1. Observation of microwave-assisted magnetization reversal in perpendicular
recording media
Lei Lu, Mingzhong Wu, Michael Mallary, Gerardo Bertero, Kumar Srinivasan et al.
Citation: Appl. Phys. Lett. 103, 042413 (2013); doi: 10.1063/1.4816798
View online: http://dx.doi.org/10.1063/1.4816798
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2. Observation of microwave-assisted magnetization reversal in perpendicular
recording media
Lei Lu,1
Mingzhong Wu,1,a)
Michael Mallary,2
Gerardo Bertero,2
Kumar Srinivasan,2
Ramamurthy Acharya,2
Helmut Schultheiß,3
and Axel Hoffmann3
1
Department of Physics, Colorado State University, Fort Collins, Colorado 80523, USA
2
Western Digital Technologies, San Jose, California 92630, USA
3
Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
(Received 19 March 2013; accepted 12 July 2013; published online 25 July 2013)
This letter reports microwave-assisted magnetization reversal (MAMR) in a 700-Gbit/in2
perpendicular media sample. The microwave fields were applied by placing a coplanar waveguide on
the media sample and feeding it with narrow microwave pulses. The switching states of the media
grains were measured by magnetic force microscopy. For microwaves with a frequency close to the
ferromagnetic resonance (FMR) frequency of the media, MAMR was observed for microwave power
higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a
broad microwave frequency range which covers the FMR frequency and is centered below the FMR
frequency. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4816798]
In the presence of microwave magnetic fields, magnet-
ization reversal or switching in magnetic materials can be
realized with relatively low magnetic fields. This effect is
called microwave-assisted magnetization reversal (MAMR).
The MAMR effect was first observed by Thirion et al. in
2003.1
They demonstrated microwave-assisted switching in
a 20-nm cobalt particle using superconducting quantum in-
terference device (SQUID) techniques. Following the experi-
ments by Thirion et al., the MAMR effect was demonstrated
in a wide variety of magnetic elements or structures. These
include (i) single-domain elements, such as micron- and
submicron-sized permalloy film elements,2,3
permalloy nano
dots,4
submicron cobalt particles,5
and cobalt nanoparticles
with a diameter of only 3 nm,6,7
and (ii) multi-domain ele-
ments, such as cobalt strips,8
permalloy wires,9,10
permalloy
and FeCo thin films,11–13
permalloy layers in magnetic tun-
nel junctions,14,15
and Co/Pt multilayered structures.16–18
The experiments on these elements all demonstrate that the
application of appropriate microwaves can remarkably
reduce the magnetic field required for the magnetization re-
versal. The underlying physics for this MAMR response is as
follows: The microwave magnetic fields excite large-angle
magnetization precession; the large-angle precession lowers
the energy barrier for the rotation reversal in single-domain
elements and that for domain nucleation in multi-domain
materials.
In addition to its interesting fundamental physics, the
MAMR effect is also a very promising mechanism for the
realization of next-generation magnetic recording at several
terabits per square inch. Numerical simulations have demon-
strated the feasibility of MAMR operation in perpendicular
recording media.19,20
Experimental demonstrations, how-
ever, are rather challenging, as the media typically require
relatively large switching fields and have substantially larger
damping in comparison with the above-mentioned magnetic
materials. Nevertheless, very recently two groups reported
the studies of MAMR in perpendicular media. Boone et al.
used the anomalous Hall effect (AHE) to measure the hyster-
esis loop of a perpendicular media bar and studied the effects
of microwaves on the AHE loop.21,22
They observed a
microwave-caused reduction in the coercivity field of up to
8%. Nozaki et al. demonstrated that the exposure of a per-
pendicular media sample to microwaves could produce a no-
table shift in the sample’s ferromagnetic resonance (FMR)
field, which indicated the microwave-assisted switching of
certain grains in the sample.23
These two studies indicated
the feasibility of MAMR in perpendicular media.
This letter reports on the observation of MAMR
responses in a sample cut from a commercial quality 700-
Gbit/in2
media disk. Narrow microwave field pulses were
applied by placing a coplanar waveguide (CPW) structure on
the media sample. The switching states of the grains in the
media were measured by magnetic force microscopy
(MFM). For the microwaves with a frequency close to the
FMR frequency of the media, the microwave-assisted
switching was observed when the microwave power was
higher than a certain threshold level. For the microwaves
with a certain high power level, the MAMR was observed
for a relatively wide microwave frequency range which cov-
ers the FMR frequency but is centered at a frequency below
the FMR frequency. The effects of both the microwave pulse
duration and repetition rate were also examined. The results
indicated that the observed MAMR response was not attrib-
uted to heating effects.
