| dc.description.abstract | Room-temperature multiferroics, possessing ferroelectricity and ferromagnetism simultaneously in one phase, hold great promise in miniaturized devices including sensors, actuators, transducers, and multi-state memories. However, single-phase multiferroics are scarce because of the drastically different orbital requirements for ferroelectricity (requiring empty d-orbital) and ferromagnetism (coming from partially filled d-orbitals). Combining two cations possessing ferroelectric and ferromagnetic ordering respectively into one phase is one of the effective routes towards creating single-phase multiferroic materials, such as the Bi-based perovskites BiFeO3 and Bi2FeMnO6. For Bi-based perovskites, the ferroelectricity comes from the high stereochemical activity of the lone-pair electrons of the Bi^3+ cation and the B-site cation provides the magnetism. Bi3Fe2Mn2O10+δ supercell (BFMO322 SC) is a layered structure with enhanced ferroelectricity and magnetism compared to the conventional pseudocubic Bi2FeMnO6 phase. BFMO322 SC has been fabricated on LaAlO3 (001) substrate and can also be fabricated on CeO2 buffer layer.
In this dissertation, the influence of CeO2 thickness to the growth and magnetic property of BFMO322 SC has been first investigated. The result shows that a CeO2 buffer layer as thin as 6.7 nm is sufficient to trigger the growth of BFMO322 SC and the sample exhibits the best magnetic properties with both highest magnetization and anisotropy. The growth of BFMO322 SC with high phase purity and superior magnetic properties on CeO2 with a thickness of 6.7 nm is attributed to the lattice match between Ce-Ce and Bi-Bi bond as well as the smooth surface of CeO2 buffer layer.
Next, the influence of Fe/Mn molar ratio to the growth and magnetic property of Bi-based layered supercell structure has been studied by both experimental and theoretical methods. It was found that that Mn is more important than Fe in facilitating the growth of Bi-based layered supercell structures. With more Fe than Mn in the structure, the layered supercell structure cannot be formed. The three-dimensional distribution of Young’s modulus of the Bi-based layered supercell structures is calculated based on density functional theory. The theoretical calculation indicates that the strain energy is too high to keep the layered supercell structure if there is more Fe than Mn. In particular, the layered supercell structure with Bi2Ox slabs can also be obtained on CeO2 buffer layer and SrTiO3 (001) for single-perovskite BiMnO3 under well controlled growth conditions.
Then tunable layered supercell (SC) structures have been designed and achieved in both BiMnO3 and Bi2NiMnO6 thin films. More specifically, both supercells with two layer BiOx-slabs (2-Bi SC) and three layer structure BiOx-slabs (3-Bi SC) have been achieved on both LaAlO3 (001) and SrTiO3 (001) under deposition parameter tuning. The novel layered supercell structures consist of alternative layered stacking of Bi2Ox (or Bi3Ox) slabs and Mn-O (or Ni-Mn-O) octahedra layers along out-of-plane direction, respectively. Both the BiMnO3 and Bi2NiMnO6 layered supercell structures exhibit robust multiferroic response at room temperature and tunable ferromagnetic and optical properties attributed to the variable SC structures.
Finally, a new layered supercell structure with Bi3Ox slabs has been designed and fabricated from the new material system Bi2AlMnO6 (BAMO). The new BAMO layered supercell structure is self-assembly grown by alternative layered stacking of three-layer-thick Bi-based slabs [Bi3O3+δ] and one-layer-thick [MO2]∞ layer (M = Al/Mn). It can be fabricated on single-crystal substrates SrTiO3 (001) and LaAlO3 (001), with or without CeO2 (001) and La0.7Sr0.3MnO3 (001) buffer layers. Robust room-temperature multiferroic responses have been observed for the new BAMO misfit (incommensurate) layered structure with non-magnetic cations Al^3+ and magnetic cations Mn^3+.
The Bi-based layered supercell structures present great composition flexibility and hold great significance towards the design and creation of new two-dimensional layered materials with a wide range of potential functionalities, such as single-phase multiferroic materials, thermoelectrics, and layered materials with tunable band gaps. | en |
