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Lubricant reflow after laser heating in heat assisted magnetic recording

Authors:
Haoyu Wu at Western Digital Corporation
Haoyu Wu
Alejandro Rodriguez Mendez at Western Digital Corporation
Alejandro Rodriguez Mendez
Shaomin Xiong at HGST, A Western Digital Company
Shaomin Xiong
David Bogy at University of California, Berkeley
David Bogy
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Abstract

In heat assisted magnetic recording (HAMR) technology for hard disk drives, the media will be heated to about 500 °C during the writing process in order to reduce its magnetic coercivity and thus allow data writing with the magnetic head transducers. The traditional lubricants such as Z-dol and Z-tetraol may not be able to perform in such harsh heating conditions due to evaporation, decomposition and thermal depletion. However, some of the lubricant depletion can be recovered due to reflow after a period of time, which can help to reduce the chance of head disk interface failure. In this study, experiments of lubricant thermal depletion and reflow were performed using a HAMR test stage for a Z-tetraol type lubricant. Various lubricant depletion profiles were generated using different laser heating conditions. The lubricant reflow process after thermal depletion was monitored by use of an optical surface analyzer. In addition, a continuum based lubrication model was developed to simulate the lubricant reflow process. Reasonably good agreement between simulations and experiments was achieved.
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Lubricant reflow after laser heating in heat assisted magnetic recording
Haoyu Wu, Alejandro Rodriguez Mendez, Shaomin Xiong, and David B. Bogy
Citation: Journal of Applied Physics 117, 17E310 (2015); doi: 10.1063/1.4914073
View online: http://dx.doi.org/10.1063/1.4914073
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/17?ver=pdfcov
Published by the AIP Publishing
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Lubricant reflow after laser heating in heat assisted magnetic recording
Haoyu Wu, Alejandro Rodriguez Mendez, Shaomin Xiong,
a)
and David B. Bogy
Computer Mechanics Lab, University of California at Berkeley, Berkeley, CA 94720, USA
(Presented 6 November 2014; received 11 September 2014; accepted 30 October 2014; published
online 6 March 2015)
In heat assisted magnetic recording (HAMR) technology for hard disk drives, the media will be
heated to about 500
C during the writing process in order to reduce its magnetic coercivity and
thus allow data writing with the magnetic head transducers. The traditional lubricants such as Z-dol
and Z-tetraol may not be able to perform in such harsh heating conditions due to evaporation,
decomposition and thermal depletion. However, some of the lubricant depletion can be recovered
due to reflow after a period of time, which can help to reduce the chance of head disk interface fail-
ure. In this study, experiments of lubricant thermal depletion and reflow were performed using a
HAMR test stage for a Z-tetraol type lubricant. Various lubricant depletion profiles were generated
using different laser heating conditions. The lubricant reflow process after thermal depletion was
monitored by use of an optical surface analyzer. In addition, a continuum based lubrication model
was developed to simulate the lubricant reflow process. Reasonably good agreement between simu-
lations and experiments was achieved.
V
C
2015 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4914073]
I. INTRODUCTION
In current hard disk drives (HDDs), nanometer-thick
lubricant layers are applied on the surface of the media to
provide protection for the heads and disks, by reducing the
friction and wear during accidental slider disk contact.
1
The
lubricant films are synthesized from Perfluoropolyether
(PFPE) molecules, e.g., Z-tetraol, which are stable enough to
protect the disk at and around room temperature for at least
five years.
On the other hand, high magnetic anisotropy materials
need to be used to break the limit of superparamagnetism in
order to increase storage areal density beyond 1Tb/in.
2
. The
magnetic state of this kind of media is so stable at room tem-
perature that current magnetic transducers may not be able to
switch its orientation. Therefore, heat assisted magnetic re-
cording (HAMR) technology
2,3
has been proposed to solve
this probl em. In HAMR, the magnetic layer is heated up to
its Curie temperature with a laser such that the magnetic
coercivity of the media is reduced and data writing with the
magnetic transducers is possible.
Since the lubricant layer is on top of the magnetic layer,
it will also be heated locally to a similar temperature. The
harsh heating condition can damage traditional lubricants
and reduce their lifetime due to evaporation, decomposition
and thermal depletion.
4,5
However, some of the lubricant
depletion can be recovered due to reflow after some period
of time. The reflow behavior can help to cure the lubricant
depletion and reduce the chance of hard head disk interface
(HDI) failure. It is therefore important to understand the
mechanisms and characteristics of the lubricant reflow
behavior for HAMR systems.
In this paper, experimental studies of lubricant reflow
were performed in a HAMR test stage for a Z-tetraol type
lubricant. Section II describes the experimental setup. The
observed reflow behavior of the lubricant is discussed in
Sec. III.SectionIV introduces a numerical model for lubri-
cant reflow and compares the numerical simulation w ith our
experimental results.
