Graduation Year

2014

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Electrical Engineering

Major Professor

Sanjukta Bhanja

Keywords

nanomagnetic disk, programmable nanomagnetic grid

Abstract

The Spin-Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM) has opened new doors as an emerging technology with high potential to replace traditional CMOS-based memory technology. This has come true due to the density, speed and non- volatility that have been demonstrated. The STT-MRAM uses Magnetic Tunnel Junction (MTJ) elements as non-volatile memory storage devices because of the recent discovery of spin-torque phenomenon for switching the magnetization states. The magnetization of the free layer in STT-MRAM can be switched from logic "1" to logic "0" by the use of a spin-transfer torque. However, the STT-MRAMs have till now only been used as universal memory. As a result, STT-MRAMs are not yet commercially used as computing elements, though they have the potential to be used as Logic-In-Memory computation applications.

In order to advance this STT-MRAM technology for computation, we have used different MRAM devices that are available as memory elements with different geometries, to use it as computing elements. This dissertation presents design and implementation of such devices using different multilayer magnetic material stacks for computation. Currently, the design of STT-MRAMs is limited to only memory architectures, and there have been no proposals on the viability of STT-MRAMs as computational devices. In the present work, we have developed a design, which could be implemented for universal logic computation. We have utilized the majority gate architecture, which uses the magneto-static interaction between the freelayers of the multilayer nanomagnets, to perform computation.

Furthermore, the present work demonstrates the study of dipolar interaction between nanomagnetic disks, where we observed multiple magnetization states for a nanomagnetic disk with respect to its interaction energy with its neighboring nanomagnets. This was achieved by implementing a single layer nanomagnetic disk with critical dimension selected from the phase plot of single domain state (SDS) and vortex state (VS). In addition, we found that when the interaction energy between the nanomagnetic disks with critical dimension decreases (increase in center-to-center distance) the magnetization state of the nanomagnetic disks changes from single domain state to vortex state within the same dimension. We were able to observe this effect due to interaction between the neighboring nanomagnets.

Finally, we have presented the design and implementation of a Spin-Torque driven Re- configurable Array of Nanomagnets (STRAN) that could perform Boolean and non-Boolean computation. The nanomagnets are located at every intersection of a very large crossbar array structure. We have placed these nanomagnets in such a way that the ferromagnetic free layers couple with each other. The reconfigurable array design consists of an in-plane (IP) free layer and a fixed polarizer [magnetized out-of-plane (OP)]. The cells that need to be deselected from the array are taken to a non-computing oscillating state.

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