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Vol.42 No.1
JournalJournal of the Magnetics Society of Japan
Electronic Journal URL: https://www.jstage.jst.go.jp/browse/msjmag

2018

Hard and Soft Magnetic Materials
Optimum Conditions for Synthesizing Fe Substituted Hibonite
発行（公社）日本磁気学会 〠101‒0052 東京都千代田区神田小川町2丁目8番地
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印刷（株）国 際 文 献 社 〠169‒0075 東京都新宿区高田馬場3の8の8☎(03)3362-9741

H. Nagumo, K. Kakizaki, and K. Kamishima…01

Measurement Technique, High-Frequency Devices
AC Magnetic Field Imaging of Perpendicular Magnetic Write Head without Image Distortion
on Alternating Magnetic Force Microscopy using a Cone-Shape FePt-coated Tip

広告◇計測エンジニアリングシステム株式会社

S. Yoshimura, F. Zheng, S. Yasui, G. Egawa, and H. Saito◇ネオアーク株式会社◇東英工業株式会社

…05

42

HP: http://www.magnetics.jp/ e-mail: msj@bj.wakwak.com
Electronic Journal: http://www.jstage.jst.go.jp/browse/msjmag

1

2018

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Journal of the Magnetics Society of Japan
Vol. 42, No. 1
Electronic Journal URL: https://www.jstage.jst.go.jp/browse/msjmag

CONTENTS
Hard and Soft Magnetic Materials
Optimum Conditions for Synthesizing Fe Substituted Hibonite……………………………………………………………… H. Nagumo, K. Kakizaki, and K. Kamishima

1

Measurement Technique, High-Frequency Devices
AC Magnetic Field Imaging of Perpendicular Magnetic Write Head without Image Distortion
on Alternating Magnetic Force Microscopy using a Cone-Shape FePt-coated Tip……………………………………………… S. Yoshimura, F. Zheng, S. Yasui, G. Egawa, and H. Saito

Board of Directors of The Magnetics Society of Japan
President:

K. Takanashi

Vice President:

K. Nakagawa, S. Nakamura

Director, General Affairs:

Y. Miyamoto, K. Niiduma

Director, Treasurer:

K. Aoshima, K. Ishiyama

Director, Planning:

Y. Saito, S. Nakagawa

Director, Editorial:

K. Kobayashi, T. Ono

Director, Public Relations:

H. Itoh, S. Greaves

Director, International Affairs:

Y. Takemura, M. Nakano

Auditor:Y. Suzuki, R. Nakatani

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J. Magn. Soc. Jpn., 42, 1-4 (2018)<Paper>Optimum conditions for synthesizing Fe substituted hibonite
H. Nagumo, K. Kakizaki, and K. Kamishima

Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Saitama 338―0825, Japan

We investigated the synthesis conditions and magnetic properties of Fe substituted hibonite with initial
compositions of CaAlxFey xO19 (1 ≤ x ≤ 3, 5 ≤ y ≤ 9) and CaAlxFe8-xO19 (0.5 ≤ x ≤ 1.6) sintered at 1200–1300ºC. The
optimum conditions for synthesizing the best magnetic hibonite were found to be the initial composition of Ca:Al:Fe =1:0.6:7.4 and the sintering temperature of 1250ºC. The best magnetic hibonite was magnetized at 75.0 A·m2/kg at T =5 K and µ0H = 7 T. This magnetic moment can be interpreted with a model of the collinear magnetic structure. The
Curie temperature of the best magnetic hibonite was 330ºC, which was the highest among those of iron-substituted
hibonite samples.
−δ

−

−δ

Keywords: M-type, hexaferrite

1. Introduction
Ca-based M-type ferrite has been attracting a lot ofinterest because the other alkaline-earth-based M-typeferrites (Ba, Sr)Fe12O19 have been used as a permanent
magnet for a long time.1-3) In spite that pure M-type
CaFe12O19 phase does not exist in the CaO-Fe2O3
diagram,4-6) the M-type phase becomes stable with theaddition of a rare-earth element of lanthanum to
CaFe12O19.7,8) The optimum synthesis condition of the(Ca,La)-based M-type ferrite with the highest
magnetization was recently clarified.9)
Resources of rare-earth elements are, however,limited in comparison with those of the other elementsof calcium, iron, and oxygen. Therefore, it is desirable to
avoid the use of rare-earth elements in the material.
On the other hand, the crystal structure of M-typeferrite is similar to that of hibonite (CaAl12O19).
Therefore, instead of lanthanum, aluminum canstabilize the M-type structure even with iron elements.
This approach is consistent with “element strategy”
because aluminum, calcium, iron, and oxygen areabundant elements in Earth's crust.10)
We previously reported the study of synthesis andmagnetic properties of Fe substituted hibonite.11) In thisreport, we changed initial composition from
CaAl12-xFexO19 to CaAl10-xFexO19- in order to prevent theexcess of α−Fe2O3 and to improve the magnetic
properties. But it was insufficient to optimize synthesis
conditions because α-Fe2O3 still remained in themagnetic samples of CaAl10 xFexO19 .
In this study, we have investigated the optimum
synthesis conditions and magnetic properties of Fe
substituted hibonite in order to produce arare-earth-free Ca-based ferromagnet.

