NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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of this value would be required to remove completely the internal
dipolar
interactions.
7.4.2 MEASUREMENTS
A specimen of powder (a) was used to measure the variation
in the width of the resonance spectrum with rotation speed about the
magic axis. The foil bearing rotor system was employed in a magnet-
ic field of 1.45 T. As this type of rotor became warm after it had
been running for some time it was not possible to measure the sample
temperature accurately, but from T1 measurements it was estimated
on average to be about 325 K. Figure 7.2 shows the decay shapes
together with the corresponding lineshapes in the frequency domain
for two different rotation speeds. Measurements were limited to
speeds above 2 kHz because below this the spinning frequencies could
not be determined to any reasonable accuracy. The variation in
the half-height linewidth of the central resonance line is shown in
Figure 7.3.
The experimental linewidth plotted includes contributions from
lifetime broadening and homogeneity of the magnetic field. Because
of the large magnet gap the homogeneity of the applied field was
relatively poor. The field gradient contribution to the static
linewidth was estimated to be about 80 Hz, although the effective
value would have been reduced upon rotation. Assuming that the
spin-lattice relaxation time of aluminium at room temperature is
6 ms, the lifetime broadening contribution to the linewidth is
approximately
50 Hz.
From Figure 7.3 it can be seen that at a rotation rate of 7.7 kHz
the central component of the absorption lineshape is still narrowing