Modeling of Spintronic Devices: From Theory to Applications

Kabir, Mehdi, Electrical Engineering - School of Engineering and Applied Science, University of Virginia
Stan, Mircea, Department of Electrical and Computer Engineering, University of Virginia

In recent years, there has been a paradigm shift ushered in by “More-than-Moore” technologies which has focused on functional diversification of modern circuits rather than geometric scaling. One of the promising technologies in this field has been spintronics devices which exploits the spin of an electron instead of its charge. Furthermore, the integration of magnetic spintronics devices with MOSFET circuits--demonstrated by commercial devices such as STT-MRAM--has opened the possibility of Systems-on-Chip (SoC) integrated circuits with both types of components.

While there are many physics-based simulators which can study the detailed dynamics of magnetic materials and many CAD tools for large scale circuit design, there is a dearth of simulation tools for circuit designers working with spintronics devices. This dissertation proposes a Verilog-A behavioral hardware model of multiferroic and spin transfer torque (STT) devices which can be incorporated within traditional large scale CAD tools such as Synopsys HSPICE. Using this simulation platform, this work explores how spintronics devices can implement several More-than-Moore applications and proposes circuit and architecture-level designs to realize those applications. This dissertation considers two promising developments in magnetic spintronics devices--logic using multiferroic materials and applications using spin-torque nano-oscillators (STNO).

Multiferroic materials describe a class of materials which exhibit both ferroelectric and ferromagnetic behaviors. The combination of these two attributes allows for the control of the magnetic state of the material using an electric field rather than a magnetic field—a process known as electrically assisted magnetic switching (EAMS). This work develops a Verilog-A model which captures the EAMS process in multiferroic materials through a compact thermodynamic model. This model demonstrates that multiferroic nanopillars can not only be used to represent binary logic bits, but they also provide a third state that can be used for reconfiguration similar to traditional field programmable gate arrays (FPGAs). This dissertation describes the operations of a reconfigurable array of magnetic automata (RAMA) based on multiferroic nanopillars which can perform ultra-low power computation.

Yet another method to control the magnetization of materials using electrical currents is through the spin transfer torque (STT) effect. The STT effect manifests in magnetic tunnel junctions (MTJ) when a DC current is applied through the junctions. The modularity of the proposed Verilog-A model can be modified to include the STT effect to simulate the behavior of spin torque nano-oscillators. Furthermore, this model shows that connecting multiple STNOs leads to complex behaviors such as synchronization. In an array of parallel-connected STNOs, this synchronization can be exploited for pattern recognition applications. Finally, this dissertation explores applications using STNOs as on-chip RF components such as bandpass and bandstop filters. The nanoscale dimensions, electrically tunable frequencies and integration with MOSFETs make STNOs an attractive option for future RF components of SoC integrated circuit.

PHD (Doctor of Philosophy)
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