1. IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 7, JULY 2014 3001008
Head and Media Challenges for 3 Tb/in2 Microwave-Assisted
Magnetic Recording
Mike Mallary1, Fellow, IEEE, Kumar Srinivasan1, Gerardo Bertero1, Dan Wolf1, Christian Kaiser1,
Michael Chaplin1, Carl Elliott1, Mahendra Pakala1,5, Qunwen Leng1, Francis Liu1,
Yiming Wang2,4, Tom J. Silva3, Justin M. Shaw3, and Hans T. Nembach3
1Western Digital Corporation, San Jose, CA 95138 USA
2Carnegie Mellon University, Pittsburgh, PA 15213 USA
3National Institute of Standards and Technology, Boulder, CO 80305 USA
4Headway Technology, Inc., Milpitas, CA 95035 USA
5Applied Materials Inc., Santa Clara, CA 95054 USA
A specific design for microwave assisted magnetic recording (MAMR) at about 3 Tb/in2 (0.47 Tb/cm2 or 4.7 Pb/m2) is discussed
in detail to highlight the challenges of MAMR and to contrast its requirements with conventional perpendicular magnetic recording
(PMR). In particular, it has been determined that MAMR-optimized media should have: higher damping than today’s PMR media
upon which ferromagnetic resonance measurements are reported, very low intergranular exchange coupling, and somewhat stronger
layer to layer exchange coupling. It was found that with exchange-coupled composite type media (i.e., graded anisotropy with
controlled exchange), a spin torque oscillator in the write gap of a wider shielded pole cannot adequately define the written track
at high track density. Adequate lateral field gradient, however, is achieved by modifying the pole tip geometry. Other details of this
conceptual design are also discussed.
Index Terms—Damping, field generating layer, microwave-assisted magnetic recording (MAMR), spin torque oscillator (STO).
I. INTRODUCTION
THE USE of microwave-assisted magnetic recording
(MAMR) to provide an adequate write field for fine grain,
high anisotropy, thermally stable media has been previously
proposed and investigated [1]–[11]. A MAMR head architec-
ture, which applies a circularly polarized microwave field to
the media in the write region just downstream of the write pole
tip corner, has been proposed by Zhu [7], [11] and is shown
in Fig. 1. With this approach, a spin torque oscillator (STO)
is located in the gap of a wide track shielded pole write head
[12]–[14]. With this architecture, the cross-track width of the
STO determines the magnetic write width (MWW) at 50%.
The STO consists of a polarizing layer (PL) in electrical
contact with the pole tip and is magnetized in the direction of
the gap field of the head, a conductive spacer layer to conduct
the polarized current, and a field generating layer (FGL) whose
magnetization precesses in the plain of the film because of the
gap field and the polarized current. Zhu’s original proposal
also had a bias layer (BL), with perpendicular anisotropy,
on top of the FGL to stabilize its precession in the absence
of an applied perpendicular field. However, this paper has
determined that the BL is not needed because of the presence
of the gap field of the head (so it is not in Fig. 1).
With the STO shown in Fig. 1, a negative (i.e., the electrons
flow from right to left) polarized electric current flows from
the FGL to the PL. The partial polarization of this current
Manuscript received July 15, 2013; revised August 28, 2013; accepted
January 28, 2014. Date of publication February 11, 2014; date of current
version July 7, 2014. Corresponding author: M. Mallary (e-mail: mikemal-
lary@verizon.net).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2014.2305693
Fig. 1. Side view (top) of a spin torque oscillator in the write gap
with electrical current (black arrows) flowing between the poles. ABS view
(bottom) shows that the pole width is much greater than the STO width which
determines the track width.
in the direction of the in-plane FGL magnetization can be
resolved into perpendicular components that are parallel and
antiparallel to the PL magnetization. The parallel (to the PL
magnetization) component selectively propagates into the PL
while the other is selectively reflected by the surface and bulk
GMR effects. This antiparallel reflected component torques the
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2. 3001008 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 7, JULY 2014
Fig. 2. Simulation of a 2 MFCI pattern on single-layer media.
