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Multiferroics

Multiferroics are multifunctional materials which combine two or more of the ferroic order parameters – spontaneous polarization, magnetization and strain, exhibited individually in ferroelectric, magnetic and ferroelastic materials respectively. An important related class of these materials, magnetoelectrics (MEs), are remarkable because of their potential application for new multifunctional devices, including novel sensors, spin-valves, biosensors and memory storage. Yet our understanding of the full industrial impact of these novel materials is only just emerging. The key to their advanced functionality is the ability to control magnetic order with electric field, and electric polarisation via magnetic fields. Whilst this capability in any single-phase materials may provide interesting insights into fundamental condensed matter physics, there are many issues that make the use of these materials impracticable in real life applications. For instance, instrinsic multiferroic magnetoelectricity is rare, occurring in only a small number of currently studied compound families, and the inherent coupling between the multiferroic ordering parameters is not particularly strong. More importantly for device design considerations, the coupling is often not accessible anywhere near room temperature.

Research into single-phase multiferroic ME systems has consequently been surpassed recently by the advent of ME composites. The combination of magnetostrictive and piezoelectric materials is allowing researchers to mimic the effects of real multiferroic materials, with coupling parameters hundreds of times larger. This enables much greater device design flexibility. The net result has been heralded a revival of the ME effect, something that x-ray and neutron scatterers have picked up on in recent years. These diffraction and spectroscopy techniques use the atomic structure and bonding as a theoretical understanding of the macroscopic properties of any given crystal. While neutrons and x-rays have been used to probe the behaviour of magnetic systems for over half a century, their use in understanding ME systems is a relatively new game. And it leads to a comparatively new challenge – the experimental difficulties of how to control and measure the magnetic, electric, thermal, stress and strain variables within the confines of the sample housing environment on an experiment beamline.