Diffusion analyses of non-ferromagnetic
element in the cap-layer of post-annealed
CoCrPt perpendicular media
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齊藤 伸
IEEE transactions on magnetics
41
10
3187-3189
2005
http://hdl.handle.net/10097/35360
doi: 10.1109/TMAG.2005.855286
IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005
3187
Diffusion Analyses of Non-Ferromagnetic Element
in the Cap-Layer of Post-Annealed
CoCrPt Perpendicular Media
Norikazu Itagaki1 , Shin Saito1 , and Migaku Takahashi1;2 , Member, IEEE
Department of Electronic Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8579, Japan
This paper reports on the factors that dominate diffusion phenomena in post-annealed CoCrPt media with various cap-layer elements
X. Hf- and Zr-capped media were found to show low diffusion temperature and have thin reacted layer in this experiments. These results
are explained with following relationships revealed by detailed analyses of boundary diffusion and interdiffusion: 1) X with high melting
point in cap-layer hardly starts boundary diffusion and interdiffusion, 2) X-capped media with higher melting point of CoX compound
forms thinner reacted layer, and 3) X with low enthalpy of Co-X formation lead to low diffusion temperature.
Index Terms—Hafnium, perpendicular magnetic recording media, post-annealing, zirconium.
I. INTRODUCTION
OR CoCr-based perpendicular media, to achieve both
thermal stability and low intergranular exchange coupling
is indispensable [1]. To solve this problem, several attempts,
especially post-annealing, have been made focusing on the diffusion of non-ferromagnetic element from under- or cap-layer
into the grain boundaries [2], [3]. We have already reported
that remarkable exchange decoupling among magnetic grains
was realized in the case of 410 C post-annealed media with
Ti under- and cap-layers [4], [5]. This decoupling originates
from not only boundary diffusion of Ti but also Co-Ti cohesions in grain boundaries. However, post-annealing causes
interdiffusion, which leads to non-ferromagnetic reacted layer
of CoTi phase with the B2 typed structure. The reacted layer
brings magnetic spacing loss in recording. Furthermore, high
post-annealing temperature is undesirable from a view point
of mass production. In this paper, in order to reduce thickness
of the reacted layer and the post-annealing temperature for
post-annealed media, we investigated non-ferromagnetic material X, superior to Ti based on the detailed analyses of boundary
diffusion and interdiffusion.
TABLE I
PROPERTIES OF CAP-LAYER MATERIALS X
F
II. EXPERIMENTAL PROCEDURE
All the media were fabricated by the dc magnetron sputtering
method in the ultraclean process [6]. The stacking structure of
nm]/Ru
the media was C[6 nm]/X[5 nm]/Co Cr Pt [
[5 nm]/Ti[25 nm]/crystallized glass disk substrate. Here, the
were varied from 5 to
nominal thickness of CoCrPt layer
30 nm. Ru underlayer was utilized on the purpose of both epitaxial growth of magnetic layer [7] and suppression of formation
of reacted layer at the bottom of CoCrPt layer. Note that Ru has
low reactivity with Co, Cr Pt, and Ti. The cap-layer materials
X were Al, Hf, Nb, Ta, Ti, and Zr, which are insoluble in Co.
, and dominant factors over chemical
Melting point of
Digital Object Identifier 10.1109/TMAG.2005.855286
coupling between Co and X, such as melting point of CoX compound,
, and enthalpy of formation of Co-X interatomic
are summarized in Table I.
cohesion,
The disks were heated by an infrared lamp run by constant
power without exposure to the atmosphere. The substrate
heating was performed up to 350 C for 13 s just after the deposition of the Ti under-layer. The post-annealing was carried
out up to 160–420 C for 2–13 s just after the deposition of the
cap-layer. The heating time is sufficiently short to suit the mass
production.
For structural analysis, X-ray diffraction profiles were obtained by grazing incident angle method (in-plane XRD;
scan) using Cu-K radiation. The incident angle was 0.4 ,
which corresponds to about 20-nm-thick penetration of X-ray
from the incident surface. The coercivities and saturation magnetizations were evaluated by vibrating sample magnetometer
(VSM).
III. RESULTS AND DISCUSSION
A. Post-Annealing Effects for the Media With Various
Cap-Layer Materials
Fig. 1 shows coercivity normalized with that of media without
post-annealing
as a function of post-annealing temperature
. Here, the value of
slightly changes in the
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005
Fig. 1. Coercivity normalized with that of media without post-annealing as a
function of post-annealing temperature for the media with X cap-layer. d
of all the media is 20 nm.
range of 4.5–5.1 kOe, depending on X. For the media without
. On the
cap-layer, over 250 C post-annealing degenerates
other hand, for the media with any cap-layer materials,
value
clearly increases by post-annealing of adequate temperature.
