Giant improper ferroelectricity is discovered in the ferroaxial magnet, CaMn7O12

Part of the structure of CaMn7O12 that shows the ferroaxial rotation

Part of the structure of CaMn7O12 that shows the ferroaxial rotation
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Neutron diffraction studies at ISIS have given scientists new insights into the nature of certain magnetic materials, work that has led to a major breakthrough in the field of multiferroics. Ferroelectricity is a term that refers to the property of materials that undergo spontaneous electric polarisation, which underpins a range of technologies such as smart cards and FRAMs. Magnetic ferroelectrics (also known as multiferroics) are currently an intense focus of research in Physics, with their unique properties potentially making them a suitable material for new and improved technology.

For several years, scientists have been investigating multiferroics with the goal of identifying a material that displays a large ferroelectric polarisation strongly coupled to the magnetic structure, at high temperatures. Materials demonstrating these characteristics would be suitable for applications in technology as they could, for example, significantly reduce the power consumption of data storage devices or allow for the development of novel 4-state memory.  Encouragingly, several multiferroric compounds have been found to display coupling between ferroelectricty and magnetism, for example TbMnO3, Ni3V2O8 and MnWO4. However, the electric polarisation in these materials tends to be small and the Néel temperature - the temperature below which long-range antiferromagnetic ordering exists- tends to be too low. Therefore, the aim has been to identify a mutiferroric material that has a large electric polarisation close to room temperature. Such a material has recently been identified that may represent a significant step towards reaching these goals.

Roger Johnson and co-workers at the University of Oxford, along with collaborators in France, have successfully attained the largest measured, magnetically induced ferroelectric polarisation in the multiferroric material CaMn7O12. What’s more, the team have measured this giant polarisation at a reasonable temperature of 90 K.

Not only have the team achieved groundbreaking results through the identification of the giant polarisation in CaMn7O12, but they have also resolved the microscopic origin of the multiferroic coupling. Data measured at the WISH instrument at ISIS was used in determining the magnetic structure of CaMn7O12.  Surprisingly, Johnson et al. found that in CaMn7O12 the magnetic moments (spins) form a helicoidal spiral, where in conventional multiferroics such as TbMnO3 the magnetic ordering that gives rise to ferroelectricity adopts what is known as a cycloidal spiral of spins. Helicoidal multiferroics are rare and all such materials studied so far have only been weakly ferroelectric. Johnson et al. propose that the giant ferroelectricity induced by helical magnetic ordering arises from a novel “ferroaxial coupling” mechanism.

To conclude, Johnson et al. have discovered that in CaMn7O12, a helicoidal magnet, a giant ferroelectric polarisation is induced at a Néel temperature of 90 K. The research group have been able to elucidate the relationship between helicity, crystal structure and ferroelectricity for this material. Future research goals for Jonhson et al. involve studying other multiferroic materials that could demonstrate similar properties to CaMn7O12 at room temperature. This work will therefore underpin the development of new multiferroic technology.

Amandeep Hundal

Research date: January 2012

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