materials. First, Al2O3 powder was heated at 500ºC for
an hour in order to remove water molecules on thematerial. The starting materials were mixed in thedesired proportions of CaAlxFey xO19 (1 ≤ x ≤ 3, 5 ≤ y ≤10) and CaAl8 xFexO19 (6.4 ≤ x ≤ 7.5) in a ball-milling
pot for 24 h. The mixed powder was pressed into a disk
shape. The disk was pre-sintered in air at 900ºC for 5 h.
The sintered sample was pounded in a mortar and thenground into fine powder using a planetary ball mill for
10 minutes at 1100 rpm (Fritsch, P-7 Premium line).
The powder was pressed into a disk shape again. The
disks were heated at 1100–1400ºC for 5 h. X-ray
diffraction (XRD) analysis with Cu-Kα radiation was
performed to characterize the crystalline samples. The
magnetic properties were measured by using a vibrating
sample magnetometer (Tamakawa TM-VSM2130HGC)
and superconducting quantum interference device(SQUID) magnetometers (Quantum Design MPMS-XL).
The chemical composition was examined throughenergy dispersive X-ray analysis (EDX).
−δ

−

−

−δ

δ

−

−δ

2. Experimental procedure
Samples were prepared by a conventional ceramic
method. We used CaCO3, Al2O3, and α-Fe2O3 as starting

Fig. 1 X-ray diffraction patterns of initial
composition samples of CaAlxFey xO19 (1 ≤ x ≤ 3,
y = 8) sintered at TS = 1300ºC.

Journal of the Magnetics Society of Japan Vol.42, No.1, 2018

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3. Results and discussion
Figure 1 shows the X-ray diffraction patterns of the
Ca:Al:Fe = 1:x:y−x (1 ≤ x ≤ 3, y = 8) composition samples
sintered at TS = 1300ºC. The sample with the initial
composition at x = 1 and y = 8 showed the single
hibonite phase. The diffraction pattern of hibonite isalmost identical to that of Sr-based M-type ferrite. The
other samples of x = 2 and 3 contained minority phasesof α-Fe2O3 and CaAl2O4 although the main phase wasthat of hibonite. This situation is same for otherstarting composition samples with Ca:Al:Fe = 1:x:y−x (1≤ x ≤ 3, 5 ≤ y ≤ 9). But the single hibonite phase wasobserved only for the sample at x = 1 and y = 8.
Figure 2 shows the room-temperature saturation
magnetization (MS) of initial composition samples of
CaAlxFey xO19 (1 ≤ x ≤ 3, 5 ≤ y ≤ 12) sintered at TS =1300ºC, where MS was estimated from themagnetization measurements at −2 T ≤ µ0H ≤ 2 T. The
sample at x = 1 and y = 8 had the highest Ms among
these samples. This is consistent with the fact that the
sample at x = 1 and y = 8 contained no minority phasesof α-Fe2O3 and CaAl2O4.
These experimental results of CaAlxFey xO19
strongly suggest that the suitable (Al+Fe)/Ca ratio is 8
for the formation of the iron-substituted magnetic
hibonite. This led us to the next experiments todetermine the optimum Al:Fe ratio so as to synthesize
the best magnetic hibonite with the highest
magnetization and the highest Curie temperature.
Figure 3 shows the X-ray diffraction patterns ofinitial composition samples of CaAlxFe8 xO19 (0.5 ≤ x ≤
−

−δ

−

−

−δ

−δ

Fig.
2
Room-temperaturesaturation
magnetization of initial composition samples of
CaAlxFey xO19 (1 ≤ x ≤ 3, 0 ≤ y ≤ 12) sintered at TS= 1300ºC.
−−δ1.6) sintered at TS = 1250ºC. The maximum ironsubstitution amount corresponds to xmax = 0.6 where thesecondary phase of hematite was not left. Here, thesintering temperature TS was decreased to 1250ºC from
1300ºC because the samples sintered at TS = 1300ºC
showed the secondary hematite phase at x < 1.0. HighTS can remove iron and calcium elements from thehibonite structure with high concentration of Fe3+,
possibly due to a low melting point of CaFe2O4. On theother hand, the single hibonite phase was not observed
for the samples at TS ≤ 1225ºC, suggesting the lowest
limit of TS for the formation of the single phase ofhibonite.

Fig. 3 X-ray diffraction patterns of initial composition samples of CaAlxFe8 xO19 (0.5 ≤ x ≤ 1.6) sintered at TS =1250ºC.
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Figure 4 shows the lattice constants of the hibonite
phase in initial composition samples of CaAlxFe8 xO19
(0.5 ≤ x ≤ 1.6) sintered at TS = 1250ºC. The lattice
constants of a and c were obtained by the use of Cohen’s
least square method.12) Both a and c became maximum
at x = 0.6, which is in agreement with theabove-mentioned xmax. The maximum lattice constants
at x = 0.6 implied that Fe3+ ions maximally replaced Al3+ions in the hibonite structure because the ionic radius of
an Fe3+ ion r[Fe3+] is larger than that of an Al3+ ionr[Al3+].
−−δ

