Superstructural arrangement and half-metallic semiconductor nature of the ferromagnetic material Ca2TiFeO6
Description
The modern technological applicability of materials depends substantially on their structural and physical properties. In the area of spintronics, materials design focuses on attributes such as multiferroicity, colossal magnetoresistance, ferromagnetism in semiconductors and half-metallicity, among others, which are conducive to the development of devices based on spin-polarized currents. For these purposes, the perovskite family has proven useful for spin technologies. In this manuscript, the synthesis of the material Ca2TiFeO6 is reported. Crystallographic analysis reveals that it adopts a monoclinic structure (space group P21/n), classifiable as a double perovskite type due to the ordered distribution of Ti and Fe cations along the crystallographic axes, forming a superstructure. Magnetic response measurements show the ferromagnetic feature and band structure, and density of electronic states calculations suggest the occurrence of half-metallicity. For one of the spin orientations, the material behaves like a metal due to strong hybridizations of the 4d-Fe orbitals with 2p-O, and for the other like a semiconductor with band gap of 2.3 eV, thanks to the availability of 3d-Fe and 3d-Ti states in the conduction band. The results demonstrate the multifunctionality of the material for use in spintronics technology.
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Samples production: Ca2TiFeO6 samples were synthesized through solid-state reactions using high-purity CaO, TiO2, and Fe3O4 precursor powders. These powders were dried at 120 °C, weighed in stoichiometric proportions, ground in an agate mortar, and pressed into 9 mm diameter pellets under 20 MPa of pressure. The pellets were first calcined at 800 °C for 48 hours. Afterward, the pellets were macerated for 30 minutes and then calcined again at 1000 °C for an additional 48 hours. Theoretical calculations: Total energy calculations were performed using Density Functional Theory (DFT), specifically employing the Projector Augmented Wave (PAW) method [23-24], implemented via the VASP code [25]. The Generalized Gradient Approximation (GGA), as proposed by Perdew-Burke-Ernzerhof (PBE) [26], was used to evaluate the exchange and correlation energies. Given the involvement of 3d cations, a Hubbard correction (GGA+U) was applied, with a UFe value of 5.3 eV, the validity of which has been well established in previous studies [27-28]. A plane-wave energy cutoff of 520 eV was set for the PAW-PBE potentials, ensuring high convergence in the total energy calculations, with deviations of less than 0.001 eV per atom. The first Brillouin zone was sampled using a Monkhorst-Pack scheme with a 7×7×5 k-point grid [29], yielding highly convergent energies with fluctuations below 1 meV per atom. The partial occupancies of the electronic states near the Fermi level were calculated using the Methfessel-Paxton method [30], with a smearing width of 0.05 eV. These parameters ensured full energy convergence within 1 meV. Additionally, the lattice parameters and ionic positions were optimized until the forces on the ions were reduced to less than 30 meV/Å.