THERMALLY ASSISTED FLUX FLOW IN MgB2
A.Sidorenkoa,b, V. Zdravkovb, V. Ryazanovc, S. Klimma, S. Horna, R.Tidecksa, A. Wixfortha
a
Institut für Physik, Universität Augsburg, 86159 Augsburg, Germany
b
c
Institute of Applied Physics, 2028 Kishinev, Moldova
Institute of Solid State Physics, 142432 Chernogolovka, Russia
The origin of the resistive transition broadening has been investigated for MgB2 thin
films. Thermally activated flux flow is found to be the mechanism responsible for the
resistivity at the lower part of the transitions.
1.
Introduction
The discovery of superconductivity in MgB2 [1], a material with hexagonal layered crystal
structure, immediately raised questions about its transport properties. The layered structure
influences the magnetic flux penetration and motion. In particular, thermally activated flux flow
(TAFF) processes should play a crucial role in the resistive transition broadening similar to the
case of high-Tc superconductors [2], and artificially multilayered systems [3]. Thus, studying this
phenomenon has a great importance for possible applications of the novel superonductor.
In the present work we report about an experimental investigation of the resistive transition
broadening for magnesium diboride thin films.
2.
Samples
The MgB2 films were prepared by dc-magnetron sputtering from Mg-MgB2 composite targets on
single crystalline (100) oriented sapphire substrate. The temperature was kept during the process
of sputtering at 2000C and then raised to 6000C. Next, the deposited film was annealed ex situ at
8500C in a Mg vapour atmosphere.
Further details of the sample preparation have been
described in [4]. The X-ray diffraction study revealed a textured (101)-oriented structure of the
films. The resistive transitions, R(T), in constant external magnetic fields were measured by a
conventional four-terminal resistive method using a 12T superconducting magnet.
1
3.
Results and Discussion
Fig.1 shows the resistive transitions R(T) at several magnetic fields, B , perpendicular to the
MgB2 film plane for one of the investigated samples. The width of transition is about 0.3K in
zero and low magnetic fields and increases up to ~2K for high fields. Usually, the broadening of
the resistive transition in a magnetic field for layered superconductors is interpreted in terms of a
dissipation of energy caused by the motion of vortices. This interpretation is based on the fact
that for the low-resistance region, R<<Rn (where Rn is the resistivity in the normal state just
above the transition), the R(T) dependences are of the thermally activated type and may be
described by:
R  R0 exp  U ( B, T ) / k BT 
(1)
Here, U(B,T) is the activation energy and R0 is a proportionality factor. Investigation of high-Tc
superconductors and artificial multilayers showed, that U(B,T) has different power-law
dependences on magnetic field and temperature [2,3].
102
70
101
60
100
50
7
5
4
3
2
1
0
R (W)
R (W)
80
40
10-1
7
10-2
30
5
10-3
20
10-4
10
0
20
22
24
26
28
30
32
34
36
T (K)
Fig. 1. The resistive transitions, R(T) , for a 500
nm thick MgB2 film at different values of the
magnetic field perpendicular to the film: curve 0
to 7 correspond to B=0, 1, 2, 3, 4, 5, 7 Tesla.
10-5
0
0.030
1
2
0.035
3
4
0.040
0.045
0.050
1 / T (K-1)
Fig. 2. Arrhenius plot of R(T)B=const for the
sample, presented in Fig.1. From the slope of the
linear parts of the curves the value of the
activation energy U(B,T) is obtained.
Fig. 2 presents an Arrhenius plot of the data of Fig.1, i.e. lnR is plotted against 1/T. The stright
line parts indicate that the resistive behavior of the MgB2 film is caused by TAFF-process
described by the Arrhenius law given in Eq.(1).
2
A fit of the experimental results R(T)B=const of Fig.2 by Eq.(1) yields values of the activation
energy, ranging from U/kB = 300K in low magnetic field up to 10000K in the high field region.
Compared to the power law U(B)~Bn with the exponent n ~ 1, usually observed for other layered
systems [3,5], MgB2 demonstrates a much stronger field dependence of the activation energy.
4.
Conclusion
One of the possible reasons of the unusual strong field dependence of the activation energy of
the TAFF-process for MgB2 observed in the present work, may be a very complex flux
dynamics, such as the dendritic flux instability in MgB2 films, found recently [6] for c-axis
textured films in a magnetic field perpendicular to the film plane. The magneto-optcal imaging
measurements demonstrate a „fractal-like“ structure of the flux penetration with strong increase
of the flux-dendrits quantity with increasing magnetic field. This unusual flux creep behavior of
magnesium diboride needs further investigation.
Acknowlegments
This work was partially supported by the INTAS-Project Nr.99-00585.
References
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