SUPERPARAMAGNETISM IN GRANULAR COMPOUNDS AS SEEN BY MÖSSBAUER SPECTROSCOPY

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SUPERPARAMAGNETISM IN GRANULAR COMPOUNDS AS SEEN BY MÖSSBAUER

SPECTROSCOPY

J. Dormann, P. Renaudin, P. Gibart, C. Sella

To cite this version:

J. Dormann, P. Renaudin, P. Gibart, C. Sella. SUPERPARAMAGNETISM IN GRANULAR COM-

POUNDS AS SEEN BY MÖSSBAUER SPECTROSCOPY. Journal de Physique Colloques, 1979, 40

(C2), pp.C2-96-C2-96. �10.1051/jphyscol:1979234�. �jpa-00218632�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au n O 3, Tome 40, mars 1979, page C2-96

SUPERPARAMAGNETISM I N GRANULAR COMPOUNDS AS SEEN B Y MOSSBAUER SPECTROSCOPY~

J.L. Dormann, P. Renaudin, P. Gibart and C.

el la*

Laboratoire de Magne'tisme, C . N . R. S., 1, Place A r i s t i d e Briand, 92190 Meudon, France

f

Laboratoire de Physique des M a t e ' r i m , C. N. R. S . , Meudon, Prance

RQsum6.- Les f i l ~ s granulaires / I / sont des matSriaux composites constituds de particules de mdtal (15 P 200 A) dispersds dans une matrice amorphe et isolante. Les composds ltudiSs Fe- SiOl 1 2 1 , Fe-A1203 et Fe-ZrOn ont Cite prdpards par copulvdrisation de Fe et de SiOn (ou A1203 ou Zr02). La microstructure et le diamstre des grains de fer dgpendent de la proportion rela- tive de fer et de matrice. L'effet Mzssbauer sur le ''~e permet d'ltudier la transition ferro superparamagndtisme et de prgciser des propridtds de l'interface mctal-isolant en corrdlation avec la microscopie Slectronique.

Diffdrentes compositions ont St6 dtudides de 4,2K 1 300K. Pour les matdriaux peu riches en fer, les grains sont de petit diamztre et les composds sont presque amorphes. Pour 12s composds contenant de 50 B 60 % de fer, les diamstres des grains varient entre 50 et 100 A et plu- sieurs sites de fer sont mis en dvidence : deux sites, d'intensitds faibles, correspondent d'une part B du fer paramagn6tique dilu6 dans la matrice, d'autre part B des composds non dd- finis Fe-0 matrice (H = 200 B 250 kOe B 4,2K); un site principal correspond au fer massif mltallique; deux sites simulant une rdpartition de champs hyperfins (370

<

^H

<

420 kOe) correspondent B des atomes de fer B l'interface 131. Les intensitds de ces sites varient beaucoup avec le diamgtre des grains et la nature de la matrice.

Abstract.- Granular films are composite materials made of small metals grains (15-200

i)

^{in an}

amorphous insulating matrix /I/. The granular compounds studied here were Fe-Si02 /2/, Fe A1203 and Fe-Zr02 with different ratio Felinsulating matrix. These samples were prepared by cosputte- ring of Fe and SiO2 (or Al2O3 or ZrOp). The microstructure was studied by electron microscopy.

Gssbauer effect was used to study the transition superparamagnetism

*

ferromagnetism as a function of T and to get information about the interface Fe-matrix /2/.

In 0.35 Fe, 0.65 Zr02, the spectrum shows a large distribution of Hhpf around the value of metallic iron, this sample is amorphous and relaxation effects are present at 4.2K. In 0.53 Fe, 0.45 A1203 the distribution of Hhpf is narrow, this sample is almost amorphous. In other samples with higher concentration of Iron, a magnetically ordered well defined spectra appear at 4.2K.

The following features are shown : a paramagnetic site (low intensity) was found which corresponds to very tiny Fe clusters. A magnetic site (low intensity) with Hhpf = 200 to 250 kOe corresponds to some iron-oxygen-matrix compound. The magnetic site with Hhpf

"

340 kOe corresponds to bulk metallic iron. Other sites with Hhpf = 370 kOe and 410 kOe correspond to iron atom at the surface of the grains 131.

Spectra at different temperature on samples 0.6 Fe, 0.4 A1203 and 0.5 Fe, 0.5 ZrO2 are typical of relaxation behaviour with transition temperature around 300 and 500K respectively.

The nature of the matrix is of significant importance on the behaviour of the magnetic grains.

For a given grain diameter the number of sites with Hhpf>340 kOe depends strongly of the nature of the matrix. Amorphous material appears when the concentration of iron is very low. The size diameter measured from electron diffraction photograph is consistent with the size deduced from ~Essbauer experiments. Fe-A1203 gives the narrowest linewidth, this means that the granular compound is well defined : interface effect are the lowest, concentrations of paramagnetic Fe and Fe compound are very low. Furthermore in Fe-A1203 there is a narrow rangeof grains diameter.

Fe-A1203 appears to be the best defined granular compound, i.e. : magnetic metal grains in an insulating matrix with a sharp distribution of diameters.

References

/I/ Review on granular compound is given in "structural and electrical properties of granular metals films", Abeles, B., Sheng, P., Coutts, M.D.and Arie, T., Adv.

Phys. (1975) 407.

/ 2 / Dormann, J.L., Gibart, P. and Renaudin, P., J. Physi- que Colloq.

37

(1976) C6-281.

/3/ Shinjo, T., Hine, S., Takada, T., Proc. 7th Intern.

Vac. Comp. and 3rd 1nter.Conf. Solid Surfaces (Wien 1977), 2655.

t ~ n extended version of this abstract is submited to thin Solid Films.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979234