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Nonvolatile Spin Memory based on Diluted Magnetic Semiconductor and Hybrid Semiconductor Ferromagnetic Nanostructures
Hani Enaya
Date: 2008-05-23
Degree: PhD - Electrical Engineering
Advisory Committee
Jacqueline Krim - Committee MemberJohn Zavada - Committee Member
Ki Wook Kim - Committee Chair
Salah Bedair - Committee Member
Venna Misra - Committee Member
Abstract
The feasibility of nonvolatile spin-based memory device concepts is explored. The first memory device concept utilizes the electrically controlled paramagnetic-ferromagnetic transition in a diluted magnetic semiconductor layer (quantum well or dot) when the ferromagnetism in the diluted magnetic semiconductor is mediated by itinerant holes. The specific structure under consideration consists of a diluted magnetic semiconductor quantum well (or quantum dot) and a nonmagnetic quantum well, which acts as a hole reservoir, separated by a permeable barrier. The quantitative analysis is done by calculating the free energy of the system. Formation of two stable states at the same external conditions, i.e., bistability, is found feasible at temperatures below the Curie temperature with proper band engineering. The effects of scaling the magnetic quantum well to quantum dot on bistability are analyzed. The bit retention time, i.e., lifetime, with respect to spontaneous leaps between the two stable states is calculated. The write/erase and read operations are discussed as well as the dissipation energy. Also, potential logic operations are proposed. In the second memory concept, the active region is a semiconductor quantum dot sharing an interface with a dielectric magnetic layer. The operating principle of the device is based on the spontaneous magnetic symmetry breaking due to exchange interaction between the magnetic ions in the magnetic layer and the spins of the itinerant holes in the quantum dot. Room temperature operation is possible given the availability of insulating ferromagnetic or antiferromagnetic materials whose Curie temperature is above room temperature. The specific range of material parameters where bistability is achieved is found. Analysis is extended to different quantum dot and magnetic dielectric materials and designs. Influence of material choice and design on the memory robustness, i.e., lifetime, is discussed.
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