II. EXPERIMENTAL CONDITIONS AND PROCEDURE
A HAMR test stage was built to provide HAMR-like
heating conditions on the disk and study the lubricant deple-
tion and reflow behavior. The test stage contains the follow-
ing three parts: an illumination module that can generate a
laser beam at different power levels and focus the laser spot
onto the disk with a size of a few microns, a spindle stage
that can spin a disk at a controlled speed, and a servo motor
that can control the radial movement of the laser spot such
that different parts of a disk can be heated by the laser. A
schematic drawing of the test system is shown in Fig. 1.
FIG. 1. Schematic drawing for the HAMR test stage. The Central Controller
is for the spindle, the Laser Generator, and the Servo Motor. The Laser
Generator illuminates the spinning disk with a laser light. The Servo Motor
controls the objective lens such that different parts can be exposed to the
laser.
a)
Author to whom correspondence should be addressed. Electronic mail:
xshaomin@berkeley.edu.
0021-8979/2015/117(17)/17E310/4/$30.00
V
C
2015 AIP Publishing LLC117, 17E310-1
JOURNAL OF APPLIED PHYSICS 117, 17E310 (2015)
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Since HAMR disks were unavailable to us, commercial
Perpendicular Magnetic Recording (PMR) disks were used
instead in this investigation. The disks were 3.5 in. in diame-
ter with aluminum substrates. The lubricant type was Z-
tetraol with A20H additives, 60% bonding ratio and thick-
ness of 9.5 A
˚
.
As shown in Fig. 1, a laser spot generated from the illumi-
nation module was focused on the spinning disk. The laser spot
heats up the disk and provides a HAMR-like condition. This
illumination procedure contains three controllable parameters:
laser power incident on the spinning disk (P
inc
), disk’s spinning
speed (x
disk
) and number of disk revolutions (repetitions) dur-
ing laser illuminat ion (n
illum
). P
inc
was controlled by optical fil-
ters between the laser generator and disk; x
disk
and n
illum
were
controlled by an in-house designed electronic controller based
on a field programmable gate array (FPGA) board. The optical
encoder in the spindle was used to count the number of revolu-
tions of the disk. The laser ill umination repetitions (n
illum
) can
be precisely controlled. The relationship between repetitions
and laser illumination time is shown in Fig. 2.
The disk was exposed to the laser at a constant P
inc
while spinning at a constant x
disk
. However, we used differ-
ent values of n
illum
from 1000 to 1 on different tracks. Soon
after the laser exposure, the disk was measured by a Candela
optical surface analyzer (OSA). The Q-Phase channel was
used to measure the lubricant thickness change.
6
Scans by
the OSA were taken periodically at room temperature at
intervals of about 95-s up to about 22 min in total such that
the lubricant profile could be recorded at different times. A
scan was also taken again after 24 h to see the final state of
the lubricant.
III. LUBRICANT REFLOW PROCESS
Due to spindle run-out, the Q-phase image of the lubri-
cant showed some curvature and background. A script was
developed to post process the images and eliminate the run-
out curvature and non-uniform background. Examples of the
processed OSA images are shown in Fig. 3.
As can be seen in Fig. 3(a), the parallel lines represent
the exposed tracks to the laser for different n
illum
. The
increase of reflectivity in the Q-Phase indicates a lubricant
thickness decrease. This is mainly due to lubricant depletion.
Higher n
illum
causes significantly more lubricant depletion as
shown in Fig. 3 where the tracks on the top have a larger
change of reflectivity. Figures 3(b) and 3(c) show the OSA
Q-phase images after some time has elapsed. The reflectivity
change of the tracks shown in Figs. 3(b) and 3(c) becomes
smaller compared to Fig. 3(a), indicating that the lubricant
flows back to the depleted region. Fig. 3(d) shows the reflec-
tion of the lubricant after 24 h. Fig. 3(d) shows no apparent
reflection when n
illum
100, which means that the lubricant
has recovered back to its initial state. However, when
n
illum
> 100, there still remain some changes of reflectivity
which were not recovered in 24 h of lubricant reflow. This
final state condition may be due to degradation of the carbon
overcoat (COC) or magnetic layers.
4
To eliminate possible
non-lubricant effects, only the n
illum
100 conditions are
discussed below.
Figure 4 shows the maximum lubricant depletion depth
as a function of time for one set of experiments. The
FIG. 2. Modulation of laser by spindle index.
FIG. 3. OSA scanning images of relative reflectivity on a disk after certain
repetitions of illumination by laser at: (a) 0 min, (b) 3 min, (c) 9 min, and
(d) 24 h, respectively. The x-axis is the angular position in units of degrees
and the y-axis is the relative radial position in units of lm. The relative
reflectivity slowly fades as time elapses. The n
illum
from top to bottom for
the seven tracks are 1000, 500, 100, 50, 10, 5, 1, respectively.