FGL magnetization against the applied gap field causing it to
precess in the plane. This generates an elliptically polarized
microwave field in the media. Its chirality is the same as that
of the precession of the magnetization of the media grains
in the region between the STO and the pole tip (which are
antiparallel to the pole tip field before they switch). As such,
it causes them to precess to larger angles to their easy axis and
therefore switch at a pole tip field strength that is much lower
than that which would be needed if there were no microwave
field [10]. Thus, MAMR can be used to affect switching of
very high anisotropy media with thermally stable fine grains
to achieve higher areal density (AD).
In Section II we report on high AD micromagnetic write
simulations of single-layer PMR-like media. Section III reports
simulation results on continuous granular coupled (CGC) [15]
two-layer media with properties that approximate present PMR
media. Section IV reports on ferromagnetic resonance (FMR)
measurements of CGC-like PMR media samples and micro-
magnetic simulations of FMR on similar media. Section V
reports measurements of STOs and compares them to simula-
tion results. Section VI details simulation results for a high AD
exchange-coupled composite (ECC) media [16], design with
a write pole geometry that produces a cross-track pole field
gradient to improve fringing. Section VII gives overall con-
clusions and comments.
II. SINGLE-LAYER MEDIA SIMULATIONS
Initially micromagnetic studies of the MAMR write process
were conducted on a single-layer media design. The code
for the LLG Voronoi model and read back sensitivity used
was provided by the Data Storage System Center at Carnegie
Mellon University [17].
Fig. 2 shows the simulation results for writing a 2 million
flux changes per inch (MFCI) pattern with: head media spacing
of HMS = 3 nm (magnetic surface to surface); maximum
μ0 Hperp = 1.3 T (i.e., highest pole tip field); 5 nm average
center-center Voronoi grains with a log-normal area σ = 35%
(i.e., size σ = 16%); no intergranular exchange coupling
but no allowance for grain boundaries (which is very opti-
mistic); μ0 HK = 2.7 T; Gaussian σ HK = 3% (also very
optimistic); Ms = 500 kAm−1 (or 500 emu/cc); average
R.T. energy barrier of KuV/kT = 53 with 15 nm thickness;
FGL = 25 × 25 × 15 nm; with Ms = 1800 kAm−1; STO
frequency optimized at 41 GHz; and the media Landau–
Lifshitz–Gilbert (LLG) damping parameter is set at α = 0.10.
It should be noted that the FGL in the code for this write
process model is a single magnetic element, so its precession
Fig. 3. Jitter at 2 MFCI (79 Mfcpm) versus σ HK (1σ error bars).
frequency was directly proportional to the applied current as
is expected from an analytic model [18]. Therefore, arbitrarily
high frequencies were generated with higher applied current.
As is discussed below, reality is not so kind in that there is
a maximum frequency for every FGL geometry owing to the
production of spin waves at high current.
Fig. 3 shows the jitter versus σ HK . As with the PMR sim-
ulations of Gao [19], the variability in HK severely degrades
SNR which is inverse with jitter squared. This variability
increases the MWW as well because the pole field must
increase for large σ HK in order to write the grains in the high
anisotropy tail of the distribution. For σ HK = 3%, 6%, and
9%, the MWW is 32, 35, and 46 nm with maximum μ0 Hperp
of 1.3, 1.4, and 1.5 T, respectively.
With a 15% 1 T jitter criteria (i.e., 15% of the minimum bit
space) for a low density parity check (LDPC) error correction
code with a 0.87 rate and an MWW = 80% of pitch criteria,
the predicted AD is approximately 2.76, 2.28, and 1.33 Tb/in2
(i.e., 4.3, 2.5, and 2.0 Pb/m2) for σ HK of 3%, 6%, and 9%,
respectively. For σ HK = 3% and HMS = 3 nm, this consists
of 635 KTPI × 4350 KFCI [i.e., 25 million-tracks-per-meter
(Mtpm) × 171 million-flux-changes-per-meter (Mfcpm)].