Fig. 4 Lattice constants of initial composition
samples of CaAlxFe8-xO19- (0.5 ≤ x ≤ 1.6) sintered at TS= 1250ºC. Dotted lines are connected between lattice
constants of CaAl12O19 at x = 8 and those of SrFe12O19(instead of CaFe12O19 that does not exist) at x = 0,
where a conversion of x = 8x’ / 12 is employed for
CaAl12-x’Fex’O19 due to the change of (Al+Fe)/Ca from12 to 8.
δFigure 5 shows the room-temperature saturation
magnetization (MS), the Curie temperature (TC) and the!𝐼𝐼relative
intensity𝐼𝐼ofinitialcomposition samples of CaAlxFe8 xO19 (0.5 ≤ x ≤ 1.6).
With decreasing x from 1.6, TC was linearly increased
up to 330ºC at x = 0.6 and then slightly decreased at x =0.5. This x dependence of TC is comparable to thevariation of the lattice constants as shown in Fig. 4. The
highest TC strongly suggests that the sample with x =0.6 sintered at 1250ºC contains the maximum amount ofiron cations in the hibonite structure. Also, MS wasbasically increased with decreasing x from 1.6 to 0.6
except that MS deviated from this linear tendency at x =1.4 and 1.3 because of the formation of a minority phaseof α−Fe2O3. The sample with x = 0.6 had the maximum
MS of 44 A·m2/kg and the highest TC of 330ºC.
The EDX analysis of the sample with x = 0.6
provides the result of Ca:Al:Fe = 1.00±0.04 : 0.62±0.04 :7.81±0.23 (average value ± one sigma estimation). The
composition of Fe is slightly larger than the initialamount, which may be caused by low-melting-pointcalcium-iron oxides such as CaFe2O4 that can be eluted
off from the hibonite grain. The chemical formula of thissample can be expressed as CaAl0.6Fe7.8O13.6 where the−composition ratio of oxygen is estimated from the chargebalance with the concentration of Ca2+, Al3+, and Fe3+
cations. The form of this chemical formula is muchdifferent from the reference materials of hibonite(CaAl12O19) and M-type ferrite (SrFe12O19).
On the other hand, the x dependences of the lattice
constants are similar to the dotted lines connected
between lattice constants of CaAl12O19 and those of
SrFe12O19 as shown in Fig. 4. This fact suggests that theframework of the hibonite structure is maintained evenin the change of (Al+Fe)/Ca ratio. The crystal structure
consists of a close-packed framework of large ions (Ca2+and O2-) with intervening small ions (Al3+ and Fe3+).
Therefore, we can assume that the total number of largeions (Ca2+ and O2-) in the hibonite structure is kept at 20(= 1+19). Based on this assumption, the chemical
formula of the sample with x = 0.6 is estimated to be
Ca1.4Al0.8Fe10.7O18.6. The total number of Al3+ and Fe3+
cations becomes 11.5 that is smaller than 12.
This picture can be applied to the case of
Al-substituted (Ca,La)-based M-type ferrite. Shigemura
et al. recently reported that the Curie temperature of
Ca0.88La0.12Fe8.80Al1.01O15.8 was about 350ºC.13) Thischemicalformulacanbeconverted
toCa1.05La0.14Fe10.5Al1.20O18.8 on the assumption that the
total number of large ions (Ca2+, La3+, and O2-) is kept at
20. The estimated amounts of Fe3+ in chemical formulas−δFig. 5 Room-temperature saturation
magnetization (MS) and Curie temperature(TC) of initial composition samples of
CaAlxFe8-xO19- (0.5 ≤ x ≤ 1.6) sintered at TS= 1250ºC. Intensity ratio of α-Fe2O3 (102)plane to hibonite (203) plane is also shown.
δJournal of the Magnetics Society of Japan Vol.42, No.1, 2018

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INDEXaresimilarbetween
Ca1.4Al0.8Fe10.7O18.6
andCa1.05La0.14Fe10.5Al1.20O18.8, which may cause thesimilarity in the Curie temperature.

Batlle et al. pointed out that the substitution of
Co2+-Ti4+ for Fe3+ in BaFe12O19 can progressively breakthe collinearity of the magnetic structure of
BaFe12-2xCoxTixO19 at x > 0.7 although the overallbehavior remains ferrimagnetic.15) In our case, thecollinearity of our sample can be also weakened because
the estimated amount of the Fe3+ cations in our sample
is close to this threshold of 10.6 (= 12−2×0.7) and oursample does not contain magnetic Co2+ cations. The
high-field susceptibility of our sample is relatively highas shown in Fig. 6, suggesting the weakened
collinearity.
Therefore, our result is consistent with the previous
studies of M-type ferrite.
4. Conclusion

