Physica B 294}295 (2001) 388}392
High-"eld #ux jumps in BSCCO at very low temperature
A. Milner*
High Magnetic Field Laboratory, School of Physics and Astronomy, Sackler Institute of Condensed Matter Physics,
Tel Aviv University, 69978 Tel Aviv, Israel
Abstract
Magnetization of single crystals of high-¹ superconductor BSCCO-2212 comprises #ux jumps (FJs) that di!er
qualitatively from well-studied magneto-thermal instabilities in conventional superconductors. FJs start above full
penetration "eld, demonstrating clear periodicity in the slowly changing magnetic "eld. Magnetization process exhibits
high reproducibility, which is, however, prone to sudden and uncontrollable breaks. Thin plate-like samples were
examined in "elds up to 17 T at temperatures down to 0.3 K, using magnetometric and calorimetric techniques. 2001
Elsevier Science B.V. All rights reserved.
PACS: 74.25.Ha; 74.60.Ec; 74.60.Ge
Keywords: High-¹ superconductors; BSCCO; Flux jumps
1. Introduction
In a type-II superconductor, nonequilibrium #ux
distribution being constantly built up by a slowly
ramping magnetic "eld continuously scatters at
not too low temperature. Relaxation mechanisms
involved are #ux #ow, #ux creep and temperature-activated #ux avalanches (TAA). The
last mechanism has been investigated in detail in
high-temperature
superconductor
(HTSC)
Bi Sr CaCu O (BSCCO-2212) which we study
here, as well as in YBCO, and in conventional
superconductors (SC) [1}3]. It was shown that
TAA is triggered by temperature-activated depinn* Correspondence address: School of Physics and Astronomy,
Sackler Institute of Condensed Matter Physics, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel. Fax: #972-364229-79.
E-mail address: milner@ccsg.tau.ac.il (A. Milner).
ing of single vortex line with the following correlated movement of a bundle of vortices. The
magnetization curve consists of relatively small
irregular steps [3]. TAA shows power-law distribution of the amplitude and duration of the events in
good agreement with the theory of self-organized
criticality (SOC) [3}5].
In the case of very strong pinning, magnetothermal instabilities in SC may manifest themselves
di!erently [6,7]. Macroscopic #ux jump (FJ) overspreads the substantial sample volume and is followed by a huge heat release which can even bring
the sample into the normal (resistive) state. Being
started by a local #uctuation of magnetic induction
or temperature, FJ proceeds as a process with
a positive feedback. The sequence of FJs on the
magnetization loop is normally located between
zero and some moderate "eld (for a typical hysteresis loop with FJs in the conventional SC see, e.g.,
Ref. [8]). The picture is irregular, independent of
0921-4526/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 6 8 4 - 0
A. Milner / Physica B 294}295 (2001) 388}392
magnetic history, irreproducible and highly sensitive to the sweep rate of magnetic "eld. The "rst FJ
in the increasing "eld may happen below full penetration "eld H only, otherwise, the magnetiz$.
ation curve remains smooth.
Magnetization of HTSC at very low temperatures was reported a few times [9}12] to di!er
qualitatively from the above-mentioned picture.
The sequence of giant FJs has been observed also,
but its appearance cannot be described in the
framework of the traditional model of magnetothermal instabilities: the "rst FJ occurs at a "eld
strength much higher than H ; events of almost
$.
the same amplitude look quite regular and are
essentially sensitive to magnetic history and independent of "eld ramp rate. There is no consensus so
far about the nature of this phenomenon.
Here experimental results on BSCCO single
crystals in the magnetic "elds up to 17 T at the
temperatures below 2 K are presented.
2. Experimental results
The following results were obtained on several
plate-like c-oriented BSCCO-2212 single crystals
with a characteristic cross-section of a few square
mm and thickness 0.06}0.2 mm. Temperature dependence of the resistivity exhibited a two-step
transition into the superconducting state, at 112
and 93 K. The external "eld orientation was always
parallel to the crystal c-axis. In most cases, the
sample was glued to a thin sapphire plate in a
home-made calorimeter using GE-Varnish. A small
RuO thermometer and a calibrated heater were
also glued close by. Above the sample, a Hall sensor
was located spaced by 0.8 mm. Sample magnetization M was estimated using the di!erence between
the Hall sensor reading and the external "eld
strength. Magnetic "eld ramp rate was controlled
from 10 to 53 Oe/s. A calorimeter was arranged in
the vacuum space of the He cryostat.
