The complex magnetic behavior of holmium is a well-established experim
ental fact and has been studied by a variety of experimental technique
s, including magnetization, x-ray diffraction, thermal expansion, and
heat capacity. Our recent studies have focused on using neutron diffra
ction to study the temperature and field dependence of the helimagneti
c structure. The pitch of the helix is described by tau, the spiral wa
ve vector. tau usually varies smoothly with temperature but tends to l
ock in at various values that are commensurate with the lattice when a
field is applied. These lock-ins provide clues to the nature of the d
ifferent magnetic phases and, up to now, all the lock-ins have corresp
onded to features of the magnetic phase diagram obtained from magnetiz
ation measurements. However, recent experiments have revealed a previo
usly unobserved lock-in at tau=2/9 rlu in a 1.4 T b-axis field. It sha
res the same general features of other lock-ins except that it does no
t correspond to any feature of the magnetic phase diagram.