Of course the lack of grain boundaries and the very low HMS
and HK dispersion assumed above is optimistic. This will be
dealt with below. Another source of optimism is the assumed
media damping of α = 0.10 (which is much higher than the
measurement results from our FMR study discussed below).
If media damping is much lower, a thinner FGL (i.e., ∼50%)
can be used but jitter and MWW are greatly increased. This
increase in jitter at low damping is shown in Fig. 4. Another
source of AD reduction is higher HMS. The AD reductions
per nanometer for increased HMS are −8.4%, −7.8%, and
−5%, for σ HK = 3%, 6%, and 9%, respectively.
III. CONTINUOUS GRANULAR COUPLED MEDIA
To better understand the damping issue and MAMR per-
formance on conventional media, micromagnetic studies of
the write process and FMR were performed on CGC-like
media. A granular layer with μ0 HK = 1.6 T (i.e., 16

3. MALLARY et al.: HEAD AND MEDIA CHALLENGES FOR 3 Tb/in2 MAMR 3001008
Fig. 4. Jitter at 2 MFCI (79 Mfcpm) versus media intrinsic damping.
Note that the statistical error bars are 1 S.D. and are calculated from the
jitter divided by the square root of the degrees of freedom (i.e., number of
transitions minus 1).
Fig. 5. Jitter at 1 MFCI versus bottom layer damping for: top damping =
bottom (blue diamonds), top damping of αtop = 0.10 (yellow triangles), and
top damping of αtop = 0.20 (red squares).
kOe) was exchange coupled (Eex = 1.5 mJm−2 or 1.5
ergs/cm2) to a continuous layer with μ0 HK = 0.8 T. The
intergranular exchange field in the cap was 0.4 T (all of
the neighbors together). The top and bottom layers were 3
and 11 nm thick and their saturaion magnetization, Ms, was
600 and 640 kAm−1 (i.e., 600 and 640 emu/cm3), respec-
tively. The average center-to-center grain separation was 9 nm
with a 2.8 nm grain-to-grain boundary on the lower layer.
These parameters approximate those of conventional CoCrPt
PMR media.
The growth of jitter and MWW with reduced damping for
this media is shown in Figs. 5 and 6. Here, HMS was a more
realistic 5.5 nm and the FGL thickness was optimized for
minimum jitter (i.e., 7 < FGL thickness < 15 nm) for each
damping configuration. The frequency optimized at 21 GHz in
all cases. From these results, it can be seen that high damping
(i.e., αtop = 0.20) in the top layer is sufficient to minimize
Fig. 6. MWW versus bottom layer damping for: top damping = bottom
damping (red squares) and top damping of αtop = 0.20 (blue diamonds).
Fig. 7. Simulated OVW writing on a 1 MFCI (39.4 Mfcpm) pattern
overwritten with a density from 10 to 900 KFCI (0.4–35.5 Mfcpm).
jitter and MWW even when the bottom layer has low damping
(i.e., αbottom = 0.01). The exchange coupling between the
layers is strong enough (i.e., 1.5 mJm−2) to damp the bottom
one when the top has high damping.
To quantify the potential advantage of MAMR over con-
ventional PMR, overwrite (OVW) and adjacent track erasure
(ATE) were investigated but with high damping (αtop = 0.20
and αbottom = 0.04). Fig. 7 shows the results for OVW
of a low frequency pattern written on top of a high fre-
quency pattern where the low frequency pattern transi-
tion linear density varied from 250 KFCI to 900 KFCI
(i.e., 9.8–35 Mfcpm) which was written on top of a 1 MFCI
(i.e., 39 Mfcpm). OVW varied from −24 to −33 dB in this
range. This result was limited by the number of transitions in
the string (i.e., 18), so it should be taken as a lower limit (i.e.,
longer strings would have shown better OVW owing to better
SNR in the Fourier analysis).
