Half-metallic ferromagnets (HMFs), whose spin-polarization is theoretically ±100% at the Fermi level, are very important spintronic materials. These materials have huge application potential in spintronic devices such as magnetoresistance random access memory (MRAM), read-write magnetic head, spin transistors and magnetic sensors [1] -[3] . Furthermore, they are selected as high quality magnetic electrodes, through which the highly spin-polarized current may be injected into semiconductors. Then spintronic semiconductor devices may be manufactured [4] . FeO, as one oxide of late transition metals, is the classic example of strongly correlated materials. It is very necessary to carry on theoretical, experimental and application studies on FeO, although it is very difficult to prepare pure FeO sample due to Fe segregation [5] . Firstly, the partially filled d-shell of Fe-ions makes FeO challenging for electronic structure theory. Secondly, it is very important component of many spintronic materials prepared successfully. In 2010, magnetic nanocomposites with (NiFe₂O₄/CuO/FeO) crystal phases were prepared using the sol-gel method [6] . In 2011, nanoparticles composed of magnetite and wustite were successfully synthesized using the pulsed laser irradiation [7] . In 2014, FeO/Ni_0.5Mn_0.5Re₂O₄ nano-composite was prepared using glycine assisted combustion method [8] . In 2015, CoFe₂O₄/FeCo-FeO core-shell nanoparticles were prepared [9] . On the other hand, stoichiometric FeO had been prepared with the high pressure (similar to 5.5 GPa) by Yoshikawa et al. [10] . This shows that it is possible to prepare pure FeO with the development of experimental technique in future. Therefore, more and more attention is attracted on pure FeO or its composites, especially the first-principles calculations based on the density function theory, which are used to design new materials and calculate their physical properties including magnetic, electric and luminescent properties [11] - [14] . The geometric and electronic structures of the FeO/Pt (111) surface, and edges of FeO/Pt (111) interface were first-principles studied by Zhang et al. [11] [12] . In 2011 the ground and excited states of FeO, FeO⁺, and FeO⁻ were studied by Sakellaris [13] . Screened exchange band structures for FeO were calculated by first-principles method [14] . Although many successes had been gotten about FeO and its composites, magnetic and electric properties of FeO, their micro-mechanisms are not still well understood. In this paper, the magnetic orders were studied based on the first-principles calculations.

For probing the magnetic sequence, the FeO supercell with the NaCl structure containing four Fe-ions and four O-ions was selected and shown in Figure 1(a), where these ions were signed by the corresponding element symbols. The space group was Fm3-m. The initial magnetic moment configurations of Fe-ions were set as (1) One up and three down, (2) two up and two down and (3) all up. Then their geometrical structures were optimized. The optimization was performed by the module “castep” based on density functional theory. The optimi- zation quality “fine” was elected, corresponding to the cutoff energy 340 eV and the k-point set 5 × 5 × 5. The wave functions in the software were expanded with plane-wave pseudo-potentials, and the exchange correlation functions were the Perdew-Burke-Ernzerh functions (PBE) of three nonlocal gradient-corrected exchange-cor- relations (GGA). The calculated electronic structures of the corresponding isolated atoms were Fe 3d⁶4s² and O 2s²2p⁴, respectively. The minimum energy was −5205.076, −5204.952, −5205.852 eV for these three spin configurations (1), (2) and (3), respectively. This means that the ferromagnetic state, namely the spin configuration (3) is most stable for NaCl structure FeO. Therefore, the ferromagnetic state was studied in detail in this paper.

For studying magnetic and electric properties of ferromagnetic FeO, the bigger supercell containing eight Fe-ions and eight O-ions was optimized and shown in Figure 1(b). The optimized crystal constants under the ferromagnetic state are a = b = c = 0.609 nm, which are 0.97% smaller than the experimental values a = b = c = 0.615 nm. This shows that optimized results agree with experimental values. Then magnetic and electric properties of the optimized supercell were calculated in detail.

From our calculation, ferromagnetic FeO is a kind of half-metallic ferromagnet. Its primitive magnetic moments are 4.00 μ_B, which arise from spin polarization of Fe 3d-orbits. The electronic structures of Fe-ions are Fe²⁺t_2g³↑e_g²↑t^*_2g¹↓.

JunLiu,WeiKang,XiaolanYang, (2015) First-Principles Calculations on the Half-Metallicity of Rock-Salt Ferromagnetic FeO. Journal of Materials Science and Chemical Engineering,03,1-7. doi: 10.4236/msce.2015.39001