The observation and suppression of an unusual equilibrium in the molecular nanomagnet Fe-8

Low temperature susceptibility and magnetic relaxation measurements show that an unusual equilibrium phase occurs below 0.9 K in the spin S = 10 molecular nanomagnet Fe-8. A plateau in the susceptibility results from a competition between ordering of spins governed by thermodynamics and quantum tunneling of the spins which tends to disorder the system. Very unusual dynamic behaviour is observed along the plateau and the relaxation is non-monotonic. The giant spins do not freeze even at very low temperature due to tunneling and, as a result, the spin entropy remains very large. If the tunneling is "turned off" by the application of an appropriate transverse field, the above effects are suppressed, and the system behaves as a normal super paramagnet.