An example of an ATE simulation is shown in Fig. 8,
in which 400 KFCI (i.e., 15.8 Mfcpm) was written 36 nm
away from a 1 MFCI track (i.e., 39.4 Mfcpm). Thus, the jitter
increased from 6.5% to 7% (i.e., −0.6 dB loss in transition
SNR). With this as the maximum acceptable encroachment,
and allowing 10% for track mis-registration (TMR), a track
density of 660 KTPI or 26 Mtpm can be supported with this

4. 3001008 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 7, JULY 2014
Fig. 8. ATE simulation with 400 KFCI (15.8 Mfcpm) written 36 nm from
a 1 MFCI (39.4 Mfcpm) track increasing its jitter from 6.5% to 7% of the
25.4 nm bit space.
25 × 25 nm FGL sitting on a 100 nm wide pole tip. For
an allowed 1 T jitter of 15%, the 7% (ATE-corrupted jitter)
at 1 MFCI (i.e., 39.4 Mfcpm) could support 1.9 MFCI (i.e.,
74.9 Mfcpm). Including the LDPC rate of 0.87 gives an AD of
about 1.2 Tb/in2 or 1.9 Pb/m2. This is a moderate gain on what
could be achieved with PMR but at a greatly reduced applied
field. For PMR, this media needed maximum μ0 Hperp > 1.1 T
whereas the MAMR write needed only ∼0.5 T. Therefore,
even with damped media, MAMR on today’s PMR media is
expected to yield only a moderate gain in AD. However, a
large gain in OVW is expected because of the much lower
pole field requirement from a pole that is much wider (i.e.,
100 nm here) than would be needed for PMR at high TPI.
The MAMR gain is mainly in TPI with an FGL width of
25 nm, so wider FGLs would just break even with PMR.
IV. MEDIA FMR STUDIES
The above studies clearly indicate the need for high damp-
ing for MAMR media. But the small sample thickness (i.e.,
14 nm data layer) makes it particularly difficult to deter-
mine the intrinsic damping by use of FMR measurements.
In addition, the large amount of inhomogeneity due to σ HK
and irregular grain size (local demagnetization field variation)
compounds the difficulty. Mo et al. [20] made complex fits
to angle-dependent FMR measurements on thick, continuous,
low anisotropy CoCr films, and concluded that α = 0.004.
Recently, Hinata et al. [21] reported FMR data on commercial-
like CoCrPt media with the aim of extracting the intergranular
exchange field. Even with the sources of line broadening
discussed above, close examination of [21, Fig. 4] indicates
that residual inhomgenetity and intrinsic damping resulted in
a half-width at half-height of 0.023 (i.e., after integration
to get a Lorentzian-like curve whose Lorentzian like half-
width at half-height would be the LLG damping if there
were no inhomogeneities). Therefore the intrinsic damping is
less than 0.023.
To further investigate the variation of damping in mul-
tilayer media, we performed perpendicular geometry FMR
measurements for a variety of media samples with a precision,
70 GHz bandwidth FMR spectrometer that uses amplitude-
detection with a vector network analyzer [22]. The applied
magnetic field was perpendicular to the surface plane. Such
a measurement geometry minimizes the contribution of two-
magnon-scattering (linear spin-wave generation) to the mea-
sured linewidth [23]. Data are shown in Figs. 9 and 10
for two samples. The first sample has exchange-decoupled
Fig. 9. Measured FMR field versus frequency for an 8.7 nm thick granular
layer (blue diamonds) and a similar sample with a 9 nm thick continuous
capping layer (red squares).
Fig. 10. Measured FMR line width versus frequency for granular layer
without a capping layer (blue diamonds) and with a 9 nm thick capping layer
(red squares).
grains (8.7 nm thick) without a capping layer, and the second
sample is identical except for the inclusion of a 9-nm-thick
continuous capping layer strongly exchange coupled to it (i.e.,
no exchange break layer). Guided by micromagnetic simula-
tions (discussed further below), the capping layer thickness,
which is about three times larger than typical for commercial
media, was chosen to maximize the intergranular exchange
coupling to minimize the inhomogeneous line broadening.
