Laser reveals new type of magnetic ordering
Oct 12, 2007 2 comments
A new type of magnetic ordering in which the magnetic moments of a material are frozen head-to-tail in tiny vortices has been discovered by researchers in Germany and Switzerland. The team say that the direction of these vortices – clockwise or anticlockwise – could be changed using a special combination of electric and magnetic fields. As a result, the ordering could be used to create a new and very stable type of data storage device (Nature 449 702).
The effect, which is called ferrotoroidicity, was spotted in the oxide material LiCoPO4 by physicists Bas Van Aken and Manfred Fiebig from the Max-Born Institute in Berlin together with chemists from the University of Geneva.
The team discovered that the material contains regions (or domains) where all of these vortices were either clockwise or anticlockwise. Each vortex is not much bigger than the spacing of atoms in the crystal lattice and their extremely small size distinguishes them from the much larger (and unrelated) vortices that have been seen in disks of magnetic material.
The domains were detected using a technique called second harmonic generation in which a laser beam shone onto the sample is doubled in frequency as it passes through. When the beam hits a ferrotoroidic domain oriented in the opposite direction to an adjacent domain, part of the beam is phase shifted by 180°. Studying the light leaving the sample, the researchers found regions of constructive and destructive interference, indicating that such oppositely-aligned domains, and hence ferrotoroidicity, were present.
In theory, the direction of the vortices can be flipped using a combination of electric and magnetic fields. The team therefore believe that ferrotoroidics could be used in data storage devices with clockwise and anticlockwise domains representing the 0s and 1s of data bits. Because both electric and magnetic fields are needed to flip a vortex, such bits would be less prone to flipping by stray magnetic fields than conventional ferromagnetic bits.
Ferrotoroidicity is the latest “ferroic” ordering effect to be discovered. The other ferroics are ferromagnetism, which is the spontaneous ordering of magnetic moments in one direction; ferroelectricity, which is a spontaneous electric polarization of a material; and ferroelasticity, which is a spontaneous strain on a material.
About the author
Jon Cartwright is a reporter for physicsworld.com