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Double perovskite-like materials which include transition elements have relevance due to the technological perspectives in electronics and spintronics engineering. In this study, we report the investigations of the electronic and magnetic properties of Sr2CrWO6 and Sr2FeReO6 by using the first-principles density functional theory (DFT). The electronic and magnetic results predict proper half-metallic (HM) and ferromagnetic (FM) ground states in Sr2CrWO6 and Sr2FeReO6 with total magnetic moments of 2.0 and 3.0 μB, respectively. Therefore, these materials seem to possess HM and FM properties, making them useful candidates for applications in spintronics and energy devices.
Double perovskite materials, which include transition elements, are relevant because of the technological perspectives in electronics and spintronics engineering. In this study, we report the research on the electronic and magnetic properties of Sr2CrWO6 and Sr2FeReO6 using first density functional density theory (DFT). The electronic and magnetic results predict suitable semi-metallic (HM) and ferromagnetic (FM) ground states in Sr2CrWO6 and Sr2FeReO6 with total magnetic moments of 2.0 and 3.0 μB, respectively. Therefore, these materials appear to possess HM and FM properties, making them useful candidates for applications in spintronics and energy devices.
The double perovskite materials, here include transition elements, are relevant because of the technological perspectives in electronics and spintronics engineering. In this study, we report the research on the electronic and magnetic properties of Sr2CrWO6 and Sr2FeReO6 using first density functional density theory (DFT). The electronic and magnetic results predict appropriate semi-metallic (HM) and ferromagnetic (FM) ground states in Sr2CrWO6 and Sr2FeReO6 with magnetic total moments of 2.0 and 3.0 μB, respectively. Therefore, these materials appear to possess HM and FM properties, making them useful candidates for applications in spintronics and energy devices.
Respected of their requests in spintronics and energy devices, half-metallic (HM) double perovskites are appropriate based on their unique features; (i) full spin-polarization at the Fermi level (EF), (ii) quantization of spin magnetic moment, and (iii) zero spin susceptibility. Some transition-metal double perovskites (TMDPs) with chemical formula A2MNO6 (A=alkali-earth; MN=transition-metals) have been recently designated to exhibit ferromagnetism (FM) and HM with (SP=100%) of conduction electrons at the EF, making them promising candidates as materials suitable for spintronics technologies such as magnetic recorders, magnetic sensors, computer memories, and solar cell devices. Also, very interesting properties are detected in TMDPs family, such as magnetoresistance (MR) in (Sr2FeMoO6) and (Sr2FeReO6) [1,2], HM above room-temperature (RT) in (A2FeMoO6; A=Ca, Sr, Ba) [3,4] and (Sr2CrMoO6) [5,6] and high Curie temperature (TC) [1,7]. The ordered TMDPs (Sr2FeMoO6), (Sr2FeReO6), (Sr2CrMoO6), (Sr2CrWO6), etc., are among the very few materials that allow conduction electrons of one spin direction to move through them, while blocking the electrons with opposite spin direction. In this study, the structural, electronic and magnetic properties of two Sr-based double perovskites (Sr2CrWO6 and Sr2FeReO6) are reported by using the density functional theory (DFT) calculations within the exchanged and correlated local spin density approximation (LSDA+U).
The ideal crystal structure of Sr2CrWO6 and Sr2FeReO6 can be viewed as an ordered arrangement of corner-sharing Cr(Fe)O6 and W(Re)O6 octahedra (6-coordinate system), alternating along the three directions of the crystal space, with the large cations Sr2+ (12-coordinate system) occupying the cavities in between these octahedra. Sr2CrWO6 and Sr2FeReO6 crystallize in cubic structure with (Fm-3m) symmetry and Cr3+–W5+ and Fe3+–Re5+ systems arranged in rock-salt ordering. Their lattice parameters are a=7.8587 Å and a=7.8858 Å, respectively, around the ideal values (a=8.0 Å). Each Cr3+ (W5+) or Fe3+(Re5+) is coordinated by W5+(Cr3+) or Re5+(Fe3+) and each has an O2– in between forming Cr3+/Fe3+O2–6 and Re5+O2–6 octahedra with bond-lengths of Cr3+/Fe3+–O2–=1.981 Å/1.949 Å and Re5+–O2–=2.016 Å/1.928 Å. The atomic positions in the unit cell are Sr2+ at 8c (¼, ¼, ¼), Cr/Fe at 4a (0, 0, 0), Re at 4b (½, ½, ½) and O at 24e (u, 0, 0), where u=0.252 and u=0.256 for Cr and Fe compound, respectively.
Figure 1 shows the total densities of states (TDOSs) of Sr2CrWO6 and Sr2FeReO6 with an energy-gap in spin-up of (Eg=2.14 eV) and (Eg=2.31 eV), respectively, falls between the occupied Cr/Fe (3d) and unoccupied W/Re (5d) states. From the partial densities of states (PDOSs) in Figure 2, it can be see that the spin-down conduction states are created mainly from the contributions of W (5d) and Re (5d) states with tiny contributions come from Cr (3d) and Fe (3d) states, respectively. The small variation between two TDOSs is due to the extra electron in Re (5d2) than in W (5d1). Also, since the Eg produces from the antiferromagnetic coupling between Cr/Fe (3d) and W/Re (5d) states (Figure 2), their peaks emerge as Cr/Fe (3d)↑ and W/Re (5d)↓ near EF. Therefore, the spin-up electrons are insulating while the spin-down ones are metallic, resulting in SP = 100% of their conduction electrons at the EF. Accordingly, Sr2CrWO6 and Sr2FeReO6 allow electrons of spin-down direction to move through them as though they were passing through a regular metal, while blocking electrons with spin-up direction. The obtained results of Sr2CrWO6 and Sr2FeReO6 are agreement with previous results [8,9].
The most contributions to the electronic and magnetic structures of Sr2CrWO6 and Sr2FeReO6 come from the super-exchange interaction (SEI) between the energetic orbitals of 3d and 5d in Cr3+ (3d3)/Fe3+ (3d5) and W (5d1)/Re (5d2), respectively. The spin configurations of ground states in two compounds are stabilized in Cr3+ (3d3; t2g3↑eg0↑; S=3/2 μB) and W5+ (5d1; t2g1↑ eg0↑; S=1/2 μB); Fe3+ (3d5; t2g3↑eg2↑; S=5/2 μB) and Re5+ (5d2; t2g2↑ eg0↑; S=2/2 μB). Thus, the ferromagnetic structures can be assigned primarily to the SEI between Cr/Fe and W/Re via intermediated O atoms in 180° long-chain paths; Cr (3d-t2g3 ↑ eg0↑)–O (2pπ)–W (5d-t2g1↓) and Fe (3d-t2g3↑ eg2↑)–O (2pπ)–Re (5d-t2g2↓). Where, the band filling of spin-up and spin-down sub-orbitals in t2g and eg govern these interactions. The calculated magnetic moments for Sr2CrWO6 are MCr=2.919 μB, MW=–1.044 μB with a total magnetic moment per unit cell of MTot.=1.878 μB, in agreement with the LSDA+U value MTot.=2.01 μB [10,11] and theoretical (S=2 μB). For Sr2FeReO6, MFe=4.578 μB, MRe=–1.344 μB and MTot.=3.184 μB, also in agreement to the LSDA+U result, MTot.=3.06 μB  and theoretical (S=3 μB). The obtained 100% SP, HM and FM features in Sr2CrWO6 and Sr2FeReO6 makes them suitable for many potential applications like spintronics, where the spin currents are utilized as well as charge currents.
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