MÖSSBAUER STUDIES ON GRANULAR Fe-SiO2

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MÖSSBAUER STUDIES ON GRANULAR Fe-SiO2

J.-L. Dormann, P. Gibart, P. Renaudin

To cite this version:

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JOURNAL DE PHYSIQUE CoIloque C6, supplgment au no 12, Tome 37, DLcembre 1976, page C6-281

MOSSBAUER STUDIES

ON GRANULAR Fe-Si02

J.-L. DORMANN, P. GIBART and P. RENAUDIN Laboratoire de Magnetisme, C. N. R. S. 92190 Meudon-Bellevue, France

Rbum6.

-

Les composes granulaires Fe-SiOz ont kte prepares par copulverisation cathodique en utilisant des cibles segment& Fe-SiOz, un plasma d'argon (ou argon

+

Hz) ?I10-3 torr. Leurs spectres Mossbauer mettent en kvidence la transition superparamagnetisme-ferromagnktisme. Cette temperature de transition depend de la taille des grains de fer dans la matrice de SiOz. Une analyse fine des spectres Mossbauer montre qu'il existe plusieurs sites superparamagnetiques. Ceci s'inter- prete par des effets d'interface. I1 existe aussi plusieurs sites magnetiques, ceci est dil h une distribution des tailles de grains k l'intkrieur du compose. Les rtsultats de la spectrometrie Mossbauer don- nent des details sur la nature des composes granulaires qui n'avaient jamais et6 mentionnes aupa- ravant.

Abstract.

-

Granular compounds Fe-SiOz were prepared by cosputtering using Fe and SiOl targets, Argon (or Argon i- Hz) plasma under 10-3 torr. Mossbauer spectra from 4 to 300 K

clearly show the transition superparamagnetism-ferromagnetism. The transition temperature depends on the size of the iron grain in the Si02 matrix. A careful1 analysis of the Mossbauer spectra show that several superparamagnetic sites exist. This was interpretated by interface effects. Several magnetic peaks exist too, which are due to a distribution of grains size among the sample. Mossbauer data gives details on the nature of granular compounds which were never seen before.

1. Introduction.

-

Ultrafine particles of magnetic compounds have unusual magnetic properties, they are typically superparamagnetic. Superparamagnetism is due to thermal fluctuations of the magnetic moment of a particle which lead to zero average moment of the sample. The relaxation time of the magnetic moment of single domain particle is approximately given by : z = 7, exp(Kv/kT) (1) where

v : is the volume of the particle, K : the anisotropy constant,

=

1 0 - ~

Depending upon the size of the particle (v), z varies between 1 and 10-" S. Thus, superpararnagnetic properties depend on the measuring time. In y resonance spectroscopy on 57Fe the observation time is 2 X 10-a S. Using y resonance it is possible to study the transition to superparamagnetism as a function of v and T.

Recent papers reported superparamagnetism properties of ultrafine iron [l], y-Fe,03 [2,3] and Fe,O, [4] particles studied by Mossbauer spectroscopy. It was found that thermal fluctuations of the magnetic moment lead to the disappearence of the hyperfine structure of the spectra. A broadening of the lines was found. Very small iron clusters (< 13 pm) have been prepared by reduction of zeolites containing ferrous ions [5]. They behave like superparamagnets, but the

volume of the clusters is .so small that no hyperfine structure appear in Mossbauer spectra even at 4 K. Furthermore in small iron clusters, most of the atoms belong to the surface.

The present study reports the superparamagnetic properties of granular compounds Fe-SiO,. Granular compounds are of recent interest. These materials are small metal clusters dispersed in an amorphous matrix. i. e. Si0,-Ni or Si0,-MO. Magnetic and electrical properties of granular compound have been recently reported [6, 7, 81. Depending upon the composition

these compounds are either ferromagnetic (F) or superparamagnetic (SP). A broad maximum of the magneto- resistance occurs near the transition temperature. Mossbauer studies where not carried out on Ni-SiO,. We report here Mossbauer data on granular Fe-SiO,.

y resonance is a very sensitive way to get information on size repartition, on interface effect.

