Head and Media Challenges for 3 Tb/inSq Microwave Assisted Magnetic Recording

0018-9464 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/TMAG.2014.2305693, IEEE Transactions on Magnetics

Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States

Abstract — A specific design for Microwave Assisted Magnetic

Recording (MAMR) at about 3Tb/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

(FMR) 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 (STO) 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 — microwave assisted magnetic recording, spin

torque oscillator, field generating layer, damping

I. INTRODUCTION

HE 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 architecture,

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 J.G. Zhu [7, 11] and is shown in

Figure 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

a) Corresponding author. E-mail: [email protected], 4 Osgood Rd., Sterling, MA 01564 .

b) Presently a consultant.

c) Presently at Applied Materials Corporation. d) Presently at Headway Technology, Inc.

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 due to

the gap field and the polarized current. Zhu’s original proposal

also had a Bias Layer (BL), with perpendicular anisotropy, on

Figure 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.

Head and Media Challenges for 3 Tb/in2

Microwave Assisted Magnetic Recording

Mike Mallarya), IEEE Fellow, Kumar Srinivasan, Gerardo Bertero, Dan Wolf, Christian Kaiser,

Michael Chaplin, Carl Elliottb)

, Mahendra Pakalac), Qunwen Leng, Francis Liu, Western Digital

Corporation

Yiming Wangd)

, Carnegie Mellon University

T.J. Silva, IEEE Senior Member, Justin M. Shaw , Hans T. Nemnbach, National Institute of Standards

and Technology, Boulder

T



top of the FGL to stabilize its precession in the absence of an

applied perpendicular field. However, the study reported here

has determined that the BL is not needed because of the

presence of the gap field of the head (so it is not in Figure 1).

With the STO shown in Figure 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 in the direction of the in-plane FGL magnetization can

be resolved into perpendicular components that are parallel

and anti-parallel 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 anti-parallel reflected component

torques the 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 anti-parallel 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 in order 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 micromagnetic simulations of FMR on similar

media. Section V reports measurements of STOs and

compares them to simulation results. Section VI details

simulation results for a high AD exchange coupled composite

media, ECC [16], design with a write pole geometry that

produces a cross track pole field gradient to improve fringing.

Section VII gives overall conclusions 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].

Figure 2 Simulation of a 2 million flux changes per inch

(MFCI) pattern on single layer media.

Figure 2 shows the simulation results for writing a 2 MFCI

pattern with: head media spacing of HMS = 3nm (magnetic

surface to surface); maximum 0Hperp = 1.3T (i.e. highest pole

tip field); 5nm average center-center Voronoi grains with a

log-normal area = 35% (i.e. size = 16%); no intergranular

exchange coupling but no allowance for grain boundaries

(which is very optimistic); 0HK = 2.7T ; Gaussian HK = 3%

(also very optimistic); Ms = 500 kA m-1

(or 500 emu/cc);

average R.T. energy barrier of KuV/kT = 53 with 15nm

thickness; FGL = 25x25x15nm; with Ms = 1800 kAm-1

; STO

frequency optimized at 41 GHz; and the media Landau-

Lifshitz-Gilbert (LLG) damping parameter is set at = 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

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 due to the

production of spin waves at high current.

Figure 3 shows the jitter vs HK. As with the PMR

simulations 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 HK in order to write the grains in the

high anisotropy tail of the distribution. For HK = 3, 6, and

9%, the MWW is 32, 35, 46nm with maximum 0Hperp of 1.3,

1.4, and 1.5T , respectively.

Figure 3 Jitter at 2 MFCI (79 Mfcpm) vs HK (1 error

bars).

