First-Principles Study of Electronic Properties and Robust Half Metallicity of Mixed Sr1-xCaxBeO3 (x = 0.25) Perovskite

I. INTRODUCTION

Half-metallic compounds are characterized by the coexistence of metallic behavior of one spin electrons and the insulating/semiconducting behavior of other. The electronic density of states of such compounds is 100% spin polarized at Fermi level (EF) and in these compounds, the metallic single spin charge carriers dominate the conductivity. After the remarkable discovery of HM ferromagnetism in NiMnSb by de Groot et al. [1], the research in this field has made significant developments. The HM character can also be confirmed very well experimentally these days [2-4]. ABO3 perovskite oxides exhibit a broad spectrum of physical properties such as piezoelectricity, semiconductivity, ferroelectricity, superconductivity, metallic conductivity, paramagnetism, ferromagnetism, half metallicity etc. [5] and these properties can be controlled by several ways such as by changing ion size, electronic configuration and preparative conditions. Metallic SrRuO3 has attracted much interest of researchers due to its unusual high Curie temperature (Tc = 160 K) which is unique among 4d/5d transition metal oxides [6,7]. In the past, RTFM was reported in various kinds of perovskites e.g. BaTiO3 [8], double perovskites Sr2FeMoO6 [9] and doped perovskites Bi0.5Na0.5TiO3 [10] etc. A-site substituted Sr1−xCaxRuO3 was found metallic, but it suffers a substantial lattice contraction and a gradual loss of FM [11,12] due to the lowering of the DOS at EF. Mahmood et al. [13] investigated the Half-metallic ferromagnetism and optical behaviors of XBeO3 (X=Mg, Ca, Sr and Ba) perovskites. They showed thermodynamically stability, half metallicity and room temperature (RT) ferromagnetism of all compounds by computing their Curie temperatures and spin polarizations [13]. After reviewing the literature, we planned to check whether the half metallicity and magnetism get retained by SrBeO3 after Ca-doping at Sr-site or not using density functional theory (DFT) approach [14,15].

We performed electronic structure calculations of pristine SrBeO3 and mixed Sr1-xCaxBeO3 (x = 0.25) using the all-electron full-potential linearized augmented plane-wave (FPLAPW) method [16] based on DFT as implemented in WIEN2k package [17]. The exchange and correlation effects were taken into account by Generalized Gradient Approximation (GGA) within Perdew--Burke--Ernzerhof parameterization [18]. The radii of the Muffin-tin sphere (RMT) for various atoms were taken in such a way to ensure nearly touching spheres and thus to avoid the charge leakage. The plane wave cut-off parameters were decided by RMTkmax = 7 (where kmax is the largest wave vector the atomic sphere was lmax = 10. The k-space integration was carried out using the modified tetrahedron method [19] and the self-consistency was obtained with 10×10×10 k-mesh in the Brillouin zone (IBZ) for both compounds. The calculations were based on the supercell approach for mixed perovskite where one Sr atom at (0,0,0) in the (2×2×2) supercell of SrBeO3 was replaced by Ca-atom to obtain Sr0.75Ca0.25BeO3 as depicted in Fig. 1

The most of the ABO3 type perovskites have as an ideal cubic structure within pm3m (221) space group, similar to that of the mineral perovskite (CaTiO3), which consists of a corner sharing BO6 octahedral where A and B cations are coordinated by 6 and 12 O-anions, respectively. To start with, we considered the lattice parameter of SrBeO3 (3.67 Å) as calculated by Mahmood et al. [13] and performed the structural optimization for SrBeO3 and Sr0.75Ca0.25BeO3. We have obtained a lattice parameters of 3.674/3.663 Å for SrBeO3/Sr0.75Ca0.25BeO3. With the optimized parameters, the final calculations of density of states (DOS) were performed. Before analyzing the mixed perovskite, it is important to study the nature of DOS and contributions of various atoms in it for pristine SrBeO3 compound. For this, we have examined total and partial DOS of SrBeO3 in Fig. 2. It is clear that the total DOS is highly spin polarized with 100% spin polarization at EF. A suitable band gap in majority spin channel was observed for it. After observing Sr-p, Be-p and O-p states, we found strong exchange splitting near the EF for majority spin channel. On the other hand, these states cross the EF in minority spin channel. Therefore, SrBeO3 is half metallic in nature. The calculated bandstructure and total DOS for the mixed Sr1-xCaxBeO3 (x = 0.25) perovskite is shown in Fig. 3. The total DOS of mixed compounds is exactly similar to that of pristine case. Therefore, the origin of half metallicity and magnetism is expected to be same for both compounds. 100% spin polarization at 0.25) perovskite.

Further, the existence of HM gap (a minimal gap for spin excitation) instead of band gap in one spin channel is a striking feature of the HM compounds. This gap is of unique importance for creating a hole and an electron and is defined as the minimum value out of (EF-Etop) and (Ebot-EF) where Etop/Ebot represents the energy corresponding to top of valence band (VB)/ bottom of conduction band (CB). A true HM ferromagnet is governed by a non zero HM gap [20]. We observed a HM gap for SrBeO3/Sr0.75Ca0.25BeO3 as 0.22/0.18 eV. A small decrease in this gap is expected for mixed compound as the atomic radius of Ca is somewhat larger than Sr.

In the present work, we predicted the electronic and magnetic properties of SrBeO3 and mixed Sr1-xCaxBeO3 (x = 0.25) perovskites using DFT approach. It was found that both compounds are half metallic ferromagnets with 100% spin polarization at Fermi level. The magnetism in these materials results essentially due to hybridization of Ca/Sr-p states and O-p states. The total magnetic moments for both compounds come out to be integer values (2 µB), which confirm the ferromagnetic character. Due to the interesting half-metallic properties and sufficient magnetism, mixed Sr1-xCaxBeO3 (x = 0.25) perovskite may be a potential candidate for spintronic applications.

R.S. would like to acknowledge the financial support from Council of Scientific & Industrial Research (CSIR), New Delhi (India) in the form of junior research fellowship (JRF).

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Ms. Renu Singla is pursuing Ph.D. from Department of Physics, Kurukshetra University, Kurukshetra (Haryana), INDIA. My research area is Theoretical Condensed Matter Physics and she mainly works on DFT simulations of novel low dimensional systems. Dr. Manish K. Kashyap is Assistant Professor at Department of Physics, Kurukshetra University, Kurukshetra (Haryana), INDIA. His research area is Theoretical Condensed Matter Physics and he perovskites materials.

Department of Physics, Kurukshetra University, Kurukshetra, India renusdft@gmail.com