APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 1
5 JANUARY 2004
Enhancement at low temperatures of the critical current density
for Au-coated MgB2 thin films
Eun-Mi Choi, Hyun-Sook Lee, Heon-Jung Kim, and Sung-Ik Lee
National Creative Research Initiative Center for Superconductivity, Department of Physics,
Pohang University of Science and Technology, Pohang, 790-784, Korea
Hyeong-Jin Kim
Material Science Laboratory, Korea Basic Science Institute, Daejeon, 305-333, Korea
W. N. Kang
Department of Physics, Pukyong National University, Pusan, 608-737, Korea
共Received 5 September 2003; accepted 11 November 2003兲
We measured the superconducting critical current densities (J c ) from the magnetization hysteresis
(M – H) loop for Au-coated MgB2 thin films. The purpose of this experiment was to determine
whether the vortex avalanche phenomenon which suppresses the J c for low temperature (T
⬍15 K) and low field (H⭐1000 Oe) could be cured by gold deposition. This avalanche, called flux
noise, has been a headache in applications of MgB2 thin films. Fortunately, the flux noise in the
M – H loop is suppressed with increasing Au-film thickness and finally disappears when the
thickness of the gold becomes 2.55 ␮m. We found a way to remove one obstacle for applications of
MgB2 thin films as an superconducting device. © 2004 American Institute of Physics.
关DOI: 10.1063/1.1637944兴
The recent discovery of the binary metallic MgB2 superconductor with T c ⫽39 K is very interesting for basic science
and applications. This transition temperature is much higher
than those of conventional metallic superconductors, such as
Nb3 Sn and Nb–Ti alloy used in superconducting magnets.
Also, films1 and wires2 have been successfully fabricated,
which is a good sign for technological applications. The relatively high upper critical field (Hc2 )(20– 30 T) of MgB2 3
and its extremely high critical current density J c (J c
⬃107 A/cm2 ), 4 especially in thin films, suggests that MgB2
should be a very important superconducting material. The
high J c is believed to be related to strongly linked nature
between grains4 and the high charge carrier density.5
Although MgB2 is much superior to conventional superconductors, it has one critical drawback called the vortex
avalanche phenomenon. In particular, for T⬍15 K and for
H⭐1000 Oe, magnetic vortex flux abruptly invades MgB2
thin films and reduces the J c . 6,7 This invasion is also called
vortex avalanche, vortex jump, or magnetic flux noise. This
phenomenon, which is scientifically very interesting, has
been studied vastly by using Magneto-Optics imaging
method.6 When first image of the magnetic invasion at low
temperature was observed, Johansen et al. found the dendritic growth of the flux structure, which was totally unexpected for conventional superconductors.6,8 This dendritic
structure had previously been observed on rare occasions
such as Nb thin films.9
Even though the growing patterns of dendrite is very
attractive to physicists, this phenomenon itself is a defect for
superconducting devices. The value of the J c is reduced
when the avalanche appears; therefore, for applications, finding a method to suppress this phenomenon is important. This
phenomenon is believed to be related to thermal instability.
How to understand this phenomenon? When a vortex invades
the superconductor, the vortex motion is accompanied by
heat dissipation. If the heat is not removed immediately, then
the pinning force is reduced along the paths of the vortices. It
also induces vortex motion. Eventually, a large-scale invasion of a depinned flux, called the vortex avalanche, occurs.
If thermal instability of superconductor is really the cause of
the avalanche, there should be a way to prevent this in MgB2
thin films. If it is cured, the problem of J c suppression can be
solved. For this purpose, we deposited gold films on top of
the MgB2 thin films.
In this research, we determined the effect of a gold coating on MgB2 thin film by measuring the magnetic hysteresis
and calculating the J c . We believe that the good thermal
conduction of the gold film helps the thermal stability of the
MgB2 thin film. When the thickness of the thin gold film was
increased, the suppression of the J c due to flux jump was
drastically reduced; thus, it was possible to eliminate the
vortex avalanche problem. Surprisingly, full recovery of the
J c was achieved at T⫽5 K when the thickness of the thin
gold film was about 2.55 ␮m, which was quite a bit thicker
than expected, being about 6 times thicker than the MgB2
thin film.
For this study, the MgB2 thin film was fabricated using a
two step method.1 Briefly, an amorphous boron thin film was
deposited on a Al2 O3 (1 1̄ 0 2) substrate at room temperature by using pulsed laser deposition. The B film was put into
a Nb tube with high-purity Mg 共99.9%兲, and the Nb tube was
then sealed in an Ar atmosphere. The heat treatment was
carried out at 900 °C. The rectangular sample with a size of
2.48⫻4.34 mm2 was chosen for the magnetic measurement.
The 490 nm thickness and the c-axis orientation were confirmed by using scanning electron microscopy 共SEM兲.10
Also, the thin film showed a sharp T c at 38.5 K, which was
observed by using a SQUID magnetometer 共Quantum De-
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© 2004 American Institute of Physics
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Appl. Phys. Lett., Vol. 84, No. 1, 5 January 2004
Choi et al.
83
FIG. 1. Zero field cooled magnetization at 10 Oe vs temperature (6⭐T
⭐40 K) of a bare MgB2 thin film 共circles兲 and a MgB2 thin film with a 2.55
␮m Au-film coating 共triangles兲.
sign, MPMSXL兲, and was very highly oriented to the c axis.