With a 15% 1T 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

1.33Tb/in2 (i.e. 4.3, 2.5, and 2.0 Pb/m

2) for HK of 3%, 6%,

https://www.researchgate.net/publication/268397275_Physics_and_Micromagnetic_Analysis_of_Advanced_Recording_Technologies?el=1_x_8&enrichId=rgreq-d80769d9-2bbb-4f14-9039-c2b6249c9104&enrichSource=Y292ZXJQYWdlOzI2MTE3NzQxNTtBUzoxMDI4NzI5NDM0MzE2OTRAMTQwMTUzODIzMDI0Mw==



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and 9%, respectively. For HK = 3% and HMS = 3nm, this

consists of 635 KTPI x4350 KFCI (i.e. 25 Million-tracks-per-

meter x 171 Million-flux-changes-per-meter or 25 Mtpm

x171 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= 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

Figure 4. Another source of AD reduction is higher HMS. The

AD reductions per nm for increased HMS are -8.4%, -7.8%,

and -5%, for HK = 3, 6, and 9%, respectively.

Figure 4 Jitter at 2 MFCI (79 Mfcpm) vs media intrinsic

damping. Note that the statisical 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).

III. CONTINUOUS GRANULAR COUPLED MEDIA

In order to better understand the damping issue and MAMR

performance on conventional media, micromagnetic studies of

the write process and FMR were performed on CGC-like

media. A granular layer with 0HK = 1.6 T (i.e. 16 kOe) was

exchange coupled (Eex = 1.5 mJ m-2

or 1.5 ergs/cm2) to a

continuous layer with 0HK = 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 11nm thick

and their saturation magnetization, Ms, was 600 and 640 kA

m-1

(i.e. 600 and 640 emu/cc), respectively. The average

center-to-center grain separation was 9nm with a 2.8nm 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 Figures 5 and 6. Here, HMS was a

more realistic 5.5nm and the FGL thickness was optimized for

minimum jitter (i.e. 7 < FGL thickness < 15nm) 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. top = 0.20) in the top layer is sufficient to minimize jitter

and MWW even when the bottom layer has low damping (i.e.

bottom= 0.01). The exchange coupling between the layers is

strong enough (i.e. 1.5 mJ m-2

) to damp the bottom one when

the top has high damping.

Figure 5 Jitter at 1 MFCI vs bottom layer damping for: top

damping = bottom (blue diamonds), top damping oftop= 0.10

(yellow triangles), top damping of top= 0.20 (red squares).

Figure 6 MWW vs bottom layer damping for: top damping

= bottom damping (red squares); and top damping of top =

0.20 (blue diamonds) .

In order to quantify the potential advantage of MAMR over

conventional PMR, Overwrite (OVW) and Adjacent Track

Erasure (ATE) were investigated but with high damping (top

= 0.20 and bottom = 0.04). Figure 7 shows the results for

OVW of a low frequency pattern written on top of a high



frequency pattern where the low frequency pattern transition

linear density varied from 250 KFCI to 900 KFCI (i.e. 9.8 to

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 due to better SNR in

the Fourier analysis).

Figure 7 Simulated OVW writing on a 1 MFCI (39.4

Mfcpm) pattern overwritten with a density from 10 to 900

KFCI ( 0.4 to 35.5 Mfcpm).

Figure 8 ATE simulation with 400 KFCI (15.8 Mfcpm)

written 36nm from a 1 MFCI (39.4 Mfcpm) track increasing

its jitter from 6.5 % to 7% of the 25.4 nm bit space.

An example of an ATE simulation is shown in Figure 8, in

which 400 KFCI (i.e. 15.8 Mfcpm) was written 36nm away

from a 1 MFCI track (i.e. 39.4 Mfcpm). In this case, the jitter

increased from 6.5% to 7% (i.e. -0.6 dB loss in transition

SNR). Taking 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

25x25nm FGL sitting on a 100nm wide pole tip. For an

allowed 1T 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 1.2Tb/in2 or 1.9 Pb/m

2. 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 0Hperp > 1.1 T

while 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 due to the much lower pole

field requirement from a pole that is much wider (i.e. 100nm

here) than would be needed for PMR at high TPI. The MAMR

gain is mainly in TPI with an FGL width of 25nm, so wider

FGLs would just break even with PMR.