A potential explanation for this effect (i.e., the suppression
of anisotropy dispersion induced line broadening), observed
in the simulation results, is that large intergranular exchange
causes clusters of grains to precess in unison thus averaging
down the granular inhomogeneities.
We present data for resonance field versus excitation fre-
quency in Fig. 9. The data indicate the extrapolated zero-
applied-field resonance frequency was raised by the addition of
the capping layer from 25.2 to 31.7 GHz. Note that simulations
indicate that STO operation at the FMR frequency extrapolated
to zero-applied-field is optimal for MAMR performance. The
fitted values for net effective perpendicular anisotropy field
μ0 Hk were 0.8 and 1.0 T, and the fitted values for the
spectroscopic g-factor were 2.25 and 2.17, respectively.
Fig. 10 shows the corresponding FMR linewidths for the
granular and the granular-plus-cap samples. Linear fits yielded
zero-frequency-intercepts (i.e., the nominal component due

5. MALLARY et al.: HEAD AND MEDIA CHALLENGES FOR 3 Tb/in2 MAMR 3001008
Fig. 11. Simulation of FMR line width for a granular-plus-capping-layer
system with α = 0.01, σ HK = 12%, and with various values of intergranular
capping-layer exchange fields and interlayer exchange energies. These are
μ0 Hex = 0.4, 1.2, and 1.2 T for the blue diamonds with error bars, triangles,
and square, respectively; and interlayer coupling energy of Eex = 1.5, 1.5,
and 10 mJm−2, respectively. Even the case of very large interlayer exchange
(black square), the linewidth is still larger than that expected for a purely
homogeneous film with α = 0.01 (purple line).
to inhomogeneous broadening) of 14 ± 51 and 55 ± 4 mT,
respectively. The fitted slopes yield α = 0.053 ± 0.016 and
α = 0.017 ± 0.001, respectively. We found that the uncapped
sample yielded substantially poorer signal-to-noise than that
with the cap, which resulted in large error bars for both the
damping and the zero-frequency intercept.
Simulations of FMR indicate that reduction of inhomoge-
neous line-width broadening requires a combination of both
large intergranular and large interlayer exchange coupling,
as shown by the FMR simulation results in Fig. 11. The
simulation parameters were similar to those for the CGC media
discussed above with: α = 0.01; σ HK of 12% (to exaggerate
its effect); and μo HK = 1.6 and 0.8 T for the bottom and top
layers, respectively. The blue curve (with maximum likelihood
fit error bars), with typical intergranular exchange coupling
strength for the capping-layer of 0.4 T (i.e., the average
effective field exerted on a given grain because of all nearest
neighbors), does not converge to the purple line (that would be
expected for a homogeneous film with α = 0.01) even out to
120 GHz. Progressively increasing the intergranular exchange
field from 0.4 to 1.2 T (red triangles) partially suppresses
the effect of HK dispersion but does not eliminate it. Even
a 1.2 T intergranular capping layer exchange field combined
with 10 mJm−2 interlayer exchange (see the black square at
70 GHz in Fig. 11) does not fully nullify the inhomogeneous
contribution to the linewidth.
From the results in Fig. 11, it is clear that intergranular
exchange coupling of the capping-layer, in combination with
interlayer exchange coupling between the capping and data
layers, partially suppresses the effect of σ HK on inhomoge-
neous broadening. In contrast, the FMR data shown in Fig. 10
are inconclusive as to whether the inclusion of the capping
layer has any effect on the measured inhomogeneous broad-
ening, in large part because the reduced signal-to-noise for the
uncapped material results in substantial error bars for the zero-
frequency intercept. Nevertheless, the larger signal-to-noise
afforded by the addition of the capping layer does substantially
improve the precision of the fitted value for the damping.
