Fe–Ta–C soft underlayer for double-layered perpendicular recording media
A. Perumal, Y. K. Takahashi, and K. Hono
Citation: J. Appl. Phys. 105, 07A304 (2009); doi: 10.1063/1.3058613
View online: http://dx.doi.org/10.1063/1.3058613
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JOURNAL OF APPLIED PHYSICS 105, 07A304 共2009兲
Fe–Ta–C soft underlayer for double-layered perpendicular recording media
A. Perumal,a兲 Y. K. Takahashi, and K. Hono
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 3050047, Japan
共Presented 14 November 2008; received 21 September 2008; accepted 16 October 2008;
published online 4 February 2009兲
We report the investigation of microstructure, magnetic domain structure, and magnetic properties
of Fe–Ta–C thin films for the application of soft underlayer 共SUL兲 of perpendicular magnetic
recording 共PMR兲 media. As-deposited Fe80Ta8C12 film showed an amorphous structure with low
saturation magnetization 共M S ⬃ 600 emu/ cc兲 and high coercivity 共HC ⬃ 18 Oe兲, while the optimally
annealed 共艋500 ° C兲 films exhibited high value of M S 共⬃1350 emu/ cc兲 and low HC 共⬃0.3 Oe兲.
The magnetic domain structures and their correlation with the microstructures suggest that the
refinement of the average ␣-Fe grain size below 15 nm and the magnetic properties of the
intergranular residual amorphous matrix are very important in obtaining improved soft magnetic
properties. These results indicate that Fe–Ta–C films would be a suitable candidate for SUL of FePt
PMR media, for which the annealing for L10 ordering is essential. © 2009 American Institute of
Physics. 关DOI: 10.1063/1.3058613兴
I. INTRODUCTION
High magnetocrystalline anisotropy 共Ku ⬃ 108 ergs/ cc兲
materials such as L10 ordered 共001兲 equiatomic FePt and
CoPt based thin films are expected to be required for future
ultrahigh-density perpendicular magnetic recording 共PMR兲
media.1 One of the key aspects of the PMR is the utilization
of the media with soft magnetic underlayer 共SUL兲.2 Since the
past decade, extensive studies have been carried out on various SULs, such as CoTaZr, CoNbZr, FeTaN, FeAlSi, FeCoNi, synthetic antiferromagnetic 共SAF兲 SUL, laminated
type SUL, and hard magnet-biased SUL 共HM-SUL兲, in terms
of noise properties. In addition, several issues on the design
of the SUL were discussed.3 These studies revealed that the
media with nanocrystalline SUL show lower noise than the
media with the other SULs.4 Also, note that a high temperature annealing 共⬎350 ° C兲 is demanded for acquiring the L10
ordered phase of FePt and CoPt, and hence, the L10 based
PMR media will require a heat resistant SUL. This condition
limits the use of SAF SUL and HM-SUL due to the degradation of soft magnetic properties by the deprivation of interface in the multilayer structure.
Nanocrystalline magnetic thin films are considered to be
suitable for the SUL application due to their large saturation
magnetization 共M S ⬎ 1200 emu/ cc兲, low coercivity 共HC
⬍ 1 Oe兲, and high thermal stability.5,6 The soft magnetism of
nanocrystalline magnetic thin films originates from both their
grain size and strong intergranular ferromagnetic exchange
coupling.7 Earlier reports on Fe–TM–C or Fe–TM–N 共TM:
transition metal兲 films suggested that the Fe grain sizes of the
films, with their excellent soft magnetic properties, are less
than 10 nm, which is less than the ferromagnetic exchange
length.8,9 However, a detailed correlation between the microstructure, the domain structure, and the development of soft
magnetic properties in Fe–Ta–C films is still not understood
a兲
Author to whom correspondence should be addressed. Electronic mail:
a.perumal@nims.go.jp.
0021-8979/2009/105共7兲/07A304/3/$25.00
clearly. In addition, the recent studies10,11 of FePt–C film
suggested that the realization of FePt based PMR media is
approaching and hence prompted an instant search for ideal
SUL materials of the PMR media. In this work, we have
studied the change in the microstructure and magnetic properties of amorphous Fe–Ta–C films annealed at different
temperatures, and optimized the magnetic properties for the
application of SUL. We have also discussed the relationship
between the microstructures and magnetic domain structures
for the development of the soft magnetic properties in Fe–
Ta–C films. The optimally annealed Fe–Ta–C films show
high values of M S 共⬃1350 emu/ cc兲 and low HC 共⬃0.3 Oe兲.