Fig. 6 Magnetization curves at T = 5 K
and 300 K for sample at x = 0.6 sintered at
TS = 1250ºC.
Figure 6 shows the magnetization curves at T = 5 K
and 300 K for the sample with x = 0.6 sintered at TS =1250ºC. Gradual increase of magnetization wasobserved above µ0H > 2 T. At the maximum external
magnetic field of µ0H = 7 T, the magnetizations at T = 5
K and 300 K are 75.0 A·m2/kg and 47.3 A·m2/kg,
respectively. The magnetization at T = 5 K corresponds
to the magnetic moment per formula unit of 13.1 µB/f.u.
We would like to discuss the magnetic structure of
the best magnetic hibonite in our study. The magnetic
structure of this iron-substituted hibonite can be similar
to that of the M-type ferrite because the crystal
structure of hibonite is similar to that of the M-typeferrite. The M-type ferrite has a collinear magnetic
structure where eight of the Fe3+ cations are
antiparallel to the other four Fe3+ cations. The totalmagnetic moment of the M-type ferrite is equivalent tofour Fe3+ cations (20 µB).
Here, the chemical formula of the best magnetic
hibonite is estimated to be Ca1.4Al0.8Fe10.7O18.6. Albanesedemonstrated that Al3+ cations in BaAlxFe12-xO19 tend tooccupy the up-spin 12k site at x ≤ 1.14) Also, theestimated chemical formula with the excess of Ca2+
cations suggests that Ca2+ replaces O2- (possibly in the
R-block), which can produce vacancy at a small cationsite due to the local electroneutrality (the Paulingprinciple). Therefore, we can assume that the magnetic
hibonite has the collinear magnetic structure where theup-spin-sites contain 0.8 Al3+ and 0.5 vacancy. Then, 6.7
Fe3+ cations are antiparallel to the other four Fe3+
cations. The total magnetic moment of the magnetic
hibonite becomes equivalent to 2.7 Fe3+ cations (13.5 µB).
This value is close to the experimental result of 13.1
µB/f.u. at T = 5 K and µ0H = 7 T.
The slight difference between the experimental
result and the estimated value can be caused by thedeviation from the collinear magnetic structure. In fact,

4

We have successfully synthesized Fe3+ substituted
hibonite-phase samples by a conventional ceramic
method. The optimum synthesis conditions of the best
magnetic hibonite are found to be the initial
composition of Ca:Al:Fe = 1:0.6:7.4 and the sintering
temperature of 1250ºC. The magnetization of the best
magnetic hibonite was 75.0 A·m2/kg at T = 5 K and µ0H= 7 T. This magnetic moment can be basicallyinterpreted with the model of the collinear magnetic
structure. The Curie temperature of the best magnetic
hibonite was 330ºC, which is the highest among those ofiron-substituted hibonite samples.
References
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
14)
15)

J. Smit and H. P. J. Wijn: Ferrites, pp. 182–184, pp. 193–
194 (Philips Technical Library, Netherlands 1959).
S. Chikazumi: Physics of Ferromagnetism, p. 210 (Oxford
University Press, Oxford, 2009).
Ü. Özgür, Y. Alivov, and H. Morkoc: J. Mater. Sci-Mater.
El., 20, 789 (2009).
B. S. Boyanov: J. Min. Met., 41 B, 67 (2005).
M. Hillert, M. Selleby, and B. Sundman: Metall. Trans. A,
21A, 2759 (1990).
B. Philips and A. Muan: J. Am. Ceram. Soc., 41, 445(1958).
N. Ichinose and K. Kurihara: J. Phys. Soc. Jpn., 18, 1700
(1963).
H. Yamamoto, T. Kawaguchi, and M. Nagakura: IEEE
Trans. Magn., 15, 1141 (1979).
M. Shigemura, K. Watanabe, K. Kakizaki, and K.
Kamishima: J. Magn. Soc. Jpn., 41, 10 (2017).
F. W. Clarke and H. S. Washington: The Composition of
the Earth's Crust, pp. 20–21 (United States Geological
Survey, Washington, 1924).
H. Nagumo, K. Watanabe, K. Kakizaki and K.
Kamishima: J. Magn. Soc. Jpn., 41, 20 (2017).
B. D. Cullity: Elements of X-ray Diffraction, 342(Addison-Wesley, 1967).
M. Shigemura, K. Kakizaki, and K. Kamishima: J. Magn.
Soc. Jpn., 41, 94 (2017).
G. Albanese: J. Magn. Magn. Mater., 147, 421 (1995).
X. Batlle, X. Obradors, J. Rodríguez-Carvajal, M. Pernet,
M. V. Cabañas, M. Vallet: J. Appl. Phys., 70, 1614 (1991).

Received Jun. 18, 2017;
Accepted Oct. 30, 2017

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J. Magn. Soc. Jpn., 42, 5-10 (2018)<Paper>

AC Magnetic Field Imaging of Perpendicular Magnetic Write Head
without Image Distortion on Alternating Magnetic Force Microscopyusing a Cone-Shape FePt-coated Tip
S. Yoshimura1, F. Zheng2,3, S. Yasui1, G. Egawa1, and H. Saito1

Research Center for Engineering Science, Graduate School of Engineering Science, Akita University,

1

Tegata Gakuen-machi 1-1, Akita 010-8502, Japan

Venture Business Laboratory, Akita University, Tegata Gakuen-machi 1-1, Akita 010-8502, Japan

2

School of Physics and Electronic-Electrical Engineering, Ningxia University, Yinchuan 750021, China

