Abstract
Multiferroic topologies are an emerging solution for future low-power magnetic nanoelectronics due to their combined tuneable functionality and mobility. Here, we show that in addition to being magnetoelectric multiferroic at room temperature, thin-film Aurivillius phase Bi6TixFeyMnzO18 is an ideal material platform for both domain wall and vortex topology-based nanoelectronic devices. Utilizing atomic-resolution electron microscopy, we reveal the presence and structure of 180°-type charged head-to-head and tail-to-tail domain walls passing throughout the thin film. Theoretical calculations confirm the subunit cell cation site preference and charged domain wall energetics for Bi6TixFeyMnzO18. Finally, we show that polar vortex-type topologies also form at out-of-phase boundaries of stacking faults when internal strain and electrostatic energy gradients are altered. This study could pave the way for controlled polar vortex topology formation via strain engineering in other multiferroic thin films. Moreover, these results confirm that the subunit cell topological features play an important role in controlling the charge and spin state of Aurivillius phase films and other multiferroic heterostructures.
Highlights
Room-temperature multiferroic materials, possessing coupled ferroelectric and ferromagnetic states, have exciting potential for use in future low-energy data-storage devices such as magnetoelectric spin orbit logics for recurring neural networks.[1,2] No such commercial devices presently exist, as single-phase multiferroic materials are extremely rare, due to the fundamental contraindication between ferroelectricity and ferromagnetism.[3]
Aurivillius phases are established ferroelectric materials with strong in-plane polarizations,[7] high Curie temperatures (>600 °C), and fatiguefree energy storage performance.[8−10] The rare demonstration of room-temperature ferromagnetism within a ferroelectric framework is achieved in Aurivillius phases with the introduction of magnetic ions within the scaffold.[11−15] Our previous studies have shown that when B-site Ti (x) is maintained between 2.80 and 3.04, Fe (y) between 1.32 and 1.52, and Mn (z) between 0.54 and 0.64, thin-film samples on sapphire display saturation magnetization (MS) values as high as 215 emu/cm[3], in-plane saturation polarization (Ps) values of >26 μC/cm[2], and demonstrate magnetoelectric switching at room temperature,[15−17] as shown in Supporting Information
The 100 nm thick Aurivillius films in this study were synthesized by liquid injection chemical vapor deposition and the magnetoelectric multiferroic properties were confirmed in our previous reports.[15,16]
Summary
Room-temperature multiferroic materials, possessing coupled ferroelectric and ferromagnetic states, have exciting potential for use in future low-energy data-storage devices such as magnetoelectric spin orbit logics for recurring neural networks.[1,2] No such commercial devices presently exist, as single-phase multiferroic materials are extremely rare, due to the fundamental contraindication between ferroelectricity (empty d0 electronic structures) and ferromagnetism (occupied dn electronic structures).[3]. Received: September 9, 2021 Accepted: January 9, 2022 Published: January 19, 2022. Direct piezoresponse force microscopy visualization of ferroelectric switching under the influence of a full in-plane magnetic field cycle demonstrated both irreversible and reversible magnetoelectric domain switching in Bi6TixFeyMnzO18 (B6TFMO).[16] Great care was taken to perform detailed micro- and nanostructural analysis combined with rigorous statistical analysis, which concluded that ferromagnetic secondary phase impurities do not affect the measurements observed with a confidence level of ≥99.5%.19
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