Usually the structural distortion which gives rise to the ferroelectricity occurs at high temperature, and the magnetic ordering, which is usually antiferromagnetic, sets in at lower temperature.
The prototypical example is Bi Fe O=76 K) is also type-I, although its ferroelectricity is so-called improper, meaning that it is a secondary effect arising from another (primary) structural distortion.
In addition other types of primary order, such as ferroic arrangements of magneotelectric multipoles is an example, have also been recently proposed. (An article on how Spaldin arrived at the question is here The graph to the right shows in red the number of papers on multiferroics from a Web of Science search until 2008; the exponential increase continues today.
Besides scientific interest in their physical properties, multiferroics have potential for applications as actuators, switches, magnetic field sensors or new types of electronic memory devices. To place multiferroic materials in their appropriate historical context, one also needs to consider magnetoelectric materials, in which an electric field modifies the magnetic properties and vice versa.
Larger polarizations occur when the non-centrosymmetric magnetic ordering is caused by the stronger superexchange interaction, such as in orthorhombic Ho Mn O In both cases the magnetoelectric coupling is strong because the ferroelectricity is directly caused by the magnetic order.
While most magnetoelectric multiferroics developed to date have conventional transition-metal d-electron magnetism and a novel mechanism for the ferroelectricity, it is also possible to introduce a different type of magnetism into a conventional ferroelectric.
We will not however discuss these phenomena in more detail because it seems that till present, presumably, they have not been observed in any substance.” One year later, I. Dzyaloshinskii showed using symmetry arguments that the material Cr Over the next decades, research on magnetoelectric materials continued steadily in a number of groups in Europe, in particular in the former Soviet Union and in the group of H. Schmid in the proceedings of the 1993 MEIPIC conference (in Ascona).
To be defined as ferroelectric, a material must have a spontaneous electric polarization that is switchable by an applied electric field.
While ferroelectric ferroelastics and ferromagnetic ferroelastics are formally multiferroics, these days the term is usually used to describe the magnetoelectric multiferroics that are simultaneously ferromagnetic and ferroelectric.
Sometimes the definition is expanded to include non-primary order parameters, such as antiferromagnetism or ferrimagnetism. This work explained the origin of the contraindication between magnetism and ferroelectricity and proposed practical routes to circumvent it, and is widely credited with starting the modern explosion of interest in multiferroic materials.