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Ferroelectricity is the lesser-known twin of ferromagnetism. Iron, cobalt and nickel are examples of common ferromagnetic materials. The electrons in such materials function as small magnets, dipoles, with a north pole and a south pole. In a ferroelectric, the dipoles are not magnetic but electric and have a positive and negative pole.
In absence of an applied magnetic (for a ferromagnet) or electric (for a ferroelectric) field, the orientation of the dipoles is random. When a sufficiently strong field is applied, the dipoles align with it. This field is known as the critical (or coercive) field. Surprisingly, in a 'ferroic' material the alignment remains when the field is removed: the material is permanently polarized. To change the direction of the polarization, a field at least as strong as the critical field must be applied in the opposite direction. This effect is known as hysteresis: the behaviour of the material depends on what has previously happened to it. The hysteresis makes these materials highly suitable as rewritable memory, in for example hard disks.
Piece - Material - Piece - Polarization - Field
For a piece of ideal ferroelectric material, the whole piece switches its polarization when the critical field is reached and it does so with a well-defined speed. In real ferroelectric materials, different parts of the material switch polarization at different critical fields, and at different speeds. Understanding this non-ideality is key to the application in memories.
A model for ferroelectricity and ferromagnetism was developed by the German researcher Franz Preisach as early as 1935. The purely mathematical Preisach model describes ferroic materials as a large collection of small independent modules called hysterons. Each hysteron shows ideal ferroic behaviour, but has its own critical field that can differ from hysteron to hysteron. It has been generally agreed that the model gives an accurate description of real materials, but scientists have not understood...
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