Experimental Study of Exchange-Coupled Nanomagnets

We study novel ways of doing computation by using nanomagnetic systems. Extending the nanomagnet logic (NML) concept, which is based on dipole coupling between neighboring nanomagnets, we attempt to use inter-layer exchange coupling as an additional coupling mechanism between two laterally adjacent nanomagnets. Simulations show that more than 4 times stronger coupling can be achieved between magnets in such a system compared to a conventional dipole-field coupling based system. The simulated system was successfully fabricated, and more than 90% of the fabricated devices worked reliably. Using the coupling scheme as a building block, a three-input majority gate has been successfully simulated. This coupling mechanism can tremendously benefit miniaturization of nanomagnets, enhancing its immunity to thermally induced errors at the same time, and this opens new perspectives for magneto-logic devices.

Additionally, we study the reversal properties of nanomagnets patterned from exchange and/or dipole-coupled magnetic multilayers using vibrating sample magnetometry (VSM). The joint effects of shape anisotropy, exchange, and dipole coupling between the multilayers have been explained by micromagnetic simulations, and transverse magnetization metrology (TMM) was used for the first time to study the switching behavior of such systems. Gradual/macro-spin type and abrupt/nucleation-growth type switching have been observed in exchange- and dipole-coupled nanomagnets, respectively. The role of thermal fluctuations in these systems has also been explored.

Part of my work was to develop the capability to measure magnetic processes in the GHz range. Ferromagnetic resonance (FMR) was used to study dynamic properties of nanomagnets with strong emphasis on application drivers. These measurements were the first dynamic, high-speed characterization of nanomagnet devices by the Notre Dame group. Moreover, new device concepts based on spin waves and the interactions of spin waves with magnetic fields and magnets of different shapes have been explored. This dissertation describes the technologies for fabrication and characterization of nanomagnet arrays and their integration with waveguides for high-frequency studies. Simulations were done to reveal the details of the switching process, and the interaction of nanomagnets with microwave components.