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 -