Saturation Remanent Polarization Research Articles

Chemical Solution Deposition 방법을 이용한 BiFeO3/Pb(Zr0.52Ti0.48)O3다층박막의 전기적 특성에 대한 연구

(BFO/PZT) multilayer thin films have been prepared on a Pt/Ti//Si(100) substrate by chemical solution deposition. BFO single layer, BFO/PZT bilayer and multilayer thin films were studied for comparison. X-ray diffraction analysis showed that the crystal structure of all films was multi-orientated perovskite phase without amorphous and impurity phase. The leakage current density at 500 kV/cm was reduced by approximately four and five orders of magnitude by bilayer and multilayer structure films, compared with BFO single layer film. The low leakage current density leads to saturated P-E hysteresis loops of bilayer and multilayer films. In BFO/PZT multlayer film, saturated remanent polarization of was obtained at room temperature at 1 kHz with the coercive field() of 681.4 kV/cm.

High-Performance Bi0.5Na0.5TiO3 Single Crystals Grown by High-Oxygen-Pressure Flux Method

High-PO2 (oxygen pressure) crystal growth is developed for Bi0.5Na0.5TiO3 single crystals based on defect chemistry at high temperatures. Thermogravimetric analysis shows that the vacancy formations of Bi and O at high temperatures are controlled by the surface reaction (oxygen desorption), which is suppressed at a higher PO2 . Bi0.5Na0.5TiO3 crystals grown at a PO2 of 1 MPa exhibit a saturated remanent polarization of 44 µC/cm2 along [110]cubic, suggesting a spontaneous polarization of 54 µC/cm2 along [111]cubic (polar direction) of rhombohedral Bi0.5Na0.5TiO3. High-PO2 heat treatments are proposed to be effective for fabricating high-quality and high-performance ferroelectric/piezoelectric devices using Bi-based ferroelectric oxides.

Reduced leakage current in La and Ni codoped BiFeO3 thin films

La (2.5%) and Ni (2.5%) codoped BiFeO3 (BLFNO) thin films were formed by chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) structures. BiFeO3 (BFO), La(5%)-doped BFO, and Ni(5%)-doped BFO films were studied for comparison. X-ray diffraction analysis showed that the crystal structure of all films was single perovskite phase, however, structural modifications were observed in La-doped films. The leakage current density at 500kV∕cm was reduced by approximately three orders of magnitude by codoping La and Ni atoms, compared with BFO films. In BLFNO films, well saturated remanent polarization of 70μC∕cm2 was obtained at room temperature at 10kHz with the reduced coercive field by approximately 30%.

The influence of La-substitution on the micro-structure and ferroelectric properties of chemical-solution-deposited BiFeO3 thin films

La-substituted BiFeO3 (BFO) thin films were formed by depositing sol–gel solutions on Pt/Ti/SiO2/Si (1 0 0) structures. No secondary phase was observed by La-atom-substitution up to 20% in BFO thin films. The micro-structures of the films were changed with an increase in La concentration, which influenced the ferroelectric properties of the films. The dielectric constant of a BFO film was approximately 90 at 1 MHz and it increased to 125 in a 20% La-substituted BFO film, which was probably due to the size effect of grains. The substitution of La atoms in BFO films reduced the leakage current as well as remanent polarization. The leakage current contribution in the hysteresis loop was negligible when the loops were measured at above 20 kHz. The obtained saturated remanent polarization was 50 µC cm−2 in a 5% La-substituted BFO film at 100 kHz.

