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Register a new userOrigin and control of ferromagnetism in dilute magnetic semiconductors and oxides
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Authors
Tomasz Dietl
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The author reviews the present understanding of the hole-mediated ferromagnetism in magnetically doped semiconductors and oxides as well as the origin of high temperature ferromagnetism in materials containing no valence band holes. It is argued that in these systems spinodal decomposition into regions with a large and a small concentration of magnetic component takes place. This self-organized assembling of magnetic nanocrystals can be controlled by co-doping and growth conditions. Functionalities of these multicomponent systems are described together with prospects for their applications in spintronics, nanoelectronics, photonics, plasmonics, and thermoelectrics.
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This paper reviews the present understanding of the origin of ferromagnetic response of diluted magnetic semiconductors and diluted magnetic oxides as well as in some nominally magnetically undoped materials. It is argued that these systems can be grouped into four classes. To the first belong composite materials in which precipitations of a known ferromagnetic, ferrimagnetic or antiferromagnetic compound account for magnetic characteristics at high temperatures. The second class forms alloys showing chemical nano-scale phase separation into the regions with small and large concentrations of the magnetic constituent. To the third class belong (Ga,Mn)As, heavily doped p-(Zn,Mn)Te, and related semiconductors. In these solid solutions the theory built on p-d Zeners model of hole-mediated ferromagnetism and on either the Kohn-Luttinger kp theory or the multi-orbital tight-binding approach describes qualitatively, and often quantitatively many relevant properties. Finally, in a number of carrier-doped DMS and DMO a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localisation boundary lead to an electronic nano-scale phase separation.
We study the ferromagnetism of Ga1-xMnxAs by using a model Hamiltonian, based on an impurity band and the anti-ferromagnetic exchange interaction between the spins of Mn atoms and the charge carriers in the impurity band. Based on the mean field approach we calculate the spontaneous magnetization as a function of temperature and the ferromagnetic transition temperature as a function of the Mn concentration. The random distribution of Mn impurities is taken into account by Matsubara and Toyozawa theory of impurities in semiconductors. We compare our results with experiments and other theoretical findings.
Magnetic properties of Ga$_{1-x}$Mn$_x$N are studied theoretically by employing a tight binding approach to determine exchange integrals $J_{ij}$ characterizing the coupling between Mn spin pairs located at distances $R_{ij}$ up to the 16th cation coordination sphere in zinc-blende GaN. It is shown that for a set of experimentally determined input parameters there are no itinerant carriers and the coupling between localized Mn$^{3+}$ spins in GaN proceeds via superexchange that is ferromagnetic for all explored $R_{ij}$ values. Extensive Monte Carlo simulations serve to evaluate the magnitudes of Curie temperature $T_mathrm{C}$ by the cumulant crossing method. The theoretical values of $T_mathrm{C}(x)$ are in quantitative agreement with the experimental data that are available for Ga$_{1-x}$Mn$_x$N with randomly distributed Mn$^{3+}$ ions with the concentrations $0.01 leq x leq 0.1$.
Over the last decade the search for compounds combining the resources of semiconductors and ferromagnets has evolved into an important field of materials science. This endeavour has been fuelled by continual demonstrations of remarkable low-temperature functionalities found for ferromagnetic structures of (Ga,Mn)As, p-(Cd,Mn)Te, and related compounds as well as by ample observations of ferromagnetic signatures at high temperatures in a number of non-metallic systems. In this paper, recent experimental and theoretical developments are reviewed emphasising that, from the one hand, they disentangle many controversies and puzzles accumulated over the last decade and, on the other, offer new research prospects.
We show through density functional theory calculations that extended magnetic states can inherently occur in oxides as the size of the crystals is reduced down to the nanometer scale even when they do not explicitly include intrinsic defects. This is because in nanoscale systems crystallographically perfect crystallites paradoxically result in nonstoichiometric compositions owing to the finite number of constituting atoms. In these structurally perfect but stoichiometrically imperfect nanocrystallites, the spin-triplet state is found to be more stable than the spin-singlet state, giving rise to an extended spin distribution that expands over the entire crystal. According to this picture, long-range magnetic order arises from the combined effect of crystal symmetry and nonstoichiometry that can coexist exclusively in nanoscale systems. The idea can also give reasonable explanations for the unprecedented ferromagnetic features observed commonly in nanoscale oxides, including ubiquity, anisotropy, and diluteness.
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