Details of Ferroelectric material

A material capable of reversing its polarity when voltage is applied. This polarization inversion is usedin various applications such as memory and piezoelectric actuators

A ferroelectic material is a material that exhibits, over some range of temperature, a spontaneous electic polarisation that can be reversed or reorientated by application of an electric field [poling].

   A necessary criterion is the requirement of an everpresent spontaneous polarisation, with the requirement of reversibility or reorientation of that spontaneous polarisation being a sufficient criterion for a ferroelectric phase.

   An exclusion from the definition of ferroelectrics is those materials belonging to non-polar crystal classes at all temperatures, and in which a metastable polarisation can be induced by an external electric field

In recent years, ferroelectric materials and thin films have attracted much attention and exhibited potential in many important applications such as dynamic random access memories (DRAMS), non-volatile ferroelectric random access memories micro-armours and infrared sensors. At present, the ferroelectric materials suitable for these devices are Pb(Zr,Ti)O₃ (PZT) systems, SrBi₂Ta₂O₉ (SBT) systems, Bi₄Ti₃O₁₂ (BIT) systems and BaTiO₃ (BT) systems that are studied with a great deal of interest. In these ferroelectric materials, BaTi_0.91(Hf_0.5,Zr_0.5)_0.09O₃ (BTHZ-9), one of the BT systems, which has several advantages such as an extremely low coercive field, a high remnant polarization, better mechanical strength and small deviation in composition, could have a strong potential application for ferroelectric thin film devices.

In previous reports, the remnant polarization (Pr) and coercive field (Ec) of these materials were determined to be 15 µC/cm² and 0.65kV/cm respectively, showing a satisfactory piezoelectric property. Also, the BTHZ-9 was shown to have a quadratic characteristic in the strain as a function of electric field in AC field of above 1kV/cm, due to an extremely low coercive field and an excellent piezoelectric property. Especially, the BTHZ-9 with rhombohedral phase at room temperature had a longer endurance against repetition of polarization reversal than that of the tetragonal. It may be due to its low coercive field, which is required for polarization reversal.

In addition, the BTHZ systems, lead and/or bismuth-free material, also present a great interest both for applications in the field of environmental protection and for fundamental studies. Therefore, the BTHZ system is expected to be one of the attractive materials suitable for ferroelectric thin film devices.

Actually,Ferroelectricity is a phenomena which was discovered in 1921. The name refers to certain magnetic analogies, though it is somewhat misleading as it has no connection with iron (ferrum) at all. Ferroelectricity has also been called Seignette electricity, as Seignette or Rochelle Salt (RS) was the first material found to show ferroelectric properties such as a spontaneous polarization on cooling below the Curie point, ferroelectric domains and a ferroelectric hysteresis loop. A huge leap in the research on ferroelectric materials came in the 1950's, leading to the widespread use of barium titanate (BaTiO₃) based ceramics in capacitor applications and piezoelectric transducer devices. Since then, many other ferroelectric ceramics including lead titanate (PbTiO₃), lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), and relaxor ferroelectrics like lead magnesium niobate (PMN) have been developed and utilized for a variety of applications. With the development of ceramic processing and thin film technology, many new applications have emerged. The biggest use of ferroelectric ceramics have been in the areas such as dielectric ceramics for capacitor applications, ferroelectric thin films for non volatile memories, piezoelectric materials for medical ultrasound imaging and actuators, and electro-optic materials for data storage and displays