Micro-structure and ferroelectric properties of BiFeO3 thin films formed on Pt-coated r-plane sapphire substrates

Magnetoelectric BiFeO3 (BFO) materials exhibit ferroelectric and ferromagnetic properties simultaneously, therefore they have a potential to be applied in magnetic as well as ferroelectric devices. BFO thin films were formed by depositing sol-gel solutions on Pt-coated r-plane sapphire dielectric substrates. We did not observe any secondary phase such as Bi2Fe4O9 on the r-plane sapphire substrates, which is generally observed on Si substrates. We observed small ferroelectric grains of about 0.1 μm on Pt/sapphire structures. The leakage current density in BFO films was found to be decreased dramatically after optimizing process conditions of stoichiometric BFO chemical solution. The leakage current densities were in the range of 10− 7 A/cm2 at room temperature and 10− 9 A/cm2 at 80 K under 0.4 MV/cm applied electric field. The main reason for low leakage current is considered to be reduction of oxygen vacancies due to the presence of exclusive Fe3 + valance state in the films. An applied electric field higher than 0.5 MV/cm was required to pole the BFO films, which made it difficult to obtain the saturated polarization at room temperature. We could measure the saturated remanent polarization in the BFO films at 80 K and the obtained remanent polarization was 100 μC/cm2.

Ferroelectric Properties of Na0.5K0.5NbO3 Films at Low Temperatures

The temperature dependence of polarization and dielectric behaviour of Na0.5K0.5NbO3 thin films were studied from 25 K to room temperature. NKN films were deposited on Pt80Ir20 substrates by RF–magnetron sputtering. The freezing of domain walls with decreasing temperature makes domain walls switching difficult and suppresses the domain walls vibration. Both saturation remanent polarization and coercive field increased with decreasing the temperature. A pronounced low-temperature dielectric relaxation process was observed below 100 K; the relaxation rate fellows the Arrhenius law. Low temperature dielectric behaviour has been studied for the NKN films at various frequencies and bias electrical fields. Constant phase element model is introduced and can interpret the dielectric behaviour well. Dielectric constant, ac conductivity, loss tangent and universal law dielectric equation can be easily deduced from our model. The measured loss tangent and fitted result at various temperature and bias electrical fields agreed very well for our NKN films.

Structure dependence of ferroelectric properties of bismuth layer-structured ferroelectric single crystals

Ferroelectric properties along the a(b) axis and c axis in single crystals of various bismuth layer-structured ferroelectrics (BLSFs) were investigated. By measuring saturated P-E hysteresis curves of BLSFs, values of the saturated remanent polarization and the coercive electric field were found to be related to the Curie temperature and the number of BO6 octahedra (m) between bismuth layers, respectively, along the a(b) axis. The saturated remanent polarization was larger in the BLSF with a higher Curie temperature. This is attributed to a large atomic displacement accompanied by a high Curie temperature. In contrast, the saturated coercive electric field was smaller in the BLSF with a larger number of m. This phenomenon is assumed to be caused by the decrease in the strain energy of the octahedra from the bismuth layer, which leads to an easy movement of the octahedral cations in the direction of an applied external electric field. Along the c axis, the BLSFs with an odd number of m had the same relationship, that is, the saturated remanent polarization was larger with the high Curie temperature, and the saturated coercive field was smaller with the large m number. However, no remanent polarization was confirmed in the BLSFs with an even number of m.

Dielectric and ferroelectric properties of SrBi4Ti4O15 single crystals

SrBi 4 Ti 4 O 15 single crystals were grown, and their dielectric and ferroelectric properties were investigated along the a(b) axis and c axis, separately. The dielectric permittivity at 1 MHz was 1900 along the a(b) axis at the Curie temperature of 520 °C. This value was ten times higher than that along the c axis. With respect to the ferroelectricity, the saturated remanent polarization was 29 μC/cm2 and the saturated coercive field was 26 kV/cm along the a(b) axis under an electric field of 59 kV/cm, and ferroelectricity was not observed along the c axis.

Enhanced Ferroelectric Properties of V-Doped BaBi4Ti4O15 Single Crystal

Single crystals of barium bismuth titanate (BaBi4Ti4O15) and BaBi4Ti4O15 partially substituted with V5+ and Ba2+ for Ti4+ and Bi3+ sites, respectively, at 5 at.% (Ba1.2Bi3.8Ti3.8V0.2O15), were grown using slow cooling. The incorporation of vanadium into the B-site of the layered pseudo-perovskite block resulted in a shift of the Curie point to a higher temperature, from 410°C to 425°C, and an increase in dielectric permittivity from 8000 to 9500 along the a(b)-axis direction and from 150 to 160 along the c-axis direction at 1 MHz and at their respective Curie points. The increase in Curie temperature brought about an increase in remanent polarization, and saturated remanent polarization was enhanced from 14.8 µC cm-2 to 15.5 µC cm-2.