IV. MEDIA FMR STUDIES

The above studies clearly indicate the need for high

damping for MAMR media. But the small sample thickness

(i.e. 14 nm data layer) makes it particularly difficult to

determine the intrinsic damping by use of FMR

measurements. In addition, the large amount of inhomogeneity

due to HK and irregular grain size (local demagnetization

field variation) compounds the difficulty. N. Mo et al. [20]

made complex fits to angle-dependent FMR measurements on

thick, continuous, low anisotropy CoCr films and concluded

that = 0.004. Recently, S. 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 Figure 4 of ref. 21 indicates that residual inhomgenetity and

intrinsic damping resulted in a half width at half height

of (i.e.after integration to get a Lorenzian-like curve

whose Lorenzian 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

multilayer 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

measured linewidth [23]. Data are shown in Figures 9 and 10

for two samples. The first sample has exchange-decoupled

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

simulations (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 in order 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 vs. excitation

frequency in Figure 9. The data indicate the extrapolated zero-

applied-field resonance frequency was raised by the addition

of the capping layer from 25.2 GHz to 31.7 GHz. Note that

simulations indicate that STO operation at the FMR frequency

extrapoplated to zero-applied-field is optimal for MAMR

performance. The fitted values for net effective perpendicular

anisotropy field 0Hk were 0.8 T and 1.0 T, and the fitted

values for the spectroscopic g-factor were 2.25 and 2.17,

respectively.



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.

Figure 9 Measured FMR field vs frequency for an 8.7nm

thick granular layer (blue diamonds) and a similar sample with

a 9nm thick continuous capping layer (red squares).

Figure 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 to

inhomogeneous broadening) of 14 ± 51 and 55 ± 4 mT,

respectively. The fitted slopes yield a = 0.053± 0.016 and

a = 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.

Figure 10 Measured FMR line width vs frequency for

granular layer without a capping layer (blue diamonds) and

with a 9nm thick capping layer (red squares).

Simulations of FMR indicate that reduction of

inhomogeneous line-width broadening requires a combination

of both large intergranular and large interlayer exchange

coupling, as shown by the FMR simulation results in Figure

11. The simulation parameters were similar to those for the

CGC media discussed above with: = 0.01; HK of 12% (to

exaggerate its effect); and oHK = 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 due to all

nearest neighbors), does not converge to the purple line (that

would be expected for a homogeneous film with = 0.01)

even out to 120 GHz. Progressively increasing the

intergranular exchange field from 0.4 T 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 mJ m-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 Figure 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 HK on

inhomogeneous 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

broadening, 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 indicate

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 non-

uniform modes (a.k.a. spin waves) even if the degeneracy

condition w k = 0( ) = w k ¹ 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 vs. 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

towards 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%, while 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 Figure 10 should be taken as



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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 further refine our

knowledge of intrinsic damping in recording media.

Figure 11 Simulation of FMR line width for a granular-

plus-capping-layer system with = 0.01, HK = 12%, and

with various values of intergranular capping-layer exchange

fields and interlayer exchange energies. These are: 0Hex = 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 mJ m-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

= 0.01 (purple line).

Use of FMR as a tool to directly measure 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 HK, then its calculated value is

about 5% (i.e. half the line width of 190 mT at 50 GHz

divided by 0HK = 2T, as calculated from the data in Figure 9).

Including intrinsic damping would lower the calculated

HK. Including residual intergranular exchange would raise it.

Measurement of the intergranular exchange by the technique

of ref. 21 and using it in micromagnetic simulations of FMR

can yield a more reliable result. Alternately, measurement of

the FMR linewidth vs. 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 23x23x5 nm. Figure 12 shows the

frequency vs. applied field for applied currents up to ~ 3.5

mA. The applied field was magnified ~ x4 by the shields up to

their saturation at » 0.25 T. The decreasing slope for

frequency vs. perpendicular field is attributed 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

proportional to the current. A weakening dependence of

frequency on current, as the current is increased, is apparent

in the micromagnetic simulation results shown in Figure 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

.

Figure 12 Measured STO frequency vs applied field at

currents of 2 (diamonds), 3 (squares) and 3.5 mA (triangles).

