DIPOLAR MAGNETISM IN A DIAMOND LATTICE - DIRECT OBSERVATION OF ZERO-POINT MOTION EFFECTS

We report the first observation of dipolar magnetic order in a diamond lattice, in the RPO4(MoO3)12 . 30H2O compounds (R = Gd, Dy, Er). Clas sical theories predict antiferromagnetic order for a diamond lattice o f dipoles, with a constant susceptibility chi0 perpendicular-to below T(N) which depends only on lattice symmetry. Unusually large zero poin t motion effects reduce chi0 perpendicular-to by a factor which depend s additionally only on spin. Experimental susceptibilities for ions wi th different effective spin provide an unusual direct measurement of t he magnitude of zero point motion effects. We compare these quantitati vely with theory.