Magnetic Rings for Memory and Logic Devices

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In this project, the properties of mesoscopic magnetic rings will be investigated, and their utility in magnetic devices such as memory or logic cells will be explored. Thin-film magnetic rings, with diameters of 200 nm and above, can be magnetized in several distinct states. For example the magnetization can be oriented circumferentially around the ring (a 'vortex' state), or the ring can contain two head-on walls at opposite ends of a diameter (an 'onion' state). Recently, another state (a 'twisted' state) was observed which consists of a vortex state containing a 360 wall. Rings can be reproducibly switched between these states, introducing the possibility that rings could be candidates for magnetic data storage or data manipulation devices. In this project, circular and elliptical rings will be fabricated using electron-beam lithography combined with ion-beam sputtering. Rings will be made from single magnetic layers, exchange-biased layers and spin-valve multilayers. Their magnetic states will be determined as a function of geometry. In particular the stability of the vortex, onion and twisted magnetization states, the possibility of moving walls around the ring, and pinning of the walls will be examined. Micromagnetic modeling will be used to help interpret the observations. Magnetoresistance measurements on small rings will be made by electrically contacting the rings in a two-level lithography process. Based on the results, a magnetic random access memory cell will be prototyped, in which data bits are stored in the ring as different magnetization states. Bits will be written by applying a field and read back using magnetoresistance. The possibility of designing a magnetic logic gate will also be assessed, in which data bits (represented by domain configurations) can be written, manipulated, and read back. Education and communication activities will be carried out in combination with these research activities. These will include the participation in research of two graduate and several undergraduate students, the incorporation of research findings into teaching, and the continued development of a series of local school outreach activities. In addition, a software tool will be designed suitable for senior high school or freshman college students, which will illustrate magnetic fields, flux density and magnetization, and allow students to explore hysteresis phenomena, improving their understanding of magnetic materials and devices.