Frustration in the Magnetic Molecules W$_{72}$V$_{30}$ and Pr$_{13}$ Probed by NMR and Tunnel-diode Resonator

The magnetic molecules Pr$_{13}$ and W$_{72}$V$_{30}$ have been studied by DC and AC magnetic susceptibilities and NMR. These molecules exhibit geometrical frustration resulting from antiferromagnetic intramolecular coupling of nearest neighbor paramagnetic ions. In Pr$_{13}$, 12 Pr$^{III}$ ions (S=1) interact along edge sites of an icosahedron centered about an additional Pr$^{III}$ ion. For W$_{72}$V$_{30}$, 30 V$^{IV}$ ions (S=1/2) interact along corner sharing sites of a near perfect icosidodecahedron, a spherical representation of a 2-D Kagome Lattice. Characterizing the DC magnetic response requires a distribution of exchange constants for both molecules at low temperatures. Results are compared to similar frustrated systems which feature a variety of magnetic phenomena - metamagnetic phase transitions, magnetic hysteresis without anisotropy, spin glass behavior, and superparamagnetism.