Kinetic Exchange Vs. Room Temperature Ferromagnetism in Diluted Magnetic Semiconductors
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Kinetic exchange vs. room temperature ferromagnetism in diluted magnetic semiconductors J. Blinowski1, P. Kacman2 and T. Dietl2 1 Institute of Theoretical Physics, Warsaw Universit y, ul. Ho_za 69, PL-00-681 Warszaw a,P oland 2 Institute of Physics, Polish Academy of Sciences, al. Lotnikow 32/46, PL-02-668 Warszaw a, Poland
Abstract
Guided by the internal-reference rule and the known band o sets in III-V and II-VI diluted magnetic semiconductors, we discuss the feasibility of obtaining p-type conductivity, required for the carrier-induced ferromagnetism, as well as the cases for which the doping by shallow impurities may lead to the ferromagnetism driven by the double exchange. We consider the dependence of kinetic exchange on the p-d hybridization, on the electronic con gurations of the magnetic ions, and on the energies of the charge transfer betw een the valence band of host materials and the magnetic ions. In the case of Mn-based II-VI compounds, the doping by acceptors is necessary for the hole-induced ferromagnetism. The latter is, how ev er,possible without any doping for some of Mn-, Fe- or Co-based III-V magnetic semiconductors. In nitrides with Fe or Co carrier-induced ferromagnetism with TC > 300 K is expected in the presence of acceptor doping.
The discov ery of the ferromagnetic order in Mn-based III-V [1{4] and II-VI diluted magnetic semiconductors (DMS) [5, 6] started intensive studies of these materials and their layered structures [7, 8]. The recently demonstrated phenomena, such as spin-injection from (Ga,Mn)As contacts [9], tuning of magnetic properties by an electric eld [10], and large tunneling magnetoresistance [11], brought closer the idea of new semiconductor spin devices combining complementary features of semiconductor and magnetic systems. F or this purpose it is, however, of great importance to increase the Curie temperatures of ferromagnetic DMS. The hopes were awak enwhen the Curie temperatures abov e room temperature were predicted for materials containing light anions [12]. Much tec hnological e ort has been recently put in the search for di erent DMS exhibiting ferromagnetism at room temperature. Besides the standard II-VI and III-V materials, the variet y of other semiconducting compounds were used in the pursuit [13]. T o explain the origin of the ferromagnetism in tetrahedrally coordinated DMS as well as the values of the Curie temperatures the mean- eld Zener model was employed [12]. In this model, the ordering of spins results from the p-d kinetic exchange in teraction betw een the magnetic ions and the delocalized or weakly localized holes. Accordingly, the large value of the exchange energy N0 is one of the crucial conditions for high-temperature ferromagnetism, along with the large (of the order of few 1020 cm 3 ) hole and Mn concentrations. We note that in n-type DMS, only v ery low Curie temperatures can be expected because of small s-d exchange energy and low density of states in the conduction band [14]. Indeed, no ferromagnetism was detected above 1 K in (Zn,Mn)O:Al
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