MAGNETIC STRUCTURES - Rare Earth Magnetism: Structures and Excitations - page 129

Fig. 2.10. The helifan(3/2) struc-
ture in Ho at 50 K. The moments lie
in planes normal to the c -axis and
their relative orientations are indi-
cated by arrows. A magnetic field of
11 kOe is applied in the basal plane,
and moments with components re-
spectively parallel and antiparallel
to the field are designated by filled
and open arrow-heads. This compo-
nent of the moments has a period-
icity which is 3/2 that of the corre-
sponding helix, and the helicity of
the structure changes regularly.
A detailed consideration (Mackintosh and Jensen 1990) of the mag-
netization curves measured in Ho indicates that the metastable heli-
fan(2) may replace or co-exist with the stable (3/2)-structure, if the
measurements are made so rapidly that complete thermodynamic equi-
librium is not attained. Other stable or metastable helifans may be
involved in the five phases observed by Akhavan and Blackstead (1976).
In addition, the very pronounced hysteresis which they observed is con-
sistent with the existence of a large number of phases which have almost
the same energy, but are not easily transformed into each other.
The stability of the various periodic structures is determined by the
form of the two-ion coupling, especially the long-range component. If