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Group Activities

Most Recent Highlights


Modelling of the vertical deflection of ferroelectric bending tongues loaded at their free end

Juliette Cardoletti, Aldin Radetinac, Daniel Thiem, Julian Walker, Philipp Komissinskiy, Bai-Xiang Xu, Helmut Schlaak, Susan Trolier-McKinstry, and Lambert Alff

AIP Advances 9, 025017 (2019)
doi: 10.1063/1.5082392

Ferroelectric bending tongues loaded at their free end have been modelled to calculate their vertical deflection. The model relies on a ferroelectric switching criterion and on the Euler-Bernoulli beam theory. It takes into account the geometry of the bending tongue, the mechanical and piezoelectric material properties, the applied electrical field, the crystallographic state of the ferroelectric thin film and its built-in strain. The developed model differentiate the different contributions to the bending tongue vertical deflection. The results are characteristic of a ferroelectric with the modelled butterfly loop incorporating strain saturation at high electrical field. It allows accurate estimates of the vertical deflection for ferroelectric bending tongue based applications and show the importance of the ferroelectric thin film crystallographic orientation.

Electrochemical Performance of All-Solid-State Sodium-Ion Model Cells with Crystalline NaₓCoO₂ Thin-Film Cathodes

All-solid-state sodium batteries are attractive due to the abundance of sodium and advantageous for safe battery operation by avoiding flammable organics and liquids and suppressed dendrite formation. Herein, we present a NaxCoO2/Nasicon/Na thin-film model sodium solid-state battery using a Sc-substituted Nasicon solid electrolyte. Excellent chemical stability of this solid electrolyte at high voltages of up to 4.2 V increases the accessible sodium (de)intercalation range and battery capacity. Direct extraction of the interface resistances between the electrode materials of the thin-film model cell using electrochemical impedance spectroscopy gives a unique opportunity of correlation the electrochemical performance with properties of electrode materials and their interfaces.

doi: 10.1016/j.jpowsour.2018.10.089
doi: 10.1149/2.0491903jes

Magnetic attraction

The mixed German-Indian team of ATFT @ JEMS.
The mixed German-Indian team of ATFT @ JEMS.

The 9th Joint European Magnetic Symposia was organized by our RMU-colleagues from University of Mainz. Several members of the ATFT group (Berthold Rimmler, Jasna Palakkal, Shalini Sharma, Thorsten Schneider, Supratik Dasgupta) presented their recent work on magnetism related topics and informed themselves about the research in the field. ATFT always supports young researchers from the Master stage to the PhD and postdoc phase to attend international scientific conferences to boost their career possibilities and come into contact with world-wide leading researchers in the field.

Oxide electronics on top


The 25th International Workshop on Oxide Electronics took place in the mountains of Switzerland. During the interdisciplinary workshop the state-of-the-art of mostly epitaxial functional oxide materials and their potential for electronic application are discussed. The ATFT group was represented by Patrick Salg, Lukas Zeinar and Lambert Alff. Former group member Vikas Shabadi now working for his own technology company also joined the foto. Our contributions to the conference were focusing on the perovskite SrMoO3 as outstanding electrode material for high-frequency devices.

Doctoral graduation in the ATFT group

We congratulate our colleague Sharath S. Ulhas to his outstanding doctoral defense (January 31,2018). The title of his PhD thesis is Defect engineering in HfO2/TiN-based resistive random access memory (RRAM) devices by reactive molecular beam epitaxy. As external examiners we had Thomas Schroeder (IKZ Berlin) and Klaus Hofmann (Electrical Engineering department of TU Darmstadt) as our guests. The defense was followed by a social event with food catering in the presence of Dr. Sharath’s parents and wife which traveled all the way from India and US. We are extremely thankful to Sharath for his remarkable advances in science and for having been a much valued member of our research group. We look forward to continuing scientific exchange and wish him all the best for his future career!