The sample was a 4 mm by 4 mm rectangle element cut
from an exchange coupled composite (ECC) media disk. The
core components of the ECC media include a 4.5-nm-thick
“soft” magnetic layer, an 8.5-nm-thick “hard” magnetic
layer, and a 0.8-nm-thick weakly magnetic exchange-break
layer in-between the “soft” and “hard” layers. Both of the
two magnetic layers are CoPtCr-based granular films.
Figure 1 shows the static and FMR properties of the
sample. Graph (a) presents a hysteresis loop measured by
SQUID techniques. The measurement was carried out with a
static magnetic field (H) normal to the sample plane. The
a)
Author to whom correspondence should be addressed. Electronic mail:
mwu@lamar.colostate.edu
0003-6951/2013/103(4)/042413/5/$30.00 VC 2013 AIP Publishing LLC103, 042413-1
APPLIED PHYSICS LETTERS 103, 042413 (2013)

3. data indicate a saturation induction (4pMs) of about 9.2 kG, a
nucleation field in the 2–3 kOe range, and a coercivity field
of about 5.4 kOe. There is also a vertical dashed line in (a),
which indicates the strength of the switching field used in the
MAMR experiments. Graphs (b) and (c) present the FMR
data measured by broadband vector network analyzer techni-
ques.24
Graph (b) shows the (static) FMR field as a function
of frequency (f). The circles show the data, while the line
shows a fit to the Kittel equation
f ¼ jcjðH þ HintÞ; (1)
where |c| is the absolute gyromagnetic ratio. The term Hint
denotes an effective internal field, which is the sum of the
perpendicular magneto-crystalline anisotropy field (Ha), the
dipolar field, and the intergranular exchange field. One can
see that the fitting in graph (b) is almost perfect. The fitting
yielded |c| ¼ 3.28 GHz/kOe and Hint ¼ 7.4 kOe. Note that the
gyromagnetic ratio here is higher than the standard value
(2.8 GHz/kOe). Similar ratios were also reported for CoCrPt
alloy films25
and CoCr granular films.26
Note also that
Hint ¼ 7.4 kOe indicates an anisotropy field of Ha % 16.6 kOe
if one assumes Hint % Ha-4pMs. Graph (c) presents the half-
power FMR linewidth (DH) as a function of f. The circles
show the data, while the line shows a linear fit to
DH ¼
2a
jcj
f þ DH0; (2)
where a is the effective Gilbert damping parameter and DH0
describes sample inhomogeneity-caused line broadening.
The fitting yielded a ¼ 0.061 6 0.003 and DH0 ¼ 761
6 64 Oe. The a value is close to that reported for CoPtCr
alloy films (0.06).27
The inhomogeneity line broadening is
relatively large and is mainly attributed to the variation of
Hint on individual grains.28
The ratio of DH0 to Hint is about
10%, which is close to the switching field distribution of the
sample (7%–8%).29
Figure 2 illustrates the experimental approach. Graph
(a) shows a schematic of the experimental configuration.
Graphs (b) and (c) show a photograph and an atomic force
microscopy (AFM) image, respectively, of the portion of the
CPW structure where the signal line is a 5.5-lm-wide, 100-
lm-long narrow strip. Graph (d) gives a representative MFM
image of the media. The bright strip shows the region where
the media surface was in contact with the CPW signal line
and the grains were switched, due to the MAMR operation.
Graph (e) presents the two MFM images of the media show-
ing opposite contrasts. The left image was taken from an
area in the media where the MAMR operations were con-
ducted at four different locations. The four bright strips in
the image show the regions where the media grains are
switched due to the MAMR effect. The right image in (e)
was taken from an area where the MAMR operations were
carried out at two locations. The two dark strips show the
regions where the MAMR effect resulted in the switching.
For both the cases, the MAMR operations were carried out at
the same conditions except that for the right image (1) the
media sample was initially saturated in an opposite direction
and (2) the switching field during the MAMR operations was
also in an opposite direction, in comparison to the left
image.
