UNIAXIAL ANISOTROPY OF AMORPHOUS CoZrNb FILMS

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UNIAXIAL ANISOTROPY OF AMORPHOUS

CoZrNb FILMS

G. Heller, G. Bayreuther, H. Hoffmann

To cite this version:

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JOURNAL DE PHYSIQUE

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

UNIAXIAL ANISOTROPY OF AMORPHOUS CoZrNb FILMS

G. Heller, G. Bayreuther and H. Hoffmann

Institut fur Angewandte Physik, Universitat Regensburg, 8400 Regensburg, F.R.G.

Abstract. - The effect of substituting Zr by Nb in amorphous C O ~ . ~ ~(0.0 Z5 ~y

5

~0.11) films prepared by . ~ ~ ~ ~ N ~ ~ r.f. sputtering has been studied. Saturation magnetization, M S , coercive field, Hc, anisotropy field, HK, and structure constant, S, decrease with increasing Nb content. The annealing behaviour of the uniaxial anisotropy remains essentially unchanged.

1. Introduction

For magnetic thin film heads materials with low coercive force, Hc, large susceptibility and large saturation magnetization, Ms, are desired. The uniaxial anisotropy field, HK

,

should be a few Oersteds. Amor- phous CoZr films are known to have these properties [I-41. A disadvantage of CoZr films is the relatively high saturation magnetostriction of 3

x

[I, 41 which limits a further reduction of Hc. In the case of CoZrNb films it is possible to vary the saturation magnetostriction from positive t o negative values by changing the Nb concentration [5].

2. Experimental

Amorphous CoZrNb films were prepared by r.f. sputtering in the presence of an inplane magnetic field. Film thickness and composition were determined by X-

ray fluorescence analysis. The films have the composition C O O . ~ ~ Z ~ O . ~ I - ~ N ~ ~ with 0.0

5

y

< 0.11. The film

thickness was between 20 and 500 nm. Halo diffraction patterns typical for amorphous materials were ob- served by X-ray and electron diffraction. Saturation magnetization, Ms, and coercive field, H c , were mea- sured with a vibrating sample magnetometer and a magneto-optical hysteresis loop tracer. The anisotropy field, H K , was determined from the field dependence of the transverse biased susceptibility. This method is based on the ripple theory [6]; it has been described in detail in previous papers [7, 81. One also obtains the structure constant, S, which is a measure of inhomoge- neous local anisotropies. Lorentz microscopy was used to observe domain walls and the magnetization ripple. The domain structure of the films was determined by using a Kerr microscope.

3. Results

3.1 As PREPARED FILMS. -Figure 1 shows the concentration dependence of the saturation magnetization for CoZrNb films with constant concentration of Co. The saturation magnetization decreases with increasing Nb concentration. The values for Ms agree with the data of Attina [5] and Materne [9].

Fig. 1. - Dependence of the saturation magnetization on the concentration of Nb for C o o . s g Z r ~ . ~ l - ~ N b ~ ( 0 ) present

results; (A) Attina [5]; ( m ) Materne [9].

The values of the coercive field, Hc, of CoZrNb films vary between 0.1 and 0.2 Oe and are therefore slightly smaller than in CoZr films (0.1-0.3 Oe 17, 81).

The structure constant, S, of CoZrNb films

(S = 0.6

-

1.9 x erg/cm2) in general is smaller than in CoZr films (S = 1.5

-

2.2 x erg/cm2 [8]). This is attributed to the reduced magnetostriction [4] in these alloys. No ripple structure could be detected in films with S

5

1 x erg/cm2 thus con- firming the conclusion that these films are very close t o the ideal case of homogeneous magnetization.

Figure 2 shows the dependence of the anisotropy field, H K , on the Nb concentration. H K decreases with increasing concentration of Nb. This means that the anisotropy energy constant,

K,

=

HK

.Ms /2, decreases even faster with increasing Nb content.

Films were investigated immediately after prepara- 3.2 ANNEALING BEHAVIOUR. - The films were an- tion and after annealing in UHV with a magnetic field nealed at constant temperature (160 "C, 200 OC and

applied in the film plane. 240 OC) with the magnetization saturated along the

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C8 - 1746 JOURNAL DE PHYSIQUE

Fig. 2.

-

Dependence of the anisotropy field on the concentration of Nb for C O ~ . ~ ~ Z ~ O . I I - ~ N ~ ~ .

hard axis. Previously they had been "stabilized" by field annealing with the magnetization along the easy axis at the same temperature for 39 hours. The initial anneal reduced the structure constant, S, by 20 %

-30 %

.

Figure 3 shows typical annealing isotherms for a CoZrNb film.

- a slower reversible process with an activation energy of 0.1 eV. Thls process contributes 70-75 % to the total anisotropy.

The values for the activation energy in CoZrNb films are comparable t o those found in CoZr [8] and are considerably smaller than those reported by Maehata

et al. [3] (1-2 eV). This fact and the smaller values

of

Hx

indicate a different microstructure in their sam- ples.

3.3 FILMS WITH LOW UNIAXIAL ANISOTROPY.

-

When the anisotropy field decreased below 3 Oe during isothermal hard axis annealing we found that further annealing did not only decrease HK but also rotate the original easy axis. Observing these films in a Kerr microscope we found that there are four stable directions of the magnetization in specific applied fields. This indicates a complex superposition of anisotropies with different origins.

4. Conclusions

Amorphous CoZrNb films show very small ripple ef- fects and therefore approach the ideal uniaxial film. The uniaxial anisotropy can be decreased by isothermal hard axis annealing. The calculated activation energies are very low and indicate an atomic difhsion along the surface of the free volume of the amorphous films.

Fig. 3.

-

Annealing isotherms: hard axis field annealing of a ~00.89Zr0.08Nb0.03 film of 470 nrn thickness.

After this hard axis anneal the films were again annealed with the magnetization saturated along the original easy axis. The &l anisotropy field was lower than the initial value before annealing.

From the annealing isotherms we can deduce the relaxation times of several relaxation processes and their relative contributions t o the total anisotropy [8].

We find three different processes of atomic reorien- tation of the uniaxial anisotropy:

- an irreversible process with very small relaxation time (T

<

10 min), which contributes about 10 % to the total anisotropy;

- a fast reversible process with an activation energy of 0.2 eV. This process amounts to about 15-20 % to the total anisotropy;

111

Yamada, K., Maruyama, T., Tanaka, H., Kaneko, H., Kagaya, I., Ito, S., J. A p p l . Phys. 55 (1984) 2235.

[2] Shimada, Y., 3. A p p l . Phys. 56 (1984) 2996. [3]Maehata, Y., Yamaori, T., Hattori, M.,

Tsunashima, S., Uchiyama, S., Rapidly Quenched Metals, Proceedings of the Fifth International Conference on Rapidly Quenched Metals, Eds. S. Steb, H. Warlimont (North Holland) 1984, p. 1279.

[4] Shimada, Y., Kojima, H., J. Appl. Phys. 53

(1982) 3156.

[5] Attina, P., Olivetti Res. Tech. Rev. 4 (1985) 29. [6] Hoffmann, H., IEEE Trans. Magn. 4 (1968) 32. [7] Hoffmann, H., Heller, G., Kammerer, K.-H.,

IEEE Trans. Magn. 23 (1987) 2731.

[8] Heller, G., Kammerer, K.-H., Bayreuther, G., Hoffmann, H., Proc. of the Int. Symp. on Trends and New Applications in Thin Films (Strasbourg) 1987, p. 567.

[9] Materne, A., Geynet, J., Moriceau, H., J. Chem.