First-principles Study of Nitrogen Vacancies in GdN
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First-principles Study of Nitrogen Vacancies in GdN Atchara Punya, 1 Tawinan Cheiwchanchamnangij, 1 Alexander Thiess, 1,2,3 and Walter R. L. Lambrecht1 1 Department of Physics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH-44106-7079 2 German Research School for Simulation Sciences, 52428 Jülich, Germany 3 Institute for Solid State Research and Institute for Advanced Simulation, Forschungszentrum Jülich, D-52425 Jülich, Germany ABSTRACT The electronic structure of nitrogen vacancies in gadolinium nitride are studied using the full-potential linearized muffin-tin orbital method in the local spin density approximation with Hubbard U corrections (LSDA+U). The vacancy is found to have two localized defect levels in the gap, one of each spin. The third electron of each vacancy in the neutral state dopes the conduction band. The single positive state is found to be the ground state for Fermi levels located anywhere within the band gap. The vacancy has a net magnetic moment of 1 μB in the neutral charge state. The presence of the vacancy is found to increase the average exchange interactions between Gd atoms and hence the Curie temperature but only by about a factor 2 compared to GdN without vacancies. INTRODUCTION Because of the high n-type carrier concentrations usually found in gadolinium nitride (GdN), it has in the past been controversial whether this material is semiconducting or semimetallic. Calculations employing the local spin density approximation or even the LSDA+U method, with Hubbard Uf and Jf parameters for f-electrons only, also find a zero gap [1,2]. Recently however, the improved growth techniques and new optical [3] and transport studies [4] have unambiguously determined semiconducting behavior. Nevertheless, the material always has unintentional residual n-type carriers. Nitrogen vacancies (VN) are often assumed to be the main source of this n-type doping, although substitutional oxygen impurities are also mentioned in some works.[5] Surprisingly, there have not yet been any computational studies of the VN in GdN. Here we present calculations of the energy of formation, electronic structure and magnetic properties of the VN. A second major reason for our interest in this defect is to investigate its role in the magnetic properties of GdN. In the early literature on GdN, it was assumed that the ferromagnetism of GdN was carrier mediated [5]. However, with the new growth techniques GdN much closer to perfect stoichiometry is obtained and still found to be ferromagnetic with a Curie temperature between 58 [6] and 70 K [4,7,8]. As pointed out by Kasuya et al. [9] this is surprising by comparison to other Gd pnictides (GdP, GdAs, GdSb, GdBi) and EuO. The other pnictides are all antiferromagnetic and semimetallic. The bulk ferromagnetic semiconductor behavior of EuO is well established and can be interpreted in terms of a double exchange mechanism of Eu-d states via oxygens combined with the intra-atomic fd-exchange coupling. However, in EuO the f-levels lie very close to the d states, f
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