Coercivity enhancement in FePt nanowires due to the suppression of available paths for domain wall propagation

This paper reports the observation of magnetization reversal in nanometric devices with perpendicular anisotropy using extraordinary Hall effect. Hall crosses were processed by scanning electron microscope lithography in FePt(001)/Pt(001) layers deposited by molecular beam epitaxy. Hysteresis loops are found to be strongly dependent on the wire width, particularly with an increase of the coercitive field at low sizes. We show that this is not due to a lower probability for extrinsic nucleation to occur in smaller structures but rather due to the suppression of available domain wall propagation paths in thin wires. An invasion percolation model, as well as simulations, is proposed to describe this coercivity enhancement, based on a quasisquare lattice of structural defects, the microtwins, being clearly responsible in our samples for the magnetic coercivity. Finally, we report the direct observation of individual Barkhausen jumps in 200-nm-wide crosses, showing that the measured amplitudes of these jumps correspond to the microtwins' areal density.