1. Correlation of microstructure, intrinsic magnetization switching properties,
and recording performance in exchange-coupled composite media
Kumar Srinivasan,a)
Eric Roddick, John Mardinly, and B. Ramamurthy Acharya
Western Digital, 1710 Automation Parkway, San Jose, California 95131, USA
(Presented 15 November 2010; received 24 September 2010; accepted 25 November 2010;
published online 30 March 2011)
The analytical model for the intrinsic coercive squareness parameter, SÃ
int was applied to hard–soft
stacked exchange-coupled composite media, and correlations with the microstructure and switching
were studied. Thickening the hard magnetic layer in the composite stack led to a decrease in SÃ
int, as
did thickening the NiW seed layers. However, this decrease was masked by thermal effects at the
normal magnetometry time-scales of measurement. Upon thickening the soft layer in the composite
stack, SÃ
int increased sharply at first and then only slightly. In contrast, the extent of incoherent
switching, estimated from the peak value of the minor loop slope, increased slowly at first, and then
sharply. The changes in SÃ
int and switching are correlated to the microstructure, particularly, grain
size effects for the NiW series and growth effects for the hard–soft composite media series. Media
signal-to-noise ratio at low recording frequencies, and adjacent track interference also show
correlations with SÃ
int. VC 2011 American Institute of Physics. [doi:10.1063/1.3556899]
I. INTRODUCTION
Advances in magnetic hard-soft stacked exchange-
coupled composite (ECC) media1,2
such as graded anisotropy
media3,4
and dual exchange-break layers5
have been instru-
mental in the recent progression of perpendicular magnetic re-
cording (PMR) media technology. While the effects of the
hard layer, the soft layer, and the exchange-break layer on
ECC media switching and performance have been well stud-
ied,6–8
it is also necessary to understand correlations between
the microstructure, incoherent reversal and intrinsic switching,
and recording properties. In regard to intrinsic switching, we
recently developed an analytical model for the intrinsic coer-
cive squareness parameter SÃ
int, which is a measure of the
time-scale (i.e. thermal agitation) independent squareness, and
correlates directly with the exchange and magnetostatic inter-
action effects.9
In this work, we applied the analytical model
for SÃ
int to ECC media and studied the correlations between the
microstructure, switching and recording properties.
II. EXPERIMENTAL DETAILS
All PMR media samples consisted of NiP plated AlMg
substrates, antiferromagnetically coupled soft underlayers
(SUL), NiW seed layers, Ru intermediate layers and hard-soft
ECC recording layers with an exchange-break layer inserted
in-between. Three sets of samples were studied where only
the thickness of the layer of interest was varied (rest was
fixed): thickness series in (i) NiW seed layer, (ii) oxide con-
taining hard magnetic bottom layer (BL), and (iii) non-oxide
soft magnetic cap layer (CL). The magnetic properties were
evaluated by means of a polar magnetooptical Kerr effect
(p-MOKE) magnetometer. The thermal stability factor KuV/
kBT (Ku is the magnetocrystalline anisotropy constant, V is the
switching volume, kB is Boltzmann’s constant, and T is the
absolute temperature) was estimated from a Sharrock fit10
to
Hc(t) data, where Hc is the coercivity and t is the measurement
time-scale. SÃ
int was extracted from fitting the dynamic S*(t)
data to the model expression formulated in Ref. 9. To
investigate the degree of incoherent reversal across samples,
the initial minor loop slope (IMLS) method was used11
.
Microstructural analysis was carried out using a Transmission
Electron Microscope (TEM) in both the plan-view and cross-
sectional modes. Recording tests were carried out using a
Guzik spin-stand and single-pole recording heads, under con-
ditions setup for 500 Gb/in2
areal densities.
III. RESULTS AND DISCUSSION
KuV/kBT and Hc measured at a fixed sweep rate (5 kOe/s)
are plotted versus thickness in Figs. 1(a) and 1(b), respectively.
For all three sample sets, KuV/kBT increased in step with the
thickness of the layer of interest (other layers were held fixed)
following an increase in the switching volume V. In particular,
for the NiW series, V increased following an increase in core
grain size. This aspect is further discussed below. For the NiW
and BL series, both Hc and saturation field Hs (not shown here)
increased with thickness, mostly due to the larger KuV/kBT. For
the CL series, both Hc and Hs decreased upon increasing thick-
ness, probably due to the exchange-spring effect.