2. Experimental.

-

SOz-Fe granular compounds with various compositions were prepared by sputtering. Samples were prepared in 5 000

W

to 24 pm thin films deposited on SiO, substrates. The targets used in these experiments were a SiO, disc (12 cm in diameter) with several 2 mm large Armco >> iron bars. It was necessary to get uniform composition within the thickness of the film. In the case of Fe-SiO, cosputtering cone formation reported for other systems (Fe-Ag) was not observed. Sputtering was carried out using an Ar plasma or an argon

+

5

%

H, plasma under 10F3 torr.

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C6-282 J. L. DORMANN, P. GIBART AND P. RENAUDIN

tion of the target and the temperature of the substrat are relevant parameters.

In table I, some results are given : TABLE I

Results and CO-sputtering of Fe-SiO, target. Sample

-

1. 0.65 Fe 0.33 SiOz

Diameter

T substrat of grains Remarks

- R. T. < 20° A Broad Fe peak in electron diffraction 200 OC 50 A Microcrystalline Fe 300 OC 100-200 A Microcrystalline Fe (Fig. 1) R. T. 50 A Fe peak and broad

SiO2 peak (Fig.

2)

FIG. 1.

-

Electron diffraction and microscopic examination

of Fe-si02 granular films (- 100 A) : composition 0.65 Fe, Samples used for Mossbauer experiments were much

0.33 si02. _{thicker (25 pm) than for micro-diffraction experiments.}

3. Mossbauer data.

-

A large range of composition was studied for temperature between 4 and 300 K. For very tiny grains the transition SP-F occurs at very low temperatures, for large grains the samples remain F. Two compositions only are reported here (N 5 and N 3 (l)) which are of interest for the transition SP-F.

3.1 PRELIMINARY REMARKS.

-

Only static hyperfine interactions were taken into consideration. Relaxatio- nal effects are clearly present, but is almost impossible to do a correct analysis in the present case. Several expressions for the relaxation time z have been derived, but which diverge within an order of magnitude. Furthermore, theory for superparamagnetic flipping

1

were derived for an electronic spin S = - [9] and not 2

interacting particles.

At 4.2 K, five magnetic sites appear. It is believed

FIG. 2.

-

Electron diffraction and microscopic examination that a distribution of grain diameter exist. A relaxa- of Fe-Si02 : film composition 0.50 Fe, 0.50 SiOz. _{tional treatment in this Gase would imply too many} parameters for an unambiguous solution. Our analysis is only incorrect for a narrow diameter range corres- The applied voltage was 1 500 V. The substrates ponding to critical relaxation time. In a first appro- could be cooled to room temperature. Under these ximation, this gives consistent results for all tempera- conditions the speed of deposition was

-

1 pm/hour. tures. Particularly, no considerable broadening of the

There is a direct relation between the relative surface line width was observed between 4.2 and 300 K. of Fe and SiO, of the target and the composition of the

film. For instance starting from a target with 0.55 Fe and 0.45 SiO,, and assuming that the yield of sputtering at 1 500 V is 1.7 for Fe and 0.26 for SiO, the expected composition is Fe = 0.67, SiO, = 0.33. The composition deduced from the electronic microprobe data was Fe : 0.65, Si0, : 0.33 and 0.02 impurites, which is in

3.2 COMPUTING OF MOSSBAUER SPECTRA.

-

The programm used to fit the spectra can solve either paramagnetic or magnetic spectra (6 or 8 peaks) or both at the same time. In this programme any hyperfine parameter can be fixed a priori, or be deduced from a least square refinement using a method derived from

-

fair agreement with the expected composition. ₍₁₎ _N₅_:_{target with 5 iron bars on the SiO2 target, composi-} The microstructure of the granular films was studied tion and structure close to sample # 3.

