Numerical investigation of thermal effects on a HAMR head-disk …oar.a-star.edu.sg/jspui/bitstream/123456789/1361/1/post... · 2016. 4. 5. · Numerical investigation of thermal

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



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

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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

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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



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



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

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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



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



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



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



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

Huang L, Stipe B, Staffaroni M, Juang JY, Hirano T, Schreck E,

Huang FY (2013) HAMR thermal modeling including media hot

spot. IEEE Trans Magn 49:2565–2568

Stipe BC, Strand TC, Poon CC, Balamane H, Boone TD, Katine JA,

Li JL, Rawat V, Nemoto H, Hirotsune A, Hellwig O, Ruiz R, Dobisz

E, Kercher DS, Robertson N, Albrecht TR, Terris BD (2010)

Magnetic recording at 1.5 Pb m using an integrated plasmonic

antenna. Nat Photon 4:484–488

Yu P, Zhou WD, Yu SK, Zeng Y (2013a) Laser-induced local

heating and lubricant depletion in heat assisted magnetic recording

systems. Int J Heat Mass Transf 59:36–45

Yu P, Zhou WD, Yu SK, Myo KS (2013b) Modeling laser heated

thin film media for heat assisted magnetic recording. Microsyst

Technol 19:1457–1463

Zhang S, Bogy DB (1999) A heat transfer model for thermal

fluctuations in a thin slider disk air bearing. Int. J. Heat Mass Transf

42:1791–18001

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Zheng H, Li H, Talke FE (2012) Numerical simulation of thermal

flying height control sliders in heat-assisted magnetic recording.

Microsyst Technol 18:1731–1739

Zhou WD, Liu B, Yu SK, Hua W, Wong CH (2008) A generalized

heat transfer model for thin film bearings at head-disk interface.

Appl Phys Lett 92:043109

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Numerical investigation of thermal effects on a HAMR head-disk …oar.a-star.edu.sg/jspui/bitstream/123456789/1361/1/post... · 2016. 4. 5. · Numerical investigation of thermal