The micromagnetic simulation results in Fig. 11 show that
extraction of the damping parameter from FMR measurements
in highly disordered materials is still subject to substantial
systematic errors that can make the fitted results highly
dependent on the measured frequency range. One possible
explanation is that the large deviations of the simulated line
widths from a linear functional dependence on frequency, as
expected for a purely Gilbert-like damping process, is the
result of persistent two-magnon scattering effects even in
the perpendicular geometry: in the case of highly disordered
systems, it is clear that a substantial local variation in the
FMR resonance conditions within the film can still result in
large coupling between the uniform and nonuniform modes
(also known as spin waves) even if the degeneracy condition
ω (k = 0) = ω (k = 0) is not strictly met because all the
spins waves are of the forward magnetostatic volume wave
type. However, it is also clear from the simulation results
that the average slope of the linewidth versus frequency
curve approaches that of the intrinsic damping in the limit
of an extremely broad frequency range (>100 GHz). It is
also clear that the slope, and therefore the inferred damping,
converges toward that calculated from the inputted damping
of 1% as the frequency goes up. A fit for the points from
20 to 49 GHz gives effective damping of 1.7%, whereas
those from 20 to 70 GHz give 1.3%. Matching the frequency
range of the measured data by averaging the points at 49 and
70 GHz and calculating the slope to the point at 34 GHz
also gives 1.3% for the effective damping. Therefore, the
effective damping of 1.7% from the measured slope in Fig. 10
should be taken as an upper limit on the intrinsic damping
of this CGC-like media. Higher frequency data and data
on purely continuous films, such as have been obtained
with high anisotropy perpendicular multilayers [24], can fur-
ther refine our knowledge of intrinsic damping in recording
media.
Use of FMR as a tool to directly measure σ HK is also
of great importance to provide an input parameter for the
modeling of MAMR and PMR. From the above, it is clear
that intergranular exchange must be minimized for this, so the
granular-only result in Fig. 10 is the best source of data on
this. However, it is clear from the data that the error on the
zero-frequency-intercept is too large to be of use for numerical
simulations. However, the fitted value for the damping in the
case of the media with capping layer suggests that the intrinsic
damping is a relatively small component of the measured
linewidth over the measured frequency range. If we neglect
the effect of intrinsic damping and attribute all of the line
broadening at 50 GHz to σ HK , then its calculated value is
about 5% (i.e., half the line width of 190 mT at 50 GHz
divided by μ0 HK = 2 T, as calculated from the data in Fig. 9).

6. 3001008 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 7, JULY 2014
Fig. 12. Measured STO frequency versus applied field at currents of 2 mA
(diamonds), 3 mA (squares) and 3.5 mA (triangles). The gap field is × 4 of
the applied up to shield saturation at Bperp ∼0.25 T (where BSTO/ Bperp
changes from ∼4 to ∼1 so the reduction in Freq/ Bperp is due primarily
to shield saturation).
Including intrinsic damping would lower the
calculated σ HK . Including residual intergranular exchange
would raise it. Measurement of the intergranular exchange
by the technique of [21] and using it in micromagnetic
simulations of FMR can yield a more reliable result.
Alternatively, measurement of the FMR linewidth versus
average grain boundary thickness on a range of granular
samples could be used to measure the dependence of
intergranular exchange on grain segregant thickness, insofar
as increased intergranular exchange should reduce the
measured inhomogeneous line broadening.
V. STO EXPERIMENTS AND SIMULATIONS
A key element of MAMR is the STO. We fabricated STOs
with Co90Fe10 FGLs in the gap between conventional reader
shields, with dimensions 23 × 23 × 5 nm. Fig. 12 shows
the frequency versus applied field for applied currents up to
∼3.5 mA. The applied field was magnified ∼ × 4 by the
shields up to their saturation at ≈ 0.25 T. The decreasing slope
for frequency versus perpendicular field is due to saturation of
the head shields. It is clear that the dependence of frequency on
current is both negative (decreasing frequency with increasing
current) and weak for this system. This is unlike the results of
analytic models that assume uniform precession of the FGL
magnetization [18], where the frequency is directly propor-
tional to the current. A weakening dependence of frequency
on current, as the current is increased, is apparent in the
micromagnetic simulation results shown in Fig. 13 (performed
with DSSC-supplied software and computer time). Other
simulations (from hundreds of runs) show that a maximum
frequency is reached as the current is increased and the
precession of the elements becomes less coherent and more
dominated by the excitation of spin waves as shown in Fig. 14.
