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The dHvA oscillations in crystals of LuNi 2 B 2 C are known to persist deep inside the vortex state, across the region of the peak effect located at the edge of the irreversibility field normal phase boundary. This makes the identification of H c2 (T) ambiguous in magnetization hysteresis loops at temperatures at which the dHvA signal is prominent. One can overcome this difficulty if H c2 (T) value is ascertained from the measurement of isothermal quadrupole moment (Q) vs. field values using a vibrating sample magnetometer. In these crystals, one can witness dHvA oscillations up to about 8 K and we show that Q(T) data in LuNi 2 B 2 C can be used to construct the vortex phase diagram over the entire (H, T) range.

Using small-angle neutron scattering we have studied the magnetic flux line lattice in YNi2B2C with the field rotated 30° away from the crystalline c-axis. Previously we have reported on a square to hexagonal symmetry transition of the flux line lattice below 1 kOe for H|c . We find that the rotation of the field shifts the transition to higher fields in agreement with theoretical prediction. Two different directions in the ab-plane were chosen as axes of rotation. Rotating the field around [1,1,0] shifts the onset of the transition up to 3 kOe. Rotating the field around [1,0,0] stabilizes the hexagonal flux line lattice up to the highest measured field of 30 kOe. The difference between the two axes is also reflected in the flux line lattice distortion.

Using small-angle neutron scattering we have studied the flux line lattice and the magnetic structure in TmNi2B2C. With an applied field parallel to the crystalline c-axis several magnetic and flux line lattice symmetry transitions have been reported. The subject of this paper is the field-induced magnetic structure and how it is influenced by the flux line lattice symmetry. We find that the domain splitting of the field-induced magnetic structure is hysteretic, and appears to be determined by the history of the flux line lattice symmetry.