Properties of HoV0 4 below 1 K III. A nuclear orientation study of the Van Vleck enhanced nuclear antiferromagnet holmium vanadate

Abstract

The y-ray emission anisotropy from the long-lived isomer 166m Ho ( I = 7) has been investigated in single crystals of the enhanced nuclear paramagnet HoVO 4 . From measurements in fields up to 2 T and temperatures between 45 and 65 mK, the nuclear magnetic moment of 166m Ho is determined as 3.60(5) pN. After adiabatic demagnetization, temperatures of approximately 1 mK were reached, and the nuclear ordered state has been studied. The results confirm that it is anti-ferromagnetic, and that the ordering is consistent with the predictions of Bleaney (1980), based on an effective field treatment of the dipole-dipole interaction between the enhanced nuclear moments. In particular, the spin-flop phase has been identified in fields of order 10 -2 T.