Diluted magnetic semiconductor (DMS) nanostructures have been the subject of intense experimental and theoretical investigation for their potential use in spin-electronic and spin- photonic devices. In colloidal DMS quantum dots grown from solution, stronger quantum confinement leads to extremely large exchange fields, and correspondingly robust excitonic magnetic polarons (EMPs). An important unanticipated feature of the robust EMPs found in colloidal Mn2+-doped CdSe quantum dots was the apparent role of anisotropy. The roles of nanocrystal shape and crystalline anisotropy on carrier-mediated magnetism in diluted magnetic semiconductor nanocrystals have been examined in this work. A combination of density functional theory (DFT) and analytical perturbation theory are used to investigate the electronic structures of Mn2+-doped CdSe nanocrystals of varied shapes and sizes. Density functional calculations are used to compute the magnetic exchange energies and to analyze the band edge splitting in these materials. To understand the observed anisotropic magnetism, we have derived a perturbative relationship between the magnetic exchange and nanocrystal anisotropy to illustrate the effect in an analytical and quantum mechanical expression. The first-principles calculations and the analytical predictions are in excellent agreement, and the anisotropic driving terms in magnetic exchange are very well explained by the final perturbation equation. The analytical model derived in this work can be generalized to nanocrystals with multiple charge carriers and multiple dopants. The theory presented here lays the groundwork for using crystalline and shape anisotropies to tune the magnetism in DMS nanocrystals.

Effects of crystallographic and shape anisotropies on dopant-carrier exchange interactions in magnetic semiconductor quantum dots

248th National Meeting (2014)

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