Flux dendrites produced by avalanches in superconducting films

Yuri Galperin^1,2 and T. H. Johansen¹

¹ Department of Physics and Center for Advanced Materials and Nanotechnology, University of Oslo, Norway
² A. F. Ioffe Physico-Technical Institute of RAS, St. Petersburg, Russia

We present review of our recent experimental and theoretical studies of magnetic flux penetration in thin superconducting films. Magneto-optical imaging reveals 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 behavior is responsible for the anomalous noise found in magnetization curves, and strongly suppresses the apparent critical current [1]. The instability is of thermo-magnetic origin, as supported by linear analysis of thermal diffusion and Maxwell equations [2], which allowed us to follow the development of the fingering instability in bulk samples [2] and thin films [3]. The theory allows explaining specific anisotropy in the flux penetration pattern in the films with anisotropic pinning [4].

In addition, we will discuss interaction of the vortices in a superconducting film with Bloch walls occurring in in-plane magnetized ferrite garnet films used for magneto-optical imaging. Our theory [5] explains how vortices are attracted to such walls, and excellent quantitative agreement is obtained for the resulting peaked flux profile determined experimentally in NbSe₂ using high-resolution magneto-optical imaging of vortices. In particular, this model, when generalized to include charged magnetic walls, explains the counterintuitive attraction observed between vortices and a Bloch wall of opposite polarity.

[1] T.H. Johansen et al., Supercond. Sci. Technol. 14 726 (2001); Europhys. Lett. 59 599 (2003).
[2] A.L. Rakhmanov et al., Phys. Rev. B 70, 224502 (2004).
[3] D.V. Denisov et al., Phys. Rev. B 73, 014512 (2006); D.V. Denisov et al., Phys. Rev. Lett. 97, 077002 (2006).
[4] J. Albrecht et al., Phys. Rev. Lett. 98, 117001 (2007).
[5] J.I. Vestgarden et al., Phys. Rev. Lett. 98, 117002 (2007).