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 μB. Specifically,
the ferromagnetic character is mediated by the unoccupied states of the Sr 5p band above the Fermi
level, EF.
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.