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Submitted on 1 Jan 1988
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MAGNETIC MICROSTRUCTURES OF CoCr-FILMS
H. Mändl, H. Hoffmann
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplkment au no 12, Tome 49, decembre 1988
MAGNETIC MICROSTRUCTURES
OF CoCr-FILMS
H. Miindl and H. Hoffmann
Institut fur angewandte Physik, Universitiit Regensburg, 84 Regensburg, F.R.G.
Abstract. - Investigations on rf-sputtered CoCr-films with Lorentz-microscopy showed two typical micromagnetic struc- tures, stripe-domains and dotlike patterns. The particulate case is due t o Cr segregation into the column bounderies enhancing the Cr content by more than 6 at %
. Energy calculations for the remanent state showed that in the case of
continuous films stripedomains are stable. However in the particulate case the energy minimum was found for circular vertical domains.
1. Introduction
In a previous paper [l] it was reported, that rf- sputtered CoCr-films show a columnar structure and often chromium segregation in the column bound- aries was found. Observations by Lorentz microscopy showed two typical micromagnetic structures for the remanent state (Figs. la, b). To determine the do- main width that corresponds with the dot like struc- ture of figure lb, a model suggested by the authors [l]
was used. The resulting domain width was 0.08 pm. It could be shown by different investigations that the stripedomain structure is connected with continuous film properties, while the dotlike structure is more due to particulate films. There were reasons to assume that the transition between these two different cases depends on the amount of chromium segregation into the boundaries of the columns. Energy calculations for the observed microstructures should clear up the
conditions for the films to show a more continuous or more particulate behavior.
2. Energy calculations
2.1 THE STRIPE MODEL. - A theory for energy calculai tion of a stripe domain structure as shown in figure l a was given by Kooy and Enz 121. Using their equation for a stripe domain structure the total energy den- sity has been calculated as a function of the stripe period D for CoCr-films. The following film parame- ters were used: saturation magnetization M, = 500 G ,
anisotropy field
Hk
= 5 000 Oe and film thickness d =0.5 pm. The resultant energy us. stripe period D is shown in figure 2.
A minimum of the energy density at 4 x
lo5
erg/cm3 with a stripe period of about 0.45 pm is to be seen. This value corresponds to the observations [I].d l
-4
Fig. 1. - Two different micromagnetic structures af CoCr-films: (a) stripedomains; (b) dotlike structure.
JOURNAL DE PHYSIQUE Fig. 2. - D. A - 0 P 3 1 5 6 C W Edge Length a [ 1 0 - ~ p r n l
Total energy density a s function of stripe period Fig. 5. - Energy density in the particulate model as func- tion of a a, without p, with chromium segregation.
2.2 THE PARTICULATE MODEL. -In the case of the Par- was experimentally only found in films with dotlike ticulate model the energy density has been calculated structure, an energy dependence as shown in figure 5, for an hexagonal array of vertical domains as shown in curve 0 , was found.
figure 3. The total energy density was calculated as a function of the edge length a with a ACcr of 6 at % (Fig. 4). The other film parameters were the same as used in section 2.1. The morphological diameter of the coIumn
-. was assumed to be 0.08 pm in agreement with our
d 1 experimental findings [I].
-
Because of the increasing chromium concentration
Fig. 3. - Hexagonal array of vertical domains. in the boundary of the columns the exchange stiff-
ness is reduced in this region. Therefore, the wall en- In figure 3 D corresponds to the nearest neighbour
separation of "down" magnetized domains and a is the
length of the edge of the hexagons. For an array very similar to that one shown in figure 3 Charap [3] gave an expression for the density of the demagnetizing self energy. The calculation of the wall energy density as-
sumes Bloch walls, as in the stripe model. The total energy was calculated for two cases; a , without and
0,
with a linear increase of chromium content in the boundaries of the columns (Fig. 4), respectively.The first case led to an energy dependence as shown in figure 5, curve a. The minimum of the energy was
larger than in the stripe model. In this case an ob- served dot like structure would be only metastable. But introducing chromium segregation (Fig. 4), which
0 360 LOO
r i [ W r n
I
-
C BoundaryFig. 4.
-
Assumed change of the chromium concentration with the distance T ; from the center of the column.ergy decreases if the walls are in the boundaries of the columns. This decrease was determined with an em- pirical equation for CoCr given by Honda [4]. It can be seen that the total energy density rapidly decreases if the walls are located in the region of chromium seg- regation. As shown in figure 5 a minimum of the total energy density lower than in the stripe model results for ACcr
>
6 at %. The minimum of the energy density decreases furthermore for increasing AC,,. This gives a stable state of the dot like structure, because the total energy density is lower than in the stripe model. 3. ConclusionsEnergy calculations for two micromagnetic mod- els have been carried out. It was found that in the particulate model lower energies than in the stripe model occur, as soon as the chromium segregation into the columnar boundaries enhances 6 at % above the mean chromium concentration. For films with large chromium segregation, particulate film properties can be expected. For films with very low or no segregation into the boundaries the stripedomain structure gives the lower total energy density. These results agree with our experimental findings.
[I] Hoffmann, H., M h d l , H., Schiirmann, TH., J.
Magn. Mat. 59 (1986) 156.
[2] Kooy, C., Enz, E., Philips Res. Rep. 21 (1949)
54.
[3] Charap, S., Nemchik, J., IEEE Trans. Magn.
MAG-5 (1969) 566.
