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

-3-
Transforming to the rotating frame such that W= +w both H0 and
H1 will be static. With the choice of the i* axis of the rotating
frame to lie along H1 equation (1.3) becomes
and
au
(ýt)* = u* X y[(Ho + Y)k + Hli*]
Heff
(H0 + Yak + Hli*
Thus, in the rotating frame, p precesses in a cone about Heff with
angular frequency YHeff' and at a fixed angle 0 to the z axis where
o tan
l
H0
H1
1
At resonance w= -yH0 so Heff =H1 and p* will precess about H1
with 6= /2. Assuming that at t=0p lies along the k axis,
then if H1 is applied as a pulse at the resonance frequency for a
time duration dT the angle through which the magnetic moment nutates
is given by
ý- YH1S&r
.
The rf pulse may then be defined by this angle.
Providing. the system of spins is non-interacting the above
equations are equally applicable to the expectation value of the
macroscopic total magnetic moment M.
In the experimental situation an alternating rf voltage
V cos Wt is applied to a coil wound around the sample with its axis
perpendicular to Ho. A linearly polarized magnetic field is prod-
uced which is equivalent to two counter rotating components of
(1.6)
(1.7)
(1.8)
(1.9)