Further increasing the current beyond this point can result in
a negative dependence of frequency on current as shown in
the data of Fig. 12 and in simulations at currents beyond this
threshold of instability which were not included in Fig. 13.
The lack of a strong dependence of frequency on current
combined with a strong dependence of frequency on field
Fig. 13. Simulated STO frequency versus field for currents of 2, 3, and
4 mA (purple diamonds, red squares, and green triangles, respectively).
Fig. 14. High STO current causes disordered precession, at which the fairly
uniform precession of the FGL breaks down into complex spin waves and
moving vortices.
makes it difficult to simultaneously adjust frequency and write
field to optimize MAMR. Novel write head designs [25] and
STO designs will be needed to address this difficulty. In
addition, obtaining a high enough precession frequency in an
acceptable deep gap field will be challenging for high AD
MAMR. As the thickness of the FGL increases, the frequency
at which instability sets in decreases.
VI. EXCHANGE-COUPLED COMPOSITE
THREE LAYER MEDIA
To explore the AD limits of MAMR, we have simulated
it on three-layer ECC-type media. In this approach, HK
increases progressively from the top to bottom layer, with con-
trolled exchange coupling strength (Eex) between the layers.
This results in higher thermal stability for a given write-field
limit. It also reduces the effective switching field distribution

7. MALLARY et al.: HEAD AND MEDIA CHALLENGES FOR 3 Tb/in2 MAMR 3001008
Fig. 15. Pole tip pedestal under the STO provides lateral write field gradient
for improved ATE with three-layer media [26].
and PMR write fringing. The ECC approach is combined with
the CGC approach by using a continuous top layer to control
the intergranular exchange of the isolated grains in the bottom
layers and with graded anisotropy.
In the paper, the media parameters that were used (for each
layer from bottom to top) were: thicknesses of 2.5, 2.5, and
3.5 nm, μ0 HK of 5.2, 4.2, and 3.1 T, LLG damping of 0.01,
0.01, and 0.15, and grain boundaries of 0.7, 0.7, and 0 nm.
The interlayer exchange energy was 3 mJm−2. The capping
layer had a very weak intergranular exchange field of 40 mT to
minimize fringing, where the average effective exchange field
at a given grain is due to all neighboring grains. All layers
had: Ms = 650 kAm−1, 6 nm average center-to-center with
a log-normal area σ = 35%, Gaussian σ HK = 6%, and
σθ = 3o. The HMS was 4 nm (magnetic ABS to the
top magnetic surface of the media). The ABS-SUL distance
was 30 nm.
The STO FGL was 18×18×12 nm, Ms = 1.8 MAm−2, and
its center was positioned 9 nm from the pole corner. With such
ECC media parameters, write fringing was more problematic,
so a novel pole design shown in Fig. 15 was used [26]. The
STO sat on a 30-nm-tall pedestal with 45° side walls to provide
a lateral write field gradient. In addition, a uniform 0.1 T
perpendicular field was added to the field map to suppress
shield edge erasure due to the very low HMS [14].
With the above configuration, micromagnetic write simula-
tions were performed at 1500 KFCI (5.9 × 105 Mfcpm). The
jitter (average over 108 transitions) was 6.3%. A 900 KFCI
(3.5 × 105 Mfcpm) adjacent track write at 24 nm off-track
of one of the runs increased its jitter from 5.9 to 6.3% [i.e.,
−0.7 dB loss of transition signal to noise ratio (SNR)]. This
is shown in Fig. 16. Again, assuming a 15% 1 T jitter criteria,
10% TMR, and an LDPC code rate of 0.87, this corresponds
to an AD of about 3 Tb/in2 (i.e., 960 KTPI × 3100 KBPI or
4.7 Pb/m2 with 38 Mtpm × 122 Mbpm).
In principle, there is a lot of room for improvement in that
the maximum μ0 Hperp was optimized at 0.75 T. (Note that
PMR needs maximum μ0 Hperp = 2.2 T to write this media
while the write pole structure was capable of 1.3 T). Therefore,
much higher HK media could be written with finer grains.
