Co-existence of ferroelectricity and ferromagnetism in 1.4 nm SrBi₂Ta₂O₁₁ film

In pseudo-tetragonal strontium bismuth tantalate, SrBi₂Ta₂O₉ (SBT), with two formula units per unit cell, bismuth oxide {(Bi₂O₂)²⁺} layers alternate with double strontium tantalate perovskite layers {(SrTa₂O₇)^2-}. A unit cell of SBT is truncated to form a sub-cell or film, of composition SrBi₂Ta₂O₁₁, which is 1.4 nm thick and comprised of a bottom (BiO₂)¹⁺ layer, a central (SrTa₂O₇)^2- layer and a top (BiO₂)¹⁺ layer. Using spin-polarized first-principles calculations, it is found that this SrBi₂Ta₂O₁₁ film is multi-ferroic, magnetoelectric, i.e. it simultaneously exhibits ferroelectric and ferromagnetic characteristics. When Ta ions are collectively displaced in the ab plane and in the [110] direction, the calculated double potential energy well, with a depth of -3.1 eV/unit cell at a Ta off-centre displacement of 0.032 nm, reflects the ferroelectric character. The calculated spin-polarized electronic structure reveals that ferromagnetism stems, not from the d electrons of the Ta ions, but predominantly from the unpaired p electrons of the O ions. The O ions in the Sr-O layer have the largest magnetic moment of 1.32 μ_B. Specifically, the ferromagnetic character is mediated by the unoccupied states of the Sr 5p band above the Fermi level, E_F. These states provide a mechanism for the double exchange or hopping of highly localized O 2p (majority) spins between adjacent O ions located on both sides of the Sr ion.