Resonant Magnetization Tunneling in Single-Molecule Magnets

Abstract

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 [Mn^IV Mn^III 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.