C-axis oriented Ba-ferrite thin film with small grain for perpendicular magnetic recording

Article abstract of DOI:10.1016/j.jmmm.2003.12.632

Hexagonal Ba-ferrite(BaM) thin films with amorphous AlO(a-AlO) under-layer were prepared by a facing targets sputtering system. The grain size of c-axis perpendicularly oriented BaM/a-AlO films is about 20 nm at the thickness of 20 nm for BaM layer. The perpendicular coercivity is about 3.5-4.3 kOe with BaM layer thickness in the range from 80 to 30 nm and 2.3 kOe for BaM layer thickness of 20 nm.The in-plane coercivity for BaM/a-AlO films was less than 0. 1 kOe at the thickness ranging from 20 to 80 nm.

Full text of DOI:10.1016/j.jmmm.2003.12.632

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2191–2193
C-axis oriented Ba–ferrite thin film with small grain for
perpendicular magnetic recording
A. Morisakoa,*, N.N. Shamsa, Y. Miuraa, M. Matsumotoa, S.H. Geeb,
M.H. Parkb, Y.K. Hongb
a Department of Information Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan
b Department of Materials Science and Engineering, University of Idaho, Moscow, ID 83844-3024, USA
Abstract
Hexagonal Ba–ferrite(BaM) thin films with amorphous AlO(a-AlO) under-layer were prepared by a facing targets
sputtering system. The grain size of c-axis perpendicularly oriented BaM/a-AlO films is about 20 nm at the thickness of
20 nm for BaM layer. The perpendicular coercivity is about 3.5–4.3 kOe with BaM layer thickness in the range from 80
to 30 nm and 2.3 kOe for BaM layer thickness of 20 nm.The in-plane coercivity for BaM/a-AlO films was less than
0.1 kOe at the thickness ranging from 20 to 80 nm.
r 2003 Elsevier B.V. All rights reserved.
PACS: 75.70.S; 75.50.G; 52.75.R; 85.70.K
Keywords: Hexagonal ferrite film; Perpendicular magnetic recording; Small grain; Sputtering
1. Introduction
aluminum oxide (a-AlO) was used as an under-layer for
BaM layer.
Highdensity perpendicular magnetic recording media
should have a high perpendicular coercivity (Hc) and
small magnetic grains as well as mechanical durability.
The combination of high anisotropy field, excellent
mechanical durability and chemical stability makes
2. Experimental
The BaM/a-AlO films were deposited by facing
targets sputtering (FTS) system. The substrates were
thermally oxidized silicon wafers. A pure Al metal target
was used to deposit amorphous aluminum oxide (a-AlO)
under-layer in presence of O2 and a sintered target with
a stoichiometric composition of BaFe12Ox was used to
deposit the BaM layer. Sputtering was carried out after
hexagonal BaM
a highly attractive candidate for
perpendicular magnetic recording medium [1]. Although
preferred c-axis orientation is a prime concern for the
perpendicular magnetic recording, the BaM thin films
with c-axis perpendicular orientation exhibited fairly
large grain size [2], which should be reduced. In the
previous study, the authors used AlN as an under-layer
to increase the Hc in perpendicular direction. Although
a high Hc of about 4–5 kOe could be obtained and grain
size was reduced to around 50 nm, further reduction of
grain size was necessary [2]. In this study, an amorphous
evacuating the chamber to
a
pressure below
1 Â 10À6 Torr. The c-axis perpendicularly oriented
BaM films were obtained by depositing the thin film at
a substrate temperature of about 600ꢀC [3]. In this
study, the substrate temperature was kept at 600ꢀC to
deposit BaM layer, while a-AlO was deposited without
any heating of the substrate. To compare the results,
some BaM thin films were deposited on the amorphous
BaM(a-BaM) under-layer.
*Corresponding author. Tel./fax: +81-26-269-5481.
E-mail address: morisako@cs.shinshu-u.ac.jp
(A. Morisako).
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.12.632

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2192
3. Result
4
Fig. 1 shows the XRD diagram for BaM/a-AlO film
with the thickness of 20 nm for BaM layer. The
appearance of diffraction lines only from BaM(0 0 l)
planes implies that c-axis, which is easy axis of
magnetization of BaM, is perpendicularly oriented to
the film. The c-axis dispersion angle, Dy50; was measured
by a full width at half maximum of intensity line of the
rocking curve for (0 0 8) plane. These values decreased
slightly with the decrease of the BaM layer thickness and
it was 3.9ꢀ at the thickness of 20 nm.
Hc
3
2
1
BaM
δ
a-AlO
10nm
Substrate
Hc
//
Fig. 2 shows the coercivity values in both perpendi-
cular and in-plane directions of BaM/a-AlO films. The
perpendicular coercivity is about 3.5–4.3 kOe withBaM
layer thickness in the range from 80 to 30 nm, and
2.3 kOe for the thickness of 20 nm. Presence of Al in the
BaM layer has been observed from the XPS study. It is
not yet confirmed but some aluminum oxide may
surround the BaM grain and a part of Fe ions in
hexagonal crystallite are substituted by aluminum atom
and these result in higher coercivity in BaM/a-AlO films.
The in-plane coercivity for BaM/a-AlO was less than
0.1 kOe at the thickness ranging from 20 to 80 nm. This
is due to the narrow distribution of easy axis of
magnetization.
0
20
40
60
80
Thickness of BaM thin film, δ(nm)
Fig. 2. Coercivity value, Hc, of BaM/a-AlO thin films of
various film thickness in both perpendicular and in-plane
direction.
100
80
Fig. 3 shows the grain size of BaM/a-AlO thin films
with various BaM layer thickness. The grain size was
evaluated from the surface morphology of the films
observed by SEM. The grain size for BaM layer without
a-AlO under-layer was in the range from 80 to 100 nm,
while that for BaM/a-AlO layer decreases from 60 to
20 nm withdecrease of BaM layer thickness from 80 to
20 nm.
60
50 nm
40
20
100 nm
Fig. 4 shows an MFM image of BaM/a-AlO film with
the BaM layer thickness of 55 nm. Black and white spots
ranging from 40 to 80 nm can be observed. The delta M
plots have also measured and showed a negative peak.
0
20
40
60
80
Thickness of BaM thin film, δ(nm)
Fig. 3. Grain size and surface images of BaM/a-AlO films with
various thickness.
100
BaM
20 nm
10nm
a-AlO
Substrate
14
20
30
40
50
2θ (degree.)
Fig. 1. X-ray diffraction diagram of BaM/a-AlO thin film with
BaM layer thickness of 20 nm.
Fig. 4. MFM image of BaM/a-AlO film withtihckness of
55 nm.

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A. Morisako et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2191–2193
2193
These imply that a magnetostatic coupling is dominant
in BaM/a-AlO thin films.
BaM layer thinner than 20 nm should be increased for
ultra high density recording medium.
References
4. Conclusion
[1] T.L. Hylton, et al., J. Appl. Phys. 75 (1994) 5960.
[2] A. Morisako, et al., J. Magn. Magn. Mater. 242–245 (2002)
304.
C-axis perpendicularly oriented BaM thin films with
the grain size of about 20 nm could be deposited with
a-AlO under-layer. The perpendicular coercivity for
[3] A. Morisako, et al., IEEE Trans. Magn. 23 (1987) 2359.