, 8, 9) appear, which are absent in bulk Fe304. They can be attributed either to a defective structure of particles (especially due ot their large surfaces) or to the appearance of new non-equivalent positions of iron- in Fe304 [9].

It is known that the ratio of the line intensities of the spectra due to A and B sizes characterizes the degree of electron delocalization using a parameter D = Si/S, which is equal to the ratio of the areas of lines 1' and 1.

Thus D ' k 2 is connected with the metallic state and D ^%0.5 with the dielectric. This conclusion follows from Mossbauer spectroscopy and conductivity data for bulk samples [3].

The values D given in the table are taken from the least square fitted Mossbauer spectra (Fig. 1). One may now look how the degree of delocalization can be related to the decreasing particle size : The degree of delocalizition decreases with size and for a size of

The temperature dependence was studied for particles with d,,, = 200, 500, 2 200 and 3 500

A

in the temperature range between T = 90-300 K. Because of the onset of sintering of the particles no higher temperatures were used. The Verwey temperature was determined from the spectra with D

--

1 in a way similar as used for bulk material. One can conclude that for decreasing particle size the Verwey tempera- ture T, is increased (see Fig. 2).

Figure 2 shows the magnetization data M,, again for different particle sizes. The results are in good agree- ment with those drawn from Mossbauer data. For increasing degree of localization, M, decreases in comparison to the mean value for bulk material. The value M, for dm ⁼200

A

(in the dielectric state) is four

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SIZE EFFECTS IN FINE PARTICLES OF Fe304 C6-409

FIG. 2. -Saturation magnetization M as function of mean particle diameter in samples of Fe304 at T = 300 K (dashed line). Verwey temperature Tv as function of mean particle dia-

meter in samples of Fe304 (dotted line).

times smaller than that for bulk Fe304. It is remarkable that M, decreases only 0.5 % from its mean value for bulk Fe304 when passing the Verwey transition [lo, 111.

3. Discussion. - From our experimental data we can conclude that a metal-insulator transition occurs in fine particles of Fe30, with decreasing particle size or upon application of an external magnetic field. We consider some possibilities for explanation : this transition is a consequence of a size effect or it results from one of the mechanisms discussed below.

It is known [I, 121 that the Verwey transition is influenced by the stoichiometry of the samples (especially by impurities of y-Fe203). For example already 4 % of y-Fe20, lead to the disappearance of this transition 1121, but in this case the saturation magneti- zation M, of non-stoichiometric Fe30, is equal to that of stoichiometric Fe30,.

In our case M, drops when the electrons are loca- lized, which is in disagreement with the above assumption. Furthermore, non-stoichiometry cannot explain the influence of the magnetic field on the Mossbauer spectra, i. e. on electron localization. The presence of impurities of a-Fe can be excluded from the X-ray analysis. Also the Verwey transition would not be influenced by these impurities [13].

The observed effect cannot be explained by a cation redistribution process taking place during the growth of the particles. In fact, the magnetic moment of one molecule of magnetite [14] is

where y is a coefficient characterizing the inverse spinel structure, ^pBis Bohr's magneton. For normal Fe304 we have y ⁼0. For another cation distribution 0

<

y

<

1 and M, must increase. This is in disagree- ment with our data.

We have also to consider the possibility that the cation distribution in our particles is different from the distribution of a spinel [9]. But again the influence of

the external magnetic field on the Mijssbauer spectra and the increasing Verwey temperature with decreasing particle size cannot be explained in this way. We suppose therefore, that the observed phase transition results from the influence of particle size on the physical properties alone. Some physical arguments for this assumption are given in recent theoretical treatments of conductivity [7] and of the nature of ordering [8] of magnetite. The conductivity a was calculated [7] as a function of T on the basis of a band model for the non-localized state in the hopping regime above Tv (see ref. 8). The upper (electron) bands are assumed to be flat, the lower one (holes) to behave as

I

k2

I.

Such a dispersion law is correct only if k. c

<

1, where c is the lattice constant. This calculation agrees with experiments for magnetite above the Verwey temperature. We therefore can assume that the wave length of mobile holes

A

9 c.

