Imaging of electric-field-induced domain structure in DyMnO $$_{3}$$ nanocrystals

Discover Nano Research Imaging of electric‑field‑induced domain structure in DyMnO3 nanocrystals Mansoor A. Najeeb1 · Robbie Morrison1 · Ahmed H. Mokhtar1 · Daniel G. Porter2 · Frank Lichtenberg3 · Alessandro Bombardi2 · Marcus C. Newton1 Received: 16 August 2024 / Accepted: 4 December 2024 © The Author(s) 2024  OPEN Abstract Multiferroic materials that exhibit interacting and coexisting properties, like ferroelectricity and ferromagnetism, possess significant potential in the development of novel technologies that can be controlled through the application of external fields. They also exhibit varying regions of polarity, known as domains, with the interfaces that separate the domains referred to as domain walls. In this study, using three-dimensional (3D) bragg coherent diffractive imaging (BCDI), we investigate the dynamics of multiferroic domain walls in a single hexagonal dysprosium manganite (h-DyMnO3) nanocrystal under varying applied electric field. Our analysis reveals that domain wall motion is influenced by the pinning effects, and a threshold voltage of +3 V is required to overcome them. Using circular mean analysis and phase gradient mapping, we identified localised phase realignment and high-gradient regions corresponding to domain walls, providing insights into the behaviour of multiferroic systems under external stimuli. Keywords Multiferroic · Domain walls · Bragg coherent diffraction imaging · Machine learning 1 Introduction 1.1 Multiferroics synopsis Multiferroics are of great interest, as an understanding of the interplay between coexisting yet contrasting ferroic properties at the atomic scale that could lead to the development of novel technologies, where one ferroic property is used to control the conjugated field of another.[1–5] For example, multiferroics where ferroelectric and ferromagnetic orders are coupled, allowing the control of magnetic order through the use of an external electric fields and vice-versa.[6–10] Multiferroics also exhibit varying regions of polarity, known as domains. The interfaces that separate these domains are known as domain walls. These domain walls can be created, moved or erased with application of external stimuli. Importantly, even in the absence of external interference, the domain walls can give rise to intrinsic defects and strain within the material which results in local atomic rearrangements.[11] Domain walls in multiferroics are also 2D systems that can host functional electronic and magnetic properties, which could find utility in new generation devices due to Supplementary Information The online version contains supplementary material available at https://doi.org/10.1186/s11671-024- 04165-8. * Mansoor A. Najeeb, ; Marcus C. Newton, | 1Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK. 2Diamond Light Source, Harwell Oxford Campus, Didcot OX11 0DE, UK. 3Department of Materials, ETH Zürich, Zürich 8093, Switzerland. Discover Nano (2024) 19:203 | https://doi.org/10.1186/s11671-024-04165-8 Vol.:(0123456789) Research Discover Nano (2024) 19:203 | https://doi.org/10.1186/s11671-024-04165-8 their agility and spatial mobility.[12, 13] As a result, there is a vibrant effort to realise technologies such as integrated devices that utilise this underlying mechanisms. Multiferroics are generally classified into two types. Type-II multiferroics are those in which ferroelectricity emerges as a result of specific magnetic ordering. Type-I encompasses all others where ferroelectricity does not have a magnetic origin. Further classification is possible based on: (1) whether or not ferroelectricity is the primary order parameter; or (2) the atomic scale mechanism that gives rise to ferroelectricity, namely electronic lone pairs, geometric constraints, charge ordering or magnetic ordering with resulting inverse DzyaloshinskiiMoryia interaction.[14, 15] 1.2 BCDI synopsis In multiferroic materials, an applied external electric field can alter the ordered electric dipole moments, which leads to significant changes in the material’s microstructural characteristics which manifest as domain wall movements and associated strain. The intensity of diffraction patterns can be influenced by various aspects of crystal structure including unit cell parameters, crystal defects and grain boundaries.[16] When an external voltage is applied, new defects may arise and existing defects may shift within the crystal.[17] Conventional techniques like X-ray diffraction and scanning electron microscopy, often provide average information across larger volumes, missing local variations that are crucial for understanding strain and defect dynamics. In contrast, BCDI presents a powerful alternative, as it offers high-resolution 3D imaging capabilities that enables precise tracking of changes in the crystal structure. The process involves shining a spatially coherent X-ray on to the nanoscale crystal configured at the Bragg reflection geometry. The coherence length exceeds the dimensions of the crystal [18, 19], leading to corresponding interference patterns in the far field, thus producing a comprehensive 3D k -space diffraction pattern and the intensity is measured by a photon counting area detector. The experiment captures a two-dimensional diffraction pattern on the detector, while the third dimension is acquired by incrementally rocking the sample and recording the diffraction pattern at each step (rocking curve scan). [20] Subsequent to this, machine learning-aided iterative phase reconstruction methodologies are employed to recover the distinct 3D electron density and phase information [21]. The displacement of ions throughout the material correlates directly with the phase, enabling the derivation of strain information via the relationship 𝜙 = Q ⋅ u, where u represents atomic displacement [18, 22] 1.3 Summary of manuscript In this manuscript, we use BCDI reconstructed 3D images to reveal the phase variations within h-DyMnO 3 nanocrystals as a function of applied voltage. Although this approach offers valuable insights into the electric-field-induced domain wall structure, the direct investigation of magnetic and magneto-electric properties of multiferroic material falls outside the scope of this study. 2 Results 2.1 Bragg CDI experiment DyMnO3 nanocrystals were grown using the bulk melt-grown technique described by [23, 24] were prepared using the procedure described in the methods section. Powder XRD (SI Fig. S12) analysis at the laboratory was conducted prior to the synchrotron measurements to ensure the right hexagonal crystallographic phases were obtained with lattice parameters a = b = 6.18 Å and c = 11.4 Å [25]. The formation of ferroelectric domain structures in h-DyMnO 3 can be achieved by cooling the material from its high-symmetry paraelectric phase (P63/mmc) to the lower-symmetry ferroelectric phase (P63cm) at a critical temperature (Tc ) exceeding 1250 K. BCDI characterisat (...truncated)