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Submitted on 1 Jan 1988
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RECORDED MAGNETIZATION PATTERNS OF
HIGHLY c-AXIS ORIENTED Co-Cr FILM
OBSERVED BY ELECTRON HOLOGRAPHY
Y. Honda, M. Futamoto, S. Hasegawa†, T. Kawasaki†, F. Kugiya, M.
Koizuwi, K. Yoshida, A. Tonomura†
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988
RECORDED MAGNETIZATION PATTERNS OF HIGHLY c-AXIS ORIENTED Co-Cr
FILM OBSERVED BY ELECTRON HOLOGRAPHY
Y. Honda, M. Futamoto, S. Hasegawat, T. Kawasakit, F. Kugiya, M. Koizuwi, K. Yoshida and A. Tonomurat
Central Research Laboratory and Advanced Research Laboratoryt, Hitachi Ltd, Kokubunjz Tokyo, 185, Japan Abstract. - Magnetization structures of leakage flux of a Co-Cr perpendicular magnetic recording medium recorded at 200 and 300 kFCI are successfully visualized by electron holography. The magnetic field strength is estimated as a function of distance from the medium surface.
A quantitative and microscopic investigation of magnetization structure is required to clarify the mech- anism of perpendicular magnetic recording. We pre- viously applied an electron holography to observe the magnetization structures of single-layer Co-Cr media recorded at 100 kFCI [I, 21.
A
stronger magnetizationas well as a deeper penetration of perpendicular mag- netization was realized with the highly c-axis oriented Co-Cr film formed on a Ge layer.
In the present research, we observe the magnetiza- tion structure of a Co-Cr film recorded at linear densi- ties exceeding 100 kFCI. The distribution of magnetic leakage flux over a Co-Cr medium recorded at 200 and 300 kFCI are successfully visualized by the electron holography using a recently developed phase difference amplification method [3].
Co-Cr films were prepared by vacuum deposition on polyimide substrates after forming Ge layers to en- hance c-axis oriented columnar growth [4]. A pro- tective layer 15 nm in thickness was coated on the 200 nm thick Co-Cr film t o give durability dur- ing readlwrite processing. Perpendicular magnetic recording was carried out using a ring head. The magnetic properties and the recording characteristics are shown in figure 1. The output shown in this fig- ure was calculated from the magnitude of fundamental component measured by a spectrum analyser. Cross- sectional specimens were cut using an ultramicrotome and mounted on a standard 3 mm-diameter copper mesh. Electron holograms were recorded under the electron microscope operated at 100 keV [5]. Interfer- ence images were reproduced from the original electron holograms. The images were processed digitally using a computer to observe interference micrographs with increased phase-difference sensitivities [3].
Interference micrographs of magnetic flux straying over the Co-Cr film recorded at 300 and 200 kFCI are shown in figures 2a and b, respectively. A transmission electron micrograph of a sliced specimen is depicted in figure 2c. The TEM image was observed using an an- other electron microscope operated with an accelera- tion voltage of 200 keV. Although the columnar bound- aries are not clear, probably, due to slight plastic de-
w I I
a
10 30 50 100 300 500 IRecording Density ( kFCI
Fig. 1.
-
Magnetic properties and read/write characteris- tics of the Co-Cr medium. The output voltage was calcu- lated from the magnitude of fundamental component mea- sured by a spectrum analyser.Fig. 2.
-
Interference micrographs of magnetic flux stray- ing over the Co-Cr film recorded at 300 kFCI (a) and at 200 kFCI (b). Transmission electron micrograph and the diffraction pattern of a sliced specimen (c).formation caused duringslicing, the diffraction pattern indicates the highly c-axis oriented microstructure of the Co-Cr film. The columnar diameter is ranging be- tween 30 and 55 nm. The number of interference fringe (N) depends on the strength of recorded magnetiza- tion (MI,,)
.