The gap field is x4 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).

result in a negative dependence of frequency on current as is

seen in the data of Fig. 12 and in simulations at currents

beyond this threshold of instability which were not included in

Fig. 13



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Figure 13 Simulated STO frequency vs field for currents

of 2, 3 & 4 mA (purple diamonds, red squares, and green

triangles , respectively).

Figure 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.

The lack of a strong dependence of frequency on current

combined with a strong dependence of frequency on field

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

In order 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

controlled 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 and PMR write fringing. The ECC approach is

combined with the CGC approach by utilization of a

continuous top layer to control the intergranular exchange of

the isolated grains in the bottom layers and with graded

anisotropy.

In the study reported here, the media parameters that

were used (for each layer from bottom to top) were:

thicknesses of 2.5, 2.5 and 3.5 nm, 0HK 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 mJ

m-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 that due to

all neighboring grains. All layers had: Ms = 650 kA m-1

, 6 nm

average center-to-center with a log-normal area,

Gaussian HK = 6%, and = 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 18x18x12 nm, Ms = 1.8 MA m-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 Figure 15 was used [26]. The

STO sat on a 30 nm-tall pedestal with 45o 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

simulations 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 Figure 16. Again, assuming a 15% 1T 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 x 3100

KBPI or 4.7Pb/m2 with 38 Mtpm x 122 Mbpm ).

In principle, there is a lot of room for improvement in that

the maximum 0Hperp was optimized at 0.75 T. (Note that

PMR needs maximum 0Hperp = 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 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 top

= 0.15 to top = 0.11, which increases the jitter to 7.1% (i.e. an

11% AD loss).



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Figure 15 A pole tip pedestal under the STO provides

lateral write field gradient for improved ATE with three layer

media [26].

Figure 16 Magnetization after writing 900 KFCI (35.5

Mfcpm top) 24nm off track from a 1500 KFCI (59 Mfcpm

bottom) write which degraded transition SNR by -1dB.

VII. DISCUSSION AND CONCLUSIONS

From the simulation results provided here, it is clear that

MAMR media benefits from a damping level that is

significantly larger than the upper limit from FMR

measurements reported here (i.e. a = 0.017 ). For multi-layer

media, high damping in one of the layers can compensate for

low damping in the 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 vs damping were approximately 3%,

5% and 7% for media with one, two and three layers,

respectively. So it appears that the use of multi-layer ECC-like

media for MAMR requires higher damping (which also

necessitates an increase in FGL thickness). Therefore, the

damping of conventional CoCrPt media is sub-optimal for

MAMR.

An additional media issue uncovered by these studies is that

intergranular exchange coupling reduces AD capability.

Therefore, 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/m

2 ) and much lower

pole field (maximum 0Hperp = 0.75 T vs 1.3 T), despite the

fact that significantly more realistic and (i.e. less optimistic)

parameters were used (i.e. HMS = 4 nm vs. 3 nm, HK = 6%

vs 3%, grain center to center = 6 nm vs 5 nm, grain boundaries

= 0.7 nm vs 0 nm, and averaged KuV/kT = 77 vs 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. 63GHz vs 41GHz). Obtaining coherent rotation of

the magnetization in the FGL plane at high STO frequencies

with an FGL of adaquate magnetic thickness (i.e. 12 nm with

MS = 1.800 MA m-1

) will be quite challenging. Doing this

with a sufficiently low 0Hperp (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 sufficently high TPI without the pole-

tip pedestal design shown in Figure 15. In short, the MAMR

write head and STO design may be the limiting factor for

ultra-high AD MAMR.

ACKNOWNLEDGEMENTS

We would like to thank Jian Gang Zhu of the DSSC for

guidance and his authorship of the original Fortran code upon

which the micromagnetic write simulation code was based.

He also authored the micromagnetic code that was used to

simulate STOs .

REFERENCES:

[1] C. Thirion, W. Wernsdorfer, D. Mailly, Nature Mater. 2, 524

(2003). [2] Y. Novaki, N. Narita, T. Tanaka, K. Matsuyama, Appl.Phys.Letts., 95,

082505 (2009).

