Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 5986–5990
Part 1, No. 10, October 2002
#2002 The Japan Society of Applied Physics

Synthesis of Bi–Lu-Substituted Iron Garnet Films for Visualization of Magnetic Fluxin High-Tc Superconductors
Nobuyasu A DACHI, Keiji O BATA, Takashi O KUDA, Takato M ACHI1 and Naoki K OSHIZUKA1
Department of Material Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
1
Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan(Received April 10, 2002; accepted for publication July 2, 2002)

In order to visualize the magnetic ﬂux that penetrates into high-Tc superconductors with high spatial resolution, epitaxial
growth of the in-plane magnetic Bi–Lu-substituted iron garnet ﬁlms was investigated. The ratio of increments of Faraday
rotation to magnetic ﬁeld increased to 13 deg/(cmOe) by Bi and Ga substitutions. The as-grown ﬁlms showed aperpendicular magnetic anisotropy. The appropriate conditions for annealing treatments that change the perpendicularmagnetic anisotropy into an in-plane magnetic anisotropy were investigated. Then, the annealed (BiLu)3 (FeGa)5 O12 ﬁlm
clearly showed a high resolution image of the magnetic ﬂux that penetrates into the NdBa2 Cu3 O7 high-Tc superconductor attemperatures between 5 K and 100 K, and the behavior of the magnetic ﬂux penetrating the microcracks less than 5 m inwidth was successfully captured; the magnetic ﬂux depends on external magnetic ﬁeld and temperature.
[DOI: 10.1143/JJAP.41.5986]
KEYWORDS: magnetooptical imaging, magnetic flux, magnetic garnet, LPE, in-plane magnetic anisotropy, high-Tc superconductor

1.

Introduction

For practical applications of high-Tc superconductors, it isimportant to characterize the behavior of magnetic ﬂux thatpenetrates superconducting materials in applied magnetic
ﬁelds. One of the most attractive characterization techniques
is visualization of magnetic ﬂux using magnetooptical (MO)eﬀects. High-contrast observations of magnetic ﬂux density
using a magnetic garnet ﬁlm have been reported in the lastten years.1–3) The use of magnetic garnet ﬁlms has
advantages over the magnetic ﬂux observation in high-Tc
superconductors because of their (1) high Curie temperature
Tc above 550 K and (2) large Faraday eﬀects which givehigh contrast to diﬀerent magnetic ﬂux densities. These
properties of magnetic garnet ﬁlms enable observation of awide dynamic range of the magnetic ﬂuxes that penetrate
into high-Tc superconductors in comparison with theobservation using EuSe (Tc ¼ 5 K) or EuO (Tc ¼ 69 K).
Thin ﬁlms with in-plane magnetic anisotropy enable highresolution imaging of magnetic ﬂux predicted to be less than
1 m. However, it is diﬃcult to synthesize a complete inplane magnetic garnet ﬁlm because the growth-induced
magnetic anisotropy and crystalline magnetic anisotropy
easily generate perpendicular magnetic domains. Yokoyama
et al. reported for the ﬁrst time the behavior a magnetic ﬂux
that penetrated into YBa3 Cu3 Ox high-Tc superconductor
using Bi-substituted YIG, a perpendicular magnetic ﬁlm.1) In
this case, the magnetic domain wall limits the spatial
resolution to 10–30 m. There are several reported observations on the behavior of dynamical magnetic ﬂux in high-Tc
superconductors using in-plane magnetic garnet ﬁlms in thelast 10 years.4–8) Recently, Johansen et al. have reported theobservation of ﬂux patterns in MgB2 ﬁlms by the MO
imaging technique.8) In these reports, the ﬁlms used formagnetic ﬂux observation were prepared by the liquid phase
epitaxy (LPE) technique. For the optical devices, the LPE
technique has great advantages over conventional techniquesin achieving two dimensional crystal growth and an opticallyE-mail address: nadachi@mse.nitech.ac.jp