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MOSSBAUER STUDIES ON GRANULAR Fe-SiOz C6-283

Gauss Newton. To fit correctly the Mossbauer spectra of these compounds, it was necessary to have different linewidth for the peaks of a given site.

A good fit of the spectra was obtained only when the experimental linewidth F,, of magnetic peaks was

choosen 2.3 time larger than the

T,,

of paramagnetic peaks. Furthermore relative transition probabilities ought to be slightly changed, the best fit was obtained

1 1 1

with

-

(1

-

X ) , - (l

+

2 X), - (1

-

X) with

4 6 12

3.3 TRANSITION SP-F.

-

Mossbauer spectra at different temperatures on N 5 and N 3 (Fig. 3,4) show

that most of the SP-F transition occurs between 150

and 300 K (N 3) and for T

>

300 K (N 5). It can be

seen from the two set of spectra that N 5 spectrum at

300 K and N 3 at 150 K are similar, so are N 5 at 77 K and N 3 at 40 K. This suggest that the volume of the

grains in N 5 is twice larger than that in N 3. (This

result is consistent with electron microscopy results.) In other words N 3 spectra at Tare equivalent to N 5

at T/2.

Counts ( X 10' )

r y

FIG. 3. - Computer analyzed Mossbauer spectra of N 5 for different temperatures. The zero velocity is the barycentre of a

metallic iron-spectrum.

3 . 4 DIFFERENT SITES IN GRANULAR COMPOUNDS.

-

Mossbauer spectra are quite different from those on SP

fine particles [l-5, 101. The physical properties of gra-

nular compounds and fine particles are not exactly the same : In fact, the contributions of the interface Fe- Si02, of strains and of coupling between Fe grains can be far to be negligible [l l].

FIG. 4.

-

Computer analyzed Mossbauer spectra of N 3 for different temperatures.

To get a reliable fit of the spectra, 3 SP sites must be

considered whose intensities vs Tare given on figure 5, 6, E on figure 6, and 1 to 5 F'sites whose hyperhe fields

are given on figure 7, intensities on figure 8. E for the

magnetic sites is weak ( G 0.05 mm/s) except the so

called A site for which -0.3 (4 K) < E ,

< -

0.1 mm/s.

6 are given on table 11.

TABLE I1

6 values of magnetic peaks ( N 5, N 3)

A Fe

-

- S 1

-

S2

-

S3 300 K 0.0 0.05 0.10 -

4 K 0.12 0.12 0.17 0.20 0.30

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C6-284 J. L. DORMANN, P. GIBART A N D P. RENAUDIN

FIG. 5. -a) SP iron sites and their intensities as a function

of T : N 5 broken line, N 3 full line ; b) Temperature dependence of the experimental full linewidth at half maximum for SP sites with respect to the same parameter of a thin natural

iron absorber.

FIG. 6. - Temperature dependence of the isomer shift (S), and the quadrupole splitting E = 114 e z qQ : N 5 dashed line,

N 3 full line.

FIG. 7. -Temperature dependence of the hyperfine fields : N 5 dashed line, N 3 full line.

FIG. 8. - F iron sites and their intensities as a function of I : N 5 broken line, N 3 full line.

of the grains occur ; the magnetization lies in the plane of the film. The transition probabilities used were between 3 : 2 : 1 (random) and 3 : 4 : 1 (internal field parallel to the film). Furthermore, it appears on electron diffraction pattern a slight texture. In amorphous Fe-Si alloys such an orientation was found [12].

4. Discussion. - 4.1 TRANSITION SP-F. - From the intensities plotted on figures 5 and 8, it is possible using relation (1) to get the distribution of the volumes of the grains. We took 2 ,1OP9 ~ S, 2 =20 X 10-' S, K = 4 X 105 erg cm-3. Results are plotted on figure 9.

1

4.2 K

I

S,

Is,

:

0

2 5 5

o

75 100

A

> FIG. 9. - Repartition of grains diameter in N 3. The arrows correspond to the SP-F transition temperature for the corres-

ponding diameter.

