Domain-wall pinning and depinning at magnetic soft spots in nanowires

The controlled motion of magnetic domain walls (DWs) in nanowires is one of the fundamental issues for the realization of new concepts of high-density and ultrafast non-volatile data-storage devices. Locally welldefined confining potentials, e.g., created by notches, act as pinning sites for individual DWs . A spin polarized current, driven through the nanowire, yields the possibility to manipulate the magnetic configuration due to a transfer of spin-angular momentum. For a reliable spin-torque induced domain-wall depinning, low threshold currents are required in order to reduce stochastic temperature effects, caused by Joule heating as well as to avoid damage to the nanowire.

Here we present a concept to create pinning sites for DWs in magnetic nanowires without geometric constrictions:

The local modification of magnetic properties by means of ion irradiation (magnetic soft spots). Implantation of chromium ions into Ni80Fe20 causes a reduction of the saturation magnetization (Ms) and changes the crystalline magnetic anisotropy. The application of electron beam lithography (EBL) or focused ion beam (FIB) on the other hand enables a spatial resolution below 50 nm for this kind of magnetic patterning process. Pinning of DWs in magnetic soft spots is expected to be preferred compared to the environment, as the locally reduced Ms causes a decrease of the exchange energy associated with the DW. In order to verify the suitability of our concept, we fabricate magnetic soft spots into Ni80Fe20 nanowires by means of 15 kV Cr ion irradiation through an EBL shadow mask and examine the pinning characteristics for DWs.

In conclusion, magnetic soft spots realized via implantation of chromium are suitable as pinning sites for domain walls, avoiding a local increase of the current density in the nanowire due to supplementary spatial constriction.