Controlling the ferroelectric and resistive switching properties of a BiFeO3 thin film prepared using sub-5 nm dimension nanoparticles

Ho and Ti Co-Substitution Tailored Structural Phase Transition and Enhanced Magnetic Properties of BiFeO3 Thin Films

The polycrystalline thin films of BiFeO₃ (BFO) and Bi_0.90Ho_0.10Fe_1-xTi_xO (x = 0, 0.025, 0.05, 0.10, 0.15, and 0.20) were successfully synthesized by the simple sol-gel method. X-ray diffraction and Raman spectra revealed the substitution of Bi and Fe by Ho and Ti, respectively, and correspondingly a structural phase transition from the rhombohedral phase to orthorhombic phase. The field-emission scanning electron microscopy and transmission electron microscopy images indicated that the average size of the particles was decreased and the surface homogeneous agglomeration was enhanced with the increased concentration of Ti to x = 0.05. The X-ray photoelectron spectroscopy measurements illustrated that Fe³⁺ and O^2- ions tended to increase with the Ti concentration increase, which accounted for the enhanced super-exchange interaction between Fe³⁺ and O^2-. Because of the reduced concentration of oxygen vacancies, Ho and Ti ions with a smaller ionic radius and denser surface structure, the Ho and Ti co-substituted films with an appropriate concentration of Ti (x = 0.05) showed an optimal saturation magnetization (M _s) of 44.23 emu/cm³ and remanent magnetization (M _r) of 4.62 emu/cm³, which were approximately 1.8 times and 1.9 times than that of the pure BFO, respectively. This work opened up an effective way to modulate the structure and properties of BFO-based materials.

Enhanced Magnetic Properties of BiFeO₃ Thin Films by Doping: Analysis of Structure and Morphology

The improvement of ferromagnetic properties is critical for the practical application of multiferroic materials, to be exact, BiFeO₃ (BFO). Herein, we have investigated the evolution in the structure and morphology of Ho or/and Mn-doped thin films and the related diversification in ferromagnetic behavior. BFO, Bi_0.95Ho_0.05FeO₃ (BHFO), BiFe_0.95Mn_0.05O₃ (BFMO) and Bi_0.95Ho_0.05Fe_0.95Mn_0.05O₃ (BHFMO) thin films are synthesized via the conventional sol-gel method. Density, size and phase structure are crucial to optimize the ferromagnetic properties. Specifically, under the applied magnetic field of 10 kOe, BHFO and BFMO thin films can produce obvious magnetic properties during magnetization and, additionally, doping with Ho and Mn (BHFMO) can achieve better magnetic properties. This enhancement is attributed to the lattice distortions caused by the ionic sizes difference between the doping agent and the host, the generation of the new exchange interactions and the inhibition of the antiferromagnetic spiral modulated spin structure. This study provides key insights of understanding the tunable ferromagnetic properties of co-doped BFO.

How Ligands Affect Resistive Switching in Solution-Processed HfO2 Nanoparticle Assemblies

Advancement of resistive random access memory (ReRAM) requires fully understanding the various complex, defect-mediated transport mechanisms to further improve performance. Although thin-film oxide materials have been extensively studied, the switching properties of nanoparticle assemblies remain underexplored due to difficulties in fabricating ordered structures. Here, we employ a simple flow coating method for the facile deposition of highly ordered HfO₂ nanoparticle nanoribbon assemblies. The resistive switching character of nanoribbons was determined to correlate directly with the organic capping layer length of their constituting HfO₂ nanoparticles, using oleic acid, dodecanoic acid, and undecenoic acid as model nanoparticle ligands. Through a systematic comparison of the forming process, operating set/reset voltages, and resistance states, we demonstrate a tunable resistive switching response by varying the ligand type, thus providing a base correlation for solution-processed ReRAM device fabrication.