FIG. 4. Lubricant relaxation after laser
depletion. (a) The three different lines
show different laser illumination repe-
titions. Less repetitions result in shal-
lower initial lubricant depletion. The
reflow trends are similar for the three
different conditions. (b) Lubricant
depletion normalized with respect to
initial value. The depletion is set to 1
at t ¼ 0. Similar trends are shown.
17E310-2 Wu et al. J. Appl. Phys. 117, 17E310 (2015)
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lubricant depletion and reflow profiles were obtained from
the OSA Q-phase images, some of which were shown in Fig.
3. The experimental parameters used in Fig. 4 are x
disk
¼600
RPM and P
inc
¼165 mW. The illumination repetitions were
100, 50, and 5, respectively. The depletion curve with
n
illum
¼ 1 was too small for a reliable analysis, therefore
these result is not presented.
As shown in Fig. 4, the lubricant depletion is more
severe when the disk is illuminated for mo re repetitions. To
exclude the effect of different initial lubricant depletion on
the reflow, we normalized the lubricant depletion curves by
its initial value as shown in Fig. 4(b). It is observed that the
lubricant depletion decreases as time elapses, which indi-
cates that the lubricant flows back into the depleted area. The
reflow rate is initially fast and decreases with time. Almost
80% of the lubricant recovers within 20-min of relaxation at
room temperature.
7
IV. COMPARISON BETWEEN SIMULATION AND
EXPERIMENTS
Simulations of lubricant reflow were carried out to com-
pare them with the experimental results. The lubricant reflow
was described using continuum theory with a modified
(effective) viscosity.
8
Within the continuum approach, the
dimensions of the thin film on the disk surface make it possi-
ble to use lubrication theory and thus we obtain the govern-
ing equations described below
@h
@t
þ
1
3l
@
@x
h
3
dP h
ðÞ
dh
@h
@x

¼ 0 ; (1)
where h ¼ hðx; tÞ is the film thickness, l is the effective
lubricant viscosity, PðhÞ is the disjoining pre ssure arising
from van der Waals interaction s between the lubricant and
the solid substrate.
9
This disjoining pressure is of the form
PðhÞ¼Ah
3
, where A is the Hamaker constant. The initial
condition, as seen in Fig. 5(a), was given by the lubricant
depletion profile obtained in the experiments at time t ¼ 0s.
As boundary conditions, we considered zero volume flow at
the right and left boundaries. This condition is equivalent to
setting dh=dx ¼ 0 at the boundaries. It can be observed that
Eq. (1) depends only on the ratio of the Hamaker constant to
lubricant visco sity. This ratio was adjusted to give the best
match to the experimental results. The simulation results of
lubricant reflow are shown in Fig. 5.
It can be seen from Fig. 5(b) that the simulation results
fit adequately the experimental data. However, there exist
regions of some discrepancy. In the first 400 s of reflow, the
simulation results show a faster recovery rate than the experi-
ments. After this time, the reflow in the simulation slows
down relative to the experiments. This discrepancy may be
explained by noting that the lubricant viscosity of thin films
can be thickness dependent as discussed in Ref. 10. This phe-
nomenon was not included in the present simulation model.
V. CONCLUSION
In this paper, the thermal depletion behavior of Z-tetraol
due to a free laser beam heating condition as well as the re-
covery behavior after heating was studied. The initial lubri-
cant depletion was different for different laser heating
conditions, i.e., a longer heating duration causes more lubri-
cant depletion. However, a similar trend was found regard-
less of initial lubricant depletion. Almost 80% of lubricant
reflows back within 20 min at room temperature. Simulation
results show a reasonably good agreement with experiments.
Real HAMR laser conditions use a near field transducer
(NFT) as a heating method to achieve heated spots of tens of
nanometers rather than a few microns. So the NFT heating
has a spot size a few orders of magnitude smaller and its du-
ration is a few orders of magnitude shorter than our free laser
beam heating. Further study will be made with the NFT heat-
ing and HAMR disks as soon as the needed components
become available.
ACKNOWLEDGMENTS
This research was supported by the Computer
Mechanics Laboratory (CML) of the University of
California at Berkeley.
1
B. Marchon, “Thin-film media lubricants: Structure, characterization, and
performance,” Developments in Data Storage: Materials Perspective
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2
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3
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17B702 (2014).
4
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(2014).
FIG. 5. (a) Film thickness profile at
selected times obtained from simula-
tions. (b) Depth of the maximum
depletion point in the film as obtained
from experiments and results. The
experiment parameters are x
disk
¼ 600
RPM, P
inc
¼ 165 mW and n
illum
¼ 100.
The simulation parameters are l
¼ 1:5Pa s and A ¼ 1 10
21
J.
17E310-3 Wu et al. J. Appl. Phys. 117, 17E310 (2015)
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6
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7
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8
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9
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10
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17E310-4 Wu et al. J. Appl. Phys. 117, 17E310 (2015)
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... On the other hand, the lubricant has a self-healing ability, and it is also important to the reliability. Some of the depleted lubricant recovers gradually due to the lubricant reflow, which has mostly been studied using ex-situ methods [148,149]. An in-situ measurement of the lubricant reflow is important to the tribological longevity of HAMR hard disk drives. ...
