Unusual magnetization reversal of [Co(4 Å)/Pt(10 Å)]4 multilayers with perpendicular magnetic anisotropy has been revealed macroscopically by magnetometry measurements and microscopically by magneto-optical Kerr effect microscopy and magnetic force microscopy (MFM) imaging. During the first-order reversal process, the magnetization first decreases, then reaches a plateau, and finally rises back to saturation, corresponding to expanding bubble domains, stationary domains, and fading contrast but unchanged boundary domains, respectively. MFM imaging reveals the existence of many submicron-scaled unreversed channels within the boundary of the “bubble” domains. The magnetization reversal behavior can be accounted for by the evolution of the unusual domain structures in different field regimes

Cheng, Xuemei

Chien, C. L.

Nikitenko, V. I.

Shapiro, A. J.

Shull, R. D.

English

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Bryn Mawr College
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2006
Unusual Magnetization Reversal in [Co/Pt]4
Multilayers with Perpendicular Anisotropy
Xuemei Cheng
Bryn Mawr College, xcheng@brynmawr.edu
V. I. Nikitenko
A. J. Shapiro
R. D. Shull
C. L. Chien
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Custom Citation
X. M. Cheng et al., J. Appl. Phys. 99, 8C905 (2006).
Unusual magnetization reversal in †Co/Pt‡4 multilayers with perpendicular
anisotropy
X. M. Chenga
Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218
V. I. Nikitenko, A. J. Shapiro, and R. D. Shull
Magnetic Materials Group, National Institute of Standards and Technology, Gaithersburg,
Maryland 20899
C. L. Chien
Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218
Presented on 1 November 2005; published online 21 April 2006
Unusual magnetization reversal of Co4 Å /Pt10 Å4 multilayers with perpendicular magnetic
anisotropy has been revealed macroscopically by magnetometry measurements and microscopically
by magneto-optical Kerr effect microscopy and magnetic force microscopy MFM imaging. During
the first-order reversal process, the magnetization first decreases, then reaches a plateau, and finally
rises back to saturation, corresponding to expanding bubble domains, stationary domains, and fading
contrast but unchanged boundary domains, respectively. MFM imaging reveals the existence of
many submicron-scaled unreversed channels within the boundary of the “bubble” domains. The
magnetization reversal behavior can be accounted for by the evolution of the unusual domain
structures in different field regimes. © 2006 American Institute of Physics.
DOI: 10.1063/1.2166608
I. INTRODUCTION
Recently, Co/Ptn multilayers have been extensively
studied because of the unique attribute of perpendicular mag-
netic anisotropy PMA and the potential applications in per-
pendicular magnetic recording.1–10 The properties of
Co/Ptn multilayers depend intricately on the number n of
bilayer repeats. Many previous studies on these multilayers
with larger values of nn10 Refs. 5–7 show that during
magnetization reversal, the reversed domains nucleate from
isolated defects and then develop into dendritic patterns.
Magnetization reversal in the Co4 Å /Pt7 Å50 multilay-
ers has also been studied by the first-order reversal curve
technique.5 At the other extreme, thin Pt/Co/Pt trilayers re-
vealed very different behaviors, where magnetic domains ex-
pand by the propagation of smooth and well-defined domain
walls.8,9 Under a reversed field, the domain walls retract to-
ward the nucleation centers. However, there has been little
work on Co/Ptn multilayers with an intermediate n, al-
though several papers reported other materials with PMA
and intermediate thickness.11–13
In this work, we have studied in detail the magnetization
reversal behavior of Co4 Å /Pt10 Å4 multilayers with
an intermediate n=4, and revealed very unusual reversal be-
havior. The reversal has been studied macroscopically by
vibrating-sample magnetometry VSM, and microscopically
by both magneto-optical Kerr effect MOKE
microscopy9,14,15 and magnetic force microscopy MFM
imaging.7,10 The first-order reversal curves FORCs for
Co/Ptn multilayers with an intermediate repeat number n
=4 are very different from those for Co4 Å /Pt7 Å50
multilayers.5 The unusual magnetization reversal behavior
can be explained by the evolution of the domain structure in
different external field regimes, as confirmed by the direct
domain observations by MOKE and MFM.
II. EXPERIMENT
The Pt100 Å / Co4 Å /Pt10 Å4 /Pt20 Å multi-
layers were deposited on silicon substrates by dc magnetron
