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Nanoring is a ring-shaped nanoscale entity. The diameter is between 50 nm and 1 um. It can be made of normal metal, magnetic metal, semiconductor, oxides and even polymers. I am working on the magnetic nanorings, typically made of cobalt or permalloy, that have novel properties. Magnetic nanoring was first proposed by Prof. J.-G Zhu in 2000 (JAP 87, 6668) as a new structure for magn3tic random access memory (MRAM). Since then the magnetic properties and fabrication processes have been greatly explored. |
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Magnetic nanorings are both scientifically and technologically important. Microscale and nanoscale magnetic elements, such as circular disks and rectangles, have rich domain structures determined by their geometry. Very small entities usually have the single domain state, in which all spins point to the same direction; Large enetities can stay in the so-called vortex state, as shown on the right, whose magnetic moments form a closure structure. No stray field is leaked out. Sitting at the center of a vortex is the vortex core with magnetic moments pointing out of the plane, either up or down. Vortex state becomes less and less stable when the laterial size shrinks, and eventually transforms to single domain state when the size is below the critical size. For example, the critical diameter of thin Py disk is about 100 nm. This transition is the manifestation of the competition between the exchange energy (dominant at small scale) and the magetostatic energy (dominant at large scale). In nanorings, however, stable vortex states can be achieved due to the lack of vortex core. All moments are completely comfined within the plane and form a closure, as shown on the right. The chirality of the vortex, clockwise and counterclockwise, can be unitilized to store information.
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Electron beam lithography (EBL) is the mostly commonyl used method to fabricate nanorings. However, only a small number of rings (<103) over a small area (um2) at once. The area density and throughout are both low. We have developed a fast, cheap and efficient method to fabricate billions of nanorings at once on a macroscopic area with very high areal density. Step 1: Cover the substrate with a monolayer of non-contacting nanospheres. Step 2: Sputter a thin film of magnetic material on the substrate. Step 3: Use Ar ion beam to mill the sample at a normal angle or oblique angle. Step 4: Sputter 5 nm Au on top of everything to prevent oxidation. This process uses nanospheres as template, often referred as "natural lithography". The diameter, areal density, thickness and composition of nanorings can be easily controlled. The symmetry of nanorings can also be engineered to obtain asymmetric nanorings. |
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