To study fluctuating two-level spin systems, which limit performance of nanodevices, MRFM (magnetic resonance force microscopy) is a technique with a high potential. MRFM detects spins by measuring...Show moreTo study fluctuating two-level spin systems, which limit performance of nanodevices, MRFM (magnetic resonance force microscopy) is a technique with a high potential. MRFM detects spins by measuring their coupling to a micrometer sized magnet at the tip of a soft cantilever, leading to a shift in resonance frequency and quality factor. In this thesis, we present easyMRFM, a probe-head design for a SQUID-detected MRFM, operating at cryogenic temperatures. We determine a coupling-free resonance frequency of f0 = 393.42134 Hz ± 0.06 mHz and a quality factor of Q = (3.50 ± 0.03) · 104, and determined the temperature dependence thereof. An experiment is proposed and set up to detect the temperature dependent shift in these parameters, induced by spin-magnet coupling to a copper sample, thus realising easyMFM (magnetic force microscopy). A lower and upper limit for the coupling of cantilever deflection to measured signal are experimentally determined at 0.022 mV/μm < dVsignal dx < 0.486 mV/μm.Show less
Current imaging is crucial to condensed matter physics, materials research and industry. State-of-the-art current imaging setups revolve around SQUID-on-tip (SOT) probes, that scan over a sample to...Show moreCurrent imaging is crucial to condensed matter physics, materials research and industry. State-of-the-art current imaging setups revolve around SQUID-on-tip (SOT) probes, that scan over a sample to locally measure magnetic fields and temperature. The resolution of such systems is presently limited by the lack of a robust method to control the probe-sample distance. In this thesis, we develop probes for hybrid microscopy that combine SOT with STM. We theoretically investigate interesting systems, and find that our approach would considerably improve on past magnetic investigations of vortex matter. We use focused-ion-beam milling to fabricate SOT probes on top of a commercial AFM-cantilever, and show these to be very sensitive to changes in applied magnetic field and temperature. We develop a novel readout scheme to simultaneously measure a magnetic and a tunneling signal. We present a proof-of-concept STMSOT probe that displays magnetic sensitivity inside a cryogenic STM setup, and use it as an STM probe to see the topography of a NbSe$_2$ crystal. Our approach will culminate in the development of a STMSOT setup in the near future.Show less
This project follows a design that aims at enabling multi-frequency readout of STM current noise in the MHz regime, with future prospects to amplify and record such signals with a SQUID amplifier....Show moreThis project follows a design that aims at enabling multi-frequency readout of STM current noise in the MHz regime, with future prospects to amplify and record such signals with a SQUID amplifier. The design consists of multiple LC resonators to allow multi-frequency readout. Design choices, freedoms and restrictions are noted and a method to built and test the model is given. Steps are taken to allow testing of a SQUID readout experiment inside a dry dilution cryostat and the dynamic range of the SQUID is calculated and compared to typical STM (noise) currents. Mainly the feasibility of the design is studied and further effort to quantitatively explore the design is suggested.Show less
Magnetic Resonance Force Microscopy (MRFM) is a sensitive method to investigate spin systems, which uses a flexible cantilever as mechanical amplifier of the forces on its magnetic tip. However,...Show moreMagnetic Resonance Force Microscopy (MRFM) is a sensitive method to investigate spin systems, which uses a flexible cantilever as mechanical amplifier of the forces on its magnetic tip. However, MRFM is generally limited in its application at milliKelvin temperatures because existing devices rely on laser interferometry to detect cantilever deflection, which heats the cantilever, leaving many condensed matter systems out of reach for MRFM. Furthermore, lower temperatures correspond to lower cantilever force noise, so samples with more diluted spins could be investigated. SQUID-detected MRFM, using the flux induced by a moving cantilever tip, does allow for operation at milliKelvin temperatures. Yet, SQUID-detecting setups have still been limited in sample accessibility because the detection loop is printed on the sample. This thesis reports on the construction of a SQUID-detected MRFM device that employs a single probe head design to overcome the issue. The design choices and assembly methods for this device, called the easyMRFM, are discussed, as well as models to predict the sensitivity. It was found that the coupling is large enough to do optimisations in liquid helium dipstick experiments, although the thermal cantilever motion signal will only barely rise above the flux noise level. Lastly, a room-temperature magnetometry setup for cantilever chips is discussed that has proven useful in characterising cantilevers before mounting them in more permanent setups.Show less
