Oxygen Deficiency and Migration-Mediated Electric Polarization in Magnetic Fe,Co-Substituted SrTiO3−δ

Abstract

We use density functional theory (DFT) calculations to show that oxygen vacancies (vO) and mobility induce noncentrosymmetric polar structures in SrTi1−x−yFexCoyO3−δ (STFC, x=y=0.125) with δ={0.125,0.25}, enhance the saturation magnetization, and give rise to large changes in the electric polarization |ΔP|. We present an intuitive set of rules to describe the properties of STFC, which are based on the interplay between (Co/Fe)-vO defects, magnetic cation coordination, and topological vacancy disorder. STFC structures consist of layered crystals with sheets of linearly organized O4,5,6-coordinated Fe–Co pairs, sandwiched with layers of O5-coordinated Ti. (Co/Fe)-vO defects are the source of crystal distortions, cation off-centering and bending of the oxygen octahedra which, considering the charge redistribution mediated by vO and the cations’ electronegativity and valence states, triggers an effective electric polarization. Oxygen migration for δ=0.125 leads to |ΔP|>∼10 µC/cm2 due to quantum-of-polarization differences between δ=0.125 structures. Increasing the oxygen deficiency to δ=0.25 yields |ΔP|, the O migration of which resolved polarization for δ=0.25 is >∼3 µC/cm2. Magnetism is dominated by the Fe,Co spin states for δ=0.125, and there is a contribution from Ti magnetic moments (∼1 μB) for δ=0.25. Magnetic and electric order parameters change for variations of δ or oxygen migration for a given oxygen deficiency. Our results capture characteristics observed in the end members of the series SrTi(Co,Fe)O3, and suggest the existence of a broader set of rules for oxygen-deficient multiferroic oxides.

First-principles based Monte Carlo modeling of oxygen deficient Fe-substituted SrTiO$_3$ experimental magnetization

“…Our screening value µ was chosen similar to References [10,31] to compare with STC/STO results, and the U ef f = U − J terms were used within Dudarev's GGA+U approach for the d-Fe electrons [52]. In the latter case, the acting forces were reduced below 10 −5 eV/ Å, and the U ef f values were chosen within the range of validity of recent simulations for STFC i.e., 4 and 7 eV for 3d-orbitals were applied to Fe and Ti, respectively [34]. In the case of the ab-initio calculations, the supercell of the solid solutions consisted of 2 × 2 × 2 unit cells with {40, 39} ions for δ = {0.0, 0.125}, as displayed in Figure 1.…”

“…According to each configuration, we have considered all the atomic valence states, which are reflected in several possible high and low Pauli states for the TM, as well as the possible combinations for the FM or AFM Fe-Fe exchange coupling. The valences of the cations are selected to maintain a neutral supercell for a given oxygen deficiency, and point/bulk-charging effects due to these defects are negligible as suggested by DFT calculations in STF and STFC [33,34]. We also refer in this work, without loss of generality, to V a,p,d (1,2) as the vacancies corresponding to the Fe cations aligned along the [1, 0, 0] (above), [1, 1, 0] (plane) and [1, 1, 1] (diagonal) crystalline directions, respectively.…”

“…Although here we are interested in the magnetic degrees of freedom and V p1 and V p2 behave similarly from that viewpoint, among those two v o the first one turns out to be more interesting for multiferroic purposes as the 3d hybridization is slightly shifted just below E f and the system is a semiconductor with a 2.15 eV band-gap, which is adequate for searching of an effective electric polarization in non-metallic ferroelectrics, as it was explored in STFC [34]. In this last work, the authors showed how oxygen migrations and deficiency variations can modulate the band-gap and electric polarization in Fe, Co-substituted STO systems with defects such as the ones studied here.…”

“…This indicates that ions-proximity is not a predominant factor in the metallic-like effects due to cluster formation in perovskite solid solutions of this kind when we consider v o . For instance, in semi-metallic SCO a gap opens up and increases with δ while perovskites such as Fe, Co-substituted STO present a maximum at intermediate vacancy density [34] for different cations nn distances.…”

“…However, experimental results have shown that O-deficiency is capable of turning a magnetic semimetal such as SrCoO 3 (SCO) into a semiconductor as well as converting the insulating paramagnetic STO into a magnetic semiconductor or weak ferroelectric [9,20,31]. 3d-orbitals in transition metal (TM) cations are distorted by incomplete oxygen octahedral coordination O 5,4 , and both the covalent and ionic bondings characters through O-A/B-v O defects plays a relevant role [32], as in the case of the multi-interpreted SCO spin-states [31] and the magnetism in Fe,Co-substituted STO [33,34].…”