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Fig. 2.9.
Mean-field calculation of the free energy per ion for different
magnetic structures in Ho at 50 K, as a function of the magnetic field
along an easy
b
-axis. Thefreeenergyisineachcaseminimizedwithre-
spect to the wave-vector which characterizes the structure, as illustrated
for the fan phase in the insert.
even though the underlying exchange function is constant, and they
agree very well with those observed by neutron diffraction. For the helix,
fan and helifan(3/2) structures, the experimental (theoretical) values of
Q
are respectively 0.208 (0.211), 0.170 (0.168), and 0.063 (0.066), times
2
π/c
. The period of the fan phase increases relative to that of the he-
lix because of the resulting increase in the opening angle of the fan,
expressed by the relation (1.5.21). This allows a decrease in the ex-
change energy which is greater than the concomitant increase of the
Zeeman energy. The change in
Q
in the various helifan phases is there-
fore to a very good approximation proportional to their magnetization.
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