Ipek Efe

Last Name

Efe

First Name

Ipek

ORCID

0000-0003-2004-162X

Organisational unit

03918 - Fiebig, Manfred / Fiebig, Manfred

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Publications 1 - 10 of 12

Strong magnetoelectric coupling at an atomic nonmagnetic electromagnetic probe in bismuth ferrite
Schell, Juliana; Schmuck, Merlin; Efe, Ipek; et al. (2022)
Physical Review B
Isolated nonmagnetic substitutional defect ions experience huge coupled electric magnetic interaction in the single-phase multiferroic BiFeO3. In the ferroelectric state above the magnetic Néel temperature TN, the electric environment generates a single symmetric electric field gradient (EFG) parallel to the electric polarization direction. Below TN, a distinct magnetic interaction arises, monitored by the probe nuclei via their magnetic moment. Two magnetic environments arise, given by the relative angle of the local magnetic moment within its easy magnetic plane with respect to the EFG orientation. The angle between field gradient orientation and magnetic field direction is the most stable fitting parameter. The magnetic interaction concomitantly increases the EFG dramatically which reflects an outstandingly large local magnetoelectric coupling. In the set of best fits, two different electric environments form concurrently with two distinctly different local magnetic fields. The magnetic ordering in BiFeO3 thus completely distorts the electric environment of the nonmagnetic probe nucleus. The implications for the local effect of dopants in BiFeO3 are discussed. A third probe environment arising independent of temperature is identified and associated with an iron vacancy.
Salt-In-Wood Piezoelectric Power Generators with Circular Materials Design for High-Performance Sustainable Energy Harvesting
Garemark, Jonas; Ritter, Maximilian; Dreimol, Christopher; et al. (2025)
Advanced Functional Materials
The nanowatt-level power density of current biobased piezoelectric energy harvesters restricts their applicative potential for the efficient conversion of biomechanical energy. A high-performing, fully renewable piezoelectric device incorporating green piezo-active Rochelle salt in a laser-drilled wood template is demonstrated to form ordered crystal pillar arrays by melt crystallization. Investigating the effect of different crystal pillar configurations on the piezoelectric response, a shearing design (45 degrees-oriented pillars) shows potential of up to 30 V and a current of 4 mu A - corresponding to a 10-fold power increase compared to single-crystalline Rochelle salt. A concept of direct laser graphitization on the crystal surfaces are demonstrated using a fully renewable ink to create electrodes of low resistance (36 Omega sq-1). The entire device can be disassembled, fully recycled, and reused. This nanogenerator outperforms state-of-the-art biobased ones and competes with conventional lead-based devices in power generation while showing a significantly lower environmental footprint, as indicated by life-cycle assessment.
Magnetoelectric effect in hydrogen harvesting: magnetic field as a trigger of catalytic reactions
Kim, Donghoon; Efe, Ipek; Torlakcik, Harun; et al. (2021)
arXiv
Magnetic fields have been regarded as an additional stimulus for electro- and photocatalytic reactions, but not as a direct trigger for catalytic processes. Multiferroic/magnetoelectric materials, whose electrical polarization and surface charges can be magnetically altered, are especially suitable for triggering and control of catalytic reactions solely with magnetic fields. Here, we demonstrate that magnetic fields can be employed as an independent input energy source for hydrogen harvesting by means of the magnetoelectric effect. Composite multiferroic CoFe2O4-BiFeO3 core-shell nanoparticles act as catalysts for the hydrogen evolution reaction (HER) that is triggered when an alternating magnetic field is applied to an aqueous dispersion of the magnetoelectric nanocatalysts. Based on density functional calculations, we propose that the hydrogen evolution is driven by changes in the ferroelectric polarization direction of BiFeO3 caused by the magnetoelectric coupling. We believe our findings will open new avenues towards magnetically induced renewable energy harvesting.
Magnetoelectric Effect in Hydrogen Harvesting: Magnetic Field as a Trigger of Catalytic Reactions
Kim, Donghoon; Efe, Ipek; Torlakcik, Harun; et al. (2022)
Advanced Materials