... The black and red dots correspond to the reflow dynamics on the two disks with initial lubricant thicknesses of 8.0 and 12.5Å, respectively. The governing equation for the reflow dynamics can be derived using continuum theory as [149,154]: ...
Thermal Transport, Thermal Protrusion, and Thermally-Induced Material Transfer in the Head-Disk Interface of Heat-Assisted Magnetic Recording
Thesis
Full-text available
  • Dec 2021
As data generated worldwide are growing explosively, it is crucial to increase the areal density of traditional storage devices to satisfy the requirements. Conventional hard disk drive (HDD) technology, perpendicular magnetic recording (PMR), has reached the superparamagnetic limit of ~ 1 Tb/in2. To realize the areal density over 1 Tb/in2, the size of the media bits must be further decreased to tens of nanometers, which requires high coercivity magnetic media. The high coercivity can avoid superparamagnetism and thus store data safely at the small bit size under room temperature, but it makes data writing challenging. To assist the writing process, energy is input to the media to lower its coercivity temporarily. Current technologies such as heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR) utilize two different methods to lower the coercivity. HAMR integrates a laser to locally heat the media to its Curie temperature (400–500 °C), while MAMR uses a spin torque oscillator to induce ferromagnetic resonance in the media grains. In the HAMR head-disk interface (HDI), a recording head flies over a rotating disk with a relative velocity of 5–40 m/s and an initial spacing of 10–15 nm controlled by an air bearing. Then, the spacing is reduced by energizing a joule heater inside the head. The heater generates a protrusion on the head surface to lower the initial spacing to 1–2 nm so that data reading/writing can be performed using the read/write transducers in the head. The head is also integrated with a laser diode, a waveguide (WG) and a near-field transducer (NFT) for laser delivery. The laser beam is launched from the recording head and is focused on the recording disk to locally heat the disk (400–500 °C), which is even hotter than the head temperature (150–250 °C). Therefore, the head-disk interface of HAMR is a system that combines nanoscale spacing (< 15 nm), high temperatures (head ∼ 150–250 °C, disk ∼ 400–500 °C), steep thermal gradient (∼ 10 K/nm), and a high-speed sliding condition (5–40 m/s). Furthermore, the introduction of the laser affects thermal transport and thermal protrusion, and causes thermally-induced material transfer in the interface, which needs to be investigated both for fundamental understanding and for practical applications such as HAMR and other microelectronics devices. To study the thermal transport across a closing gap between the head and the disk, we conducted static touchdown experiments using a custom-made setup where the disk is not rotating to exclude the air cooling effect. The head temperature rise was measured as a function of the heater power under various conditions such as different substrate materials, relative humidity and laser on/off. 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Ge2Sb2Te5 undergoes a crystalline transition at 149 °C with changes in its density and optical reflectivity. By use of these changes, we can map surface temperatures from nanoscale to microscale with minimal calibration, which is demonstrated using a recording head. Finally, we propose a near-field thermal transport based scheme for lubricant thickness measurement. The thermal effect of the lubricant is investigated when the head approaches the disk in the flying touchdown experiments, which is then used to determine the lubricant thickness. Most previous lubricant measurements require an ex-situ tool such as optical surface analyzer (OSA), but the proposed scheme is an in-situ method with a sub-angstrom resolution and a faster response time. Using the scheme, we performed in-situ measurements of the lubricant depletion and reflow dynamics under HAMR operations.
View
... This scheme requires implementing a laser diode in the recording head. It becomes critical to study and understand the thermal process of all components in a HAMR system, such as the protrusions on the head [5,6,7], media temperature rise [8], a range of nanoscale heat and material transfer phenomena [9][10][11], and thermal degradation of the head-disk interface [12,13]. ...
... The temperature rise on the media is a very critical factor in successful data writing. If the temperature rise is less than Tcr, the magnetic writing will not occur, but if the temperature rise is significantly higher than Tcr, it could damage the head disk interface [12]. As the media rotates at the operational speed, each point on an exposed track experiences a flash laser irradiation when the laser spot travels through. ...
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Heat assisted magnetic recording (HAMR) promises to deliver higher storage areal density than the current perpendicular magnetic recording (PMR) product. A laser is introduced to the HAMR system to heat magnetic media to reduce the media coercivity. The thermal response of the media becomes very critical for the success of the magnetic writing process. The study of thermal response time in HAMR relies on the setup configurations, such as laser spot sizes, the way that laser energy is delivered to media and the media structures. In this paper, the thermal response time of HAMR media under three different heating methods is systematically investigated through experiments and numerical analysis. A lumped model is built to simplify the heat conduction problem to understand the difference in thermal responses under various experimental conditions. Dominant layers are identified under those experimental conditions. The transient thermal response is mainly determined by the dominant layers. Engineering the dominant layers helps the most in optimizing the thermal performance of the media. Our study clearly suggests that, for HAMR systems, optimizing the thermal properties of the heat sink layer is the key to reducing variations in the transient thermal process resulting from changes in the linear speed.
