Absence of evidence ≠ evidence of absence: statistical analysis of inclusions in multiferroic thin films

Charged Domain Wall and Polar Vortex Topologies in a Room-Temperature Magnetoelectric Multiferroic Thin Film

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 Bi₆Ti_xFe_yMn_zO₁₈ 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 Bi₆Ti_xFe_yMn_zO₁₈. 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.

Probing Ferroelectric Behavior in Sub-10 nm Bismuth-Rich Aurivillius Films by Piezoresponse Force Microscopy

Sub-10 nm ferroelectric and multiferroic materials are attracting increased scientific and technological interest, owing to their exciting physical phenomena and prospects in miniaturized electronic devices, neuromorphic computing, and ultra-compact data storage. The Bi6Ti2.9Fe1.5Mn0.6O18 (B6TFMO) Aurivillius system is a rare example of a multiferroic that operates at room temperature. Since the formation of magnetic impurity phases can complicate attempts to measure ferromagnetic signal intrinsic to the B6TFMO multiferroic phase and thus limits its use, herein we minimize this by utilizing relatively large (49%) bismuth excess to counteract its volatility during sub-10 nm growth. X-ray diffraction, electron microscopy, and atomic force microscopy show sample crystallinity and purity are substantially improved on increasing bismuth excess from 5 to 49%, with the volume fraction of surface impurities decreasing from 2.95–3.97 vol% down to 0.02–0.31 vol%. Piezoresponse force microscopy reveals 8 nm B6TFMO films are ferroelectric, with an isotropic random distribution of stable in-plane domains and weaker out-of-plane piezoresponse. By reducing the volume fraction of magnetic impurities, this work demonstrates the recent progress in the optimization of ultra-thin B6TFMO for future multiferroic technologies. We show how the orientation of the ferroelectric polarization can be switched in 8 nm B6TFMO and arrays can be “written” and “read” to express states permitting anti-parallel information storage.

Compositional Tuning of the Aurivillius Phase Material Bi5Ti3-2xFe1+xNbxO15 (0 ≤ x ≤ 0.4) Grown by Chemical Solution Deposition and its Influence on the Structural, Magnetic, and Optical Properties of the Material

A series of Aurivillius phase materials, Bi₅Ti _{3 - 2x} Fe _{1 + x} Nb_xO₁₅ ( [Formula: see text], 0.1, 0.2, 0.3, and 0.4), was fabricated by chemical solution deposition. The effects of aliovalent substitution for the successful inclusion of Fe ³⁺ and Nb ⁵⁺ by replacing Ti ⁴⁺ were explored as a potential mechanism for increasing magnetic ion content within the material. The structural, optical, piezoelectric, and magnetic properties of the materials were investigated. It was found that a limit of x = 0.1 was achieved before the appearance of secondary phases as determined by the X-ray diffraction. Absorption in the visible region increased with increasing values of x corresponding to the transition from the valence band to the conduction band of the Fe- [Formula: see text] energy level. Piezoresponse force microscopy measurements demonstrated that the lateral piezoelectric response increased with increasing values of x . Magnetic measurements of Bi₅Ti_2.8Fe_1.1Nb_0.1O₁₅ exhibited a weak ferromagnetic response at 2, 150, and 300 K of 2.2, 1.6, and 1.5 emu/cm³ with H_c of  ∼ 40 , 36, and 34 Oe, respectively. The remanent magnetization M_R of this sample was found to be higher than the range of reported values for the Bi₅Ti₃Fe₁O₁₅ parent phase. Elemental analysis of this sample by energy-dispersive X-ray analysis did not provide any evidence for the presence of iron-rich secondary phases. However, it is noted that a series of measurements at varying sample volumes and instrument resolutions is still required in order to put a defined confidence level on the Bi₅Ti_2.8Fe_1.1Nb_0.1O₁₅ material being a single-phase multiferroic.

Progress and Perspectives on Aurivillius-Type Layered Ferroelectric Oxides in Binary Bi4Ti3O12-BiFeO3 System for Multifunctional Applications

Driven by potentially photo-electro-magnetic functionality, Bi-containing Aurivillius-type oxides of binary Bi4Ti3O12-BiFeO3 system with a general formula of Bin+1Fen−3Ti3O3n+3, typically in a naturally layered perovskite-related structure, have attracted increasing research interest, especially in the last twenty years. Benefiting from highly structural tolerance and simultaneous electric dipole and magnetic ordering at room temperature, these Aurivillius-phase oxides as potentially single-phase and room-temperature multiferroic materials can accommodate many different cations and exhibit a rich spectrum of properties. In this review, firstly, we discussed the characteristics of Aurivillius-phase layered structure and recent progress in the field of synthesis of such materials with various architectures. Secondly, we summarized recent strategies to improve ferroelectric and magnetic properties, consisting of chemical modification, interface engineering, oxyhalide derivatives and morphology controlling. Thirdly, we highlighted some research hotspots on magnetoelectric effect, catalytic activity, microwave absorption, and photovoltaic effect for promising applications. Finally, we provided an updated overview on the understanding and also highlighting of the existing issues that hinder further development of the multifunctional Bin+1Fen−3Ti3O3n+3 materials.

