IMPROVEMENT OF CORROSION RESISTANCE OF Tb-Fe-Co FILMS BY COATING WITH Tb AND Fe-Co LAYERS

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IMPROVEMENT OF CORROSION RESISTANCE OF

Tb-Fe-Co FILMS BY COATING WITH Tb AND Fe-Co

LAYERS

N. Saito, M. Takenouchi

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supplkment au no 12, Tome 49, dkcembre 1988

IMPROVEMENT OF CORROSION RESISTANCE OF Tb-Fe-Co FILMS BY

COATING WITH T b AND Fe-Co LAYERS

N. Saito and M. Takenouchi

R@ D DIV., Sony Magnetic Products Inc., 3-4-1, Sakuragi Tagajo-City, Miyagi-Pref., Japan

A b s t r a c t . - The magneto-optical properties and corrosion resistance of amorphous Tb-Fe-Co thin films by coating with T b and Fe-Co layers are discussed. T b layers protect Tb-Fe-Co layers agaist corrosion in a wet environment, and Fe-Co layers are effective against surface oxidation in a dry environment.

1. Introduction

For an application to magneto-optical memories, amorphous thin films of rare-earth (RE)/transition metal (TM) alloys have been investigated [I]. These alloys, however, are oxidized easily and their magnetic properties are significantly degraded. Several efforts have been tried to protect RE-TM alloys against ox- idation and corrosion [I-61. This paper shows an ap- proach to improve oxidation and corrosion resistance of Tb-Fe-Co thin films by coating with Tb and Fe-Co layers, without using any other metals.

2. Experiments

Amorphous Tb-Fe-Co thin films were prepared by co-sputtering Tb and Fe-Co using two independent magnetron sputter guns in a high vaccum system. A 100 nm thick TbI9Fe77Co4 alloy thin film was chosen as magneto-optical (MO) layer, and was coated with T b and Fe-Co layers. Tb and Fe-Co layers were de- posited within the same deposition run, and the thick- ness of these coating layers was from 1 to 10 nm. The Kerr rotation angle Bk and the coercive force Hc were measured by Kerr loop tracer (A = 780 nm)

.

We esti- mated the corrosion resistance by the following mya- surements; (1) the changes of Hc and Ok after ageing

in dry air a t 120 "C; (2) the electrochemical proper- ties, polarization resistance (R,)

,

corrosion potential (Ecofr)

,

and pitting potential (E,it)

,

in 3 % -NaCl(aq). Potentiostat and galvanostat were used for measure-

Fig. 1. - Changes of H, after ageing in dry air at 120 OC

(A) TbFeCo (MO); (B) Tb/MO/Tb; (C) FeColMOfFeCo (MO: d = 100 nm, T b or FeCO layer: d = 3 nm).

ments of electrochemical properties. Rp was measured by current control method and E,,,,, EPit were mea- , sured by potentiodynamic method [7-91.

To analyze the oxidation, the oxygen depth profiles were measured by Auger electron spectroscopy. 3. Results a n d discussion

In the first place, three samples, (A) Sub./Tb- Fe-Co (MO); (B) Sub./Tb/MO/Tb; (C) Sub./Fe- Co/MO/Fe-Co, were prepared and compared.

Figure 1 shows the changes of Hc after ageing in

DEPTH (

A

)

Fig. 2. - Oxygen depth profiles by Auger electron spec- troscopy after aging in dry air at 120 "C for 3 hours; (B)

Tb/MO/Tb; (C) FeCo/MO/FeCo.

JOURNAL DE PHYSIQUE

dry air at 120 OC. The coated layer (Tb or Fe-Co) thikeness is 3 nm. As compared with MO film (A), drastic decrease of the Hc /H, 0-values is observed in the case of T b coated film (B). On the other hand, these values are smaller in the case of Fe-CO coated film (C). The oxygen depth profiles of the samples after ageing in air a t 120 OC for 3 hours are shown in figure 2. The oxidized layer of Tb coated film extends from the surface t o 50 nm depth (B). In contrast to this, the oxidized layer of Fe-Co coated film is limited t o the surface of MO layer (C). These results show that Fe- Co layer protects MO layer against oxidation, but T b layer causes to a more rapid surface oxidation.

Table I shows the electrochemical properties of these three samples before againg. The values of Rp and AE

of T b coated film are larger than those of MO film. Particularly the value of R, is ten times large. Anodic oxidation of T b is easy to take place, and then passive oxide of Tb is formed. This passive oxide appears to protect MO layer against corrosion. These values of Fe-Co coated film are the same as those of MO film, and Fe-Co layer has no effects to corrosion resistance in a wet environment.

In the next place, three samples, (D) Sub./Tb- Fe-Co-Cr (MO); (E) Sub./Tb/Fe-Co-Cr/MO/Fe-Co- Cr/Tb; (F) Sub./Fe-Co-Cr/Tb/MO/Tb/Fe-Co-Cr, were prepared and the corrosion resistance of these films were estimated as same as before.

The results are shown in figure 3. MO layer thick- ness is 100 nm, and each of coated layers is 2 nm thick. In the case of double layers coated film with inner Fe- Co-Cr layer and outer T b layer (E), both oxidation and corrosion resistance are improved as compared with MO film (D). And in this case, these coating layers do not decrease the Kerr rotation angle Bko.

4. Conclusion

It is possible to improve the corrosion resistance of Tb-Fe-Co (MO) thin fiIms by coating with a few nanometers thick T b and Fe-Co layers:

1) T b layer protects MO layer against corrosion in a wet environment but accelerates oxidation in a dry;

2) Fe-Co layer protects MO layer against oxidation in a dry environment but has no effects in a wet;

3) Double layer, the inner layer is Fe-Co and the outer is Tb, is effective to protect MO layer against both oxidation and corrosion.

Acknowledgments

The authors thank Mr. M. Kikkawa for this help with Auger electron spectroscopy.

Table I. - Electrochimicat properties (in 3 %-NaC1, RE:

SCE, CE: Pt).

Sample Rp Epit Ecorr AE (Epit - &or=)

(KR cm2) (v) (v) _(V)

(A) TbFeCo (MO); (B) Tb/MO/Tb; (C) FeCo/MO/FeCo. Table 11. - Corrosion resistance of TbFeCoCr films

coated with double layer of Tb and FeCoCr.

Sample Hc /Hco Bk e k /&O Rp AE

-

(dee) (-> (KO cm2) (V)

(D) TbFeCoCr (MO); (E) Tb/FeCoCr/MO/FeCoCr/Tb; (F) FeCoCr/Tb/MO/Tb/l?eCoCr. Hc /Hco and Bk / Bko are the value after ageing in dry air 120 OC for 20 hours. (MO: d = 100 nm, Tb or FeCoCr layer: d = 2 nm).

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