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Numerical investigation of thermal effects on a HAMR head-disk interface
Kyaw Sett Myo
Email [email protected]
Weidong Zhou
Xiaoyang Huang
Shengkai Yu
Data Storage Institute, (A*STAR) Agency for Science, Technology
and Research, DSI Building, 5 Engineering Drive
1, Singapore, 117608 Singapore
School of Mechanical and Aerospace Engineering, Nanyang
Technological University, 50 Nanyang
Avenue, Singapore, 639798 Singapore
Abstract
Heat-assisted magnetic recording (HAMR) is one of prospective high
density recording technologies in current hard disk industry. In this
paper, we incorporate HAMR optical structures into a finite element
model of a thermal flying height control slider and study thermal
effects of these structures on the HAMR head-disk interface. Our
focus will be on the slider flying height (FH) changes due to the heat
loss in the waveguide and near field transducer (NFT). According to
our results, we find that large heat dissipation in NFT alone could
affect the slider’s FH due to the additional thermal protrusion induced
around the writer. The heat dissipation in the waveguide could also
influence on FH drop. In addition, the media back heating effect on
the slider temperature and thermal deformation is also analysed in
this paper. The numerical results shows that the respective thermal
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deformation caused by media back heating is about 0.09 nm with
72 nm of heat spot size. This amount could be significant when the
slider’s FH is below 1 nm. Therefore, it is very important to improve
the efficiency of laser optical delivery system and reduce thermal
effects on a HAMR head-disk interface.
AQ1
1. Introduction
Heat-assisted magnetic recording (HAMR) is one of prospective high
density recording technologies in current hard disk industry. It requires
heating a spot on the recording media with thea laser beam to overcome
the superpara-magnetic limit. The coercivity of media material grains
require very high magnetic writing field in order to conduct high
density recording. The current writer designs cannot deliver higher field
than the media coercivity. With the localized laser heating within the
desired recording spot on the disk at around Curie temperature
(~700 K), the magnetic domain of media is able to be switched for
writing. However, the additional demands of delivering laser energy to
the media not only require an integrated HAMR slider with laser
delivery components but also result in very high peak temperature at
media heat spot region.
The heat loss caused by the optical delivery system located inside a
slider may cause unwanted thermal protrusion on the slider body, which
may affect slider’s flying stability in the end. At the same time, the heat
produced by laser beam causes the temperature field on the disk surface
to be highly non-uniform, which may lead to unexpectedly severe
lubricant loss, or even the failure of the entire HAMR system. On the
other hand, the heat spot shining under the slider may also cause
additional thermal protrusion on the slider body. This heat spot back
heating effect is expected to be more serious at very low slider flying
height for high density recording.
Currently, some research has been conducted to study the thermal
effects on a HAMR head-disk interface. For example, Yu et al. ( 2013a ,
b ) numerically studied the lubricant depletion caused by the laser
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heating on the media of a HAMR system. Zheng et al. ( 2012 )
investigated the effect of heat dissipation in a waveguide on the thermal
deformation and slider’s flying characteristics based on a finite element
(FE) model built by ANSYS software. They concluded that the heat
dissipated in the waveguide generates an undesired thermal protrusion
with a small radius at the read/write element. However, no thermal
effect caused by the near field transducer (NFT) was considered in their
study. Huang et al. ( 2013 ) studied the slider temperature rise due to
media hot spot back heating. They found that head temperature
increases linearly with the media temperature and spot size. However,
the detailed temperature distribution and slider deformation profiles of
integrated HAMR head model due to media back heating was not
presented and discussed in the previous analyses.
In this paper, we will incorporate HAMR optical structures into a FE
model of a thermal flying height control (TFC) slider and study the
thermal effects of these structures on the HAMR head-disk interface.
Our focus will be on the slider flying height (FH) changes due to the
heat loss in the waveguide and NFT. Besides, the effect of media spot
back heating on the slider temperature and head deformation is also
studied to understand further on the heat transfer process in HAMR
head disk interface.
2. Numerical method
A schematic model of a femto-sized HAMR TFC slider with its
magnetic and optical head components at the deposited edge of the
slider is shown in Fig. 1 . The head components include the upper,
lower pole, write coil, upper shield, lower shield, the heater, cladding
layer, NFT and waveguide. The structure of NFT is modelled as an
E-shape one and its material property is set as gold. The waveguide core
material is set as tantalum pentoxide (Ta O ) and the core is separated
from NFT by a 36 nm-thick SiO layer. The waveguide core and NFT
are sandwiched by the waveguide cladding layer which is made of
glass. The HAMR head design is mainly based on the one in Stipe et al.
paper ( 2010 ). The commercial ANSYS software is used to carry out
finite element analysis (FEA) simulation on the thermal protrusions due
to the heat dissipations inside the NFT and waveguide. For thermal
2 5
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boundary conditions of the slider, we set heat transfer coefficient as a
constant of 100 W/m K at non-ABS surfaces of the slider and the heat
transfer coefficient at the ABS surface will be calculated by applying
the heat transfer model adopted by Zhou et al. ( 2008 ) and Huang et al.
