The structure and magnetic anisotropy of amorphous Gd-Co films

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The structure and magnetic anisotropy of amorphous Gd-Co films

Y.-J. Wang, F.-H. Li, Zh.-Q. Wang, J.-J. Gao

To cite this version:

Y.-J. Wang, F.-H. Li, Zh.-Q. Wang, J.-J. Gao. The structure and magnetic anisotropy of amorphous Gd-Co films. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-239-C5-242.

�10.1051/jphyscol:1979587�. �jpa-00219004�

The structure and magnetic anisotropy of amorphous Gd-Co films

Y.-J. Wang, F.-H. Li, Zh.-Q. Wang and J.-J. Gao Institute of Physics, Academia Sinica, Peking, China

Abstract. — The structure of amorphous Gd-Co alloy films has been investigated by electron diffraction. The films were prepared by bias sputtering on liquid nitrogen-cooled NaCl substrates. The films were in the 400- 800 A thickness range. Magnetic domains were observed by Lorentz microscopy. Hysteresis loops were measured by Faraday magneto-optic effect with the applied field perpendicular to the film surface. Elastically scattered electron intensity and reduced radial distribution functions are significantly different for the perpendicular ani- sotropic and in-plan anisotropic films. It can also be deduced from these experimental results that the perpen- dicular anisotropy is associated with a predominance of Gd-Co atom pairs.

1. Introduction. — Amorphous Gd-Co films pre- pared by bias sputtering frequently have uniaxial magnetic anisotropy perpendicular to the film plane.

The origin of this positive K

has been one of the important problems in the field of amorphous magne- tism. Various mechanisms have been proposed about the origin of the positive K

[1-3], so the relation between the macro-anisotropy and the structure of the amorphous GdCo-based films has been studied.

Cargill [4] and Wagner [5] have used X-ray scattering to investigate possible atomic scale structural ani- sotropy (pair ording) in ~ 20 um thick bias-sputtered GdCo films. Because of experimental uncertainties, no convincing evidence concerning the origin of such an anisotropy was obtained. Graczyk [6] observ- ed anisotropic phase separation in very thin (500-

1000 A) bias-sputtered, magnetically anisotropic GdCo-based films by small-angle scattering, and explained the origin of positive K

on the basis of anisotropic phase separation. On the other hand, Hoffman's experiments [7] show that a columnar structure in uniaxial amorphous Gd-Co films may exist, however, the anisotropy does not necessarily depend on the columnar structure. Cargill's [8]

. X-ray measurements on 1 um and 20 um thick bias-sputtered films indicated isotropic small-angle scattering, but no anisotropy in the microstructure.

Therefore, Cargill concluded that atomic-scale struc- ture anisotropy is responsible for the magnetic ani- sotropy in these films.

In this paper, we report studies made on the struc- ture and magnetic anisotropy of three types of

Table I. — Films investigated.

Film A B C D(*) E F

Target (nominal comp.)

GdCo

GdCo

GdCo

GdCo

GdCo

GdCo

(*) About 400 A thickness, the thinnest of the

d.c. bias (V) - 80 - 100 - 120 - 80

50 0

six.

Composit Co

76 78 82 76 74 72

ion Gd

24 22 18 24 26 28

Anisotropy type perpendicular perpendicular

in-plane in-plane in-plane mixed

JOURNAL D E PHYSIQUE Colloque C 5 , supplément au n° 5, Tome 40, Mai 1979, page C5-239

Résumé. — Nous avons étudié la structure de couches amorphes Gd-Co par diffraction électronique. Les couches ont été préparées par pulvérisation en biais sur des substrats de NaCl refroidis à l'azote liquide. Les couches avaient une épaisseur de l'ordre de 400 à 800 Â. Nous avons observé les domaines magnétiques par microscopie de Lorentz. Nous avons mesuré les boucles d'hystérésis par effet Faraday avec le champ appliqué perpendi- culaire à la surface de la couche. L'intensité des électrons diffusés élastiquement et la fonction de distribution radiale réduite sont nettement différentes pour les films à direction d'anisotropie perpendiculaire et dans le plan.

On peut aussi déduire de ces résultats expérimentaux que l'anisotropie perpendiculaire est associée avec la pré- dominance de paires Co-Co.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979587

C5-240 Y . J . WANG, F.-H. LI, Zh.-Q. WANG AND J.-J. GAO

amorphous Gd-Co films (perpendicular anisotropy, in-plane anisotropy and a mixture of the two) pre-

pared by bias sputtering. Our purpose is to find the

connection between the magnetic anisotropy and

the structure of these films.

2. Experimental.

Films of GdCo alloy were I/

prepared by bias sputtering from an arcimelted

target on to freshly cleaved, liquid-nitrogen cooled, /I

~ a substrates ~ l

The thickness of these films ranged from 400-800A. The films were floated free of their substrates in water and collected on copper grids for examination.