The MAMR experiments consist of the following four
steps. (1) Saturate the media sample with a strong perpendic-
ular magnetic field. (2) Apply a switching field which is op-
posite to the magnetization in the media and is close to the
nucleation field (lower than the coercivity field). (3) Apply
microwave pulses to the CPW to assist the switching of the
grains in the media. (4) Turn off both the switching field and
the microwave signals and use the MFM system to determine
the switching status of the grains in the media. When the
FIG. 1. Properties of the ECC media
sample. (a) A hysteresis loop. (b) FMR
field vs. frequency. (c) FMR linewidth
vs. frequency.
042413-2 Lu et al. Appl. Phys. Lett. 103, 042413 (2013)

4. microwave pulses are applied, the narrow portion of the
CPW signal line (see Fig. 2(c)) produces relatively strong
microwave magnetic fields. These microwave fields lower
the energy barrier for magnetization reversal in the grains
right above the signal line and thereby induce the switching
of these grains. Such switching manifests itself as a bright
strip in the MFM image, as shown in Fig. 2(d) and in the left
graph of Fig. 2(e), or a dark strip, as shown in the right graph
of Fig. 2(e).
Three facts should be pointed out. First, the bright strip
in the MFM image in Fig. 2(d) has the same length and width
as the narrow signal line of the CPW. Second, towards the
left end of the bright strip in the MFM image in Fig. 2(d),
one sees a gradual increase in the strip width and a gradual
decrease in the strip contrast. This agrees with the expecta-
tions that the wider the signal line is, the weaker the micro-
wave field is and the fewer grains are switched. Third, the
MFM image shows a completely opposite contrast if one ini-
tially saturates the media along the opposite directions, as
shown in Fig. 2(e). These facts together clearly demonstrate
the validity of the above-described MAMR measurement
approaches.
Figures 3 and 4 show the core results of this work. The
MFM image in Fig. 3 was taken from a 100 lm  100 lm
square area of the sample where six separate MAMR experi-
ments were conducted at six different locations. The micro-
wave power Pmw used for each MAMR experiment is
indicated at the corresponding location. For all the experi-
ments, the carrier frequency fmw of the microwave pulse was
kept the same, at 13 GHz. For the MFM images in Fig. 4, in
contrast, the MAMR experiments at different locations were
carried out at the same microwave power, which was
31 dBm, but different microwave frequencies, as indicated.
Except for Pmw and fmw, the other parameters were the same
for all the experiments. The field used to saturate the sample
was 20 kOe. The switching field Hsw was 3 kOe. The micro-
wave pulses had a width of 11 ns and a repetition rate of
0.1 kHz. Note that the power levels cited above are nominal
power applied to the CPW. The field values given in Fig. 3
and below are microwave magnetic fields from the narrow
CPW signal line, which were estimated based on the input
microwave power, the reflection coefficient of the CPW, and
the width of the CPW signal line. The dashed lines in Fig. 4
indicate the positions of the CPW signal line in the corre-
sponding experiments.
The MFM image in Fig. 3 shows strips with rather low
contrasts for Pmw ¼ 23 dBm and 25 dBm and strips with high
contrasts for Pmw ! 27 dBm. This indicates a power thresh-
old of about 27 dBm (109 Oe) for MAMR operations with
microwave pulses of fmw ¼ 13 GHz. The MFM images in
Fig. 4 show high-contrast strips for fmw ¼ 8 GHz, 9 GHz,
10 GHz, 11 GHz, 12 GHz, 13 GHz, 14 GHz, and 15 GHz but
FIG. 3. An MFM image of the area of a media sample where six separate
MAMR experiments were carried out at six different locations. The micro-
wave power level used in each experiment is indicated at the corresponding
location.
FIG. 2. (a) Experimental configuration.
(b) A photograph of the portion of the
CPW where the signal line is narrow.
(c) An AFM image of the CPW signal
line. (d) An MFM image of the media
sample which shows the MAMR
effect. (e) Two MFM images which
show MAMR-caused strips but have
opposite contrasts.
042413-3 Lu et al. Appl. Phys. Lett. 103, 042413 (2013)

5. show no strips for fmw ¼ 5 GHz, 6 GHz, 7 GHz, 17 GHz,
and 19 GHz. This indicates that when Pmw ¼ 31 dBm, the
MAMR occurs over a frequency range of 8–15 GHz. Note
that according to Eq. (1) and the parameters cited above, the
field Hsw corresponds to an FMR frequency of 14.4 GHz for
the media, which is within the 8–15 GHz frequency range.