( 2013 ), respectively, in their work.
Fig. 1
Schematic model of HAMR-TFC slider and diagram of HAMR magnetic
and optical system. a The cross-section diagram of a waveguide and near
field transducer (NFT) with dimensions
AQ2
We applied a coupled-field analysis method, which includes an air
bearing model, a heat transfer model and a thermal-structural FE model
developed in ANSYS to investigate the flying and thermal
performances of a HAMR slider at various surface temperatures and air
bearing surface profiles. An iterative solution is required to obtain the
thermal protrusion and temperature distribution on the slider body. This
is because the thermal protrusion of the slider will change the pressure
2
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and heat flux distributions across the ABS, and these changes will
inversely affect the thermal protrusion. Once the thermal protrusion of
the slider is determined, the respective FH of the slider can be obtained
from our self-developed ABSolution software which is employed for
computing the slider flying performance. The heat transfer model
applied in calculating the heat flux across the HDI in the couple-field
analysis is based on an expression derived by Zhang and Bogy ( 1999 ).
The heat flux between the slider and the air bearing disk can be
described as,
where x and y are the coordinates of the slider, T and T are the
temperatures of the slider and the disk, respectively, k is the thermal
conductivity of air, h is the slider-disk spacing, λ is the mean free path
of air, b = 2(2 − σ )γ/σ (γ + 1)Pr, σ is the thermal accommodation
coefficient, γ is the ratio of the specific heat, and Pr is the Prandtl
number. During the case studies, it is observed that the resulted heat
fluxes of different cases are ranged from about 5 × 10 to 2 × 10 W/m .
A detailed introduction of iteration process between ABSolution and
ANSYS software can be found in Ref (Zhou et al. 2009 ).
In order to study the effect of media heat spot on the head temperature
increment, firstly HAMR media surface temperature profile is obtained
from the optical absorption model derived from Maxwell equations (Yu
et al. 2013b ). The heat spot temperature distribution is assumed to
follow the Gaussian distribution. The peak temperature is 673 K and the
diameters of spot sizes which are based on full width at half maximum
(FWHM), are set as about 48 and 72 nm, respectively. The heat spot
temperature distribution profile used in studies is as shown in Fig. 2 b.
This thermal profile is applied to assemble as a heat spot on the disk in
HAMR slider FE model structure to study the back heating effect of
media spot on the slider temperature rise and deformation.
Fig. 2
q(x, y) = −k(x, y) −Ts Td
h(x, y) + 2bλ(x, y)
s d
T T T
7 8 2
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a The simulation model and setup for media back heating study and b the
temperature distribution profile (peak temperature: 673 K) of a heat spot
on the media applied in our numerical study
3. Results and discussions
In all simulation case studies, the heater is powered at 40 mW. Different
ranges of energy power settings are allocated on the waveguide and
NFT to study their thermal effects caused by heat dissipation in these
components. Figure 3 shows the slider thermal deformation profile due
to the heat dissipation in NFT and waveguide when the heater power is
set as 40 mW. Two protrusion peaks are clearly observed at the trailing
edge of slider and they are caused by the heater and NFT/waveguide
heat losses, respectively. It is also found that the highest temperature
occurs around NFT region and the slider body could be deformed
significantly due to heat dissipation in the optical delivery system
components. Therefore, it is necessary to study the slider flying
behaviour change caused by this deformation in HAMR interface.
Fig. 3
Thermal protrusion profile as shown in ANSYS with powers applied to
NFT, waveguide and the heater
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To differentiate the FH change caused by the heat loss in NFT from that
in waveguide, we first set different energy loss values at NFT only with
the heater power on. Thermal deformation results are shown in Fig. 4 .
It is observed that thermal protrusion increment is about 1 nm at 1 mW
of NFT heat loss and its profile looks pointed with narrower width. This
is because NFT is positioned in the middle of cladding layers which are
made of low thermal conductivity material; SiO . When the NFT power
loss is set at 2.0 mW and above, we find that the thermal protrusion
around the NFT region increases significantly with the temperature rise.
Temperature of NFT is increased up to about 1000 K with the power of
4.0 mW assigned to NFT. Such high temperature is more than the
melting point of gold material which makes NFT component. Therefore,
it is necessary in practice to reduce the power loss at NFT for those
undesired slider deformation and reliability problem. The method could
be to enlarge NFT volume or design additional heat sink layer for
reducing temperature rise. We also study the FH changes with different
NFT power loss settings as shown in Fig. 5 . It is found that with higher
heat dissipation in NFT, FH drop rate at writer location is obviously
steeper than that at reader location. FH change at NFT location follows
up with the similar gradient of FH drop at writer but it is closer to the
disk due to the protruded deformation around optical component region.
At 4.0 mW of NFT power loss, the FH at NFT, writer and reader
positions are around 0.88, 1.15 and 1.8 nm, respectively.
