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1. Switching the spin cycloid in BiFeO3 with an electric field

   https://doi.org/10.1038/s41467-024-47232-5  

   Meisenheimer, Peter; Moore, Guy; Zhou, Shiyu; Zhang, Hongrui; Huang, Xiaoxi; Husain, Sajid; Chen, Xianzhe; Martin, Lane W; Persson, Kristin A; Griffin, Sinéad; et al (December 2024, Nature Communications)

   Abstract Bismuth ferrite (BiFeO₃) is a multiferroic material that exhibits both ferroelectricity and canted antiferromagnetism at room temperature, making it a unique candidate in the development of electric-field controllable magnetic devices. The magnetic moments in BiFeO₃are arranged into a spin cycloid, resulting in unique magnetic properties which are tied to the ferroelectric order. Previous understanding of this coupling has relied on average, mesoscale measurements. Using nitrogen vacancy-based diamond magnetometry, we observe the magnetic spin cycloid structure of BiFeO₃in real space. This structure is magnetoelectrically coupled through symmetry to the ferroelectric polarization and this relationship is maintained through electric field switching. Through a combination of in-plane and out-of-plane electrical switching, coupled with ab initio studies, we have discovered that the epitaxy from the substrate imposes a magnetoelastic anisotropy on the spin cycloid, which establishes preferred cycloid propagation directions. The energy landscape of the cycloid is shaped by both the ferroelectric degree of freedom and strain-induced anisotropy, restricting the spin spiral propagation vector to changes to specific switching events. 

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2. Large bi-axial tensile strain effect in epitaxial BiFeO3 film grown on single crystal PrScO3

   https://doi.org/10.1038/s41598-023-45980-w  

   Bae, In-Tae; Lingley, Zachary R.; Foran, Brendan J.; Adams, Paul M.; Paik, Hanjong (December 2023, Scientific Reports)

   Abstract A BiFeO₃film is grown epitaxially on a PrScO₃single crystal substrate which imparts ~ 1.45% of biaxial tensile strain to BiFeO₃resulting from lattice misfit. The biaxial tensile strain effect on BiFeO₃is investigated in terms of crystal structure, Poisson ratio, and ferroelectric domain structure. Lattice resolution scanning transmission electron microscopy, precession electron diffraction, and X-ray diffraction results clearly show that in-plane interplanar distance of BiFeO₃is the same as that of PrScO₃with no sign of misfit dislocations, indicating that the biaxial tensile strain caused by lattice mismatch between BiFeO₃and PrScO₃are stored as elastic energy within BiFeO₃film. Nano-beam electron diffraction patterns compared with structure factor calculation found that the BiFeO₃maintains rhombohedral symmetry, i.e., space group ofR3c. The pattern analysis also revealed two crystallographically distinguishable domains. Their relations with ferroelectric domain structures in terms of size and spontaneous polarization orientations within the domains are further understood using four-dimensional scanning transmission electron microscopy technique. 

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3. Beyond Substrates: Strain Engineering of Ferroelectric Membranes

   https://doi.org/10.1002/adma.202003780  

   Pesquera, David; Parsonnet, Eric; Qualls, Alexander; Xu, Ruijuan; Gubser, Andrew_J; Kim, Jieun; Jiang, Yizhe; Velarde, Gabriel; Huang, Yen‐Lin; Hwang, Harold_Y; et al (September 2020, Advanced Materials)

   Abstract Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high‐quality substrates. Here, using the ferroelectric BaTiO₃, production of precisely strain‐engineered, substrate‐released nanoscale membranes is demonstrated via an epitaxial lift‐off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide‐metal/ferroelectric/oxide‐metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm⁻¹and improving switching times to <5 ns for a 20 µm diameter capacitor in a 100‐nm‐thick film). In devices integrated on flexible polymers, enhanced room‐temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1% strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS‐compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth. 

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4. Giant tuning of ferroelectricity in single crystals by thickness engineering

   https://doi.org/10.1126/sciadv.abc7156  

   Chen, Zibin; Li, Fei; Huang, Qianwei; Liu, Fei; Wang, Feifei; Ringer, Simon P.; Luo, Haosu; Zhang, Shujun; Chen, Long-Qing; Liao, Xiaozhou (October 2020, Science Advances)

   null (Ed.)

   Thickness effect and mechanical tuning behavior such as strain engineering in thin-film ferroelectrics have been extensively studied and widely used to tailor the ferroelectric properties. However, this is never the case in freestanding single crystals, and conclusions from thin films cannot be duplicated because of the differences in the nature and boundary conditions of the thin-film and freestanding single-crystal ferroelectrics. Here, using in situ biasing transmission electron microscopy, we studied the thickness-dependent domain switching behavior and predicted the trend of ferroelectricity in nanoscale materials induced by surface strain. We discovered that sample thickness plays a critical role in tailoring the domain switching behavior and ferroelectric properties of single-crystal ferroelectrics, arising from the huge surface strain and the resulting surface reconstruction. Our results provide important insights in tuning polarization/domain of single-crystal ferroelectric via sample thickness engineering. 

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5. Electronic Transport and Ferroelectric Switching in Ion‐Bombarded, Defect‐Engineered BiFeO ₃ Thin Films

   https://doi.org/10.1002/admi.201700991  

   Saremi, Sahar; Xu, Ruijuan; Dedon, Liv_R; Gao, Ran; Ghosh, Anirban; Dasgupta, Arvind; Martin, Lane_W (November 2017, Advanced Materials Interfaces)

   Abstract Despite continued interest in the multiferroic BiFeO₃for a diverse range of applications, use of this material is limited by its poor electrical leakage. This work demonstrates some of the most resistive BiFeO₃thin films reported to date via defect engineering achieved via high‐energy ion bombardment. High leakage in as‐grown BiFeO₃thin films is shown to be due to the presence of moderately shallow isolated trap states, which form during growth. Ion bombardment is shown to be an effective way to reduce this free carrier transport (by up to ≈4 orders of magnitude) by trapping the charge carriers in bombardment‐induced, deep‐lying defect complexes and clusters. The ion bombardment is also found to give rise to an increased resistance to switching as a result of an increase in defect concentration. This study demonstrates a systematic ion‐dose‐dependent increase in the coercivity, extension of the defect‐related creep regime, increase in the pinning activation energy, decrease in the switching speed, and broadening of the field distribution of switching. Ultimately, the use of such defect‐engineering routes to control materials will require identification of an optimum range of ion dosage to achieve maximum enhancement in resistivity with minimum impact on ferroelectric switching. 

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