This work focuses on the design, synthesis, characterization and integration of multiferroic composite materials via radical-enhanced atomic layer deposition (RE-ALD). Specifically, ferrimagnetic CoFe2O4 is integrated with multiferroic BiFeO3 and ferroelectric HfO2 (FE-HfO2), respectively, to create two distinct composite systems. The use of multiferroic BiFeO3 as the ferroelectric phase offers the potential of employing two interfacial coupling phenomena simultaneously. On the other hand, desirable ferroelectric property and superior Si-compatibility that FE-HfO2 can offer make it an intriguing material system for further implementation into device processing at an industrial scale.
In the CoFe2O4/BiFeO3 system, high-quality CoFe2O4 and BiFeO3 were synthesized on SrTiO3 (001) substrates via RE-ALD using TMHD-based metalorganic precursors (TMHD = 2,2,6,6-tetramethylheptane-3,5 dione) and atomic oxygen. With post-deposition thermal treatments, BiFeO3 exhibited epitaxial single-crystalline growth in its (001)pc orientation. Ferroelectric switching and measurable ferromagnetism confirmed the multiferroicity of BiFeO3. CoFe2O4 exhibited textured-polycrystalline growth with a ~10-nm epitaxial transition layer, which led to tunable ferrimagnetism with a thickness-related strain relaxation process. The CoFe2O4 thin films exhibited magnetic behavior that is comparable with the ones synthesized by other processing techniques as well as bulk crystals. Nano-laminates of CoFe2O4/BiFeO3 in 2D-2D configuration were synthesized on SrTiO3 (001) and Si (001) substrates. By fixing the nanolaminate total thickness at 55 nm and CoFe2O4-BiFeO3 ratio at a constant of CoFe2O4:BiFeO3 = 15:40 while increasing the number of alternating layers up to 5 layers, the tri-layer BiFeO3/CoFe2O4/BiFeO3 structure exhibits the most promising functional properties with an optimized polarization ~17 μC/cm2 and saturation magnetism (Ms) ~125 emu/cm3. Both strain and magnetic interactions were observed at the interface for the nano-laminates due to the multiferroic nature of BiFeO3. The tri-layer structure exhibited a converse magnetoelectric coupling coefficient (αconverse) of ~22 Oe cm/kV. By scaling the nano-laminate from ~55 nm to ~16 nm in total thickness, αconverse is further improved to ~64 Oe cm/kV, comparable with systems reported with much higher thicknesses. As the first demonstration of a fully ALD-synthesized multiferroic composite, this part of the work reveals the possibility to utilize ALD to optimize multiferroic nano-laminates for further integrations into magnetoelectric devices.
For CoFe2O4/FE-HfO2 composites, HfO2 thin films were synthesized with tetrakis(dimethylamido)hafnium(IV) (TDMAH) and atomic oxygen. In this design, CoFe2O4 served not only as the mechanical confinement layer but also as an active magnetic layer that contributed to overall magnetism. For ~6nm HfO2 annealed at ~700 ï¿½C, the CoFe2O4/FE-HfO2 composites exhibited a remnant polarization (Pr) ~5.5 μC/cm2 and an electrical coercivity (Ec) ~2000 kV/cm as well as an out-of-plane magnetic anisotropy with a saturation magnetization (Ms) of ~155 emu/cm3 and a magnetic coercivity (Hc) ranging from ~1000-3400 Oe. Magnetoelectric characterization revealed promising magnetoelectric coupling, with αconverse ranged 55-168 Oe cm/kV at room-temperature, once again is comparable with other systems reported with much higher thicknesses. It is believed that the CoFe2O4/FE-HfO2 system here opens many new avenues for developing future magnetoelectric composites and related devices.