Oxygen-vacancy tuning of magnetism in SrTi0.75Fe0.125Co0.125O3−δ

Abstract

We use density functional theory to calculate the structure, band-gap and magnetic properties of oxygen-deficient SrTi1−x−yFexCoyO 3−δ with x = y = 0.125 and δ = {0, 0.125, 0.25}. The valence and the high or low spin-states of the Co and Fe ions, as well as the lattice distortion and the band-gap, depend on the oxygen deficiency, the locations of the vacancies, and on the direction of the Fe-Co axis. A charge redistribution that resembles a self-regulatory response lies behind the valence spin-state changes. Ferromagnetism dominates, and both the magnetization and the band gap are greatest at δ = 0.125. This qualitatively mimics the previously reported magnetization measured for SrTiFeO 3−δ , which was maximum at an intermediate deposition pressure of oxygen.Reprinted with permission from

Oxygen deficiency and migration mediated electric polarization in Fe,Co-substituted SrTiO$_{3-δ}$

“…For STC, although its FE properties have not been explored, preliminary work 21 suggests that STC could share some FE characteristics in common with O-deficient STO. STC has a relatively large band gap and room temperature magnetization as well as off-centering mechanisms induced by incorporation of Co 22 or through the Co-v O -Ti distortion 23 .…”

“…The generalized gradient approximation (GGA) within the Dudarev approach for the Hubbard +U correction (GGA+U) was used for d electrons of the TM 30 . The U values used for Fe, Co and Ti are 3, 9 and 8 eV, respectively, and were chosen based on an extensive search for lattice parameters, local magnetic moments and band gap behaviors that would best represent the end members SrTiO 3 , SrFeO 3 and SrCoO 3 , as well as the intermediate solutions SrTi 1−x Fe x O 3 and SrTi 1−x Co x O 3 with x = {0.125, 0.25} 4,5 ; such values also capture key aspects of hybrid simulations for STFC 23 , and are used to calculate the migration features. The electric polarization was calculated for resulting semiconductor solid solutions by using the Berry's Phase approach 31 , as implemented also in VASP.…”

“…Pre-and post-processing were performed with VESTA 32 , CrystalMaker R 33 and pymatgen 34 . 4,5,23 , and to other behaviors such as the anionic-redox paradigm and voltage decay in Li-ion batteries 35,36 . v O are created by removing one (δ = 0.125 ⇒ δ 01 ) or two (δ = 0.25 ⇒ δ 02 ) oxygen atoms from the relaxed δ = 0 ⇒ δ 0 system and following a symmetry analysis of the configuration space with a tolerance of 10 −3 Å.…”

“…We focus here on O-deficient STFC perovskites because oxygen deficiency is commonly observed experimentally. Likewise, we also focused on materials with large local magnetization, narrowing our final analysis to 57 of the configurations mentioned above, displayed in Figure 1 and Figure S1, which present at least one v O coordinating Fe or Co 39 favoring the high-spin state 5,23 .…”

“…Magnetic TM cations are able to accommodate a wider range of spin states than their diamagnetic counterparts, though both respond through their valence states to stoichiometry conditions, which could nevertheless also lead to non-stoichiometry and hopping conductivity. Because mobile charges would screen out the electric polarization and therefore not be suitable for our investigation of ferroelectric response, if a conducting gs is found, the energetically-closest semiconductor solution is then considered unless hybrid results predicted otherwise 23 .…”