Magnetic fields have been regarded as an additional stimulus for electro- and photocatalytic reactions, but not as a direct trigger for catalytic processes. Multiferroic/magnetoelectric materials, whose electrical polarization and surface charges can be magnetically altered, are especially suitable for triggering and control of catalytic reactions solely with magnetic fields. Here, it is demonstrated that magnetic fields can be employed as an independent input energy source for hydrogen harvesting by means of the magnetoelectric effect. Composite multiferroic CoFe2O4-BiFeO3 core-shell nanoparticles act as catalysts for the hydrogen evolution reaction (HER), which is triggered when an alternating magnetic field is applied to an aqueous dispersion of the magnetoelectric nanocatalysts. Based on density functional calculations, it is proposed that the hydrogen evolution is driven by changes in the ferroelectric polarization direction of BiFeO3 caused by the magnetoelectric coupling. It is believed that the findings will open new avenues toward magnetically induced renewable energy harvesting.
Magnetoelectric Phase Control at Domain-Wall-Like Epitaxial Oxide Multilayers
Gradauskaite, Elzbieta; Yang, Chia-Jung; Efe, Ipek; et al. (2025)
Advanced Functional Materials
Ferroelectric domain walls are nanoscale objects that can be created, positioned, and erased on demand. They often embody functional properties that are distinct from the surrounding bulk material. Enhanced conductivity, for instance, is observed at charged ferroelectric domain walls. Regrettably, domain walls of this type are scarce because of the energetically unfavorable electrostatics. This hinders the current technological development of domain-wall nanoelectronics. Here this constraint is overcome by creating robust domain-wall-like objects in epitaxial oxide heterostructures. Charged head-to-head (HH) and tail-to-tail (TT) junctions are designed with two ferroelectric layers (BaTiO3 and BiFeO3) that have opposing out-of-plane polarization. To test domain-wall-like functionalities, an ultrathin ferromagnetic La0.7Sr0.3MnO3 layer is inserted into the junctions. The interfacial electron or hole accumulation at the interfaces, set by the HH and TT polarization configurations, respectively, controls the LSMO conductivity and magnetization. Thus it is proposed that trilayers reminiscent of artificial domain walls provide magnetoelectric functionality and may constitute an important building block in the design of oxide-based electronic devices.
On the happiness of ferroelectric surfaces and its role in water dissociation: The example of bismuth ferrite
Efe, Ipek; Spaldin, Nicola; Gattinoni, Chiara (2021)
The Journal of Chemical Physics
We investigate, using density functional theory, how the interaction between the ferroelectric polarization and the chemical structure of the (001) surfaces of bismuth ferrite influences the surface properties and reactivity of this material. A precise understanding of the surface behavior of ferroelectrics is necessary for their use in surface science applications such as catalysis as well as for their incorporation in microelectronic devices. Using the (001) surface of bismuth ferrite as a model system, we show that the most energetically favored surface geometries are combinations of surface termination and polarization direction that lead to uncharged stable surfaces. On the unfavorable charged surfaces, we explore the compensation mechanisms of surface charges provided by the introduction of point defects and adsorbates, such as water. Finally, we propose that the special surface properties of bismuth ferrite (001) could be used to produce an effective water splitting cycle through cyclic polarization switching. © 2021 Author(s).
Engineering of ferroelectricity in thin films using lattice chemistry: A perspective
Efe, Ipek; Yan, Bixin; Trassin, Morgan (2024)
Applied Physics Letters
Ferroelectric materials hold significant potential for ultralow-energy-consuming oxide electronics and have recently been pointed out as a suitable platform for next-generation neuromorphic and reservoir computing schemes. We provide a brief overview of the progress in engineering electric dipole textures of epitaxial ferroelectric oxide thin films, with an emphasis on the technologically relevant ultrathin regime. In epitaxial films that are only a few unit-cells thick, surface chemistry and interfacial electrostatics are commonly considered limiting factors in ferroelectric device integration, as they may suppress the net ferroelectric behavior. Here, we highlight how nanoscale lattice chemistry control, including off-stoichiometry and layer polarization in oxides, can, in fact, emerge as powerful tools for engineering ferroelectricity in thin films. We also discuss the potential of such an approach in the context of recent trends in the field, such as the design of ferroelectric freestanding membranes and the optical control of polarization in thin films. Hence, with our Perspective article, we aim to provide key insights on the use of lattice chemistry for ferroelectricity engineering in thin films to facilitate exciting developments in ferroelectric-based applications.
