Thin films of multiferroic (Bi0.6Tb0.3La0.1)FeO3 were grown on Pt∕Ti∕SiO2∕Si substrate under various oxygen pressures by pulsed laser deposition technique. X-ray diffraction patterns show that the crystallinity of the thin film is improved with decreasing depositing oxygen pressure and the thin film grown at a lower oxygen pressure of 0.01torr exhibits a single perovskite phase with preferred (001) orientation. Leakage current of the as-deposited thin films decreases with decreasing grown oxygen pressure. Significantly, the leakage current of the thin films can be reduced largely by an annealing process at 800°C with flowing oxygen. The annealed thin films show a relatively high resistivity and stable polarization loops with double remnant polarization of about 3μC∕cm2. The conduction properties of the thin films have been well analyzed, and it is indicated that the dominant conduction mechanism is the space-charge-limited conduction for the thin film grown at higher oxygen pressure and the Poole-Frenkel conduction for the thin films grown at lower oxygen pressure.

Self-aligned-gate Pt/SrBi₂Ta₂O₉/HfAlO/Si metal/ferroelectric/insulator/semiconductor (MFIS) field-effect transistors (FETs) were fabricated. Drain current (I _d) versus gate voltage curves of the MFIS FETs showed almost the same steepness as that of a non-self-aligned-gate Pt/HfAlO/Si metal/insulator/semiconductor FET. Long retention times with large on- and off-state I _d ratios were obtained for the MFIS FETs. The retention times more than 33 days with an on- and off-state I _d ratio over 10⁵ were demonstrated for the self-aligned-gate MFIS FET with a 2-µm-long and 80-µm-wide gate. This device showed the longest memory retention among all of the ferroelectric-gate FETs reported ever before.

We have investigated ferroelectric-gate field-effect transistors (FeFETs) with Pt/SrBi₂Ta₂O₉/(HfO₂)_x(Al₂O₃)_1−x (Hf-Al-O) and Pt/SrBi₂Ta₂O₉/HfO₂ gate stacks. The fabricated FeFETs have excellent data retention characteristics: The drain current ratio between the on- and off-states of a FeFET was more than 2 × 10⁶ after 12 days, and the decreasing rate of this ratio was so small that the extrapolated drain current ratio after 10 years is larger than 1 × 10⁵. A fabricated self-aligned gate Pt/SrBi₂Ta₂O₉/Hf-Al-O/Si FET revealed a sufficiently large drain current ratio of 2.4 × 10⁵ after 33.5 day, which is 6.5 × 10⁴ after 10 years by extrapolation. The developed FeFETs also revealed stable retention characteristics at an elevated temperature up to 120 °C and had small transistor threshold voltage (V_th) distribution. The V_th can be adjusted by controlling channel impurity densities for both n-channel and p-channel FeFETs. These performances are now suitable to integrated circuit application with nonvolatile functions. Fundamental properties for the applications to ferroelectric-CMOS nonvolatile logic-circuits and to ferroelectric-NAND flash memories are demonstrated.

Stable organic biradical crystals PNNNO, ${\mathrm{F}}_2\mathrm{PNNNO},$ and PIMNO of the PNNNO family were synthesized. $\{\mathrm{PNNNO}=2-[4^{\prime }-$ $(N-\mathit{tert}-\mathrm{butyl}-N-\mathrm{oxyamino})\mathrm{phenyl}]-4,4,5,5-\mathrm{tetramethyl}-4,5-\mathrm{dihydro}-1H-\mathrm{imidazol}-1-\mathrm{oxyl}$ $3-\mathrm{oxide},$ ${\mathrm{F}}_2\mathrm{PNNNO}=2-[2^{\prime }{,6\mathrm{}}^{\prime },-\mathrm{difluoro}-4^{\prime }-(N-\mathit{tert}-\mathrm{butyl}-N-\mathrm{oxyamino}\left)\mathrm{phenyl}\right]-\mathrm{}4,4,5,5-\mathrm{tetramethyl}-4,$ $5-\mathrm{dihydro}-1H-\mathrm{imidazol}-1-\mathrm{oxyl}$ $3-\mathrm{oxide},$ $\mathrm{PIMNO}=2-\left[4^{\prime }-\right(N-\mathit{tert}-\mathrm{butyl}-N-\mathrm{oxyamino})-\mathrm{phenyl}]-4,4,5,5-\mathrm{tetramethyl}-4,5-\mathrm{dihydro}-1H-\mathrm{imidazol}-1-\mathrm{oxyl}.\}$ PNNNO and PIMNO crystallize to form quasi-one-dimensional lattices, but ${\mathrm{F}}_2\mathrm{PNNNO}$ to form a quasi-two-dimensional lattice. The temperature dependences of the susceptibility and the high-field magnetization process up to 34 T were measured down to 0.5 K. The results are analyzed by comparing with the theoretical calculations based on the crystal structures. PNNNO and PIMNO are considered to be antiferromagnetic Heisenberg spin chains consisting of $S=1/2\mathrm{}$ spin pairs (dimers) in which the two spins are coupled ferromagnetically. At low temperatures, an antiferromagnetic ordering occurs in these crystals, which is confirmed by the thermodynamic discussion through specific heat measurements. On the other hand, ${\mathrm{F}}_2\mathrm{PNNNO}$ is thought to be a two-dimensional Heisenberg system, in which the spin pairs are connected by two types of antiferromagnetic interactions. The ground state is singlet. The high-field magnetization process shows a two-step saturation with a plateau of the half value of the saturation magnetization.

Retention characteristics of metal-ferroelectric-insulator-semiconductor (MFIS) structures have been studied theoretically by considering effects of charge injections derived from the difference between leakage current densities in the ferroelectric and insulator layers. The calculated curves for time-dependent capacitance have shown good agreements with experimental results. The numerical results for the MFIS structure have indicated that excess current over a certain value through the ferroelectric and the insulator layers causes the retention time to rapidly degrade. An idea of inserting an insulator film between the metal and the ferroelectric layers in an MFIS has also been examined in order to cut down the currents through the ferroelectric layer. The calculations based on our model have found this metal-insulator-ferroelectric-insulator-semiconductor (M-I-FIS) structure to exhibit much longer retention time than the conventional MFIS.