Pinning Force Scaling Analysis of Polycrystalline MgB 2
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ORIGINAL PAPER
Pinning Force Scaling Analysis of Polycrystalline MgB2 M. R. Koblischka1,2
· A. Wiederhold1 · A. Koblischka-Veneva1,2 · C. Chang3
Received: 30 November 2019 / Accepted: 8 February 2020 © The Author(s) 2020
Abstract Flux pinning force scaling f = Fp /Fp,max vs. h = Ha /Hirr was performed on a variety of pure MgB2 samples, including a spark plasma sintered (SPS) one and a series of samples sintered at various reaction temperatures ranging between 775 and 950 ◦ C. The SPS sample exhibits a well-developed scaling at all temperatures, and also the sintered samples prepared at 950 ◦ C; however, the obtained peak positions of the pinning force scalings are distinctly different: The SPS sample reveals dominating pinning at grain boundaries, whereas the dominating pinning for the other one is point-pinning. All other samples studied reveal an apparent non-scaling of the pinning forces. The obtained pinning parameters are discussed in the framework of the Dew–Hughes’ pinning force scaling approach. Keywords MgB2 · Flux pinning forces · Scaling of flux pinning forces · Grain boundary pinning · Core pinning
1 Introduction The MgB2 superconductors are interesting for various applications as they offer the metallic character which enables a simple processing route and no involved rare earth materials, which reduces the costs involved [1, 2]. The modern cryocooling techniques enable a temperature of 20 K to be reached, which is commonly accepted as the optimum one for applications of MgB2 . Besides these advantages, the still limited critical current densities and the presence of flux jumps require further studies on the acting flux pinning mechanism(s) in MgB2 superconductors [3–10]. The pinning force scaling as introduced by Kramer [11] and Dew–Hughes (DH) [12] is an important tool to study the pinning mechanisms acting in a given superconductor sample. This applies not only for the conventional metallic superconductors, where the pioneering work was performed
M. R. Koblischka
[email protected]; [email protected] 1
Experimental Physics, Saarland University, P.O.Box 151150, 66041, Saarbr¨ucken, Germany
2
Present address: Superconducting Materials Laboratory, Department of Materials Science and Engineering, Shibaura Institute of Technology, Tokyo, 135-8548, Japan
3
Institut Jean Lamour, UMR CNRS-Universit´e de Lorraine, 54506, Vandœvre-l`es-Nancy, France
on, but also for the high-Tc counterparts [13–16] as well as MgB2 [16–20] and the iron-based superconductors [21]. The main difference is given by the use of the irreversibility field, Hirr instead of the upper critical field, Hc2 for the scaling, as Hirr determines the upper limit of the flux pinning (= irreversible region in the H-T diagram). The flux pinning can be obtained from the scaled pinning force data f = Fp /Fp,max vs. h = Ha /Hirr by a fit to the functional dependence given by the following: f = A(h)p (1 − h)q .
(1)
The resulting parameters, p and q, determine the dimensionality and the type of the dominating flux pinning, whe
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