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Different crystallization mechanisms of GeO2 thin films on sapphire substrates under different annealing temperatures. The lattice rotation of the crystals gradually decreasing with increasing temperature and finally vanish in single crystals.
Different crystallization mechanisms of GeO2 thin films on sapphire substrates under different annealing temperatures. The lattice rotation of the crystals gradually decreasing with increasing temperature and finally vanish in single crystals.
- “Crystallization of GeO2 thin films into α-quartz: from spherulites to single crystals” by S. Zhou, J. Antoja-Lleonart, P. Nukala et al.

Acta Materialia (11 June 2021) DOI:10.1016/j.acta mat.2021.117069

Piezoelectric quartz (SiO2) crystals are widely used in industry as oscillators. As a natural mineral, quartz and its relevant silicates are also of interest in geoscience and mineralogy. However, the nucleation and growth of quartz crystals are difficult to control and not fully understood. Here we report successful solid-state crystallization of thin film of amorphous GeO2 into quartz on various substrates, including Al2O3, MgAl2O4, MgO, LaAlO3 and SrTiO3. At relatively low annealing temperatures, the crystallization process is spherulitic: with fibers growing radially from the nucleation centers and the crystal lattice rotating along the growth direction with a linear dependence between the rotation angle and the distance to the core. For increasingly higher annealing temperatures, quartz crystals begin to form. The edges of the sample play an important role in facilitating nucleation followed by growth sweeping inward until the whole film is crystallized. Control of the growth allows single crystalline quartz to be synthesized, with crystal sizes of hundreds of microns achieved on sapphire substrates, which is promising for further piezoelectric applications. Our study reveals the complexity of the nucleation and growth process of quartz and provides insight for further studies.

Electron microscope images on the electrode layer of (La,Sr)MnO3 in direct contact with the ferroelectric Hf0,5Zr0.5O2: left sample with oxygen atoms (some indicated with arrows), right sample with many oxygen vacancies (some indicated with arrows).
Electron microscope images on the electrode layer of (La,Sr)MnO3 in direct contact with the ferroelectric Hf0,5Zr0.5O2: left sample with oxygen atoms (some indicated with arrows), right sample with many oxygen vacancies (some indicated with arrows).
- “Reversible oxygen migration and phase transitions in hafnia-based ferroelectric devices” by P. Nukala et al.

Science (15 April 2021) DOI:10.1126/ science.abf3789

It is established by now that ferroelectricity in hafnia is novel and unconventional. Here we dive into understanding the mechanisms of ferroelectric switching through in-situ atomic resolution microscopy as well as synchrotron nanobeam measurements. Especially we utilize the potential of DPC-STEM technique to image and reveal the dynamics of oxygen vacancies while operando biasing, which turn out to be crucial for this new type of ferroelectricity.

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- “Progress and perspective on polymer templating of multifunctional oxide nanostructures” by J. Xu, A.I. Berg et al.

Journal of Applied Physics (20 November 2020) DOI: 10.1063/5.0025052

Metal oxides are of much interest in a large number of applications, ranging from microelectronics to catalysis, for which reducing the dimensions to the nanoscale is demanded. For many of these applications, the nano-materials need to be arranged in an orderly fashion on a substrate. A typical approach is patterning thin films using lithography, but in the case of functional oxides, this is restricted to sizes down to about 100 nm due to the structural damage caused at the boundaries of the material during processing having a strong impact on the properties. In addition, for applications in which multifunctional or hybrid materials are requested, as in the case of multiferroic composites, standard top-down methods are inadequate. Here, we evaluate different approaches suitable to obtain large areas of ordered nano-sized structures and nanocomposites, with a particular focus on the literature of multiferroic nanocomposites, and we highlight the polymer-templating method as a promising low-cost alternative.

XRR scans and fits (red dashed lines) of the six films, from which the thicknesses were extracted.
XRR scans and fits (red dashed lines) of the six films, from which the thicknesses were extracted.
- “Atomic layer deposition of SiO2–GeO2 multilayers” by J. Antoja-Lleonart et al.

Applied Physics letters (28 July 2020) DOI: 10.1063/5.0009844

Despite its potential for CMOS applications, atomic layer deposition (ALD) of GeO2 thin films, by itself or in combination with SiO2, has not been widely investigated yet. Here, we report the ALD growth of SiO2/GeO2 multilayers on silicon substrates using a so far unexplored Ge precursor. The characterization of multilayers with various periodicities reveals layer-by-layer growth with electron density contrast and the absence of chemical intermixing, down to a periodicity of two atomic layers.

Scaling exponent (n) as a function of in-plane strain (εxx) in the metallic phase of epitaxial NdNiO3 films.
Scaling exponent (n) as a function of in-plane strain (εxx) in the metallic phase of epitaxial NdNiO3 films.
- “Tunable resistivity exponents in the metallic phase of epitaxial nickelates” by Q. Guo et al.

