Extraordinary viseous magnetic wall motion in a wide range of velocities

The velocity v of extraordinary viscous wall motion in oxidized 83 at% NiFe films is investigated by Lorentz microscopy as function of field H and temperature T in the range 10−8 to 103 m/s. The velocity exponentially depends on H for eight decades up to 30 m/s, this range decreasing with T. With H approaching Hc the velocity drops to zero, whereas at high fields v saturates. At even higher fields reversal proceeds by incoherent rotation. The slope of the linear part of ln v = f(H) is proportional to T and is independent from the wall area. All effects can be explained by a model using relaxing ferrimagnetic inclusions which cause a time-dependent exchange anisotropy. The law ln v ∼ H + const emerges for intermediate velocities whenever the distribution of activation energies E of the inclusions is a slowly varying function of E.