Physica C 357±360 (2001) 568±571
www.elsevier.com/locate/physc
Magneto-optical imaging of vortex lattice melting
transition in Bi2Sr2CaCu2O8‡y
T. Tamegai *, M. Yasugaki, K. Itaka, M. Tokunaga
Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Received 16 October 2000; accepted 25 November 2000
Abstract
Magneto-optical di€erential imaging technique is applied to the observation of the vortex lattice melting transition in
Bi2 Sr2 CaCu2 O8‡y . Contrary to the naive expectation, the nucleation of the vortex liquid paddle occurs in a rather
inhomogeneous manner. The front of the vortex solid±liquid interface propagates re¯ecting the intrinsic inhomogeneities of the crystal. Direct compositional mapping of the constituent atoms indicates that the change in the chemical
composition have some correlation with the behavior of the vortex solid±liquid interface. Changes in the superconducting parameters due to the variation of the local doping level could be the origin of such an e€ect. Ó 2001 Elsevier
Science B.V. All rights reserved.
PACS: 74.72.Hs; 74.60.Ge; 74.80.-g
Keywords: Magneto-optics; Vortex lattice melting; Bi2 Sr2 CaCu2 O8‡y
1. Introduction
Extensive studies of the vortex matter phase
diagram in high temperature superconductors
have clari®ed the details of the vortex states [1].
Thermal ¯uctuations drive the vortex lattice to
melt into the vortex liquid states at the ®rst-order
phase transition [2,3]. In the case of Bi2 Sr2 CaCu2 O8‡y (BSCCO), the vortex lattice melting
transition (VLMT) occurs at low ®elds because
of high anisotropy parameter [2], and hence the
transition could be a€ected by many factors such
as the geometrical barrier [4] and the disorder in
the crystals. How the actual VLMT proceeds over
*
Corresponding author. Tel./fax: +81-3-5841-8886.
E-mail address: tamegai@ap.t.u-tokyo.ac.jp (T. Tamegai).
the whole crystal in the presence of such factors
would be of great interest. However, this is a
rather challenging task, because the change in
physical parameters accompanied with the VLMT
is very small. Magnetic measurements such as using scanning Hall probe microscope (SHPM)
could provide important information. Coexistence
of the solid and liquid vortex states is observed
using this technique [5]. However, the typical size
of the crystal is beyond the scanning range over
which SHPM can reliably give meaningful information. Magneto-optical (MO) observations of
the magnetic induction pro®le have been used to
characterize the pinning properties of superconductors [6]. Lack of ®eld resolution has been a
stumbling stone for the application of this technique to the VLMT. A recent study by Soibel et al.
has overcome this problem by a sophisticated
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 3 1 0 - 0
T. Tamegai et al. / Physica C 357±360 (2001) 568±571
di€erential technique and reports complicated behavior of the phase evolution [7].
In this paper, we report on the di€erential MO
studies of the VLMT in BSCCO with the emphasis
on the relation between the behavior of the solid±
liquid interface and the inhomogeneities in the
crystal. We make compositional mapping of the
crystal after the MO observations, and discuss
the in¯uence on the VLMT. Typical forms of the
evolution of the vortex solid±liquid interface are
described.
2. Experimental
Di€erential MO images are taken by subtracting images at H ˆ Ha dHa =2 and H ˆ Ha ‡
dHa =2 with dHa ˆ 1 Oe [4]. We use a high-speed
cooled CCD camera (Apogee 6E) with 1024 1024 pixels and 14-bit resolution. To resolve the
magnetization step (0.3 G) accompanied by the
vortex lattice melting under the typical background ®eld of 100 Oe, we need to have a light
intensity resolution better than 0.1%. This number
can be achieved by accumulating more than 106
photons into each CCD pixel. This means that we
need to average more than 100 images for each
®eld settings with 14-bit CCD camera. All the
procedure including the image acquisition and the
setting of the current for the magnet are controlled
by a computer.
The crystals used in the present study are grown
by the ¯oating zone method using an image furnace [8]. They are carefully cleaved to a thickness
of about 20 lm and cut into approximate dimensions of 0:5 0:5 mm2 . We always keep edges of
the crystal either parallel or perpendicular to the
crystal growth direction (a-axis). One of the corners is cut to make it easier to identify the orientation of the crystal. Compositional mappings of
the crystals are performed using energy dispersive
X-ray spectroscopy after the MO observations.