Figure 2(a) is a plot of SÃ
evaluated at a fixed sweep rate
(5 kOe/s) versus thickness. For the BL and NiW series, SÃ
increased with thickness. This is unexpected because an
increase in SÃ
is generally associated with an increase in inter-
granular (IG) exchange, which leads to reduced Hc from an
enhanced cooperative reversal.12
However, Hc increased with
thickness, as noted in Fig. 1(b). In order to clarify on the phys-
ical basis for this observation, SÃ
int was extracted from fitting
SÃ
(t) data (not shown here) and is plotted versus thickness in
Fig. 2(b). It was observed that SÃ
int in fact decreases with the
increasing thickness for both the NiW and BL series. For the
NiW series, TEM based plan-view images (not shown here)a)
Electronic mail: kumar.srinivasan@wdc.com.
0021-8979/2011/109(7)/07B734/3/$30.00 VC 2011 American Institute of Physics109, 07B734-1
JOURNAL OF APPLIED PHYSICS 109, 07B734 (2011)
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

2. of the recording layer indicated an increase in core grain size
from 7.2 6 1.3 nm (on thin NiW) to 8.6 6 1.4 nm (on thick
NiW). When core grain size increases at fixed volume fraction
of segregant oxide, adjacent grains become more loosely
packed leading to decreased IG exchange interactions and
decreased SÃ
int values. For the BL series, the basis for the
decrease in SÃ
int with increasing thickness is less clear and
could be related to either IG segregation effects,13
or to a
change in the interlayer exchange coupling between the bot-
tom and cap layers.8
In Figs. 2(a) and 2(b), note that for the
CL series, both SÃ
and SÃ
int increased with thickness. How this
correlates with the incoherent switching and exchange interac-
tion is discussed next.
Figure 3(a) is a plot of the normalized IMLS peak value
vs. Ms Á tcap for the CL series. Ms and tcap refer to the
saturation magnetization and thickness of the cap layer,
respectively. Ms Á tcap influences the vertical exchange cou-
pling between the bottom and cap layers8
and larger IMLS
peak values are typically associated with more incoherent re-
versal.11
It was observed that the IMLS peak value increased
slowly at first with Ms Á tcap, and then sharply. When the
IMLS peak value was plotted vs. SÃ
int, shown in Fig. 3(b),
two switching regimes became evident: (i) Regime I where
SÃ
int increases rapidly, but the IMLS peak value increases
only slightly, i.e., incoherent reversal is small; (ii) Regime II
where SÃ
int increases only slightly, but the IMLS peak value
increases rapidly, i.e., incoherent reversal is large.
Microstructural observations were made for further
understanding. Figure 4(a) is a TEM-based cross-sectional
image of the film stack. The BL grains are columnar and sep-
arated by an oxide grain boundary phase (light contrast).
However, the oxide phase extends into the CL too. Such
observations have also been noted previously.8
This oxide
must have probably migrated upward, since the composition
of the CL was oxide-free. Figure 4(b) is a cartoon of the pro-
posed growth model. In regime I, the initial CL starts out
FIG. 1. Plot of (a) thermal stability factor KuV/kBT and (b) coercivity Hc vs
thickness for the three sample sets. The lines guide the eye.
FIG. 2. (a) Time-scale dependent SÃ
and (b) time-scale independent SÃ
int
(intrinsic) are plotted vs thickness for the three sample sets. The lines guide
the eye.
FIG. 3. The peak value of initial minor loop slope (normalized) is plotted vs
Mstcap product for the cap layer series in (a) and SÃ
int in (b).
FIG. 4. (a) Cross-sectional TEM image of media showing nonoxide soft cap
layer on oxide hard bottom layer. The presence of oxide phase along the bot-
tom layer’s grain boundary and in the cap layer is indicated. (b) Proposed
growth model.
07B734-2 Srinivasan et al. J. Appl. Phys. 109, 07B734 (2011)
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

3. decoupled due to the oxide and IG exchange progressively
increases with CL thickness. In regime II, the CL grains are
completely coupled and effective toward causing incoherent
reversal via interlayer exchange coupling. The two growth
regimes are correlated with the two switching regimes.
