Magnetic interactions ofCr−CrandCo−Coimpurity pairs inZnOwithi

Abstract

The well-known "band-gap" problem in approximate density functionals is manifested mainly in an overly low energy of the conduction band ͑CB͒. As a consequence, the localized gap states of 3d impurities states in wide-gap oxides such as ZnO occur often incorrectly as resonances inside the CB, leading to a spurious transfer of electrons from the impurity state into the CB of the host, and to a physically misleading description of the magnetic 3d-3d interactions. A correct description requires that the magnetic coupling of the impurity pairs be self-consistently determined in the presence of a correctly positioned CB ͑with respect to the 3d states͒, which we achieve here through the addition of empirical nonlocal external potentials to the standard density functional Hamiltonian. After this correction, both Co and Cr form occupied localized states in the gap and empty resonances low inside the CB. In otherwise undoped ZnO, Co and Cr remain paramagnetic, but electron-doping instigates strong ferromagnetic coupling when the resonant states become partially occupied.

Microscopic picture of Co clustering in ZnO

“…16 The importance of a correct description of the electronic structure for the determination of the magnetic properties has been emphasized in other recent studies. 24,25 In addition, these studies suggest that the weak antiferromagnetic coupling observed in the undoped Co:ZnO could be turned to a ferromagnetic coupling by electron doping, the stabilization of the FM state being due to the partial occupation of the t 2 -like minority bands. The connection between the electronic structure and the magnetic properties of transition-metal oxides has been discussed in more general terms by Solovyev.…”

The quest for dilute ferromagnetism in semiconductors: Guides and misguides by theory

“…One such example is our earlier work in which we designed empirical nonlocal external potentials (NLEP) [38], in the spirit of the method of Christensen [39], where external potentials are placed on host atoms (and interstitial sites) and the potential parameters are adjusted to reproduce the correct host band structure. Applying such band-gap corrections to self-consistent impurity calculations allows one to place the impurity levels in the correct manifold of host states.…”

Polaronic hole localization and multiple hole binding of acceptors in oxide wide-gap semiconductors

“…(4), and our (occupation-independent) non-local external potential V nlep of Ref. [18]. The hole-state potential V hs , eq.…”