Direct observation of magnetically induced phase separation in Co-W sputtered thin films

Abstract

Phase separation of Co-W sputtered thin films having a large magnetocrystalline anisotropy energy have been investigated. A nanoscale compositional fluctuation caused by magnetically induced phase separation was directly confirmed in the films deposited on a heated substrate in analogy with Co-Cr-based alloys. The difference between the phase separation features in Co-W and Co-Cr is attributed to the difference in their elastic energy. It is expected that the phase separation is enhanced by selecting optimum sputtering conditions. The Co-W system, therefore, is considered to be a promising candidate as a base alloy system for high-density recording media. © 2004 American Institute of Physics. [DOI: 10.1063/1.1793354] Co-Cr-based sputtered films are in the current of high-density longitudinal magnetic recording media, 1 and also promising materials for perpendicular magnetic recording media. 2 The Co-Cr-based film is made up of Co-rich ferro-magnetic hexagonal-closed-packed (hcp) nanosized grains surrounded by a Co-poor paramagnetic hcp phase. 3 This unique compositional modulation weakens the interparticle exchange interaction between the ferromagnetic grains, re-sulting in improvements of recording resolution as well as significant recording noise reductions. Such a compositional modulation due to the magnetically induced phase separation 4,5 in a hcp phase is easily developed during the thin-film growth process on heated substrates around at 500– 700 K. 3 For increase of recording densities, both much higher magnetocrystalline anisotropy and enhancement of magnetic isolation between constituent ferromagnetic grains are re-quired. Therefore, a number of intensive studies have been focused on how to promote the magnetic isolation as well as the enhancement of magnetocrystalline anisotropy energy (MAE) by utilizing high magnetocrystalline anisotropy ma-terials, such as FePt 6 and CoPt. 7 According to systematic studies on the MAE of Co-based hcp alloy thin films, the value of the MAE for Co-Mo, 8 Co-W, 9 and Co-Pt 10 alloy films is larger than that of Co-Cr films. It should be noted that the MAE of the Co-W alloy films is several times higher than that of the Co-Cr films in the composition range Co -5–20 at. % W. 9 In addition, the metastable miscibility gap associated with the Curie temperature line has been pre-dicted from the thermodynamic calculations for the Co-W system 11 in analogy with the Co-Cr system. Accordingly, it is expected that a nanoscale compositional fluctuation of W is easily developed during the deposition of Co-W thin films on a heated substrate. Namely, the Co-W thin films are expected to be promising as next generation high-density magnetic recording media. In our recent work, 11 an annealing of Co-W hcp alloy films at 773 K enhances the saturation mag-netization and the Curie temperature, suggesting that the magnetically induced phase separation is enhanced by an-nealing. In the present study, the nanoscale compositional fluc-tuation of W developed in Co-W films is directly observed by a scanning transmission electron microscope equipped with an energy dispersive x-ray analyzer (STEM-EDX). Fur-thermore, the thermodynamic calculations are carried out by considering the effect of elastic energy on the phase separa-tion behavior in the film. A Cr100 buffer layer 20 nm in thickness and a Co-W11· 0 layer of 50 nm thickness were epitaxially grown on a NaCl100 substrate by a sputtering method. The sputtering chamber had a base pressure of less than 5 10 −7 Torr. The depositions were carried out at an argon gas pressure of 5m Torr. The Cr layer was deposited at 423 K by using a dc sputtering and the Co-W layer was deposited at room tem-perature and 573 K by an rf sputtering. The Co-W11· 0 layer showed a bicrystal growth on Cr001 with the crystal-lographic orientation NaCl001100 /Cr001110 / Co-W11· 000· 1. The film compositions were determined as Co-11.9 at. % W by an electron probe micrcoanalyzer. The magnetic properties were measured with a vibrating sample magnetometer. The STEM-EDX was used to investigate the local compositions in the films. For electron transparency, the substrate was dissolved in water and the Cr layer was removed by ion milling. Each point analysis was conducted at 200 kV with a spot size of 0.7 nm.

