Magnetic properties of a new spin cluster topology with high-spinground state:the spin cluster $\rm [Mn^{II}_4Mn^{III}_3(teaH)_3(tea)_3](ClO_4)_2\cdot3MeOH

The new spin cluster $\rm [Mn^{II}_4Mn^{III}_3(teaH)_3(tea)_3](ClO_4)_2\times 3MeOH$ realizes a topological structure of mixed-valence manganese clusters which is especially favorable for a high-spin ground state. The magnetic properties of the spin cluster are studied numerically by exact diagonalization of the spin Hamiltonian. By magnetic susceptibility two exchange constants are found: $J_1/k_{\rm B}=-1.28 \;{\rm K}$ and $J_2/k_{\rm B}=+4.25\;{\rm K}$, which lead to a S=11 high-spin ground state. Electron spin resonance (ESR) measurements at three frequencies 95, 190 and 285 GHz confirm the large spin of the ground state and reveal an Ising anisotropy of the ground state which is characterized by the spin Hamiltonian $H=\alpha(S_z)^2+\beta(S_z)^4$ with $\alpha /{\rm h.c.}=-0.08\;{\rm cm^{-1}}$ and $\beta=-2.1\times 10^{-4}\;{\rm cm}^{-1}$. Ac-susceptibility shows thermally activated relaxation of the magnetization for temperatures above $T=1\;{\rm K}$ with an activation energy of $\Delta E/k_{\rm B}=-19.5\;{\rm K}$ and a relaxation time of $\tau_0\approx 10^{-8}\;{\rm s}$.