Nature Communications (11 June 2020) DOI:10.1038/s41467-020-16740-5

Nickelates are a hot topic in condensed matter physics due to the recent discovery of superconductivity in an infinite-layer nickelate [Nature 572, 624 (2019)] and their metal-insulator transitions, which make them interesting candidates for resistive switching memory and logic devices, including neuromorphic applications. However, the nature of the metallic state in these strongly-correlated electron systems is not yet fully understood. Here we show that thin films of NdNiO3 under low strain conditions show a linear dependence of the resistivity versus temperature, consistent with a classical Fermi gas ruled by electron-phonon interactions. In addition, the apparent temperature exponent, n, can be tuned with the epitaxial strain between n= 1 and n= 3. We discuss the critical role played by quenched random disorder in the value of n. Our work shows that the assignment of Fermi/Non-Fermi liquid behaviour based on experimentally obtained resistivity exponents requires an in-depth analysis of the degree of disorder in the material.

Check also the press release by Rene Fransen on EurekAlert and on the University website!

(left) AFM topographical image and (right) correspondent low-temperature MFM phase of a CaFe2O4 thin film on TiO2 substrate collected at 12 K.
(left) AFM topographical image and (right) correspondent low-temperature MFM phase of a CaFe2O4 thin film on TiO2 substrate collected at 12 K.
- “Structure and magnetic properties of epitaxial CaFe2O4 thin films" by S. Damerio et al.

npj Quantum Materials (01 June 2020) DOI:10.1038/s41535-020-0236-2

CaFe2O4 is a highly anisotropic antiferromagnet reported to display two spin arrangements with up–up–down–down (phase A) and up–down–up–down (phase B) configurations. The relative stability of these phases is ruled by the competing ferromagnetic and antiferromagnetic interactions between Fe3+ spins arranged in two different environments, but a complete understanding of the magnetic structure of this material does not exist yet. In this study, we investigate epitaxial CaFe2O4 thin films grown on TiO2 (110) substrates by means of pulsed laser deposition (PLD). Structural characterization reveals the coexistence of two out-of-plane crystal orientations and the formation of three in-plane oriented domains. The magnetic properties of the films, investigated macroscopically as well as locally, including highly sensitive Mössbauer spectroscopy, reveal the presence of just one order parameter showing long-range ordering below T = 185 K and the critical nature of the transition. In addition, a non-zero in-plane magnetization is found, consistent with the presence of uncompensated spins at phase or domain boundaries, as proposed for bulk samples.

(left) Bright-field TEM image of a BaTiO3 film on NdScO3 substrate showing domain contrast (a/c domains) at the top of the film gradually decreasing towards the bottom part of the film. (right) In-plane deformation map obtained by dark-field electron holography. The inset in the bottom left corner shows a Piezoelectric Force Microscopy (amplitude) image of the a/c domain structure.
(left) Bright-field TEM image of a BaTiO3 film on NdScO3 substrate showing domain contrast (a/c domains) at the top of the film gradually decreasing towards the bottom part of the film. (right) In-plane deformation map obtained by dark-field electron holography. The inset in the bottom left corner shows a Piezoelectric Force Microscopy (amplitude) image of the a/c domain structure.
- “Temperature-independent giant dielectric response in transitional BaTiO3 thin films” by A. S. Everhardt et al.

Applied Physics Reviews (21 January 2020) DOI: 10.1063/1.5122954

Ferroelectric materials exhibit the largest dielectric permittivities and piezoelectric responses in nature, making them invaluable in applications from supercapacitors or sensors to actuators or electromechanical transducers. The origin of this behavior is their proximity to phase transitions but these transitional states should be frozen in such that the large responses become robust against temperature changes . Here we recreate a smear in-transition state and leads to a giant temperature-independent dielectric response in BaTiO3 films. Similar mechanisms could be utilized in other materials to design low-power ferroelectrics, piezoelectrics, dielectrics, or shape-memory alloys, as well as efficient electro- and magnetocalorics.

Current as a function of bias voltage in the high resistance state (black symbols) and low resistance state (red symbols), showing Tunneling Electroresistance (TER) values as large as 10^6%. Measurements shown are done on a 30μm × 30μm junction at 50 K
Current as a function of bias voltage in the high resistance state (black symbols) and low resistance state (red symbols), showing Tunneling Electroresistance (TER) values as large as 10^6%. Measurements shown are done on a 30μm × 30μm junction at 50 K
- “Magneto-ionic control of spin polarization in multiferroic tunnel junctions” by Y. Wei et al.

npj Quantum Materials (17 December 2019) DOI: 10.1038/s41535-019-0201-0

Ferroelectric (FE) synaptic (memristive) devices are fast and energy efficient. The plasticity (change in resistance) originates from the Tunneling Electroresistance Effect (TER). We have shown that TER can be as large as 10000 in tunnel junctions made with only 2nm thick Hafnia-based FE tunnel barriers. Thanks to the unique properties of FE hafnia, the junctions are so resistive that they can be electroded and wired-bonded, one of the greatest challenges in other FE tunnel junctions. See our paper by Yingfen Wei et al. at https://www.nature.com/articles/s41535-019-0201-0.