Fig. 4
Comparison of thermal protrusion profiles along the center line of the
slider with various NFT power loss settings
2
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Fig. 5
The respective flying height changes near the locations of NFT, writer
and reader with various NFT power loss settings
Afterwards, varied energy powers are assigned to the waveguide only to
study the effects of heat dissipations in the waveguide. According to the
deformation outcomes shown in Fig. 6 , higher power dissipation in
waveguide causes wider structural deformation around writer position
than that in NFT. The protrusion increment is almost in linear manner
and around 0.75 nm of protrusion is produced with every additional
6 mW dissipation power to waveguide. The protrusion peak increases
with waveguide power loss and it becomes about same or higher level
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than another protrusion peak resulted from the heater. Therefore,
thermal dissipation from waveguide could also influence the slider’s
flying performance largely. The FH changes at the reader, writer and
NFT positions with different waveguide power settings are calculated
by using ABSolution software and compared in Fig. 7 . It is found that
FH becomes smaller with higher power loss in waveguide. The FH
drops at the writer and NFT positions are steeper with rising power
dissipation to waveguide. Critically, with high power loss at 30 mW in
waveguide, we observe that the contact between the slider and disk
happens around the writer location. The combination effect of heat
dissipations in NFT and waveguide should definitely influence more in
head disk interface. Therefore, it can be concluded that higher heat
energy loss in optical delivery components may lead severe slider/disk
contact and affect the slider flying stability.
Fig. 6
Comparison of thermal protrusion profiles along the center line of slider
with various waveguide power loss settings
Fig. 7
The respective FH changes near the locations of NFT, writer and reader
with various waveguide power loss settings
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As for the study of back heating effect, we assume that no power loss
happens in optical components. We also turn off TFC power in the
simulation studies. This is to observe temperature rise and slider
protrusion caused by back heating effect more clearly. The gap spacing
between the slider trailing edge and the disk is fixed at 2 nm as shown
in Fig. 2 a, which means that the spacing between NFT component and
disk is about 4 nm. The peak Curie temperature of recording media is
set as 673 K or 400 °C. The heat spot center is designed purposely to be
located under the NFT position. Two different diameters of heat spots
(FWHM) such as 48 and 72 nm are adopted in case studies. The initial
slider body temperature is set as 300 K.
The resulting slider temperatures in both down-track (x-direction) and
cross-track (y-direction) are shown in Fig. 8 . Maximum temperature
rises are 1.5 and 9 K for 48 and 72 nm of spot sizes respectively. The
down-track temperature distribution is asymmetric across the
waveguide and NFT region because it pasts different thermal properties
of optical components on the slider air bearing surface. Applying with
larger heat spot, wider area or almost all area of optical components are
exposed with high temperature and it results higher temperature rise
with 72 nm of spot size. Consequently, thermal protrusions increase up
to 0.09 nm with back heating from a heat spot, which is about 8 times
larger than that with 48 nm of heat spot size, as shown in Fig. 9 .
Although this protrusion value is relatively small, it could become
significant when slider’s FH drops to below 1 nm. From Fig. 9 a, we
also find that the protrusion profile is not smooth due to varying thermal
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expansion coefficients (TEC) of head components in the region affected
by back heating. It is observed that the peak protrusion in down-track
direction occurs around NFT component which is made of gold and has
higher TEC value. This deduction is supported by observing thermal
protrusion profile in cross-track direction. Two shallow dips at the sides
of protrusion profiles (in x-direction) are caused by very low TEC of
cladding material (SiO ).
Fig. 8
The comparison of slider temperature distributions due to the media back
heating in a x-direction (down track) and b y-direction (cross track) using
different heat spot sizes
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Fig. 9
The comparison of slider thermal protrusion profiles due to the media
back heating in a x-direction (down track) and b y-direction (cross track)
using different heat spot sizes
4. Conclusions
In this paper, the numerical study of thermal effects of optical
components on slider flying behaviour in HAMR head disk interface are
carried out using ANSYS FE analysis coupled with ABSolution
software. Different power settings are assigned in NFT and waveguide
as varied heat energy loss together with the powered heater. The results
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show that higher heat dissipation in NFT alone could affect the slider’s
FH due to the additional thermal protrusion induced around the writer.
The heat dissipation in waveguide could also influence on FH drop.
Larger waveguide heat dissipation could cause the slider contacting
with the disk and the drive reliability could be affected. Therefore, it is
very important to improve the efficiency of laser optical delivery
system so that less heat dissipation occurs in its components. Slider
temperature rise and thermal protrusion increment by back heating of a
media heat spot could be significant with larger diameter spot at low
flying height. Currently, in improved HAMR slider head designs, an
additional heat sink or high thermal conductivity layer(s) is placed to
dissipate heat to the surrounding for better efficiency of optical delivery
system. We will study its thermal effect on the interface using those
HAMR head designs in near future.
References
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