View
... For this reason, Dahl et al. [24] carried out a detailed parameter study of the effect of each factor on lubricant depletion groove caused by laser heating using the diffusion equation, which takes into account disjoining pressure, temperature characteristics of surface tension, heated evaporation, temperature and film thickness effects on viscosity, etc. On the other hand, Wu et al. [25] measured the replenishment process of the depleted groove produced by laser for Z-tetraol lubricant with thickness of 0.98 nm and bonded ratio of 0.6 and compared it with the calculated results using the conventional Equation (3) with d 0 = 0, A = 10 −21 J/m 2 , and µ m = 1.5 Pa·s. The replenishment speed appears to be close to the experimental value, but this may be due to the fact that the lubricating film had a small bonded ratio and the extremely small Hamaker constant was used. ...
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Full-text available
  • Mar 2023
Dinamika pembalikan magnetisasi pada nanoblock berbasis kobalt teredam kuat yang distimulasi oleh pemanasan pulsa telah dieksplorasi secara numerik untuk model parallelepiped dengan ukuran 50×50×20 nm3. Pengaturan simulasi diimplementasikan untuk mengevaluasi dua parameter penting dalam perekaman magnetik dengan bantuan panas, yaitu stabilitas termal, dan konsumsi medan tulis. Stabilitas termal diukur melalui medan magnet induksi yang meningkat secara linear hingga 2 Tesla selama 2,5 ns pada kesetimbangan termal terhadap lingkungan. Dalam simulasi sistem dengan stimulasi medan panas, konsumsi medan tulis diselidiki menggunakan dua jenis pulsa, yaitu medan magnet, dan pulsa medan termal. Simulasi ini didasarkan pada persamaan Landau-Lifshift-Gilbert yang memasukkan teorema disipasi fluktuasi dalam menghitung efek fluktuasi termal. Selain itu, parameter-parameter material yang tergantung suhu, yaitu saturasi magnetik, anisotropi magnetik, dan interaksi pertukaran, juga diperhitungkan. Sebagai hasilnya, nanoblock berbasis kobalt yang teredam kuat memiliki penghalang energi magnetik yang tinggi untuk mencegah batasan superparamagnetik. Selain itu, medan penulisan dapat dikontrol melalui pengaturan periode pemanasan, dan juga melalui manipulasi sifat magnetik seperti redaman magnetik intrinsik.
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... The diamond-like carbon (DLC) protective film and perfluoropolyether (PFPE) lubricant film of the disk are heated to high temperatures reaching 200-500°C, which causes damage such as evaporation, thermal decomposition, and thermal oxidation. However, considering that the application of highcapacity disks using HAMR technology is storage for near-line data servers such as cloud computing, this is not considered to be a major problem because most of it is a write-once usage mode, and the damaged lubricating film is repaired in a short time [2,3]. In contrast, evaporated and thermally decomposed lubricant and DLC-derived head smears adhere to the magnetic head slider surface owing to heating [4,5], causing more serious problems for hard disks, such as head slider vibration and troublesome effects on read/write elements. ...
Effects of Oxygen on Smear Formation in Heat Assisted Magnetic Recording System
Article
Full-text available
  • Dec 2022
  • Tribol Online
Heat-assisted magnetic recording (HAMR) is expected to be a realistic next-generation technology for increasing the recording density of hard disks. However, the magnetic layer is heated above the Curie temperature, and, as a result, the heated lubricant is desorbed from the disk by decomposition and evaporation, which causes a problem as it adheres to the air-bearing surface (ABS) as a smear. In this study, pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) analysis was performed in helium and air environments to investigate the decomposition mechanism of perfluoropolyether (PFPE) lubricant D-4OH by heating and in the presence of oxygen. In the helium environment, thermal decomposition of the end groups was confirmed at 350°C and above with a possibility of main chain decomposition at 450°C. In the air environment, decomposition of the end group was confirmed at 250°C and above, and decomposition of the main chain was confirmed at 450°C. Experiments using a pin-on-disk tester were conducted to confirm what happens to the area of smear when a thin film of D-4OH lubricant coated on an actual disk is laser heated. As a result, it was confirmed that the area of smear decreased even at an oxygen concentration of 5%.
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... The lubricant on the disk can recover and the lubricant on the air-bearing surface (ABS) can be redistributed [8]. Recently, reflow of the lubricant was observed for a HAMR disk after laser heat induced depletion [9][10][11]. Interestingly, the above recovery, redistribution and reflow mechanisms of the lubricant can be explained and predicted by a viscous flow model based on continuum fluid mechanics theory [8,11]. ...
