Dendritic magnetic instability in superconducting MgB2 films

– Magneto-optical imaging reveal that below 10 K the penetration of magnetic flux in MgB2 films is dominated by dendritic structures abruptly formed in response to an applied field. The dendrites show a temperature-dependent morphology ranging from quasi-1D at 4 K to large tree-like structures near 10 K. This behaviour is responsible for the anomalous noise found in magnetization curves, and strongly suppresses the apparent critical current. The instability is of thermo-magnetic origin, as supported by our simulations of vortex dynamics reproducing the variety of dendritic flux patterns. The new superconductor [1], MgB2, has already proved to be a promising candidate for technological applications due to success in fabrication of thin films [2] and wires [3] with high current carrying capabilities. At the same time, such films and wires, as well as polycrystalline MgB2 are reported to show exceptional magnetic behaviour displaying numerous and “noiselike” jumps in the magnetization as a function of applied field [3–5]. Magnetization jumps in type-II superconductors are usually associated with a thermo-magnetic instability of the flux lines (vortices) penetrating the material. Motion of vortices is accompanied by a heat dissipation, which reduces flux pinning and facilitates further motion, eventually leading to a large-scale avalanche invasion of depinned flux [6]. In high-temperature superconductors (HTSs) the flux jumps have been observed only in bulk materials, where the first jump occurs typically at applied fields of 1 Tesla, and with subsequent ones coming nearly periodically as (∗) E-mail for correspondence: [email protected] (∗∗) E-mail for request of the materials: [email protected]