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 μ_B, 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.

INTRODUCTION

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 (E_F), (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 E_F, 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 (T_C) [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).

CRYSTAL STRUCTURES

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 O^2– in between forming Cr³⁺/Fe³⁺O^2–₆ and Re⁵⁺O^2–₆ octahedra with bond-lengths of Cr³⁺/Fe³⁺–O^2–=1.981 Å/1.949 Å and Re⁵⁺–O^2–=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.

HALF-METALLIC PROPERTIES

Figure 1 shows the total densities of states (TDOSs) of Sr₂CrWO₆ and Sr₂FeReO₆ with an energy-gap in spin-up of (E_g=2.14 eV) and (E_g=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 E_g 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 E_F. Therefore, the spin-up electrons are insulating while the spin-down ones are metallic, resulting in SP = 100% of their conduction electrons at the E_F. 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].

FERROMAGNETIC PROPERTIES

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; t_2g³_↑e_g⁰_↑; S=3/2 μ_B) and W⁵⁺ (5d¹; t_2g¹_↑ e_g⁰_↑; S=1/2 μ_B); Fe³⁺ (3d⁵; t_2g³_↑e_g²_↑; S=5/2 μ_B) and Re⁵⁺ (5d²; t_2g²_↑ e_g⁰_↑; S=2/2 μ_B). 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-t_2g³ _↑ e_g⁰_↑)–O (2p_π)–W (5d-t_2g¹_↓) and Fe (3d-t_2g³_↑ e_g²_↑)–O (2p_π)–Re (5d-t_2g²_↓). Where, the band filling of spin-up and spin-down sub-orbitals in t_2g and e_g govern these interactions. The calculated magnetic moments for Sr₂CrWO₆ are M_Cr=2.919 μ_B, M_W=–1.044 μ_B with a total magnetic moment per unit cell of M_Tot.=1.878 μ_B, in agreement with the LSDA+U value M_Tot.=2.01 μ_B [10,11] and theoretical (S=2 μ_B). For Sr₂FeReO₆, M_Fe=4.578 μ_B, M_Re=–1.344 μ_B and M_Tot.=3.184 μ_B, also in agreement to the LSDA+U result, M_Tot.=3.06 μ_B [8] and theoretical (S=3 μ_B). 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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