3

The AC magnetic field of a perpendicular magnetic write head having three surrounding shields was
successfully imaged without image distortion on our alternating magnetic force microscope (A-MFM) using a newly
developed cone-shape Si tip coated with an L10-FePt film. In contrast, a conventional quadrangular pyramidal Si tip
coated with the L10-FePt film showed a distortion of the AC magnetic field image for the same magnetic write head.
The image distortion depended on the spatial configuration between the tip and the write head. It is concluded that around magnetic symmetry of the cone-shape FePt-coated tip is most effective for taking a clear AC magnetic field
image of the perpendicular magnetic write head having three surrounding shields without distortion.
Key words: perpendicular magnetic write head, alternating magnetic force microscopy, image distortion, cone-shape
FePt-coated tip

1. Introduction
In perpendicular magnetic recording, the demandfor high magnetic recording density requires the super
performance of magnetic recording media andmagnetic write/read head. To achieve high recording
density, the write head design has to be optimized toachieve the large field magnitude and a high field
gradient in both the down-track and cross-track
directions as required. So far, most characterizationsof the magnetic field of write head have been done bytheoretical modeling1)-4). However, the simulation of
the exact working conditions of a write head in a drive
has encountered difficulty when using theoreticalmodeling. At the same time, the characterization of
write head has been done by experimental spin-standmeasurements. The results obtained from thisspin-stand measurement not only include the effect of
the real magnetic field distribution of the write head,but also the magnetic property and microstructure of
the recording media.
Magnetic force Microscope (MFM) is a powerfultool to understand the microscopic magnetic
domain/bit structures of high density magnetic
recording media and nanoscale magnetism5). The
maximum resolution of an MFM is approximately 10nm6). For this reason, MFM applications for thedevelopment of magnetic materials and magnetic
devices have received more and more attention overthe past ten years.
To detect and image an AC magnetic field, theconventional MFM has to employ a frequency which is
close to the mechanical resonant frequency of theMFM cantilever7),8). Because the cantilever acts as amechanical filter near its resonant frequency, thesignals at the frequencies that are not close to the
cantilever’s resonant frequency will not be picked up.
In our previous work, we have developed a new
MFM to image AC magnetic field with a wide
frequency range that is referred to as alternating
magnetic force microscopy (A-MFM)9). The A-MFM
uses a frequency modulation (FM) of the cantilever
oscillation by applying an AC magnetic field over it.
The A-MFM can measure the vertical component of an
AC magnetic field when the magnetization direction of
MFM tip is perpendicular to the sample surface.
Previously, we achieved high-resolution AC magnetic
field images for a perpendicular magnetic write head
having a one-side trailing shield by such A-MFM witha conventional quadrangular pyramidal FePt-coated
tip10),11). The L10-FePt film having a coercivity of morethan 10 kOe has been used for the MFM tip in thiscase, because the magnetic field from the write head isoverly strong to be able to change magnetization of an
MFM tip. The development of MFM tips coated withhard magnetic materials such as Fe-Pt12)-16), Fe-Pd16),17),
Co-Pt18)-22), and Sm-Co23) were reported by several
groups. However, the shape effect of the MFM tip withhard magnetic coatings has barely taken into account.
In the present study, we developed a new
FePt-coated tip with a special shape for thecharacterization of a perpendicular magnetic recording
head having three surrounding shields. The headdesign having three surrounding shields can generate
more focused magnetic field for high recording density.
The conventional quadrangular pyramidal shape tip
with FePt coating usually causes image distortion forthis type of head. The newly-developed tip was

Journal of the Magnetics Society of Japan Vol.42, No.1, 2018

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supposed to fix the issue with the image distortion of
the AC magnetic field. The image distortion is definedas the disaccord between the position, shape, andspatial symmetry of main pole in topographic image
and those of strong signal in amplitude image of
A-MFM. We analyzed the cause for the image
distortion of the AC magnetic field when using thequadrangular pyramidal-shape tip and found the way
to make the AC magnetic field successfully imaged
without distortion by using the developed FePt-coatedtip. In this paper, we will also show a necessity toincrease the coercivity of hard magnetic coating tocharacterize future perpendicular magnetic writehead.
2. Experimental Procedure
The A-MFM was built from a conventionalscanning probe microscope (JSPM-5400 (JEOL Ltd.)
and/or SPI3800N･SPA300HV (SII-NT Ltd.)). All of themeasurements were done in air atmosphere. The
cantilever was oscillated by using a piezoelectricelement. The value of the resonant frequency of thecantilever with the MFM tip was approximately 330
kHz. The oscillation frequency (fc) of the piezoelectricelement was set at 325 kHz which is close to theresonant frequency of the tip, and the value of Q wasaround 500.
A perpendicular magnetic write head having a
one-side trailing shield and a perpendicular magnetic
write head having three surrounding shields were
sampled for this work. The write head was driven by asinusoidal AC current with a zero-to-peak amplitude of
20 or 40 mA at the frequency (fm) of 100 Hz.
The AC magnetic field frequency modulated thecantilever resonant frequency. The cantileverdeflections were sensed by using laser beam deflection.
The AC magnetic field measurement was achieved by
the lift mode after topographic measurement. The lift
height was 10 nm. The amplitude and phase
information of the alternating force between thesample and the tip was extracted by using a lock-in
amplifier where the input signal was the frequency
demodulated signal of cantilever oscillation from aphase-locked loop (PLL) circuit and reference signal
was the frequency signal of fm from signal
generator8),9).
Both conventional high-coercivity MFM tip(SI-MF40-Hc, Nitto Optical Co. Ltd.) with aquadrangular pyramidal shape (DF-40, SII Co. Ltd.)and a 30 nm-thick L10-FePt coating, and a new
made-in-house MFM tip in cone-shape (SS-ISC, Team
Nanotec Co. Ltd.) coated with the same thickness
L10-FePt film were used in this work. The MFM tips
were magnetized to saturation along the tips axis
before use to make sure the magnetization direction of
the tips were vertical to the write head surface.