On the definition of diffusion temperature as the one at which
value gains 5%, the Al-, Nb- and Ta-capped media show
higher diffusion temperature of 370, 390, and 390 C than that
of 360 C for the Ti-capped medium. In contrast, the Hf- and
Zr-capped media show lower diffusion temperature of 240 or
280 C. The distinction of exchange decoupling among caplayer materials must be due to the different potential barrier to
diffuse.
Fig. 2 shows in-plane XRD profiles for the X-capped media
post-annealed up to 420 C. The Nb- and Ta-capped media have
diffracted lines from single phase of Nb and Ta. In cap-layer
, the chemical coupling of each atom is
material with high
so strong and stable that interdiffusion hardly proceeds by annealing up to 420 C. On the other hand, the profiles of the Zr-,
Hf-, Al-, and Ti-capped media show the diffracted lines from
CoX reacted layers of the B2 typed structure. This suggests that
interdiffusion occurs around X/CoCrPt interface in the same
way as Ti-capped media [4].
Fig. 2. In-plane XRD profiles for the media with X cap-layer post-annealed
to 420 C. The histogram below corresponds to powder pattern of hcp
Co Cr Pt .
Fig. 3. Schematic views of diffusion phenomena in the media (a) before and
(b) after post-annealing.
the X/CoCrPt interface. As a result, boundaries become non-ferromagnetic, and CoX reacted layer is formed. Taking account of
the structural change mentioned above, we can simply derive the
and averaged
thickness of reacted layer in magnetic layer
saturation magnetization of magnetic layer
. These terms
have a relation like the following equation:
(1)
B. Quantitative Evaluations for Boundary Diffusion and
Interdiffusion Phenomena
Next, we tried quantitative evaluations of boundary diffusion and interdiffusion separately through analysis of saturation magnetization. Fig. 3 shows a schematic view of diffusion phenomena in the post-annealed media. Light and dark
gray circles represent CoCrPt and X atoms, respectively. Before
post-annealing, the CoCrPt layer has hcp ferromagnetic grains
surrounded by amorphous Cr-rich boundaries. With post-annealing, X atoms diffuse into amorphous boundaries and cohere
with Co atoms. In addition, Co and X atoms interdiffuse around
is total saturation magnetization normalized with a
where
is saturation magnominal volume of CoCrPt layer, and
netization of CoX reacted layer. Furthermore, CoX is non-feris equal to zero. Therefore, (1) can
romagnetic, namely,
be rewritten in the following form:
(2)
Using a linear portion of the
versus
plot,
from the intersection of the extended
we can determine
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ITAGAKI et al.: DIFFUSION ANALYSES OF NON-FERROMAGNETIC ELEMENT
3189
Fig. 4.
Reacted layer thickness plotted against melting point of CoX
compound for the post-annealed media with X cap-layer.
Fig. 5. Averaged saturation magnetization plotted against enthalpy of Co-X
formation for the post-annealed media with X cap-layer.
line with the
axis and
from the gradient of the line.
indicates formability of reacted layer, and a
A value of
value of
by post-annealing is an index of exchange decoupling among ferro-magnetic grains. Therefore, we investigate the relationship between diffusion indices defined above
and dominant factors of chemical coupling.
plotted against the melting point of
Fig. 4 shows the
for the media annealed up to 420 C.
CoX compound
tend to have thin
The media with X cap-layer of high
, except for the Ta- and Nb-capped media, which form no
reacted layer. This result provides us with interdiffusion mechanism around X/CoCrPt interface: Once CoX reacted layer with
is formed, further interdiffusion of atoms hardly prohigh
gresses due to high structural stability of CoX phase.
plotted against
for the
Fig. 5 shows the
of each annealed medium
media annealed up to 420 C.
is reduced from the one of nonannealed medium. In particular,
of
the Zr- and Hf-capped media show large reductions of
is equivalent to the difficulty
99 and 130 emu/cm .
of Co-X interatomic cohesion. Therefore, this result suggests
not only diffuse into the grain
that X atoms with low
boundaries but also form lots of Co-X cohesions and make
the boundaries much non-ferromagnetic. The facility of Co-X
cohesion is the reason why Zr- and Hf-capped media have low
diffusion temperature.
Consequently, for the X-capped CoCrPt media, magnetic
property and structure widely vary, depending on the chemical
hardly start
coupling of Co and X. The X atoms with high
boundary diffusion and interdiffusion. The cap-layer with low
leads into low diffusion temperature and with high
into a thin reacted layer. Therefore, Hf and Zr are found
to be more effective materials than Ti in this experiment from
a view point of mass production. Additionally, we found that
only 0.5 nm Hf cap-layer is sufficient for exchange decoupling.
ACKNOWLEDGMENT
The authors would like to thank N. Goto from OHARA Inc.
for contributing all the glass substrates used in the experiment.
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=
Manuscript received February 7, 2005.
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