Atomic Layer Deposition Enabled Synthesis of Multiferroic Nanostructures
- Author(s): Pham, Calvin Dinh-Tu
- Advisor(s): Chang, Jane P
- et al.
Complex metal oxides exhibit remarkable tunability in their ferromagnetic, ferroelectric, and multiferroic properties that enable future applications such as non-volatile memory, miniaturized antenna, sensors and actuators. Motivated by the promise of high magnetoelectric coupling from nanostructured multiferroics, atomic layer deposition (ALD) processes were developed to synthesize single-phase ferroics that would be further integrated to form composite multiferroics. The highly conformal ALD coatings promised intimate interfaces of the various ferroic phases to realize tunable magnetoelectric coupling.
In this work, a radical enhanced ALD process was used to synthesize the complex oxide nanostructures, using metalorganic precursors Y(tmhd)3 (tmhd = 2,2,6,6-tetramethylheptane-3,5 dione), Mn(tmhd)3, Bi(tmhd)3, Co(tmhd)2, and Fe(tmhd)3 as well as oxygen atoms produced from a microwave powered atomic beam source. The processing-structure-property relations were systematically studied for three material systems: YMnO3 (YMO), BiFeO3 (BFO), and CoFe2O4 (CFO). For YMO, it was found that the crystal structure forming between orthorhombic or hexagonal configurations was due to the substrate and that Si(111) and Y:ZrO2 (111) substrates preferentially formed the orthorhombic and hexagonal phases respectively. The magnetic susceptibility versus temperature of YMO was characterized and it was determined that the magnetic anomalies at ~48 K and ~80 K corresponded to orthorhombic and hexagonal phases respectively which matched the reports given from literature. The ALD BFO films were grown on SrTiO3 (STO) (001) substrates and were found to crystallize epitaxially in the (001) pseudocubic orientation when annealed at 650 °C. The ferroelectric properties were confirmed via PFM while the weak ferromagnetic coupling showed a magnetic saturation (Ms) of approximately 27 emu/cm3. The ALD CFO films were grown on STO (001) substrates and were mostly polycrystalline with a textured (001) orientation. The magnetic properties were studied and the Ms ranged from 260 to 550 emu/cm3 and the magnetic coercivity (Hc) ranged from 200 to 2180 Oe depending on the direction of the magnetic field, annealing condition, and thickness which matched values of those found in bulk and other thin film studies.
Synthesis of multiferroic composites with nanostructure was enabled by ALD. 2-2 multiferroic composite configurations using BiFeO3 and CoFe2O4 were synthesized by ALD and it was determined that for 40 nm thick films, a change in the easy axis of magnetization could be controlled by changing the size and number of repeating BFO/CFO bilayers. 0-3 multiferroic composite configurations using both BFO/CFO and CFO/PZT were synthesized by a hybrid approach first using evaporation induced self-assembly to deposit a porous metal oxide thin film followed by the ALD film. It was confirmed by SEM that the ALD coating was conformal and the magnetic properties were studied. The CFO template film with ALD BFO was found to have an Ms of 151.1 emu/cm3 out-of-plane and 89.4 emu/cm3 in-plane while Hc was about 2.0 kOe for both orientations. For the PZT template film with CFO overlayer, it was found that the Ms began to saturate at about 40-45 emu/cm3 once the CFO thickness exceeded 4 nm due to the restricted pore necks. These volumetric values exceeded thin films found in literature with 0-3 configurations that were synthesized using wet techniques.