Elsevier

Physica C: Superconductivity

Volume 363, Issue 3, 15 November 2001, Pages 170-178
Physica C: Superconductivity

Flux dynamics and vortex phase diagram of the new superconductor MgB2

https://doi.org/10.1016/S0921-4534(01)00936-4Get rights and content

Abstract

Magnetic relaxation, critical current density and transport properties have been investigated on MgB2 bulks from 1.6 K to Tc at magnetic fields up to 8 T. A vortex phase diagram is depicted based on these measurements. Two phase boundaries Hirrbulk(T) and Hirrg(T) characterizing different irreversible flux motions are found. The Hirrbulk(T) is characterized by the appearance of the linear resistivity. A large separation between the bulk irreversibility field at 0 K and the upper critical field Hc2(0) has been found, it is interpreted as either due to a quantum vortex liquid induced by strong quantum fluctuation of vortices at 0 K, or flux flow through weak-link channels. It is further found that the magnetic relaxation rate is weakly dependent on temperature but strongly dependent on field indicating a trivial influence of thermal fluctuation on the vortex depinning process. Therefore the phase line Hirrbulk(T) may be attributed to quantum vortex melting in the rather clean system at a finite temperature. The second boundary Hirrg(T) reflects the irreversible flux motion in some local regions due to either very strong pinning or the surface barrier on the tiny grains.

Introduction

The recently discovered new superconductor MgB2 generates enormous interests in the field of superconductivity [1]. Many important thermodynamic parameters have already been derived, such as the upper critical field Hc2(0)=13–20.4 T [2], [3], [4], [5], the Ginzburg–Landau parameter κ≈26 [5], and the bulk critical superconducting current density jc≈8×104 A/cm2 at 4.2 K and 12 T [6] in thin films. One big issue concerns however how fast the critical current will decay under a magnetic field and in which region on the field-temperature (HT) phase diagram the superconductor can carry a large critical current density (jc). This jc is controlled by the mobility of the magnetic vortices, and vanishes at the melting point between the vortex solid and liquid. A finite linear resistivity ρlin=(E/j)j→0 will appear and the relaxation rate will reach 100% at this melting point showing the starting of the reversible flux motion. In this paper we present an extensive investigation on the flux dynamics by magnetic relaxation and transport measurement. A vortex phase diagram will be depicted based on these measurements.

Section snippets

Experimental

Samples investigated here were fabricated by both high pressure (HP) (P=6 GPa at 950°C for 0.5 h) and ambient pressure (AP) synthesis which was described very clearly in a recent publication [7]. HP synthesis is a good technique for producing the MgB2 superconductor since it can make the sample more dense and uniform (in sub-micron scale) and also prevent the oxidization of Mg element during the solid reaction. Our HP samples are very dense and look like metals with shiny surfaces after

Results

Fig. 1 shows the diamagnetic transition of one of the HP samples measured in the field-cooled (FC) and zero-field-cooled (ZFC) process. All other samples show almost similar quality. In the FC process, the temperature was lowered from above Tc to a desired temperature below Tc under a magnetic field, and the data are collected in warming up process with field. Its signal generally describes the surface shielding current and the internal frozen magnetic flux profile. In the ZFC process, the

Vortex phase diagram of MgB2

The phase lines of Hirr(T) and Hc2(T) determined by following the different methods mentioned above are shown in Fig. 6. It is clear that the bulk irreversibility line Hirrbulk(T) determined from MHL measurement at the point just before the appearance of the small tail shown in Fig. 3 terminates at about 8 T at 2 K, which is very close to that determined by resistive measurement and from the M–T measurement (Fig. 4). This strongly indicates that the Hirrbulk(T) is a vortex melting line which

Conclusion

In rather pure samples of MgB2 the irreversibility field is rather low comparing to the upper critical field in low temperature region. This effect has been attributed to either the possible existence of the quantum vortex liquid due to strong quantum fluctuation of vortices, or the easy flux flow through the weak link channels. The temperature and field dependence of the relaxation rate may further suggest that the vortex melting at a finite temperature is also induced by strong quantum

Acknowledgments

This work is supported by the National Science Foundation of China (NSFC 19825111) and the Ministry of Science and Technology of China (project: NKBRSF-G1999064602). HHW gratefully acknowledges Prof. B. Ivlev and Dr. A.F.Th. Hoekstra for fruitful discussions, and continuing financial support from the Alexander von Humboldt foundation, Germany.

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