Thin Films & Physics of Nanostructures

Spin transport, spin caloritronics, magnetooptics, x-ray resonant magnetic reflectivity

Group of PD Dr. Timo Kuschel

In spintronics, one of the main goals is to use the electron spin as the information carrier in electronic devices instead of the charge. This will offer smaller and faster devices with much less heat convection. Therefore, new applications for the generation, the manipulation and the detection of spin currents have been developed in the last years.

In the new and active research field of spin caloritronics new spin- and temperature-dependent effects have been investigated. One of the most important effects in this area is the spin Seebeck effect, which describes the generation of spin currents by temperature differences in a magnetic nanostructure. This effect was firstly observed in 2008 in Japan and supported the rise of the spin caloritronics research. In addition, we explore the Seebeck effect in magnetic tunnel junctions, so-called tunnel magneto-Seebeck effect. Find an introductory review article on the tunnel magneto-Seebeck effect here:

Tunnel magneto-Seebeck effect, T. Kuschel, M. Czerner, J. Walowski, A. Thomas, H. W. Schumacher, G. Reiss, C. Heiliger, M. Münzenberg, J. Phys. D: Appl. Phys. 52, 133001 (2019) https://doi.org/10.1088/1361-6463/aafa5f

Another new spin manipulating effect is the spin Hall magnetoresistance, which was firstly observed in 2013. Here, the resistance of a non-magnetic material depends on the magnetization of an attached ferromagnetic material due to spin absorption from one material into the other. This effect can be explained by a non-equilibrium proximity effect, which describes the spin polarization at the interface due to a combination of spin Hall and inverse spin Hall effect. Further information and recent research highlights can be found here:

Static magnetic proximity effects and spin Hall magnetoresistance in Pt/Y₃Fe₅O₁₂ and inverted Y₃Fe₅O₁₂/Pt bilayers, S. Geprägs, C. Klewe, S. Meyer, F. Schade, M. Schneider, D. Graulich, S. Francoual, S. Collins, K. Ollefs, F. Wilhelm, A. Rogalev, Y. Joly, S. T. B. Goennenwein, M. Opel, T. Kuschel, R. Gross, Phys. Rev. B. 102, 214438 (2020) https://doi.org/10.1103/PhysRevB.102.214438

In order to observe pure spin effects, one needs ferro- or ferrimagnetic material, which should be insulating or semiconducting. Thus, parasitic charge based effects can be excluded. In our group we investigate thin films of yttrium-iron-garnet (magnetic insulator), nickelferrite (magnetic semiconductor) and magnetite (weak magnetic conductor) in order to study the influence of parasitic effects on these very new spin-dependent effects. Thanks to careful material development, we found strong spin Seebeck effects in lattice-matched nickelferrite as reported here:

Enhancement in thermally generated spin voltage at the interfaces between Pd and NiFe₂O₄ films grown on lattice-matched substrates, A. Rastogi, Z. Li, A. V. Singh, S. Regmi, T. Peters, P. Bougiatioti, D. Carsten né Meier, J. B. Mohammadi, B. Khodaddadi, T. Mewes, R. Mishra, J. Gazquez, A. Y. Borisevich, Z. Galazka, R. Uecker, G. Reiss, T. Kuschel, A. Gupta, Phys. Rev. Appl. 14, 014014 (2020) https://doi.org/10.1103/PhysRevApplied.14.014014

Additionally, we use different magnetooptic techniques based on the magnetooptic Kerr effect. Recently, we developed a dual-beam vectorial magnetometer using different wavelengths and a three-dimensional magnet. With this machine we characterize the magnetic nanostructures, develop new measurement techniques and explore new quadratic electric and magnetooptic effects. The outcomes can be used, e.g., to characterize the magnetic and structural ordering of Heusler compounds:

Scaling of quadratic and linear magnetooptic Kerr effect spectra with L2₁ ordering of Co₂MnSi Heusler compound, R. Silber, D. Král, O. Stejskal, T. Kubota, Y. Ando, J. Pistora, M. Veis, J. Hamrle, T. Kuschel, Appl. Phys. Lett. 116, 262401 (2020) https://doi.org/10.1063/5.0008427

Finally, we are regularly using synchrotron techniques such as x-ray resonant magnetic reflectivity to analyze the magnetic depth profiles and buried magnetic layers of thin film sample stacks. Thus, we found enhanced magnetic moments at the magnetite surface, possibilities to manipulate the magnetic moment in platinum as well as quantitative agreement between x-ray resonant magnetic reflectivity and other synchrotron techniques:

Cation- and lattice-site-selective magnetic depth profiles of ultrathin Fe₃O₄(001) films, T. Pohlmann, T. Kuschel, J. Rodewald, J. Thien, K. Ruwisch, F. Bertram, E. Weschke, P. Shafer, J. Wollschläger, K. Kuepper, Phys. Rev. B. 102, 220411(R) (2020) https://doi.org/10.1103/PhysRevB.102.220411

Asymmetric modification of the magnetic proximity effect in Pt/Co/Pt trilayers by the insertion of a Ta buffer layer, A. Mukhopadhyay, S. K. Vayalil, D. Graulich, I. Ahamed, S. Francoual, A. Kashyap, T. Kuschel, P. S. A. Kumar Phys. Rev. B 102, 144435 (2020) https://doi.org/10.1103/PhysRevB.102.144435

Quantitative comparison of the magnetic proximity effect in Pt detected by XRMR and XMCD, D. Graulich, J. Krieft, A. Moskaltsova, J. Demir, T. Peters, T. Pohlmann, F. Bertram, J. Wollschläger, J. R. Linares Mardegan, S. Francoual, T. Kuschel, Appl. Phys. Lett. 118, 012407 (2021) https://doi.org/10.1063/5.0032584

We are always looking for motivated students to explore the wide field of spin transport, spin caloritronics, magnetooptics and x-ray resonant magnetic reflectivity. Please, do not hesitate to contact Timo Kuschel (tkuschel@physik.uni-bielefeld.de) for further information.

Bachelor- and Masterthesis

Please contact me directly via email (tkuschel@physik.uni-bielefeld.de) to ask for the latest opportunities for bachelor's and master's theses.

PhD and PostDoc positions

All open positions can be found here.

Vita

PD Dr. Timo Kuschel studied physics and received his PhD at Osnabrück University. Afterwards, he worked as a research assistant in the group of Prof. Günter Reiss at Bielefeld University leading the "Spin Caloritronics Team" that investigates the interaction between charge, spin and heat in magnetic nanostructures. From 2015 to 2017 he was on a research stay at Groningen University in the Netherlands. In the group of Prof. Bart van Wees, he explored new nonlocal spin transport phenomena in magnetic nanostructures. After finishing his habilitation in January 2023, he is now working as substitute professor in Bielefeld in the "Thin Films and Physics of Nanostructures" group.

List of publications