The quantum size effect becomes essential, if the wave length of current carriers becomes comparable or larger than the size of particles. We consider the particle to represent a 3-dimensional square-well. The gap between valence and conduction band will appear with the width

where m* is the effective mass of current carriers, d is the size of particles. This phenomenon has been des- cribed theoretically for thin films [15].

If an external magnetic field is applied to a substance with semimetal or semiconductor properties, this may lead to the origin or to the increasing of the gap and consequently to a metal-insulator transition under certain conditions, such a condition is shown in reference [16]. In bulk material the magnetic field must be of the order of 1060e to cause the metal-insulator transition. Apparently in small particles already a small increase of A E can induce the transition.

AE is on the order of kT for Bi-type semimetals with a particle size d

-

^{lo-' cm} ^(m*is small in this case [17]). For magnetite, however, the conduction band is narrow due to the large effective mass m*.

Therefore the nature of the metal-insulator transition must be of quite a different type as for semimetals or semiconductors, which contain no transition elements.

The transition in fine particles of Fe304 seems to be similar to a Mott transition. The transition is caused by the appearance and the increasing of the band gap.

The observed metal-insulator phase transition for decreasing particle size or upon application of an external magnetic field is apparently a first order transition. This can also explain the distortion of the spinel structure for particles in the insulating state.

Let us return now to the magnetization data.

At the Verwey transition the saturation magnetization decreases only

-

^0.5^%from its mean value. In our case, however, we find a drop from 110 emu/g to

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C6-410 YU. F. KRUPYANSKII AND I. P. SUZDALEV

26 emu/g. We conclude, therefore, that for small particles the order in the insulating state is quite different from the Verwey order in bulk material. In small particles the non-collinearity in the magnetic sublattice [18] seems to become effective, and conse- quently the Fez+ ions of lattice B may couple anti- ferromagnetically. This can lead to a reduction of saturation magnetization.

4. Conclusion. - A metal-insulator transition was observed in fine particles of Fe,O, for decreasing particle sizes or upon application of an external magnetic field. At T = 300 K without applied field the transition takes place for a particle size of d

-

²⁰⁰

^A.

The experimental data allow to conclude that the order in the insulating state of highly dispersed magnetite is quite different from the Verwey order.

References

[I] VERWEY, E. J. W., Nature 144 (1939) 327. [9] ALIEV, A. A., POVITSKII, V. A,, STUCAN, R. A., Proceedings [2] CALHOUN, D. A., Phys. Rev. 94 (1954) 1577. of the conference on application of Mijssbauer spec- [3] KUNDIG, W., HARGROVE, R. s., Solid. State Commun. 7 troscopy. Dresden 1971, p. 455.

(1969) 223. [lo] WEISS, P., FORRER, R., Ann. Phys. 12 (1929) 279.

[Ill BICKFORD, L. R., _Jr.,Rev. Mod. Phys. 25 (1953) 75.

r41 HARGROvE, R. S . y KUNDIG, W., "lid State Cornmu'. [12] DANIELS, J. N., ROSENCWAIG, A., J. Phys. Chem. Sol. 30

(1970) 303. (1969) 1561.

[5] RUBINSTEIN, M., FORESTER, D. W., Solid State Commun. 9 [13] GREENWOOD, N. N., HOWE, A. T., J. Chem. Soc. (Dalt.

(1971) 1675. Trans.) 1 (1972) 116.

[6] R O G W I L L ~ , P., KUNDIG, W., Solid State Commun. 12 [I4] N'EL, L., knnrs de dhys. (Ig4') 137.

(1973) 901. [15] SANDOMIRSKII, V. B., JETP 48 (1962) 2309.

[16] AZBEL, M. YO., BRANDT, N. B., JETP 48 (1965) 1206.

[7] CULLEN, J. R., CALLEN, E. R., CHOUDHURY, B., Phys. Left. [17] TAVGER, B. A., DEMICHOVSKII, V. Ya., Usp. Fis. Nauk 96

38A (1 972) 11 3. (1968) 61.

[8] CULLEN, J. R., CALLEN, E. R., Phys. Rev. B 7 (1973) 397. [IS] YAFET, Y., KITTEL, Ch., Phys. Rev. 87 (1952) 290.