Full text

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Direct observation of magnetically induced phase separation in Co-W
sputtered thin films
K. Oikawaa)and G. W. Qin
National Institute of Advanced Industrial Science and Technology, Sendai 983-8551, Japan
M. Sato, S. Okamoto, O. Kitakami, and Y. Shimada
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
Sendai 980-8577, Japan
K. Fukamichi and K. Ishida
Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579,
Japan
T. Koyama
National Institute for Materials Science, Ibaraki 305-0047, Japan
(Received 27 April 2004; accepted 26 July 2004)
Phase separation of Co-W sputtered thin films having a large magnetocrystalline anisotropy energy
have been investigated. A nanoscale compositional fluctuation caused by magnetically induced
phase separation was directly confirmed in the films deposited on a heated substrate in analogy with
Co-Cr-based alloys. The difference between the phase separation features in Co-W and Co-Cr is
attributed to the difference in their elastic energy. It is expected that the phase separation is enhanced
by selecting optimum sputtering conditions. The Co-W system, therefore, is considered to be a
promising candidate as a base alloy system for high-density recording media. © 2004 American
Institute of Physics. [DOI: 10.1063/1.1793354]
Co-Cr-based sputtered films are in the current of high-
density longitudinal magnetic recording media,1and also
promising materials for perpendicular magnetic recording
media.2The Co-Cr-based film is made up of Co-rich ferro-
magnetic hexagonal-closed-packed (hcp) nanosized grains
surrounded by a Co-poor paramagnetic hcp phase.3This
unique compositional modulation weakens the interparticle
exchange interaction between the ferromagnetic grains, re-
sulting in improvements of recording resolution as well as
significant recording noise reductions. Such a compositional
modulationduetothemagnetically
separation4,5in a hcp phase is easily developed during the
thin-film growth process on heated substrates around at
500–700 K.3
For increase of recording densities, both much higher
magnetocrystalline anisotropy and enhancement of magnetic
isolation between constituent ferromagnetic grains are re-
quired. Therefore, a number of intensive studies have been
focused on how to promote the magnetic isolation as well as
the enhancement of magnetocrystalline anisotropy energy
(MAE) by utilizing high magnetocrystalline anisotropy ma-
terials, such as FePt6and CoPt.7According to systematic
studies on the MAE of Co-based hcp alloy thin films, the
value of the MAE for Co-Mo,8Co-W,9and Co-Pt10alloy
films is larger than that of Co-Cr films. It should be noted
that the MAE of the Co-W alloy films is several times higher
than that of the Co-Cr films in the composition range Co
-?5–20 at.%? W.9In addition, the metastable miscibility
gap associated with the Curie temperature line has been pre-
dicted from the thermodynamic calculations for the Co-W
system11in analogy with the Co-Cr system. Accordingly, it is
expected that a nanoscale compositional fluctuation of W is
inducedphase
easily developed during the deposition of Co-W thin films on
a heated substrate. Namely, the Co-W thin films are expected
to be promising as next generation high-density magnetic
recording media. In our recent work,11an annealing of
Co-W hcp alloy films at 773 K enhances the saturation mag-
netization and the Curie temperature, suggesting that the
magnetically induced phase separation is enhanced by an-
nealing.
In the present study, the nanoscale compositional fluc-
tuation of W developed in Co-W films is directly observed
by a scanning transmission electron microscope equipped
with an energy dispersive x-ray analyzer (STEM-EDX). Fur-
thermore, the thermodynamic calculations are carried out by
considering the effect of elastic energy on the phase separa-
tion behavior in the film.
A Cr?100? buffer layer 20 nm in thickness and a Co-
W?11·0? layer of 50 nm thickness were epitaxially grown on
a NaCl?100? substrate by a sputtering method. The sputtering
chamber had a base pressure of less than 5?10−7Torr. The
depositions were carried out at an argon gas pressure of
5m Torr. The Cr layer was deposited at 423 K by using a dc
sputtering and the Co-W layer was deposited at room tem-
perature and 573 K by an rf sputtering. The Co-W?11·0?
layer showed a bicrystal growth on Cr?001? with the crystal-
lographicorientation NaCl?001??100?/Cr?001??110?/Co-
W??11·0??00·1?. The film compositions were determined as
Co-11.9 at.% W by an electron probe micrcoanalyzer. The
magnetic properties were measured with a vibrating sample
magnetometer. The STEM-EDX was used to investigate the
local compositions in the films. For electron transparency,
the substrate was dissolved in water and the Cr layer was
removed by ion milling. Each point analysis was conducted
at 200 kV with a spot size of 0.7 nm.
