Resonant Magnetization Tunneling in Single-Molecule Magnets

Data are presented for three different types of single-molecule magnets (SMM's): [Mn12O12(O2CR)16(H2O)4],[Cation][Mn12O12(O2CR)16(H2O)4], and the distorted cubane [MnIV MnIII 3O3X(O2CR)3L3] complexes. All three types of complexes exhibit slow magnetization relaxation at temperatures below 5 K. Each molecule can change the direction of its magnetization only slowly at these low temperatures. Manisfetations of this are seen in magnetization hysteresis loops and in the presence of frequency-dependent out-of-phase ac magnetic susceptibility signals (X′M). It is shown that the neutral Mn12 SMM's exist in isomeric forms that differ in the positoning of the 4H2O and 16 carboxylate ligands. The orgin of the two X′M signals for the neutral Mn12 complexes is discussed. All Mn12 complexes exhibit magnetization hysteresis loops with steps seen at regular increments of magnetic field. The step heights and coercive fields for the loops vary from one Mn12 SMM to another. Step-structured hysteresis loops are also seen for both the S = 19/2 [Mn12 −] and S = 9/2 [Mn4] SMM's. The steps seen at zero field are interesting since these are half-integer spin complexes that should not tunnel in zero field. Apparently, an internal magnetic field due to the nuclear spins in the complex is responsible for the tunneling of these half-integer spin complexes.