[3] T. Yoshioka, T. Nozaki, T. Seki, M. Shiraishi, T. Shinjo, Y. Suzuki,

Y. Uehara, Appl. Phys. Exp., vol. 3, 2010. [4] Y. Nozaki, M. Ohta, S. Taharazako, K. Tateishi, S. Yoshimura, K.

Matsuyama, Appl. Phys. Lett., 91, 082510-1 (2007).

[5] Y.Wang, Y. Tang, J. G. Zhu, J. Appl. Phys., 105, 07B902 (2009). [6] M. Igarashi, Y. Matsubara, M. Shimoto, K. Nagasaka, Y. Sugiura, M.

Sato, Y. Udo, K. Hattori, Y. Nishida, H. Hoshiya, k. Nakamoto, I.

Tagawa, IEEE Trans.Magn., 46, 2507 (2010). [7] J.G. Zhu, X. ZIEEE Trans. Magn. 44, 125 (2008).

[8] Y.M. Wang and J.G. Zhu, J.Appl.Phys., 105 (2009).

[9] J.G. Zhu and Y. M. Wang, IEEE Trans.Magn., 46, 751 (2010). [10] Y. Tang, J.G. Zhu,hu, Y. Tang, IEEE Trans.Magn., 44, 125 (2008).

[11] J.G. Zhu, USPat.# 7,616,412 , 11/10/09 .

[12] M. Mallary and S. Das, USPat.# 4,656,546, 4/7/1987. [13] M. Mallary, A. Torabi, M. Benakli, IEEE TransMagn., 38, 1719

(2002).

[14] M. Mallary and M. Benakli, USPat.7,511,921 - B2, 3/31/2009.



9

Pag

e9

[15] A.M. Goodman, J.S. Greaves, Y. Sonobe, M. Muraoka, Y.

Nakamura, J. Appl. Phys., 91, 8064 (2002).

[16] R.H. Victora and X. Shen, IEEE Trans.Magn., 41, 537 (2005).

[17] Y.M. Wang, Physics and Micromagnetic Analysis of Advanced

Recording Technologies, thesis dissertation, Carnegie Mellon University, April 2011.

[18] X. Zhu and J.G. Zhu, IEEE Trans.Magn., 42, No.10, 2670 (2006).

[19] K. Gao and N.H. Bertram, J. Appl. Phys., 91, 8369 (2002). [20] N. Mo, J. Hohlfeld, M.U. Islam, C.S. Brown, E. Girt, P. Krivosik, W.

Tong, A. Rebei, C.E. Patton, Appl. Phys. Letts. 95, 052509 (2009).

[21] S. Hinata, S. Saito, M. Takahashi, J. Appl. Phys., 111,07B722 (2012). [22] H.T. Nembach, T.J. Silva, J.M. Shaw,M.L. Schneider, M.J. Carey, S.

Maat, J.R. Childress, Phys. Rev. B, vol. 84, 054424 (2011).

[23] R. D. McMichael and P. Krivosik, IEEE Trans. Magn., vol. 40, 2 (2004).

[24] J. M. Shaw, H.T. Nembach, T.J. Silva, Phys. Rev. B, vol. 85, 054412

(2012)]. [25] K. Yamada, H. Iwasaki,M. Takagishi, USPat. Appl. #2010/0134922-

A1 (2010).

[26] M. Mallary, USPat.# 8,456,967, 6/4/13.

[27] M. Benakli, A.F. Torabi, H. Zhou, M. Mallary,H.N. Beretram IEEE

Trans. Magn., 37, 1564 (2001)

[28] W.A. Challener, C. Peng, A.V. Itagi, D. Karns, W. Peng, Y. Peng,

X.M. Yang, X. Zhu, N.J. Gokemeijer, Y.T. Hsia, R.E. Rottmayer,

M.A. Seigler, E.C. Gage, Nature Photonics, 3, 303 (2009).

Head and Media Challenges for 3 Tb/inSq Microwave Assisted Magnetic Recording

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