(711c) Atomic Layer Deposition Enabled Synthesis of Nanostructured Composite BiFeO3/CoFe2O4 Thin Films for Multiferroic Applications

Multiferroic materials, that can either exist as single-phase materials or multi-phase composites, exhibit two or more forms of ferroic order such as (anti)ferroelectricity, (anti)ferromagnetism, or ferroelasticity and have been proposed for use in future non-volatile memory technology.  Atomic layer deposition (ALD) is proposed as a scalable approach to synthesize multiferroic thin films and to enable the synthesis of multiferroic composites which utilize conformal deposition onto 3-D nanostructures.  Challenges that must be overcome in the ALD of multiferroic materials is the amorphous nature of as-deposited films and the difficulty in attaining the desired crystallinity and structure that would enable multiferroic properties to emerge from these materials.

In this work, multiferroic BiFeO₃ was deposited by ALD as a single-phase multiferroic thin film as well as the ferroelectric component in a composite multiferroic using a ferrimagnetic CoFe₂O₄ mesoporous template that was synthesized using an evaporation induced di-block copolymer self-assembly technique.  The ALD process used the metallorganic precursors Bi(tmhd)₃ (tmhd = 2,2,6,6-tetramethylheptane-3,5 dione) and Fe(tmhd)₃ alongside oxygen atoms produced from a coaxial waveguide microwave powered atomic beam source.  A variety of ALD process conditions were studied, such as the effects of process temperature, precursor pulsing time, and precursor pulsing ratio on film composition, growth rate, and crystallization.  The ALD films were able to be grown with a composition ratio Bi:Fe close to unity and with a controlled nanostructure and growth rate of ~0.7 Å/cycle.  In order to achieve the desired crystalline material after rapid thermal processing, the composition and nanostructure of the as-deposited films must first be controlled via the ALD process to fit within narrow windows.

To compare the performance of the multiferroic ALD films to more well established synthesis methods, measurements of magnetic and ferro/piezoelectric properties were accomplished using SQUID magnetometry and piezoresponse force microscopy, respectively.  Magnetic measurements showed that the out-of-plane remnant magnetization of a composite film at room temperature was approximately 66.4 emu/cm³ while the coercive field was approximately 1950 Oe which was comparable to epitaxial films grown by other methods such as PLD.  The magnetoelectric coupling effects in the composite films were studied to assess the effectiveness of the nanostructured material approach.