Electronic and magnetic structure of transition-metal-doped -hematite Julian Velev Department of Physics, University of Nebraska, Lincoln, Nebraska 68588-0111, USA A. Bandyopadhyay Department of Electrical and Computer Engineering, University of Maryland, College Park, Maryland 20742-3285, USA W. H. Butler* Center for Materials for Information Technology and Department of Physics, University of Alabama, Tuscaloosa, Alabama 35487-0209, USA and Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6114, USA S. Sarker Department of Physics, University of Alabama, Tuscaloosa, Alabama 35487-0209, USA Received 9 October 2004; revised manuscript received 18 January 2005; published 24 May 2005 We investigate the electronic and magnetic structure of transition-metal-doped -hematite using the local density approximation with local correlations LDA+ U. The dopants in this study are the 3d transition metals Sc-Zn and Ga and are assumed to substitute on an Fe site. The calculated net moment per substitutional impurity is found to be |z D - z Fe |, opposite to that of the replaced Fe, where z D and z Fe are the numbers of valence electrons of the dopant and Fe, respectively. The dopants, D, substitute in an effective charged state D 3+ except for Ti 4+ and Zn 2+ . In the case of Ti, the extra electron converts a neighboring Fe 3+ atom to Fe 2+ . In the case of Zn, the missing electron generates a relatively diffuse hole at the top of the valence band spread over neighboring O atoms. DOI: 10.1103/PhysRevB.71.205208 PACS numbers: 75.10.Lp, 71.20.Ps, 71.55.Ht I. INTRODUCTION There is currently considerable interest in magnetic semi- conductors. We are particularly interested in a class of semi- conductors, such as -hematite corundum structure Fe 2 O 3 , that are magnetically ordered at room temperature and have relatively large moments. However, pure -hematite is an antiferromagnetic insulator. In order to make it a useful semiconductor, appropriate doping is necessary. There has been substantial effort in doping -hematite with various el- ements including Ti, 1 Sn, 2,3 and Zn. 4 Until recently, there have been few first-principles-based electronic structure cal- culations to complement these experimental studies. One reason for the absence of such studies is that the transition-metal oxides are strongly correlated materials due to strong local repulsion between electrons occupying the 3d orbitals of the magnetic ions. It is well known that ab initio studies based on density functional theory within the local density approximation LDA or the generalized gradient ap- proximation GGA do not work very well for strongly cor- related materials. 5 For example, a study of Ti-doped -hematite within the GGA approximation 6 predicted spuri- ous metallic behavior and strong hybridization between the impurity and the O bands. Much effort has been directed towards accounting for these electron correlations. The LDA+ U approximation 5 is one such approach that accounts for local electron-electron repulsion at the price of introducing a phenomenological pa- rameter U, which is the measure of this repulsion. In a pre- vious paper we studied Ti-doped -hematite using the LDA+ U method. 8 LDA+ U gives better agreement with ex- periment than LDA or GGA not only in relation to band gaps and local moments but also qualitatively. It predicts, for ex- ample, that Ti substitutes for Fe in -hematite as Ti 4+ and that the “extra” electron localizes on a neighboring Fe 3+ , converting it to a localized Fe 2+ . Conduction would then be expected to occur by activated hopping of this electron be- tween neighboring Fe 3+ ions, a picture which is consistent with experimental understanding. 1 In this paper, we continue the work on -hematite doping using the LDA+ U approximation. In the role of the dopant we use the entire range of 3d impurities Sc-Zn and Ga. In each case we assume that the dopant atom substitutes for an Fe. We have not investigated in this paper the possibility of anion vacancy formation or other extended defects that may form in complex oxides. II. ELECTRONIC STRUCTURE OF SUBSTITUTIONAL IMPURITIES IN -HEMATITE We investigate the electronic structure of systems of the type Fe 2-x A x O 3 where A is a 3d transition metal ion using a well-known and much used ab initio projector-augmented wave code. 7 Most of our calculations treat a single substitu- tional impurity in a 30-atom cell for which x =1/12. The 30-atom hexagonal cell used in our calculations consists of six layers with three coplanar O atoms each separated by six slightly buckled layers with two Fe atoms each. For most systems reported here, charge densities and total energies were evaluated using 50 k points in the irreducible zone. The host material, corundum structure R3-CH Fe 2 O 3 , was studied by us 6,8 and by other groups. 9,10 References 6 PHYSICAL REVIEW B 71, 205208 2005 1098-0121/2005/7120/2052087/$23.00 ©2005 The American Physical Society 205208-1