Andy Thomas' group uses deposition techniques such as atomic layer deposition and (magnetron) sputtering to prepare thin film heterostructures. In addition, we utilize focussed ion beam cutting to obtain micro structures out of single crystal bulk materials provided by our collaborators. Our main focus are quantum materials such as topological insulators, Weyl semimetals and 2D materials. After in-situ characterization, the stacks are further processed by lithographic patterning to receive transport devices. These devices are measured at low temperatures (>2K) and magnetic fields (<14T), in particular, the longitudinal and transverse transport response to an electric field and/or a temperature gradient are evaluated (conductance, Hall, Nernst, Seebeck …). In the future, we will make use of the 3-d conformity of atomic layer deposition to prepare sophisticated devices, notably in the field of magnetism. Here, spin textures and magnon transport are predicted to be affected by curved surfaces.
K. Geishendorf, P. Vir, C. Shekhar, C. Felser, J.I. Facio, J. van den Brink, K. Nielsch, A. Thomas, S.T.B. Goennenwein
Weyl semimetals exhibit interesting electronic properties due to their topological band structure. In particular, large anomalous Hall and anomalous Nernst signals are often reported, which allow for a detailed and quantitative study of subtle features. We pattern single crystals of the magnetic Weyl semimetal Co3Sn2S2 into nanoribbon devices using focused ion beam cutting and optical lithography. This approach enables a very precise study of the galvano- and thermomagnetic transport properties. Indeed, we found interesting features in the temperature dependency of the anomalous Hall and Nernst effects. We present an analysis of the data based on the Mott relation and identify in the Nernst response signatures of magnetic fluctuations enhancing the anomalous Nernst conductivity at the magnetic phase transition.
G. Park, K. Nielsch, A. Thomas:
Adv. Mater. Inter. 6 (2019) 1800688
Ultrathin 2D transition metal dichalcogenide (TMD) thin films have attracted much attention due to their very good electrical, optical, and electrochemical properties. Chemical vapor deposition (CVD) and atomic layer deposition (ALD), which is in some regards an enhanced version of CVD, are techniques that can provide exceptionally conformal large‐area coatings, even for complex surface geometries. Besides, these techniques include the transport of one or more precursor chemicals in the gas phase onto a substrate. Subsequently, a chemical reaction occurs, resulting in the deposition of a film of a solid material on the substrate. One of the advantageous aspects of chemical deposition methods, such as CVD and ALD, is the growth of thin films onto a variety of substrates as well as 3D structures. Because of their chemical approach, these techniques are well suited to synthesizing 2D materials (2DMs) with a low defect concentration. Furthermore, the scalability would allow industrial application, as opposed to, e.g., micromechanical cleavage. Here, the recent progress in 2D TMD thin films is reviewed and the current applications of these materials fabricated by CVD and ALD are surveyed.
R. Schlitz, P. Swekis, A. Markou, H. Reichlova, M. Lammel, J. Gayles, A. Thomas, K. Nielsch, C. Felser, S.T.B. Goennenwein
Nano Lett. 19 (2019) 2366-2370
The presence of nontrivial magnetic topology can give rise to nonvanishing scalar spin chirality and consequently a topological Hall or Nernst effect. In turn, topological transport signals can serve as indicators for topological spin structures. This is particularly important in thin films or nanopatterned materials where the spin structure is not readily accessible. Conventionally, the topological response is determined by combining magnetotransport data with an independent magnetometry experiment. This approach is prone to introduce measurement artifacts. In this study, we report the observation of large topological Hall and Nernst effects in micropatterned thin films of Mn1.8PtSn below the spin reorientation temperature TSR ≈ 190 K. The magnitude of the topological Hall effect ρxyT = 8 nΩm is close to the value reported in bulk Mn2PtSn, and the topological Nernst effect SxyT = 115 nV K–1 measured in the same microstructure has a similar magnitude as reported for bulk MnGe (SxyT ∼ 150 nV K–1), the only other material where a topological Nernst was reported. We use our data as a model system to introduce a topological quantity, which allows one to detect the presence of topological transport effects without the need for independent magnetometry data. Our approach thus enables the study of topological transport also in nanopatterned materials without detrimental magnetization related limitations.
K. Geishendorf, R. Schlitz, P. Vir, C. Shekhar, C. Felser, K. Nielsch, S.T.B. Goennenwein, A. Thomas
Appl. Phys. Lett. 114 (2019) 092403
Magnetic Weyl semimetals exhibit intriguing transport phenomena due to their non-trivial band structure. Recent experiments in the bulk crystals of the shandite-type Co3Sn2S2 have shown that this material system is a magnetic Weyl semimetal. To access the length scales relevant to chiral transport, it is mandatory to fabricate microstructures of this fascinating compound. We therefore have cut micro-ribbons (typical size 0.3 × 3 × 50 μm3) from Co3Sn2S2 single crystals using a focused beam of Ga2+-ions and investigated the impact of the sample dimensions and possible surface doping on the magnetotransport properties. The large intrinsic anomalous Hall effect observed in the micro-ribbons is quantitatively consistent with the one in bulk samples. Our results show that focused ion beam cutting can be used for nano-patterning single crystalline Co3Sn2S2, enabling future transport experiments in complex microstructures of this Weyl semimetal.