Nitrogen induced ferromagnetism in Cobalt doped BaTiO3

The electronic structure and magnetism of Cobalt doped BaTiO 3 (BaTi 1 − x Co x O 3 ) is investigated. Substitutional Nitrogen on an Oxygen site is found to play an important role in inducing net magnetic moments in the system. The presence of a Nitrogen atom as nearest neighbour to a Cobalt atom is crucial in producing spin splitting of both the Nitrogen and Cobalt states thereby introducing a net local magnetic moment. The introduction of Nitrogen is further found to enhance ferromagnetic interactions between Cobalt atoms. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License . [http://dx.doi.org/10.1063/1.4747820]

The electronic structure and magnetism of Cobalt doped BaTiO 3 (BaTi 1−x Co x O 3 ) is investigated. Substitutional Nitrogen on an Oxygen site is found to play an important role in inducing net magnetic moments in the system. The presence of a Nitrogen atom as nearest neighbour to a Cobalt atom is crucial in producing spin splitting of both the Nitrogen and Cobalt states thereby introducing a net local magnetic moment. The introduction of Nitrogen is further found to enhance ferromagnetic interactions between Over the past decade considerable interests have evolved around dilute magnetic semiconductors, as these materials seemed promising candidates for device applications. Ever since the discovery of ferromagnetism in p-type Mn-doped IV-VI, 1 III-V 2, 3 and II-VI 4 a lot of scientific efforts were driven towards the discovery and design of multifunctional materials that combine the functionalities of both semiconductors and magnetic transition as well as rare-earth metal ions. 5 There has also been an extensive effort to increase the Curie temperature of these systems by various means such as doping 6 as well as to look for new candidates for high T c ferromagnetism. Although there have been various reports of dilute magnetic semiconductors, it is only recently that experimental works have been devoted towards transitional metal doped insulators. Of particular interest have been perovskite oxides such as BaTiO 3 and SrTiO 3 which exhibit a wide range of interesting properties, by themselves, such as ferroelectricity 7 and superconductivity. 8 First principles calculations of transition metal doped BaTiO3 have been previously performed by Nakayama et al. 9 They reported the stabilization of a nonmagnetic state in Co doped BaTiO 3 and attributed the reason to the hybridization of the Co d and Oxygen p states. In later experimental reports, 10 however, ferromagnetism in Co doped BaTiO 3 was observed. Lin et al. did not observe the formation of Co clusters and attributed the origin of ferromagnetism to defects such as Oxygen vacancies.
The purpose of the present work is to design Co doped BaTiO 3 in a way so as to enhance ferromagnetism in this system. We thereby propose the introduction of substitutional Nitrogen on Oxygen sites as promising candidates.
In this work we perform first principles calculations to show that changes in the local chemical environment as well as the local lattice relaxation, results in the modification of the electronic structure of the Co states which in turn gives rise to a net local magnetic moment. Furthermore a ferromagnetic coupling between Co atoms is also favored in the presence of an intermediate N atom.
All calculations have been done using the Vienna Ab-initio Simulation Package (VASP) code. 11 We use projector augmented wave pseudopotentials. 12 For Ba and Ti, 5s 2 5p 6 6s 2 and 3s 2 3p 6 4s 2 3d 2 are included as valence electrons respectively. For Co 4s 2 3d 7 have been included as valence electrons. A plane wave cut off energy of 600 eV is used and a 4x4x4 Monkhorst-Pack special kpoint grid 13 is chosen for integration over the Brillouin zone. All structural relaxations are performed keeping the lattice constant of cubic BaTiO 3 fixed, which is taken to be the experimental value of (3.995 Å). We allow the internal atomic coordinates to relax. The energies are converged to within a E-mail: cmitra@physics.utexas.edu In order to study correlation effects of d electrons on Co, we add an onsite Colomb correlation parameter, U, on the d electrons. Here we employ the LSDA+U formalism as introduced by Dudarev et al. 14 For Co atom the U parameter is chosen according to Reference 15 where this parameter has been extracted from photoemission data of Co metal. An effective U (U-J) value of 4 eV has been used for all the calculations.
We first study the electronic structure of a single Co atom doped in a 135 atom supercell of BTO. This corresponds to a Co concentration of 3.7 %. We find that Co maintains a cubic symmetry with all the octahedrally coordinated Oxygen atoms sitting at equal distances from the Co atom. In Fig. 1(a) we show the spin resolved orbital projected density of states (DOS) of Co. Triply degenerate t 2g and doubly degenerate e g orbitals are seen which is to be expected and the system is metallic. There is no spin splitting and the system is found to be non-magnetic. We also find that the O p's hybridize with the Co d states in agreement to previous work 9 ( Fig. 1(b)).
