[1] Univ Leipzig, Dept Superconduct & Magnetism, Inst Expt Phys 2, D-04103 Leipzig, Germany
[2] Univ Leipzig, Dept Nucl Solid State Phys, Inst Expt Phys 2, D-04103 Leipzig, Germany
[3] AF Ioffe Phys Tech Inst, St Petersburg 194021, Russia
[4] Umea Univ, Dept Expt Phys, S-90187 Umea, Sweden