Qianjin Wang

Magnetic Properties and Spontaneous Polarization of La-, Mn- and N-Doped Tetragonal BiFeO3: A First-Principles Study

Multiferroic materials have been receiving attention for their potential applications in multifunctional devices. Chemical substitution is an effective method for improving the physical properties of BiFeO3 (BFO). However, different experimental results have been reported for Lanthanum- (La-) and Manganese (Mn) -doped BFO ceramics. Here, we systematically studied the magnetic properties and spontaneous polarization of La-, Mn-, and Nitrogen (N) -doped tetragonal BiFeO3 using density functional theory with the generalized gradient approximation and U-value method. The calculated results demonstrated that the systems show ferromagnetism with Mn and N doping, whereas no magnetization was found with La doping in G- and C-type antiferromagnetic orderings. Our research further revealed that the ferromagnetism is attributed to the p-d orbital hybridization. Berry-phase polarization calculations predicted a large polarization of 149.2 µC/cm2 along the [001] direction of pure tetragonal BFO. We found that La and N substitution had little influence on the spontaneous polarization, whereas Mn substitution reduced the spontaneous polarization. The reduced energy barrier heights of the doped systems indicate the reduced stability of the off-centering ferroelectricity against the thermal agitation. These findings provide greater understanding for controlling and tuning the multiferroic properties of BFO.

Carbon Nanotube Nonvolatile Thin-Film Transistors With (Bi,Nd) 4 Ti 3 O 12 Gate Insulators

Single-walled carbon nanotube (SWCNT) nonvolatile thin-film transistors (TFTs) with (Bi,Nd)₄Ti₃O₁₂ (BNT) gate insulators were fabricated. The electrical properties of BNT films and SWCNT/BNT TFTs were investigated. The subthreshold swing, the threshold voltage, the channel mobility, and the ON/OFF ratio of SWCNT/BNT TFTs reach to 62.5 mV/decade, 0.45 V, 1.3 × 10³ cm²/Vs, and 1.5 × 10⁷, respectively. Notably, the device shows a memory window of ~4.1 V and a long retention time of ~10⁷ s. These mainly attribute to the SWCNTs channel and BNT ferroelectric gate insulator, which induce much larger charge in channel layer. These results suggest that the SWCNT/BNT TFTs are suitable for the next-generation nonvolatile memory devices and integrated circuits.