Field distribution and flux-line depinning in the mixed-state of MgB2   probed by Conduction Electron Spin Resonance
  We report the first observation of the internal magnetic-field distribution and flux-line depinning in the vortex-state of a type-II superconductor probed by conduction electrons spin resonance (CESR) technique. The CESR measurements were performed in the recently discovered MgB2 type-II superconductor compound with transition temperature Tc ~ 39 K using microwave sources at 4.1 (S-band) and 9.5 GHz (X-band) corresponding to resonance fields of Hr ~ 1455 and 3390 Oe for g ~ 2.00 in the normal state, respectively. From the distortion of the CESR line in the superconducting state, the field distribution function, n(H), in the vortex-state was inferred, and from the broadening of the line a direct estimate of the standard deviation, sigma ~ 14 Oe, was obtained at ~ 28 K and 7 K for S-band and X-band, respectively. Furthermore, our experiments allowed the determination of the flux-line lattice depinning temperature for both employed microwave frequencies. 
R.R. Urbano, P.G. Pagliuso, C. Rettori, Y. Kopelevich, N.O. Moreno and   J.L. Sarrao