Ferroelectricity

A ferroelectric is a material with a spontaneous electric polarization in zero applied field — its bound charges have collectively decided to point in one direction even when nothing is pushing on them. Apply an external field strong enough, and the polarization flips to the new direction; remove the field, and the new polarization stays. The material remembers. The polarization-versus-field curve traces out a hysteresis loop almost identical in shape to the magnetisation-versus-field curve of a ferromagnet, which is why the phenomenon is called ferroelectricity even though no iron is involved.

The microscopic origin in barium titanate (BaTiO₃) — the canonical ferroelectric — is a small off-centre displacement of the central titanium ion within its octahedral oxygen cage. Below a critical temperature (the ferroelectric Curie temperature, around 120 °C for BaTiO₃), the lattice spontaneously distorts so that all the Ti⁴⁺ ions in a domain shift slightly in the same direction, producing a uniform spontaneous polarization. Above the Curie temperature the distortion vanishes by thermal fluctuation and the material reverts to ordinary high-permittivity behaviour. Each crystal contains domains of opposite polarization that can be flipped by external fields — the same domain physics as in ferromagnets, with bound electric charges playing the role of magnetic moments.

Ferroelectric materials are everywhere in modern electronics. Their colossal dielectric constants (κ in the thousands) make them the standard material for surface-mount ceramic capacitors. Their hysteresis loop makes them candidate memory elements (FeRAM — ferroelectric RAM — stores a bit in the polarization direction of a tiny BaTiO₃ cell). Their pyroelectric response (polarization changes with temperature) underpins every passive infrared motion sensor in burglar alarms and automatic faucets. And their piezoelectric coefficients are typically large, which is why most modern piezoelectric ceramics (lead zirconate titanate, PZT) are also ferroelectric.

Joseph Valasek, a graduate student at the University of Minnesota, discovered ferroelectricity in 1920 in Rochelle salt while measuring its piezoelectric response — he noticed a hysteresis loop in the polarization curve identical in form to a magnetisation hysteresis loop. The discovery sat largely ignored for two decades; barium titanate was identified as the second ferroelectric in 1944 (independently in the USSR, USA, and Japan) and turned ferroelectricity into a major technological field. Today there are hundreds of known ferroelectrics.