sputtering. The multilayer structures were verified by small-
angle x-ray reflectivity measurements using a four-circle
x-ray diffractometer Philips X’Pert-MRD with Cu K 
=1.5405 Å radiation.16
Macroscopic magnetic properties were measured by a
VSM with the magnetic field applied perpendicular to the
film plane. Direct observation of the domain pattern evolu-
tion during the FORC procedure was made by MOKE imag-
ing under an external field. To study the domain pattern on a
submicron scale, MFM imaging was performed using a
Veeco multimode MFM Ref. 16 on the sample initially
magnetized to the desired magnetic state by an electromag-
net. Careful preexamination showed that the MFM tip did
not alter the magnetic patterns of the sample.
III. RESULTS AND DISCUSSION
The small-angle x-ray reflectivity of the Co/Pt4
multilayer Fig. 1 confirmed the specifically designed
multilayer structure. The periodicity of peaks at small angles
reflects the overall thickness, 170 Å, of the deposited
Pt100 Å / Co4 Å /Pt10 Å4 /Pt20 Å, which agrees
with the expected thickness of 176 Å.
The Co/Pt multilayers are among the few materials
with well-established PMA.17 The Co/Pt4 multilayers usedaElectronic mail: xmcheng@pha.jhu.edu
JOURNAL OF APPLIED PHYSICS 99, 08C905 2006
0021-8979/2006/998/08C905/3/$23.00 © 2006 American Institute of Physics99, 08C905-1
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in this work also exhibit PMA. A square hysteresis loop with
sharp reversal was observed in Co4 Å /Pt10 Å4 with a
coercivity HC=16.4 kA/m, as shown in Fig. 2. To reveal the
details of the reversal process, we resorted to the method of
FORC measurements, where the field was decreased from a
positive saturation field to a negative field defined as the
returning field −HT HT is the magnitude of the returning
field, and then increased to the original positive saturation
field.
Several FORCs for the Co/Pt4 sample shown in Fig. 2
share common features, represented by the one with HT
=15.5 kA/m. Five numbered reference points are marked on
the FORC with HT=15.5 kA/m, as illustrated in Fig. 2. As H
is decreased from the positive saturation field to −HT, the
magnetization M begins to decrease at a nucleation field
point 1 and continues to decrease to point 2. As H is now
increased, M first decreases precipitously between points 2
and 3, then reaches a plateau between points 3 and 4, and
finally increases towards the saturation value between points
4 and 5. In contrast, in most FORC studies of magnetic thin
films, including those of Co/Ptn multilayers with large n,5
M rises with increasing H from the returning field −HT.
To understand the unusual macroscopic FORC behavior,
MOKE imaging of microscopic domain was performed in
the external field following the same procedure as that for
the marked FORC. Eight representative images, shown in
Fig. 3, are closely related to the five reference points in Fig.
2: Images a–d were taken between points 1 and 2 in Fig. 2,
image e was taken at point 3 but represents the domain pat-
tern between points 3 and 4, and images f–h were taken
between points 4 and 5.
Between points 1 and 2, the reversed domains the dark
area in Fig. 3 first nucleated from a few sites within the
view Fig. 3a. The domains then grew in size with the
outer boundary expanding to form larger reversed “bubbles”
Figs. 3a–3d. This domain aspect is similar to the re-
versed bubbles observed in Pt/Co/Pt trilayers8 except that
the outer boundaries in Co/Pt4 are not as smooth as those
in Pt/Co/Pt trilayers, but is quite different from the den-
dritic patterns in thicker Co/Ptn multilayers.5–7 In this
stage, the reduction in magnetization is due to both the ex-
pansion of the existing domains and the nucleation of new
domains Fig. 3b. As H was increased from points 2 to 3,
the reversed domains kept growing instead of shrinking until
the domain pattern was formed, as shown in Fig. 3e, at
point 3. Between points 3 and 4, although H continued to
increase, the reversed regions kept a constant area repre-
sented by Fig. 3e, thus the magnetization has a plateau
value. Most surprisingly, as H was increased from points 4 to
5 Figs. 3f–3h, the reversed dark domains maintain the
same outer boundary, while the regions inside became pro-
gressively brighter and finally disappeared. This is very un-
usual behavior, different from that in Pt/Co/Pt trilayers
where the outer smooth boundaries retract toward the nucle-
ation centers.8
The MOKE imaging results suggest much finer unre-
solved structures inside the outer boundaries of the reversed