The growth and improved magnetoelectric response of strain-modified Aurivillius SrBi4.25La0.75Ti4FeO18 thin films

In this study, we grew 5-layered SrBi4.25La0.75Ti4FeO18 (SBLFT) polycrystalline thin films (80-330 nm thick) via pulsed-laser deposition to study their ferroelectric and magnetoelectric response. Structural/microstructural analysis confirmed the formation of orthorhombic SBLFT with good crystallinity and randomly oriented Aurivillius phases. Detailed scanning transmission electron microscopy analysis of 120 nm film revealed a predominantly five-layered structure with the coexistence of four-layer stacking. Such stacking defects are found to be pertinent to the high structural flexibility of Bi-rich Aurivillius phases, alleviated by lattice strain. Raman spectral features at ambient temperatures depict the signature of the orthorhombic-tetragonal phase transition. SBLFT films have a strong ferroelectric nature (remanent polarization 2Pr of 35 μC cm-2) with a fatigue endurance up to 1010 cycles and strongly improved, switchable magnetization as opposed to its antiferromagnetic bulk counterpart. The scaling behavior of dynamic hysteresis reveals that ferroelectric domain reversal has good stability and low energy consumption. We observed the presence of SBLFT nanoregions (1-5 nm), distributed across the film, with Bi and Fe-rich compositions and oxygen vacancies that contribute to the weak ferromagnetic behavior mediated by the Dzyaloshinskii-Moriya interactions. Subtle changes in the structural strain and lattice distortions of thin films with varied thicknesses led to distinct ferroic properties. Stronger ferroelectric polarization of 80 nm and 120 nm films compared to that of thicker ones can be due to structural strain and the possible rearrangement of BO6 octahedra. The observation of the improved magnetoelectric coefficient of 50 mV cm-1 Oe-1 for 120 nm film, as compared to that of several Aurivillius oxides, indicates that the structural strain modification in SBLFT is beneficial for the fatigue-free magnetic field switching of ferroelectric polarization. The structural strain of the unit cell as well as the presence of Bi- and ferromagnetic Fe-rich nanoregions was found to be responsible for the improved multiferroic behaviour of the SBLFT films.

Direct atomic scale determination of magnetic ion partition in a room temperature multiferroic material

The five-layer Aurivillius phase Bi₆Ti_xFe_yMn_zO₁₈ system is a rare example of a single-phase room temperature multiferroic material. To optimise its properties and exploit it for future memory storage applications, it is necessary to understand the origin of the room temperature magnetisation. In this work we use high resolution scanning transmission electron microscopy, EDX and EELS to discover how closely-packed Ti/Mn/Fe cations of similar atomic number are arranged, both within the perfect structure and within defect regions. Direct evidence for partitioning of the magnetic cations (Mn and Fe) to the central three of the five perovskite (PK) layers is presented, which reveals a marked preference for Mn to partition to the central layer. We infer this is most probably due to elastic strain energy considerations. The observed increase (>8%) in magnetic cation content at the central PK layers engenders up to a 90% increase in potential ferromagnetic spin alignments in the central layer and this could be significant in terms of creating pathways to the long-range room temperature magnetic order observed in this distinct and intriguing material system.

Structure Evolution and Multiferroic Properties in Cobalt Doped Bi4NdTi3Fe1-xCoxO15-Bi3NdTi2Fe1-xCoxO12-δ Intergrowth Aurivillius Compounds

Here, we report the structure evolution, magnetic and ferroelectric properties in Co-doped 4- and 3-layered intergrowth Aurivillius compounds Bi₄NdTi₃Fe_1-xCo_xO₁₅-Bi₃NdTi₂Fe_1-xCo_xO_12-δ. The compounds suffer a structure evolution from the parent 4-layered phase (Bi₄NdTi₃FeO₁₅) to 3-layered phase (Bi₃NdTi₂CoO_12-δ) with increasing cobalt doping level from 0 to 1. Meanwhile the remanent magnetization and polarization show opposite variation tendencies against the doping level, and the sample with x = 0.3 has the largest remanent magnetization and the smallest polarization. It is believed that the Co concentration dependent magnetic properties are related to the population of the Fe³⁺ -O-Co³⁺ bonds, while the suppressed ferroelectric polarization is due to the enhanced leakage current caused by the increasing Co concentration. Furthermore, the samples (x = 0.1–0.7) with ferromagnetism show magnetoelectric coupling effects at room temperature. The results indicate that it is an effective method to create new multiferroic materials through modifying natural superlattices.