Theory for Carrier-Induced Ferromagnetism in Diluted Magnetic Semiconductors

概要

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A theory is presented for the paramagnetic-ferromagnetic phase transition induced by spin-carriers in diluted magnetic semiconductors. The localized spins of d-electrons are approximated by Ising spins which are coupled with itinerant spin-carriers through the s-d interaction. The scattering of the carriers by the randomly distributed localized spins is treated by the coherent potential approximation at finite temperatures. The magnetic moments of localized spins are evaluated from the free-energy, and the Curie temperature T_c is determined as a function of the concentration of localized spins x as well as the carrier-density n. The emergence of a narrow band of local density of states associated with the magnetic impurities at the edge, or below the edge, of the energy band of the host crystal plays a crucial role in determining the magnetic properties of the system. It is predicted that T_c is maximized for n ≃ x/2, but decreases for over-doping, and becomes zero at a critical value n_c(≥x).
2002-08-15