domains. Consequently, MFM is used to clarify the situation,
imaging the same sample premagnetized to the remnant state
FIG. 2. Major magnetic hysteresis loop and several first-order reversal
curves for a Co4 ÅPt10 Å4 sample with returning fields of 15.1, 15.5,
15.9, 16.2, and 16.7 kA/m. Reference points 1–5 and Q mark the conditions
where MOKE and MFM observations were performed.
FIG. 1. Small-angle x-ray reflectivity data for Co4 ÅPt10 Å4 showing
that the overall thickness of 170 Å is in agreement with the designed
structure.
FIG. 3. MOKE images of magnetic domain evolution
obtained simultaneously during the FORC measure-
ments, observed at the locations marked by the refer-
ence points in Fig. 2; images a–c were taken between
points 1 and 2, image d was taken at point 3, image e
was taken at point 4, and images f–h were taken be-
tween points 4 and 5.
08C905-2 Cheng et al. J. Appl. Phys. 99, 08C905 2006
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shown by point Q, between points 3 and 4 in Fig. 2. Between
points 3 and 4 not only does the magnetization remain un-
changed but also the MOKE images stay the same. The
MFM image Fig. 4 obtained at point Q clearly shows that
there are many submicron-scaled unreversed channels inside
the outer boundaries. Therefore, the apparent reversed
bubbles are, in fact, not single domains with uniform M in-
side.
Magnetic domain growth in the thin films with PMA can
be realized through two modes: the nucleation of the adja-
cent new reversed domains and the motion of the existing
domain walls.5,8,10,15 In Co/Ptn multilayers with large n,
the nucleation mode dominates, resulting in dendritic domain
patterns.5–7 The domain-wall motion mode dominates in the
Pt/Co/Pt trilayers or Co/Ptn multilayers with very small n,
resulting in magnetically uniform bubble domains with
changing sizes.8,9
In contrast, the behavior of the Co/Pt4 multilayers with
an intermediate n is between the above two limiting cases.
When H is decreased from the positive saturation to −HT
point 2 in Fig. 2, the nucleated domains grow through both
the nucleation and domain-wall motion modes, forming the
bubble domains with numerous unreversed channels inside
the apparent boundaries. When H is increased back from
−HT between points 2 and 3, the bubble domains first con-
tinue expanding by thermally activated domain-wall mo-
tions, which overcome the domain-wall pinning. This results
in a decrease in M although the external field is increasing.
In the plateau region between points 3 and 4, the external
field cannot provide sufficient energy for the domain wall to
overcome the wall pinning energy barriers, so the domain-
wall motion ceases. Therefore, microscopically the bubble
domains remain unaltered, and macroscopically a plateau on
the M-H curve is formed. When H is further increased to-
ward the positive saturation field between points 4 and 5,
the domain walls move again, but this time backwards, re-
sulting in the expansion of the previously unreversed chan-
nels. Because such domain-wall motions take place only on
the submicron scale, the MOKE images on the scale of
100 m only show contrast changes instead of changes in
the outer boundary.
IV. CONCLUSIONS
In conclusion, we have studied macroscopically and mi-
croscopically the unusual magnetization reversal behavior of
Co4 Å /Pt10 Ån multilayers with an intermediate repeat
of n=4. We have observed features that are very different
from those with either a small or a large n. When measured
in an increasing field from −HT to the positive saturation
field, the magnetization exhibits an unusual decrease and pla-
teau. Direct domain observations demonstrate the existence
of submicron-scaled unreversed channels inside the apparent
much larger macroscopic “bubble” domains. These unusual
macroscopic reversal features result microscopically from
thermally activated domain-wall motion overcoming
domain-wall pinning in Co4 Å /Pt10 Å4 multilayers.
ACKNOWLEDGMENT
This work is supported by NSF Grant Nos.
DMR0403849 and DMR0520491.
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08C905-3 Cheng et al. J. Appl. Phys. 99, 08C905 2006
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Unusual Magnetization Reversal in [Co/Pt]\u3csub\u3e4\u3c/sub\u3e Multilayers with Perpendicular Anisotropy

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Unusual Magnetization Reversal in [Co/Pt]\u3csub\u3e4\u3c/sub\u3e Multilayers with Perpendicular Anisotropy

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