On the origin of the sharp, low-field pinning force peaks in MgB2 superconductors

Michael R Koblischka,Crosby Chang,Quentin Nouailhetas,Masato Murakami,Alex Wiederhold,Bruno Douine,Anjela Koblischka-Veneva,Kévin Berger

doi:10.1063/1.5133765

Abstract

Various MgB2 thin films and single crystals were found in the literature to exhibit a sharp, narrow peak at low fields in the volume pinning force, Fp(H)-diagrams. The origin of this peak is associated with a steep drop of the current density when applying external magnetic fields and is ascribed to sample purity. We show here that bulk MgB2 prepared by spark-plasma sintering also shows the sharp, narrow peak in Fp. The peak is also seen in the volume pinning force scaling, Fp/Fp,max vs h = H/Hirr. Furthermore, polycrystalline bulk MgB2 samples prepared close to the optimum reaction temperature reveal this peak effect as well, but other samples of the series show a regular scaling behavior. The combination of magnetization data with data from electric transport measurements on the same samples demonstrates the origin of this peak effect. On increasing preparation temperature, the pinning force scaling changes from grain boundary pinning to point pinning and the grain connectivity gets worse. Hence, the sharp, low-field peak in Fp vanishes. Therefore, the occurrence of the peak effect in Fp gives important information on the grain coupling in the MgB2 samples.

Highlights

Polycrystalline MgB2 samples, it was found to be possible to establish a current flow through the entire sample perimeter as the grain boundaries (GBs) do not act as weak links, but as flux pinning sites, e.g., in Nb3Sn.[3,4]
The electric transport measurements performed on the same samples reveal a change of the grain connectivity on increasing the preparation temperature, whereas the highly dense, spark-plasma sintered sample shows a well-developed grain connectivity
From this combination of magnetic and electric measurements, we can conclude that the change of length scale of the current flow at Hpeak is responsible for the Fp-peak

Summary

Introduction

The metallic superconductor with the highest transition temperature, MgB2, is considered as a candidate material for many applications such as trapped field magnets, tapes, and wires due to its simplicity and the simple preparation route possible, even though its Tc is much lower than most of the high-Tc superconductors.[1,2] Even in sintered, polycrystalline MgB2 samples, it was found to be possible to establish a current flow through the entire sample perimeter as the grain boundaries (GBs) do not act as weak links, but as flux pinning sites, e.g., in Nb3Sn.[3,4] Together with modern cryocooling techniques, the lower Tc does not pose a serious problem.[5] On the other hand, MgB2 has its own characteristic magnetic behavior including dendrite-like flux penetration,[6] flux jump effects,[7] and peculiar flux pinning properties,[8] which may influence possible applications Two peculiar observations of the flux pinning analysis on MgB2 include (i) the change of the main acting pinning mechanism with increasing temperature found in polycrystalline samples prepared with varying reaction temperatures[11] and (ii) the appearance of a sharp, narrow peak in the Fp − H diagrams at relatively low applied fields.[12–21] Such a peak was observed first in MgB2 single crystals[22,23] and in thin films.[24–26] The occurrence of this peak at relatively low fields was ascribed to the strong decrease in jc(B) due to sample purity

Methods

Results

Conclusion