Detection of a vortex nucleation position in a circular ferromagnet using
asymmetrically configured electrodes
Xiaomin Cui, Shaojie Hu, and Takashi Kimura
Citation: Applied Physics Letters 105, 082403 (2014); doi: 10.1063/1.4894216
View online: http://dx.doi.org/10.1063/1.4894216
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/8?ver=pdfcov
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APPLIED PHYSICS LETTERS 105, 082403 (2014)

Detection of a vortex nucleation position in a circular ferromagnet using
asymmetrically configured electrodes
Xiaomin Cui,1 Shaojie Hu,1 and Takashi Kimura2,3,4,a)1

Graduate School of Information Science and Electrical Engineering, Kyushu University, 744 Motooka,
Fukuoka 819-0395, Japan2
Department of Physics, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan
3
CREST, Japan Science and Technology Agency, Sanbancho, Tokyo 102-0075, Japan
4
Research Center for Quantum Nano Spin Sciences, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan(Received 8 July 2014; accepted 17 August 2014; published online 26 August 2014)
We demonstrated that the anisotropic magneto-resistance of a ferromagnetic micro-disk measured by
the asymmetrically configured electrodes provide the information of the vortex chirality. The large
difference of the electrical resistivity between the ferromagnetic disk and the nonmagnetic electrodes
creates an inhomogeneous current distribution, allowing us to detect the local domain structure. By
focusing on the chirality-dependent magneto-resistance change due to the vortex nucleation, we canclearly detect the vortex nucleation position, leading to the chirality determination. The reliability ofthe developed detection method has been confirmed by the magneto-resistance measurement with
C 2014 AIP Publishing LLC.
symmetrically configured electrodes. V[http://dx.doi.org/10.1063/1.4894216]
Study on magnetic vortex structure stabilized in a
micron or submicron scaled ferromagnetic disk has been
paid of great attention because of its high thermal stability,
negligible magneto-static interaction, and low frequency
dispersion at the resonant state.1–6 These outstanding characteristics provide prospective application in high density
storage media7 and functional radio-frequency (RF) devices
such as reconfigurable RF filter8–10 and spin torque oscillator.5 Moreover, the magnetic vortex includes several intriguing fundamental properties such as Bloch point reversal11
and the critical slowing down of the vortex formation.12 The
vortex structure can be characterized by two important quantity: polarity2,4 (the direction of the vortex core, either up or
down), and chirality (rotational direction of the magnetization, clockwise (CW) or counterclockwise (CCW)).13 The
effective manipulation and identification of the vortex
characteristics are indispensable for the realization of aforementioned applications and further understanding the fundamental property of the magnetic vortex.
So far, numerous reports concerning the manipulation of
the vortex polarity and chirality have been developed.14,15 In
addition, the detection methods for the polarity and chirality
have also been developed by several specific measurement
techniques such as Magnetic Force Microscope (MFM),2,16,17
Lorentz microscope,18,19 or Magneto-optical Kerr effect(MOKE).20 However, in these techniques, complex experimental conditions such as measurement time, temperatures,and sample thickness have to be satisfied. These regulations
prevent the systematic and statistical considerations. The lateral spin valve also can detect the vortex chirality.21,22
However, the complicate fabrication process and a tiny
spin-dependent signal may prevent the fair evaluation of thevortex structure at room temperature. In the method using
anisotropic magnetoresistance (AMR) with four terminal
electrodes, the device can be fabricated more simply.23
a)t-kimu@phys.kyushu-u.ac.jp0003-6951/2014/105(8)/082403/4/$30.00

However, the analysis based on the micromagnetic simulation
may prevent the intuitive understanding. Therefore, to further
explore the new functionalities of spin vortex and deepen theunderstanding on fundamental property of the vortex structure, we should seek a relatively simple and reliable method
to identify the vortex chirality.
To detect the vortex chirality, in the present study, we
use an AMR effect, in which the resistance of the ferromagnet depends on the magnetic domain structure. In the ferromagnetic disk, the magnetization reversal proceeds throughthe nucleation, displacement, and annihilation of the vortex.3
The nucleation position is known to depend on the directionof the applied magnetic field and the vortex chirality. Since
the nucleation of the vortex core corresponds to the phasetransition of the domain structure from the uniform magnetization state,12 a significant AMR change is induced by the
nucleation of the vortex, especially around the vortex core.
As schematically shown in Figs. 1(b) and 1(c), the domain
structure before the nucleation can be described by a similar
curling structure with setting the vortex core outside thedisk. Since the spin distribution inside the disk seems a C
shape, this domain structure is known as C state.24 Here, we
introduce asymmetrically configured nonmagnetic Cu electrodes, which are schematically shown in Fig. 1(a). Becauseof the large difference in the resistivity between the ferromagnetic disk and the Cu electrodes, the electric current
flows favorably in the Cu electrode, leading to create an
inhomogeneous current distribution. This enables to detect
the local domain structure around the bottom side of theferromagnetic disk. Therefore, when the vortex nucleatesfrom the bottom side, we should observe the large resistancechange because the most of the spins inside the electroderotates from the horizontal direction.(Fig. 1(b)) On thecontrary, the vortex nucleation from the opposite side will
be undetectable in the AMR measurement using the same
electrode because the current flowing in the top side is sotiny.(Fig. 1(c)) In this way, we can distinguish the vortex

