Manipulation of Individual Magnetic Domain Walls with a Local Circular Field
Understanding domain wall (DW) motion in nanoscale ferromagnetic structures reveals intriguing physics, with tremendous potential applications in logic devices and racetrack memory. Work has been done to move a series of DWs uniformly, either by passing spin-polarized currents or applying non-uniform magnetic fields. One challenge remained is to create and move individual DWs in arbitrary locations. We developed a technique to generate localized circular magnetic field by applying a current through the tip of the atomic force microscope (AFM) and thereby manipulating the state of the ferromagnetic rings. Now we extend our ability to control domain walls in various structures, such as straight wires with notches and zigzag wires. By placing the tip near a 180◦ DW in a vertex of a zigzag wire, we can move the 180◦ DW along the wire and form a stable 360◦ DW in neighboring vertex. We can also separate the 360◦ DW back to two 180◦ DWs or annihilate the 360◦ DW entirely. Micromagnetic simulations have been performed at Odyssey computer cluster at Harvard University with Object Oriented Micro Magnetic Framework (OOMMF), a public domain program distributed by NIST, to study the evolution of magnetic states in various structures in a uniform or circular magnetic field. NiFe and Co nanowires have been fabricated with Electron Beam Lithography and lift-off technique. We are presently working on to experimentally pass current through AFM tip and examine magnetic states with Magnetic Force Microscope (MFM) imaging. In the future, we plan to simultaneously measure magnetoresistance while manipulating DWs in order to monitor the motion while it is occurring. We would also like to understand the probabilistic nature of DW motion in a greater depth.