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SQUID-on-tip sensors for real-space magnetic imaging of a chiral magnet

Romagnoli, Giulio. SQUID-on-tip sensors for real-space magnetic imaging of a chiral magnet. 2024, Doctoral Thesis, University of Basel, Faculty of Science.

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Official URL: https://edoc.unibas.ch/96395/

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Abstract

Nanomagnetism is an area of research in Physics that studies magnetic properties of samples which have at least one dimension in the nanometer range. The aim of nanomagnetism is to investigate properties and applications of nano-objects, such as particles, dots, wires, or thin films or bulk samples exhibiting changes in magnetism (like magnetic domains or interfaces) at the nanoscale.
Among all the applications of nanomagnetism, the most successful one has been in the field of information storage and data communication. In the last few decades, magnetic recording has drastically improved, getting able to boost both areal density and data rate. The rapid growth of this technology was made possible only by the effort in analyzing the properties of magnetic thin films and of small magnetic particles that constitute a key part of magnetic read heads and hard disk platters. Another emerging field where magnetism plays a crucial role is spintronics, for the implementation of the next generation of nanoelectronic devices with reduced power consumption, increased memory and processing capability. Such devices are based on the interaction of magnetic materials with the spin degree of freedom of an electric current and make use of films and other magnetic structures at the nanoscale.
To investigate the electronic correlations that give rise rise to macroscopic phenomena such as magnetism, superconductivity, and topological phases, it is necessary to fully understand the micro- or nanoscopic mechanism behind them. This study often requires probes with very fine spatial resolution, capable to sense changes in magnetic field over a length scale of hundreds of nanometers or less. Not many imaging techniques can provide that, especially when the information is not accessible by optical or topographic images.
Magnetic field changes not only take place on short distances, but might as well be extremely weak to probe. This happens when the electronic properties are defined by a small fraction of electrons participating in a certain order, or when the phenomena take place in material layers that are screened by other layers above them. Furthermore, perturbation of the electrons by the sensing probe can be detrimental for the observation of some phenomena. So, in many experiments, a sensitive sensor offering high spatial resolution and low invasiveness is desired.
Among the variety of magnetic probes, the one interesting this thesis work is the Superconducting Quantum Interference Device (SQUID). SQUIDs are superconducting interferometers, defined by a loop threaded by magnetic field, and are one of the most powerful magnetometers, due to their excellent sensitivity to stray magnetic field and a negligible perturbation of the sample under investigation. A further strong point is their versatility to probe a broad range of electronic orders, due to the possibility to perform local thermometry and susceptometry measurements. The success of SQUID microscopy comes from the balance between magnetic field sensitivity and spatial resolution.
A game changer in the history of Scanning SQUID Microscopy (SSM) is represented by the innovative idea, by Finkler et al. in 2010, of fabricating a SQUID loop on the apex of a laser-pulled quartz pipette, having a sharp tip with an apex diameter of few tens of nanometers. This configuration brings three main advantages: a easier, self-aligned fabrication process, the possibility to miniaturize and tune the SQUID loop down to a nanometric size and, ultimately, a design that allows to move the SQUID loop and approach it at sample to probe distances comparable to the SQUID diameter. The consequent fine spatial resolution that can be achieved has led the SQUID-on-Tip (SOT) technique to tens of remarkable achievements over the last decade, probing magnetic fields and heat dissipation with sub-100 nm resolution.
In this thesis work, a contribution to the SOT field is brought, focused on implementing a new deposition method, that extends the range of materials that can be used to coat the quartz pipettes. Such capillaries, showing resolution below 100 nm and excellent magnetic sensitivity, were applied in a extensive investigation of magnetic phases in a bulk crystal of Cu2OSeO3. This chiral magnet is the first insulating material which was found to host a lattice of topologically non-trivial spin textures called magnetic skyrmions.
Advisors:Poggio, Martino
Committee Members:Zardo, Ilaria and Finkler, Amit
Faculties and Departments:05 Faculty of Science > Departement Physik > Physik > Nanotechnologie Argovia (Poggio)
UniBasel Contributors:Poggio, Martino and Zardo, Ilaria
Item Type:Thesis
Thesis Subtype:Doctoral Thesis
Thesis no:15385
Thesis status:Complete
Number of Pages:x, 106
Language:English
Identification Number:
  • urn: urn:nbn:ch:bel-bau-diss153857
edoc DOI:
Last Modified:08 Aug 2024 04:30
Deposited On:07 Aug 2024 11:01

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