Towards magnetic force sensing of single molecules using suspended carbon nanotubes

Abstract

Measuring the magnetic moment of a single spin remains an experimental challenge. Measuring such a moment at timescales relevant to relaxation in many small spin systems, even more so. However, such a measurement would permit detailed studies of the physics of these system and could probe new avenues of technology, such as real time use of single molecule magnets for quantum information processing. This thesis presents work towards realizing fast measurements of magnetic moments on the order of a single electron spin. This will be achieved by using a suspended carbon nanotube (CNT) resonator and a CNT-magnet coupling realized through nanoscale ferromagnets.

Fabricating high quality CNT resonators for this application requires combining high quality, high throughput nanofabrication with carefully adapted growth of CNTs. The first part of this thesis describes fabrication steps developed to create full wafer arrays of CNT devices consisting of predefined contacts and fine local gates that will provide the fine magnetic structure that will allow strong CNT-magnet coupling. The growth of CNTs over these contacts is iterated to achieve long defect-free CNTs suspended over the trench between contacts. Low temperature measurements of one such device allow identification of potential fabrication improvements.

The second section of this thesis describes simulations of the proposed sensing technique. Euler-Bernoulli beam models of the CNT allow extraction of resonant frequencies as a function of device parameters, and in particular allow us to identify the impact of a single Bohr magneton magnetic moment reversal. By mapping this frequency shift as a function of the device design and operating conditions we identify favourable device designs and optimal operating conditions to obtain maximum sensitivity. By comparing the achievable frequency shifts with intrinsic resonator noise, we calculate the fundamental signal to noise ratios of this sensing technique. By also considering transient response decay we extract optimal measurement bandwidths. These calculations reveal that magnetic switching on the order of a single Bohr magneton can be observed on timescales as short as 10us with this technique.