smooth surface of the ﬁlm. In addition, various growthparameters such as melt composition, growth temperatureand rotation speed enable controls of the magnetic andmagnetooptical properties of ﬁlms.9) In order to fabricate inplane magnetic ﬁlms, we recently reported the in-planemagnetic anisotropy of the Nd- and Pr-substituted irongarnet ﬁlms prepared by the LPE technique.7) Nd- and Prsubstituted ﬁlms exhibited a negative growth-induced
magnetic anisotropy which leads to an in-plane magnetic
anisotropy.10) We utilized the growth-induced magnetic
anisotropy for the in-plane magnetic ﬁlm and tried to prepare
a ﬁlm with large amounts of Pr and Nd ions. In such a ﬁlm,
the energy of the growth-induced magnetic anisotropy is
suﬃciently large at room temperature in comparison withother magnetic anisotropies such as the crystalline magnetic
anisotropy and stress-induced magnetic anisotropy. However, with decreasing temperature, the crystalline magnetic
anisotropy energy due to increment of rare-earth ions.11)
This increased anisotropy energy may compete with growthinduced magnetic anisotropy, which causes non-linear
magnetization curve. We have observed steplike magnetization curve for the (NdBiLu)3 Fe5 O12 ﬁlm which was grownby LPE technique (see Fig. 1). At room temperature, themagnetization of the ﬁlm linearly changes up to saturation in
proportion to the magnetic ﬁeld; however, the ﬁlm shows asteplike magnetization curve at 77 K. At the present stage,
the origin of the anomaly of the magnetization curve has not
yet been clariﬁed in detail. One of the possible reasons isthat the increment of the magnetic moment of rare-earth ionsaﬀects the magnetic anisotropy with decreasing temperature.
In order to avoid the inﬂuence of the crystalline magnetic
anisotropy due to rare-earth ions, we chose the synthesis of
the Nd- and Pr-ion-free magnetic garnet ﬁlm,
(BiLu)3 (FeGa)5 O12 (BLIG). Lu3þ (4f14 ) is magneticallyinactive. Bi substitution increases the degree of Faraday
rotation. The dilution of Fe ions with Ga ions results in a
reduction of the magnetic ﬁeld to saturate magnetization,
which increases the rate of increase in the degree of Faraday
rotation against the magnetic ﬁeld. As-grown BLIG ﬁlms
show perpendicular magnetic anisotropy due to positive

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Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 10

N. ADACHI et al.

4 M (kG)

2(BiNdLu)3Fe5O1277 K
1
H plane H
0

plane

-1
-2
-4

-2

0

2

4

H (kOe)Fig. 1. Magnetic hysteresis curve for (BiNdLu)3 Fe5 O12 ﬁlm prepared by
the LPE technique. Measurements were performed at 80 K. The
magnetization curve is steplike.

growth-induced magnetic anisotropy. We expect that shapemagnetic anisotropy causes the in-plane magnetic anisotropyof the ﬁlm after removing the growth-induced anisotropy. In
this paper, we report the detailed procedure for synthesizing
the in-plane magnetic ﬁlm of BLIG.
2.

Experimental

The ﬁlms were grown on Gd3 Ga5 O12 (111) substrates(as ¼ 12:383 A). The melt compositions are summarized inTable I. Bi2 O3 was used as the ﬂux for BLIG. The
preparation conditions are summarized in Table II. The
substrate was horizontally dipped into the melt and wasrotated at 100 rpm. The ﬂux remains on the substrate when
the substrate is extracted from the melts. In order to remove
the ﬂux residue on the ﬁlm surface, the substrate was rotated
at 500 rpm above the melt for 30 s. The ﬁlms were annealedin air at temperatures between 850 and 1300 C. The
temperature was increased up to the annealing temperature
Table I. Composition of the melt (mol%) for BLIG ﬁlm.
Melt

