ABSTRACT
Diluted magnetic semiconductors, namely, semiconductors doped with magnetic impurities, or ternary alloys with transition metal (TM) components (Mn, Fe, Co) are recognized as templates for various devices in semiconductor spintronics. In this chapter we briefly discuss the physics related to the magnetic properties of wideband and narrow-gap III-V DMS bearing in mind the main subject of theoretical and experimental activity-room-temperature ferromagnetism. Material science aspects of ferromagnetism include the growth method, the choice of magnetic component and its content, and intentional manipulation of the electron spectrum of the host matrix by fabricating artificial structures like quantum wells and quantum dots.Experimental Curie temperatures and various theoretical approaches to ferromagnetic phase transitions in narrow-gap GaAs(TM) and wideband GaN(TM) have been topics of discussion for a long time now. The explanation of ferromagnetic phase transition in GaAs(TM) and GaN (TM) is normally based on the mean-field approximation (MFA) applied to the RKKY interaction (or its modification) [1-3]. It is instructive to look at the theory of ferromagnetism in DMS when MFA validity is under the question and to find an adequate picture of phase transition that can explain experimental results obtained to date. In this chapter, we Wide Bandgap Semiconductor Spintronics Vladimir Litvinov Copyright © 2016 Pan Stanford Publishing Pte. Ltd. ISBN 978-981-4669-70-2 (Hardcover), 978-981-4669-71-9 (eBook) www.panstanford.com
discuss some features of the theory when the MFA is hardly or not at all applicable to GaAs-and GaN-based DMS. 6.1 Mean-Field Approximation
Interacting spins in a magnetic field are described by the isotropic Heisenberg Hamiltonian 1= – ( – ) ,2i i jB i ji i, jH g Vm BS r r S S (6.1)where B is the external field magnetic induction, V(R) is the exchange interaction, g and mB are the magnetic impurity g-factor and the Bohr magneton, respectively. The first term in Eq. (6.1), the Zeeman energy, is written in a simplified form that does not take into account the orbital momentum of a magnetic atom. We will come back to this point in more detail in Section 6.3.1 where the g-factor is discussed.