Finite element simulations of sliding contact of the head-disk interface in magnetic storage with lubricant effects
Article
Full-text available
  • Dec 2021
Perfluoropolyether (PFPE) polymer lubricant performs an important role in protecting both the writing/reading elements in the recording head and the disk of magnetic storage devices from mechanical wear or damage, induced by direct contact. Under high-speed sliding, the nanometer thick lubricant shows complex behavior, such as shear thinning, slippage and bonding ratio, making it difficult to directly test or simulate its performance. Based on a numerical model, the present study obtains the mechanical response of the lubricant under normal and sliding contacts. The mechanical response is then entered into a finite element model (FEM) through definition of material behavior of the lubricant using a hyper-elastic constitutive model. By comparing the sliding contact simulations with lubricant and without lubricant, it is found that the presence of lubricant helps to reduce the contact stresses, mainly because the contact area is increased.
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... This figure shows that for the micron-size (and larger) laser spots, the lubricant behaves viscously, and therefore, the lubrication equation applies at this limit. Interestingly, this is where most of the experiments are conducted due to the limited resolution of the focusing and measurement systems such as OSA [22,24,25]. However, for the target of HAMR with a length scale of L = 20 nm, Fig. 4 shows that the calculated Deborah number is larger, and therefore, the elastic behavior ...
Effect of Viscoelasticity on Lubricant Behavior Under Heat-Assisted Magnetic Recording Conditions
Article
Full-text available
  • Jan 2018
  • TRIBOL LETT
In Heat-Assisted Magnetic Recording (HAMR) technology, the lubricant layer coating on the disk is exposed to severe thermal conditions, leading to evaporation, depletion, and chemical degradation. In general, those studying the effects of laser exposure on lubricant depletion and recovery have assumed the lubricant to be a viscous fluid and have modeled its behavior using lubrication theory. However, PFPE lubricant depletion and recovery behavior at the timescale of HAMR conditions (microsecond to millisecond) is known to be that of a viscoelastic fluid. In this paper, we introduce a modification to the traditional lubrication equation that accommodates viscoelastic effects. The results suggest that this method is numerically unstable for small laser spot sizes, close to the target of HAMR. Accordingly, we developed a novel approach to model the viscoelastic depletion and recovery behavior of PFPE ultra-thin films using a Finite Element Analysis. We show that this new method is able to model the entire range of material viscoelasticity, from purely viscous to purely elastic extremes. The results show that the viscoelastic effects become remarkably pronounced with a decrease in laser spot size. For the micron-size laser spots, close to typical experimental conditions, the lubricant behaves like a viscous fluid. For the laser spot size of 20 nm, close to the target of HAMR, it behaves like an elastic material. In exposing the consequences of this viscoelastic behavior, this study predicts that lubricant flow due to thermo-capillary effects will not be a significant issue in the development of the HAMR technology. Rather, future efforts should concentrate on the thermal degradation and evaporation effects upon HAMR lubricants.
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... The magnetic state of this media is stable at room temperature, and so current magnetic transducers may not be able to switch its orientation due to medium coercivity. In consequence, when the HDD is writing data, the technology of heat-assisted magnetic recording (HAMR) helps to reduce medium coercivity by heating the disc with a laser spot [1][2][3]. To protect the head-disc interface (HDI) from wear to hit, perfluoropolyether (PFPE) lubricants are used on the disc surface. ...
Modeling of formation and breaking of lubricant bridge in the head–disk interface by molecular dynamic simulation
Article
  • Jun 2018
The technology of heat-assisted magnetic recording (HAMR) has improved the storage density of hard disc drives. The PFPE molecules of lubricant layer adhered on the disc can transfer from the lubricant layer and form the lubricant bridge which can deteriorate the stability of read/write process. In this paper, the formation and breaking of lubricant bridge at the head–disc interface (HDI) affects HAMR stability and deserves to be investigated. Using molecular dynamic simulation, a full-atom model was built to evaluate the behaviour of the lubricant bridge. Moreover, the effects of lubricant temperature, heating-up time, disc rotation speed and bearing pressure on the HDI were studied. It has been found that the amount of transferring atoms sharply increased when the lubricant temperature was above 700 K. The loss rate of lubricant layer decreased gradually during the heating process and it took about 2.2 ns for the remaining lubricant to reach stability. Furthermore, transferring PFPE molecules can form the lubricant bridge. A shorter heating-up time makes the lubricant bridge thicker and more robust. And the duration of lubricant bridge is notably affected by heating-up time, rotation speed and bearing pressure. A shorter heating-up time leads to a longer duration of lubricant bridge, whereas a higher rotation speed and bearing pressure reduces the duration of lubricant bridge.