Superconducting QUantum Interference Devices (SQUIDs) play a central role in numerous applications, ranging from cancer treatment to magnetic imaging of nanoparticles. Conventional fabrication...Show moreSuperconducting QUantum Interference Devices (SQUIDs) play a central role in numerous applications, ranging from cancer treatment to magnetic imaging of nanoparticles. Conventional fabrication methods revolve around a multi-step lithography process that encompasses etching, heating, chemical cleansing and coating, thereby limiting these techniques in practice. In this thesis we present a non-destructive alternative approach that does not entail pre- or post processing, namely, a direct-write printed SQUID patterned under 20 minutes using Focus Electron Beam Induced Deposition (FEBID). The Josephson behaviour of these devices is confirmed by performing out-of-plane magnetic field sweeps and measuring the corresponding oscillations in critical current. Our endeavours pave the way for printing sophisticated quantum systems and three dimensional superconducting sensors.Show less
Testing the mass-proportional CSL model, that describes quantum-mechanical wave function collapse, by measuring a small offset in energy due to that same collapse, requires ultra-low temperatures....Show moreTesting the mass-proportional CSL model, that describes quantum-mechanical wave function collapse, by measuring a small offset in energy due to that same collapse, requires ultra-low temperatures. These temperatures can be reached, using adiabatic nuclear demagnetization as a refrigeration method. To obtain the lowest temperature possible and to do so for a long time, dissipation has to be minimized. Theoretical work in this thesis provides a way to decrease dissipation through an optimized demagnetization ramp, resulting in a final magnetic field of 5 mT and a field ramp rate of 0.5 mT/s. Experimentally, a decrease in dissipation is found by comparing demagnetization ramps with and without an LCR circuit. The ramp with such a circuit has approximately 2.5 times less dissipation. Also discussed in this thesis is SQUID thermometry, a reliable way of measuring the temperature at ultra-low temperatures. An analysis method is presented to reduce the influence of mechanical interference when determining the temperature.Show less
A common problem in Magnetic Resonance Force Microscopy (MRFM) is the spin-induced damping of the cantilever, which drastically limits the sensitivity to spin signals. In order to solve this...Show moreA common problem in Magnetic Resonance Force Microscopy (MRFM) is the spin-induced damping of the cantilever, which drastically limits the sensitivity to spin signals. In order to solve this problem, we have developed improvements to a Persistent Current Switch (PCS) that make it less dissipative and capable of creating a stronger magnetic field at the sample. On top of this, the low noise level that our detection setup requires is conserved. The improvements are based on the use of a low-temperature magnetic core material called MetGlas [1]. We have measured the full B-H curve of the MetGlas and verified that it decreases the current required to switch a Niobium wire to the resistive state by a factor of 30. Furthermore, we have used this data to calculate the performance of a transformer made using this material, and we have calculated the expected extremely low noise level that this circuit will cause in our SQUID.Show less
In this study, we employed several methods to characterize iron-oxide nanoparticles using SQUID magnetometry and MRI. With SQUID magnetometry, we measured the Isothermal Remanent Magnetization of C...Show moreIn this study, we employed several methods to characterize iron-oxide nanoparticles using SQUID magnetometry and MRI. With SQUID magnetometry, we measured the Isothermal Remanent Magnetization of C. Elegans and two human brain samples. We obtained the iron concentration from the fit. We were able to detect changes in iron concentration due to mutations in C. Elegans. For the MRI measurements, we used Quantitative Susceptibility Mapping and an Off-Resonance Saturation method for brain phantoms. These phantoms consist of different concentrations of magnetite or ferritin dissolved in an agarose gel and mimics the human brain. With QSM we observed a comparable slope of the susceptibility/µg iron/ml. For the ORS method, a good agreement is found between the obtained iron concentration and the pre-determined iron concentration in the sample.Show less
In this thesis, the necessary elements to build up a quantum switch, the central element in a quantum random access memory, are proposed and analyzed. A network with quantum switches at its nodes...Show moreIn this thesis, the necessary elements to build up a quantum switch, the central element in a quantum random access memory, are proposed and analyzed. A network with quantum switches at its nodes forms the bifurcation path that leads an address register from a root node to an array of memory cells, activating, quantum coherently, only the quantum switches that the register encounters in its path to the memory cells. Transmon qubits and SQUIDs are used to design a superconducting device capable of routing a register of microwave photons through a bifurcation network, allowing for superposition of paths. In order to give rise to all the required interactions between the device and the address register, a non-linear capacitor, composed of two plates with carbon nanotubes in between, is introduced into the transmon. The dynamic operation of the quantum switch is analyzed using Langevin equations and a scattering approach, and probabilities of reflection and transmission of photons by (or through) the switch are computed, both for single- and two-photon processes. Computations show that, with parameters taken from up-to-date similar devices, probabilities of success are above 94%. Applications of quantum random access memories are discussed, as well as other applications of quantum switches. Also, solutions are proposed to the challenges that emerge during the study of the dynamics of the quantum switch.Show less