6

3. Results and Discussions
First, the quadrangular pyramidal FePt-coated tip
was used to take the amplitude and phase images of
the AC magnetic field of the two types of write heads.
Fig.1 shows SEM images of the quadrangularpyramidal Si tip. Fig.1 (a) is an image of entire tip,
Fig.1 (b) a magnified image of the vertex region of thetip, and Fig.1 (c) a top view of the vertex region of thetip.
The results for the head having a one-sided
trailing shield and the head having three surrounding
shields are shown in the topographic image in Fig.2 (a)and (d), respectively. Fig.2 (b) and (c) are theamplitude and the phase images of the AC magnetic
field for the head having a one-sided trailing shield,
respectively. Fig.2 (e) and (f) are the amplitude andphase images for the head having three surrounding
shields, respectively. In all measurements the AC
current was fixed at 20 mA.
For the head having a one-side trailing shield, theamplitude and the phase images of the AC magnetic
field are clearly observed without distortion. In Fig.2(b), the strong amplitude of the AC magnetic field(bright area) appears at the main pole position. In
addition, a relatively large field intensity is obtained at
the trailing shield position near the gap, and a very
low intensity of near-zero is obtained at the gap
position. In Fig.2 (c), the polarity of the field can beclearly observed as a binary image. The phasedifference between the dark area and bright area isapproximately 180°. If the dark area corresponds tothe in-phase magnetic field with respect to the headcurrent, the bright area corresponds to the field in the
opposite direction. As shown in this figure, the polarityof the perpendicular component of the AC magnetic
field at the main pole region and the trailing shieldregion of the head can be clearly distinguished. The AC
magnetic field images were clearly observed without
obvious distortion for the write head having a
one-sided trailing shield.
In contrast, the image of the strong amplitude of
the AC magnetic field (bright area) around the mainpole position becomes asymmetric on certain direction
(a)

(b)

(c)

1 m

10 m

1 m

Fig. 1 SEM images of quadrangular pyramidal Si
tip: (a) entire tip, (b) magnified image of vertex
region of the tip, and (c) top view of vertex region of
the tip. The dotted line in (b) and (c) indicates theridges from the 1st vertex to the 2nd vertex of thetip.

Journal of the Magnetics Society of Japan Vol.42, No.1, 2018

INDEX

Fig. 2 (a) and (d) are topographic images, (b) and(e) are amplitude images of the AC magnetic field,and (c) and (f) are phase images of the AC magnetic
field. (a), (b), and (c) are images for the write head
having a one-sided trailing shield, and (d), (e), and(f) are images for the write head having
surrounding shields on three sides.
as shown in Fig.2 (e).
In addition, a near-zerointensity is not observed at the gap position. In Fig.2(f), the polarity of the field observed around the mainpole position is also asymmetric in certain direction. A
polarity change of the field could not be observed at
the gap position.
The observations above hints at a distortion of
the amplitude and the phase images for a head having
three surrounding shields. To investigate the cause forthe distortion of the AC magnetic field images, theshape effect of the quadrangular pyramidal FePt tip
was taken into account. This tip has two vertices, andthe dot line in Fig.1 (b) and (c) indicates the ridge line
formed by the 1st vertex and the 2nd vertex of the tip.
This ridge line is one possible factor contributing to theimage distortion, the ambiguity at the gap position,and the asymmetry of the field images.
To clarify the influence of the ridge line of the tipon the AC magnetic field imaging, the spatialconfiguration between the quadrangular pyramidal
FePt-coated tip and the write head having threesurrounding shields was adjusted and imaged the AC
magnetic field images. Fig.3 (a), (d), and (g) show
schematics of the quadrangular pyramidal FePt-coated
tip and the spatial configurations against the write
head. The dotted line and bold line in (a), (d), and (g)indicate the outline and the ridge line formed by the
1st vertex and the 2nd vertex of the tip, respectively.
Fig.3 (b), (e), and (h) show the corresponding
amplitude images of the AC magnetic field, and Fig.3(c), (f), and (i) show the corresponding phase images of
the AC magnetic field for the write head. In the case of
Fig.3 (g), the direction of the ridge line (bold line) isparallel to the direction of the cross-track of the write
head. The (h) amplitude and (i) phase images of the AC
magnetic field extend toward the direction of theFig. 3 (a), (d), and (g) are the schematics of thequadrangular pyramidal FePt tip and its spatialconfiguration against the writing head, (b), (e), and(h) are the amplitude images, and (c), (f), and (i)are the phase images of the AC magnetic field forthe writing head having three surrounding shields.
cross-track of the write head. These results suggestthat the image distortion of the AC magnetic fielddepends on the spatial configuration between the tip
and the write head. Here, the (e) amplitude and (f)phase images of the AC magnetic field were alreadydiscussed in Fig.2 (e) and (f).
Self-magnetic charge of the FePt film happens at
the edge line of film surface. As a result, the magnetic
pole line is created at each ridgeline of the tip and thecenter part of the tip end. The center part of the tip
end mainly contributes to the signal for the AC
magnetic field image in direction perpendicular to theridge line of the tip, while the ridge lines of the tip
mainly generate the signal for the AC magnetic field
image in direction parallel to the ridge lines of the tip.
In order to reduce the influence of ridge pole line
on imaging AC magnetic field, a new tip having acone-shape was developed. Fig.4 shows SEM images of
the cone-shape Si tip. Fig.4 (a) is the image of theentire tip, (b) is the magnified image of the vertex of
the tip, and (c) is the top view of the vertex of the tip.
This tip has only one vertex with a round symmetry.
Within this cone-shape tip, the magnetic pole of FePt
film forms only at the end of the tip. To see theinfluence of the improved round symmetry of the tip
magnetics on the image distortion, the cone-shape
FePt-coated tip was used for the characterization of
the write head having three surrounding shields. Fig.5
show (a) a schematic of the cone-shape FePt-coated tip
and its spatial configuration against the write head, (b)the topographic image, and (c) the amplitude and (d)Journal of the Magnetics Society of Japan Vol.42, No.1, 2018