a)Electronic mail: k-oikawa@aist.go.jp
APPLIED PHYSICS LETTERSVOLUME 85, NUMBER 13 27 SEPTEMBER 2004
0003-6951/2004/85(13)/2559/3/$22.00
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© 2004 American Institute of Physics2559

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Figure 1 shows the in-plane magnetization curves of the
films measured along the NaCl?110? direction. The satura-
tion magnetization Ms and coercivity Hcgiven by the solid
curve for the film deposited at 573 K are slightly larger than
those represented by the dashed line for the film deposited at
room temperature. Note that the relatively large value of Hc
for the film deposited at room temperature is associated with
a large value of MAE.9The temperature dependence of Ms
of the film deposited at 573 K is given in Fig. 2. The Ms
curve changes in the slope around at 300 K. Note that the Ms
behavior can be well reproduced by superposition of the two
Brillouin functions with the different Curie temperature Tc
(the dashed line with a low Tcand the solid line with a high
Tc), suggesting that two phases with the different Tccoexist
in the film. These magnetic properties are very similar to
those of Co-Cr sputtered films,12regarding as a supporting
evidence of the compositional fluctuation in films.
Figure 3 sets out the results of the STEM-EDX observa-
tion of the Co-W film deposited at 573 K. For the TEM
bright-field image, the corresponding selected area electron-
diffraction
(SAD)
pattern
diffraction pattern are shown in Figs. 3(a)–3(c), respectively.
Because of its bicrystalline structure, the SAD pattern con-
sists of two different types of domains with the c axes of hcp,
being at right angles to each other, as seen from Fig. 3(c).
Only a disordered hcp structure can be identified from the
SAD patterns of the film. The local composition was exam-
ined at the positions numbered in the STEM bright-field im-
age shown in Fig. 3(d). The nanoscale compositional fluctua-
tion of W is clearly confirmed as shown in Fig. 3(e). The
composition analysis was carried out in several local areas in
the views. Each position is given by the relative distance
from the position 1. The minimum and maximum values are
4.7 and 17.0 at.% W, respectively, bearing out that the two-
and theindexedelectron-
phase separation of the hcp phase takes place during the
deposition.
The compositional fluctuation of W in the Co-W hcp
alloy film was confirmed by STEM-EDX examinations and
magnetic measurements. According
calculations,13a metastable two-phase miscibility gap of hcp
associated with the Curie temperature line has been pre-
dicted; that is, this fluctuation is considered to occur due to
the magnetically induced phase separation, just as the Co
-Cr system.5,10However, the spread of the compositional
fluctuation of W in the Co-W film is depressed in compari-
son with that of Co-Cr films,12although the calculated mis-
cibility gap of the magnetically induced phase separation in
Co-W system is very similar to that of the Co-Cr system.11
In other words, the phase separation in the film does not
arrive at the equilibrium state, which depends not only on the
element diffusivity but also on the elastic energy introduced
by the coherent lattice misfit between two phases.14The
composition dependence of the lattice constant of the
Co-W system is larger than that of the Co-Cr system,15and
hence the effect of the elastic energy on the phase separation
cannot be ignored. Therefore, the thermodynamic calcula-
tions have been conducted by taking the effect of the elastic
energy into consideration. The free energy G is given by the
sum of the chemical energy, Gchem, the magnetic contribution
term, Gmag, and the elastic strain energy, Eelastic, as follows:
tothermodynamic
G = Gchem+ Gmag+ Eelastic,
?1?
where Gchemand Gmagare described by a subregular solution
model and the Hillert-Jarl description,16respectively, as in
the previous reports.5,11Eelasticis expressed by Cahn14as
Eelastic= ?2Y?c − c0?2Vm,
?2?
where ? is the linear lattice expansion per unit composition
change, Vmthe molar volume, c the local composition, and
c0the average composition. Y=?2C11+C33+2?C12+2C13?
−?C11+C12+C33?2/C33?/2 is the elastic constant of the elas-
tically soft direction which is the a-axis direction in hcp Co.
Here, Cijis the elastic stiffness constant. The values of Y and
? are evaluated from the available data as 3.96717and
0.1222,15respectively.
FIG. 1. Magnetization curves of Co-11.9 at % W films deposited at room
temperature and at 573 K.
FIG. 2. Temperature dependence of the saturation magnetization Msof a
Co-11.9 at% W film deposited at 573 K.
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FIG. 3. Several kinds of data for Co-11.9 at % W film deposited at 573 K:
(a) TEM bright-field image, (b) the corresponding SAD pattern, (c) the
indexed-electron-diffraction pattern, (d) STEM bright-field image, and (e)
the local composition at the numbered positions.