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A Study of the Nanoscale Heat Transfer in the HDD Head–Disk Interface Based on a Static Touchdown Experiment
Article
  • Mar 2020
Understanding the heat transfer behavior at the nanoscale head-disk interface (HDI) in hard disk drives is crucial for head design, media design, and failure analysis of the current hard disk drive (HDD) industry, especially for the emerging technologies including heat-assisted magnetic recording (HAMR), microwave-assisted magnetic recording (MAMR), and 2-D magnetic recording (TDMR). Previous experimental studies of both static touchdown technique and theoretical developments of the wave-based phonon conduction have shown enhanced heat transfer at the HDI. To better understand the heat transfer behavior across the HDI, a series of the simulation is necessary to connect the theory and the detailed geometric model of the HDI. In this article, we developed a finite-element model to explain the temperature change of the head during a static touchdown experiment. The wave-based phonon conduction theory is integrated into the simulation through iteration. The simulation results trend in agreement with the theoretical development and the experimental results. This simulation strategy can also be implemented for flying heads to predict the heat transfer behavior under HAMR conditions.
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Molecular Dynamics Study of Lubricant Depletion by Pulsed Laser Heating
Article
  • Dec 2017
  • APPL SURF SCI
In this study, molecular dynamics simulations were performed to numerically investigate the effect of pulsed laser heating on lubricant depletion. The maximum temperature, the lubricant depletion width, the number of evaporated lubricant beads and the number of fragmented lubricant chains were studied as a function of laser peak power, pulse duration and repetition rate. A continuous-wave laser and a square pulse laser were simulated and compared to a Gaussian pulse laser. With increasing repetition rate, pulsed laser heating was found to approach continuous-wave laser heating.
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Lubricant depletion under various laser heating conditions in Heat Assisted Magnetic Recording (HAMR)
Conference Paper
Full-text available
  • Sep 2014
  • Proceedings of SPIE
Heat assisted magnetic recording (HAMR) is expected to increase the storage areal density to more than 1 Tb/in2 in hard disk drives (HDDs). In this technology, a laser is used to heat the magnetic media to the Curie point (~400-600 °C) during the writing process. The lubricant on the top of a magnetic disk could evaporate and be depleted under the laser heating. The change of the lubricant can lead to instability of the flying slider and failure of the head-disk interface (HDI). In this study, a HAMR test stage is developed to study the lubricant thermal behavior. Various heating conditions are controlled for the study of the lubricant thermal depletion. The effects of laser heating repetitions and power levels on the lubricant depletion are investigated experimentally. The lubricant reflow behavior is discussed as well.
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Lubricant Flow and Accumulation on the Slider’s Air-Bearing Surface in a Hard Disk Drive
Article
Full-text available
  • Feb 2014
Lubricant accumulation on the slider’s surface of a hard disk drive (HDD) has a detrimental effect on its read/write performance. Air flow through the slider-disk clearance moves some of the lubricant from the air-bearing surface (ABS) toward the slider’s lateral walls where it accumulates. In this article, we show by numerical simulations that the lubricant accumulation characteristics are strongly dependent on the slider’s flying height, skew angle and ABS design. The lubricant flow on the slider’s surface is quantified numerically. Air shear stress, air pressure and disjoining pressure are used as driving forces in the simulations. The lubricant thickness profile and volume evolution are calculated for two states of the HDD: operating and at rest. In the first state, lubricant is driven by air shear stress toward the trailing edge of the slider where it accumulates on the deposit end. In the second state, lubricant from the deposit end flows back into the ABS driven by the action of disjoining pressure. Lubricant accumulation on the four lateral walls of the slider is taken into account. The lateral walls are unfolded to study the flow using a two-dimensional lubrication model. The effects of flying height, skew angle and slider design on the accumulation removal of lubricant from the ABS are determined for the two states of the drive.
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Lubricant Spin-Off from Magnetic Recording Disks
Article
Full-text available
  • Jan 2001
As the rotation rate of magnetic recording disks increases over the next few years, lubricant spin-off from the disk surface may be significant. Lubricant thickness was measured as a function of spin time at 10000 rpm on typical carbon overcoated magnetic recording disks initially lubricated with 10–135 of perfluoropolyether Zdol. The viscosity of the lubricant film increased as the film thickness decreased with spin time. Lubricant spin-off in response to air shear stress on the free surface was approximately described by viscous flow. The rate of lubricant removal by evaporation was compared to the spin-off removal rate in films between 10 and 50 thick. Dispersion interaction and chemisorption are expected to retain a molecularly thin film of lubricant on the disk surface.
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Heat-assisted magnetic recording by a near-field transducer with efficient optical energy transfer
Article
Full-text available
  • May 2009
Although near-field microscopy has allowed optical imaging with sub-20 nm resolution, the optical throughput of this technique is notoriously small. As a result, applications such as optical data storage have been impractical. However, with an optimized near-field transducer design, we show that optical energy can be transferred efficiently to a lossy metallic medium and yet remain confined in a spot that is much smaller than the diffraction limit. Such a transducer was integrated into a recording head and flown over a magnetic recording medium on a rotating disk. Optical power from a semiconductor laser at a wavelength of 830 nm was efficiently coupled by the transducer into the medium to heat a 70-nm track above the Curie point in nanoseconds and record data at an areal density of ~375 Tb m-2. This transducer design should scale to even smaller optical spots.