7

INDEX

(a)

(b)

(c)
vertex

1 m
10 m

1 m

Fig. 4 SEM images of the cone-shape Si tip: (a)image of entire tip, (b) magnified image of vertex
region of the tip, and (c) top view of vertex region of
the tip.
phase images of the AC magnetic field of the write
head having three surrounding shields. The dot line
and bold point in (a) indicate the outline and the end
point of the tip, respectively. The amplitude and thephase images of the AC magnetic field are clearlyobserved without image distortion. In Fig.5 (c), a
strong intensity, relatively large intensity, and very
low intensity of nearly zero can be seen at the mainpole position, the trailing shield position near the gap,
and the gap position, respectively. In Fig.5 (d), thepolarity of the field can be observed clearly. The phasedifference between the main pole and the threesurrounding shields is approximately 180°. It is clearthat the amplitude and phase images of the AC
magnetic field are not distorted for the write head
having three surrounding shields.
As described earlier, there was no image distortion
observed for the head having one-side trailing shieldby using the quadrangular pyramidal FePt-coated tip.
The magnetic field of the head having one-side trailingshield is not strong enough compared with that of thehead having three surrounding shields, and mainly
focuses at the 1st vertex region of the tip. Moreover,the shape of the quadrangular pyramidal tip aroundthe 1st vertex has a near-cone shape from the detailFig. 6 (a) and (b) are the amplitude images of AC
magnetic field for the write head having threesurrounding shields at the head driving current of
20 mA and 40 mA, respectively. (c) is the down
track line profile of amplitude signal of the whiteline shown in (a) and (b).
SEM observation. Therefore, the MFM measurementof the write head having one-side trailing shield byusing the quadrangular pyramidal FePt-coated tip isalmost same situations as that of the write head
having three surrounding shields by using the cone
shape FePt-coated tip. On the other hand, themagnetic field of the head having three surrounding
shields focuses not only at the 1st vertex region but
also the ridgeline of the quadrangular pyramidal tip.
This is why the AC magnetic field image of the write
head having three surrounding shields shows no
distortion when using the cone-shape FePt-coated tip.
This means that the MFM tip with round symmetry
for the magnetic pole charge is very important to
reduce image distortion when characterizing strong,
widely-spread magnetic field generated from a write
head. These results generally indicate that the MFM
tip with asymmetry for the magnetic charge is effective
to get a MFM image without distortion only for thecase of samples which generate the not so strong andwidely-spread magnetic field.
A sinusoidal AC current of 20 mA with azero-to-peak amplitude was used for the above A-MFM
characterization of the magnetic write head. This issmaller than that of the actual motion of magnetic
write head working in hard disk drive.
Fig.6 (a) and (b) show the amplitude images of AC
magnetic field taken by the cone-shape FePt-coated tip
with a film thickness of 40 nm for the write head
having three surrounding shields. In this run, thehead driving current of 20 mA and 40 mA were applied
respectively. Fig.6 (c) is the down track line profile ofamplitude signal of the white line shown in Fig.6 (a)20
0Fig. 5 (a) is a top view of the cone-shaped FePt tip,(b) is a topographic image, (c) is an amplitude
image, and (d) is a phase image of the AC magnetic
field for the write head with three surrounding
shields.

8

20

(a)

0

-20

-20

-40

-40

-60

-60

-80

-80

-100
329.3 329.4 329.5 329.6 329.7 329.8 329.9 330 330.1

Frequency (kHz)

-100

(b)

329.3 329.4 329.5 329.6 329.7 329.8 329.9 330

Frequency (kHz)Fig. 7 Spectra of the cantilever oscillation with thehead current of (a) 20 mA and (b) 40 mA.