BLIG

Bi2 O383.00

Fe2 O3

13.99

Lu2 O3

1.84

Ga2 O31.17

5987

at a rate of 5 C/min in air and was kept for 2 h, then it was
decreased at a rate of 5 C/min. The lattice mismatch (as )was determined by X-ray diﬀraction analysis. The compositions of the ﬁlms were analyzed using an energy dispersive
X-ray spectrometer (EDX). Faraday rotation was measured
at a wavelength of 670 nm. The magnetization was measured
with a vibrating sample magnetometer (VSM). The FMR
was measured in the X-band microwave region. A polarized
microscope was used for the observation of the magnetic
ﬂux that penetrated the high-Tc superconductor. We used a
NdBa2 Cu3 O7 (Nd123) specimen in this experiment. The
specimen having Tc  94 K, Jc  20 kA/cm2 (at 77 K in
0 Oe) was prepared by the self-ﬂux method.12) The 1:2
1:0  0:3 mm3 specimen, where c-plane was faced below thegarnet, was mounted on the cold ﬁnger of a helium gas ﬂow
type optical cryostat and the temperature was varied from
5 K to 300 K. The ﬁlm is grown on both sides of thesubstrate. For the observation of magnetic ﬂux, one side of
the ﬁlm is polished and removed, then the other side iscoated with gold for reﬂecting light. The gold-coated sides
contacted with the specimen. The magnetic ﬁeld was applied
up to 8 kOe.
3.

Results and Analysis

The saturated Faraday rotation angle F of the BLIG ﬁlm
was 13  103 deg/cm, which did not change before andafter annealing treatments. The increment ratio of the F of
the annealed ﬁlms against the magnetic ﬁeld was 13 deg/(cmOe).
As-grown BLIG ﬁlms showed a perpendicular magnetic
anisotropy and a labyrinth domain structure, as observed
under a polarized microscope [see Fig. 2(a)]. Annealing
treatments changed the magnetic anisotropy of the ﬁlm. The
labyrinth structure disappeared with annealing above 900 C
for 1 h [see Fig. 2(b)]. Annealing above 1000 C induced
grain nucleation, which resulted in a rough surface. Thus, theoptimal annealing temperature was determined to be 930 C.
Figure 3 shows the magnetic hysteresis loops of the asgrown and annealed BLIG ﬁlms. The magnetization of theannealed ﬁlm easily saturates below 200 Oe in the magnetic
ﬁeld parallel to the ﬁlm, which indicates that the magnetic
anisotropy of the ﬁlm changed into an in-plane magnetic
anisotropy after the annealing treatment. From the saturation

Table II. Chemical analysis data and growth and annealing conditions for
Bi–Lu-substituted iron garnet ﬁlms.
As-grown

Annealed

Lattice constant a (A)12.371

Lattice mismatch a (A)

0.002

Thickness (m)

10.4

½Bi (f./u.)½Ga (f./u.)

0.68
0.96

0.65
0.97

Saturation temp. ( C)

969—

Growth temp. ( C)

824—

Annealing temp. ( C)—

930

Fig. 2. Micrograph of the domain structure of the BLIG ﬁlm. The
labyrinth pattern is clearly observed in (a) as-grown ﬁlm and the domain
pattern disappears in (b) the ﬁlm annealed at 930 C.

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 10

200annealed0

as-grown

-200
-400
-2

-1

0

1

2

H (kOe)Fig. 3. Magnetic hysteresis loops of as-grown and annealed BLIG ﬁlms
with an applied magnetic ﬁeld parallel to the plane at room temperature.
It is clear that the annealed ﬁlm shows in-plane magnetic anisotropy.

magnetization and ferromagnetic resonance ﬁelds, theuniaxial magnetic anisotropy energy Ku of the as-grownﬁlm at room temperature was estimated to be þ6:1
103 erg/cm3 using
H A MS
5
Ku ¼
K1 ;
þ
18
2
2
HA ¼ ðHk  H? Þ þ 4MS ;3
where K1 is the cubic crystalline anisotropy. The value of K1(¼ 2:0  103 erg/cm3 ) for Y3x Bix Fe5y Gay O12 (x ¼ 0:7,
y ¼ 1) is used13,14) on the assumption that there is noremarkable diﬀerence in eﬀect between Y and Lu. HA is theanisotropy ﬁeld, Hk and H? are the parallel and perpendicular resonance ﬁelds. The stress-induced anisotropy Ku ðÞis estimated to be 7:14  102 erg/cm3 using
3Ea111 ;
Ku ðÞ ¼ 
2 1  111 a

