Slow Magnetic Relaxation of [Et3NH]2Mn(CH3CN)4(H2O)2] [Mn10O4(biphen)4Br12] (biphen=2,2-biphenoxide) at Very Low Temperature

The high-field EPR spectra of a mixed valence cluster compound of formula [Et3NH]2[Mn(CH3CN)4(H2O)2] [Mn10O4(biphen) 4Br12] confirm that the high-spin ground state (S=12) of the cluster has a negative zero-field splitting, D=-0.037(1) cm-1. The reversal of the magnetization at low temperature should therefore overcome an energy barrier corresponding to DS2, which corresponds to ca. 7.7 K. Slow relaxation of the magnetization is detected below 1 K by ac magnetic susceptibility and the characteristic time follows approximately an Arrhenius law, where the height of the barrier is 7.0 K. The counterion, constituted by a manganese(II) complex, is easily oxidized, and aged samples seem to contain amorphous Mn3O4.