3. Results and discussion
Ideally, the nucleation of the vortex liquid
phase starts at the center of the crystal and the
569
vortex solid±liquid interface propagates in a concentric manner. However, the actual nucleation
does not start from the center nor expands regularly as shown in Fig. 1(a)±(d) for BSCCO #1. In
this case, the nucleation starts at slightly o€ the
center (Fig. 1(a)) and the interface propagation
making complicated patterns. In these ®gures,
white regions indicate the location where the vortex lattice has melted with the increase in H from
Ha to Ha ‡ dHa . A broader white region means
that the vortex liquid region expands more easily,
whereas the white region can be discontinuous
when the boundary is completely pinned. Actually,
such a pinning of the boundary can be seen at
several locations in the crystal (Fig. 1(c) and (d)).
Fig. 2 shows the scanning electron micrograph
of BSCCO #1. It is clear that this crystal has no
apparent defect on the surface. Microanalyses of
the chemical compositions using energy dispersive
X-ray spectroscopy have been performed along the
vertical and horizontal lines on the crystal. The
area of each mapped region is 5 5 lm2 . We plot
the compositional ratio of Sr to Cu along the two
lines. The error bar is estimated by the standard
deviation of 10 measurements at the same location.
It should be noted that if the same analysis is applied to SrTiO3 , the ratio of Sr to Ti is 1 within an
error of 1%. So the variation of the Sr/Cu ratio
re¯ects the actual variation of the composition.
A close inspection of the relation between the
chemical composition variation and the boundary
propagation reveals that there is a certain correlation. For example, Sr/Cu ratio decreases abruptly
at x ˆ 160 lm in Fig. 2(b). At the same location,
left protrusion of the solid±liquid interface is disturbed. Another example is that the upward interface motion is pinned at around y ˆ 250 lm (see
scale in Fig. 2(c)) in Fig. 1(c). Almost at the same
location, the Sr/Cu ratio increases sharply as
shown in Fig. 2(c). Variation of the chemical
composition would lead to the local change in the
superconducting parameters. This, in turn, changes
the local melting ®eld and disturbs the interface
propagation. It is not easy to say what the increase
in Sr/Cu ratio leads to, since depending on the
absolute number and the location of the strontium
atoms, changes in the above ratio can make the
system either overdoped or underdoped.
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T. Tamegai et al. / Physica C 357±360 (2001) 568±571
Fig. 1. Di€erential MO images of the vortex lattice melting transition in BSCCO #1 at T ˆ 70 K and at (a) 101 Oe, (b) 102 Oe, (c) 103
Oe and (d) 104 Oe.
stripes have some curvature. In other words, they
form arc-like structures. One of the possible origins for such structures would be related with
boundary of the grown crystallite and the molten
region during the crystal growth. Actually the radius of the arc is about 15 mm and is consistent
with the direct observation of the boundary after
the crystal growth. Considering the slow speed of
the crystal growth (0.2 mm/h), it would be possible
to have chemical inhomogeneities parallel to the
growth direction.
Finally, it should be stressed that the inhomogeneities that we have described in this paper have
almost no e€ect on the critical state ®eld pro®le at
low temperatures. So, they are considered to be
very weak perturbation.
Fig. 2. (a) Scanning electron micrograph of BSCCO #1. Spatial
dependence of compositional variation of the ratio of Sr to Cu
along (b) horizontal and (c) vertical dotted lines in (a).
4. Summary
In some of the crystals the vortex liquid nucleates along a stripe-like region as shown in Fig. 3
for BSCCO #2. The direction of the stripe is perpendicular to the crystal growth direction and the
The di€erential MO imaging technique reveals
the complicated nucleation and propagation of the
vortex solid±liquid interfaces. The inhomogeneities of the chemical composition frozen in the
T. Tamegai et al. / Physica C 357±360 (2001) 568±571
571
Fig. 3. Di€erential MO images of the vortex lattice melting transition in BSCCO #2 at T ˆ 70 K and at (a) 99 Oe, (b) 100 Oe, (c) 101
Oe and (d) 102 Oe.
crystal during the growth in¯uences the propagation of the interfaces. In some cases, the interfaces are found to follow one-dimensional arc-like
structures perpendicular to the growth direction,
which are considered to be related to the boundary
between the crystallite and molten region during
the crystal growth.
Acknowledgements
The authors acknowledge A. Soibel and E.
Zeldov for stimulating discussion. This work is
supported by CREST and Grant-in-Aid for Scienti®c Research from the Ministry of Education,
Science, Sports and Culture of Japan.
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