15

F(

where E and 111 , are the Young modulus (¼ 2:05
1011 erg/cm3 ) and Poisson constant (¼ 0:30) of pureY3 Fe5 O12 , respectively.14) 111 is the magnetostriction
constant of the [111]-oriented ﬁlm (1:34  106 ).13,14)
Ku of the annealed ﬁlm at room temperature wasdetermined to be 11:4  103 erg/cm3 . The negative sign
conﬁrms the in-plane magnetic anisotropy of the ﬁlm. The
magnetic properties of the as-grown and annealed ﬁlms are
summarized in Table III.
The change of the magnetic anisotropy can be considered
as follows; the magnetic anisotropies of the ﬁlm consist of

the growth-induced magnetic anisotropy, stress-induced
anisotropy, crystalline magnetic anisotropy and shapemagnetic anisotropy. The energies of these anisotropies
except for the growth-induced anisotropy are eﬀective forthe in-plane magnetic anisotropy of the ﬁlm. For the case of
the as-grown ﬁlm, the dominant magnetic anisotropy is
considered to be the growth-induced magnetic anisotropy.
After the growth-induced anisotropy was removed byannealing treatments, other magnetic anisotropy energies
caused the change from the perpendicular magnetic
anisotropy to the in-plane magnetic anisotropy.
With decreasing temperature, Ku increases. At a temperature of 78 K, Ku of the as-grown BLIG was þ12:9  103 erg/
cm3 and that of the annealed ﬁlm was þ5:9  103 erg/cm3 .
According to the study by Hansen et al., the uniaxial
anisotropy of Lu–Bi ﬁlms increases with decreasing
temperature due to the increments of growth-induced
anisotropy.15) In our case, the growth-induced anisotropy
may not be completely removed with annealing, which
probably results in the increment of Ku . The sign of Kuchanged to positive at 78 K, however, the anisotropy ﬁeld of
2Ku =Ms is smaller than the demagnetizing ﬁeld of Ms =0(0 is permeability of vacuum), which indicates that the ﬁlm
maintains the in-plane magnetic anisotropy. In comparison
with the increase of the saturation magnetization at low
temperature, the shift of the ferromagnetic resonance ﬁeld issmall, which may lead to increasing Ku .
The hysteresis loop of the Faraday rotation at 635 nm isshown in Fig. 4. The saturation magnetic ﬁeld was about1000 Oe and the ratio of increments of Faraday rotation
against the magnetic ﬁeld was 13 deg/(cmOe) for theannealed ﬁlm.
By using the annealed BLIG ﬁlm, the images of magnetic
ﬂux that penetrated into Nd123 single crystal weresuccessfully visualized with high spatial resolution [seeFigs. 5(a)–5(f)]. The rectangular Nd123 single crystal wascooled to 5 K in a zero magnetic ﬁeld. Then, a magnetic ﬁeld
was applied perpendicularly to the (010) plane of thespecimen up to 8 kOe. With increasing magnetic ﬁeld, the

3

R. T.
H plane400

4 M (G)

N. ADACHI et al.

10 deg/cm)

5988

R. T.

10
5

635 nmas-grown
annealed

0-5-10Table III. Magnetic properties of BLIG as-grown and annealed ﬁlms.
As-grown
4Ms (G)-15

Annealed

R.T.

77 K

R.T.

77 K

498

705

475

650

Hk plane (Oe)

3446

3304

3002

3246

H? plane (Oe)Ku (103 erg/cm3 )

3560
7.3

3646
12.9

4577
11:4

3847
5.9

‘‘Hk plane (Oe)’’ and ‘‘H? plane (Oe)’’ denote the FMR resonance ﬁelds
in the the parallel and perpendicular magnetic ﬁelds, respectively.

-2

-1

0

1

2

H (kOe)Fig. 4. Faraday hysteresis loops of as-grown and annealed BLIG ﬁlms at awavelength of 635 nm. The saturation magnetic ﬁeld of the annealed ﬁlm
is about 1000 Oe and the ratio of the increments of the Faraday rotation
against the magnetic ﬁeld is 13 deg/(cmOe).

