Magnetic and ferroelectric materials are essential components in dozens of electronic devices and machines we use every day but some of the mechanisms underlying their behaviour are not well understood. This study by ANSTO s
Magnets are used in components ranging from speakers, electric motors and some power plugs through to computer hard drives and automatic teller machine cards. Ferroelectric materials are not only used in some types of Random Access Memory (RAM) devices and in Radio Frequency Identification (RFID) equipment, but also for electronic toll collections on toll roads, security and access management systems and for tracking goods and animals.
Magnetic materials respond to changes in an applied magnetic field. These changes are usually remembered after the field has been turned off. Similarly, ferroelectric materials have a spontaneous electric polarisation that can be reversed by applying an external electric field, and the new polarisation remains when the electric field is removed.
Magnetism and ferroelectricity are usually mutually exclusive, but there is a handful of materials that are known to possess both magnetic and ferroelectric properties they are called multiferroic . Multiferroic materials have the potential to be extremely useful, for example, in the control of the magnetic components in memory-storage devices through electric manipulation, which provides faster storage and retrieval of information.
Rather than arising from covalent bonding between ions, ferroelectric polarisation may instead arise from frustrated charge order, as reported for LuFe2O4. This compound is of particular interest as the magnetism originates from the same Fe ions as the ferroelectricity, suggesting that there will be strong magneto electric coupling. In addition, both ferroelectric and magnetic ordering take place at or near ambient temperature, opening the way to room-temperature applications.
LuFe2O4 forms a series of stacked Fe-containing bilayers. Charge ordering occurs when the valence of each Fe ion forms a regular pattern, as indicated by the red Fe3+ and blue Fe2+ atoms in the figure. This charge order results in a net polarisation of the bilayer along both the 110 (diagonal within the layer) and 001 directions (perpendicular to the layers). The purple arrows indicate the magnetic moments of the iron ions.
These bilayers are stacked parallel or antiparallel along the c-axis to form a ferroelectric or antiferroelectric alignment. Long-range magnetic order is signalled by the appearance of peaks in neutron diffraction patterns, and intensity changes in these peaks give insight into the nature of the ordering.
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