Journal of Magnetism and Magnetic Materials 140-144 (1995) 425-424

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agnetic and structural properties of fine G. Pet6 a, I. Szab6 a, I?. KisdiXosz6 ap*, 6. Zsoldos a, L. Guczi b, P.

a 3%~ KFKl Reh. hr. jar hfareriais Scrence, Ehdapest, Hungary b MTA Research Institrrte for Isotop, Btipest, Iiuqury

’ PJ. $afLrih Uniuersiry, KoSice, SIouak Republic

Abstract The structural and magnetic properties of finemet thin films and bulk (rapidly quenched) ribbons show tba

and differences can be found in the amorphous and in the nanocrystalline state. Crystalliiation begins earlier n t magnetic properties calt be achieved only during co-existence of a-Fe and amorphous matrix.

1. Introduction

Investigation of amorphous magnetic thin films is very common, offering new aspects in the understanding of these materials and a good possibility for structural investi- gations with TEM, since one does not need to thin the sample. On the other hand extra work is needed to ascer- tain whether the structure is the same as in bulk material. The same is true for the magnetic properties which are also studied intensively because of their interesting magnetic applications.

In this paper a comparison is made betweeu bulk (rapidly quenched ribbon) and thin film (sputtered from that ribbon) fmemet type alloy in the amorphous and (nano-) crystalline state in order to see the similarities and differences.

2, Results and discussion

Finemet type (Fe,s, Cu,Nb,Si,,,B,) 400 nm fitms were deposited by ion beam sputtering on a water-cooled SiO/Si surface for magnetic measurements and on a NaCl/Si surface for TEM investigations - these latter requiring a thickness of 70 nm. As it is allways the case when an alloy target is used, we supposed that the chemi- cal composition is the same as that of the target [l], The as-prepared thin fdms and the heat treated samples were investigated by X-ray diffraction to determine their struc- ture; the as-prepared films were all amorphous. The elec- tronic structure was measured by X.R.D. When the energy distribution of photoeletrons excited by He I was mea-

* Corresponding author Fax: +36-1-155-0694; email: kis- dine@rl.atki.kfki.hu.

sured, the as-prepared film gave a spectrum $0

the amorphous state, and after heat treatme i-d 500°C for 30 min. some crystalliaa!io~ could be seen. The density of states, DOS, at the Fermi level was very sirn~~ to previous results observed on Fe-Si-B crystallized alloys [2]. The magnetic properties were inves- tigated by Foner magnetometer and a Kerr hysteresisgraph [3].

In Fig. 1 the results of X-ray investigations are for both the thin films and the rapidly after heat treatments. It is only a qualitative de of the existence of the phases as from X-ray ~~e~~~ it would be diicult to determine the the given phases. One can see that the tryst a-Fe and Fe,B begins earlier in the fi This is understandable because in the crystallization begins on the surface as well [4J, but it can be revealed much more easily in the film than in the ribbon. The crystallization of Fe-B in the film the tetragonal Fe,,B but for temperatures hi 550°C the orthorhombic Fe,B will crystallize.

There is another interesting difference between the and the thin film: in the film the Si content of the a-Fe nanosrystals increases much slo*wer than in the ri _. maybe in connection with this - the ~~~bo~ m exists longer, although it gets smaller; in addition Fe&I crystallites are already appearing. This also has a notice- able effect in the magnetic pro

Fig. 2 shows the coercive fie of the rapidly quenched ribbon m after various heat treatments. ribbon are similar to other data [5] The coercive forces of the magnitude higher than that of cate that there must be some s the fti and the ribbon. TEM PI

0304-8853/9S/W9.50 Q 1995 i%evier Science B.V. All rights reserved SSDI 0304~8853(94)01010-2

6. P&et 01. /Jamal ofMagnetism and Magmic Materials 140-144 (1995) 425-426

in tiemet thin film and rapidly quenched ray ~v~~~~ idler heat treatments at

Annealing time is always 1 h.

wed some differences and further inves- ss, the result of them will be pub-

Fig. 2 coercive force as a ion of T, armtaling temperature rapidly quenched ribbon. Annealing time

X: ribban. The insert shows variatio of coercive force of the riibon.

emphasizes the role of the amorphous matrix which pro- vides the magnetic coupling between the a-Fe nanocrys- tals. It can therefore be said that it is not the appearance of Fe,,S crystallites that causes the increase of I-f, but the disappearance of the magnetic coupling between the nanocrystals.

For the magnetic investigation of thin films, one of the most suitable m&& is ?he Kerr m3ge!ocptica! hjstcrc- sisgraph as this gives information from the about 30-50 nm thick surface layer 121. We investigated our 400 nm thick film in the as-prepared state and after heat treatment at 550°C for 1 h. For the as-prepared state this surface coercive force, H,” = 1600 A/m is slightly higher than that obtained for the whole cross section of the film by the Foner magnetometer (H, = 1000 A/m) but for the an- nealed film the difference is higher: H,” = 840 A/m in- stead of H, = 128 A/m. This may be because there are more Fe,,B crystallites at the surface.

As a very brief summary of our results, in thin films the dynamics of crystallization and thus the appearance of soft magnetic properties differ from tha! of bu!k ribhns. The two orders of magnitude difference between the coercive force of film and ribbon in the amorphous and nanocrys- talline state suggests that there must be some structural difference between them. The role of the amorphous ma- trix, which gives the magnetic coupling between the @-Fe nanocrystals, is emphasized.

Acknowledgements: We are grateful to the Institute of Physics of the Slovak Academy of Sciences, Bratislava, for providing us with the finemet amorphous ribbon. The work was sponsored in part by the OTKA Fund of Eiun- gary through grants Nos. T-7574 and T-2954.

[I] L. Maissel, Handbook of Thin Film Technology, Ch. 4. (McGraw-Hill, New York, 1970).

[2] 1. Kanski, G. Petii, Solid State Commun. 51 (19841747. (31 P. Kolllr, P. Skyba, L. Potockf, 2. JurCnek, Electrotecha. Gas.

38 (1987) 283. [4] J. Kanski, G. Petii, i. Zsoldos, J. Non-Cryst. Solids 88 (1986)

[5] LyMiiller, N. Mattem, L. II&en, J. Magn& Magn. Mater. 112 (1992) 263.