Current-Induced Distortion and Displacements of Bloch Walls in Nickel-Iron Films

The interaction between an electric current and a Bloch wall has been studied experimentally by sending short current pulses through Ni-Fe films. Irreversible wall displacements start at a critical current density j_{c}. For a 743 nm film in zero magnetic field, these displacements are as large as 48 μm at a current density 53% above j_{c}. When the film thickness w is varied, j_{c} shows a variation given by j_{c }~ w^{-2.08}. Also, j_{c} is independent of the duration tau of the rectangular pulse, when tau is varied between 50 and 300 ns, and decreases monotonically when a hard -axis field is applied to the film. When applying an easy -axis field H_{z} at the same time as the current pulses of density j_ {x}, the boundary between regions of static and moving walls in the (j_{x}H _{z}) plane can be determined. The shape of this boundary confirms the presence of a current -induced coercivity-reduction effect already found in the thesis work of Hung. The thickness dependence of j_ {c} indicates that the main interaction between the current and the Bloch wall involves a wall -distortion mechanism. In this mechanism, the circumferential field of the current distorts the wall to an S shape, therefore increasing its energy and leaving it in a non-equilibrium state. Wall oscillations, induced by the falling edge of the current pulse, reduce the coercivity to zero for a certain time interval after the pulse, and the domain wall moves during this interval of ballistic overshoot. Based on these ideas and a one-dimensional wall model, the ( j_{x}H_{z}) phase boundary is calculated, and is in good agreement with the experimental one. Also, a calculation of the ballistic overshoot displacements is performed. This calculation predicts correctly the order of magnitude of maximum wall displacements observed, and the shape of the curve representing the variation of the wall displacement as a function of current density.