Slow magnetic relaxation of [Et3NH]2[Mn(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)(4)Br-12] 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 \D\S-2 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. (C) 1999 Academic Press.