Domain-wall motion in permalloy nanowires with magnetic soft spots

The controlled motion of magnetic domain walls in nanowires is of scientific and technological relevance since it is necessary for new concepts of ultrafast, high-density, and nonvolatile data storage devices. Locally well-defined confining potentials, e.g., created by notches, act as pinning sites for individual domain walls. Magnetic configurations can be manipulated by external magnetic fields and by spin-polarized currents via transfer of spin-angular momentum.
The local modification of magnetic properties by ion irradiation opens the possibility to create pinning sites without geometric constrictions. Implantation of chromium ions into permalloy nanowires causes alloying and structural defects which leads to a reduction of the Curie temperature and the saturation magnetization Ms as well as small changes of the magnetocrystalline anisotropy.
The energy associated with the domain wall is locally reduced and a position at the so-called magnetic soft spot is energetically favorable in comparison to the environment, Field- driven pinning and depinning of a domain wall at the soft spots has been directly observed using magnetic transmission soft X-ray microscopy (MTXM) at beamline 6.1.2. of the Advanced Light Source in Berkeley, CA, USA. Magnetic contrast is provided via X-ray magnetic circular dichroism (XMCD) at the Ni L3-absorption edge. We have shown that the strength of the pinning potential and thus the depinning field can be tuned by the chromium ion fluence applied to induce the soft spots.