If you don't know what ESR is, click here.
Conventional ESR systems tend to operate at frequencies below about 10 GHz.
In theory, systems operating at higher frequencies are just the same, but
simply 'scaling up' conventional instruments to the mm-wave region leads to
serious practical problems.
The main problems are:
- High losses and other problems in conventional waveguides.
- Need for very high applied magnetic fields.
Circuit diagram of simple mm-wave ESR system.
The St. Andrews system gets around the problems by employing the
Quasi-Optical Circuit methods which our group specialise in. The instrument
we have developed was built in conjunction with Prof. P. C. Riedi's ESR
group at St. Andrews. They contributed considerable experience in ESR and in
the use of the superconducting magnets required to reach the high applied
fields needed for mm-wave ESR.
In the St. Andrews MM-Wave ESR system the sample is placed inside a
supercooled (to 4 Kelvins) magnet which we can use to apply H-fields up to 8 Teslas.
Using Quasi-Optics we can illuminate the sample with a controlled signal in
the 80 GHz to 300 GHz frequency range. This field and frequency are much
higher than conventional ESR, hence this type of system is often called a
'high-field ESR' instrument. The high field/frequency allows the instrument
to have a sentivity and resolution which is between 100 times and 100,000
times better than conventional instruments.
The system is used for 'in house' research on magnetic materials and thin
films. It is also available for use by external researchers who wish to make
measurements on other samples. The system is of particular value to chemists
and biologists who wish to analyse small samples of weakly interacting
materials, and materials whose resonant details are too fine to be resolved
by conventional commercial instruments.
The QO arrangement means the same basic system can be used over the whole
frequency range of interest. It has losses, etc, which are much lower than
in competing instruments which try to use ordinary waveguide at these
frequencies. As a result, it is comparatively easy to use and gives very
clear results. The optical approach also means it can employ a novel
polarisation discrimination approach which significantly improves its
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.