Instrumental aspects of high-field force-detected electron spin resonance
Abstract
Magnetic resonance force microscopy (MRFM) is a new measurement technique combining
scanning probe microscopy (SPM) and MR spectroscopy, offering the potential
of high resolution chemical specific imaging. MRFM is based on the principle of force
detection of magnetic resonance (FDMR) in which the magnetisation of a sample in a
magnetic field is coupled to an atomic force microscopy cantilever via a field gradient.
Magnetic resonance is used to modulate the sample magnetisation at the cantilever
resonant frequency and the resulting oscillating force on the cantilever leads to oscillations
which may be detected optically. The high sensitivity of force detection offers
the potential for single electron spin sensitivity.
This thesis describes instrumental aspects of ESR based FDMR experiments and
presents the first results at high fields (3.3T). High fields are advantageous for sensitivity
and spectral resolution. However, they pose significant technical challenges.
FDMR measurements on the organic conductor (fluoranthene)2PF6 were carried out
in experiments based around an existing quasi-optical high field ESR spectrometer.
Further measurements on (FA)2PF6 and DPPH are presented together with
progress towards the construction of a high field MRFM system, based on a commercial
SPM instrument. Experiments were performed with both magnet-on-cantilever
and sample-on-cantilever configurations with the former the favoured method for potential
imaging applications. Signal detection uses a novel fibre-optic interferometer.
Cantilever magnets of low conductivity ferrite appear to be more promising for high
Q measurements than the metallic magnets favoured by most other groups.
Experiment sensitivities are estimated at around 4.4 x 10⁸ polarised electron spins,
comparable to conventional commercial ESR spectrometers. Experimental consistency
was difficult, especially regarding the positioning of probe and sample, an area
in which refinement is essential for repeatable and sensitive experiments. The potential
for imaging is attractive and the prospect of single spin detection is discussed.
Type
Thesis, PhD Doctor of Philosophy
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