Occurrence of spintronics behaviour (half-metallicity, spin gapless semiconductor and bipolar magnetic semiconductor) depending on the location of oxygen vacancies in BiFe0.83Ni0.17O3

The current communication signifies the effect of oxygen vacancies (OVs) both qualitatively and quantitatively in multiferroic BiFe0.83Ni0.17O3 by an in-depth atomic-level investigation of its electronic structure and magnetization properties, and these materials have a variety of applications in spintronics, optoelectronics, sensors and solar energy devices. Depending on the precise location of OVs, all the three types of spintronic material namely half-metallic, spin gapless semiconductor and bipolar magnetic conductor have been established in a single material for the first time and both super-exchange and double-exchange interactions are possible in accordance with the precise location of OVs. We have also calculated the vacancy formation energies to predict their thermodynamic stabilities. These results can highlight the impact and importance of OVs that can alter the multiferroic properties of materials.

Occurrence of spintronics behaviour (half-metallicity, spin gapless semiconductor and bipolar magnetic semiconductor) depending on the location of oxygen vacancies in BiFe 0.83 Ni 0. 17  The current communication signifies the effect of oxygen vacancies (OVs) both qualitatively and quantitatively in multiferroic BiFe 0.83 Ni 0.17 O 3 by an in-depth atomic-level investigation of its electronic structure and magnetization properties, and these materials have a variety of applications in spintronics, optoelectronics, sensors and solar energy devices.
Depending on the precise location of OVs, all the three types of spintronic material namely half-metallic, spin gapless semiconductor and bipolar magnetic conductor have been established in a single material for the first time and both super-exchange and double-exchange interactions are possible in accordance with the precise location of OVs. We have also calculated the vacancy formation energies to predict their thermodynamic stabilities. These results can highlight the impact and importance of OVs that can alter the multiferroic properties of materials.
2017 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

Introduction
Recently, multiferroic materials have been perceived as an ideal candidate for novel applications that include but are not limited to spintronics, magnetic field sensors and multiple state memory elements [1]. Researchers are making extensive efforts in fabricating a robust, high performance and relatively less energy expensive memory storage device as a result of huge increasing technological demand over the past decade. Consequently, multiferroic materials surge in importance for the current technological evolution as it shows significant coupling of ferroelectricity and ferromagnetism specifically at room temperature. Bismuth ferrite (BiFeO 3 -BFO), a class of single-phase multiferroic materials is one of the most promising material for next-generation technological devices owing to its very high both antiferromagnetic Neel temperature of approximately 640 K and ferroelectric Curie temperature of approximately 1100 K [1]. However, ferroelectric polarization measurements of BFO samples show high electrical leakage currents thereby limiting its applications in memory storage devices [2]. It has been well established that by means of site-engineering approach (doping at Bi and Fe sites) the leakage current can be significantly controlled [3,4] and in particular, doping of aliovalent ions at Fe site greatly influences the electronic structure, releases net magnetization by destroying the cycloid-type magnetic structure and enhances the optical properties of BFO [5]. One of the most successful aliovalent ions doped in BFO is examined to be Ni 2+ and experimental reports of Ni-doped BFO shows enhanced multiferroic properties [6,7] and also predict that the introduction of Ni 2+ is expected to create more oxygen vacancies (OVs) and prevent the formation of Fe 2+ ions. Our previous density functional theory (DFT) calculations on stoichiometric BiFe 0.83 Ni 0.17 O 3 (approx. 16.67 at% of Ni) with zero OVs displayed half-metallic behaviour which holds applications in spintronics [8]. Nevertheless, it is very difficult to determine half-metallicity by experimental measurements owing to various reasons which includes uncertainty in measurements, incomplete spin polarization [9], and existence of structural disorders [10,11]. In particular, OVs do occur in the synthesis of BFO samples and both qualitative and quantitative presence of OVs play a vital role in modulating the electronic, magnetic and optical properties of Ni-doped BFO. Nevertheless, the accurate and in-depth role of OVs is still puzzling at an atomic level and an understanding of the modulated behaviour of Ni-doped BFO is required, therefore in this short communication, we have tried to address the influence of OVs both nearer and farther to Ni ion in BiFe 0.83 Ni 0.17 O 3 . By employing first principles DFT calculations, we have made an effort in exploring the behaviour of BiFe 0.83 Ni 0.17 O 3 by varying the OVs concentration both qualitatively and quantitatively. The details of the calculations performed are mentioned below in the computational methodology section.

Computational methodology
In our previous calculations [8], we have modelled a hexagonal cell having molecular formula  package [12,13]. The projector augmented wave method [13,14] was adopted in our calculations by considering the valence electrons for Bi (6s 2 6p 3 ), Fe (3d 6 4s 2 ), Ni (3d 8 4s 2 ) and O (2s 2 2p 4 ). The generalized gradient approximation (GGA) as a revised version of Perdew, Burke and Ernzerhof [15] was employed to treat the exchange and correlation effects of electrons. A plane wave kinetic energy cutoff for the plane wave basis set of 500.00 eV was used throughout the calculations and denser Γ -centred k-point mesh density of 4 × 4 × 1 was generated for the integration of Brillouin zone. Relaxation of the ionic positions are stopped until the Hellmann-Feynman forces are less than 10 −2 eV/Å and the SCF iterations are completed with the total energy convergence of 10 −6 eV. The relaxation of all the configurations was carried out by means of conjugate gradient method [16] with Gaussian broadening of 0.1 eV. The strong correlations effects have been treated by performing GGA + U calculations and in our calculations, we have followed the approach of Dudarev et al. [17] where an effective Hubbard parameter U eff = U-J enters the Hamiltonian, by considering J as the exchange interaction parameter. The effective Hubbard parameter U eff for Fe is fixed to 4.00 eV and for Ni is fixed to 5.00 eV, in accordance with our previous calculations [8].

Results and discussion
The structural modifications which include bond angles, bond lengths, lattice parameters and cell volume of all the configurations with OVs will be discussed elaborately in our next article. In the current communication,

Conclusion
In summary, we present the impact of OVs both nearer and farther to the Ni ion in BiFe 0.83 Ni 0.17 O 3 and its existence will modulate both the electronic structure and magnetization properties of BiFe 0.83 Ni 0.17 O 3 significantly according to their precise locations, and these results can boost up the further investigation in experiments. The existence of half-metallicity, SGS and BMS in BiFe 0.83 Ni 0.17 O 3 depends on the exact location of OVs. We have found the formation energy to be lowest for the configuration in which OV is farther away from the nickel atom indicating greater stability of this configuration compared to others. Further work is heading towards the impact of OVs on its charge density patterns with bonding, optical and electrical properties.