These results clearly demonstrate the MAMR operation
in the media. Moreover, they show that, for certain high
microwave power, the MAMR operation can take place over
a relatively broad frequency range which covers the FMR
frequency but is centered below the FMR frequency. This
agrees with previous experimental observations.21–23
The
reason for such a broad frequency range is due to the fact
that the media have a rather broad FMR linewidth as shown
in Fig. 1(c). The broad FMR field linewidth indicates a broad
FMR linewidth in the frequency domain. At 14 GHz, for
example, the above cited damping parameter corresponds to
a frequency linewidth of about 1.7 GHz; the field linewidth
DH0 corresponds to a frequency linewidth of about 2.5 GHz.
The broad linewidth in the frequency domain indicates that
the microwaves can drive the magnetization to precess over
a broad frequency range as long as the microwave field is
sufficient strong. Also, one can expect that this frequency
range increases with an increase in the microwave power.
The observation that the frequency range is centered below
the FMR frequency is mainly due to the fact that during the
switching process, the anisotropy field takes lower values
since the projection of the magnetization along the sample
normal direction is reduced. As the anisotropy field is
smaller, the effective internal field is smaller, and the preces-
sion frequency is also smaller during the switching.
Figure 5 presents representative MFM images that show
the effects of the microwave pulse repetition rate and dura-
tion. The left image shows four strips resulted from MAMR
with microwave pulses of the same duration (98 ns) but sig-
nificantly different repetition rates, as indicated. The right
images show two strips resulted from MAMR with micro-
wave pulses of the same repetition rate (0.1 kHz) but signifi-
cantly different durations, as indicated. The field used to
saturate the sample and the switching field was the same as
cited above. The microwave pulses had fmw ¼ 13 GHz and
Pmw ¼ 31 dBm. The strips in each image show almost the
same contrast. This demonstrates that the effects of the
microwave pulse repetition rate and duration are negligible.
This result indicates that the above-presented MAMR
responses were not attributed to a heating effect. Besides, the
closeness of the MAMR responses for two different micro-
wave pulse durations also indicates that the switching time is
no longer than 11 ns. Note that 11-ns-wide pulses are the nar-
rowest microwave pulses available in the authors’ labora-
tory. Future work on MAMR experiments with narrower
pulses is of great interest.
Three important points should be emphasized. (1) This
work made use of MFM techniques rather than the AHE and
FMR techniques used in previous work,21–23
to directly mea-
sure the switching status of the grains in the media in the
presence of microwaves. In this sense, this work presents a
direct demonstration for MAMR in perpendicular media. (2)
The data presented above were obtained from a commercial
quality 700-Gbit/in2
media disk sample rather than custom-
ized samples. Thus, this work is of practical significance. (3)
The data reveal the connection of the frequency response of
the MAMR operation to the FMR frequency as well as the
microwave power required by the MAMR operation for per-
pendicular media. These results provide far-reaching impli-
cations for the future of microwave-assisted magnetic
recording using perpendicular media.
In summary, this work demonstrated MAMR operation in
a 700-Gbit/in2
perpendicular media sample. For microwaves
with a frequency close to the FMR frequency of the media,
the MAMR operation was observed for microwave power
higher than a certain threshold level. For microwaves with
certain high power, the MAMR effects were observed for a
broad microwave frequency range which covers the FMR fre-
quency and is centered below the FMR frequency. It was also
demonstrated that the MAMR operation was independent of
both the microwave pulse duration and repetition rate.
This work was supported in part by the U. S. National
Institute of Standards and Technology (60NANB10D011)
FIG. 5. MFM images for the areas of a sample where separate MAMR
experiments were carried out at different locations. Left: the experiments
were carried out with microwave pulses of different repetition rates, as indi-
cated. Right: the experiments were carried out with microwave pulses of dif-
ferent durations, as indicated
FIG. 4. MFM images for the areas of a sample where separate MAMR experi-
ments were carried out at different locations. The experiments were carried
out with microwave pulses of different carrier frequencies, as indicated.
042413-4 Lu et al. Appl. Phys. Lett. 103, 042413 (2013)

6. and Western Digital Technologies. Work at Argonne and use
of the Center for Nanoscale Materials was supported by the
U.S. Department of Energy, Office of Basic Energy Sciences
(DE-AC02-06CH11357).
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