Journal of the Magnetics Society of Japan Vol.42, No.1, 2018

INDEXand (b). The amplitude image (a) and its line profile forthe head current of 20 mA were clear with a highamplitude signal at the main pole position. In
comparison, the amplitude image (b) and its line
profile for the head current of 40 mA were unclear andvery low amplitude signal at the main pole position.
Especially, there is an amplitude signal of near-zerowith the head current of 40 mA at the position which
has the highest amplitude signal with head current of
20 mA. These indicate that the magnetization state of
the magnetic tip is different between the head currentsof 20 mA and 40 mA.
To understand the difference of signal intensity
withdifferenthead
running
currents,
themeasurement of frequency spectra of tip oscillation foreach head current was carried out by using spectrum
analyzer. Fig.7 (a) and (b) show the spectra of the tip
oscillation with the head running current of 20 mA and
40 mA, respectively. The oscillation frequency (fc) of
the tip was 329.71 kHz (on the head current of 20 mA)
and 329.64 kHz (on the head current of 40 mA), andthe frequency (fm) of the AC magnetic field 100 Hz. As
seen in the figures, the sideband spectra with thefrequency of fc±nfm (n: an integer), which is modulated
by AC magnetic field from head, are observed near the
baseband spectra of the cantilever oscillation with thefrequency of fc. The frequency of fc±fm is dominatingwhen coercivity of the magnetic component of tip ishigher than the AC magnetic field. And the frequencyof fc±2fm is dominating when coercivity of the magnetic
component of the tip is smaller than the AC magnetic
field. This is because, the spectrum intensity at fc±fm is
proportional to the gradient of magnetic fieldcomponent which is perpendicular to the sample
surface9), and this indicates that direction of tip
magnetization does not change when the fc±fm isdominant. In Fig.7 (a), the modulated frequency withhighest peak of the spectra was fc±fm. This indicatesthat hard magnetic behavior of the magnetic tip isdominant in the case of head current of 20 mA. By
comparison, the modulated frequency with highest
peak of the spectra was fc±2fm as shown in Fig.7 (b).
This indicates that soft magnetic behavior of themagnetic tip is dominant in the case of head current of
40. The reason for the low amplitude signal at themain pole position in Fig.6 (b) is due to the incrementof oscillation intensity at modulated frequency of thefc±2fm. In addition, the modulated frequencies of fc±nfm(n>2) were observed in both cases with different
intensities. This suggests that the magnetization
direction of the FePt tip sways in a non-linear way. It
is clear that the coercivity of the FePt coating is lower
than the magnetic field generated by the head with thecurrent of 40 mA. The magnetization direction of the
FePt tip is swayed by the magnetic field of the headwith the current of 20 mA although the coercivity of
FePt coating is larger than the head field. Therefore,
the increase of tip coercivity is necessary to takeaccurate MFM images of future magnetic write head.
We will describe the effect of increased tip coercivity ina future paper.
4. Conclusion
We analyzed the cause of the MFM image
distortion of the AC magnetic field for the magnetic
recording heads having 3 surrounding shields bycomparing a conventional quadrangular pyramidal
FePt-coated tip with our newly developed cone shape
FePt-coated tip. The cone shape FePt-coated tip makes
it able to image the AC magnetic field of perpendicularmagnetic write head having three surrounding shields
without image distortion.
In comparison to the quadrangular pyramidal
shape structure of the conventional FePt tip, thecone-shape FePt-coated tip and its round magnetic
symmetry are very effective in imaging the AC
magnetic field of the write head having threesurrounding shields without image distortion. In
addition, it is necessary to increase the coercivity ofhard magnetic coating in order to characterize the very
high magnetic field for the future magnetic write head.
Acknowledgements This work was supported by
JST/SENTAN.
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International Magnetics Conference (INTERMAG Europe2014), EG-6 (2014).

Received Nov. 23, 2016; Accepted Oct. 30, 2017

Journal of the Magnetics Society of Japan Vol.42, No.1, 2018

INDEX

Editorial Committee Members・Paper Committee Members
K. Kobayashi and T. Ono (Director), T. Kato, K. Koike and T. Taniyama (Secretary)
A. Fujita
H. GotoH. Hashino
S. Honda
S. Inui
S. Kasai
A. Kikitsu
H. Kikuchi
T. Kimura
T. Kubota
T. Nagahama
H. Naganuma
M. Naoe
M. Ohtake
N. PhamT. SatoT. SatoS. Seino
K. Sekiguchi
M. Sekino
Y. Shiratsuchi
M. Sonehara
T. Tanaka
K. Yamamoto
H. Yuasa
N. Adachi
N. Inaba
S. Kokado
T. Nakagawa
K. Tajima
K. Yamazaki

K. Bessho
S. Isogami
Y. Kota
H. Nakayama
M. Takezawa
S. Yoshimura

M. DoiK. KamataT. MakiT. Narita
T. Takura

T. DoiH. KatoE. Miyashita
D. Oyama
M. Tsunoda

Y. Kanai
K. Miura
Y. Sasayama
T. Shima

M. EndoK. KatoT. Morita
T. Saito
S. Yabukami

T. Hasegawa
T. KodaS. Muroga
R. Sugita
T. Yamamoto

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