Doctoral graduation in the ATFT group

We congratulate our colleague Dominik Gölden to his very successful defense (January 24, 2018). The title of his PhD thesis is Magnetocrystalline anisotropy of iron thin films with interstitial nitrogen and boron. The defense was followed in the evening by a social event in the department with food and liquid relaxation medicine after all the excitement. We thank Dominik very much for his advances in science and for having been an important member of our group in many respects. It is good news that Dominik immediately obtained an attractive job in science related industry showing the value of the doctorate at ATFT and TU Darmstadt.

Evolution of anisotropy in bcc Fe distorted by interstitial boron

Dominik Gölden, Hongbin Zhang, Iliya Radulov, Imants Dirba, Philipp Komissinskiy, Erwin Hildebrandt, and Lambert Alff

Phys. Rev. B 97, 014411 (2018)
doi: 10.1103/PhysRevB.97.014411

The evolution of magnetic anisotropy in bcc Fe as a function of interstitial boron atoms was investigated in thin films grown by molecular beam epitaxy. The thermodynamic nonequilibrium conditions during film growth allowed one to stabilize an interstitial boron content of about 14 at.% accompanied by lattice tetragonalization.The c/a ratio scaled linearly with the boron content up to a maximum value of 1.05 at 300 °C substrate growth temperature, with a room-temperature magnetization of. In contrast to nitrogen interstitials, the magnetic easy axis remained in-plane with an anisotropy of approximately −5.1 × 106 erg/cm3. Density functional theory calculations using the measured lattice parameters confirm this value and show that boron local ordering indeed favors in-plane magnetization. Given the increased temperature stability of boron interstitials as compared to nitrogen interstitials, this study will help to find possible ways to manipulate boron interstitials into a more favorable local order.

CeCo₅ thin films with perpendicular anisotropy grown by molecular beam epitaxy

Shalini Sharma, Erwin Hildebrandt, Márton Major, Philipp Komissinskiy, Ilya Radulov, and Lambert Alff

J. Magn. Magn. Mater. 452, 80–85 (2018)
doi: 10.1016/j.jmmm.2017.12.042

Buffer-free, highly textured (001) oriented CeCo5 thin films showing perpendicular magnetic anisotropy were synthesized on (001) Al2O3 substrates by molecular beam epitaxy. Ce exists in a mixture of Ce3+ and Ce4+ valence states as shown by X-ray photoelectron spectroscopy. The first anisotropy constant, K1, as measured by torque magnetometry was 0.82 MJ/m3. A maximum coercivity of 5.16 kOe with a negative temperature coefficient of -0.304%K-1 and a magnetization of 527.30 emu/cm3 was measured perpendicular to the film plane at 5 K. In addition, a large anisotropy of the magnetic moment of 15.5% was observed. These magnetic parameters make CeCo5 a potential candidate material for spintronic and magnetic recording applications.

Video Abstract Online

S.U. Sharath, S. Vogel, L. Molina-Luna, E. Hildebrandt, C. Wenger, J. Kurian, M. Duerrschnabel, T. Niermann, G. Niu, P. Calka, M. Lehmann, H.-J. Kleebe, T. Schroeder, and L. Alff
Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfOx based Memristive Devices
Adv. Funct. Mater. 27, 1700432 (2017)
doi: 10.1002/adfm.201700432

Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfOₓ based Memristive Devices

Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfOx based Memristive Devices – © Foto: Gabi Haindl

S. U. Sharath, S. Vogel, L. Molina-Luna, E. Hildebrandt, C. Wenger, J. Kurian, M. Duerrschnabel, T. Niermann, G. Niu, P. Calka, M. Lehmann, H.-J. Kleebe, T. Schroeder, and L. Alff
Adv. Funct. Mater. 27, 1700432 (2017) (DOI: 10.1002/adfm.201700432)

S. U. Sharath is the first author of a publication on resistive random access memory (RRAM) in the high-impact journal Advanced Functional Materials. This type of non-volatile memory has the potential to replace the well-known Flash technology approaching its physical limits. The paper describes the fundamental role of oxygen vacancies in the switching process in hafnium oxide based memory cells. It also reports the stabilization of quantized conductance states at room temperature. RRAM could play a role in all kind of digital memory, in quantum technology and neuromorphic applications. Funding came from German Research Foundation (DFG), the European Union and BMBF through the project PANACHE.

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