In this experiment, we first measured the magnetization
M – H loop of the pristine MgB2 thin film. Then, we added a
gold film on top of the MgB2 thin film and measured M – H
curves, in the field range of ⫺5 T⭐H⭐5 T for H 储 c axis at
temperatures of 5 and 10 K. This procedure was repeated
until the M – H curves does not change with thickness of
gold. For the Au deposition, we used an ion-deposition machine with a beam voltage of 900 V and a beam current of
1.5 mA/cm2 . The deposition rate was 5 nm/min. To make
sure that the thin film did not deteriorate during the gold
deposition, we measured the temperature dependence of dc
magnetization after each Au deposition.
Figure 1 shows the temperature dependent low-field
magnetic susceptibility for MgB2 thin films with and without
a thin gold film deposited. The low field magnetic susceptibility of the MgB2 thin film coated with a 2.55-␮m-thick Au
film was basically the same as that for the uncoated MgB2
thin film, which implied that the superconductivity did not
deteriorate during the gold deposition. The values of T c were
38.5 K, for all Au-film thicknesses up to 2.55 ␮m.
Figure 2 shows the M – H curves for various Au film
thicknesses at 共a兲 5 K (⫺2000 Oe⭐H⭐2000 Oe) and 共b兲 10
K (⫺1500 Oe⭐H⭐1500 Oe). As one can see in Fig. 2共a兲,
generally, the height of the M – H loop (⌬M ) increased with
the thickness of the Au film, which implies that the flux
avalanche was suppressed when the Au film was deposited.
Even for a thickness of 0.15 ␮m, the suppression was quite
dramatic. However, the additional suppression of the vortex
avalanche when the thickness was increased from 0.15 to
0.45 ␮m was less than that for the 0.15 ␮m deposition,
which implies that the rate of suppression does not increase
linearly with the thickness of the Au film. The suppression
basically stopped at T⫽5 K once the thickness of the gold
thin film had reached 2.55 ␮m.
The disappearance of the avalanche for magnetic fields
higher than 1000 Oe is also observed previously from magnetooptics image and the magnetic relaxation.6 – 8 The suppression of the J c shows only at low fields and low temperatures. For fields higher than 1000 Oe or temperatures higher
FIG. 2. Magnetization hysteresis (M – H) loop for different thicknesses of
Au films on MgB2 thin films 共a兲 at 5 K in the field range of ⫺2000⭐H
⭐2000 Oe and 共b兲 at 10 K in the field range of ⫺1500⭐H⭐1500 Oe. The
inset shows the full M – H loop.
than 15 K, the vortex avalanche does not appear; thus, the
metallic coating does not affect the M – H curves.
Figure 2共b兲 shows M – H loops at 10 K. The thicknesses
of the gold films are less than 1.05 ␮m. A reduction of J c is
also observed as with the case of T⫽5 K. At 10 K, the avalanche stops for a Au thickness of 1.05 ␮m, which is less
than half the thickness for T⫽5 K. However, since the vortex avalanche is weakened for bare MgB2 thin films at T
⫽10 K, reduction of the avalanche after gold deposition is
quite reasonable.
Figures 3共a兲 and 3共b兲 show the values of J c estimated
from the M – H loops by using the Bean’s critical state
model: J c ⫽30⌬M /r, where ⌬M is the height of the M – H
loop. We choose the effective sample size r as the radius of
the circle whose total area is the same as a sample size by
using ␲ r 2 ⫽2.48⫻4.34 mm2 . We used an effective sample
size, r, of 1.85 mm, which is orders of magnitude larger than
the grain size. In Fig. 3, the magnitude of the critical current
densities is clearly seen in Fig. 2. From Bean’s model, basically the shapes of the curves in Figs. 2 and 3 are the same
except a proportional constant. Figure 3共a兲 shows J c at 5 K
and 共b兲 shows at 10 K. For bare MgB2 , the value of J c is
6⫻106 A/cm2 at T⫽5 K, but it is 8.6⫻106 A/cm2 at T
⫽10 K. Since the vortex avalanche is severe at low tempera-
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84
Choi et al.
Appl. Phys. Lett., Vol. 84, No. 1, 5 January 2004
with a thickness of 1.05 ␮m. This is very promising news for
device applications because the headache of J c reduction for
T⬍15 K and H⭐1000 Oe can be completely removed.
We believe that the vortex avalanche occurs because of
the thermal instability. As expected, the cause of thermal
instability is removed after using metallic deposition to increase the thermal conduction. We believe this instability
prevails only at low temperatures for bare MgB2 thin film
because the high Debye temperature due to light elements in
MgB2 makes the specific heat very small, at low temperatures thus introducing a very interesting vortex avalanche
phenomenon.
In summary, we obtained the J c from the M – H loops of
MgB2 thin films with Au deposited on their tops. We confirmed that the vortex avalanche causing the reduction of J c
could be reduced once metallic thin films had been deposited
onto the MgB2 thin films. Thus, the obstacles associated with
of J c reduction in MgB2 thin films are removed.
This work is supported by the Creative Research Initiatives of the Korean Ministry of Science and Technology.
1
FIG. 3. Critical current density (J c ) 共a兲 at 5 K in the field region 0⭐H
⭐2000 Oe and 共b兲 at 10 K in the field region 0⭐H⭐1200 Oe. The inset
shows the J c in the region 0⭐H⭐3 T.
tures, the critical current density is reduced by 2/3 when the
temperature is reduced from 10 to 5 K. As explained earlier,
when the thickness of the Au film is 2.55 ␮m, J c reach
1.22⫻107 A/cm2 (T⫽5 K) and J c is fully recovered. Also,
the value of J c at 10 K reaches 1.13⫻107 A/cm2 a Au film
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