Fig. 1 presents a typical magnetization curve,
obtained on the relatively large rectangular sample,
1.6;7.2 mm. The most prominent property of this
graph is the beginning of #ux jumps series at the
"eld strength much higher than H on the increas$.
ing branches of the "eld cycle (second and fourth
389
Fig. 1. Magnetization of sample C2 at T"0.32 K; dB /
dt"0.0053 T/s.
quadrants). The absence of FJs below 6 T is, however, not the essential attribute of these patterns.
The basic one is high periodicity and reproducibility
of the FJs sequence at high enough "elds. This is
seen well in Fig. 2. There is also another peculiarity
in the dependence of the step frequency B versus
B , which needs special attention. The periodicity is
di!erent in the increasing and decreasing magnetic
"elds and it is even sensitive to the "eld sign After
many experiments, we failed to establish a rule
concerning the period and to correlate it to some
external parameter. Sometimes, it resembles Fig. 2,
sometimes on the contrary, FJs on the descending
branch occur more often than on the ascending
branch, sometimes the periods coincide. The relation between the "eld strengths of the "rst FJ on the
ascending branch and the last FJ on the descending
branch remains also unpredictable (cf. Ref. [12]
where H 'H
was repeatedly observed,
which served as one of the cornerstones in the
treatment of the phenomenon by the model of
SOC). Occasionally, after a well-marked &comb' in
the growing "eld only few (sometimes, only one)
high-"eld big steps occur on the falling branch.
The appreciable di!erence between step sizes B
in the positive and negative growing "elds is even
more enigmatic. The result presented in Fig. 2,
where this di!erence exceeds 10% between 9 and
15 T, was not exceptional. It was observed to reach
up to 35% a few times in di!erent experimental
390
A. Milner / Physica B 294}295 (2001) 388}392
Fig. 2. Distances between FJs, B"B
!B versus absolute
L>
L
value of B . B !and B #mean di!erent sign (direction) of
magnetic "eld. I}IV are the numbers of quadrants on the magnetization loop graph (see Fig. 1). Sample C2; T"0.37 K;
dB /dt"0.0053 T/s.
runs with di!erent samples. More often, however,
the loops were symmetrical. It should be mentioned
here, that having been set for the maximal sweep
rate the magnet power supply provided plus and
minus current scanning speeds with a small di!erence of a few percent. Considering this as a reason
for the asymmetry e!ect, a very strong dependence
of the FJs frequency on the magnetic "eld sweep
rate must be assumed. This, however, has not been
con"rmed directly in our measurements. The reverse dependence exists, but it is much less than
that which could explain the$asymmetry.
Fig. 2 contains data for a full "eld cycle and a
quarter of the following one. The increasing negative branch of the second cycle (symbols#) does
not include irregular low-"eld events, but starts
with the quite ordered FJs above 6 T. This is analogous to the loop in Fig. 1, although these experiments were about two months apart with the
in-between sample remounting. The e!ect of a threshold in the 4}7 T range has been observed in many
of our experiments on the other samples as well.
Curiously enough, we encounter the same value in
Ref. [12] for YBCO at the same temperature. To "ll
up a canvas, a record of temperature of a small
sample 0.9;1.6 mm, mounted in the calorimeter is
presented in Fig. 3. Energy dissipation of up to
Fig. 3. Temperature variations of sample C1e during the magnetization process; dB /dt"0.0026 T/s.
1 mW/g takes place. It increases while the critical
state "lls the sample and stabilizes after full penetration. Then, the metastable state collapses with
the accompanying energy release. The "rst FJ produces as much as 1.5 J/g, increasing the temperature of a 2 mg sample up to &40 K. At the
following FJs, thermal pulses decrease to about
0.2 J/g. In Fig. 3 not all pulses are caught because
experimental points were taken every 3}5 s, while
the thermal relaxation of the calorimeter with
a sample took less than 1 s. Pulse events have been
measured and calibrated by means of an oscilloscope. After every FJ, the process starts from the
same conditions: homogeneous #ux distribution in
the sample and minimal temperature. Looking at
Fig. 3, a natural question arises: could this quasistationary heating cause the FJ? It looks like the
answer is no. To check this statement, the sample
was driven into the critical state, just before the
expected FJ, then, the strength of magnetic "eld
was "xed and the sample was heated slowly. It
appeared that only at ¹"2 K very slow #ux redistribution started, and only after 6 K this process
happened quickly.
The frequency of FJs on the magnetization curve
is temperature dependent: the lower the temperature, the less the distance between collapses of the
critical state. Finally, the e!ect of sample size has
also been found. On the small rectangular 0.9;
0.4 mm sample C1d, originating from the same
A. Milner / Physica B 294}295 (2001) 388}392
parent crystal as C1e shown in Fig. 3, FJs have not
been observed at all. This is one more coincidence
with the results on YBCO presented in Ref. [12].