Turning next to spin-stand based recording measure-
ments, it is expected that enhancing the cooperative reversal
of grains will improve media background noise at all fre-
quencies. DC-noise is a fundamental contributor to total
noise in media at all areal densities.14
For this purpose, the
total integrated noise originating from a written dc pattern
was measured. A signal-to-dc-noise ratio (SNRDC) was cal-
culated, where the signal is from a medium frequency pat-
tern. SNRDC is plotted versus fixed sweep rate SÃ
in Fig. 5(a)
and versus SÃ
int in Fig. 5(b). Once again, thermal effects are
responsible for the misleading trend of SNRDC vs SÃ
for the
NiW and BL series. Once the most appropriate SÃ
int was cal-
culated, all the media series follow the same trend of an
increase in SNRDC with the increasing SÃ
int.
Adjacent track interference (ATI) is generally thought of
as being related to thermal stability effects and cooperative re-
versal.15
Typically, ATI is a measure of the error rate degrada-
tion on track n, due to fringing fields from repeated aggressor
patterns on tracks nþ1 and nÀ1. Less degradation or smaller
ATI is expected of more thermally stable media or those that
have less exchange interactions, since they are less susceptible
to fringing fields. Figure 6(a) is a plot of the error rate degra-
dation, per decade of write operations on the adjacent tracks
versus KuV/kBT. For the NiW and BL series, the most ther-
mally stable media have smaller ATI, as expected. However,
for the CL series, the most thermally stable media have worse
ATI. Once again, when viewed from a cooperative reversal of
grains standpoint, while there is no clear trend across samples
when ATI is plotted versus SÃ
as in Fig. 6(b), all three media
series follow the same trend of worse ATI with the increasing
SÃ
int, plotted in Fig. 6(c). The above observations suggest that
cooperative reversal more strongly affects the ATI perform-
ance than the thermal instability effects. This aspect needs to
be further investigated.
IV. CONCLUSIONS
The correlation between microstructure, intrinsic
switching, incoherent reversal, and recording properties was
analyzed for ECC media. Media with thicker NiW seed
layers showed a decrease in SÃ
int, which was associated with
an increase in grain size. Media with thicker hard layers in
the ECC composite stack also showed a decrease in SÃ
int,
whereas media with thicker soft layers showed an increase.
Based on the correlation between the incoherent reversal and
SÃ
int, a model was proposed for the growth of the soft layer
on top of the hard layer. Large values of SÃ
int were also
correlated with high SNRDC and poor ATI performance.
1
R. H. Victora and X. Shen, IEEE Trans. Magn. 41, 537 (2005).
2
D. Suess et al., J. Magn. Magn. Mater. 321, 545 (2009).
3
D. Suess, Appl. Phys. Lett. 89, 113105 (2006); G. T. Zimanyi, J. Appl.
Phys. 103, 07F543 (2008).
4
G. Choe et al., IEEE Trans. Magn. 47, 55 (2011).
5
G. Bertero, B. R. Acharya, S. S. Malhotra, K. Srinivasan, E. Champion, S.
Lambert, G. Lauhoff, and M. Desai, in The 21st Magnetic Recording Con-
ference (TMRC 2010), A2, San Diego, Aug 16–18, 2010.
6
A. Berger et al., Appl. Phys. Lett. 93, 122502 (2008).
7
K. Tanahashi et al., IEEE Trans. Magn. 45, 799 (2009).
8
G. Choe et al., IEEE Trans. Magn. 45, 2694 (2009).
9
K. Srinivasan et al., J. Appl. Phys. 107, 113912 (2010).
10
M. P. Sharrock, J. Appl. Phys. 76, 6413 (1994); IEEE Trans. Magn. 35,
4414 (1999).
11
Y. Ikeda et al., IEEE Trans. Magn. 46, 1852 (2010).
12
D. Suess et al., IEEE Trans. Magn. 45, 88 (2009).
13
H. S. Jung et al., J. Appl. Phys. 103, 07F515 (2008).
14
A. M. Taratorin, Magnetic Recording Systems and Measurements (Guzik
Technical Enterprises, Mountain View, CA), p. 258 (2004).
15
Y.-C. Feng et al., IEEE Trans. Magn. 45, 905 (2009).
FIG. 5. SNRDC is plotted vs (a) time-scale dependent SÃ
and (b) time-scale
independent SÃ
int (intrinsic), for the three sample sets.
FIG. 6. ATI degradation slope is plotted vs (a) thermal stability factor KuV/
kBT (b) time-scale dependent SÃ
and (c) time-scale independent SÃ
int (intrin-
sic), for the three sample sets. The lines guide the eye.
07B734-3 Srinivasan et al. J. Appl. Phys. 109, 07B734 (2011)
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp