Dipolar Magnetic Order on a Diamond Lattice.

This work reports the first observation of dipolar magnetic order on a diamond lattice. The rare earth phosphomolybdates RPO_4(MoO_3)_{12 }cdot30H_20 (R=Gd, Er, Dy, Ho, Tb) are insulators with negligible exchange interaction between the rare earth ions. The rare earth ions form a diamond lattice of interacting magnetic dipoles with a nearest neighbor separation distance of 10.0A. Classical and quantum spin wave theories predict antiferromagnetic order for this system. The value of the susceptibility at T=0 K is predicted to depend solely on the lattice symmetry and the effective spin of the rare earth ion. The samples were cooled inside the mixing chamber of a dilution refrigerator and experimental susceptibilities were measured using fluxgate magnetometers. Susceptibility measurements are consistent with antiferromagnetic order, showing transition temperatures at approximately 30 mK (Dy), 21 mK (Gd), and 17 mK (Er). Although the Ho and Tb compounds have magnetic ground states, their transition temperatures lie below the operating range of our dilution refrigerator. These observations agree with theory and reveal large quantum spin wave fluctuations due to zero point motion effects. These fluctuations substantially exceed those of the Heisenberg antiferromagnet.