2560 Appl. Phys. Lett., Vol. 85, No. 13, 27 September 2004Oikawa et al.

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The calculated free-energy composition curves are illus-
trated in Fig. 4(a). Since Eelasticis proportional to the square
of compositional deviation, the difference between the free
energy in the coherent and incoherent states increases with
increasing W content. Figure 4(b) contours the calculated
coherent and incoherent phase diagrams. The miscibility gap
of the coherent phase diagram is narrower than that of the
incoherent phase diagram. If the elastic constants vary with
the composition, the miscibility gap of the coherent phase
diagram should further be depressed, namely, the elastic en-
ergy is considered as the main reason for the depression of
the magnetically induced phase separation in the film. It is
expected that the elastic energy is relaxed by the addition of
the third element and the deposition conditions. In addition,
for magnetic recording media, it is preferred that a W-rich
paramagnetic phase surrounds a Co-rich ferromagnetic
phase. The microstructure is sensitive to the volume of each
phase. The increase of the W-rich phase would bring about
such desirable microstructures by changing the volume ratio
of the matrix phase.
In summary, the magnetically induced phase separation
of Co-W sputtered thin films has been investigated. The
compositional fluctuation of W was directly confirmed in the
film deposited on a heated substrate. The difference between
the phase separation features in Co-W and Co-Cr originates
from the difference in their elastic energy. It is expected that
the phase separation is enhanced by selecting optimum sput-
tenng conditions. Consequently, the Co-W sputtered films
are promising as next generation high-density magnetic re-
cording media, because they exhibit the magnetically in-
duced phase separation with a large magnetocrystalline an-
isotropy energy.
The present study was supported by the CREST Program
of the Japan Science and Technology Agency, and the IT21
Program under Grant No. RR2002 and SRC.
1D. Weller and M. F. Doerner, Annu. Rev. Mater. Sci. 30, 611 (2000).
2S. Iwasaki and K. Ouchi, IEEE Trans. Magn. 14, 849 (1978).
3Y. Maeda and M. Takahashi, Jpn. J. Appl. Phys., Part 1 28, L248 (1989).
4K. Oikawa, G. W. Qin, O. Kitakami, Y. Shimada, K. Fukamichi, and K.
Ishida, Appl. Phys. Lett. 79, 644 (2001).
5K. Oikawa, G. W. Qin, T. Ikeshiji, R. Kainuma, and K. Ishida, Acta Mater.
50, 2223 (2002).
6S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 278,
1989 (2000).
7O. Kitakami, Y. Shimada, K. Oikawa, H. Daimon, and K. Fukamichi,
Appl. Phys. Lett. 78, 1104 (2001).
8K. Oikawa, G. W. Qin, M. Sato, O. Kitakami, Y. Shimada, J. Sato, K.
Fukamichi, and K. Ishida, Appl. Phys. Lett. 83, 966 (2003).
9N. Kikuchi, O. Kitakami, S. Okamoto, Y. Shimada, A. Sakuma, Y. Otani,
and K. Fukamichi, J. Phys.: Condens. Matter 11, L485 (1999).
10O. Kitakami, N. Kikuchi, S. Okamoto, Y. Shimada, K. Oikawa, Y. Otani,
and K. Fukamichi, J. Magn. Magn. Mater. 202, 305 (1999).
11M. Sato, G. W. Qin, K. Oikawa, S. Okamoto, O. Kitakami, and Y. Shi-
mada, J. Magn. Soc. Jpn. 28, 245 (2004).
12Y. Maeda and M. Takahashi, J. Appl. Phys. 68, 4751 (1990).
13A. F. Guillermet, Metall. Trans. A 20A, 935 (1989).
14J. W. Cahn, Acta Metall. 10, 107 (1962).
15K. H. J. Buschow, P. G. von Engen, and R. Jongebreur, J. Magn. Magn.
Mater. 38, 1 (1983).
16M. Hillert and M. Jarl, CALPHAD: Comput. Coupling Phase Diagrams
Thermochem. 2, 227 (1978).
17E. S. Fisher and D. Dever, Trans. Metall. Soc. AIME 48, 239 (1967).
FIG. 4. Free-energy curves and phase diagrams of the Co-W alloy system:
(a) Calculated free-energy composition curves at 573 K (details are given in
the text), and (b) calculated phase equilibria. The dotted lines represent the
stable phase diagram. The solid and dashed lines are the incoherent and
coherent magnetically induced phase separations, respectively. The dot-
dashed line stands for the Curie temperature TCof the homogeneous hcp
alloy.
Appl. Phys. Lett., Vol. 85, No. 13, 27 September 2004Oikawa et al.
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