Wave reversal mode: A new magnetization reversal mechanism in magnetic nanotubes

We have investigated the magnetic properties of 14 nm thick and 1 mu m long nickel and permalloy nanotubes with external diameters of 40 and 100 nm as a function of the angle theta at which the external magnetic field is applied. Our results show that the coercivity of 40 nm diameter nickel nanotubes follows a non-monotonic behavior from theta = 0 degrees up to theta = 60 degrees, while that corresponding to permalloy displays an increasing monotonic trend at the same angular range. At theta = 90 degrees, both materials evidence a sharp drop of the coercivity to zero, indicating that the reversal mechanism has changed to a pseudo-coherent rotation. On the other hand, nickel and permalloy nanotubes with 100 nm in diameter exhibit a similar angular dependence of the coercivity, reversing their magnetization through the nucleation and propagation of vortex domain walls for angles lower than 75 degrees. For theta = 90 degrees, a novel striking mechanism, the wave reversal mode (W), arises. This phenomenon leads to an unusual S-type shape in the hysteresis curves at those given parameters, which is until now an effect that has not been reported for these nanostructures.