Engineering the Self-Assembly of CoFe₂O₄ - BiFeO₃ Nanocomposites

Magnetoelectric multiferroics are promising materials for the development of low-power logic and memory devices. This is due to the coupling between ferroelectric and ferromagnetic orderings, which enables the magnetization to be reoriented by applying an electric field and vice versa [1]. To date, multiple mechanisms have been proposed to create both single-phase and two-phase systems. Vertically aligned self-assembled nanocomposite thin films consisting of magnetic spinel CoFe₂O₄ (CFO) and ferroelectric perovskite BiFeO₃ (BFO) are attractive because of the strain-mediated indirect magnetoelectric coupling at the interface between magnetic nanostructures and the ferroelectric matrix [2]. Although these epitaxial self-assembled structures are well studied, precise control of their order and geometry remains challenging, limiting the implementation of these nanocomposites into devices. In this study, we use focused ion beam (FIB) and pulsed laser deposition (PLD) to direct the self-assembly of CFO - BFO on (111)-oriented conductive 0.7% Nb-doped SrTiO₃ and (111)-oriented insulating SrTiO₃ substrates [3]. First, we use FIB to pattern Au-coated substrates, then remove the Au and etch then anneal the substrates to make 2 nm deep pits or trenches with spacings of 100 nm and above. When ~1 nm of CFO is deposited by PLD, it nucleates preferentially in the shallow pits in the substrate. Subsequently, we co-deposit both BFO and CFO. BFO forms a perovskite matrix on the mesa, while CFO continues to grow on top of the initial spinel CFO seeds. Unlike growth on (001)-oriented substrates which yields square or rectangular CFO pillars, the CFO nanostructures preferentially elongate along &lt;110&gt; directions to form fin-shaped features. By patterning trenches along these directions, parallel alternating fins of CFO and BFO were made.<br/>We compare the self-assembled CFO - BFO structure to that made on templates prepared by an additive approach based on electron beam lithography, in which STO topography is generated by patterning a few nm thick STO film on an STO substrate using liftoff followed by crystallization. Finally, we use probe microscopy to characterize the microstructure, ferroelectric, and magnetic properties of these nanocomposites. The BFO nanostructures are taller than the CFO due to a higher growth rate, but the magnetic domain structure of the CFO can be imaged, showing in-plane domains parallel to the fins at remanence that orient parallel to an in situ applied field of 2 kOe.<br/><br/>[1] Spaldin, N. A.; Ramesh, R. <i>Nature Mater</i> <b>2019</b>, <i>18</i> (3), 203–212.<br/>[2] Comes, R.; Liu, H.; Khokhlov, M.; Kasica, R.; Lu, J.; Wolf, S. A. <i>Nano Lett.</i> <b>2012</b>, <i>12</i> (5), 2367–2373.<br/>[3] Su, T.; Yu, Y.; Ross, C. A. <i>Nano Lett. 24 195</i> <b>2024</b>.