(634a) Atomic Layer Deposition Enabled Synthesis of Nanostructured Composite Oxide Thin Films for Multiferroic Applications

Multiferroic materials are a class of material which exhibit two or more forms of ferroic order such as (anti)ferroelectricity, (anti)ferromagnetism, or ferroelasticity and have been proposed for devices in which magnetism is switched upon the application of an electric field.  While the existence of single-phase multiferroic materials, such as BiFeO₃, has been demonstrated, composite multiferroics offer substantially improved switching performance, consisting of a piezoelectric and a magnetostrictive material coupled together via strain at the interface. In addition, nanoscale multiferroic composites have been shown in literature to have even greater coupling performance when compared to bulk composites or composite bilayers. Our group therefore proposes the use of atomic layer deposition (ALD), which consists of alternatively flowed precursor gases, thus permitting the precise control of the elemental composition and film thickness by manipulating the pulsing sequence of the precursors and through careful selection of the precursor chemistry and process parameters.  In addition, two separate approaches to multiferroic composites emphasizes the flexible aspects of the ALD technique; for a 2-D composite approach, the ability to deposit nanoscale laminates; while for a 3-D composite approach, the ability to uniformly coat films over a nanoscale mesoporous CoFe₂O₄ template consisting of 14 nm diameter pores.   Challenges that must be overcome in the ALD of multiferroic materials are 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, piezoelectric BiFeO₃ and magnetostrictive CoFe₂O₄ were deposited by ALD to synthesize 2-D nanoscale composite multilayers. The ALD processes used the metallorganic precursors Bi(tmhd)₃ (tmhd = 2,2,6,6-tetramethylheptane-3,5 dione), Co(tmhd)₂, and Fe(tmhd)₃ alongside oxygen atoms produced from a microwave power coaxial waveguide atomic beam source.  In addition, lead zirconate titanate (PZT, Pb(Zr_xTi_1-x)O₃), a strong ferroelectric, was deposited via ALD on a mesoporous cobalt ferrite (CFO, CoFe₂O₄) matrix to observe the magnetoelectric effect of the composite heterostructure.  PZT thin films were synthesized by depositing alternating layers of PbO, ZrO₂, and TiO₂ layers using Pb(tmhd)₂, Zr(tmhd)₄, and Ti(O.i-Pr)₂(tmhd)₂ as the metallorganic precursors and water as the oxidizing agent.  The growth sequence of a(Pb-O)-b(Ti-O)-c(Pb-O)-d(Zr-O) was utilized and the composition and thickness of PZT thin films were tailored by regulating the a:b:c:d ratio of local cycles and number of global cycles, respectively. 

To compare the material performance of the 2-D and 3-D nanoscale multiferroic composites to more well-established synthesis methods, measurements of magnetic and ferroelectric properties were accomplished using SQUID magnetometry and Sawyer-Tower circuit methods, respectively.