The relationship is given in the followingC8 - 1970 JOURNAL DE PHYSIQUE equation [2],
7
e N = n-PO M,,, tl T h 0 0 sinh (TX)where, n is the phase amplification factor [5], e is the electron charge, h is Planck's constant, po is the per- meability of vacuum, t is the thickness of sliced spec-
imen, 1 is the recorded bit length, and 6 is the Co- Cr film thickness. Each fringe observed in figure 2a represents the change in the magnetic leakage flux strength of 6.9 x 10-l7 Wb. The recorded magneti- zation a t the 300 kFCI recording is calculated t o be 160 f 30 kA
/
m from the equation by putting numer- ical values: n = 60, t = 80 nm, 1 = 85 nm, and 6 = 200 nm. The recorded magnetization is as high as 63 f 12 % of M, (254 kA/m), though a considerablespacing loss during recording process was expected as 0.1 pm
the spacing was at least greater than 15 nm which Fig. 3.
-
Distribution of magnetic field strength in the was the thickness of the protective layer. The strong vertical direction to the medium surface, (30° kFC1). Changes in By at distances of 10(a-a), 30(bb), and 50(c- magnetization is probably due to the facts that the de-,,
f,, the surface are shown as lines of a, b, c, respec- magnetization factor becomes small (Nd S 0.1) at the tively.300 kFCI recording and that the easy axis of magneti- zation is well oriented perpendicularly to the medium
surface. men. The results are shown in figure 3, as a-a (10 nm),
b-b (30 nm), and c-c (50 nm) lines. The By value is It can be noted in 2a and b, the decreasing with increasing the distance from the spec-
bit length is not constant. It has a variation ranging imen surface following the relation of -55 ( d /A) [dB], from 72 t o 87 nm for the kFC1 recording and 120 where d is the spacing between the measuring point to 135 nm for the 200 kFCI recording. The expected ,d the
co-cr
film surface, is the wave length ofbit lenGhes are 85 for 300 kFC1 and 127 nm for magnetic recording. The By value becomes, for ex-
200 kFCI recordings, respectively. Since the average ample, less than 30 k q m at a greater distance ,.ban columnar diameter of the c 0 - C ~ film is 43 nm, thus, 50 nm from the top surf- of the specimen, or 65 nm only 2 or 3 columns are included within a magnetic bit from ,.he
co-cr
film surface.length. The magnetic inhomogeneties in the medium 1, the magnetization structure of leak- which are caused by the presence of columnar bound- age flux of a Co-Cr perpendicular magnetic recording aries or the segregation of Cr atoms are presumed to medium recorded at 200 and 300 kFCI are successfully give influence on the formation of domain observed by electron holography. Strong magnetiza- boundary. A future study using a high voltage elec- tion a t 300 kFCI recording probes the fact that the tron microscope which enables us to observe the mi- demagnetization effect decreases greatly at this high crostructure as well as the electron holograms from the recording density. The magnetic field strength above same area will make clear this point. the medium is calculated from the interference image
The magnetic field strength in the parallel (B,) and as a fuction of distance from the medium surface. the vertical (By) direction with respect to the Co-Cr
surface can be calculated from the electron holography [l] Honda, Y., Futamoto, M., Kawasaki, T., Yoshida, data. The detail of this procedure will be reported K., Koizumi, M., Kugiya, F. and Tomomura, A., elsewhere [3] and only a brief result is presented here. Jpn J. A p p l . Phys. 26 (1987) L923.
The distribution of By above the specimen surface is [2] Yoshida, K., Honda, Y., Kawasaki, T., Koizumi, shown in figure 3. The distribution of magnetic leak- M., Kugiya, F., Futamoto, M. and Tomomura, A., age flux (By) is given in figure 3 in a gray scale image, IEEE Trans. Magn. MAG-23 (1987) 2073.
where the white part corresponds to the region with [3] Hasegawa, S. et al., submitted to J. A p p l . Phys.
a large +B, value of which vector is oriented upward [4] Futamoto, M., Honda, Y., Kakibayashi, H. and while the darker part represents the oposit tendency. Yoshida, K., IEEE Trans. Magn. MAG-21
It is clearly shown in this figure that the magnetic field (1985) 1426.