II. EXPERIMENTAL DETAILS
200 nm thick amorphous Fe80Ta8C12 films were deposited at 1 mTorr Ar gas pressure with rf magnetron sputtering
at room temperature on glass and thermally oxidized Si substrates. The choice of the composition is dictated by the fact
that this composition exhibits good soft magnetic properties
under optimum annealing conditions.8,12 The films were in
situ annealed at different annealing temperatures TA using a
lamp heater. The structures of the films were examined by
x-ray diffraction 共XRD兲. The microstructures and magnetic
domain structures of the films were characterized by the FEI
Technai F30 transmission electron microscope 共TEM兲. The
domain structures were observed using Fresnel mode. The
specimen was tilted to an angle of 30° to experience the
in-plane field component. The magnetic property of the films
was measured by using a vibrating sample magnetometer
共VSM兲 with a Helmholtz coil.
III. RESULTS AND DISCUSSION
Figure 1 shows the XRD profiles of the as-deposited and
annealed Fe80Ta8C12 films. The as-deposited film exhibits no
sharp peaks peculiar to any crystalline phase, but a broad
halo is observed around 2␪ = 43°. This suggests that the asdeposited Fe80Ta8C12 film has an amorphous structure. With
105, 07A304-1
© 2009 American Institute of Physics
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07A304-2
Perumal, Takahashi, and Hono
J. Appl. Phys. 105, 07A304 共2009兲
FIG. 1. 共Color online兲 XRD patterns of the as-deposited and annealed
Fe80Ta8C12 films.
increasing TA up to 300 ° C, the XRD profiles do not show
any changes, revealing that the amorphous structure is stable
up to 300 ° C annealing. A remarkable change in the halo is
observed for the films annealed above 300 ° C. The diffraction peak around 2␪ = 44° sharpens and its peak value approaches the ␣-Fe diffraction angle 共2␪ = 44.78° 兲 with increasing TA up to 600 ° C. The formation of a face centered
cubic 共fcc兲 TaC phase was also observed for the film annealed above 500 ° C.
Figure 2 shows the room temperature magnetic hysteresis 共M-H兲 loops 关Fig. 2共a兲兴 and variation of M S and HC with
various TA 关Fig. 2共b兲兴 for the Fe80Ta8C12 films. The asdeposited film shows a rapid increase in magnetization ini-
FIG. 2. 共Color online兲 共a兲 Room temperature M-H loops and 共b兲 variations
of HC and M S with annealing temperature for the Fe80Ta8C12 films.
FIG. 3. Bright field TEM micrographs and SAED patterns of the 共a兲 asdeposited and annealed 关400 ° C 共b兲, 500 ° C 共c兲, and 600 ° C 共d兲兴
Fe80Ta8C12 films.
tially with the applied field followed by a linear variation
before it saturates. Such behavior has been observed in a film
with Curie temperature TC just above room temperature.8
The values of M S and HC are about 600 emu/ cc and 18 Oe,
respectively. The linear region at which the magnetization
increases linearly decreases with increasing TA and the film
saturates at lower fields. This could be due to the enhancement of TC with increasing TA. The values of the HC decrease rapidly from 20 to 0.6 Oe with increasing TA up to
300 ° C. The Fe80Ta8C12 films annealed at 400 and 500 ° C
show a large value of M S 共⬃1350 emu/ cc兲, low HC of
0.3 Oe, and high squareness. For the film annealed at
600 ° C, the magnetic properties change drastically, i.e., the
value of HC increases to 20 Oe, but the value of M S remains
constant.
The development of the soft magnetic properties is
known to be closely related to their microstructural evolution. Therefore, we have investigated the microstructures of
the Fe80Ta8C12 films using TEM, as shown in Fig. 3. The
selected area electron diffraction 共SAED兲 pattern of the asdeposited film shows a halo diffraction ring corresponding to
the amorphous structure. On the other hand, the films annealed at 400 and 500 ° C show the formation of ␣-Fe crystallites with average grain size of less than 15 nm and fine
TaC nanocrystallites distributed randomly in the amorphous
matrix. The fraction of crystallites increases with increasing
TA. The SAED patterns illustrate the diffraction rings corresponding to 共110兲, 共200兲, and 共211兲 planes of ␣-Fe. Also,
there is a secondary halo ring, which reflects the presence of
atomic distance close to the 共111兲 or 共200兲 plane distance of
TaC, inside the main halo ring. Note that the presence of any
clear peaks from the TaC phase was not detected in the XRD
profiles of the films annealed below 600 ° C. This might be
probably due to the small volume of the TaC phase. The
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07A304-3
J. Appl. Phys. 105, 07A304 共2009兲
Perumal, Takahashi, and Hono
matrix.13,14 For the film annealed at 600 ° C, the domain size
varies between 0.5 and 2 ␮m, and a magnetization ripple is
seen inside the domains. The presence of fine domain walls
was observed even after applying the field of 390 Oe 关Fig.