Ferroelectric Thin Films for Oxide Electronics
Müller, Marvin; Efe, Ipek; Sarott, Martin F.; et al. (2023)
ACS Applied Electronic Materials
Ferroelectric materials have set in motion numerous ultralow-energy-consuming device concepts that can be integrated into state-of-the-art complementary metal–oxide–semiconductor technology. Their nonvolatile, spontaneous electric polarization makes them promising candidates to control functionalities at the nanoscale with energy-efficient electric fields only. In this spotlight article, we start with a brief introduction to ferroelectric materials, the challenges involving the design of thin films and review the state-of-the-art of their integration into various electronic applications. Revolutionary in situ and operando diagnostic tools allowing the monitoring of the technology-relevant polarization state during the material design, or its operation will be detailed. Concepts such as chiral states in ferroelectrics and neuromorphic-type switching will be addressed to provide a comprehensive view on the evolution of ferroelectric states for the next generation of low-energy-consuming electronics. Finally, we discuss the most recent developments in the field, including the emergence of ferroelectricity at the nanoscale and in two-dimensional systems.
Multiferroic bismuth ferrite: Perturbed angular correlation studies on its ferroic α-β phase transition
Marschick, Georg; Schell, Juliana; Stöger, Berthold; et al. (2020)
Physical Review B
Work of numerous research groups has shown different outcomes of studies of the transition from the ferroelectric α-phase to the high temperature β-phase of the multiferroic, magnetoelectric perovskite Bismuth Ferrite (BiFeO3 or BFO). Using the perturbed angular correlation (PAC) method with 111mCd as the probe nucleus, the α to β phase transition was characterized. The phase transition temperature, the change of the crystal structure, and its parameters were supervised with measurements at different temperatures using a six detector PAC setup to observe the γ−γ decay of the 111mCd probe nucleus. The temperature dependence of the hyperfine parameters shows a change in coordination of the probe ion, which substitutes for the bismuth site, forecasting the phase transition to β-BFO by either increasing disorder or formation of a polytype transition structure. A visible drop of the quadrupole frequency ω0 at a temperature of about Tc≈820∘C indicates the α−β phase transition. For a given crystal symmetry, the DFT-calculations yield a specific local symmetry and electric field gradient value of the probe ion. The Pbnm (β-BFO) crystal symmetry yields calculated local electric field gradients, which very well match our experimental results. The assumption of other crystal symmetries results in significantly different computed local environments not corresponding to the experiment.
Layer and spontaneous polarizations in perovskite oxides and their interplay in multiferroic bismuth ferrite
Spaldin, Nicola; Efe, Ipek; Rossell, Marta D.; et al. (2021)
The Journal of Chemical Physics
We review the concept of surface charge, first, in the context of the polarization in ferroelectric materials and, second, in the context of layers of charged ions in ionic insulators. While the former is traditionally discussed in the ferroelectrics community and the latter in the surface science community, we remind the reader that the two descriptions are conveniently unified within the modern theory of polarization. In both cases, the surface charge leads to electrostatic instability—the so-called “polar catastrophe”—if it is not compensated, and we review the range of phenomena that arise as a result of different compensation mechanisms. We illustrate these concepts using the example of the prototypical multiferroic bismuth ferrite, BiFeO3, which is unusual in that its spontaneous ferroelectric polarization and the polarization arising from its layer charges can be of the same magnitude. As a result, for certain combinations of polarization orientation and surface termination, its surface charge is self-compensating. We use density functional calculations of BiFeO3 slabs and superlattices, analysis of high-resolution transmission electron micrographs, and examples from the literature to explore the consequences of this peculiarity.

Publications 1 - 10 of 12

Ipek Efe

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