The cover picture of the October 2012 issue of Nature Physics shows a crystalline lattice of a magnetic material Ba2CoGe2O7. A team of scientists including the Terahertz Spectroscopy group of NICPB discovered that the chirality of the magnetic structure shows up in the absorption of electromagnetic radiation by spin waves in this material.
A chiral object has two forms and one of them is a mirror reflection of the other – like left and right hand screws for example.
In the published paper it was shown that when a static magnetic field is applied on the Ba2CoGe2O7 single crystal in certain directions, the absorption of terahertz radiation depends on whether the light propagates along the magnetic field or against the direction of the magnetic field: in one case all is absorbed and in the other case almost nothing. This phenomenon is possible if spin waves interact strongly with polarization waves in a crystal with a chiral magnetic structure.
Simplified explanation:
It is possible to determine the chirality of a substance by measuring the difference of absorbances of right- and left circularly polarized light, since circularly polarized light is itself also chiral.
Electromagnetic radiation has another kind of chirality. According to Maxwell equations the three vectors k, E, and B (light propagation direction, electric and magnetic fields of light) are related by a vector product: B = ( k x E ) / c . If the propagation direction k is reversed then one of the components of the oscillating electromagnetic field has to be reversed as well.
The electric field component of light interacts with electric charges in the substance and the magnetic field component of light interacts with magnetic moments. Usually the charges and the magnetic moments move independently of each other and visible light is absorbed by moving charges while microwave and radio-frequency radiation is absorbed by the magnetic moments.
New phenomena appear when charges and magnetic moments do not move independently of each other, a situation encountered in some multiferroic materials. An example is the interaction of polarization waves of electric charges (hybridized Co and O electrons) and spin waves (magnetic moments of Co) in the Ba2CoGe2O7 single crystals. In the October issue of Nature Physics it is described how in this material a strong magnetochiral dichroism develops: the absorption of linearly polarized THz light depends on the propagation direction of light with respect to an external static magnetic field. The static magnetic field determines the relations between the electric polarization and magnetization of the crystal; the relation between the electric and magnetic field components of light depend on the propagation direction. Thus if the THz radiation propagates in one direction the phase relations are favorable for exciting the hybridized polarization- and magnetization waves and light is absorbed, but by reversing the propagation direction phase relations change and there is no absorption.
