| The most recent work on the magnetic susceptibility of rare earth phosphomolybdates, RPO4(MoO3)12·30H 2O (or RPMO) is described here. R denotes the trivalent ions Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, or Yb. Some data on iron phosphomolybdate (FePMO) and chromium phosphomolybdate (CrPMO) is also included. The paramagnetic susceptibilities of FePMO, CrPMO and GdPMO are Curie-Weiss like from 300 K to 0.1K. The paramagnetic susceptibility data from 300 K to 0.1 K of the other RPMO salts was compared to theoretical curves based on the assumption that the magnetic ions have 23 (T) site symmetry. There was good agreement between these curve fits and the susceptibility data of DyPMO, ErPMO, and YbPMO. TbPMO and HoPMO have magnetic ground states due to mixing caused by the nuclear hyperfine interaction and the data of HoPMO, in particular, could be fit reasonably well once the hyperfine interaction was included in the curves. The hyperfine interaction seems to be weak in TmPMO. The susceptibility of the lighter RPMO could not be fit with curves based on the assumption that the magnetic ions have 23 (T) site symmetry. Magnetic ordering was observed in FePMO, GdPMO, DyPMO, ErPMO and YbPMO. There may be some exchange magnetism in FePMO but GdPMO, DyPMO, ErPMO and YbPMO are likely to be dipolar magnets. If these salts have the Fd3¯ ( T4h ) space symmetry above the ordering temperature then a classical dipolar calculation predicts that a monoclinic PBb(Cc) magnetic structure has the lowest energy. The susceptibilities of FePMO, DyPMO, ErPMO and YbPMO near absolute zero are equal to or less than that predicted for a classical PBb(PCc) dipolar magnet. A "diamond" like spin wave calculation shows some qualitative agreement with these susceptibilities. The susceptibility of GdPMO is much higher than that of a classical P Bb(PCc) magnet. |