105, 082403-1

C 2014 AIP Publishing LLC
V

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Cui, Hu, and Kimura

Appl. Phys. Lett. 105, 082403 (2014)

FIG. 1. (a) Scanning electron microscope image of the fabricated device
together with a probe configuration for
the AMR measurement and numerically obtained 2-dimensional currentdensity distribution in a ferromagnetic
disk with 4 lm diameter. The magnetic
field is applied along the horizontal
axis, parallel to the average current
direction (longitudinal configuration).
Schematic illustrations of the vortex
nucleation processes for CW chirality(b) and CCW chirality (c). Here, themagnetic field is reduced from the positive saturation field.

chirality by monitoring the resistance of the local area of theferromagnetic disk.
We have prepared the sample consisting of a Permalloy(Py) disk, a diameter of 4 lm and a thickness of 40 nm and apair of the nonmagnetic Cu electrodes, 200 nm thickness.
Figure 1(a) shows a scanning electron microscope image ofthe fabricated sample together with the schematic configuration used for the AMR measurement. The Py disk was fabricated by using a conventional lift-off method combined withthe electron beam lithography. The Cu electrodes were
deposited by a Joule heat evaporator after the surface cleaning of the Py disk by a low energy Ar ion milling. Here, thevertical position of the Cu electrodes is shifted by 800 nm
from the center of the disk. The electrical resistivity for the
Py is 29 lX cm, much larger than that for the Cu electrode(2.2 lX cm). The AMR measurement was performed by
two-terminal resistance measurement with a low bias ac of
55 lA. Here, the magnetic field is applied along the horizontal direction, meaning the longitudinal configuration.
To confirm that the large difference in the resistivity
between the Py and Cu makes an inhomogeneous current
distribution in the ferromagnetic disk, we have calculated
two dimensional distribution of the current density in the
AMR device configured similarly to the fabricated device byusing COMSOL multiphysics. As shown in Fig. 1(a), thecurrent in the Py disk is found to flow mainly in the bottom
part of the Py disk. This enables to detect the local domain
structure of the Py disk.
Figure 2 shows a representative result of the AMR curvemeasured at room temperature. In both the forward andbackward field sweeps, when the magnetic field is reduced
from the saturation state, the sudden resistance change isobserved at 10 Oe. These sudden changes correspond to the
nucleation of the vortex core in the ferromagnetic disk. Afterthe sudden change, the resistance gradually increases withapproaching to the value for the saturation state. The inset in
Fig. 2 represents the domain structures calculated by
OOMMF just after the vortex nucleation. As mentioned
above, since the resistance change is dominated by the domain structure in the bottom side of the ferromagnetic disk,
the sudden resistance changes observed in Fig. 2 indicatethat the vortex nucleates from the bottom edge of the disk
both in the forward and backward sweeps. Therefore, thevortex chiralities in the backward and forward sweeps can be

FIG. 2. Longitudinal AMR curve of the circular disk measured at room
temperature. Solid and dotted lines correspond to the forward and backward
field sweeps, respectively, while the inset shows the numerically calculated
magnetization reversal process of the disk for the backward and forward
sweeps.

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082403-3

Cui, Hu, and Kimura

determined as CCW and CW, respectively, as schematicallyshown in the inset of Fig. 2.
From the aforementioned consideration, when the vortex
nucleates from the top side, the nucleation of the vortex may
become undetectable. However, as mentioned above, theobserved MR curve always involves the sudden changearound 10 Oe in the conventional field sweep, indicating thevortex always nucleates from the bottom side. To realize
the formation of the reversed vortex, the maximum magnetic
field during the field sweep was reduced. This is because theC-state just after the annihilation of the vortex is known to
induce the nucleation of the vortex from the annihilatedside.24,25 Figure 3 shows the magnetoresistance curve in thefield range 150 Oe < H < 200 Oe. As expected above, inthe backward sweep, the MR curve without the sudden resistance change has been observed, which means the vortex
nucleates from the top side and the chirality should be CW
for both backward and forward sweeps. In addition, it should
be noted that the small resistance jump has been observed at
132 Oe. This small jump can be understood by the annihilation of vortex from the bottom side. The experimental factthat the small resistance change due to the vortex annihilation has not been observed in Fig. 2 is also consistent withthe local detection of the domain structure. Thus, we cansimply distinguish the vortex chirality from the shape of the
AMR curve.
By using the simple evaluation method, we extend toevaluate how the vortex chirality distributes with changing
the magnitude of the reversed magnetic field Hr. Figure 4
shows the probability of the formation of CW and CCWchiralities as a function of Hr. Here, the probability at eachpoint was calculated by repeating the AMR measurementfor 10 times. It can be clearly seen that the probability of
CCW chirality formation increases with increasing Hr andreaches 100% at jHr j > 160 Oe. The probability is 50% at
jHr j  135 Oe, indicating that two possible chiralities CW
and CCW were randomly formed. Since in an ideal