Electrical Properties of a Bismuth Layer‐Structured Ba2Bi4Ti5O18 Single Crystal

Ba2Bi4Ti5O18 single crystals were grown, and their dielectric permittivity, conductivity, and ferroelectricity were investigated along the a‐(or b‐)axis and the c‐axis separately. The dielectric permittivity at 1 MHz along the a‐(or b‐)axis was 2000 at the Curie temperature (360°C); this value was 8 times greater than that along the c‐axis. The dc conductivity was greater along the a‐(or b‐)axis than that along the c‐axis, by one order of magnitude. In regard to the ferroelectricity, the saturated remanent polarization was 120 mC/m2 and the saturated coercive field was 3 MV/m along the a‐(or b‐)axis; values of 8.5 mC/m2 and 0.81 MV/m, respectively, were observed along the c‐axis. The Ba2Bi4Ti5O18 single crystals had large electrical anisotropies, which were due to the layered structure.

Direct pulse technique for spontaneous polarization dynamics and molecular reorientation processes in ferroelectric liquid crystals

A method has been devised for studying the spontaneous polarization, dynamics of polarization reversal, and the molecular reorientation processes that occur in the bulk and surfaces of ferroelectric liquid crystals. These studies have been carried out by analyzing the current response to triangular- and square-wave forms. The current response to a triangular wave yields the spontaneous polarization by conveniently subtracting the background contribution due to the conductive and capacitive currents. Molecular reorientation processes which take place within the bulk and the boundary surface layer can be studied by the appearance of polarization current peaks on the triangular-wave response. The dynamics of polarization reversal between bistable states and of the polarization change between the zero-field relaxed state and either of the saturation remanent polarization states have been studied by applying ‘‘symmetric’’ and ‘‘asymmetric’’ square waves. The technique can also be successfully applied to solid ferroelectrics for studying the spontaneous polarization and the domain dynamics during switching.

Preparation of ferroelectric PZT films by thermal decomposition of organometallic compounds

Transparent PZT thin films with perovskite structure were successfully obtained by thermal decomposition of organometallic compounds at the temperatures of 500 to 700° C. The films deposited on platinum substrates were smooth and uniform, but microcrackings were observed in the films deposited on fused silica substrates. The ratio of metal composition in the PZT film agreed with that in the mixture of starting materials. Films obtained at 700° C on platinum substrate showed a hysteresis loop. A spontaneous polarization was 35.65μC cm−2, a saturation remanent polarization was 30.56μC cm−2 and a coercive field was 45 kV cm−1. Dielectric constant and dielectric loss angle were about 300 and 0.05, respectively.

Ion-beam deposition of thin films of ferroelectric lead zirconate titanate (PZT)

Lead zirconate titanate is a solid solution with nominal composition Pb(ZrxTi1−x)O3. Ferroelectric thin films of this material have been successfully deposited by a focused ion-beam sputtering technique in an O2 atmosphere. These films were characterized according to composition, crystal structure, and dielectric and ferroelectric properties. The effects of deposition temperature, heat treatment after deposition, and substrate and target material were investigated. The composition of the films was within 1 at.% of the composition of the multicomponent PZT target for substrate temperatures up to 200 °C. The Pb concentration decreased for substrate temperatures above 200 °C. Films deposited at substrate temperatures below 250 °C were microcrystalline. At 300 °C, the pyrochlore phase was obtained. Films with the ferroelectric perovskite structure were deposited above 400 °C, which is the lowest deposition temperature ever reported for this phase. O2 losses during postdeposition annealing resulted in a collapse of the perovskite structure. Ferroelectric hysteresis loop measurements indicated a saturation remanent polarization of 4.9 μC/cm2 and a coercive field of 25.2 kV/cm.