MAGNETIC PROPERTIES OF ALUMITE MAGNETIC FILMS

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MAGNETIC PROPERTIES OF ALUMITE

MAGNETIC FILMS

K. Arai, Y. Ohoka, K. Ishiyama, H. Kang

To cite this version:

K. Arai, Y. Ohoka, K. Ishiyama, H. Kang.

MAGNETIC PROPERTIES OF ALUMITE

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JOURNAL DE PHYSIQUE

Colloque C8, SupplGment au no 12, Tome 49, dhcembre 1988

MAGNETIC PROPERTIES OF ALUMITE MAGNETIC FILMS

K. I. Arai, Y. Ohoka, K. Ishiyama and H. W. Kang

Research Institute of Electrical Communication, Tohoku University, Sendai 980, Japan

Abstract.

-

Magnetic properties were measured for anodic oxide films containing electro-deposited iron or cobalt particles

in pores. The coercive force perpendicular to films containing iron particles is in inverse proportion to the square of the particle diameter. For films containing cobalt particles, the magnetic crystalline anisotropy affects considerably on the coercive force.

1. Introduction

Alumite magnetic films containing fine needleshaped iron particles have high wear and corrosion resistance

as well as excellent magnetic properties, and these films have attracted special interest recently as a perpendicular magnetic recording medium [l]. There are, however, few reports concerning the coercive force and the magnetic anisotropy of these anodic oxide magnetic films.

This paper reports the coercive force and the anisotropy energy of alumite films containing electre deposited iron or cobalt particles.

2. Experimental

Aluminum plates, 99.99 % pure and 0.1 mm thick, were used as substrates for anodic oxidation, and pores were formed on the substrate in a solution of 1.6 moll1 sulfuric acid, 0.24 moll1 oxalic acid or 0.52 moll1 phos- phoric acid at 20 OC. After these pre-treatments, iron or cobalt was electredeposited into pores in a solution containing 0.20 moll1 ferrous ammonium sulfate and 0.48 moll1 boric acid or in a solution containing 0.10 moll1 cobalt (11) sulfate and 0.32 moll1 boric acid at 20 OC.

3. Experimental results and discussion

function of the pore diameter D,, i.e. the diameter of the particle with packing density of 0.2 and size ratio k of the length to the diameter of the particle over 15. The coercive force of the film containing iron particles is a high 2200 Oe when the pore diameter is 200 A. This value rapidly decreases as a function of 0,'and becomes about 200 Oe for pore diameters over 800

a.

When the pore diameter of films containing cobalt particles is 200 .&, the coercive force is about 2000 Oe and decreases with a gradient of D , ~ . ~ . The mechanism of magnetization reversal in a single domain particle of needle shape is generally explained by the curling 121 or fanning mode. In the curling mode, the coercive force H, along the needle axis is described as follows:

H, = 2 . 1 6 n ~ , R ~ / ~ '

+

CKo/M,

where Ms is the saturation magnetization of the particle, R is the needle particle diameter, R, is the crit- ical diameter, C is a constant and KO is a magnetic crystalline anisotropy constant. For magnetic films containing iron particles, the longitudinal axis of the needle shaped particle is parallel to the [I101 crystal axis, however, the degree of I1101 orientation is ex- tremely low from the observation of the the rocking curve width of X-ray diffraction. Magnetic crystalline anisotropy, therefore, contributes little to the coercive force. The coercive force of films containing iron particles is in proportion to DL', provided the pore di- Figure 1 shows the coercive force perpendicu- ameter is over-350 A, and'this relation agrees with lar t o the film containing iron or cobalt particles as a that between the coercive force and the needle diame-

100

u

100 2 0 0 5 0 0 1000 2 0 0 0 PORE DIAMETER DP (A)

ter R - ~ in the curling mode. For magnetic films containing cobalt particles, the [001] crystal axis of the particle has a tendency to become perpendicular to the needle axis, and the magnitude of the crystalline anisotropy constant KO is a large 5 x

lo6

erg/cm3. The magnetic crystalline anisotropy, therefore, contributes a large part of the coercive force. For both films containing iron and cobalt particles with pore diameter under 200

A,

some concavities and convexities are ob- served on the needle iron particles in TEM images, and this unevenness may cause the magnetization reversal by the fanning mode.

Fig. 1. - Relation between coercive force HCI perpendic-

ular to the film containing iron or cobalt particles and the Figure 2 shows the experimental values of magnetic pore diameter D,. anisotropy energy as a function of the packing den-

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JOURNAL DE PHYSIQUE b - 2 - [Z \ \

Y

- 4 - W \

2

-6 - MEASURED \ 0 \ \

-'

- CALCULATED - - - \

q

-10 - \ a \

Fig. 2. - Relation between packing density P and magnetic anisotropy energy Ku for film containing iron particles.

sity for films containing iron particles with diameter of 440

k

and length of 0.5-1.0 pm. In this exper- iment, the magnetic anisotropy energy was a maximum 6x

lo5

erg/cm3 at a packing density around 0.15, was zero a t the packing density of 0.32 and then became negative with a further increase in packing density. With regard to magnetic anisotropy energy, it could be phenomenologically accounted for by demagnetizing energy due t o the shape of iron particles, and interaction energy between particles, and a magnetic crystalline anisotropy energy of iron. In these films the orientation of the [I101 crystal axis is so low that the magnetic crystalline anisotropy contributes little t o the magnetic anisotropy of the films. The demagnetizing and interaction energy was calculated by Masuda et al. [3] as shown in figure 2, and this value coincides with the experimental values exceeding well.

For films containing cobalt particles (450 k , 1.5 pm) as shown in figure 3, the experimental value took a

maximum 4

x

105erg/cm3 at a packing density of 0.17 and became negative at a packing density of 0.26. In these films, the [001] crystal axis of cobalt particles have a tendency to be arranged perpendicular to the needle axis of the particles, and as a result, the magnetic anisotropy energy of films was lower than the calculated value [3]. -6 - MEASURED -8 - C A L C U L A T E D - - - - -10 -

Fig. 3. - Relation between packing density P and magnetic anisotropy energy Ku for film containing cobalt particles.

[I] Shiraki, M., Wakui, Y., Tokushima, T. and Tsuya, N., IEEE Trans. Magn. MAG-21 (1985)

1465.

[2] Frei, E. H., Shtrikman, S. and Treves, D., Phys.

Rev.

106 (1957) 446.

[3] Masuda, M., Shiorni, S. and Shiraki, M., ~ d n J.