FIG. 3. Longitudinal AMR curve in the field range from 190 Oe to 150 Oe.
The magnetic field is swept from 190 Oe to 150 Oe and then swept back to
190 Oe. Solid and dotted lines correspond to the forward and backward fieldsweeps, respectively, with the inset of numerically calculated magnetization
reversal process of the disk for the backward and forward sweeps.

Appl. Phys. Lett. 105, 082403 (2014)

FIG. 4. (a) Probability of the CW (solid square) and CCW (open square)vortex formation as a function of the reversed magnetic field. (b)Representative AMR curve for chirality of CW (solid line) and CCW (dottedline) during sweep from 150 Oe to 190 Oe and the C-state domain structure
after the annihilation field.

ferromagnetic disk, the chirality should be randomized
because of its symmetric shape, this result indicates that theenergy barrier for the formation of the vortex from the bottom side is reduced. To explain this, we consider the influence of slight geometrical asymmetry due to the surface
cleaning. As explained above, the surface of the Py electrode
underneath the Cu electrode was cleaned by Ar ion milling.
This geometrical modification reduces the formation energyof the vortex from the bottom side although the etched depth
is less than 2 nm, about 5% of the total thickness. The reasons for the field dependence of the probability may be due
to a residual domain structure, namely, C-shaped domain
structure shown in the inset of Fig. 4(b), just after the annihilation. The domain structure of the ferromagnetic disk is
known to remain the curling feature when the external magnetic field is not sufficiently large for the saturation of themagnetization, similarly to the situation just before nucleation. This remaining domain structure tends to form the vortex from the top side. Thus, 50% probability around theannihilation field can be understood by the competition
between the non-saturated and the geometrical asymmetriceffects.
To obtain further evidence for the advantage of Py disk
with asymmetrically configured electrodes, we also fabricated another AMR device consisting of a Py disk with the
Cu electrodes located in the central position. Here, thedimension of the ferromagnetic disk is exactly same as thatin the previous experiment. Since the Cu electrodes arelocated in the center of the disk, the vortex formation fromboth the top and bottom sides can be observed as a sudden
resistance change with the same magnitude. Therefore, thechirality cannot be distinguished from the AMR measurement. To confirm this scenario, we evaluate the AMR curveunder the field sweep from 135 Oe to 190 Oe. Since theannihilation field is 130 Oe, we expect that the domain
structure is not fully saturated at 135 Oe. Figure 5 shows a

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082403-4

Cui, Hu, and Kimura

Appl. Phys. Lett. 105, 082403 (2014)

characterization of magnetic vortex and its future spintronic
application.
This work was partially supported by KAKENHI(26630162), CANON Foundation and CREST. The authors(X.C. and S.H.) acknowledge the China Scholarship Council
for Young Scientists.

1

FIG. 5. Longitudinal AMR curve of the Py circular disk measured by thesymmetrically configured electrode. Since the reversed magnetic field is
close to the annihilation field, the position of the vortex nucleation may bereversed in each sweep.

representative longitudinal AMR curve under the fieldsweep. The observed AMR curve for each sweep is almost
symmetric with respect to the field sweep direction.
Importantly, the large resistance jump at 10 Oe due to thevortex nucleation is clearly observed in both sweep. As mentioned above, the C-state domain structure tends to maintainthe chirality in the reversed field sweep, the vortex nucleation side in the forward and backward sweep should be opposite each other. However, the observed AMR curve shows
the same feature during 10-time repetition of the measurements. This is consistent with the above expectation that thevortex nucleation from the top and bottom sides provides thesame contribution in the AMR. Thus, we cannot distinguishthe vortex chirality from the AMR curve measured by thesymmetrically configured electrode.
In short, we have studied the AMR curve for a single Py
circular disk measured by nonmagnetic Cu electrodes withthe resistivity much lower than that for the Py disk. We
showed that the asymmetrically configured electrode makes
it possible to distinguish the vortex chirality in the relatively
simple way. By extending this simple detection method, thevortex chirality in the AMR device is found to depend on themagnitude of the maximum magnetic field. The importanceof the asymmetric configuration of the electrode is examinedby evaluating the AMR device with the symmetrically configured electrodes. This simple and reliable method for thechirality detection may accelerate the development on the

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