Artistic view of a logistic map showing a sequence of bifurcations of the periodicity as the system evolves between order and chaos
Artistic view of a logistic map showing a sequence of bifurcations of the periodicity as the system evolves between order and chaos

- “Periodicity-Doubling Cascades: Direct Observation in Ferroelastic Materials” by A. S. Everhardt, S. Damerio et al.

Physical Review Letters (22 August 2019) DOI: 10.1103/PhysRevLett.123.087603

In this paper, we use piezoresponse force microscopy (PFM) to investigate a recently found phase transition in BaTiO3 thin films. The relatively slow dynamics of this transition allows to follow the changes of the ferroelastic domain structure as it evolves towards its equilibrium periodicity. We observe sequences of period doublings/halving that are often found in dynamical phase transitions and are associated with the onset of apparent chaos. Is this an example of “spatial chaos” proposed by Jensen and Bak? An introduction to this paper has been published in APS-Physics by Pol Lloveras (see here). See also what reporters have to say about this here.

Artistic view of pulsed laser deposition of hafnia-based thin films
Artistic view of pulsed laser deposition of hafnia-based thin films
- “A rhombohedral ferroelectric phase in epitaxially strained Hf0.5Zr0.5O2 thin films” by Y. Wei et al.

Nature Materials (22 October 2018) DOI: 10.1038/s41563-018-0196-0

Reducing the size of ferroelectric materials has been a research topic for more than 20 years. For ferroelectric materials to have any tangible application in microelectronics, ferroelectricity should be stabilized at nanoscopic sizes. Such nanosized-ferroelectrics became a reality with HfO2 based materials. This is also unconventional ferroelectricity because it becomes robust with decreasing size. In this work, we investigated the origins of this new-type of ferroelecticity. In the process we discovered a new polar rhombohedral phase in hafnia-based systems, that can be stabilized by large compressive strain from the substrates and the particle pressure due to nanoscopic sizes . These results also point towards arriving at guidelines to engineer this new-type of ferroelectricity in other types of simple oxides, potentially generating a vastly unexplored class of nanoscale ferroelectrics.

Advanced Electronic Materials cover January 2016
Advanced Electronic Materials cover January 2016
- “Ferroelectric BaTiO3 thin films under low strain” by A.S. Everhardt et al.

Advanced Electronic Materials (18 November 2015) DOI: 10.1002/aelm.201500214

In this paper, low-strain ferroelectric BaTiO3 films are experimentally realized for the first time, showing a fantastic agreement with the theoretical predictions of Khoukhar, Pertsev and Waser (2000).

Fig. 4
Fig. 4
- “Novel phases at domain walls” by S . Farokhipoor et al.

Nature  (20 November 2014) DOI: 10.1038/nature13918

Due to large local stresses, domain walls can promote the formation of new phases and function in some sense as nanoscale chemical reactors. We have synthesized a novel 2D ferromagnetic phase at the domain walls of the orthorhombic perovskite TbMnO3, which was grown in thin layers under epitaxial strain on SrTiO3 substrates. This phase has never been observed before and cannot be made by other chemical routes. The density of the domain walls can be tuned by changing the epitaxial strain. In this way, the distance between ferromagnetic sheets can be made as small as 5 nanometres in ultrathin films, such that the new phase at domain walls represents up to 25% of the film volume. The general concept of using domain walls of epitaxial oxides to promote the formation of unusual phases is applicable to other materials systems, thus giving access to new classes of nanoscale materials for applications in nanoelectronics and spintronics.

Commented by Philippe Ghosez and Jean-Marc Triscone in News & Views (Nature): Reactive walls

Explained for a general reader by René Fransen: Science Linx- RUG
Press releases: University of Groningen

Fig. 5
Fig. 5
- By Sylvia Matzen, Oleksiy Nesterov et al.

Nature Communications (14 July 2014) DOI: 10.1038/ncomms5415

With shrinking device sizes, controlling domain formation in nano ferroelectrics becomes crucial. Periodic nano domains that self-organize into ‘superdomains’ have been recently observed, mainly at crystal edges or in laterally confined nano objects. Here we show that in extended, strain-engineered thin films, superdomains with purely in-plane polarization form to mimic the single-domain ground state, a new insight that allows a priori design of these hierarchical domain architectures. Importantly, superdomains behave like strain- neutral entities whose resultant polarization can be reversibly switched by 90°, offering promising perspectives for novel device geometries

Last modified:05 July 2021 10.12 a.m.