Date of Award
Wiley, John B.
This research work focuses on the geometry and shape effects on submicron magnetic material. A web based micromagnetics program is written to model the hysteresis loop of nano magnetic samples with arbitrary geometry shapes and multiple magnetic materials. Three material samples have been modeled with this program along with nano magnets with a variety of geometric shapes. Shape anisotropy has been introduced to a permalloy ring by adding a cross-tie structure with various widths. The in-plane hysteresis loop and reversal behavior have no notable difference in direction parallel to the cross-tie, but greatly changed in perpendicular and diagonal directions. The switching field distribution is significantly reduced. The two distinct "onion" bit states of the modified ring elements are stabilized in the hysteresis in the diagonal direction. The changes in the modified rings make them better candidates for Magnetic Random Access Memory elements. Two Pac-Man elements, PM I and PM II, geometrically modified from disc and half disc respectively, are modeled. The PM I element undergoes a magnetic reversal through a two-stage mechanism that involves nucleation in the left and right middle areas followed by vortex core formation and vortex core motion in the lower middle area. The reversal process of the PM II element lacks the vortex core formation and motion stage. The switching field of the PM I and PM II elements are the same but the switching field distribution of the PM II elements is much narrower than that of the PM I element. Only the PM II element meets MRAM application requirements. The thickness dependence of the magnetic properties of a core-shell structure has been studied. The nano particles have a cobalt core and a permalloy shell. The nano spheres are the same size but with various shell thickness. Simulations reveal a multi-stage reversal process without the formation of a Bloch wall for thin-shell structure and smooth reversal process with the formation and motion of a Bloch wall for thick-shell structure. Gradual transition of the hysteresis loop patterns has been observed.
Zhao, Zhidong, "Modeling Shape Effects in Nano Magnetic Materials With Web Based Micromagnetics" (2005). University of New Orleans Theses and Dissertations. 157.