4共i兲兴, which supports the relative evidence of the TaC precipitates. According to the ripple theory,15 the magnitude of
the magnetization ripple depends on the Keff. This suggests
that the magnetization ripple in the Fe80Ta8C12 film is due to
the increase in the Keff, which is probably not averaged out
because of the formation of the fcc TaC phase. Also, the
formation of the TaC phase can pin the domain walls. Therefore, the deterioration of the soft magnetic properties is
caused mainly by the structural inhomogeneities rather than
by the increase in the mean ␣-Fe grain size.8
FIG. 4. Magnetic domain structures of the as-deposited 关共a兲–共c兲兴 and annealed 共400 ° C 关共d兲–共f兲兴 and 600 ° C 关共g兲–共i兲兴兲 Fe80Ta8C12 films observed at
different applied fields. A white line in 共a兲 indicates the direction of the
applied field component 共H sin ␪, where ␪ is the tilting angle兲 on the plane
of the specimen. The horizontal bars in the figures correspond to a scale of
5 ␮m.
cross-sectional TEM micrographs 共not shown here兲 of the
films show the nanocrystalline structure with a random orientation. For the film annealed at 600 ° C, the crystallites are
formed uniformly along with some precipitations. The SAED
pattern shows the diffraction rings corresponding to both
␣-Fe and fcc TaC phase. These observations are in good
agreement with the results obtained from XRD profiles.
Also, the variation of the magnetic properties with the annealing temperature can be well understood from the microstructural results. This study suggests that the two-phase
structured Fe80Ta8C12 films, i.e., presence of ␣-Fe with average size less than 15 nm and fine TaC nanocrystallites in
the ferromagnetic amorphous matrix, result in very good soft
magnetic properties. In addition, the observed nanocrystalline structure with a three-dimensional random orientation is
likely to provide low dc noise from the Fe80Ta8C12 SUL.4,12
Therefore, the optimally annealed Fe80Ta8C12 film exhibits
promising results suitable for the application of SUL of L10
ordered FePt based granular PMR media.
To investigate the correlation between the microstructure
and magnetic domain structures, we observed the magnetic
domains of the Fe80Ta8C12 films at different fields using Lorentz microscopy, as shown in Fig. 4. The as-deposited film
shows large sized domains with an average domain size
ranging between 3 and 5 ␮m. By increasing the field, the
domain wall moves out rapidly at the initial field followed by
a slow movement between 50 and 200 Oe, and disappears
completely 关Fig. 4共c兲兴 when the saturation field was applied.
The size of the domains is found to be larger 共⬎10 ␮m兲 and
the domain walls are relatively straight in the optimally annealed 共between 400 and 500 ° C兲 films 关Figs. 4共d兲–4共f兲兴.
Note that the domain walls are observed only near the edges
of the TEM specimen. This suggests that the effective anisotropy 共Keff兲 is averaged out by the intergranular magnetic
interactions through the ferromagnetic amorphous
IV. CONCLUSION
We have investigated the correlation between the microstructure, the magnetic domain structure, and the development of the soft magnetic properties in the Fe–Ta–C films for
the application of SUL of the PMR media. The results of our
magnetic properties, domain observation, and their correlation with the microstructure suggest that the refinement of
the mean grain size below 15 nm, the magnetic properties of
the intergranular amorphous matrix, as well as the threedimensional random orientations of the grains are very much
important in obtaining the suitable soft magnetic properties.
ACKNOWLEDGMENTS
One of the authors 共A.P兲 acknowledges financial support
from the Japan Society for the Promotion of Science 共JSPS兲
in the form of a JSPS postdoctoral fellowship. This work was
in part supported by the World Premier International Research Center Initiative 共WPI Initiative兲 on Materials
Nanoarchitronics, Ministry of Education, Culture, Sports,
Science and Technology 共MEXT兲, Japan.
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