DISCOVERY OF TAU=2 9 LOCK-IN IN HOLMIUM

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.