The hysteresis loops of these films were measured by Faraday magneto-optic effect with the field perpendicular to the film surface. Electron diffraction was made with a Philips EM-400 electron micro-

/'

scope. We used 120 kV TEM to study the structure of the films and to observe the magnetic domains.

The composition of the films was obtained by X-ray xi

I part the data

these films.

3. Results and discussions. - The hysteresis loops of the six films were measured by the Faraday magneto- optic effect. The loops of films A, C and F are shown in figure

Domain patterns by Lorentz microscopy are shown in figure 2. From these results, it can be concluded that films A and B were of perpendicular anisotropy, especially because magnetic bubbles appeared a11 over the plane of the films.

a ) b) ^C) d )

Fig. 2. - Magnetic domains observed by Lorentz microscopy. (a) to ( d ) are for the films A, C, E and F respectively.

a ) b) c )

Fig. 3. - Electron diffraction patterns. (a) to ( d ) are for the films A, C, E and F respectively.

STRUCTURE AND MAGNETIC ANISOTROPY OF AMORPHOUS Gd-CO FILMS C5-241

Films C, D and E were of in-plane anisotropy, e.g.,

- 7.5 x lo4 erg/cm3, for film C from magneto-optic and torque measurements. In these films, domains of in-plane magnetization were observ- ed (figure 2). Film F contains a mixture of domains of two types, for which K,

and

< 0 respectively. Both magnetic bubbles and domains of in-plane magnetization were observed (figure 2).

The electron diffraction pattern consists of a few broad rings for all three types of films, but the inten- sity and position of corresponding peaks are some- what different. These patterns for films A, C, E and F are reproduced in figure 3. Figure 4 shows the scat- tered intensity Z(S) versus scattering vector S curves.

It can be seen that the first peaks of the three film types are quite similar, but the second peaks are different. The shift in position of the second peaks of films A and F towards small S-values is clearly visible, these films being wholly and partly of per- pendicular anisotropy respectively.

The reduced radial distribution functions of the three film types are shown in figure 5. It can be seen that the first peak of all G(r)'s is split into three peaks at

2.4-2.5 A, r,

2.9-3.0 A, r,

3.4- 3.55 A. These distances correspond to the nearest

-

,?sine ^(%,-f

-

Fig. 4. - The scattered intensity I ( S ) versus scattering vector I

S I.

Fig. 5. - Reduced radial distribution function G ( r ) versus r.

neighbor separations Co-Co, Co-Gd and Gd-Gd respectively. Similar observation was made by Grac- zyk [lo] on Gd-Co films (350 A).

Figures 6 and 7 compare the scattered electron

intensity Z(S) and the reduced radial distribution

function G(r) respectively for films A and D, which

are quite close in composition but are different in

magnetic anisotropy. These results are in contrast

with those of Wagner [5]. It can be seen in figure 7

that the Gd-Co peak for film A is higher than that

for film D while the Co-Co and Gd-Gd peaks of both

films show no difference. It is not clear, however,

why there is a shift in position between the two

C5-242 Y.-J. WANG, F.-H. LI, Zh.-Q. WANG AND J.-J. GAO

Fig. 6. - Comparison of the scattered intensity I ( S ) of films A and D.

curves. It may also be noted in figure 5 that the Co-Co peak for films C. D and E (of in-plane anisotropy), which are of different compositions, is considerably higher than the corresponding Gd-Co and Gd-Gd peaks, while for films A and B (of perpendicular anisotropy) the Gd-Co peak seems to be dominant.

On the basis of these evidences, we consider that the perpendicular anisotropy is connected with a predominance of Gd-Co atom pairs. This conclusion

Fig. 7. - Comparison of the reduced radial distribution function G ( r ) of films A and D.

seems to be in agreement with Chaudhari and Cro- nemeyer's [ll] result, in which the Gd-Co coupling decides the positive K,.

References

[I] GAMBINO, R. J., ZIEGLER, J. and CUOMO, J. J., Appl. Phys.

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[2] HEIMAN, N., ONTON, A., KYSER, D. F., LEE, K., GUARNI, C. R., AZP Conf. Proc. 24 (1974) 573.

[3] ESHO, S. and FUJIWARA, S., AIP Conf. Proc. 34 (1976) 331.

[4] CARGILL 111, G. S., AIP Conf. Proc. 24 (1975) 138.

[5] WAGNER, C. N. J., in Rapidly Quenched Metal, Edited by Grant and Giessen. Second International Conference, section 1.

[6] GRACZYK, J. F., J. Appl. Phys. 49 (1978) 1738.

[7] HOFFMANN, H., OWEN, A,, SCHROPF, F., International magne- tics Conference 1978.

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[9] WANG, Y. J., CHEN, S. C., ZHENG, D. Z. et al., Acta Physica Sinica 26 (1977) 187.

[lo] GRACZYK, J. F., AZP Con$ Proc. 34 (1976) 343.

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