We then introduce a second Co atom in order to study the magnetic interaction between two Co atoms. We introduce the second Co atom as first and second nearest neighbors in order to study the magnetic interaction as a function of distance between the Co atoms. For each Co-Co inter atomic distance we compare the energies between a ferromagnetic and an anti-ferromagnetic configuration. We find that in the absence of any defect the nearest neighbor Co atoms move slightly close to each other by an amount 0.05 Å compared to the ideal structure. Furthermore the the antiferromagnetic configuration is found to be favored over the ferromagnetic configuration by an amount of 0.16 eV. This is in accordance with the Goodenough-Kanamori-Anderson rule [18][19][20] where a superexchange antiferromagnetic interaction takes place for a 180 o cation-anion-cation bridge when a virtual electron transfer takes place between two half-filled (or empty orbitals). As second nearest neighbors the Co atoms show negligible interaction as both the ferromagnetic and antiferromagnetic configurations are found to be almost degenerate. This suggests the presence of extrinsic sources, in the form of defects, to make Co doped BaTiO 3 a promising candidate for room temperature ferromagnets.
In this work we propose that a N atom (which acts as a p-type dopant) substituted on an O site, as a nearest neighbor atom to Co, plays an important role in enhancing ferromagnetism in BaTi 1−x Co x O 3 . However, it must first be verified that N would indeed be energetically favorable to be present nearest to Co rather than far away. In order to check this we compare the formation energy of a supercell with a Nitrogen atom sitting nearest to a Co atom, which we call E near and that sitting far away, which we call E far . The formation energy is computed using the Zhang-Northrup formalism as described in. 16 It is obtained from the following equation: where the first two terms within the square bracket represents the difference in total energies between the supercell containing Nitrogen and that without it. Since Nitrogen is substituted on an Oxygen site one also needs the chemical potential of Nitrogen and Oxygen (μ N and μ O respectively) chosen as references. Here we choose these values as half of N 2 and O 2 molecules respectively. 17 Upon plugging in all the values in the above equation we obtain E near = 2.785 eV and E far = 4.225 eV. Hence the formation energy gets significantly lowered when the Nitrogen atom sits as a nearest neighbor to Co. This suggests that upon doping BaTi 1−x Co x O 3 with Nitrogen, the Nitrogen atom will be more likely to be found closer to the Cobalt atom thus enabling it to introduce important magnetic effects in the system.
The relaxation effect and the electronic structure of N doped BaTi 1−x Co x O 3 is now analysed. Upon relaxing the structure it is found that the N atom moves closer towards the Co atom (Fig. 2) and hence the Co-N bond length now decreases to 1.7Å compared to the ideal Co-O bond length of 1.99Å. In Figs. 3(a) and 3(b) we present the orbital resolved DOS plots of N and Co atom. Contrary to the O atom there is a clear spin splitting of the N spin up and spin down states resulting in a net magnetic moment of 1μ B . Due to the change in its chemical environment and also due to the breaking of its octahedral symmetry there is also a spin splitting in the Co states. This results in a magnetic moment of 0.8 μ B on the Co atom. The presence of Nitrogen plays an important role as it suggests that even in a very dilute limit of Co concentration one may obtain a net local magnetic moment moment of about 2μB/Co atom in the system. Upon placing the Nitrogen atom far away from Cobalt we find the system goes back to the non-magnetic state with no net magnetic moments being induced on either Nitrogen or Cobalt. We further introduce another Co atom in order to check the magnetic interaction between them in the presence of a N atom as seen in Fig. 4. We find that the ferromagnetic configuration is now favored by as much as 0.5 eV to the antiferromagnetic interaction, in the presence of N. This suggests that a p-type dopant in Co doped BaTiO 3 can indeed induce ferromagnetism in the system.
In conclusion, we have performed first principles calculation of Co doped BaTiO 3 and find that Nitrogen doping can play a significant role in making it a promising candidate for room temperature ferromagnetism. While in the absence of N the nonmagnetic state is stabilized in the cubic phase of BaTiO 3 , inclusion of N induces important changes in its local symmetry as well as shortening the Co-N bond length by 0.3 Å compared to the ideal structure. This brings about spin splitting of its electronic states which in turn results in stabilizing a net local magnetic moment. The presence of N also brings about ferromagnetic interaction between the Co atoms.
The author would like to thank Dr. Alexander A. Demkov and Dr. C Lin for useful discussions. This work is supported by the National Science Foundation under grant DMR-0548182, the US Department of Energy (DOE) under grant DE-SC0001878. All computations were performed at the Texas Advanced Computing Center (TACC).