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Translate the text. Ferroelectricity is a property of certain materials which possess a spontaneous electric polarization that can be reversed by the application of an external

Ferroelectricity is a property of certain materials which possess a spontaneous electric polarization that can be reversed by the application of an external electric field. The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was already known when ferroelectricity was discovered in 1920 in Rochelle salt by Valasek. Thus, the prefix ferro, meaning iron, was used to describe the property despite the fact that most ferroelectric materials do not contain iron. Typically, materials demonstrate ferroelectricity only below a certain phase transition temperature, called the Curie temperature, T_c, and are paraelectric above this temperature.

The nonlinear nature of ferroelectric materials can be used to make capacitors with tunable capacitance. Ferroelectric capacitors are small in physical size compared to dielectric (non-tunable) capacitors of similar capacitance.

The spontaneous polarization of ferroelectric materials implies a hysteresis effect which can be used as a memory function, and ferroelectric capacitors are indeed used to make ferroelectric RAM for computers and RFID cards.

Ferroelectric materials are required by symmetry considerations to be also piezoelectric and pyroelectric. The combined properties of memory, piezoelectricity, and pyroelectricity make ferroelectric capacitors very useful, e.g. for sensor applications. Ferroelectric capacitors are used in medical ultrasound machines, high quality infrared cameras, fire sensors, sonars, vibration sensors, and even fuel injectors on diesel engines.

An important ferroelectric material for applications is lead zirconate titanate (PZT), which is part of the solid solution formed between ferroelectric lead titanate and anti-ferroelectric lead zirconate. PZT-based compounds are used in the manufacturing of ultrasound transducers, in the manufacturing of ceramic capacitors, STM/AFM actuators (tubes), and the like.

In 1979 Swedish Sven Torbjörn Lagerwall discovered ferroelectric liquid crystals in collaboration with Noel Clark. The ferroelectric liquid crystal (FLC) supermolecular structure is very appealing for nonlinear optics (NLO) applications.

Another idea of recent interest is the ferroelectric tunnel junction (FTJ) in which a contact made up by nanometer-thick ferroelectric film placed between metal electrodes. Yet another hot topic is multiferroics, where researchers are looking for ways to couple magnetic and ferroelectric ordering within a material or heterostructure. Multiferroics are explored for magnetic field sensors and electrically tunable microwave devices such as filters, oscillators and phase shifters in which the ferri-, ferro- or antiferro-magnetic resonance is tuned electrically instead of magnetically.

Give the summary of the text using the words below: to possess a spontaneous electric polarization; to be discovered in 1920 in Rochelle salt; below a certain phase transition temperature; to imply a hysteresis effect; ferroelectric liquid crystals; multiferroics.