Asymmetric Metal-Insulator Transition in Disordered Ferromagnetic Films

We present experimental data and a theoretical interpretation of the conductance near the metalinsulator transition in thin ferromagnetic Gd films of thickness b approximate to 2-10 nm. A large phase relaxation rate caused by scattering of quasiparticles off spin-wave excitations renders the dephasing length L phi less than or similar to b in the range of sheet resistances considered, so that the effective dimension is d = 3. The conductivity data at different stages of disorder obey a fractional power-law temperature dependence and collapse onto two scaling curves for the metallic and insulating regimes, indicating an asymmetric metal-insulator transition with two distinctly different critical exponents; the best fit is obtained for a dynamical exponent z approximate to 2.5 and a correlation (localization) length critical exponent v_ approximate to 1: 4 (v(+) approximate to 0.8) on the metallic (insulating) side.