360 Degree Domain Walls

A 360 degree domain wall (360DW) is a metastable structure in which the magnetization rotates through 360 degrees, separating domains with the same magnetization direction. A 360DW is formed by two 180DWs of opposite sense. We are interested in the response of 360DWs to field and current, and in whether 360DWs could provide a token for data storage.

We use a specially designed feature to form a 360DW, in which an arc-shaped wire with width of 100-200 nm is connected to a circular pad. A first 180DW is formed by applying a high field in the +Y direction then removing the field. The second 180DW of opposite sense is formed by a subsequent field in the –Y direction whose amplitude is big enough to reverse the magnetization in the pad while small enough to maintain the magnetization in the wire. Due to magnetostatic interactions, the two 180DWs are attracted to each other and form a 360DW at the center of the arc. This behavior is confirmed by magnetic force microscope images in a cobalt sample. The behavior of a 360DW under field and spin current is studied using both simulations and experiments. We made 360DWs in Ni80Fe20 and used a four contact method to measure the magnetoresistance of the sample, providing an more convenient in-situ way to detect the DWs compared to magnetic force microscopy. We are also concerned with searching for innovative methods to control the DW configurations and dynamics by tuning the structure of the nanowire, for example by placing a simple gold pad on top of the nanowire to shunt the current and lead to a filter for domain walls. When a current pulse is injected into the nanowire, a fraction of it will go through the gold and generate an Oersted field in the Y direction underneath the gold pad. This will then have different effects on an approaching 360DW depending on its chirality. By carefully selecting the thickness of the gold pad, 360DWs with one chirality will be repelled by the gold pad while 360DWs of opposite chirality will pass through. This result provides a promising chirality filter concept for DW-based applications and gives extra freedom to DW-based devices.

360DW pic1The structure of a 360 degree domain wall, and observation by magnetic force microscopy.

360DW pic2

3D image of the pad+wire structure used to generate a 360 degree domain wall.

360DW pic3

Gold shunt pad on a magnetic wire can act as a filter for the passage of a 180 or 360 degree domain wall.