Exchange Interactions and Nanoscale Phase Separations in Magnetically Doped Semiconductors

We have discussed the origin of the spin-dependent sp-d(f) coupling in DMSs as well as emphasized that it should be considered on equal footing with sp-sp exchange interactions in a number of experimentally important cases. Furthermore, it becomes increasingly clear that the effect of magnetic impurities on hole extended states changes dramatically if the ions give rise to hole bound states. At the same time, no hole-mediated ferromagnetism operates in the regime where holes are strongly localized. According to results summarized in this chapter, semiconductors which exhibit ferromagnetic features, such as spontaneous magnetization, can be grouped into five classes:1Diluted ferromagnetic semiconductors such as (Ga,Mn)As, heavily doped p-(Zn,Mn)Te, and related systems containing randomly distributed substitutional magnetic ions and delocalized or weakly localized holes. In these solid solutions the theory built on p-d Zener's model of hole-mediated ferromagnetism and on either the Kohn-Luttinger kp theory or the multiorbital tight-binding approach describes qualitatively, and often quantitatively, thermodynamic, micromagnetic, optical, and transport properties. Moreover, the understanding of these materials has provided a basis for the development of novel methods enabling magnetization manipulation and switching.2Carrier-doped DMSs in which a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localization boundary lead to an electronic nanoscale phase separation driven by competing interactions and/or mesoscopic fluctuations in the density of carrier states. These materials exhibit characteristics similar to colossal magnetoresistance oxides.3Alloys showing chemical nanoscale chemical phase separation into the regions with small and large concentrations of the magnetic ions. Here, high-temperature ferromagnetic-like properties are determined by the regions with high ion concentrations, whose crystal and chemical structure is imposed by the host.4Composite materials in which precipitation or contamination by nanoparticles of magnetic metal or of a ferromagnetic, ferrimagnetic, or antiferromagnetic compound account for ferromagnetic-like characteristics.5Finally, in magnetic semiconductors, such as EuO or CdCr2Se4, where most of cation sites is occupied by magnetic ions, short-range super- or double-exchange can, in specific cases, result in a ferromagnetic ordering. While semiconductors belonging to the first and fifth class show magnetic and micromagnetic properties specific to standard ferromagnets, materials encompassed by remaining classes exhibit rather characteristics of superparamagnetic systems. Importantly, the incorporation and spatial distribution of magnetic ions and, therefore, the relative importance of behaviors specific to particular classes, depend on the growth conditions and co-doping by shallow impurities. This explains a variety of magnetic properties observed in a given material system as well as elucidates why the magnetic response is often a superposition of ferromagnetic, superparamagnetic, and paramagnetic contributions. At the same time, the sensitivity to growth conditions and co-doping may serve to control the self-organized assembly of the magnetic nanocrystals. It has already been demonstrated that the use of embedded metallic and semiconducting nanocrystals can enhance the performance of various commercial devices, such as flash memories and low-current quantum-dot lasers as well as, by allowing to manipulate single charges and single spins in a solid state environment, has opened new research directions. As we have underlined in this chapter, a number of novel functionalities have been proposed for magnetic and metallic nanocrystals obtained during DMSs growth or processing. With no doubts, however, the question about the origin of high temperature ferromagnetism in semiconductors and oxides as well as the interplay between antiferromagnetic couplings, ferromagnetism, carrier localization, and superconductivity in transition metal compounds remain among the most intriguing topics in today's condensed matter physics and materials science. We may expect a fast development of novel tools for 3D element-specific and spin-sensitive analyses at the nanoscale, which will brought a number of unexpected developments in the years ahead. © 2008 Elsevier Inc. All rights reserved.