Ferromagnetism, whereby electron spins collectively
align throughout the crystal, is a fairly rare property, especially in
insulators. On the other hand ferroelectricity, whereby electric dipoles
collectively align and can be reversed with an external electric field,
requires the system to be insulating to allow for the existence of a
macroscopic polarization. This apparent dichotomy has hindered the concept of
next generation memory devices based on simultaneously ferroelectric and
ferromagnetic (multiferroic) materials.
In a recent work published in Nature Comunications, we set out to design new materials which could display
the elusive ferromagnetic and ferroelectric ground state, and help keep the
multiferroic-memory dream alive. Using fully predictive quantum mechanical
calculations, we focused on transition-metal layered perovskites, which are
known to sometimes exhibit a type of unconventional ferroelectricity, called
(hybrid) improper ferroelectricity.
As expected, we found the improper ferroelectric
phase in the layered perovskites, albeit with an unusually large electric
polarization. However, what caught us by surprise was that the ground state
d-electron spin ordering was not the expected antiferromagnetic state, as
usually dictated by superexchange interactions in insulators, but the rare
aligned ferromagnetic one. The ferromagnetic phase even appeared to be
universally favored across a wide range of chemistries. Furthermore the origin
behind the ferromagnetic ordering was very odd.
The inter-site
spin alignment was argued to be favored on the basis of intra-site Hund’s rules. This unusual scenario is allowed due to an
intricate charge and orbital ordering of the d electrons in these layered
perovskites. The orbital ordering was in
turn found to be created via certain combinations of (non-polar) distortions to
the lattice – the same non-polar distortions which also enable the improper
ferroelectricity.
The discovery of this complex interplay between the
various degrees of freedom (spin, charge, orbital and lattice) not only
provides a new mechanism to achieve ferromagnetism, but could also be used to
design new multiferroic systems with optimized device properties.
Ferromagnetism induced by entangled charge and
orbital orderings in ferroelectric titanate perovskites. N.C. Bristowe, J. Varignon, D. Fontaine, E. Bousquet and Ph.
Ghosez, Nature Communications 6, 6677 (2015). - OPEN ACCESS -