For the magnetic sites,

re,

is 2.3 times larger than the linewidth of SP peaks. This large value of

re,

for F peaks is due to a distribution of hyperfine parameter among iron atoms. It has been observed in granular Ni-SiO, compounds an expansion of the lattice parameter depending on the size of the grain [7]. The thermal expansion coefficient of SiO, and Fe are quite different, so that in granular compounds prepared by sputtering some strain will appear a t iron atoms [l l].

A statistical distribution of grains sizes results from the

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MOSSBAUER STUDIES ON GRANULAR Fe-SiOz C6-285

the

re,

observed corresponds to a fluctuation of temperature. It appears that there is a given hyperfine

+

15 kOe [13]. field for each range of diameters. The field increases A maximum of

re,

was observed for T = 220 K for low diameters (Fig. 7). There is no contradiction (Fig. 5b). This broadening is related to relaxation between interface effect discussed in 4.2, but due to the effects. A relaxational analysis, if possible, should not large

r,,

_-.-this effect cannot be analyzed.

give an increase of rex. This shows that most of the

U The A site has parameters close to Fe-Si alloys or

grains undergo the "ansition F-SP at about 220 K. _{amorphous Fe-Si alloys [I()], it corresponds to the}

The curve

re,

is to be compared to the repartition _{largest grain}_size_{( Z}_{g5 A).}

of grain sizes (Fig. 9).

_-

_{The Fe site corresponds to metallic iron (size 75} 4.2 SUPERPARAMAGNETIC STATE. - Three SP sites

were observed for all the temperature range studied. The P, site, with a constant relative intensity (0.05) whatever is T (for N 5 and N 3) could be due to an unknown compound Fe-Si-0. Its E is large (Figure 6).

However the microprobe analysis shows no evidence of departure from the formula Fe-SiO,. Other possibi- lities do exist, but we had no experimental evidence to identify unambiguously this site.

The intensities of P, (6

<

0, E

-

0) and P, (6 close to

metallic Fe) decrease with decreasing T: the grains become ferromagnetic. But due to the relative size of the grains (several tens of A) towards the iron atom unit cell 2.86

A,

a large fraction of iron atoms of the grains belong to the surface, or are very close to the surface. Atoms at the interface Si0,-Fe do not behave like core atoms. From the repartition of grains size and the transition temperature SP-F, the relative intensity of P, and P, can be calculated. It is deduced that a thickness of

-

6

W

(i. e. : two atoms layers) fit the results.

4.3 FERROMAG~TIC STATE.

-

1 to 5 sites are observed (A, Fe, S,, S,, S,) (Figures 7, 8, table 11). This large number of sites has never been observed in fine grains, so that a correct identification of each site is difficult.

The intensity of the Si sites increase with decreasing

to 85

A).

- The Si sites correspond to grains with diameter 60-75 L% (S,), 40-60

A

(S,),

<

40

A

(S,). Hyperfine fields (HF) are in the order A, Fe, S,, S,, S,. For S, the H F is close to values found in oxides. In small grains, the ratio surface/volume increases atoms at the interface are predominant and the atoms are involved in Fe-Si-0 chemical bound. H F of such small grains seem to be mostly produced by surface atoms.

5. Conclusions. - It appears that granular compounds do not behave like fine grains. The transition temperature SP-F depends on the grain size as expected, but several SP and F sites appear. The different SP sites can be related to core iron atoms and iron atoms at the interface respectively. The results are consistent with the grain size measured by electron microscopy.

The different F sites are due to a distribution of hyperfine fields, which is related to a grain size distribution.

Mossbauer spectroscopy is the only method which is able to give pertinent details on all kinds of iron atoms. Other granular compounds were reported before, but the influence of interface, strains, distribution of grain size could not be studied.

Acknowledgments.

-

The authors wish to express their thanks to C . Sella for the sputtering facilities.

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