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 10

N. ADACHI et al.

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Intensity (arb. units.)

Fig. 5. Images of the magnetic ﬂux that penetrated into the single-crystal superconductor Nd123 observed at 5 K using the annealed
BLIG ﬁlm. The garnet ﬁlm was placed onto the ac-plane of the specimen and the magnetic ﬁeld was applied after zero-ﬁeld cooling[see (a), (b) and (c)], then the magnetic ﬁeld returned to zero (d).

0200

400 600
H (Oe)

800 1000

Fig. 6. Light intensity transmitted in the garnet ﬁlm vs applied magnetic
ﬁeld. Solid circles are experimental results and solid line represents theﬁtting curve expressed by I ¼ I0  A cos2 F ðHÞ. Light intensity seems tosaturate above 500 Oe due to the charge saturation of CCD detector.

area of the bright part of the specimen increased. The brightpart of the specimen corresponds to the area where magnetic
ﬂux penetrated the specimen. The dark part is the area where
magnetic ﬂux is excluded due to the Meissner eﬀect. Whenthe applied magnetic ﬁeld was decreased to zero from 8 kOe,
the magnetic ﬂux remained at the edge and around the cracksof the specimen [see Fig. 5(d)]. The spatial resolution isestimated to be about 1 m in these images, which isdiﬃcult to achieve by using perpendicular magnetic ﬁlms.
The ﬁeld dependence of the intensity I of the transmission
light through the garnet ﬁlm is shown in Fig. 6. The intensity
at zero magnetic ﬁeld is considered to be the background
intensity at exposure time of 3 s. The solid line indicatesexperimental results and the dashed line indicates the ﬁtting
curve using I ¼ I0  A cos2 F ðHÞ, where I0 and A are theinitial light intensity and ﬁtting coeﬃcient, respectively. The
light intensity of the experimental result seems to besaturated above H ¼ 600 Oe, which is due to the charge
saturation of the CCD detector. Taking this relation into
account, magnetic ﬂux density B that penetrated theFig. 7. Temperature dependence of the remanent magnetic ﬂux in theNd123 specimen. With increasing temperature, the penetrated magnetic
ﬂux around the edge of the specimen decreases and it disappears above
100 K.

specimen was estimated as follows: on the specimen, theapplied magnetic ﬁeld Hext is modiﬁed by the shielding ﬁeld
Hshield , and the ﬁlm picks up the modiﬁed Hmod ¼ Hext þ
Hshield correspond to the penetrated magnetic ﬂux density B(¼ Hmod ). Thus, the magnetic ﬂux density that penetrated
around the center of the specimen in H ¼ 3000 Oe [see Fig.
5(d)] can be estimated to be approximately B ¼ 400 G. The
magnetic ﬂux that penetrated around the cracks outside thespecimen is expected to be greater than B ¼ 600 G becauseof the saturation of the CCD detector.
The temperature dependence of the trapped magnetic ﬂux
in zero magnetic ﬁeld is shown in Fig. 7. The brightness ofthe edge of the specimen decreases with increasing
temperature and the trapped magnetic ﬂux was completely
excluded at approximately 100 K.

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4.

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 10

N. ADACHI et al.

Conclusions

The (BiLu)3 (FeGa)5 O12 ﬁlms were synthesized by the
LPE technique. Annealing of the as-grown perpendicularmagnetic ﬁlm above 900 C remarkably reduced the size ofthe perpendicular magnetic domain and the perpendicularmagnetic anisotropy of the ﬁlm changed to the in-planemagnetic anisotropy (Ku changed from þ6:1  103 erg/cm3
to 11:4  103 erg/cm3 at room temperature.) By using thisﬁlm, the visualization of the magnetic ﬂux with a spatial
resolution of approximately 1 m revealed the behavior of
the penetrated magnetic ﬂux in the high-Tc superconductorin the temperature range between 5 K and 100 K.
Acknowledgements
The authors are grateful to Mr. Sakai and other staﬀ of theInstitute of Molecular Science for SQUID measurement.

4)

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