May 3, 2013
Multiferroics feel the strain
Multiferroics, which are both ferroelectric and magnetic, are promising for a wide range of applications, such as ferromagnetic resonance devices, photovoltaics and magnetoelectric memory cells. Most of these devices would exploit the ferroelectric properties of these materials, but a team in France, Russia and the US has now looked at the antiferromagnetic properties of the most well known of all multiferroics, bismuth ferrite (BiFeO3), and in particular how the perovskite oxide thin film behaves under strain. The study has revealed how the material might be used in data storage and spintronics devices, and perhaps even in a new technology called “magnonics”.
The ferroelectric properties of perovskite oxide thin films, such as BiFeO3, can be crafted using strain engineering. Until now, however, the technique had rarely been applied to modify the antiferromagnetic properties of these materials.
Antiferromagnets are routinely employed in a variety of fields – for example, in magnetic read heads to block the magnetization of an adjacent ferromagnetic layer, to name but one application. Many researchers now believe that antiferromagnets might also be interesting for spintronics (a technology that exploits the spin of an electron as well its charge). This is because they are sensitive to spin torques and because they show magnetoresistance under certain circumstances.
Another advantage of antiferromagnets is that spins in these structures can change direction (from “up” to “down”) much faster than in ferromagnets, which means that they might be better in magnonics and spintronics devices operating at high, terahertz frequencies. Magnonics is a relatively new technology that would rely on spin waves to carry and process information. An analogy can be made with photonics, which relies on light waves to do the same.
A team led by Manuel Bibes of the CNRS, the Université Paris-Sud and Thales began by determining how magnetically ordered BiFeO3 is using Mossbauer and Raman spectroscopy techniques combined with theoretical calculations. The researchers also looked at how mechanical strain can be used to engineer this order and also tune properties such as spin dynamics and giant magnetoresistance (GMR) in adjacent metallic multilayers.
“Our most exciting result is probably the discovery that tiny strains in a multiferroic film can produce dramatic variations in its magnetic order and spin dynamics,” Bibes told nanotechweb.org. “Harnessing such ‘instabilities’ between different physical states could lead to new device concepts using these materials.”
The researchers also found that they could suppress some so-called spin dynamics excitation modes in BiFeO3 by increasing the strain in the film and that they could tune the GMR in this way too. The results confirm that strain engineering is a new way to control magnetism, something that could be important for data storage (which relies on reversing the magnetization to “write” bits of information – strings of 1s and 0s).
Bibes and colleagues say that they would now like to be able to control magnetic order and spin dynamics in BiFeO3 films using electric fields – using, for example, piezoelectrics that would produce voltage-driven strains.
The current work is reported in Nature Materials.
About the author
Belle Dumé is contributing editor at nanotechweb.org.