We report the observation of steps at regular intervals of magnetic field in the hysteresis loop of a macroscopic sample of oriented Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 crystals. The magnetic relaxation rate increases substantially when the field is tuned to a step. We propose that these effects are manifestations of thermally assisted, field-tuned resonant tunneling between quantum spin states, and attribute the observation of quantum-mechanical phenomena on a macroscopic scale to tunneling in a large (Avogadro's) number of magnetically identical molecules. [S0031-9007(96)00131-7]

In 1935, Schrodinger attempted to demonstrate the limitations of quantum mechanics using a thought experiment in which a cat is put in a quantum superposition of alive and dead states. The idea remained an academic curiosity until the 1980s when it was proposed that, under suitable conditions, a macroscopic object with many microscopic degrees of freedom could behave quantum mechanically, provided that it was sufficiently decoupled from its environment. Although much progress has been made in demonstrating the macroscopic quantum behaviour of various systems such as superconductors, nanoscale magnets, laser-cooled trapped ions, photons in a microwave cavity and C60 molecules, there has been no experimental demonstration of a quantum superposition of truly macroscopically distinct states. Here we present experimental evidence that a superconducting quantum interference device (SQUID) can be put into a superposition of two magnetic-flux states: one corresponding to a few microamperes of current flowing clockwise, the other corresponding to the same amount of current flowing anticlockwise.

Single-molecule magnets straddle the classical and quantum mechanical worlds, displaying many fascinating phenomena. They may have important technological applications in information storage and quantum computation. We review the physical properties of two prototypical molecular nanomagnets, Mn12-acetate and Fe8: Each behaves as a rigid, spin-10 object and exhibits tunneling between up and down directions. As temperature is lowered, the spin-reversal process evolves from thermal activation to pure quantum tunneling. At low temperatures, magnetic avalanches occur in which the magnetization of an entire sample rapidly reverses. We discuss the important role that symmetry-breaking fields play in driving tunneling and in producing Berry-phase interference. Recent experimental advances indicate that quantum coherence can be maintained on timescales sufficient to allow a meaningful number of quantum computing operations to be performed. Efforts are under way to create monolayers and to address and manipulate individual molecules.

We have measured the dc magnetization at low temperatures of tetragonal crystals of Mn 12 acetate complex ͓Mn 12 O 12 ͑CH 3 COO͒ 16 ͑H 2 O͒ 4 ͔, a material composed of a large ͑Avogadro's͒ number of identical magnetic molecules, each of spin 10. Exchange coupling between Mn ions within each molecule is very strong, while the interaction between molecules is negligible. A large, uniaxial anisotropy ͑ϳ60 K͒ gives rise to a doubly degenerate ground state corresponding to spin projections of Ϯ10 along the easy axis ͑c axis͒; hysteretic behavior is found below a blocking temperature T b ϳ3 K. Based on measurements of oriented crystallites at temperatures between 1.7 and 3.2 K, we report strong evidence for resonant tunneling of the magnetization: periodic steps in the hysteresis loop, and periodic marked increases in the magnetic relaxation rate at the magnetic fields corresponding to these steps. A total of seven increases in the relaxation rate were found within the temperature range of our experiments with a period of 0.46 T; we suggest that many more such steps would be found at lower temperatures. We attribute these observations to thermally assisted resonant tunneling of the magnetization and propose a detailed model to account for our results. ͓S0163-1829͑97͒00709-1͔