Photo induced Ferromagnetism and Mechanism of Ferromagnetism in Diluted Magnetic Semiconductor
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Date
2010-06
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Addis Ababa University
Abstract
Conventional electronic devices rely on manipulating charge to produce desired functions,
spintronic devices would manipulate both the charge flow and electron spin within
that flow. This would add an extra degree of freedom to microelectronics and usher in
the era of truly nanoelectronic devices. Research aimed at a whole new generation of
electronic devices is underway by introducing electron spin as a new or additional physical
variable, and semiconductor devices that exploit this new freedom will operate faster
and more efficiently than conventional microelectronic devices and offer new functionality
that promises to revolutionize the electronics industry.
In order to enable electronic devices in active part of operation efforts have been made
to develop diluted magnetic semiconductors(DMS) in which small quantity of magnetic
ion is introduced in to normal semiconductors. The first known such DMS are II-VI and
III-V semiconductors diluted with magnetic ions like Mn, Fe, Co, Ni, etc. Most of these
DMS exhibit very high electron and hole mobility and thus useful for high speed electronic
devices.
In this thesis we study a photo induced ferromagnetism and mechanism of ferromagnetism
in diluted magnetic semiconductors by solving a Hamiltonian model that consists
of localized magnetic moments interacting with photoexcited carriers. The mechanism
for photo-induced ferromagnetism is coherence between conduction and valence bands
induced by the light which leads to an optical exchange interaction. When light is incident
on the diluted magnetic semiconductors, electron and holes are created across the
band gap. Photo excited carriers mediated a ferromagnetic interaction between the localized
moments resulting in ferromagnetic state in the range of critical temperature. The
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situation is similar to the famous Rabi problem of a two state system coupled to timedependent
oscillating electric field. The time dependance of the light-matter interaction
term is eliminated by a unitary transformation and the resulting Hamiltonian is solved
by making a Bogoliubov-Valtain transformation. Since the system of electrons and holes
in contact with the photon bath is considered in a steady state, we calculate the free
energy of the system. Starting the free energy again we calculate the magnetization of
the system in self-consistent mean field way. The magnetization and magnitude of Tc is
determined by the photon energy incident on the system. By increasing the light coupling
and frequency of the light, the transition temperature is increased
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Photo induced Ferromagnetism