All ferroelectric materials exhibit piezoelectricity a piezoelectric effect. Barium titanate is ferroelectric and its dielectric constant varies with temperature. There are a few crystal structures, notably the perovskite structure, which exhibit ferroelectric behavior. Ionic polarization causes the ferroelectric effect as well as dipolar polarization. Uncooled detectors are mostly based on pyroelectric and ferroelectric materials or microbolometer technology. Materials that are both ferroelectric "and" ferromagnetic are known as multiferroics. His early work was consumed with physics of crystals like dielectric and ferroelectric materials. It is analogous to the piezoelectric effect, which is another type of ferroelectric behavior. The nonlinear nature of ferroelectric materials can be used to make capacitors with tunable capacitance. In ferroelectric class 2 ceramic capacitors capacitance decreases over time. The permittivity of ferroelectric class 2 material depends on the applied voltage. FeFET memory uses a transistor with ferroelectric material to permanently retain state. In ferroelectric Class 2 ceramic capacitors, capacitance decreases over time. The reversal occurs gradually through the nucleation and growth of ferroelectric domains with opposite polarization. Thinfilm produces rewriteable non-volatile organic ferroelectric memory based on ferroelectric polymers.