Multiferroics

Historically, multiferroics are defined as materials with at least two of the forms of primary ferroic order in the same phase. The primary forms of ferroic order are ferromagnetism, ferroelectricity, and ferroelasticity which were recently complemented by ferrotoroidicity as new possible candidate in the family. The definition of a multiferroic was further extended by including the ant-ferroic forms of order. Present investigations on multiferroics are mostly restricted to the investigation of the coexistence of magnetic order (usually antiferromagnetism with or without a weak accompanying magnetization) and ferroelectricity. The interest in these types of multiferroics is caused by their potential to exhibit large coupling effects between the magnetic and ferroelectric phase which can be the basis of devices where magnetic order is controlled by electric voltages instead of magnetic fields. In our group we are investigating a large variety of aspects of multiferroic order. Many of these experiments are done with pulsed lasers, which we use for imaging the coexisting magnetic and ferroelectric states in a multiferroic by nonlinear optics. However, other techniques like force microscopy, linear optical spectroscopy, pyrocurrent measurements, electron microscopy, neutron and x-ray diffraction also play an important role. Some of the topics on multiferroics investigated by us are:

• Distribution and coupling of magnetic and ferroelectric domains in multiferroics.
• Domain states in which the hallmarks of magnetic and ferroelectric order are inseparably mixed so that a novel type of "multiferroic domain" emerges.
• Time-resolved magnetoelectric reversal of domains in multiferroics.
• Topology of coexisting magnetic and ferroelectric domain structures.
• Unusual manifestations of ferroelectric order.
• New types of multiferroic materials (including organics).
• Domain walls in multiferroics.
• Incommensurate order in multiferroics and its theoretical description