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Submitted on 1 Jan 1988
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THE THICKNESS DEPENDENCE OF M-H
CHARACTERISTICS OF Co-Cr FILMS PREPARED
BY FACING TARGETS SPUTTERING
S. Nakagawa, Y. Kitamoto, M. Naoe
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, SuppMment au no 12, Tome 49, decembre 1988
THE THICKNESS DEPENDENCE OF
M-H
CHARACTERISTICS OF Co-Cr FILMSPREPARED
BY
FACING TARGETS SPUTTERINGS. Nakagawa, Y. Kitamoto and M. Naoe
Department of Physical EIectronics, Tokyo Institute of Technology, 2-22-1, 0-okayama, Meguro-ku, Tokyo 152, Japan
Abstract. - Co-Cr thin film with c-axis well-oriented initial layer can be prepared by using Facing Targets Sputtering system. Large squareness on in-plane hysteresis loop of them may be attributed to high term of anisotropy constant and the in-plane components of magnetization in domain walls.
Introduction
Sputtered Co-Cr thin films exhibit large perpendicu- lar magnetic anisotropy originated from a crystalline anisotropy in the direction of c-axis of the crystallites. However, a kind of magnetization jump is observed on in-plane hysteresis loop a t applied magnetic field
H around 0 Oe, resulting in large value of squareness (Mr/Ms)II
,
where the subscript//
denote "in-plane". It is difficult to explain this phenomenon by basing on a theory of magnetization mechanism of single do- main with uniaxial anisotropy. It has been considered the thin initial layer with poor c-axis orientation is formed in the early stage of film growth [I]. However, it is not acceptable for films prepared by using Facing Targets Sputtering (FTS) system t o assume such an initial layer model. In this study, theoretical analysis are performed to explain magnetization jump on in- plane hysteresis loop without basing on unique effect of initial layer. I 1 1 , 1 1 1 1 , 1 1 1 / 1 1 , , 700 21at:lo Cr-Co 0 -0 1000 10000 Thickness : D( A )
Fig. 1. - Thickness dependence of (Mr/Ms)II x D.
At first, a second order term of anisotropy constant K2 should be considered. The total energy of mag- netized film is represented by sum of magnetostatic energy, anisotropy energy and wall energy as follows,
Results and discussions +KzD sin4 6
+
(KI+
K2) D+
KO
(1)Co-Cr films deposited by using FTS system revealed excellent c-axis orientation even though in the early stage of film growth. The dispersion of c-axis A850
is about 5' even for the films as very thin as 200
A
[2]. Therefore, large (M,/M,),, of them can not be attributed t o poor c-axis orieiiation in initial layer. Even if initial layer has significant in-plane compo- nent of magnetization, the contribution of it t o that of the films with thickness D above about 1000A
will be negligible. Figure 1 shows D dependence of (Mr/Ms)lI x D which represent the reduced thickness which contributes t o in-plane magnetization. If the c-axis poorly oriented layer has constant thickness, (Mr/Ms)II
x
D will be kept at almost constant value independent of D. In fact, however, it increased with increasing of D. The films prepared by FTS system are columnless and reveal stripe domain configuration. These results suggests that there are other origins t o be considered for in-plane component of magnetization.where M,, a, w and D represent saturation magneti- zation, wall energy, domain width and thickness of the film, respectively, and constant c is about 1.05. The stable magnetization angle 0 as indicated in the inset
of figure 2 is obtained as follow (when K2
#
O),Assuming that Ms, a and K l
+
Kz of Co79Cr21 are 400 emu/cc, 1.0 erg/cm2 [3] and 1.1x
lo6
erg/cc, re- spectively, the calculated in-plane components of mag- netization are plotted for a parameter oft
= K ~ / K I as shown in figure 2. The calculated values of them give a good agreement with that of 200 .&-thick film.Next, it was assumed that the in-plane component in domain walls is not neglected, since the transition
C8 - 1988 JOURNAL DE PHYSIQUE r . - . - 1 I " " I I -0---. 0---o..--. I . . . ! I I I I , , , . I " 100 1000 Thickness
(A)
o.5Fig. 2. - Calculated effective in-plane squareness ratio 0
(Mr/Ms)ii taking into account of K2. 100 1000 10000
Thickness
( A )
:
width of magnetization between each domains 6, is comparable to the domain width w. Supposing that the in-plane component in them are oriented to the last applied field, as seen in figure 3, relatively large in-plane component will appear. Domain width w is calculated by basing on a stripe domain theory
and the distribution of magnetization angle
+
( x ) in walls is calculated in the coordinate as shown in the insets in figure 3 as follows;\I
.o.--
-
::!qii
1
where A is exchange stiffness constant. The effective
in-plane component of magnetization in the wall Mw is calculated as follows;
.
P
V
The number of walls per unit length in the direction of x is l/w and the effective squareness ratio (M,/M,)i
is calculated as follows;
The calculated results are drawn as solid line in fig- ure 3. They are more suitable for experimental results
than those determined by basing on the poor c-axis ori-
-
Fig. 3. - Calculation of the thickness dependence of
( M r / M s ) I I taking into account of the in-plane components of domain wall.
,,, Experiments
entation in initial layer as indicated in figure 3. How-
ever, the in-plane components of magnetization seems to be originated from both mechanisms.
Conclusions
The Co-Cr thin films with good c-axis orienta- tion in initial layer have been prepared by FTS sys- tem. The thickness dependence of in-plane squareness
( M M ) of them is expressed by considering two kinds of origins, of which one is in-plane component of magnetization in domain wall and other is the sec- ond order term of perpendicular anisotropy which al- lows magnetization to cant in the extremely thin film. Good agreements between calculation and experimen- tal results were obtained.
[I] Byun, C., Sivertsen, J. M. and Judy, J. H., J. Appl. Phys. 57 (1985) 3997.
[2] Niimura, Y., Nakagawa, S., Kitarnoto, Y. and Naoe, M., J. Appl. Phys. 61 (1987) 3152. [3] Wielinga, T., Lodder, J. C. and Worst, J., IEEE
