Enhanced flux pinning by magnetic CrB2 nanoparticle in MgB2 superconductor

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 CrB ₂ were introduced as magnetic pinning center to improve the flux pinning force density ( F _p), irreversible field ( H _irr), and critical current density ( J _c) of MgB ₂. Compared to the pristine sample, the CrB ₂ doped MgB ₂ with low doping levels shows a significantly improved J _c and F _p, especially in the high field region. H _irr is also substantially enhanced. It is revealed that the magnetic flux pinning behavior of CrB ₂ doped samples obviously deviates from the surface pinning law followed by most pure MgB ₂. 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 CrB ₂ nanoparticles are highly dispersed and embedded in MgB ₂ matrix, acting as effective pinning centers.