However, the STO frequency was set to 63 GHz, which is
Fig. 16. Magnetization after writing 900 KFCI (35.5 Mfcpm top) 24 nm
off-track from a 1500 KFCI (59 Mfcpm bottom) write which degraded
transition SNR by −1 dB.
the upper limit of what can be achieved with a proprietary
STO design that employs a 9-nm-thick FGL. This thinner
FGL would require a reduction of the top-layer damping from
αtop = 0.15 to αtop = 0.11, which increases the jitter to 7.1%
(i.e., an 11% AD loss).
VII. CONCLUSION
From the simulation results provided here, it is clear that
MAMR media benefits from a damping level that is signif-
icantly larger than the upper limit from FMR measurements
reported here (i.e., α = 0.017). For multilayer media, high
damping in one of the layers can compensate for low damping
in others. The average damping of the stack appears to be
the salient variable when the interlayer exchange coupling
is adequate. The values for average damping that achieve a
plateau in AD versus damping were approximately 3%, 5%,
and 7% for media with one, two, and three layers, respectively.
So it appears that the use of a multilayer ECC-like media
for MAMR requires higher damping (which also necessitates
an increase in FGL thickness). Therefore, the damping of
conventional CoCrPt media is suboptimal for MAMR.
An additional media issue uncovered by these studies is that
intergranular exchange coupling reduces AD capability. There-
fore, MAMR media should not have a continuous capping
layer. Well-isolated grains have consistently worked better in
the simulations by resulting in lower MWW.
Because of these effects, we can draw useful analogies of
the MAMR write process to that for heat assisted magnetic
recording. However, with MAMR, the spins are directly heated
(not the electron translational degrees of freedom or those
of the lattice) as an interacting (i.e., magnetic and exchange
interactions) ensemble. These interactions enable fast media
switching and high data rates in conventional recording, even
when the intrinsic damping is low [27]. Use of media with low
damping for MAMR is analogous to use of media without a
heat sink in heat-assisted magnetic recording [28]. Continuing
the analogy, use of media with high intergranular exchange
in MAMR is analogous to the use of media with high lateral
thermal conductivity. Intergranular exchange causes the spin-
temperature thermal bubble to spread, resulting in a low spin
temperature gradient that degrades both BPI and TPI.
From the AD studies with single-layer and three-layer
ECC-like media, it appears that ECC media enables somewhat
higher AD of about 3 Tb/in2 (i.e., 4.7 Pb/m2) and much lower
pole field (maximum μ0 Hperp = 0.75 T versus 1.3 T), despite
the fact that significantly more realistic and (i.e., less opti-
mistic) parameters were used (i.e., HMS = 4 nm versus 3 nm,

8. 3001008 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 7, JULY 2014
σ HK = 6% versus 3%, grain center to center = 6 nm versus
5 nm, grain boundaries = 0.7 nm versus 0 nm, and averaged
KuV/kT = 77 versus 53, for three and single-layer media,
respectively).
However, the use of the very high anisotropy ECC media
imposes a penalty of higher frequency requirements for the
STO (i.e., 63 versus 41 GHz). Obtaining coherent rotation of
the magnetization in the FGL plane at high STO frequencies
with an FGL of adequate magnetic thickness (i.e., 12 nm with
MS = 1.800 MAm−1) will be quite challenging. Doing this
with a sufficiently low μ0 Hperp (i.e., 0.75 T) while preserving a
high gradient is an additional challenge. Another head-related
issue is that the ECC media design point used here was not
capable of supporting a sufficiently high TPI without the pole-
tip pedestal design shown in Fig. 15. In short, the MAMR write
head and STO design may be the limiting factor for ultrahigh
AD MAMR.
ACKNOWLEDGMENT
The authors would like to thank J. G. Zhu of the DSSC for
guidance and his authorship of the original Fortran code upon
which the micromagnetic write simulation code was based and
the micromagnetic code that was used to simulate STOs.
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