Towards spintronics

One of the new articles recently published by Philippe Ghosez and his group is based on the discoveries made in 2008. In this article (4), involving Doctor Nicholas Bristowe, now at Imperial College London, and Doctor Julien Varignon, currently at the Unité CNRS-Thalès in Paris, the physicists show that some materials based on titanium have unique magnetic properties. This is remarkable because titanium isn’t usually magnetic. What the researchers showed was that if it is specifically combined with certain elements (for instance, barium and lanthanum within a perovskite structure: (Ba,La)TiO₃), the compound exhibits spontaneous magnetization making it a ferromagnetic material similar to a permanent magnet. While ferromagnetism is relatively rare in nature, what further increases the interest in this discovery is the mechanism at the origin of this property. The majority of ferromagnetic materials are metals. “In our oxide”, Professor Ghosez explains, “the electrons aren’t mobile and the system is therefore in fact an electrical insulator. What we discovered is a special interaction between the electrons and the atomic structure of that crystal that is able of forcing ferromagnetism. Besides its unexpected magnetic properties, this compound is also a hybrid improper ferroelectric material.  So, in fact, we have succeeded in creating - theoretically – a material that is both ferroelectric and ferromagnetic. This is of interest not only in the field of electronics but also in the emerging domain of spintronics. For instance, such a material can store information both in an electric and magnetic form, making it a very interesting material for the manufacturing of computer memories which are more powerful than the usual devices. These memories may also consume less energy."

Transforming heat into electricity

In another article (5), published in Physical Review Letters, Philippe Ghosez, in collaboration this time with Daniel Bilc, who is currently at INCDTIM in Cluj-Napoca (Romania), and the team of Professor Gian-Marco Rignanese at UCLouvain, discovered exceptional thermoelectric properties in certain iron-based Heusler compounds. Thermoelectricity is the ability of some materials to convert heat into electricity. A godsend in these uncertain times regarding our electricity supply, especially power from sustainable sources. This effect was discovered nearly two centuries ago but its applications (besides the space sector, for instance) are still rare because this manner of producing power can’t compete with either hydrocarbons or nuclear energy, for instance. “Materials capable of making this conversion are rare and relatively inefficient”, Philippe Ghosez explains. “Indeed, a ‘good' thermoelectric material must combine apparently antagonistic properties. On the one hand, it must be a good electricity conductor and a bad heat conductor, whereas good electrical conductivity usually goes hand in hand with good thermal conductivity. On the other hand, it should ideally be a semiconductor that can be ‘doped’ with charges that must be both as numerous and mobile as possible.” And that is where the problem lies. To understand it, Philippe Ghosez compares it with a motorway: if there aren’t many cars, they can travel quickly; but if there are too many of them, their speed will be reduced until there is a traffic jam and they stop. There is only one solution to solve this problem: make extra lanes. "This is more or less what we’ve managed to do”, Philippe Ghosez points out. “We have found an original solution to increase the number of charge carriers while maintaining a high level of mobility. For this purpose, we played on the chemical composition of the material in order to build an ultra-dense and three-dimensional network of motorways for the electrons by taking advantage of the directional and anisotropic nature of certain electronic orbitals". The result? The thermoelectric performance of the materials tested was considerably heightened. Simulations on an atomic scale were carried out for iron-based intermetallic compounds, with the chemical formula Fe₂YZ (like Fe₂TiSi or Fe₂TiSn). Of course, we are talking about simulations and not wide-scale manufacturing. But these materials should have a future. First, because their thermoelectricity is amongst the greatest known up until now; secondly, because they are composed of abundant, inexpensive and non-toxic elements. And we should not forget the fact that there is already an explicit demand from industry, which is seeking thermoelectric devices to recuperate the thermal energy released during many industrial processes in temperature ranges where no other alternative exists today. Part of our future is hidden within the complex equations of quantum physics.

(4) 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).
(5) Low-dimensional transport and large thermoelectric power factors in bulk semiconductors by band engineering of highly directional electronic states. D. I. Bilc, G. Hautier, D. Waroquiers, G.-M. Rignanese and Ph. Ghosez, Physical Review Letters 114, 136601 (2015).