Advanced nanoSQUIDs for the detection of small spin systems

• Applicants: Sylke Bechstein; Dr. Oliver-Franz-Otto Kieler; Professor Dr. Dieter Kölle
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2014 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 249226987

• Final Report Year: 2023

Final Report Abstract

In this research project we developed and investigated advanced submicron-sized superconducting quantum interference devices (nanoSQUIDs). NanoSQUIDs are able to directly detect the magnetic dipole field of tiny magnetic systems such as magnetic nanoparticles (MNPs) or single molecular magnets with spin sensitivity down to the order of one electron spin in a 1 Hz bandwidth. The nanoSQUIDs are fabricated with a multilayer Niobium (Nb) thin film technology with ultrasmall (down to 80-nm-diameter), high critical current density Nb/HfTi/Nb Josephson junctions (JJs) with normal-conducting HfTi barrier, instead of most frequently used JJs based on geometric constrictions. This offers the unique advantage of combining the realization of SQUIDs with very small loop inductance and hence extremely low flux noise, with a superconductor multilayer approach that offers the realization of complex devices with significantly increased functionality for various applications. Within the project we extended the fabrication process to three independent Nb layers and developed SQUID systems with nanopatterned superconducting pickup coils. We investigated basic properties of single JJs, including self-heating effects and the currentphase-relation (CPR), which links the supercurrent to the phase difference of the superconducting wave functions across the JJs. The CPR was studied in multi-terminal, multi-JJ SQUIDs, which we investigated experimentally and by numerical simulations, including the finite voltage state. This was complemented by numerical simulations of 2-JJ SQUIDs to improve our understanding of the impact of non-sinusoidal CPRs on the experimentally accessible modulation of the maximum supercurrent with applied magnetic flux to the SQUID loop. Moreover, we analyzed by numerical simulations the magnetic coupling (coupling factor) in the near-field and far-field regime between a point-like magnetic dipole and a nanoSQUID for various SQUID geometries. Our results provide quantitative information on the spin sensitivity that can be reached as a function of orientation of a MNP and its distance to a nanoSQUID. Finally, we developed an algorithm for optimizing the alignment of the external magnetic field for magnetization reversal measurements on MNPs and investigated the integration of nanoSQUIDs into microwave circuits. We improved 3-axis vector nanoSQUIDs (developed for the simultaneous detection of all three spatially orthogonal magnetic moment components of an MNP) and analyzed their coupling to MNPs, and we implemented nanopositioning of MNPs with tungsten nanotips in only few nm distance to nanoSQUIDs. Those activities pave the way for performing ultra-sensitive quantitative measurements on the magnetization reversal of individual MNPs.

Publications

• YBa2Cu3O7 and Nb NanoSQUIDs for the Investigation of Magnetization Reversal of Individual Magnetic Nanoparticles. 2019 IEEE International Superconductive Electronics Conference (ISEC), 1-3. IEEE.
  Müller, B.; Lin, J.; Linek, J.; Karrer, M.; Limberger, F.; Koch, L.; Goldobin, E.; Kleiner, R.; Koelle, D.; Morosh, V.; Weimann, T.; Kieler, O. F.; Sesé, J. & Martínez-Pérez, M.J.
  
• Highly sensitive nanoSQUIDs for investigations of magnetic nanoparticles. Dissertation, Univ. Tübingen
  Benedikt Müller
  
• Transport and Noise Properties of sub-100-nm Planar Nb Josephson Junctions with Metallic Hf - Ti Barriers for nano-SQUID Applications. Physical Review Applied, 14(5).
  Morosh, V.; Linek, J.; Müller, B.; Martínez-Pérez, M.J.; Wolter, S.; Weimann, T.; Beyer, J.; Schurig, T.; Kieler, O.; Zorin, A.B.; Kleiner, R. & Koelle, D.
  
• Nb -Based Nanoscale Superconducting Quantum Interference Devices Tuned by Electroannealing. Physical Review Applied, 15(3).
  Collienne, Simon; Raes, Bart; Keijers, Wout; Linek, Julian; Koelle, Dieter; Kleiner, Reinhold; Kramer, Roman B.G.; Van de Vondel, Joris & Silhanek, Alejandro V.
  
• Fabrication Process for Deep Submicron SQUID Circuits with Three Independent Niobium Layers. Micromachines, 12(4), 350.
  Wolter, Silke; Linek, Julian; Altmann, Josepha; Weimann, Thomas; Bechstein, Sylke; Kleiner, Reinhold; Beyer, Jörn; Koelle, Dieter & Kieler, Oliver
  
• Herstellung und Charakterisierung von hochintegrierten Nb nanoSQUIDs mit Nb/HfTi/Nb-Josephson-Kontakten. Dissertation, Univ. Tübingen
  Silke Wolter
  
• Static and dynamic transport properties of multi-terminal, multi-junction microSQUIDs realized with Nb/HfTi/Nb Josephson junctions. Superconductor Science and Technology, 35(8), 085006.
  Wolter, S.; Linek, J.; Weimann, T.; Koelle, D.; Kleiner, R. & Kieler, O.
  
• Niobium quantum interference microwave circuits with monolithic three-dimensional nanobridge junctions. Physical Review Applied, 21(2).
  Uhl, Kevin; Hackenbeck, Daniel; Peter, Janis; Kleiner, Reinhold; Koelle, Dieter & Bothner, Daniel
  
• On the coupling of magnetic moments to superconducting quantum interference devices. Superconductor Science and Technology, 37(2), 025010.
  Linek, J.; Wyszynski, M.; Müller, B.; Korinski, D.; Milošević, M. V.; Kleiner, R. & Koelle, D.