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The physics of disk lubricant in the continuum picture
Article
Full-text available
  • Mar 2005
Molecular dynamics simulation would be well suited to predict the physics of molecularly-thick lubricants on magnetic disk surfaces. The difficulty in defining suitable interatomic potentials, as well as the requirement for large computing power, makes this type of study difficult. Treating the lubricant film in the framework of fluid dynamics has proven a much better way to model its behavior. This paper describes a full numerical model of lubricant moguls and ripples formation using finite difference analysis. We demonstrate that both moguls and ripples result from slider-induced air shear. Ripples tend to form at higher disk speed and narrower slider width, whereas moguls are seen at lower speed and larger slider width. Both moguls and ripples are enhanced for thicker lubricants, higher waviness, and lower flying heights. Ripple instabilities that were predicted earlier using a stability analysis are also quantitatively confirmed using this numerical model.
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Lubricant Flow and Evaporation Model for Heat-Assisted Magnetic Recording Including Functional End-Group Effects and Thin Film Viscosity
Article
  • Oct 2013
The lubricant covering a hard disk in a heat-assisted magnetic recording drive must be able to withstand the writing process in which the disk is locally heated several hundred degrees Celsius within a few nanoseconds to reduce the coercivity of the media and allow writing of data. As a first step in modeling a robust lubricant, we have developed a simulation tool based on continuum theory that incorporates previously proposed variations of viscosity and an additional component of disjoining pressure due to functional end-groups with film thickness. Here we apply this simulation tool to a conventional perfluoropolyether lubricant, Zdol 2000, for which there exists experimental data. The simulation tool can be used equally well for other lubricants once their properties become known. Simulations at small length and time scales that are unobservable with current experimental capabilities are performed. We investigate the effect of the total disjoining pressure and thin film viscosity on evaporation and lubricant flow for different initial thickness. For films thicker than 1 nm, the inclusion of polar disjoining pressure suppresses the lubricant thickness change due to evaporation and thermocapillary shear stress compared with cases without this component. Thin film viscosity is an important property to consider for thinner lubricants. We also consider how lubricant depletion depends on laser spot size and thermal spot maximum temperature. The smaller spot profiles exhibit side ridges due to thermocapillary shear stress while the larger spot profiles show no side ridges, only a trough due to evaporation. The lubricant depletion zone width and depth increase with increasing thermal spot maximum temperature.
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A two-stage heating scheme for heat assisted magnetic recording
Article
  • Apr 2014
Heat Assisted Magnetic Recording (HAMR) has been proposed to extend the storage areal density beyond 1 Tb/in.2 for the next generation magnetic storage. A near field transducer (NFT) is widely used in HAMR systems to locally heat the magnetic disk during the writing process. However, much of the laser power is absorbed around the NFT, which causes overheating of the NFT and reduces its reliability. In this work, a two-stage heating scheme is proposed to reduce the thermal load by separating the NFT heating process into two individual heating stages from an optical waveguide and a NFT, respectively. As the first stage, the optical waveguide is placed in front of the NFT and delivers part of laser energy directly onto the disk surface to heat it up to a peak temperature somewhat lower than the Curie temperature of the magnetic material. Then, the NFT works as the second heating stage to heat a smaller area inside the waveguide heated area further to reach the Curie point. The energy applied to the NFT in the second heating stage is reduced compared with a typical single stage NFT heating system. With this reduced thermal load to the NFT by the two-stage heating scheme, the lifetime of the NFT can be extended orders longer under the cyclic load condition.
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Optical Surface Analysis of the Head-Disk-Interface of Thin Film Disks
Article
  • Jan 1995
This paper describes the design, operation, theory, and data interpretation of an Optical Surface Analyzer (OSA). The OSA can measure carbon wear, lubricant depletion/accumulation, surface roughness, and lubricant alteration on carbon coated thin film disks. This device can measure an Angstrom of carbon wear or lubricant depletion/accumulation. The OSA can also measure debris generation and lubricant degradation through a measurement of optical index change. The lateral resolution of the OSA is approximately 5 by 10 microns and the bandwidth of the device is 2 MHz. The small device size allows it to be used within a test stand environment.
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Lubricant depletion under various laser heating conditions in Heat Assisted Magnetic Recording (HAMR), " in Society of Photo-Optical Instrumentation Engineers The physics of disk lubri-cant in the continuum picture
  • Jan 2005
  • 616-620
  • H Xiong
  • D B Wu
  • Bogy
Xiong, H. Wu, and D. B. Bogy, " Lubricant depletion under various laser heating conditions in Heat Assisted Magnetic Recording (HAMR), " in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series (2014), Vol. 9201, p. 09. 8 B. Marchon, Q. Dai, V. Nayak, and R. Pit, " The physics of disk lubri-cant in the continuum picture, " IEEE Trans. Magn. 41(2), 616–620 (2005).