The important characteristic of the phenomenon
under investigation was a sudden irreversible change
of the period of FJs. It could happen after a few
remarkably reproducible successive measurements
at constant or slightly changed temperature. For
example, a magnetization cycle at the stabilized
temperature 0.46 K next to the one at 0.32 K presented in Fig. 3, showed an almost twice as rare step
sequence. In another series of measurements on
sample C1c, the period at 0.9 K appeared to be
much less compared to that at 0.32 K. In both
cases, the initial low-temperature periods have not
been restored, even after intermediate strong heating. Thus, magnetic "eld might change some intrinsic parameter determining the frequency of FJs.
3. Conclusion
The behavior of BSCCO crystal at the temperatures of 1/100th of ¹ and below, in the scanning
magnetic "eld H#c has two remarkable attributes.
The "rst is a non-homogeneous hysteresis loop
consisting of regular (equidistant) steps, which re#ect the global redistribution of the magnetic #ux
in the sample. The second is that the onset "eld
and period of FJs are determined by some hidden
parameter, which in turn is a!ected by the "eld.
Regarding the low-temperature/high-"eld periodical magnetization jumps, there are two points of
view about their origin. These are avalanches, starting without the assistance of temperature by analogy with continuously topped-up sandpile, or
a quantum process of rearrangement of the lattice
of vortices after reaching some de"nite densities.
The change of the background on which one of
these scenarios (or may be both?) plays may also
have two distinct causes. It is natural to assume
mechanical changes of the sample occurring due to
the large magnetostriction known in this kind of
materials [13]. The fact that a hidden parameter
once been changed remains "xed even after heating
up to room temperature speaks in favor of this
point. Another possibility is also admitted, namely,
that some domain structure, existing in the sample
391
and weakly sensitive to the external xeld in this
geometry participates in the process of interest. The
abrupt #ux redistribution then occurs at de"nite
junctions of two parameters: smoothly (linear)
changing with "eld vortex density and some spatial
inhomogeneity (domains?) controlled somehow by
the experimental conditions.
The idea of domains is not unique and is widely
used in discussions of the symmetry of superconducting state in cuprates, as well as magnetic "eld
in#uence on the low-temperature thermal properties, particularly of BSCCO [14,15]. Considering
domains (rather, domain walls) as a possible net of
pinning centers, the triggering of FJs is assumed to
be related to the interplay between the interpin and
intervortex spacing (the idea brought up in Ref. [3]
in a similar context concerning grain boundaries).
The absence of FJs in small samples of BSCCO and
untwinned YBCO may also speak in favor of this
point of view.
In the late 1980s and 1990s, a few magnetooptical groups set up exciting experiments hoping
to observe spontaneous circular optical anisotropy
in high-¹ superconductors resulting from the
violation of the parity and time-reversal symmetries [16]. Perhaps, the strange e!ects of asymmetry
of the magnetization process relating to the "eld
sign and of sudden changes of the #ux jumps frequency in the high "elds, are the manifestation of
that symmetry breaking. The author believes that is
a challenge to undertake new attempts of probing
these objects at the above-mentioned conditions by
means of magneto-optics.
Acknowledgements
V.H.M. Duijn and A. Menovsky are thanked for
the high-quality single crystals.
References
[1]
[2]
[3]
[4]
[5]
[6]
X.S. Ling et al., Physica C 185}189 (1991) 2181.
Z. Wang et al., Phys. Rev. B 48 (1993) 9782.
S. Field et al., Phys. Rev. Lett. 74 (1995) 1206.
O. Pla et al., Phys. Rev. Lett. 67 (1991) 919.
W. Pan et al., Phys. Rev. B 49 (1994) 1192.
S.L. Wipf, Phys. Rev. 161 (1967) 404.
392
[7]
[8]
[9]
[10]
[11]
[12]
A. Milner / Physica B 294}295 (2001) 388}392
S.L. Wipf, Cryogenics 31 (1991) 936.
L.J. Neuringer et al., Phys. Rev. 148 (1966) 231.
M. Guillot et al., Physica C 162}164 (1989) 361.
M.E. McHenry et al., Physica C 190 (1992) 403.
G.T. Seidler et al., Phys. Rev. Lett. 70 (1993) 2814.
R.J. Zieve et al., Phys. Rev. B 53 (1996) 11849.
[13]
[14]
[15]
[16]
H. Ikuta et al., Phys. Rev. Lett. 70 (1993) 2166.
R.B. Laughlin, Phys. Rev. Lett. 80 (1998) 5188.
H. Aubin et al., Phys. Rev. Lett. 82 (1999) 624.
T.W. Lawrence et al., Phys. Rev. Lett. 69 (1992) 1439, and
references therein.