Enhanced flux pinning by magnetic CrB2 nanoparticle in MgB2 superconductor

Abstract

Different from the non-magnetic pinning centers, the magnetic pinning centers have stronger flux pinning force due to the direct magnetic interaction between the magnetic pinning centers and the magnetic vortices, thus have been widely used to enhance the critical current density of various superconductors. In this article, nanoparticles of antiferromagnet CrB2 were introduced as magnetic pinning center to improve the flux pinning force density (Fp), irreversible field (Hirr), and critical current density (Jc) of MgB2. Compared to the pristine sample, the CrB2 doped MgB2 with low doping levels shows a significantly improved Jc and Fp, especially in the high field region. Hirr is also substantially enhanced. It is revealed that the magnetic flux pinning behavior of CrB2 doped samples obviously deviates from the surface pinning law followed by most pure MgB2. Through theoretical analysis and curve fitting, an extra pinning force superimposed on the surface pinning background is separated and identified, which follows the rule of point pinning. Such a pinning scenario is supported by the microstructure analyses which demonstrate that CrB2 nanoparticles are highly dispersed and embedded in MgB2 matrix, acting as effective pinning centers.