Magnetic Logic Devices: Design, Simulation and Measurement

The constant scaling of transistor sizes that has driven the extraordinary develo- pment of CMOS technology in the last decades. The reduction of transistor sizes has many advantages: the circuit area and power consumption decrease and the clock frequency increases. However the scaling of transistor sizes is rapidly moving to- wards its physical limits. The two major factors limiting the development of CMOS technology are the difficulties in the fabrication process and the unavoidable impact of leakage losses, mainly due to the gate tunnel current. To overcome the limitations of CMOS transistors and to create circuits even denser and faster, many alternative technologies have been studied. Among these possibilities, the technologies based on magnetic devices are really interesting and promising: NanoMagnet Logic (NML), Domain Wall (DW) and Spin Wave (SW) are all part of a new family of emerging magnetic logic devices. Their magnetic nature represents the main reason of this increasing interest: they can mix logic, they can have a very low dynamic power consumption. NML and DW have also no stand-by power dissipation. The main purpose of this thesis is to investigate and to design logic circuits and interconnections based on magnetic technologies. After a complete introduc- tion analyzing different magnetic technologies, the thesis work focuses on the design, validation through simulations and analysis of logic circuits and innovative intercon- nection methods. In fact interconnections between logic elements represent a limit in many of these magnetic circuits. The research work here presented has led to the development of advanced interconnections based on different magnetic technologies. At the beginning the attention has been focused on Domain Wall technology. From the magnetic point of view, a domain wall is an interface which divides diffe- rent magnetic domains, which are defined as regions with a uniform magnetization. A domain is a region inside a magnetic material where the magnetic moments of atoms have the same direction and versus between each other. A domain wall is a region of transition between different magnetic moments in which occurs a gra- dual orientation of the magnetic moments. A domain wall has been used to interface NML logic elements. The traditional interconnection composed by nanomagnets has been substituted by a new interconnection system based on domain walls. Different structures have been presented and validated through accurate simulations. A para- metrical analysis of the new interconnection methods has been done, considering the length and the width of the line. This analysis has been used to better understand and to validate the working principle, the efficiency in terms of domain wall speed and the power consumption of the new interconnection system. This analysis has been also fundamental in finding the best trade-off between power dissipation and operation frequency to achieve a circuit based on domain wall with the best possible performance. In the second part of the thesis the attention has been directed to the Spin Wave technology. A spin wave is a collective oscillation of spins along the magnetiza- tion direction of a magnetic structure. In this case, in addition to an innovative interconnection method based on spin wave, pure spin wave logic circuits have been analyzed and validated through simulations. Moreover spin wave devices based on reconfigurable magnetic patterning structures have been presented and validated through micromagnetic simulations. This innovative technique can be used in order to pattern complex structures in a continuous film, avoiding possible scattering at interfaces and introducing a reconfigurability and tunability. This last part has been carried on in collaboration with the University of Notre Dame. The last part of the thesis work focuses on the Magneto-Optic Kerr Effect (MO- KE). This activity, carried on at the University of Notre Dame, consisted in the design and assembly of a MOKE system, with the aim of measuring magnetic sam- ples with logic gates and interconnections to validate the simulation results. The reflected optical response from a magnetic media has been introduced, using the theory of light propagation inside and at interfaces between the media. Taking ad- vantage of the electromagnetic theory, the optical rotation phenomenon observed in magnetic media is described with the expression of the reflectivity. The complete experimental set-up for longitudinal MOKE, assembled at the University of Notre Dame, is described in detail, with the correspondent operating manual useful to perform a complete measurement of a magnetic hysteresis. The research work of this thesis represents an important milestone in order to improve the NML interconnections and to design innovative logic devices. Never- theless work is still necessary to reach a complex system, analyzing and validating the behavior of other interconnection structures with different geometries. Many changes are necessary to guarantee the correct operation and to further improve a complete system, composed by logic gates and interconnection wires. Innovative approaches, for example to combine all the magnetic technologies presented in this work in a new kind of magnetic circuit, is an indispensable requirement to develop really innovative technologies and systems.