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Welcome to the website of Spintronics of Functional Materials group at the Zernike Institute for Advanced Materials!

The group of Prof. Tamalika Banerjee focusses on new approaches to create and manipulate spin transport across material interfaces exploiting the phenomenon of spin-orbit coupling. The material interfaces that we study are those on complex oxide semiconductors, graphene, silicon or topological insulators with ferromagnetic materials. Electric field tunability of the Rashba spin orbit coupling at such device are investigated and are relevant for Beyond Moore applications such as spin logic, reconfigurable spintronics architecture and bioinspired computing. We fabricate our devices using the facilities available at NanoLab Groningen and study electronic and spin transport using variable magnetic fields and temperatures. We also have a state-of-the-art advanced scanning probe technique called the Ballistic Electron Magnetic Microscope to study spin transport on the nanometer scale across such device interfaces.

We contribute to teaching in several courses (Physics and Applied Physics), in the Top Master Nanoscience program, Academic Skills course and the Graduate School course on Scientific Integrity.

Research:

Our current research projects are:

1.   a.) Electric field manipulation of spin transport at oxide semiconducting interfaces: We study emergent phenomena at complex oxide interfaces. We have demonstrated electric field tunability of spin lifetime in n-doped SrTiO₃ at room temperature by carefully tuning the potential landscape at the interface without the necessity of an additional gate electrode. Recently we have shown a new phenomenon of tunneling anisotropic magnetoresistance (TAMR) coexisting with electroresistance effects in a ‘spin memristor’. Further in an ongoing research, we show that suitable engineering of the potential landscape at the interface between doped SrTiO₃ and 3d transition ferromagnets leads to a coexistence of different spin transport effects that can be tuned by temperature and an applied bias. These effects originate due to the tunability of the Rashba spin orbit coupling at the interface and its manipulation provide unique prospects for reconfigurable Spintronics architecture. Projects are currently underway to better understand the effects that leads to such competing spin transport phenomena. (Current PhD’s: Arijit Das, Former PhDs: Sander Kamerbeek, some overlap with Roald Ruiter. Master student: Anouk Goossens, Symen T Jousma)

b.) Antiferromagnetic Spintronics with complex oxides: It is a relatively young area of research and focusses on tailoring the interface by strain or (chemical) doping at interfaces with antiferromagnetic complex oxides. The growth of such interfaces are realized using Pulsed laser deposition and spin hall magnetoresistance will be used to study the electrical read out of such thin insulating films. Utilizing the broken inversion symmetry and high spin orbit coupling of the underlying substrate of SrTiO₃, we will study emergent phenomena at interface with oxide antiferromagnets. (Current PhD’s: Arijit Das, Master student: Anouk Goossens)

2.   Graphene spintronics on oxide interfaces : Graphene is a promising material in spintronics with predicted intrinsic spin relaxation time up to 1 µs and extremely high mobilities. However, in spite of extensive experimental endeavors, the spin relaxation time falls orders of magnitude below the theoretical predictions, mainly because charge conduction and associated transport parameters in two-dimensional graphene are strongly influenced by extrinsic factors related to its local environment. In this project we have used the electronically rich platform of SrTiO₃ to study its complex interdependence of the increasing non-linear dielectric constant with decreasing temperature on spin transport in graphene and across the ferroelastic transition in SrTiO₃. For this we have performed spin transport in graphene on TiO₂ terminated SrTiO₃ in a temperature range from 4 to 290 K. We show a non-trivial dependence of the spin transport parameters in graphene and their tunability with temperature, even in the absence of an externally applied gate voltage. We are currently looking at the role of surface electric dipoles in SrTiO₃ for both charge and spin transport and for strategies to isolate the different chemical and electrostatic contributions to transport in graphene. (Current PhDs: Crystal (Si) Chen, Former PhDs: Roald Ruiter , Master students: Elisabeth Duijnstee).

3.   Tuning magnetic anisotropy at complex oxide interfaces with large spin-orbit coupling : Magnetic anisotropy provides directionality and stability to magnetization and has enabled the realization of nanomagnets in conventional transition ferromagnetic materials in proximity with heavy metals. In few recent works we have shown the onset and control of large perpendicular magnetic anisotropy in SrRuO­₃ on SrTiO₃ and studied such interfaces using Anomalous Hall Effect (AHE) and magnetotransport measurements. A thorough understanding of the sign change of the AHE and its correlation with the thickness and deposition parameters used in Pulsed Laser deposition is being currently pursued. (Current PhDs: Ping Zhang, Arijit Das. Master student (exchange): Retno Du Wulandari)

4.   Novel transport phenomena using skyrmions : In a recent FOM program on skyrmion-based electronics, the focus is on controlling and stabilizing complex magnetic states at oxide interfaces. The spin textures at such interfaces will be tailored by substrate induced strain as well as with external gate voltages. Using the electrical transport scheme of topological (magnon) Hall effect and a new microscopy technique of spin-filtered scanning tunneling microscopy we will look at the different magnetic textures induced by varying the magnetic anisotropy in oxide films and interfaces. (Current PhDs: Arjan Burema, some overlap with Arijit Das, Ping Zhang)

5. Spin momentum locking and hot electron transport using topological insulators : Topological insulators are intriguing in that they can host electrically conducting surface states along with an ideally insulating bulk. Bi₂Se₃ thin films for this work were grown at Rutgers University and were fabricated into nanodevices at NanoLab NL. The intrinsic locking of the electron’s spin to its momentum is expected to lead to a dissipationless current flow in such materials. This was demonstrated using Co detector in a device design that allowed for several cross checks and we found that factors beyond spin momentum locking influences the detected spin voltage. This was highlighted in a popular article published in Nature Physics (http://www.nature.com/nphys/journal/v11/n12/full/nphys3602.html). Using the High Feld Magnet laboratory at Nijmegen, it was also established that besides the usual surface states, other conducting pathways are coupled to the bulk insulating states in such topological insulator films that were not known earlier. To understand the influence of metal contacts on topological insulators, an ongoing work done by Master students uses a hot electron transistor scheme. This geometry allows for a pristine measurement of the Schottky barrier height on Ca doped semiconducting Bi₂Se₃ using metal contacts and will be extended to using ferromagnetic metals in future. (PhD completed: Eric de Vries, Master students: Bart Zillen, and former Master Arjan Burema).

Earlier completed PhD projects can be found in the staff list / publications/theses.

Besides we also worked with Hitachi Global (HGST, San Jose) on their new read heads using the capabilities of the BEEM set up in our group.

Collaborations : Our research has benefitted from several earlier and ongoing collaborations with the groups of Harold Y Hwang and T. P. Deveraux (Stanford University), E. Y. Tsymbal (University of Nebraska), S. Oh (State University of New Jersey), J. Fabian ( University of Regensburg), B. Sanyal (Uppsala University), A. Ghosh (IISc, Bangalore) and T. Saha-Dasgupta (SNBNCBS, Kolkata). Besides we have several collaborations within the Zernike Institute for Advanced Materials and with the University of Twente in the Netherlands.